MultiScale Numerical Research On Filtration Performance Of Small High Efficiency Particulate Air Filter

Filter cartridge functions as an important part of biological protective mask to purify the air polluted by poison gas or bio-aerosol. Recently, researches in various countries have been made on how to promote the filter efficiency and decrease the pressure drop of filter cartridge. Experimental method and theoretical method may both behave well, although gifted with their own defects. While the numerical simulation method help study the factors for filtration performance of filter cartridge and predict the filter efficiency and pressure drops of filter cartridge by mathematical and physical modelling, which may greatly simplify the process of the new filter cartridge with lowpressure drop and high filter efficiency.There are two scales involved in the application of the high efficiency particulate air filter and ultra low penetration air filter,that is, the scale of micrometer or even nanometer for filter media and the scale of centimeter for filter element and cartridge. Full scale modeling may result in enormous computational complexity because of the differences between the scales, and thus is not applicable for predicting the performance of filter cartridge. While the multiscale numerical simulation method and available data transmission method between scales may help analyze the flow in various scales and full scale. But most of the researchers took the single-scale numerical method to study the performance of filter cartridge. For example, CFD method is used to simulate the micro-scale gas-solid flow between nanofibers in order to broadcast the filter efficiency and pressure drops of filter media, or to simulate the macro-scale flow in the filter cartridge so as to study the flow field. Those studies may not work well in understanding and predicting the whole performance of filter cartridge..A layering approach was used to solve the scale difference problems between fiber diameter and thickness of filter media in order to reduce the computation load of numerical simulation. The2-D and3-D models of U15filter media of local thickness in nanometer scale were built by combining Visual Basic language and the Journal file in the preprocessing software,Gambit. Based on the independence between the computational results and the size of domain and grid density. The pressure drop and filter efficiency of U15filter media of local thickness were predicted by CFD method. and the predicted results were compared with those by theoretical calculation and experiment. The results showed that pressure drop linearly increased with face velocity and the predicted values were higher than the theoretical results. For the slippage phenomenon adjacent to wall in micro-scale flow, CFD method was applied to investigate the effects of wall slip on flow behaviors and pressure drop based on two wall boundary conditions, that is, with slip wall and with no slip wall. The results showed that the pressure drop might decrease in the slip flow.A macro-scale U15filter media of entire thickness model was built by the software, Gambit based on the porous model. The pressure drops from face velocity O.Olm/s to0.06m/s were predicted by CFD method, and then the predicted results were compared with those by theoretical calcultion and experiment. It is indicated that the pressure drop linearly increased with face velocity, and the predicted values of pressure drops were higher than the theoretical results but in excellent agreement with the experimental ones with the2-D model-based relative error8.75%～10.28%and3-D model-based relative error from7.92%～9.44%, which meant that the studies above could be used to predict the pressure drops of filter media at low face velocity. The dimensionless pressure drops f(a) in Davies experimental correlation expression for pressure drops of filter media was modified by experimental vales and predicted vales in order to make it applicable to predict the pressure dorps of U15filter media. The filter efficiency for particle size0.3μm from face velocity O.O1m/s to0.06m/s were predicted by CFD method. The results indicated that the filter efficiency decreased as the face velocity raised. The predicted values of filter efficiency are lower than the theoretical results but in excellent agreement with the experimental results and with the the error less than0.27%for2-D model0.02%for3-D model. But the predicted values could not reach the demand of ULPA filter media needed whose filter efficiency is more than99.9999%.Macro-scale filter element and filter cartridge mode were built by the software, Gambit. The pressure drops of filter element from face velocity0.01m/s to0.06m/s were predicted by CFD method. With the filter element considered as porous media, the pressure drops of filter cartridge from face velocity O.Olm/s to0.06m/s were predicted by the CFD method. The results showed that the the pressure drop linearly increased with face velocity, and the predicted results were higher than experimental ones with the relative error of12.63%to21.51%. Then, it was concluded that the method could be applied in predicting pressure drop of filter cartridge.To sum up, a multiscale flow numerical simulation method was taken to study the filtration performance of filter cartridge. Pressure distribution and particle concentration distribution of different scale regimes in filter cartridge were predicted by computational fluid dynamics(CFD) in order to get the pressure drop and filter efficiency of filter media and the pressure drop of filter cartridge. Then the predicted results were compared with those by theoretical calculation result and experiment so as to offer some suggestions to optimize the filtration performance of filter cartridge.