Numerical and experimental investigation on particle deposition and distribution in ventilation duct bends

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Numerical and experimental investigation on particle deposition and distribution in ventilation duct bends


Author: Sun, Ke
Title: Numerical and experimental investigation on particle deposition and distribution in ventilation duct bends
Degree: Ph.D.
Year: 2011
Subject: Particles -- Measurement.
Ventilation -- Control.
Air ducts -- Aerodynamics.
Hong Kong Polytechnic University -- Dissertations
Department: Dept. of Building Services Engineering
Pages: xxix, 202 leaves : ill. (some col.) ; 30 cm.
Language: English
InnoPac Record:
Abstract: Nowadays, exposure to contaminant particles causes crucial problems to human beings, advanced equipments and worthful artworks. Understanding the particle flow is important for solving these indoor environmental quality (IEQ) problems and controlling the particle concentration, distribution and deposition. Along with the usage of modern materials indoor and the increasing exhaust emissions of vehicles outdoor, ventilation ducts especially bends, play a great role to filter aerosol contaminants for occupants or equipments. This dissertation aims to investigate particle phenomena in bends of mechanical ventilation (MV) systems by means of numerical models and experimental measurements. Numerical predictions were conducted to investigate the particle flow movement and deposition in a 90° bend after a straight duct, utilizing the Lagrangian particle-tracking model incorporated with a particle-wall collision model. The developed models were validated by the current measurement results and previous experimental data. Particle distribution and deposition behaviour at five size groups (1, 3, 5, 9, and 16 μm) were investigated. The simulation results showed that, compared with the traditional 'Trap' model, the particle-wall collision model postponed the emergence and slowed the increase of the 'particle free zone' as the particle diameter increases. Particle deposition velocity in the duct bend was also generally predicted by the proposed estimation equation under the simulated conditions. Particle loss, deposition velocity and number distribution in ventilation duct bends with three different wall materials were analyzed and discussed numerically. With the predicted particle relaxation time from 1.2 to 6.4, the particle deposition velocities with the material of the largest capture velocity could be 1.2 times larger than those with the material of the smallest capture velocity. These phenomena indicate that the duct bend wall with the material of the largest capture velocity is easy to accumulate contaminant particles.
Experiments were also designed to investigate the real aerosol deposition in rectangular section bends. Scanning Electron Microscope (SEM) was adopted to explore actual ventilation ducts with deposited dusts. By SEM, different diameter particles with varied shapes were clearly found. Systematic experiments were designed and conducted to measure the particle concentration change through bends. The measured results at Reynolds number Re=17900 and 35600 showed a general agreement with previous published data, models and the numerical prediction in this thesis. The bend penetration decreases from approximately 100% for dpn=1μm particles to 64% for dpn=25μm particles. Dust deposition velocity was roughly higher than previous studies probably due to the consideration of particle rebounce from wall. The changes in Reynolds number did not significantly alter the trend of deposition velocity. Based on the observed linear relationship, empirical models were proposed by fitting the experimental data. They were valid for dimensionless relaxation time τ⁺p from 0.34 to 27.6 under present experimental conditions. Potential influencing factors, including straight duct length ahead, initial mass concentration and wall materials, were experimentally measured and analyzed. The measured results revealed that particle penetrations decreased moderately within 11% with the increase of initial concentration. However, deposition velocity increased from 1.29 to 2.87 times with Cm from 11.3 to 34.5 mg/m³. Particle accumulation is much heavier near the outer bend wall and lighter near the inner wall for particles of dpn=5μm or more. Both higher initial concentration and larger particle diameter could accelerate the formation of concentration nonuniformity at bend outlet. Furthermore, apexes and concaves of the outlet distribution are inferred to be formed by the rebounding particles from the outer wall. When the initial concentration increases, the values of the apex and concave increase. Compared to penetration, deposition velocity is more sensitive to wall materials, for example, with an increase factor of 1.06-1.45 at an initial mass concentration of Cm = 34.5 mg/m³ under Reynolds number Re = 17900. Particle concentrations with steel bend near the inner and outer wall are much smaller than those with glass bend at a same given initial concentration. Quantitatively, present experiments give specific areas for particle of specific diameter range for "particle free zone" under current measuring conditions. Academically, the numerical research contributes an efficient computatial fluid dynamics (CFD) model for solving solid aerosol flow in 90° two-dimensional bends under turbulent airflow. The experimental study provides empirical models and sequences of distribution and deposition results in 90° three-dimensional bends of rectangular section. Many new phenomena (e.g. properties of 'particle free zone', and influences of initial concentration and wall materials), are found and analyzed. For engineering practice, the numerical and experimental results and models could be useful for controlling particle pollutant. The findings can benefit the understanding of particle flow in bends, guide the design of ventilation duct, and help control the particle pollution through the ventilation system.

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