Study On Inverse Radiation Problem Of Temperature Distribution Reconstruction In Participating Medium

The electricity requirement from industrial and daily life is increasing with the development of economy in our country. In our country electricity industrial, the pul-verized coal-fired power plant is dominant. It is very important for the safe, economic, and stable operation of power plant. Combustion flame temperature field is the center of the combustion diagnostics. The advanced and effective measurement technique of the combustion flame temperature field has the significant scientific meaning and practical value for the study on the combustion of furnace in power plant. Combined with radiative properties of participating medium and radiative heat transfer calcula-tion method, based on the study of inverse radiation problem, and with the solving method of the inverse radiation problem, three-dimensional (3-D) temperature recon-struction model and combustion diagnostics method are established by means of CCD camera in participating medium.The temperature field reconstruction in large participating medium has the char-acteristics as following, the medium is absorbing, emitting and scattering, and the ra-diative heat source is very large but the receiver is very small. Therefore, it is required to establish the reconstruction model for participating medium with high computing efficiency and this is the key point of this study. It is the typical inverse radiation problem that the exiting radiative information from the system boundary is used to retrieve the temperature distribution information in the system. Inverse radiation problem is normally the ill-posed problem. How to solve the large ill-posed recon-struction matrix equation efficiently, fast and reasonably is very important and this is another key point of this study.First, the ill-posed problem is defined and the effective methods for analyzing ill-posed problem (SVD method and Picard figure) are studied. Some kinds of solving methods (Tikhonov regularization method, TSVD method and LSQR method) are summarized and the L-curve method is used to give reasonable regularization pa-rameter. Based on the above, LSQR method is used in this study for solving the ill-posed temperature reconstruction equation and the simulation case is given.The Monte Carlo method is given in detail and the developed program is vali-dated by the case in the reference. The results are found to agree well with those in the reference. The combustion flame optical imaging is calculated using Monte Carlo method and lens optical imaging principle.Served as the fundamental study on the temperature field reconstruction,3-D temperature field reconstruction model based on the forward Monte Carlo method in the visible range is presented. The reconstruction equation and solving procedure are given in detail. The effects on the reconstruction accuracy are discussed.The backward Monte Carlo method is discussed.3-D temperature field recon-struction model based on the backward Monte Carlo method is deduced and presented innovatively. This model can be used for the absorbing, emitting and scattering me-dium, and the computing time is short and suitable for radiative heat transfer between the large furnace and small receiver. The model has the ability of the real-time coeffi-cient matrix calculation and the on-line matrix equation solving. The results using the forward and backward Monte Carlo methods for computing radiative heat transfer and reconstructing the temperature field are compared. It is found the backward Monte Carlo method has much higher efficiency but the reconstruction errors are almost the same. The simultaneous reconstruction model of temperature field and radiative pa-rameters based on the backward Monte Carlo method in the homogenous medium, and the simulation study of the section temperature field reconstruction in the rotary kiln for dangerous waste incineration are also discussed.Simultaneous measurement model of 3-D soot temperature and volume fraction fields in flame with CCD cameras is presented. This model considers the practical 3-D volume radiative energy emission and flame imaging. The consideration of this model is 3-D reconstruction zone, not the flame, so the reconstruction is not related with the flame form directly, which is suitable to the axisymmetric and asymmetric flames. The simulation is used to validate the model and the experiments are also car-ried out. The reconstructed temperature and volume fraction distributions are reason-able and agree with some reports in the reference. Also, the thermocouple is used to measure the flame temperature, and the reconstructed temperatures agree well with the measured ones.Temperature field of flame is reconstructed in small scale pulverized coal-fired test rig using temperature distribution fast reconstruction model based on the backward Monte Carlo method. Recontructed temperature field is in good agreement with the results from the literature. The reconstructed results are clear and the recirculating zone can be seen clearly and has some deflection. The two-dimensional flame image is converted into the pseudo temperature image, from which the recirculating zone can be seen more clearly. This validates the reconstruction results.The experimental studies on the section temperature field and 3-D temperature field reconstruction in the large pulverized coal-fired power plant furnace are performed. The Fluent simulation is used to obtain the medium radiative properties distribution according to the furnace operation parameters, which is more accurate than some ref-erences using assumed radiative parameters. For the section temperature field recon-struction, there is a obvious high temperature zone, rond which the temperatures are lower. The values of temperature are in a reasonable range. The reoncstructed results are compared with those in literature and it is found that the temperature range agrees well, but the reconstructed temperature in this paper has more details and presents more particulars. Moreover, it can be seen that the high temperature zone has deflec-tion. For the 3-D temperature field reconstruction, the temperatures in the middle part of the furnace are high and the surrounding part of the furnace is relatively low. The reconstructed temperature range is also reasonable. These characteristics of the recon-structed temperature field agree with the results in the references. The computing time is also reported. The whole computing time (include the reconstruction equation es-tablishing time, coefficient matrix reading time, and solving time of equation) is about one minute. In the on-line reconstruction, for the stable operation condition, the same coefficient matrix can be used and it is only needed to solve the equation. At this time, the reconstruction time is only 2-3s, which shows better ability for the on-line recon-struction.Finally, the terahertz time-domain spectroscopy technique was used to study the optical properties of the soot within 0.2-1.6THz and the frequency-domain spectra was obtained through the Fourier transform. The complex refractive index of the soot was deduced by the fixed-point iteration and comparisons are made. The deduced re-sults can provide the optical data of soot for the application of terahertz time-domain spectroscopy technique to the optical combustion diagnostics, and extend the optical combustion diagnostics application area.