| Author |
Ng, Maggie |
| Title |
A theoretical investigation of the effects of different levels of ab initio, density functional theory (DFT) and transition state theory (TST) calculations on computed reaction rate coefficients |
| Department | Dept. of Applied Biology and Chemical Technology |
| Year | 2016 |
| Subject |
Chemical reactions -- Mathematical models. Chemical kinetics Hong Kong Polytechnic University -- Dissertations |
| Degree | Ph.D. |
| Pages | xxxvi, 246 pages : color illustrations |
| Language | English |
| OneSearch | https://www.lib.polyu.edu.hk/bib/b2898248 |
| URI: | http://theses.lib.polyu.edu.hk/handle/200/8544 |
| Abstract |
Kinetic modeling of gas-phase reaction systems has improved enormously due to rapid advances in computational technology in the last decade. It becomes a powerful tool to understand complex chemical systems in a broad range of areas, including combustion, fire suppression, chemical vapor deposition (CVD) and atmospheric chemistry. Reliable thermochemical and kinetic data is required to incorporate accurate description of chemical reactions into kinetic models. However, this data may not be experimentally available due to some experimental limitations. In this connection, calculating these thermochemical constants and reaction rate coefficients in an ab initio manner has the advantage of not being restricted by any experimental conditions. However, the relationship between various levels of approximation in ab initio, density functional theory (DFT) and transition state theory (TST) is complicated and it is difficult to make the appropriate choices of levels of approximation in each of these areas in calculations. The aim of this work is to investigate the effects of different levels of ab initio, DFT and TST calculations on computed reaction rate coefficients, in order to ultimately establish a consistent, reliable and practical methodology for calculating reaction rate coefficients in an ab initio manner for the development of a reliably computed thermochemical and kinetic database, which complements available experimental databases, for kinetic modeling. State-of-the-art quantum chemical calculations were performed at different levels of theory to obtain reaction enthalpies, barrier heights, rate coefficients and branching ratios (if applicable) for the gas-phase reactions of importance in fire suppression, thin film preparation in the electronics industry and atmospheric chemistry. The three gas-phase reactions being presented in this work are (i) the reaction between hydrogen atom and a Halon-alternative fire suppression agent, heptafluoropropane (FM 200), (ii) the decomposition reactions of a feed gas, hexafluoropropylene oxide (HFPO), utilized in CVD for thin film preparation in the electronics industry and (iii) the Cl-initiated oxidation of formic acid (HCOOH). In particular, the effects of basis set size, the lower-level geometry and the complete basis set (CBS) extrapolation scheme on computed relative energies, the variational, tunneling and classical adiabatic ground-state (CAG) effects on computed rate coefficients, and the correlation of computed rate coefficients obtained at different TST levels with some computed critical quantities of the minimum energy paths (MEPs) were examined. Although no single level considered in this work gives good agreement with the theoretical benchmarks for all relevant stationary points of all the gas-phase reactions being studied, the BH&HLYP, MPW1PW91 and BMK functionals are recommended for reliable rate coefficients because of their good performance in reaction rate coefficient calculations. It was found that variational effect depends on both the barrier height of the classical potential energy (VMEP) curve, and the shape and position of the dip in the zero-point energy difference (ΔZPE) curve. The results also showed that multidimensional tunneling effects depend on both the barrier height and the imaginary vibrational frequency of the transition state, and CAG corrections at the TST level are similar to the variational effects at the canonical variational transition state theory (CVT) level. This work not only improves the understanding of the reaction mechanisms and the kinetics of the gas-phase reactions being studied, but also provides valuable data for building up a high-level ab initio thermodynamic and kinetic database on these reactions, and makes progress in calculating reliable reaction rate coefficients in an ab initio manner. |
| Author |
Wei, Xuexia |
| Title |
Theoretical stress analyses for rock spheres and cylinders under the point load strength test |
| Department | Dept. of Civil and Structural Engineering |
| Year | 1999 |
| Subject |
Rock mechanics Rocks -- Testing Strains and stresses -- Testing Hong Kong Polytechnic University -- Dissertations |
| Degree | Ph.D. |
| Pages | xxvi, 204 leaves : ill. ; 30 cm |
| Language | English |
| OneSearch | https://www.lib.polyu.edu.hk/bib/b1479967 |
| URI: | http://theses.lib.polyu.edu.hk/handle/200/1191 |
| Abstract | The Point Load Strength Test (PLST) is an extremely convenient and useful method for rock classification and strength estimation. Although the PLST has been extensively studied by experimental approach, there are relatively few theoretical studies for the PLST. Analytic studies for the PLST include analyses of isotropic spheres under the diametral PLST by Hiramatsu and Oka (1966), finite cylinders under the axial PLST by Wijk (1978) and by Peng (1976), and finite cylinders under the diametral PLST by Wijk (1980) and Chau(1998a). However, the exact stress field within a cylinder under the axial or diametral PLST has not been solved analytically. The only analytic results are the approximate solutions by Wijk (1978, 1980), in which the interaction between the indentors and the surfaces of the cylinder was idealized by two point forces. The finite element method has been applied to the axial PLST (e.g. Peng, 1976), but the contact problem between the indentors and the end surfaces was not considered either. The analytic solution by Chau (1998a) is for finite isotropic cylinders with zero shear displacement on the two end surfaces under the diametral PLST. Moreover, all of these analyses are restricted to considering rock as isotropic solids, there is no analytic solution for anisotropic rocks under the PLST. Therefore, this dissertation presents a series of exact analytic solutions for anisotropic spheres and finite isotropic cylinders under the PLST. More specifically, the dissertation presents: (I) an exact analytic solution for spherically isotropic spheres under the diametral PLST; (II) an exact analytic solution for finite isotropic cylinders under the axial PLST; (III) an exact analytic solution for finite isotropic cylinders under the diametral PLST; and (IV) a general analytic solution for finite isotropic cylinders under arbitrary surface load. In addition, a series of the PLST experiments have also been done on plaster, a kind of artificial rock-like material, to verify the theoretical solutions. The method of solution for spheres uses the displacement potential approach together with the Fourier-Legendre expansion for the boundary loads. The solution reduces to the classical solution by Hiramatsu and Oka (1966) in isotropic case. Numerical results show that the maximum tensile stress along the axis of loading is very sensitive to the anisotropy in Young's modulus, Poisson's ratio and shear modulus, while the pattern of the stress distribution is relatively insensitive to anisotropy of rocks. The method for finite isotropic cylinders under the axial or diametral PLST expresses displacement functions in terms of series of the Bessel and modified Bessel functions; and the contact problem between the surfaces of the cylinder and the indentors, through which the point loads are applied, is considered. Numerical results show that the tensile stress distribution along the axis of loading within isotropic cylinders, either under the axial or diametral PLST, similar to that within isotropic spheres under PLST, is not uniform, tensile stress concentrations are developed near the point loads, the maximum tensile stress increases with the decrease of Poisson's ratio and the size of loading area. The theoretical prediction for the size and shape effects of the specimen on the PLST agrees well with experimental results. More importantly, if the sizes of the specimens are comparable (ISRM, 1985), the tensile stress distributions along the axis of loading in a sphere, in a cylinder under the axial or diametral PLST, have the similar pattern. This conclusion indicates that the PLST is insensitive to the exact shape of the specimen, thus we provide the first theoretical basis for irregular lumps under the PLST. In addition, by generalizing the method of solution for axisymmetric problem of the axial PLST and for non-axisymmetric problem of the diametral PLST, a new analytic framework for analyzing stresses for finite isotropic solid cylinders under arbitrary surface load is also presented. |
| Author |
Tsang, Chun-ho |
| Title |
Theoretical studies on domain structures in ferroelectrics |
| Department | Dept. of Applied Physics |
| Year | 2006 |
| Subject |
Hong Kong Polytechnic University -- Dissertations. Ferroelectric crystals. Ferroelectricity -- Mathematical models. |
| Degree | Ph.D. |
| Pages | 137 leaves : ill. (some col.) ; 30 cm. |
| Language | English |
| OneSearch | https://www.lib.polyu.edu.hk/bib/b2059300 |
| URI: | http://theses.lib.polyu.edu.hk/handle/200/20 |
| Abstract | Common ferroelectric crystals, such as barium titanate (BaTiO3), possess two or more possible spontaneous polarization orientations in the absence of electric field. It is well known that a ferroelectric crystal tends to split into many domains in which all of the electric dipoles have an identical spontaneous polarization direction. Many ferroelectrics are used in the form of ceramics which are polycrystals comprising a large number of differently oriented single crystal grains, each of which contains one or several domains. Domains play an important role in all phenomena of polarization and deformation of ferroelectrics. This project investigates ferroelectric domain structures in the absence of external field and evolution dynamics under the action of an alternating field, both in crystals and in ceramics. First, we focus on the formation of domain pattern in a single crystal. We develop a model to study two-dimensional domain structures using the time-dependent Landau theory. The effects of external electric field, inhomogeneity in polarization and electroelasticity have been considered. The elastic strains are subjected to the elastic compatibility constraint. Unlike previous models for two-dimensional domain formation, the depolarization effect is involved via the Poisson equation. Next, we attempt to model ceramics. In the study of ceramic domains, we use a model containing variously oriented grains. The behavior of each grain is described by our foregoing model for ferroelectric crystals. The grain boundary region is assumed to be in the paraelectric phase. Finally, we use the model to simulate the evolutions of domain structures in single crystals and ceramics under an A.C. electric field. Numerical computations are based on a 128 x 128 grid with periodic boundary conditions applied to the edges. We use BaTiO3 to perform model calculations. In our simulations for single crystal BaTiO3, it is found that only head-to-tail dipole arrangements occur in the whole formation process. In particular, our simulations reproduce experimentally observed domain patterns, including 90o wedged domains, 180o antiparallel domains and 90o wedged domains with 180o sub-domains (for brevity, "90o with-180o" domains), the formation of the latter has not been reported in previous theoretical investigations. The results also indicate that 90o domains occur frequently and is most stable; 90o with-180o domains are rare. Electrostrictive energy and elastic energy are found to dominate the pattern formation process. During formation from initially random dipoles, "vortex" domains, like ferromagnetic vortices, appear in a short period. These findings offer insights into the roles of electroelasticity and the inhomogeneity in polarization in the formation process as well as provide an understanding of the mechanism for forming domain pattern and the relation among different domain configurations. For the studies on ceramics, our simulations report that the domain pattern in a grain can be influenced by the neighboring grains. In particular, a banded structure over many grains is obtained - this has been observed experimentally but not been reported in other theory work. Some cases illustrate that the domain pattern in a grain does not necessarily propagate to the surrounding grains. In general, our results show that grain size and grain boundary regions play crucial roles in the domain pattern. We are unaware of other systematic modeling of domain structures over many grains in the open literature. When an A.C. electric field is applied in the simulations, the results give quite a square P-E hysteresis loop for BaTiO3 crystal which agrees with experiment. The domain pattern shows a single domain immediately after coercivity. In the case of BaTiO3 ceramics, the domain pattern obtained differs from those in single crystals. Single domain pattern does not usually appear in the process. The simulations report that the P-E loops change with the thickness of grain boundary regions. |
| Author |
Ling, Chi Wai |
| Title |
Theoretical study of diatomic plasmonic chains : Zak phase and spatial-temporal symmetries |
| Department | Dept. of Applied Physics |
| Year | 2016 |
| Subject |
Plasmons (Physics) Nanostructured materials. Hong Kong Polytechnic University -- Dissertations |
| Degree | Ph.D. |
| Pages | xxi, 122 pages : color illustrations |
| Language | English |
| OneSearch | https://www.lib.polyu.edu.hk/bib/b2898272 |
| URI: | http://theses.lib.polyu.edu.hk/handle/200/8539 |
| Abstract | Being small and compact, plasmonic chains and arrays of metal nanoparticles have been of great interest for decades, as they were proven to support electro-magnetic energy transfer via coupled plasmon modes. In this thesis, we studied plasmonic chains with two nanoparticles in a unit cell, namely diatomic plasmonic chains. This simple dimmerization enriched plasmonic chains with many possibilities. We presented diatomic plasmonic chains with alternative separations to mimic the Su-Schrieffer-Heeger model, a simple model which is able to have topological non-trivial phase. In this case, the inverse polarizabily and the coupled dipole equation play the roles of the eigen-energy and the Hamiltonian matrix with respectively. When two diatomic plasmonic chains with different Zak phases are connected together, topologically protected edge states will be formed. We also proposed a magnetized diatomic plasmonic chain to obtain non-reciprocal dispersion relations. By hybridizing with free photons modes, such structure supports one-way propagating coupled plasmon modes over a small range of frequencies. The relationship between the crucial rotation-time-reversal symmetry and the spectral reciprocity in this structure was discussed and demonstrated. Finally, we introduced the zero extinction property induced by inversion-time-reversal (PT) symmetry, and studied the relationship between the Zak phase and the PT symmetry. The Zak phase was found remained quantized for a 2-band PT symmetric system, provided in the unbroken PT phase and a modification on the Bloch's function. Coated diatomic plasmonic chain with gain-loss-balanced shells is given as an example, which supports edge states with complex frequencies when the Zak phase is π. Our results pave the way to achieve various optical components based on plasmonic chains in the future. |
| Author |
Sze, Lip-kit |
| Title |
Thermal and emission characteristics of an inverse diffusion flame with circumferentially arranged fuel ports |
| Department | Dept. of Mechanical Engineering |
| Year | 2007 |
| Subject |
Hong Kong Polytechnic University -- Dissertations. Flame. Diffusion. Combustion. |
| Degree | Ph.D. |
| Pages | 1 v. (various pagings) : ill. (some col.) ; 30 cm. |
| Language | English |
| OneSearch | https://www.lib.polyu.edu.hk/bib/b2145875 |
| URI: | http://theses.lib.polyu.edu.hk/handle/200/1344 |
| Abstract | An inverse diffusion flame (IDF) exhibits the characteristics of both partially premixed flame and diffusion flame. However, it is less liable to liftoff or flashback compared with premixed flame, and it burns cleaner compared with diffusion flame. The study is mainly experimental in nature. The objective is to determine the structure, thermal and emission characteristics of an IDF with circumferentially arranged fuel ports (CAP). Test rigs were set up to generate the CAP IDF, to measure the flame length, the flame temperature and the combustion species concentrations of the free jet; and to measure the heat flux of the impinging CAP IDF. According to the flame appearance under the influence of the air jet Reynolds number, Reair, and the overall equivalence ratio, o, the CAP IDF was classified into six types. Among them, the Type IV and Type V which occur at Reair > 2000 are of particular interest. They consist of a short entrainment zone and an elongated flame tower, separated by a narrow neck. Fuel is entrained and impinges on the central air jet, thus enhances air/fuel mixing which improves the flame's thermal performance. Type V flame has a dual flame structure due to its high level of fuel supply. The flame lengths were measured and an empirical relationship was derived to correlate the normalized flame length with the ratio of the air and fuel jet velocities. The flame temperature and centerline O2, CO2, CO and NOx concentrations were measured to provide information on the thermal and emission characteristics of the CAP IDF. There is a significant reduction of centerline NOx concentration in the fuel-rich combustion zone of the Type V flame at o>=2.2, which is an indication of NOx reburning. The same phenomenon was also observed in the analysis of the NOx emission index (EINOx). An empirical relationship correlating the EINOx formation rate per unit volume with o was derived. The shape, thermal and emission characteristics of the CAP IDF were compared with an equivalent CoA IDF, with co-axial air and fuel jets. The results show that there is always a better air/fuel mixing in the CAP IDF. The CAP burner tends to shorten the flame length and extends the partially premixed combustion characteristics to broader ranges of Reair and o. Moreover, the temperature of the CAP IDF is generally higher, indicating more intense combustion. The heat transfer characteristics of the impinging CAP IDF were investigated. The results show that the maximum heat flux is always associated with the visual partially premixed reaction zone impinging on the target surface. While in the fuel-rich combustion region and the post-flame region, a relatively flat heat flux plateau is formed which is an advantage for heating applications. A numerical simulation on the non-reacting flow, as well as on the reacting flow, of the CAP IDF was carried out to supplement the experimental results. The non-reacting flow simulation enhances our understanding on the air/fuel mixing effectiveness. The reacting flow simulation shows that the simulated results agree well with the experimental results at both o=1.0 and o=1.6, except that the simulated temperature is under estimated at the central region. In summary, this study has clarified the visual, physical and thermodynamic structure of an inverse diffusion flame with circumferentially-arranged fuel ports, which is a novel design. The NOx-reburning mechanism has been identified in the IDF having a dual flame structure, which is useful for low-NOx combustion applications. Two useful empirical correlations, one for the flame length, and the other for the EINOx formation rate per unit volume of the flame, have been developed. |
| Author |
Zhen, Haisheng |
| Title |
Thermal and emission characteristics of an inverse diffusion flame with induced swirl |
| Department | Dept. of Mechanical Engineering |
| Year | 2010 |
| Subject |
Hong Kong Polytechnic University -- Dissertations Flame Jets -- Fluid dynamics Heat -- Transmission |
| Degree | Ph.D. |
| Pages | xviii, 257 leaves : ill. (some col.) ; 30 cm. |
| OneSearch | https://www.lib.polyu.edu.hk/bib/b2351750 |
| URI: | http://theses.lib.polyu.edu.hk/handle/200/5529 |
| Abstract |
This study aims to investigate the thermal, emission and heat transfer characteristics of a swirl-stabilized inverse diffusion flame (IDF) burning liquefied petroleum gas (LPG). The developed swirl burner operates a highly swirling IDF with a geometric swirl number of 9.12 and the swirling IDF is associated with flow recirculation. The identified governing parameters of the swirling IDF are air jet Reynolds number Re, overall equivalence ratio Φ, geometric swirl number S′ and nozzle-to-plate distance H/d. Experimental investigations were conducted to study the characteristics of the swirling IDF including flame appearance, flame structure, flame temperature, in-flame gaseous species, overall pollutant emissions and flame impingement heat transfer. The swirling IDF is a peach-shaped flame and the analysis of flame structure characterizes it into three distinctive zones. Zone 1 is the internal recirculation zone (IRZ) in the centre and close to the burner exit. The IRZ is induced by the axial adverse pressure gradient in the swirling jet flow and occupies a large portion of the swirling IDF by volume. Zone 2 is the flame boundary in the lower section, which is in contact with ambient air on the outer side and with Zone 1 on the inner side. Zone 3 is the flame boundary in the upper section. Zone 1 is differentiated from the other two zones by associating with backward flow velocities. Under the effect of induced swirl, fluid particles coming out of the burner mainly flow through Zone 2 into Zone 3. Then a large portion of the fluid particles in Zone 3 are diverted towards the burner axis and recirculate towards the burner exit to form the IRZ, i.e. Zone 1. Therefore, Zone 3 acts as a source of reversed fluid particles in the IRZ. The temperature as well as in-flame O2/CO/CO2/NOx concentration measurements suggest that the distributions of these parameters are coupled with the flow field. It is seen that Zone 1 is a large high-temperature IRZ and has uniform temperature and gaseous species concentration distributions, caused by the strong mixing of supplied air/fuel, combustion products and entrained ambient air. Zone 2 has sharp gradients in both temperature and gaseous species concentrations, due to the rapidly changing flow properties in this small region. Zone 2, always navy-blue in color, is where both intense mixing of the supplied air/fuel and intense combustion occur, indicative of location of the flame front. Zone 3 is the post-combustion region where oxidation, accumulation and dilution of the combustion products occur and the remaining fuel is burned in a diffusion mode. The effects of Re and Φ on the in-flame gaseous species concentrations show that the concentration distribution is coupled well with the combustion condition. CO2 and NOx concentrations have a trend of variation similar to that of the flame temperature and thus the thermal NO mechanism dominates the NOx formation. The data obtained from flue gas measurement reveal a moderate level of NOx emission and an ultra low level of CO emission under certain operational conditions. When the swirling IDF impinges vertically normal to a flat surface, the swirling effect affects the local heat flux in three ways. 1. The heat transfer at the stagnation point is severely suppressed. 2. The peak of local heat flux dwells at a radial distance from the stagnation point. 3. The radial position of peak local heat flux shifts farther away from the stagnation point with increasing H. There exists an optimum H at which the heat transfer to the target surface is maximum and the optimum H increases with increasing Φ while the Reynolds number and the swirl number are unchanged. The swirling IDF is compared with a non-swirling IDF under the same operational conditions. It is found that the flame length can be shortened and the flame stability can be improved by the induced swirl. The IRZ present in the swirling IDF plays an important role in flame stabilization and flame length shortening. Because of the better mixing and thus more complete combustion, the swirling IDF is partially premixed in nature, thus it is cleaner with less soot formation. Further, a slightly higher emission index (EI) of NOx and a much lower EICO are produced in the swirling IDF. The comparison of the impingement heat transfer reveals that the swirling IDF has more complete combustion and thus is accompanied by higher heat transfer rates at small H at which there exists a cool core in the non-swirling IDF. The swirling IDF, however, has worse heat transfer at higher H where the non-swirling IDF achieves more complete combustion because the swirling IDF has been much cooled by the entrained ambient air at these H. Upon comparing the swirling and non-swirling IDFs at the same Re and Φ and their respective optimum H, an unfavorable effect of swirl on the overall heat transfer rate is observed. Two typologies, pre-mixed flame (PMF) and inverse diffusion flame (IDF), operating with different mixing mechanisms, were compared under the same fuelling and airing rates. The results show that the two swirling flames have similar visual features including flame shape, size and structure because the Reynolds number and the swirl number which are important parameters representing the aerodynamic characteristics of a swirling jet flow, are almost the same. Both PMF and IDF are found to be stabilized by the IRZ and the flame stability of IDF is higher than that of PMF. The finding confirms that IDF is a combination of premixed flame and diffusion flame, thus can exploit the advantages of both flames. PMF achieves complete combustion at a higher equivalence ratio, as compared with IDF. Both temperature and gaseous species including O2, CO, CO2 and NOx in the IRZ are uniformly distributed due to the well-stirred condition deriving from mixing between the supplied air/fuel, combustion products and entrained ambient air. PMF achieves lower levels of NOx and CO emissions. The premixing of fuel and air in PMF significantly enhances the homogeneity of unburned mixtures and reduces the characteristic lifetime of burned gas pockets in the combustion region to curtail NOx formation. The low temperature associated with fuel-lean combustion is also helpful in reducing the amount of NOx formed via the thermal NO mechanism. The lower EICO is due to the premixing of fuel and air and the higher O2 concentration, both of them which support the conversion of CO to CO2. |
| Author |
Chao, Huang |
| Title |
Thermal human modelling for functional clothing design |
| Department | Institute of Textiles and Clothing |
| Year | 2015 |
| Subject |
Fashion design -- Technological innovations. Hong Kong Polytechnic University -- Dissertations |
| Degree | Ph.D. |
| Pages | xviii, 320 pages : color illustrations |
| Language | English |
| OneSearch | https://www.lib.polyu.edu.hk/bib/b2815727 |
| URI: | http://theses.lib.polyu.edu.hk/handle/200/8031 |
| Abstract | Functional and smart clothing has been an up-and-coming product for the fashion industry. The human model or mannequin, which is a bridge between customer's physical information and design, utilization, has vital importance as an practical design tool. The physiological information of the human body may be increasingly needed in funtional product develop process. The traditional geometric human models (G-model), which convey anthropometric messages, such as body shape, body volume and measurements, may be promoted to functional human model. Thermal function is highlighted in this study, based upon an emphasized potential of developing new thermal functional clothing for health enhancement and rehabilitation purpose. Based on the data acquired from latest infrared (IR) technology and 3D body scanning, the internal relationship among physically anthropometric parameters and physiological properties like body temperatures had been found. Systematic approaches on constructing visualised, quantified and individualized 3D thermal human modelling (Ti-model) with physiological features and pathophysiologic features had been accomplished. The Ti-model with physiological features accurately reflect the real thermal physiology of the human body which can be directly used for thermal related functional product development, especially for functional clothing. It will provide more efficienct and accuract solutions for 3D functional design, 3D pattern making, 3D virtual fitting and manufacturing. The Ti-model with pathophysiologic features set up a platform for people and their health care providers, which provided visualised and quantified pathophysiologic information for health status monitoring and medical diagnoses. The exact location and area of the lesions of the human body is demonstrated with accurate size and dimension, which is necessary when developing functional and smart clothing with rehabilitation or treatment purposes and can be directly adapted to 3D printing technology with efficiency and accuracy. The average 2D IR images and average 3D thermal human modelling (Ta-model) have been set up, which provides physiological information in quantification and visualization format. The scientific meaning of average IR images and Ta-model contributed advancements on the thermal physiology study to 2D and 3D epoch. The clinical meaning of it was to set up 2D and 3D thermal objective of reference to be comparable with individual's IR images and Ti-model to detect the invisible abnormity of individuals for healthcare and disease monitoring. As a bridge over thermal physiological science and thermal related ergonomic application, this multi-disciplinary study greatly broadens the field of vision for the human being to understand their thermal related physiological and pathophysiologic features from one-dimension, two-dimension and three-dimension. |
| Author |
Shek, Ka Wing |
| Title |
Thermal & indoor air quality environment on air-conditioned buses |
| Department | Dept. of Building Services Engineering |
| Year | 2010 |
| Subject |
Hong Kong Polytechnic University -- Dissertations Buses -- China -- Hong Kong Buses -- Air conditioning Indoor air quality -- China -- Hong Kong Buses -- Heating and ventilation |
| Degree | Ph.D. |
| Pages | xxiv, 220, [6] leaves : ill. ; 31 cm. |
| OneSearch | https://www.lib.polyu.edu.hk/bib/b2374514 |
| URI: | http://theses.lib.polyu.edu.hk/handle/200/5779 |
| Abstract |
Air-conditioned buses have been serving Hong Kong over two decades. There are now approximately 5750; over 95% of the service fleets. Bus manufacturers and local operators are keen on the service quality improvement by modifying the air-conditioning designs, including system performance and reliability, parameter settings as well as energy effectiveness. However, complaints about poor air quality and thermal discomfort were received since the beginning of services. Such issues suggest less-than-satisfactory environments on these buses. Further study is necessary to enhance the in-bus commuting environment. Considering the in-bus air quality and thermal comfort environment, physical measurement and questionnaire survey were conducted to review the real scenario. Data collected from physical measurement provided clues to evaluate the dynamic effects from surrounding environment. The in-bus air quality varied when the buses travelled on different routes. The average in-bus CO concentration level was the highest on tunnel routes (4.4 ppm) followed by urban district routes (3.4 ppm). It was the lowest on rural routes (1.3 ppm). However, the I/O ratios on different routes were similar (between 2.1 and 3.4). Such variation caused by the change of roadway environment, like traffic density, surrounding building density as well as the outdoor air quality. Mechanical ventilation rate is another key to the in-bus air quality. The rate is fixed on present air-conditioned buses. Measurement result showed the mechanical ventilation rate was 250 l·s⁻¹ on a stationary bus and it rose on a travelling bus. The rate was 380 1·s⁻¹ when travelling at 30 km·hr⁻¹ and it reached 535 1·s⁻¹ when traversing on a highway (at 65 km·hr⁻¹). It was equivalent to varying the outdoor air rate per person from 1.9 l·s⁻¹ to 4.1 l·s⁻¹ on a fully loaded bus. Since the bluff head generates aerodynamic drag on a travelling bus, pressure difference is induced across bus body surface. Thus ventilation rate varies with the bus travelling speed. Higher rate helps dilute in-bus air contaminants but it increases the risk of infiltrating concentrated air pollutants when travelling in congested area. Nevertheless, lower rate results as insufficient ventilation that causes air stuffiness and odour inside passenger compartment. Therefore the mechanical ventilation rate should not be fixed but adjusted depending on the roadway air quality. Empirical comfort models were developed by concluding the correlation between the physical parameters and the subjective sensation votes from passengers. They provide a convenient platform to quantify and identify the in-bus air quality and thermal comfort through the percentage of dissatisfaction. Moreover, the computation result from models can be applied in the air-conditioning system to optimize the system control. Real-time monitoring of the air quality and thermal comfort indicators are obtained by equipping sampling and data processing instruments on buses. The computation result of real-time data provides determinants for the control system to set an appropriate outdoor air intake rate and thermal comfort settings. The application can reduce the percentage of dissatisfaction level from 35% (present) to 8%. The in-bus environment can be controlled at the most comfortable condition along the journeys. Mixing-ventilation air distribution system was applied in the present passenger compartment. It aimed to provide a uniformly air distribution in the compartment. However, supply air was not well distributed due to tight space in the compartment. Thermally discomfort was caused due to temperature stratification and draft risk. Also, fresh air might not directly reach the passengers causing ineffective air contaminants dilution. Hence, personalized air supply and return system was proposed in new compartment in order to solve those faults. Diffusers were installed in front of each seat while return air grilles were located above each row for better air distribution. Also, computation result from the empirical comfort models was applied to improve the in-bus environment. Simulation result showed the new compartment minimized those thermal discomfort issues found in the present compartment. The new system improved the air distribution by increasing the ventilation efficiency (from 0.93 in present compartment) to 1.32. Moreover, the effectiveness of personalized air supply and return system was evaluated by means of thermal comfort, concentration distribution and particle transport. The tracks of coughed droplets expelled from an index person were simulated in the compartments. Simulation result showed that the level of influence caused by the particle dispersion depended on where the index person sat in the present compartment. Passengers sitting behind the index person were possibly influenced by the infectious droplets. Thus the number of influenced passengers reduced if the index person sitting closer to the rear section. However, the number was less in the new compartment, and which was similar for the index person sitting in different sections. The percentage of particles inhaled by other passengers maintained below 0.04%. It revealed the personalized air supply and return system was effective in preventing the spread of infectious droplets in the whole compartment. The air-conditioning load and energy consumption were simulated to compare the system operating under different scenarios, including in present compartment, present compartment with applying new settings and new compartment. Set-point air temperature and mechanical ventilation strategy were adjusted depending on the computation result from empirical comfort models. Such adjustment was intended to improve the in-bus environment while satisfying a substantial majority of passengers. The application of new air-conditioning settings lowered the passengers' dissatisfaction level. However, higher energy consumption in both heating and cooling was resulted in present compartment. The consumptions rose by 9.1 times and 49% respectively. The required heating and cooling capacities were increased by 3.1 times and 98% respectively. The present air-conditioning system might not be applicable to fulfil the settings adjustment. Applying the new air-conditioning settings in the new compartment, simulation result showed lower heating and cooling capacities were required. The required capacities were diminished by 55% and 36% respectively, as compared with that the new settings applied in present compartment. Also the energy consumptions in heating and cooling were reduced by 64% and 28% respectively. The results revealed the personalized air supply and return design helped to achieve energy effectiveness by increasing the ventilation efficiency. Also adjustment of air-conditioning settings improved the commuting environment while satisfying a substantial majority (above 80%) of passengers. Therefore the personalized air supply and return system and new air-conditioning settings were suggested on new buses. Finally three elements approach was developed. The three elements were the engine, passenger compartment and air-conditioning system. It assisted to improve the in-bus air quality and thermal comfort environment in the aspect of design, operation and maintenance on air-conditioned buses. |
| Author |
Fu, Yu-fang |
| Title |
Thermal stresses and associated damage in concrete at elevated temperatures |
| Department | Dept. of Civil and Structural Engineering |
| Year | 2004 |
| Subject |
Hong Kong Polytechnic University -- Dissertations Concrete -- Effect of temperature on Concrete -- Expansion and contraction |
| Degree | Ph.D. |
| Pages | 1 v. (various pagings) : ill. ; 30 cm |
| Language | English |
| OneSearch | https://www.lib.polyu.edu.hk/bib/b1745203 |
| URI: | http://theses.lib.polyu.edu.hk/handle/200/3506 |
| Abstract | This thesis is devoted to study the thermal stress field and associated fracture of cement-based composites, such as mortor and concrete. The following works have been completed. A series of experiments were conducted at a macro-level to determine the temperature-dependent stress-strain relationship of high strength concrete (HSC), and to measure the coefficient of thermal expansion (CTE) of hardened cement paste (HCP), mortar and concrete at elevated temperatures through specially designed equipment. The effects of admixtures, and unstressed and stressed conditions on the mechanical properties of HSC were studies. Further to the macroscopic experiments, a set of scanning electron microscope (SEM) experiments were carried out to study the mechanisms of thermal damage of HCP and mortor. The evolution of micro-/meso-structure with temperatures and the temperature-dependent stress-strain relations where observed and determined respectively in a real-time mode. The dehydration-induced cracks in HCP and three types of thermal cracks in mortor during heating were observed successfully. It was found that both the decomposition of hydration products and the dehydration-induced cracks in HCP and three types of thermal cracks in mortar during heating were observed successfully. It was found that both the decomposition of hydration products and the dehydration-induced cracks significantly affected the mechanical properties of HCP. As for the mortar specimens, the cracks were induced not only by the decomposition of hydration products, but also by the thermal mismatch between the HCP and the sand. A methodology of the thermal damage analysis was developed to study the thermal degradation of cement-based composite (HCP, mortar and concrete), taking into account of heterogeneity, temperature gradient and temperature-dependent properties of phase materials. Two thermo-elastic mesoscopic damage models at a meso-level were proposed based on the results of SEM experiments conducted in this research. A finite element program T-MFPA2D, that was based on the existing MPFA2D (Material Fracture Process Analysis developed in CRISR, Northeastern University, Shenyang, China) program, was developed to calculate the thermal stresses and analyse the crack development accompanying for thermo-mechanical and thermo-dehyrated damages. A series of numerical parametric studies were conducted to identify the effects of various factors on the thermal damage of a cement-based composite at elevated temperatures. The factors were heterogeneity of phase materials, thermal mismatch between the phase materials, temperature gradient and temperature-dependence. The results showed that the crack formation, propagation and coalescence were closely connected with heterogeneity. The crack patterns were dependent on the thermal mismatch as well as the temperature gradient. The effects of temperature dependence of mechanical properties of phase materials on the thermal stress and the associated cracking were analyzed and discussed. Finally, two numerical case studies were carried out to study the thermal response of a cement-based composite subjected to elevated temperature using T-MFPA2D. In the first case, a series of numerical simulations were carried out to study the effects of the severe thermal mismatch between the concrete and the fiber reinforced polymer/plastic (FRP) on the critical temperature increments of cement-based composites reinforced with single bar and multiple bars at low elevated temperatures. The second case was devoted to chanical behavior of a cement-based composite under uniaxial compressive load and thermal load. It was found that a composite material had higher rate of reduction in mechanical properties then the related single-phase material. The transition from brittleness to ductility of a composite material subjected to high-elevated temperatures was simulated. |
| Author |
Cui, Mingxian |
| Title |
Thermodynamic development of a novel integrated air-conditioning system with DOAS using liquid desiccant |
| Department | Dept. of Building Services Engineering |
| Year | 2013 |
| Subject |
Air conditioning -- Efficiency. Hong Kong Polytechnic University -- Dissertations |
| Degree | Ph.D. |
| Pages | xxi, 230 leaves : ill. ; 30 cm. |
| Language | English |
| OneSearch | https://www.lib.polyu.edu.hk/bib/b2639092 |
| URI: | http://theses.lib.polyu.edu.hk/handle/200/7073 |
| Abstract |
A dedicated outdoor air system (DOAS) used for air-conditioning has recently attracted many attentions because it can significantly improve both energy efficiency and indoor air quality. Cooling-based dehumidification has inherent weaknesses because the process air has to be cooled down below its dew-point temperature for dehumidification and reheated thereafter. Currently available DOASs with solid or liquid desiccants have their own thermodynamic weaknesses. These obstruct the wide applications of DOAS with desiccant dehumidification in practice. Therefore, thermodynamic development and study of a novel integrated air-conditioning system with DOAS using liquid desiccant is conducted in this thesis. Mathematical models of primary components of the proposed system are developed. Simplified numerical model for internally cooled/heated dehumidifier/regenerator and a robust numerical solution are developed, which is characterized by linearization of the non-linear term in the model and multi-gridding approach to guarantee convergence in a wide range of component size and operation conditions. A semi-empirical model for chiller with a recovery exchanger installed between the compressor and the condenser is developed. All the models are validated by either experimental or manufacturers' data, providing a basis for this study. New exergy analysis method is adopted to develop air-conditioning system with high thermodynamic perfection by introducing concepts of dry-, wet-, cold- and heat-exergies, beneficial and unbeneficial exergies to quantify real exergy gains and destructions/losses and to identify each factor causing exergy loss. Application of the newly proposed concepts and method to analysis of the standard liquid desiccant system with DOAS results in the proposed system. It mainly consists of a DOAS subsystem for dehumidification, heat recovery chiller for cooling, and heat pump utilizing waste condensing heat from chiller for desiccant regeneration. The DOAS subsystem is characterized by effectively harvest the sensible and latent load of the exhaust air by an enthalpy exchanger and two sensible heat recovers. The rational exergy efficiency of the proposed system is 8.0% as compared to 3.1% of the standard system. The proper design methods of the proposed system are then developed. They consist of 1) a new RTS-based design cooling load calculation method for sizing of chiller; and 2) a ε-NTU method for design of liquid desiccant dehumidifier. The new design cooling load calculation method overcomes the weakness of RTS method for intermittent cooling by accounting for additional cooling load due to intermittent operation. The ε-NTU method for dehumidifier is characterized by two performance indices of enthalpy effectiveness and mass transfer effectiveness. The iterative design procedure of the system is presented and a design tool is provided for engineer. The thermodynamic study of the proposed system indicates that energy efficiency is enhanced by more than 60% for air-conditioning in a typical office building of Hong Kong, with the average COP reaching 4.21. The thermal analysis also reveals the indispensable role of total heat exchanger to minimize and balance the dehumidification load on dehumidifier. The effects of weather conditions on the performance of the system are analyzed in detail. The sensitivity analysis of key independent operation parameters of the proposed system are carried out to identify their optimal ranges to provide guidelines for the optimal design of the proposed system. The operation parameters studied are condensing temperature of chiller, cooling and hot water flow rates and solution flow rate. The results indicate that: 1) condensing temperature of chiller affects the energy consumption of the system the most and should be as low as possible; 2) the effects of cooling and hot water flow rates are consistent and their optimal value should be about 0.5~1.0 times of fresh air flow rate; 3) the effect of solution flow rate is small and it optimal value is limited by the largest flow rate without desiccant carry-over. |
| Author |
Cheung, Pik-yuen Belinda |
| Title |
Thioredoxin reductase : its properties and regeneration |
| Department | Dept. of Applied Biology and Chemical Technology |
| Year | 2000 |
| Subject |
Thioredoxin Hong Kong Polytechnic University -- Dissertations |
| Degree | Ph.D. |
| Pages | xxiii, 211 leaves : ill. (some col.) ; 30 cm |
| Language | English |
| OneSearch | https://www.lib.polyu.edu.hk/bib/b1535260 |
| URI: | http://theses.lib.polyu.edu.hk/handle/200/23 |
| Abstract | Thioredoxin reductase (TR) is a ubiquitous multifunctional enzyme that is widely distributed through out prokaryotic and eukaryotic organisms. It was fast discovered as the hydrogen donor for ribonucleotide reductase in E.coil. Since then, lots of studies have been carried out in characterizing the enzyme and evaluating its functions. However, information on TR from mammalian tissues, especially brain still remains largely unknown. In. this study, porcine brain TR was purified and characterized. Its regeneration and possible functional roles were also investigated. Porcine brain TR was purified to homogeneity by the method developed in our laboratory without using acid and heat treatment Purified TR was a functionally active monomeric enzyme having a molecular weight of 58kDa. This is different to those in previous studies in which the TR obtained were dimers and not active in the monomeric form. Functional studies of TR show that it is active in reducing disuifide bond of small and large protein substrates in the presence of NADPH and thioredoxin. The differences in molecular weight of TR to others previously described could be attributed to harsh acid and heat treatment employed in their purification procedures. The TR enzyme could regenerate hydrogen peroxide inactivated malate dehydrogenase (MDH). This shows that MDH may be a possible substrate for TR in the brain and one possible role of TR in brain is to alleviate the effect caused by oxidative stress through regeneration, of damaged enzymes. Comparative study of porcine brain TR with porcine red cell TR showed that they have similar physiological characteristic, but may differ structurally. We suggest that there is a possible existence of isozymes of TR which may play different roles in different tissues. In the regulation, study of TR, ribonucleoside 5-triphosphates were tested as TR inhibitors while protein. disulfide isomerase (PDI) was used as its regenerator. Adenosine 5-triphosphates (ATP) and cytidine 5r-triphosphate (CTP) showed inhibitory effect on TR activity within their physiological concentrations. PDI was able to regenerate denatured TR. The PDI regeneration property was independent of ATP presence. ATP, CTP and PDI thus may act as the regulators of TR through their inhibition and regeneration effect respectively. |
| Author |
Fan, Lulu |
| Title |
Thread solidarity and power : a study of garment workers' self-organization of production in the Yangtze River Delta Region of China |
| Department | Dept. of Applied Social Sciences |
| Year | 2015 |
| Subject |
Clothing trade -- China Clothing workers -- China Labor movement -- China. Hong Kong Polytechnic University -- Dissertations |
| Degree | Ph.D. |
| Pages | 267 pages : illustrations |
| Language | English |
| OneSearch | https://www.lib.polyu.edu.hk/bib/b2827768 |
| URI: | http://theses.lib.polyu.edu.hk/handle/200/8295 |
| Abstract | Based on my participant observation in two garment factories in Jiaxing Pinghu, and participation in a large-scale study of garment industries in six cities in the Yangtze River Delta Region, this thesis explores new forms of work organization, labor process, production and reproduction in Chinese booming garment industry, and how garment workers develop solidarity and power on their own. I look into the typology of Chinese garment workers and understand their self-organization in the sphere of production and reproduction, as well as how such associations affect labour's power. To this purpose, I identify six types of garment workers in relation to skill level, average hourly wage, method of pay and level of control over the labour process, including workers in cooperative production teams, individual outsourced workers, workers in contractor teams, regular workers, older workers and student workers. Significantly, this thesis highlights that, while China approaches full maturity stage (the Toyne Mode), informal employment and outsourced production in the Chinese garment industry are not the result of underdevelopment or total manipulation by capital (as in Third World societies) as understood by existing literature. Indeed, on occasion, informal employment is an active choice of workers. As labour shortages have become increasingly severe, particularly regarding skilled workers in the garment industry, increasing numbers of skilled workers wish to become informal and independent workers in cooperative production teams or individual outsourced workers in factories. This gives them the opportunity to earn higher hourly wages and achieve more autonomy over the labour process. This situation also occurs in the informal sector as many experienced all-round workers choose to work in non-factory production units. I identify four types of non-factory units in the garment industry according to number of workers, production activities, space arrangements for production and living, and personal relationship in the organization. These non-factory units are: contractor workshops, workshops involving family members and wage earners, cooperative production teams and home workers. This thesis discusses class formation to understand relationships among class, localism, gender and skill level. Male or female garment workers, at differing points in the process of proletarianization, originating from various native places, with varying skill levels and are at differing stages of life, participate in different kinds of work organization: cooperative production teams and contractor teams. These two types of worker self-organization are diverse in terms of class structure, social organization, organizational faith and motives for taking collective action. Therefore, they have different roles in protecting workers' rights and interests. Native place and other social bonds blur class conflict in contractor teams; meanwhile, in cooperative production teams, class identity overcomes other distinctions and helps bond workers together in struggle against employers. In this research, we discover that workers with stronger market bargaining power have greater opportunity to organize democratic and equal association and this in turn enhances their associational power. |
| Author |
Ng, Wai Lam William |
| Title |
Three-dimensional model acquisition and motion synthesis |
| Department | Dept. of Electronic and Information Engineering |
| Year | 2013 |
| Subject |
Augmented reality. Human-computer interaction. Hong Kong Polytechnic University -- Dissertations |
| Degree | Ph.D. |
| Pages | 141 leaves : ill. (some col.) ; 30 cm. |
| OneSearch | https://www.lib.polyu.edu.hk/bib/b2639214 |
| URI: | http://theses.lib.polyu.edu.hk/handle/200/7086 |
| Abstract |
Augmented reality is an important extension of virtual reality that allows the interaction of real objects and computer generated graphics in a real or virtual environment. It has been a subject of extensive interest in recent years due to its widespread application in computer games, movies, simulations, etc. In this thesis, we focus on two important research areas in augmented reality: 3D model acquisition and character animation. To allow a real object to interact with computer generated objects in a virtual environment, it is desirable to obtain the three-dimensional (3D) model of the real object. One of the popular approaches to acquire high-fidelity 3D model of real objects is fringe projection profilometry. It projects parallel fringe patterns onto the object, and captures the deformed fringe by a camera for 3D reconstruction. Due to the imperfection of imaging devices and operating environment, the captured fringe images are inevitably to have many artifacts; one of them is noisy. It is especially the case when the acquisition process is carried out in real-time applications such that images are captured with short exposure time and perhaps high ISO value (for instance, in dark environment). Such artifacts can lead to serious distortion to the final reconstructed 3D model. In the first part of this thesis, we propose an efficient algorithm to enhance the noisy fringe images using 2-dimenional (2D) dual tree complex wavelet transform (DT-CWT). The proposed algorithm exploits the special analytic property of DT-CWT to obtain piecewise smooth representations of the fringe image. Then a new wavelet denoising procedure is applied for enhancing the noisy fringe image. The proposed algorithm outperforms the traditional approaches in enhancing the fringe image, and in turn significantly improves the accuracy of the reconstructed 3D model. Another commonly found artifact in the captured fringe images is bias, which is a natural phenomenon due to the changing light intensity reflected from the object surface. If there is abruptly changing bias in the captured fringe image, severe reconstruction error will be resulted. To solve the problem, one popular approach is Phase Shift Profilometry (PSP) which removes the bias using two fringe images. It however is not suitable for real-time applications especially when the target object is moving. Any slight difference in the fringe image due to object{174}s motion can lead to serious distortion to the final reconstructed 3D model. Other approaches try to remove the bias by suppressing the zero spectrum of fringe image. It is based on the assumption that the energy of bias is located around dc (i.e. zero spectrum) and the aliasing between it and that of the fringe is negligible. It however is not the case in practice. In this thesis, we proposed a novel technique to eliminate the bias using a single fringe image. The new approach successfully identifies the feature of bias, fringe and noise in the DT-CWT domain, which allows the bias to be effectively extracted from a noisy fringe image. This bias removal approach can be integrated into the fringe enhancement algorithm as mentioned above so that the overall computational complexity can be reduced. The second part of this thesis is related to character animation, in particular synthesizing motion of computer generated characters. In augmented reality, one direction of study is how to integrate computer generated characters into real environment. To simplify the process of character animation, motion capture systems are often used so as to directly export the motion of real objects to the characters. While the motion data stored in the database are always limited due to various reasons, it is desirable if they can be reused to synthesize new motions. Traditionally, the standard motion graphs are used to facilitate motion data reuse. A motion graph keeps track of all possible transitions from one motion to another. It records how motion data can be spliced to form the motion required. Traditional motion graphs allow a motion clip to transit to another one only if the characters in the two motion clips have the same or very similar body posture. Such requirement however limits the possible transitions between motion clips and hence reduces their reusability. In this thesis, we present a new data structure called the Synchronized Partial body Motion Graph (SPbMG). The new SPbMG specifies possible transitions between upper body motions and between lower body motions separately; thus the connectivity of the graph is greatly enhanced. Besides, a new algorithm is proposed for exploiting the similarity between the lower body motions in order to derive the conditions for splicing the upper body and lower body motions synchronously. It ensures the coherence between the upper and lower body motions in the synthesized motion clips. Experimental results show that the proposed SPbMG improves the reusability of motion data while preserving the quality of motions as compared with the traditional motion graph techniques. |
| Author |
Chen, Yongping |
| Title |
Three-dimensional modelling of vertical jets in random waves |
| Department | Dept. of Civil and Structural Engineering |
| Year | 2006 |
| Subject |
Hong Kong Polytechnic University -- Dissertations. Jets -- Fluid dynamics. Turbulence. |
| Degree | Ph.D. |
| Pages | 1 v. (various pagings) : ill. (some col.) ; 30 cm. |
| Language | English |
| OneSearch | https://www.lib.polyu.edu.hk/bib/b2059251 |
| URI: | http://theses.lib.polyu.edu.hk/handle/200/9 |
| Abstract | This thesis describes an investigation of flow behaviour of a buoyant or non-buoyant jet discharging vertically into random waves. In the part of experimental study, a round jet nozzle was vertically arranged at the bottom of a random wave flume. The tap water was adopted as the jet fluid for non-buoyant jet while the mixture of ethanol and water was adopted for buoyant jet to generate initial buoyancy. Velocities, concentrations and surface elevations were measured for a number of flow cases. The measured data showed that the centreline velocities of non-buoyant or buoyant jet in random waves decays faster than those in stagnant ambience, corresponding to a significant increase of jet dilution. The jet half-width increases with incident wave period and wave height. Due to the wave effect, the location of peak turbulence shifts towards the jet inlet. The extent of location shift is related to wave period and wave height, which indicates that the jet turbulence is generated by both wave and jet motions. In order to convert those findings into more general forms, two characteristic length scales, i.e., wave-momentum length scale lm and wave-momentum-buoyancy length scale lw were introduced. Several empirical formulas for prediction of jet characteristic variables, such as centreline velocity, dilution and half-width, were derived based on the method of dimensional analysis. In the part of numerical study, a fully nonlinear numerical wave tank (NWT) was first developed for modelling of random wave situations. A novel Langrage-Euler method was introduced to solve the free surface problem. The NWT with the new free surface treatment was verified by several test cases with theoretical solutions or experimental data. A non-buoyant or buoyant jet was then added to the NWT domain and the flow field was computed with turbulence modelled by the large eddy simulation (LES) method. Velocities and concentrations obtained from numerical computation were in fairly good agreement with those from laboratory experiments. As an application of the numerical model, the effect of buoyancy on vertical jets in random waves was investigated through numerical comparison between a non-buoyant and a buoyant jet with a same discharge velocity. It was found that under the buoyancy force the wave effect on jet entraining and mixing is weakened to some extent, corresponding to a slower decay rate of velocity and a narrower jet width in the buoyant jet than in the case for the non-buoyant jet. |
| Author |
Jiang, Yuwu |
| Title |
Three-dimensional numerical modelling of sediment and heavy metal transport in surface waters |
| Department | Dept. of Civil and Structural Engineering |
| Year | 2003 |
| Subject |
Hong Kong Polytechnic University -- Dissertations Sediment transport -- Mathematical models |
| Degree | Ph.D. |
| Pages | xxi, 282 leaves : ill. (some col.) ; 30 cm |
| Language | English |
| OneSearch | https://www.lib.polyu.edu.hk/bib/b1740396 |
| URI: | http://theses.lib.polyu.edu.hk/handle/200/3648 |
| Abstract | Accurate and efficient hydrodynamic and water quality numerical models are very useful in providing various types of information for hydraulic engineering and environment protection projects. In this study, three-dimensional (3D) models for hydrodynamics and the fate and transport of cohesive sediment and heavy metals have been developed and coupled together to study surface water systems. As one of the major tasks of this study, a cohesive sediment transport model which includes the sediment aggregation and breakup processes has been developed based on the work of Winterwerp (1999). With the accurate prediction of cohesive sediment concentration, a simple metal kinetic model has been included. Two types of metal kinetic mechanisms, i.e. the one-stage and two-stage adsorption and desorption mechanisms, have been considered and tested for this model. The computed results suggested that when there is not enough data to depict the two-stage mechanism, the one-stage mechanism can still perform reasonably well. The hydrodynamic model is a 3D parallel finite element model whose algorithm is proposed by Wai and Lu (2000). In this study the model has been upgraded as follows: 1) efficient parallelized solvers for large linear equation systems are used in this model, making the model perform 50 times faster than the original Wai and Lu's model; 2) the Level 2.5 turbulence closure sub-model is included to improve the accuracy of the vertical eddy viscosity coefficient estimation; 3) in the vertical direction, the finite element method instead of finite difference methods is used to solve the vertical diffusion term, which gives velocity information at the exact water bottom; 4) both the 9-node quadrilateral and the 6-node triangular elements can be used to discretize the complex spatial domain; 5)the parallel algorithm is revised to fit the Beowulf distributed memory computer system and the efficient linear equation solver, and many more. With these upgrades, this model is more versatile and efficient to deal with real world situations. Furthermore, some new strategies and techniques have also been introduced in this study, e.g. a new approach for computing drying-wetting processes which can improve mass conservation and numerical stability in 3D modelling. This coupled hydrodynamic-sediment-metal model has been applied to Xiamen Bay and the Pearl River estuary, China. The simulation results show that the model is robust in representing the hydrodynamic and sediment processes in estuaries. The present one-stage metal model successfully reveals the non-conservative behavior of dissolved metals in the Pearl River estuary, particularly in the dry season. It has been shown that this model has considerable potential in providing hydraulic and water quality information for various types of engineering construction and environment protection activities. |
| Author |
Wei, Wenbing |
| Title |
Three dimensional slope stability analysis and failure mechanism |
| Department | Dept. of Civil and Structural Engineering |
| Year | 2008 |
| Subject |
Hong Kong Polytechnic University -- Dissertations. Soil stabilization. Slopes (Soil mechanics) |
| Degree | Ph.D. |
| Pages | xiv, 221 p. : ill. ; 30 cm. |
| Language | English |
| OneSearch | https://www.lib.polyu.edu.hk/bib/b2239573 |
| URI: | http://theses.lib.polyu.edu.hk/handle/200/1662 |
| Abstract | For slope stability problem, two-dimensional analysis is commonly used for simplicity, though all slope failures are three-dimensional (3D) in nature. There are only limited applications of 3D analysis due to the various limitations of three-dimensional slope stability methods. Recently, there are various important progresses for 3D analysis. 3D limit equilibrium method (LEM) for general asymmetrical problem together with innovative optimization method for locating the general critical 3D failure surface has recently been developed. In addition, 3D strength reduction method (SRM) can now be conducted within a tolerable duration. Until now, there is still a lack of detailed investigation of 3D slope stability failure mechanism and the application of 3D LEM and SRM under different 3D conditions. This study aims to conduct an extensive three-dimensional slope stability analysis and to investigate the failure mechanism under different situations. The 3D effect considered in this study includes the important factors such as slope geometry, boundary conditions, water, soil nail and pile. Both the LEM and SRM are conducted in this study, and some interesting differences between these two methods are discovered. It is concluded that both methods have their own merits and limitations, and a good understanding of these methods are required before a good solution can be obtained. The suitability of these two methods under different conditions is investigated and precautions when these methods are applied are suggested. In this study, some results which appear to be different from common understanding are obtained. Some of the results are also different from published studies, and careful investigations have revealed that the present detailed studies have provided a better and more reasonable understanding about the failure and stabilization mechanism. For example, for a simple slope extending to infinity, the critical slip surface is still basically two-dimensional until the external loading is large to induce a three-dimensional failure. The failure mechanisms due to the self weight of soil and external loads are actually different. The discretization domain required for SRM analysis is found to decrease with the increase of external loads which is also out of expectation. The distribution of tension force in soil nail is found to be influenced by the state of the slope (service state, limit state) and the failure modes (external failure, internal failure). In general, the line of the maximum tension may not correspond to the critical slip surface as commonly believed, except when the failure mode is an internal tensile failure. For slope supported with one row of piles, the slip surface is divided into two parts when the pile spacing is very small, and these two parts gradually become connected to form a clear single slip surface mechanism with the increase of pile spacing. The point of maximum shear force in the pile which is commonly used to determine the location of the slip surface in traditional design is found to be not a valid assumption, and this is important for design of slopes reinforced with piles. Some engineers have questioned the disturbance of the seepage pattern due to the presence of soil nails and piles. Such blocking effect is however found to be negligible. With the detailed study on 3D failure and stabilization mechanism, a clearer and better understanding of three-dimensional slope failure has been achieved in the present study. |
| Author |
Li, Jing |
| Title |
Three studies on exploring the effectiveness of e-WOM on customer evaluation and behavior |
| Department | Dept. of Management and Marketing |
| Year | 2017 |
| Subject |
Hong Kong Polytechnic University -- Dissertations Internet marketing Consumer behavior |
| Degree |
Ph.D. |
| Pages | xi, 107 pages : color illustrations |
| Language | English |
| OneSearch | https://www.lib.polyu.edu.hk/bib/99102204 |
| URI: | http://theses.lib.polyu.edu.hk/handle/200/9272 |
| Abstract |
With the advent and prosperity of social media, an increasing number of customers voluntarily post their evaluations and opinions about a product or service online and e-WOM has gained extraordinary growth and aroused intensive academic attention. The current literature has examined how e-WOM, especially in the form of online customer reviews, influences a wide range of outcomes such as customer awareness, perceptions, attitudes, behavioral intentions, and product sales. In my dissertation, I developed three studies that aim to reveal the role of e-WOM in different forms (e.g., photo, text, and vote) on consumer information processing and decision-making. In the first study, I examined the interaction effects of review certainty, reviewer popularity, reviewer expertise, and the niche width of a restaurant on review usefulness, by drawing on the dual-process theory and social influence theory. Utilizing a zero-inflated negative binomial Poisson regression, I empirically tested my hypotheses based on 10,097 reviews on 2,383 restaurants from Yelp.com. My results indicated that (1) the impact of review certainty on review usefulness decreases with reviewer popularity but does not vary with reviewer expertise; (2) the niche width of a restaurant—as a contextual feature—interacts with review certainty and reviewer characteristics in influencing review usefulness. In the second study, I focused on one particular type of e-WOM - votes by prior users on a review. I examined how two numeric characteristics of online review helpfulness: 1) helpfulness ratio—the ratio of later viewers who believe that a previous review is helpful; and 2) helpfulness magnitude—the number of later viewers who vote on a previous review; influence consumers' reaction toward the product/service reviewed. Drawing on the social influence theory, this study examined the interactive impacts between these two factors and two other characteristics of online review content (i.e., review valence and type) on consumer trust and attitude. I conducted three lab-based experiments to test the research model. My research finds that regardless of the valence and type of reviews, vote ratio enhances review trustworthiness and guide corresponding product evaluation. In contrast with ratio effect, vote magnitude is significantly influential only for the negative attribute-based review. In the third study, I investigated three key characteristic of photo e-WOM: layout, sequence and density of photo relative to text in an online customer review. This study conducted both a lab experiment and a field study to demonstrate the role of photo e-WOM on consumer information processing and decision-making. In particular, I adapted the cue summation theory of multi-channel communication to examine the impact of between-channel interactions of text and photo on two outcomes of consumer information processing—diagnosticity and pleasantness of e-WOM and one outcome of consumer decision-making—product value ratings. This study employs both a lab-based experiment and a field analysis to provide robust validation of hypotheses. The experimental results show that separate layout is better than the alternate layout in perceived diagnosticity and product evaluation, especially when the photo is displayed first than the text first displayed. In contrast, for pleasantness, alternate layout is better than separate layout, regardless of the sequence of text and picture. Moreover, the field results suggest that sharing more photos especially outside photos hurts restaurant reputation while sharing photos more on food, drink and menu of a restaurant increases the restaurant's reputation; and for the restaurant as a generalist occupying multiple cuisines, the more photos shared in a review, the better its reputation will be; by contrast, for the restaurant as a specialist occupying few cuisines, the more photos shared on food and drink in a review, the better its reputation will be. Theoretically, these findings contribute to online customer review literature, provide new managerial implications for leveraging e-WOM, and add new insights into understanding the role of organization positioning in customer evaluations. |
| Author |
Baig, Faisal Nouman |
| Title |
Thyroid cancer : a novel insight into diagnostic and therapeutic approach |
| Department | Dept. of Health Technology and Informatics |
| Year | 2018 |
| Subject |
Hong Kong Polytechnic University -- Dissertations Thyroid gland -- Cancer -- Diagnosis Thyroid gland -- Cancer -- Treatment |
| Degree |
Ph.D. |
| Pages | xxiv, 209 pages : color illustrations |
| Language | English |
| OneSearch | https://www.lib.polyu.edu.hk/bib/99102206 |
| URI: | http://theses.lib.polyu.edu.hk/handle/200/9302 |
| Abstract |
There has been a substantial increase, worldwide, in the incidence of thyroid cancer over the past few decades. The diagnosis of thyroid cancer poses a clinical challenge to medical practitioners. Fine needle aspiration cytology (FNAC) is a current standard method to diagnose thyroid cancer. FNAC has a false-positive rate of 1% - 5% and a false-negative rate of 0% - 7.7%, whilst 20% - 30% of cases remain non-diagnostic. Grey-scale ultrasound is a useful imaging technique for the assessment of the thyroid gland, because it can characterize various features of thyroid nodules such as the number, site, size, shape, echogenicity, and internal architecture. However, grey-scale ultrasound has limited value in differentiating benign and malignant thyroid nodules because of its varied sensitivity (52% - 97%) and specificity (26% - 83%). The varied sensitivity and specificity of grey-scale ultrasound might be due to the qualitative analysis and subjective interpretation of the characteristics of thyroid nodules and renders it thus vulnerable to inter- and intra-observer variations. Nevertheless, certain features of thyroid nodules can be quantified, for example tissue stiffness and the vascularity index. Shear wave elastography is a novel ultrasound technique that can quantify tissue stiffness by tracking the propagation of shear waves through thyroid nodules. In general, malignant thyroid nodules tend to be stiffer than benign nodules. Depending upon the differences in stiffness values, thyroid nodules can be differentiated as benign or malignant. The first study of this thesis evaluated the feasibility of using shear wave elastography in predicting thyroid malignancy, and determined if any (and if so, which ones) of the shear wave elastography indices (Emaximum, Emean, and Eminimum) are potential predictors of thyroid malignancy. The study further evaluated the diagnostic accuracy in differentiating benign and malignant thyroid nodules when grey-scale ultrasound was combined with shear wave elastography. Vascularity is a dynamic feature usually detected on Doppler ultrasound. Central vascularity and hypervascularity are usually associated with malignant thyroid nodules. However, not all previous studies appreciate the usefulness of vascularity in predicting thyroid malignancy. Controversial results might be due to the fact that previous studies evaluated vascularity by visual assessment methods that involve subjective interpretation and thus result in high inter- and intra-observer variations. Moreover, in previous studies, the methods used to delineate a border between peripheral and central regions of a thyroid nodule were not standardized. In the second study in this thesis, we have thus developed a computer algorithm that can perform regional segmentation of thyroid nodules by using an 'offsetting' method, and quantify the overall vascularity as well as the vascularity in peripheral and central regions of thyroid nodules. Based on the differences in the vascularity indices, thyroid nodules can be differentiated into benign and malignant types. The study further evaluated the potential advantage of combining the vascularity index with grey-scale ultrasound to enhance the diagnostic accuracy of thyroid malignancy. In the first and second study, a total of 111 patients with solitary thyroid nodules were included. Each thyroid nodule was assessed with grey-scale ultrasound, shear wave elastography, and colour Doppler ultrasound. The diagnosis of the thyroid nodules was confirmed by FNAC and/or histological examination. Grey-scale ultrasound features including microcalcification (malignant: 77.8% versus benign: 7.1%), hypoechoic (92.6% versus 33.3%), irregular margins (55.6% versus 16.7%), and a height-to-width ratio > 1 (59.3% versus 13.1%) were found to be more frequently associated with malignant thyroid nodules than with benign thyroid nodules. The differences were statistically significant (all P < 0.05). Regarding shear wave elastography, the results suggested that Emaximum ≥ 67.3 kPa and Emean ≥ 23.1 kPa are independent predictors of thyroid malignancy. Emaximum was found to be the best adjunct to grey-scale ultrasound. The combination of Emaximum or Emean with grey-scale ultrasound enhanced the diagnostic accuracy of grey-scale ultrasound from 58.5% to 80.2% and 78.4%, respectively (P < 0.05). The results of the vascularity index (VI) quantification showed that a 22% offset was optimal for regional subdivision of thyroid nodules. At the optimum offset, the mean VI of peripheral, central, and overall regions of malignant nodules were significantly higher than those of benign nodules (26.5 ± 16.2%, 21.7 ± 19.6%, 23.8 ± 4.6% versus 18.2 ± 16.7%, 11.9 ± 15.1%, and 16.6 ± 1.8%, respectively, P < 0.05). The optimum cut-off points of peripheral, central, and overall VI were 19.7%, 9.1%, and 20.2%, respectively. When compared to grey-scale ultrasound alone, a combination of VI assessment and grey-scale ultrasound evaluation of thyroid nodules increased the overall diagnostic accuracy from 58.6% to 79.3% (P < 0.05). Studies 1 and 2 have clinical significance in establishing methods for accurate diagnosis of thyroid cancer. The results of Study 1 suggest that shear wave elastography has clinical importance in differentiating benign and malignant thyroid nodules. The combination of grey-scale ultrasound with Emaximum or Emean significantly improved the diagnostic accuracy in predicting thyroid cancer. Study 2 has devised a new method to perform regional segmentation of thyroid nodules and to quantify vascularity in each segment. This approach is objective and standardized to quantify thyroid vascularity and helps in differentiating benign and malignant thyroid nodules. Colour Doppler ultrasound (CDU) and power Doppler ultrasound (PDU) are widely used for detecting the vasculature of tissues or organs. In the thyroid gland, CDU and PDU have been used for the differential diagnosis of, amongst others, Hashimoto's disease, Graves' disease, and Reidel's thyroiditis. Doppler ultrasound has also been used in monitoring treatment responses during therapy of thyroid disorders. However, the detection of blood flow in minute blood vessels or vessels with low blood flow is challenging due to technical limitations of previously developed Doppler ultrasound modalities. Most recently, a new ultrasound technology, namely AngioPLUS (Planewave UltraSensitive™ imaging), provides superb sensitivity in the detection of tissue vascularity. The beauty of this technique is that all colour pixels of the tissue can be reconstructed in a single image. AngioPLUS provides high resolution and 3D wall filtering that allow efficient discrimination between blood flow and other soft tissues by analysing space, time, and amplitude information. Study 3 aimed to evaluate the feasibility of using AngioPLUS imaging in assessing thyroid vascularity when combined with CDU or PDU. The study further evaluated whether the addition of AngioPLUS to CDU or PDU enhances the sensitivity in detecting vasculature of thyroid parenchyma. It also investigated whether there is any asymmetry of vascularity between the right and left thyroid lobes. A total of 45 healthy volunteers underwent grey-scale ultrasound, CDU, CDU+AngioPLUS, PDU, and PDU+AngioPLUS evaluations of both lobes of the thyroid gland. Thyroid vascularity was evaluated using our in-house computer algorithm. The results showed that the combination of CDU+AngioPLUS (14.7 ± 9.4%) and the combination of PDU+AngioPLUS (13.4 ± 9%) had significantly higher thyroid VI than CDU (8.8 ± 7.3%) and PDU (4.7 ± 5.4%) alone (all P < 0.05). No asymmetry was found between the VI of the right and left thyroid lobes (P > 0.05). Study 3 highlights the differences in sensitivity of detecting thyroid vasculature assessed by different Doppler ultrasound modalities. The results suggest that AngioPLUS enhances the detection of vascularity when added to PDU or CDU. The clinical significance of the study lies in the detection of small blood vessels and vessels with low blood flow that may help disease diagnosis and treatment monitoring. Study 4 highlights the scope of chemotherapeutic agents in treating papillary thyroid cancer. Current strategies to treat papillary thyroid cancer are largely based on surgery, where recurrence rate is high (up to 33%). Drug development is a costly and time-consuming process. Moreover, the identification of novel therapeutic targets is challenging. Study 4 introduces the concept of 'repurposing of drugs' that evaluates the therapeutic potential of already approved drugs beyond the scope of their primary clinical usage. Cannabinoids are derivatives of the marijuana plant. They have been used for recreation and to relieve pain. Two cannabinoid receptors (CB1 and CB2) are known to be distributed over different body organs and systems in humans. Cannabinoids receptor expression has been noted in many cancers including breast cancer, prostate cancer, hepatic cancer, lung cancer, and colorectal cancer. Anti-cancer actions, including antiproliferation, antimigration, antiangiogenesis, and apoptosis, have been validated in in vitro experiments in various cancer cell lines. In a recent study, immunohistochemistry analysis of surgical specimens of 87 thyroid nodules demonstrated that CB1 and CB2 receptor expression was more significantly associated with papillary thyroid cancer than with benign thyroid nodules. It was further found that CB2 expression was significantly higher than CB1 receptor expression. In Study 4, the therapeutic potential of CB2 receptor agonist (JWH-133) in treating papillary thyroid cancer was evaluated. A normal thyroid follicular cell line (N-thy-ori-3) was used as the control, and BCPAP was the papillary thyroid cancer cell line used in this study. Both cell lines were treated with JWH-133 at 0, 5, 10, 15, 20, 25, and 30-µM concentrations for 24, 48, and 72 hours. Cellular metabolic activity and cell viability were evaluated using an MTT assay. The results demonstrated that 25 µM was the lethal dose concentration for BCPAP cells at which cell viability was reduced to 50% after the optimal 48 hours of incubation. The results also suggested that BCPAP cells were more sensitive to the JWH-133 as compared with N-thy-ori-3. The cytotoxic effect mediated by JWH-133 was not significantly inhibited by CB2 receptor antagonist SR144528 in both cell lines. The results of the study demonstrated that JWH-133 has a potent cytotoxic effect, more pronounced in a papillary thyroid cancer cell line (BCPAP) than in a normal follicular thyroid cell line (N-thy-ori-3). Study 4 suggests that JWH-133 induces cell death in papillary thyroid cancer cells, whilst the survival of normal thyroid follicular cells can be maintained at an acceptable level. |
| Author |
Chunn, Alex |
| Title |
Ties that bind : how social networks affect new product development team performance |
| Department | Faculty of Business |
| Year | 2017 |
| Subject |
Hong Kong Polytechnic University -- Dissertations New products -- Management Social networks Teams in the workplace |
| Degree |
D.B.A. |
| Pages | viii, 113 pages |
| OneSearch | https://www.lib.polyu.edu.hk/bib/99102198 |
| URI: | http://theses.lib.polyu.edu.hk/handle/200/9154 |
| Abstract | This thesis sought to conceptually compare the two network structure perspectives of closed networks, characterized by dense and redundant ties (Coleman, 1988), and open networks, described by R. S. Burt (1992) as networks that consist of unique and non-redundant ties and to empirically examine how these types of network structures impact team performance. In particular, this research focused on how the closed and open networks affected the relationship between a cross-functional new product development team and its external ties with customer. In addition, this paper examines the moderating effect of a team's tie strength and network centrality on the number of external ties on team performance. This research used the fuzzy front end of a cross functional team in new product development within a large multinational firm as its empirical setting. A fuzzy front end is where information is shared, ideas generated, and reviews and collaborations conducted that ultimately lead to new product development plans for execution. The use of cross functional teams and a fuzzy front end process have shown to have a significant effect on team performance and, by its dynamic multifunctional nature, makes an excellent setting for this research. Two types of customer ties were measured: the end user who is ultimately the product consumer and the retailer customer who provides the channel for distribution. The results indicated that the two customer ties are not the same with each having different degrees of benefit on product success. Interestingly, the moderating effects of a closed and open network position on team members with customer ties had no or negative effect on product performance. Similar results were shown when the team's network density and centralization was considered. Given that the main effects for user and retailer ties were significantly different, but both ties had similar effects when considering team members' network position and teams' network structure indicates, there is more to the interaction than simply the ties themselves. Other factors such as tie content or the core/peripheral position of the connecting members needs further study. |
| Author |
Chan, Wai Ming William |
| Title |
Till gossip do we part : gossip, emotion, and interpersonal relationships |
| Department | Faculty of Business |
| Year | 2015 |
| Subject |
Hong Kong Polytechnic University -- Dissertations Communication in organizations. Gossip. Corporate culture. Organizational behavior. |
| Degree | D.B.A. |
| Pages | v, 150 pages : color illustrations |
| Language | English |
| OneSearch | https://www.lib.polyu.edu.hk/bib/b2890574 |
| URI: | http://theses.lib.polyu.edu.hk/handle/200/8461 |
| Abstract |
"We all gossip but we feel we should not" (Sabini & Silver, 1982). Traditionally gossip has a bad reputation because it violates the privacy norm and damages the reputation of the target of gossip. However, researchers have found that gossip that entertains, informs, and influences can protect group norms and discourage free riding. Studies of gossip have focused on the tendency to gossip, its functions, and its effects on group and individual outcomes. Most of the studies are concerned with people who engage in gossip; little is known about the way in which gossip affects recipient of gossip.Defining gossip as "comments on an absent third party with or without an evaluative element, and applying the framework from appraisal theories, this study views gossip through the lens of emotion in order to explain how such seemingly casual chat affects the relationship between the people who talk and the people who listen. It is proposed that gossip is a stimulus. The gossip recipient will have a perception of the motive of a gossip and different perceived motives will lead to different appraisals eliciting different emotions. The emotion elicited should affect the perceived relational quality between the spreader and the recipient of the gossip. Applying appraisals dimensions from the appraisal theories for the analysis of gossip motives, it is proposed that gossip perceived by the gossip recipient to be for entertainment will elicit positive emotions; gossip perceived to be for influence will elicit negative emotions. Using a questionnaire survey based on 293 events from 71 subjects in managerial positions in private corporations in Hong Kong and in Mainland China, participants were asked to record five pieces of gossip about a colleague's work. The participants were requested to summarize the event and answer questions about the emotion elicited immediately after a gossip event initiated by a colleague at work. By end of work on that day, participants were asked to answer questions about the distancing effect to the gossiper and the target of the gossip. Empirical findings provide evidence that the motive of gossip perceived by the gossip recipient will determine whether and what type of emotion will be elicited. Depending on the motive of gossip perceived by the gossip recipient, gossip may have different effects on the relationship between the gossip recipient and the gossiper.The findings of this study should allow managers to understand how gossip in the workplace can be interpreted, and more importantly how to accommodate the gossip at work. Increase sensitive and awareness of the relation between perceived gossip motive and emotion and the perceived relational quality enable managers to be more proactive in establishing and maintaining relationships with their subordinates. Better manager-subordinate relations will create a more productive work environment. |