| With the rapid development of modern industry,the importance of trace gas detection technology is self-evident.Photoacoustic spectroscopy(PAS)is one of the most important technologies.It is based on the theory of absorption spectroscopy and the principle of photothermoacoustic effect to realize the qualitative and quantitative measurement of gases.It has the advantages of high sensitivity,high selectivity,fast response,on-line monitoring and no consumption of gas samples.It has broad application prospects and high scientific research value.With the development of computer science and numerical simulation technology,great changes have taken place in the research and development environment and design process of products.In this paper,by combining theoretical analysis,experimental research and numerical simulation,we focus on the coupling calculation of multi-physical fields of photoacoustic spectroscopy,the optimization design of photoacoustic cells,the continuous detection system of photoacoustic spectroscopy and the flow field and noise in the cavity of the cell.Experiments and research are carried out on sound and other issues.Based on the theory of photoacoustic spectroscopy and acoustics,a cylindrical resonant photoacoustic cell was designed and fabricated,and a photoacoustic spectroscopy experimental detection system for gas measurement was established.Taking methane gas as an example,the system is used for quantitative detection of methane gas.The performance parameters of the system,such as the signal-to-noise,sensitivity,stability,detection limit,working resonance frequency,quality factor and cell constant are given.The gas concentration,laser power,pressure and temperature are studied.There is an objective problem of photoacoustic coupling in photoacoustic spectroscopy.Based on multidisciplinary analysis and multi-physical field coupling simulation technology,the acoustic modal frequencies and corresponding visual acoustic modes of photoacoustic cells are calculated and obtained.The influence of the main structure and geometric size of photoacoustic cells on the calculation of acoustic resonance frequencies is analyzed.The results show that it is necessary to consider the influence of buffer structure to accurately calculate the acoustic resonance frequency of photoacoustic cell.By applying the loads of the Gauss laser source and setting the photoacoustic boundary conditions,the distribution characteristics of the acoustic field and temperature field in the photoacoustic cavity are solved by means of the thermo-viscous acoustic simulation program,and the frequency response law of the photoacoustic signal in the photoacoustic cell is obtained.The factors affecting the gas concentration,laser output power,pressure and temperature of the photoacoustic signal are analyzed,and the corresponding influence curve is obtained.Combining with the experimental results,the validity and feasibility of the numerical simulation of photoacoustic spectroscopy are verified and clarified,and the closed-loop design and analysis flow of the photoacoustic cell "theoretical calculation-experimental test-numerical simulation" is basically formed.In addition,the photoacoustic spectroscopy simulation program has been redeveloped,and two professional simulation platforms have been compiled.Aiming at the optimization design of photoacoustic cell structure,a multi-objective optimization design method of photoacoustic cell based on response surface proxy model and genetic algorithm is proposed.The performance index of photoacoustic cell with horn-shaped shape at both ends of the resonator is explored and calculated.The optimum solution set of Pareto with maximum quality factor and cell constant is obtained by simulation.Through optimization,the quality factor Q of photoacoustic cell increases by 48.9%and Ccell by 34.4%compared with the initial value.Based on 3D printing and numerical simulation technology,the acoustic performance of photoacoustic cell with equilateral triangle,square,Pentagon and ellipse cross section of resonator is studied.The acoustic characteristics of photoacoustic cell with exponential curve and spline interpolation curve transition are also studied.The results show that the performance of photoacoustic cell with circular cross section is the best in regular shape and in irregular shape.The photoacoustic cell with interpolation curve(waist drum type)transition has the highest design potential.The dynamic detection of the photoacoustic spectrum detection system is studied.The gas flow phenomena in the photoacoustic cell are simulated and analyzed by combining Computational Fluid Dynamics(CFD)with experimental measurements.The physical problems of gas concentration diffusion,gas exchange and gas noise in the photoacoustic cell are also studied.The derivative value of pressure to flow at the opening and the equilibrium time of gas concentration in the cavity are proposed as two indicators to evaluate the dynamic effect of the system.The effects of the radius of the inlet and outlet holes,the distance between the axial holes,the cross-section transition design,the layout of the outlet holes and the design of the shunt through holes on the indexes were studied.A structure unit of photoacoustic cell with better dynamic detection performance is presented.The results show that when the flow rate is 100 sccm,the equilibrium time of gas concentration is effectively shortened,which is about 14.3%higher than that of the original structure.The derivative value of pressure to flow at the opening decreases to about 25%of the original value,and the performance of the photoacoustic cell is basically stable.The purpose of this paper is to provide reference for the design and optimization of similar or similar spectral detection devices through the theoretical calculation and numerical simulation of photoacoustic spectral gas detection system. |
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