Nongray gas and nongray particle effects in modeling fluidized bed combustors

Ateş, Cihan
Thermal radiation accounts for the majority of the heat transferred in combusting systems where it is constituted of the contributions from both participating gases and solid particles; hence gas and particle radiative properties play an important role. In spite of the lower operating temperatures (750-950 ºC), radiation is still predominant mode of heat transfer in fluidized bed combustors (FBCs) due to the presence of higher particle loads in the flue gas. Therefore, modelling of radiative heat transfer in such systems is of considerable importance and necessitates not only accurate but also computationally efficient methods for radiative property estimation of particle-laden combustion gases. Although extensive research has been carried out for non-gray behaviour of the flue gas, non-gray behaviour of particles has been investigated in limited number of studies, where only few involved both non-gray gas and non-gray particle properties. Therefore, ultimate objective of this study is to develop CPU efficient, accurate and compatible non-gray gas and non-gray particle radiative property models such that the outcome will provide guidelines when choosing radiation models for combustion related investigations. For that purpose, several multi-dimensional radiation codes have been developed, which can simultaneously take into consideration non-gray gas and non-gray particle property estimation techniques in conjunction with radiative transfer equation (RTE) for the determination of radiative heat flux and source term distributions. Predictive accuracy and computational efficiency of different radiative property estimation techniques for both participating combustion gases and particles are tested in one comprehensive study by benchmarking their predictions against reference solutions and measurements for the conditions relevant to combusting systems, particularly FBCs. Comparisons reveal that non-gray particle radiation is of significant importance for accurate calculation of radiative heat transfer in combusting systems and it is originated from the spectral nature of the complex index of refraction. Furthermore, it is shown that chemical composition has a significant effect on non-gray particle absorption properties as well as heat flux/source term predictions. It is also demonstrated that dominance of particle radiation in total radiative heat exchange is strongly dependent on particle chemical composition. Accordingly, particle radiation does not necessarily dominate total radiation even at high particle loads. Therefore, negligence of the non-gray behavior of either combustion gases or particles can lead to significant errors in both heat flux and source term predictions even at high particle loads.
Citation Formats
C. Ateş, “Nongray gas and nongray particle effects in modeling fluidized bed combustors,” Ph.D. - Doctoral Program, Middle East Technical University, 2018.