| In recent years,China's economy has developed rapidly and people's income has been continuously improved.As of the end of 2019,the number of motor vehicles in China reached 348 million.The increasing number of motor vehicles has caused serious environmental pollution problems.The particles emitted by motor vehicles has become one of the main sources of PM2.5 pollution in China's urban atmospheric environment.Thus,China has successively added the limits of solid particle number(PN)in the emission regulations of Phase V for Heavy Vehicles and Phase VI for Light Vehicles.Therefore,the on-line PN measurement of vehicle emission is of great significance for the accurate evaluation of the vehicle particle emissions.At present,the measurement methods of vehicle emission PN are mainly divided into two categories:electrostatic method and condensation particle counting method.The electrostatic method calculates the particle number concentration by inverting the measured micro-current value,and the detection accuracy of the measurement is not high.The condensation particle counting method directly counts the individual particles,and has ultra-high measurement accuracy.Moreover,it is the mainstream method for bench test in global motor vehicle emission PN regulations.However,in the process of condensation particle counting,the particle size of ultra-fine particles usually grows above the micron level,and the single particle pulse signals may overlap,which limits the measurement range of the condensation particle counting method.Thus,with 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",key technical research on the online measurement of the ultra-fine particles number concentration emitted by motor vehicles based on condensation particle counting was carried out.Designed and developed an ultrafine particle heterogeneous condensation growth device and a high-bandwidth optical single particle counting device.A coincidence correction method based on particle pulse width accumulation was proposed,and a measurement system for the vehicle emission ultra-fine particle number concentration was established.Moreover,the performance evaluation tests of the designed system were completed,and vehicle bench tests and other experiments were carried out.The main results of the study are as follows:(1)Based on the analysis of heterogeneous condensation theory and single-particle light scattering measurement theory,a measurement scheme for the number concentration of ultrafine particle emitted by motor vehicles was proposed,which is composed of butanol-based ultrafine particle condensation growth,optical single particle counting and coincidence correction.Moreover,under the condition of small flow,high concentration,large condensed particle conditions,the proposed scheme can achieve accurate PN measurement.(2)A butanol-based ultrafine particle heterogeneous condensation growth device based on continuous full-flow scheme is designed,and a multiphysics coupling simulation model is established.By analyzing the non-uniform spatial distribution of temperature,vapor partial pressure and saturated vapor pressure,the causes of supersaturation and particle activation are deeply analyzed,and it is found that the maximum supersaturation and the minimum Kelvin particle size are both realized near the center line of the condensation chamber.The interaction between the size parameters of the condensing chamber,the sampling flow rate and the supersaturation,the non-uniform spatial distribution of the Kelvin particle size are studied to determine the key structure and operating parameters of the condensation growth device.Quantitatively analyze the relationship between the temperature window of the condensation growth device and the particle activation efficiency,and propose a method to adjust the detection cutoff particle size to meet the needs of different applications.A high-efficiency temperature control scheme with a circulating liquid chamber structure and low energy consumption is designed to achieve a temperature control accuracy of±0.2?,thereby ensuring the stability of the supersaturation area in the condensation chamber.(3)An optical single particle counting device based on a high-bandwidth particle pulse scheme is designed,which aiming at the small sampling flow,large particle size and high concentration.Through optical desin,the detection laser beam with a width of 27 meters and a 115° collection angle of particle scattered light are realized.In order to study the effect of particle pulse bandwidth on the particle coincidence probability and concentration range,two optoelectronic conversion circuits with-3db bandwidth of 190kHz and 5.4Mhz are designed,and two particle pulse hardware measurement schemes were proposed.Under the working condition of 0.3L/Min sampling flow,1mW laser power and the 15?m standard spherical,the half particle pulse width of the high-speed photoelectric conversion circuit is about 650ns,and the half particle pulse width of the low-speed photoelectric conversion circuit is about 5?s.(4)A number concentration measurement system for ultrafine particles emitted by vehicles is established,which based on the self-designed ultrafine particle heterogeneous condensation growth device,optical single particle counting device,peripheral control unit and system control program.Then,the detection efficiency,particle coincidence,concentration range,accuracy,and response time of the measurement system were experimentally studied.The results show that when the saturator temperature is 35?,condenser temperature is 10?,the particle size of 50%and 90%detection efficiency is about 4.5nm and 8nm,respectively.When the saturator temperature is 35?,condenser temperature is 20?,the particle size of 50%detection efficiency is about 10nm.When the saturator temperature is 38?,condenser temperature is 32?,the particle size of 50%detection efficiency is about 26nm.At a concentration of 15,000#/cm^3,the coincidence probability of the 10?s particle pulse signal is about 56.4%,and the coincidence probability of the 1.3?s signal is only about 9.5%.Using particle coincidence correction method of pulse width,the self-developed measurement system concentration range is0-2.65×105#/cm3,and the deviation from the reference is less than 10%.In 1L/Min high flow mode,the self-developed system 0-90%response time(Tr,90)is about 3 seconds,the concentration 90%-0 response time(Td,90)is about 3.5 seconds.In 0.3L/Min low flow mode,the system Tr,90and Td,90 is about 3.5s and 4s,respectively.(5)The system comparison test experiment and data analysis are carried out,an environmental aerosol comparison measurement experiment was conducted with TSI-3788,Airmous-A20 and self-developed measurement system.The result show that the linear correlations between the self-developed system and the two commercial devices are both better than 0.99.The diesel/petrol motor vehicle bench test comparison experiments is carried out at the China Automotive Technology&Research Center(CATARC).The experimental results show that the statistical correlation between the self-developed system and the PN measurement device specified in the CHINA ?emission regulations(MEXA-2000SPCS,HORIBA)is better than 94%.Finally,the characteristics of particles emitted by vehicles are experimentally studied at the Urban Traffic Emissions Control Research Center of Nankai University.The experimental results show that a CHNINA V and a CHNINA ? test vehicles have a large amount of particles below 23nm,and ration between sub-23nm particles and 23nm-2.5?m particles is at least 2/5. |
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