Research On Key Technologies Of On-line Measurement Of Sampling And Number Concentration Of Ultrafine Particles In Motor Vehicle

With the rapid development of China’s economy,the vehicle ownership has also been rising year by yearn The air pollution caused by vehicle exhaust emissions has become increasingly serious and has become one of the major sources of urban air pollution.Therefore,the accurate measurement of ultra-fine particles emitted by motor vehicles as a representative of mobile pollution sources is particularly important.The monitoring results can provide effective evidence for vehicle emission assessment,research on particulate matter growth mechanisms,and development of fine control measures.Under the support of the National Key Research and Development Program of China "R&D and application demonstration of mobile pollution source emission rapid on-line monitoring technology",this article is aimed at the on-line measurement requirements of high-temperature,high-concentration ultra-fine particulates emitted by motor vehicles,on-line measurement methods and key techniques for dilution sampling and concentration of ultrafine particles,designed and developed jet dilution system,unipolar diffusion charger,and electrostatic deposition device,and achieved an on-line measurement system for ultra-fine particulate matter concentration in motor vehicles that incorporates jet dilution,diffusion charging,electrostatic deposition,and micro-current detection technologies,and initially completed the system comparison test.The main results are as follows:(1)Based on the principle of jet dilution,a jet diluter was designed.The dilution sampling system was designed based on the diluter,and Comosol simulation software was used for numerical simulation to analyze the influence of its key dimension parameters on its internal flow field and dilution ratio.Adjusting the pressure difference between the inlet and outlet can achieve a dilute dilution ratio of 1:7-1:60.When the inlet and outlet pressure difference is 0,a 1:8 stable dilution ratio can be achieved.Diluter dilution ratio stability test was carried out.The test results showed that the average dilution ratio in the dilutor was 1:8 within 0.5 hours and the standard deviation was 0.032.(2)Based on the charge theory of particles,a particle charger based on the charge-discharge principle was designed to achieve efficient charging of ultrafine particles while removing excess free ions.Using Comosol simulation software to perform numerical simulation,the trajectories of different particle sizes and free ions inside the charger were analyzed,and the free ion trap voltage was finally determined.The average charge capacity,charge efficiency,and loss efficiency of different particle size particles in the charger were measured to achieve a charge efficiency of up to 62%for the 23 nm particle,which meets the requirements for particulate detection based on the principle of electromigration in the five national standards(23 nm corresponds to 50%detection efficiency).(3)Based on the electromigration theory of particles,two kinds of particulate electrostatic precipitators with different structures were designed according to the motion characteristics of particles in the flow field and the electric field coupling field.Using Comosol simulation software for numerical simulation,the motion trajectories of particles with different particle sizes inside the two electrostatic precipitators were analyzed.And the key dimensions of the two electrostatic precipitators were determined based on the simulation results.Using the laboratory standard aerosol generation and detection equipment,the particle trapping efficiency and particle loss efficiency inside the two kinds of sedimentators were studied in depth.The results show that the trapping efficiency of the 23 nm particles reaches 100%for the trapping voltages of the cylindrical and flat electrostatic precipitators that are greater than 20 V and 15 V,respectively;for particle size of 50 nm-700 nm and 800 nm-2500 nm,the internal particle loss of the cylindrical electrostatic precipitator is about 1.4 times and twice of the internal particle loss of the flat plate electrostatic precipitator,respectively.(4)The integration and comparison testing of on-line measurement systems for ultra-fine particulate matter concentration in motor vehicles was initially achieved.The results showed that the correlation with commercial instruments was better than 87.2%.