Reconstruction Of The Multi-physical Fields In Luminous Flame Based On Active And Passive Optical Detection

Combustion flame widely exists in various domestic and industrial applications,such as turbine,combustion chambers,utility boiler etc.Temperature distribution is strongly associated with the combustion process,and can directly reflect the characteristic of the combustion state.For understanding the mechanism of combustion deeply,optimizing the combustion process and reducing the pol lution,it is necessary to measure the flame temperature field quantitatively,which is still an unsolved problem,for the restrictions of equipment and reconstruction algorithm,and the high ill-posedness of simultaneous measurement of multi-physical fields.The passive measurement technique based on light-field imaging of flame radiation shows great potential for temperature measurement.Hence it is introduced into the measurement of flame temperature distribution in this thesis.This thesis aims to sloved the problem of 3D temperature and optical parameter field reconstruction of luminous flame.Firstly,the radiation transfer and light-field imaging model of flame is introduced,then the temperature field reconstruction model is studied according to the foreword model,and a simultaneous reconstruction model for temperature and absorption coefficient is also built pursuanting to the passive measurement technique based on light-field imaging.Finally,a synergy reconstruction process based on laser extinction method combined with light-field imaging is proposed.The light-field experiments are carried out to verify the proposed calculation models.The main work is as follows:The principle and characteristic of light-field imaging is illuminated,and a calculation method for the coordinate and direction of radiation ray is proposed.In order to solve the radiation transfer equation for different kinds of flame,the line-of-sight(LOS)method and the generalized sourced multi-flux method(GSMFM)are introduced to calculate the outgoing radiation intensity for absorbing flame and scattering flame respectively.Light-field imaging model of flame radiation is built on the integration of radiation transfer mechanism and principle of light-field imaging.Simulation is proposed based on the imaging model and the light-field image of the two kinds of flame is obtained finally.Flame temperature reconstruction model is carried out according to the radiation imaging model while the optical parameters are known,based on the least-square QR decomposition(LSQR)algorithm,algebraic reconstruction technique(ART)algorithm,and Landweber algorithm,respectively.Results show that the inverse model is accurate and reliable,and the Landweber algorithm is of the highest computational efficiency.Effects of value of optical parameters and form of scattering phase function on the reconstruction accuracy are investigated,and given the results,increase of optical parameters and anisotropic scattering will both cause the loss of accuracy of temperature reconstruction.Further,consider the situation that the optical parameter distribution is also unknown.A simultaneous reconstruction model is proposed according to the passive measurement technique based on light-field imaging.The Landweber algorithm is coupled with the seqential quadratic programming(SQP)algorithm to simultaneously calculate the temperature and absorption coefficient based on the single-spectrum light-field signals.Moreover,the temperature,absorption and scattering coefficient of absorbing and scattering flame are reconstructed by the Landweber-SQP hybrid algorithm according to the three-spectrum light-field signals.Results demonstrate that the simultaneous reconstruction model is feasible for the two kinds of flame.The reconstruction accuracy of temperature is higher than the optical parameters.In order to increase the computing efficiency of simultaneous reconstruction,the laser tomography based on the light extinction(LE)method is applied to the passive measurement technique based on light-field imaging,to build a synergy reconstruction model.The laser signals are used to calculate the absorption coefficient,and then the results are introduced to reconstruct the temperature field.Simulation results demonstrate that the synergy reconstruction model is feasible for absorbing flame,and is much more efficient than the passive measurement technique.Besides,ideal temperature results can be obtianed even when the absorption coefficient changes in a large range of values.Finally,the light-field experimental investigations for different kinds of flame are carried out based on the focused light-field camera.The platform of flame light-field imaging,femtosecond CARS temperature measurement system,and thermocouple are used to measure the temperature of methane Hencken flame.The deviation between the reconstructed temperature distribution and measurement results of CARS as well as thermocouple is small,and the relative mean deviation is not bigger than 4%,hence the reconstructed results are acceptable.Then the temperature and absorption coefficient field of ethylene diffusion flame are inversed according to the light-field image.The results are compared with the thermocouple and show good accuracy.Reconstructed distribution of absorption coefficient is also in agreement with the light distribution of flame image.Therefore the results are reasonable.Hence the proposed reconstruction model and measurement technique for 3D flame temperature and optical parameter field based on light-field imaging is of good feasibility.