Research On Air Flow Uniformity Of High Efficiency Particulate Air Filter

As terminal equipment installed in the clean room,HEPA has a direct influence on the cleanliness class of the cleaning room because of the flow velocity and distribution of air flow through it.And the quality of product depends on indoor air quality affected by air distribution in the cleaning room.According to most of international standards,spot velocity in the unidirectional air flow zone,if any,has to be controlled close to the average velocity with ±20%allowable deviation.However,based on many experimental test results,most of the cleaning room cannot meet the standards.One of reasons is that inside configuration of HEPA is an unreasonable design.This thesis focuses on HEPA and analyzes its configuration to optimize the outlet air distribution and make the cleanliness of clean rooms meet the standards.Numerical simulation is used to research and analyze the inside flow field of HEPA.According to the physical model of the HEPA,numerical model is set up after some reasonable assumptions and simplifications to the HEPA's inside flow field.The turbulence model applies to the numerical model.The upwind differencing scheme(UDS)is used in governing equations.Wall functions are chosen as the Near-wall treatment.The filter materials are considered as porous media.SIMPLE are used to solve the governing equations.Then,the simulation results show the HEPA's inside flow field and outlet flow velocity distribution.Based on the result,several improved configurations of HEPA are brought up.They are analyzed through numerical simulation.Then,compared to the simulation result of original model,the best improved model is finalized.Numerical simulations at different flow rate through one HEPA are administratedto validate the wide availability of improved model.Research results show that uniform airflow distribution is guaranteed by the appropriate percentage of open area on uniform flow orifice and its right position set inside the HEPA.After comparison of numerical simulations at different working conditions,it's possible to finalize the best percentage of open area and its best position,which are 50%and 50 mm to the top of HEPA.In this case,the allowabledeviation is 10%,which meets the standards.As for the HEPA with side outlet,its outlet airflow has more uniform velocity distribution after installing baffles.It's discovered that HEPA installing three baffles gets the best uniform airflow distribution even if it works at 50 percent,120 percent of rated flow rate.In terms of diffusers,there are three optional sorts,which are grille diffuser,swirling diffuser,and porous diffuser.Airflow through grille diffuser diffusesvertically in a long distance because vertical average velocity decreases slowly.Swirling diffuser makes airflow diffuse in a large area.Airflow through porous diffuser has less and smaller eddy,and the airflow distribution is uniform in the clean room.