Study On Particulate Matter Detector Based On Radioactive Source

Real-time monitoring of the particle size distribution of atmospheric aerosol particles and the concentration of dust from fixed pollution sources are important aspects of environmental air pollution monitoring.In order to design the detector system for measuring atmospheric aerosol particles and dust from fixed pollution sources,this thesis carried out the research and design of charger based on radioactive source and particle size classifier,and based on this,designed the active detector system?pumping type?for measuring atmospheric aerosol particles.A passive?non-powered?detector system for measuring dust from fixed pollution sources was developed and tested.It laid the foundation for the development of real-time measuring device for particulate matter by using isotope technology.In order to design the charger,the theory of diffusion charging of aerosol particles and the structure and parameters of the charger were studied.Based on theoretical calculation and computational fluid dynamics simulation,the structure and parameters of the charger:main body of the structure is a cylinder with a length of 8 cm and a diameter of 5 cm,and the air inlet section is gradually expanded by a smooth curve with an expansion angle of less than 15^o,the radioactive source is ²⁴¹Am?source with a total activity of less than 10⁴ Bq,arranging the alpha source evenly in the middle of the cylinder.The ion concentration in the charger is about 2.5×10 ¹²/m³,and the gas sampling flow rate is less than 3 L/min.In order to design the particle size classifier,the simulations of the flow field distribution,electric field distribution and the motion trajectory of charged particles in the classifier were carried out by using ANSYS Fluent software.The simulation results show that the flow field is laminar,the flow field and electric field are symmetrically distributed along the central axis,and the axial movement distance of the particles decreases with the increase of electromigration.The measurable particle size range of the classifier increases as the length of the classifier increases,the flow rate decreases,and the voltage increases.Increasing the flow rate and lowering the voltage can reduce the measured particle size range while increasing particle size resolution.Based on the research and design of charger based on radioactive source and particle size classifier,an active detector system is designed,which consists of charger based on radioactive source,ion trap,splitter segment and particle size classifier.Based on the structural study of the charger,the development and testing of the passive detector system was carried out.The passive detector adopts a structure similar to that of the charger.The design parameter is that the inlet diameter is 8mm.When using a Ni-63?source,the source activity is 2.22?10⁷Bq and the detector cavity is 2 cm wide.Environmental temperature and humidity,dust concentration impact test results show that temperature and humidity have a significant impact on the detector output signal:When the temperature is raised from 34 ^oC to240^oC,the detector's output signal is increased by about 50%,and when the moisture content is increased from 6.52%to 10.65%,the detector's output signal is reduced by about 30%.The“concentration”value of the dust measured by the detector increases with the increase of the environmental dust concentration,and the filter weighing method is used as a comparative measurement,and the measured value is 13.5mg·m^-3?26.8 mg·m^-3?33.6 mg·m^-3?47.5 mg·m^-3?63.3 mg·m^-3,the detector“concentration”values correspond to16.6 mg·m^-3?30.4 mg·m^-3?38.1mg·m^-3?45.3 mg·m^-3?53.2 mg·m^-3,and preliminary measurements of passive detectors have been verified.The feasibility of dust concentration lays the foundation for further research on passive detectors.