Three-dimensional inhomogeneous temperature tomography of confined-space flame coupled with wall radiation effect by instantaneous light field

Optical tomography has been demonstrated to be a powerful tool for the three-dimensional (3D) measurement of the dynamic characteristics of turbulent flame. Many practical visualizations of reaction flows require imaging through optical windows on the combustor, which have complex interactions with wall temperature, reflection characteristics, and limited optical access, which may seriously influence the imaging quality. Previous work focused on unconfined flame imaging, and lacked the study of the multiple reflection effects of the wall and the tomographic reconstruction within a wide temperature range. To solve these problems, the effect of chamber wall radiation on the photothermal information is analyzed in the forward problem, and the flame temperature with wall radiation is reconstructed in the inverse problem. In the forward problem, the light field acquisition model of spontaneous emission of the confined flame is developed through the backward Monte Carlo method based on the radiation distribution factor, which can comprehensively consider the essential effects such as radiative attenuation, wall reflection, medium scattering interference, and limited optical access. The recorded light field signals indicate that the reflection pattern and transfer mechanism of the wall material could affect the contribution of radiative source terms in different discrete domains to the detected energy. A novel tomography algorithm, namely, the adaptive threshold segmentation iterative regularization (ATSIR) method, has been proposed to extend the temperature range of tomography. The key of this method is to carry out a two-step reconstruction of the high and low-temperature regions through the temperature threshold, and introduce the prior smoothing information to alleviate the ill-posedness of tomography. The visualization results show that the well-established algorithms (e.g., least-square QR-factorization and regularization) have lower reconstruction quality for the combustion field within a wide temperature range, even if the regularization term is added. However, the proposed ATSIR method has the advantages of better suppression of fluctuations, smaller reconstruction error, and stronger anti-noise ability. The detailed analysis of the radiative transfer mode and 3D temperature visualization of the combustion field in a confined space can provide a valuable guideline for the safety design of the engine combustion chamber and fuel utilization.