Influence Of Soot Agglomerates On The Loading Performance Of Nanofibers Composite Air Filter Media

Particles in air can decrease the combustion efficiency of the internal combustion engine and lead to the engine abrasion.The fibrous filter media are widely used to prevent the particles from entering into the cylinder in the engine air intake system.The filtration efficiency and the service time are the two most concerned parameters during the filter application process.The service time of the air filters is usually evaluated by the dust holding capacity tested under standard conditions.Different standards verified the ISO-12103 A2 dust,of which the size range is from 0.5 to 60?m,as the challenge aerosol during the loading test so far.However,many research results show that the nanoparticles(<100 nm)are more than70 percent in the ambient air.Therefore,the A2 fine dust can't be used to evaluate the dust holding capacity of the filters veritably.Soot agglomerates are a kind of the pollutant in the air.Their mainly source is the uncomplete combustion of the fuel.A large quantities of application cases show that the filter media are clogged very fast in the ambient with high soot concentration.Thus,research on the influence of soot agglomerates on the loading performance of air filter media is of great significance.Only several researches focus on the filtration efficiency of filter media against the soot agglomerates.However,there are only a few studies about the soot loading performance of the fibrous filter media.In order to investigate the soot loading performance of different filter media,the test system for soot,A2 dust and soot and A2 dust mixture loading was designed in this research.The soot,A2 dust and the mixture particles can be used as the challenge aerosol to test the filter media loading performance,which can mimic the real condition in which the filter media are applied.We can evaluate the loading performance of the filter media more actually basing on the test results.Based on this test system,the filtration characteristic of different filter media against the soot agglomerates was studied firstly.Then the loading performance of four different kinds of nano-fibers composite filter media against the soot agglomerates and A2 dust and the soot and A2 dust mixture particles was studied too.Finally,the structural model of the filter media which have long service time in the ambient with high nanoparticles concentration was proposed.The filter papers were made according to the model and the dust loading performance of them was studied.The results show that(1)Compared with the NaCl particles with the same mobility diameter,the soot agglomerates have lower penetration in the same filter media under the same test condition.This was because that the soot agglomerates had lager interception diameter leading to higher filtration efficiency of single fiber.(2)There were two phases during the pressure drop increasing process of the filter media during the soot loading experiments.(1)The nano-sized soot agglomerates deposited on the surface of fibers and in the void between them during which the pressure drop of the filter media increased slowly.(2)The soot agglomerates formed cakes on the surface of the filter media which make the pressure drop of them increased faster during the second phase.When the concentration of soot agglomerates was 7.8×10~6#/cm~3,the pressure drop of the filter media increased slower than that when the soot agglomerates concentration was 4.7×10~6#/cm~3 under the condition with the same face velocity.The soot cakes were compressed more under the higher face velocity which made the cake with more compact structure and the pressure drop increased faster.(3)When the soot agglomerates concentration was 6.5×10~6#/cm~3 and the face velocity was 6.7 cm/s,the pressure drop of the four kinds of filter media increased slowest during the soot loading process.The dust holding capacity of the PTFE membrane composite filter media(4#)was the largest and it was 5.71 g/m~2.It was 3.48 g/m~2 of the electrospinning nanofibers composite filter media(1#)which was the lowest.The pressure drop of filter media 1#and 4#increased fastest and the dust holding capacity of them was 1.82 g/m~2 when the soot concentration was 4.5×10~6#/cm~3 and the face velocity was 11.1 cm/s.The pressure drop of the filter media 2#and 3# increased fastest when the soot concentration was 4.5×10~6#/cm~3 and the face velocity was 6.7 cm/s.The dust holding capacity of them was 0.61 g/m~2 and 0.98g/m~2,respectively.(4)The pressure drop of the four types of composite filter media increased linearly during the A2 fine dust loading process which indicated that the mainly filtration mechanism of them was surface filtration during the loading process.The pressure drop increasing rate was the smallest of the four types of filter media when the A2 dust concentration was 4.5×10~4#/cm~3 and the face velocity was 6.7 cm/s.The pressure drop of 2#increased slowest and the dust holding capacity of which was 24.05 g/m~2 while that of 1#was 19.70 g/m~2 which was the smallest.The pressure drop of the four types of filter media increased fastest when the A2 dust concentration was 1.6×10~4#/cm~3 and the face velocity was 6.7 cm/s.The dust holding capacity of 4#was the largest and that of 3#was the smallest under this condition.They were 17.61 g/m~2 and 17.12 g/m~2,respectively.(5)The pressure drop increased faster when the filter media were loaded with soot and A2 dust mixture than that in the case of pure A2 dust loading process.The higher percentage of the soot in the mixture,the faster the pressure drop of the filter media increased.The dust holding capacity of 1#,2#,3#and 4#was 19.3 g/m~2,21.2g/m~2,19.1 g/m~2 and 18.9 g/m~2,respectively.(6)The loading results showed that the filter media with long service time for the application in the ambient with high soot concentration should have the characteristics as following:1.They should be the nanofibers composite materials which will ensure the filtration efficiency against the soot agglomerates to prevent the particles from penetrating into the void in the inner part of the filter media which make the pressure drop increase fast.2.The nanofibers layer should have a certain thickness and an open structure to guarantee their dust holding capacity.(7)The new nanofiber composite filter media were made by the wet-laid process according to the proposed structural model with long service time.The loading performance of them against the soot agglomerates and A2 dust and the soot agglomerates and A2 dust mixture was better than the 2#filter media which performed best during the loading process among the commercial products.The dust holding capacity of it was one time higher than the other four types of filter media under the same test condition.The average pressure drop of the four types of filter media during the mixture particles loading process was 253.6 Pa,244.9 Pa,283.9 Pa and 305.6 Pa while it was 176.6 Pa of the new nanofibers composite filter papers.This showed that the filter media which have the similar structure as them are good for lower the energy consumption during application process.