Research On Two-dimensional Reconstruction Of Combustion Based On Laser Absorption Spectroscopy Using Virtual Ray Reprojection Method

Tunable diode laser absorption spectroscopy（TDLAS）is the one of the representative measurement method,with features of fast response,high precision,and strong anti-interference ability over a wide temperature range.It has broad prospects in application of combustion diagnosis for aero-propulsion and coal-fired power plant.Traditional line of sight TDLAS measurement results in an approximate average of gas parameter.Actually the combustion flow is inhomogeneous due to the gas turbulence and non-uniform fuel supply.In order to obtain the gas parameter distribution,it is necessary to study the optical design and algorithm of two-dimensional reconstruction system.In this paper,the theoretical and experimental research on gas concentration and temperature distribution in non-uniform combustion has been carried out by monitoring H₂O spectral lines.The outlines and conclusions are given as follows:（1）Virtual beams are introduced by difference method to solve the problem of insufficient projections in real measurement.The projections of virtual beams are calculated by the difference between adjacent real projections in parallel according to relative locations.Complete projections incorporating actual ones and virtual ones are utilized to infer the reconstruction of gas concentration and temperature distribution.The fine depicting of combustion with mesh refinement can be realized.The reasonable beams arrangement is designed by simulation.The advantages and disadvantages of the differential virtual beams method by beams arrangement are analyzed.（2）The reprojection with differential virtual beams is proposed to deal with the problems of incomplete projection angles and parameters mutate due to the projection contradiction between virtual beams.The gas parameter distribution result by differential virtual beams method is treated as initial value to conduct reprojection calculation of virtual beams at other necessary angles.The accurate results would be obtained by incorporating all projections after limited iterations.The simulation shows that temperature error and concentration error are 5.38%and 8.93%respectively with measurement error of 5%in the gird of 49×49,which verifies the feasibility of the reprojection with differential virtual beams method.（3）A two-dimensional reconstruction experimental system is set up to verify the algorithm proposed in this paper.A total of 21 real beams are arranged at three projection angles.Spectral lines at 7185.6cm^-1 and 7444.4cm^-1 are scanned with wavelength division multiplex technology to conduct the research on combustion exhausted distribution using a small gas burner.The combustion exhausted distributions during combustion pulsation cycle are reconstructed to infer the temperature and concentration changes,compared with images from high speed camera.The experimental results show that the system can provide accurate information about combustion center and distribution in complex environment.Through numerical simulation and experimental research,a novel algorithm is developed and verified to reconstruct gas concentration and temperature distribution in non-uniform combustion,which is of great significance to the engineering application of TDLAS technology in combustion diagnosis.