Investigation On Characteristics Of Particle Dispersion And Deposition In A 90° Elbow Of Square Air Duct

Aerosol particles deposit on the inner surface of air duct and form a foul layer.The layer can disturb the air flow and grow microbial which can cause indoor air pollution.Therefore,studying the deposition characteristics of particles in air duct is of great significance to optimize duct design and improve indoor air quality.A lot of research on particles migrating and depositing process have being done at home and abroad.However,compared with the straight pipe,the flow field at the elbow is more complex and leading to the change of particles deposition characteristics.Thus,this dissertation studies the deposition characteristics of particles in the 90°elbow of square air duct and the influencing factor by numerical simulation.First,RNG k-εmodel and RSM model are selected as turbulence models in this dissertation,and they both are modified to simulate boundary layer accurately.By calculating and comparing,it shows that the flow field calculated by RNG k-εmodel is not accurate enough to be used to predict particles deposition.While the performance of the modified RSM model is good.Then,this dissertation carried out a numerical simulation about flow field and particles motion in 90°elbow of square duct with 200 mm hydraulic diameter and 400 mm curvature radius.The results show that there are pressure and velocity gradients in the elbow which increase with the wind speed in the horizontal direction,and secondary flow can be observed clearly at the same time.Particles deposition velocity in the elbow increases with particle diameter and air velocity,while the penetration almost only related to Stokes number（St）.In this dissertation,the correlation between particles penetration rate and St is fitted.Compared to the straight pipe,the penetration rate of particles in the elbow is lower when St<10^-3 and higher when St>10^-3.Finally,the influence of hydraulic diameter and curvature radius on the particles deposition characteristics is studied.The results show that increasing the hydraulic diameter or decreasing curvature radius will increase the particles deposition velocity.While the rule will be opposite when the particle diameter over 50μm.What’s more,the fittingη-St function mentioned above can be used to predict particles penetration rate in the 90°elbow of square duct with different geometric parameters（hydraulic diameter and curvature radius）with relative error of±5%.