Reconstruction Of The Optical And Thermal Properties In Participating Media Based On Light Field Imaging Theory

Participating media are widely present in nature and industry production process. Developing the non-contact measurement technique and theory for the optical and thermal properties(radiation properties and temperature field) of participating media, and building the underlying database of optical and thermal properties with high confidence plays an important role to promote the development in many engineering field, for instance the aerospace engineering, energy and power, defense technology, biomedicine, and chemical metallurgical etc. Therefore, a simultaneous reconstruction method of the optical and thermal properties in participating media based on the light field imaging is developed by combining the advanced light field imaging theory and solving methods for forward and inverse problem of the radiative transfer in participating media.Firstly, the radiative transfer mechanism in participating media and the light field imaging principle are introduced systematically in this thesis. For the multi-directional radiative information can be obtained by the light field imaging, the quickly and accurately calculating methods of radiation intensity in arbitrary direction for absorbing media and absorbing, emission, and scattering media are established based on the line-of-sight method(LOS) and the generalized sourced multi-flux method(GSMFM) respectively. And further combined with the light field imaging theory, the complete numerical simulation method for simulate radiative transfer process is developed to obtain the radiation intensity 4D light field information in participating media. Then it is used to simulate the light field information collection of flame successfully, and the original light field simulation image is also obtained by this method.For the research of radiative transfer inverse problem in participating media, several kinds of inverse problem solving methods which commonly used at present are summarized, such as regularization methods, gradient based methods and intelligent optimization algorithms. On this basis, several kinds of improved algorithms are developed and applied to solve the radiative transfer inverse problem, and the simulate results prove the improved algorithms have a higher accuracy, efficiency and stability.Based on the theory research of radiative transfer forward and inverse problem mentioned above, a reconstruction method of 3D temperature distribution is developed combined with the least-square QR decomposition algorithm(LSQR). And the 3D temperature distribution can be reconstructed according to the radiation intensity distribution of media with known radiative properties. A large number of numerical simulations prove that this reconstruction method has high accuracy and efficiency. Moreover, the effects of radiation properties, reconstruct resolution, and imaging resolution on reconstruct precision of 3D temperature distribution are also investigated in this thesis.Further, a simultaneous reconstruction method of the optical and thermal properties in participating media based on the hybrid LSQR-SPSO optimization algorithm is developed to reconstruct the 3D temperature field and uniform radiation properties(absorption coefficients and scattering coefficients) simultaneously according to two groups of radiative intensities in different directions. And a large number of numerical simulations prove that this reconstruction method is accurate and effective. Moreover, the effects of radiation properties on reconstruct precision of optical and thermal properties are also investigated in this thesis.The simultaneous reconstruction method of the optical and thermal properties in participating media based on the hybrid LSQR- SPSO optimization algorithm is optimized by making full use of the spectral information in the light field image of the participating media. And the reconstruct precision of 3D temperature distribution and radiative intensities could increase substantially by this method. Further considering the conjugate gradient(CG) algorithm which has some degree of optimization abilities of filed parameters, a new simultaneous reconstruction method of the 3D temperature distribution and non- uniform radiation properties distribution is developed based on the multi-spectral technology and the hybrid LSQR-CG optimization algorithm. A large number of numerical simulations prove that this method could reconstruct the non-uniform optical and thermal properties in participating media simultaneously and effectively. And the reconstruct precision of 3D temperature distribution could increase substantially by this method compared with the reconstruction mode of using uniform radiation properties. Therefore, this method is more suitable for the reconstruction of 3D temperature distribution in real project.Finally, the experiment research of simultaneous reconstructing the optical and thermal properties in ethylene diffusion flame by using the solving method investigated above is introduced. The 3D temperature distribution and non-uniform radiation properties distribution of the simple ethylene diffusion flame and the ethylene diffusion flame doped Al2O3 particle are all reconstructed according to a light field image respectively. And the comparison between the measuring results using thermocouple and the reconstruction results in literature proves that the simultaneous reconstruction method of 3D temperature field and radiation properties distribution in participating media based on light field imaging technique developed in this thesis is reasonable and feasible.