| Author: | Dong, Chuanshuai |
| Title: | Enhancing the dehumidification performance of plate liquid desiccant dehumidifier with super-hydrophilic coating considering the solid-liquid interaction effect |
| Advisors: | Lu, Lin (BSE) Yang, Hongxing (BSE) |
| Degree: | Ph.D. |
| Year: | 2018 |
| Subject: | Hong Kong Polytechnic University -- Dissertations Humidity -- Control Air conditioning |
| Department: | Department of Building Services Engineering |
| Pages: | xxxii, 243 pages : color illustrations |
| Language: | English |
| Abstract: | Humidity control of indoor air is very important for both indoor occupants and indoor building materials, especially in hot and humid regions. Falling film liquid desiccant air dehumidification is a promising alternative to traditional vapor compression air-conditioning system due to its lower energy consumption and less pollution. Besides, falling film is widely used in other engineering fields, such as falling film heat exchangers, evaporators and cooling towers, for simple structure and high efficiency. However, the incomplete wetting condition of falling film deteriorates the operating performance and limits the wide application of falling film devices. Therefore, this thesis aims to study the effect of plate surface properties on flow characteristics and heat/mass transfer performance and then try to develop a novel super-hydrophilic coating using nanoscale anatase TiO₂ particles to improve the surface wettability and enhance the operating performance of falling film liquid desiccant dehumidifiers accordingly. The principles of the dehumidification performance enhancement are investigated experimentally and theoretically in this thesis. Firstly, to investigate the surface properties on flow and heat/mass transfer characteristics in falling film dehumidifiers, three commonly-used types of falling film dehumidifiers, i.e., stainless plate dehumidifier, titanium plate dehumidifier and polytetrafluoroethylene (PTFE) plate dehumidifier, with distinctive surface properties but same geometrical size are chosen for study. The surface properties of plate dehumidifiers are investigated through FESEM test of microstructures and contact angle test of surface wettability. The surface free energy of Titanium plate, Stainless plate and PTFE plate is calculated to be 50.61 mJ/m², 36.98 mJ/m² and 30.34 mJ/m², respectively, indicating that Titanium plate possesses the superior wettability compared with the other plates. It is observed that the falling film shrinks seriously along the flow direction in PTFE plate dehumidifier while a little shrinkage occurs in Titanium plate dehumidifier due to the strong adhesion between solid surface and desiccant solution. The effect of surface properties on heat/mass transfer characteristics in falling film dehumidifiers are experimentally investigated. The experimental results prove that surface wettability demonstrates positive effect on dehumidification performance. Therefore, it is very important to improve the surface wettability of falling film dehumidifiers. Based on this research, a novel super-hydrophilic coating is developed using nanoscale anatase TiO₂ particles to improve the surface wettability of liquid desiccant plate dehumidifiers. The microstructures of the new coating on substrates are investigated through XRD, FESEM and HRTEM test. The test results indicate that pure and highly-dispersed TiO₂ paste is well developed. Then, the thin TiO₂ film is well coated on substrates with high hardness and durability. The surface wettability of TiO₂ film before and after UV illumination is investigated through surface free energy test and the super-hydrophilicity of TiO₂ coating is effectively activated by UV illumination. Then, the highly-dispersed TiO₂ paste is successfully coated onto dehumidifier plates to enhance the heat and mass transfer performance. The contact angle test shows that the surface wettability of plate dehumidifier is significantly improved by TiO₂ super-hydrophilic coating with the contact angle decreasing from 84.6° to 8.8°. The flow characteristics of falling film in coated plate dehumidifier are also investigated. Little shrinkage occurs along the flow direction in coated plate dehumidifier, while severe shrinkage occurs in the uncoated plate dehumidifier. The maximum wetting ratio increased from 75% to 100% by super-hydrophilic coating. The effect of surface wettability on falling film thickness and its fluctuation is also investigated. A novel and simplified correlation of falling film wave is developed to estimate the actual interfacial area between liquid desiccant and processed air. With more than 200 experimental conditions, the experimental results indicate that the dehumidification performance is significantly improved by surface treatment with the average performance enhancing ratio of 60%. The dehumidification performance enhancement is mainly attributed to the increasing of wetting area and decreasing of falling film thickness. Based on the experimental results, a novel empirical correlation of mass transfer coefficient is developed using non-linear regression. The surface wettability is considered in the correlation to account for the effect of surface wettability on dehumidification performance. Based on the experimental analysis, an improved heat and mass transfer model considering the solid-liquid interaction effect is developed to stimulate the heat and mass transfer process in an internally-cold falling film dehumidifier. A shrinkage model of falling film is proposed to accurately estimate the actual wetting area of liquid desiccant solution in falling film dehumidifiers with different surface wettability. Then, the effect of surface wettability on dehumidification performance is investigated theoretically. As the contact angle decreases from 85° to 5°, the moisture removal rate increases from 2.0 g/kg to 2.56 g/kg, i.e., by a performance enhancing factor of 28 %. The performance enhancement is attributed to the increase of the wetting area and the decrease of the falling film thickness. The total wetting area increases significantly from 0.145 m² to 0.176 m² with the enhancing ratio of 21.4%, while the mean falling film thickness decreases from 0.952 mm to 0.889 mm. Finally, a dynamic model of solar-assisted liquid desiccant air-conditioning system is developed using C++ to investigate the effect of solid-liquid interaction effect on energy consumption. Three iteration loops, i.e. iteration loop of liquid desiccant solution, iteration loop of cooling water and iteration loop of heating water, are proposed in the dynamic model. The effect of surface properties for falling film dehumidifiers on energy consumption is also analyzed. Improving the surface wettability of falling film dehumidifiers could reduce the energy consumption effectively. Around 9.1% (100 MW·h ) of total energy consumption could be saved for solar-assisted liquid desiccant air-conditioning system with the surface contact angle reducing from 110° to 10°. The energy saving is attributed to the dehumidification performance enhancement by higher surface wettability of falling film dehumidifiers. Besides, the introduction of solar energy could save around 15.4 % (200 MW·h ) of electricity consumption for liquid desiccant air-conditioning system. In summary, this thesis develops a novel TiO₂ super-hydrophilic coating to improve the surface wettability and investigate the effect of solid-liquid interaction on dehumidification performance of falling film dehumidifiers. As the incomplete wetting problem is encountered in other falling film applications, such as heat exchangers, evaporators and cooling towers, the findings in this research is very useful to improve the efficiency of falling film devices by improving the surface wettability. The correlation of falling film wave can be adopted in other falling film devices to accurately estimate the actual interfacial area between liquid and gas. Besides, the newly-developed model in this research is also salutary to the theoretical investigation of other falling film applications. |
| Rights: | All rights reserved |
| Access: | open access |
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|---|---|---|---|---|
| 991022168757303411.pdf | For All Users | 6.38 MB | Adobe PDF | View/Open |
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