The Design, Simulation And Experimental Evaluation Of Aerodynamic Lens Sampling Device

Aerosols are observable and measurable solid or liquid particles suspended in aprolonged period in the air. Aerosol particle are ubiquitous in the atmosphere, andcan result in hazy weather in the city when the concentration and size of theseparticulate matter reach in a certain range. Generally, the method of measuringaerosol particle are off-line and on-line detection. Off-line detection is traditionaltechnology requiring a period time to collect particulate matter in the different sizerange. This method is undesirable because it need much time and particlecomposition may change far away from original information. On the contrary,on-line detection is advanced technology immediately measuring the compositionand size of particulate matter for they can be introduced directly into the instrument.This technology can significantly reduce the situation that particulate matter happenvolatilization, gas-particle transformation, crystallization and so on. On-linedetection technology has an important point of technical requirements, that is asampling device, by which particle dispersed in the atmosphere can be introducedfrom the atmospheric pressure into the vacuum, forming a high concentration ofsmall area of the beams. The narrow particle beam is called particle beam. Singleparticle mass spectrometry require highly particle beam collimation technology forthe limitation of pulse laser spot ionizing particles, meaning particle beam shouldkeep a highly collimated beam within sub-millimeter diameter at dozens ofcentimeters downstream of the sampling device. The earliest sampling types used forsingle particle mass spectrometry are capillary and convergence nozzle with thelimitation of narrow focusing size range, and the focusing efficiency will decreaserapidly when the particle size excess this range. Compared with orifice and capillary,aerodynamic lens is a widely used sampling device affording smaller divergenceangle and higher sampling efficiency for particle beam. Aerodynamic lens consist ofseries of different diameter and thickness holes which called lens in the cylinder. The gas will form convergence and expansion flow field when pass through each hole.Then, the trajectory of particle will gradually converged toward the centerline in thisspecial flow field. Once converged at the centerline, the particle will alwaysmaintain along the centerline even in the divergence flow downstream of thesampling device in the vacuum system. This thesis is primary divided into three partsfocusing on aerodynamic lens sampling device focusing particle, that is theoreticalanalysis, simulating optimization and experimental evaluate.(1) Theoretical analysis of aerodynamic lens focusing particle. This is the secondchapter of the thesis, elaborating theoretical analysis about how aerodynamic lensfocus particle and design computational process, including the basic flow fieldparameters of focusing particle, relationship between particle stokes value andcontraction and important calculation formula of lens.(2) Simulation and optimization of aerodynamic lens sample device. This is the thirdchapter,and a key chapter of this thesis. Firstly, FLUENT as the simulation softwarewill be introduced, with the specific steps of how to use this software, mesh theaerodynamic lens model and set boundary conditions. The aerodynamic lens iscomposed of five different sizes of holes, each one can be seen as a single lens, andfocused particle size are determined jointly by the single lens shape, gas pressure atimport and export of lens and mass flow rate. So, the lens size, optimal stokes valueof single lens and effect of pressure change on single lens focusing particle areexpounded in this chapter. In the process of design, the cylinder dimension shouldbekept the same value, and the optimal dimension will be shown in this chapter inthe aspect of the effect of cylinder dimension change on particle convergence andtransmission efficiency in the first and last single lens. This chapter also show thesimulation result of aerodynamic lens-nozzle for the performance of the whole,including the distribution of gas pressure, velocity and temperature, flow streamline,particle trajectory in flow field and particle terminal velocity. Simulation resultsdemonstrate that the performance of particle in0.3-3μm range in the optimal design sampling device is very well. Particle beam diameter at a distance of24cmdownstream of the nozzle exit is less than0.03mm. Besides, the four typical shape ofnozzle will be discussed in this chapter, simulating the effect of the flow field formedby these nozzle on particle velocity and divergence.(3) The experimental evaluation of aerodynamic lens. This is the content in thefourth chapter of the thesis, and will introduce the experimental equipment design toevaluate the aerodynamic lens performance and provide experimental results. Themain purpose is to obtain a series of parameters of particle beam downstreamsampling device, such as focused degree and export beam width, compared withsimulation result by building appropriate experimental device, and then theperformance of the aerodynamic lens can be assessed. The design and simulation ofsampling device can be better verified and optimized by combining theory andpractice. The principle of experiment is that the width of particle beam is measuredby the displacement of moving knife edge block when signal change from maximumto minimum which measured by particle charge that reach faraday cup.