Evaluation of weighted-sum-of-gray-gases models and radiation characteristics analysis for gas-ash particle mixture in ash deposition

Abstract

One of the serious challenges in the efficient utilization of heat exchangers is the ash deposition on heat transfer surfaces, which leads to a drastic decrease in heat transfer efficiency. This work evaluates a comprehensive dynamic deposition model for flue gas-ash particle mixtures developed based on different forms of weighted-sum-of-gray-gases radiation models and elucidates the radiation characteristics and deposition performance of the mixture. The trajectories of fly ash particles are tracked using the discrete phase model. The coupling of sub-models in the deposition model is achieved through user defined functions, and the radiative transfer equation is solved using the discrete ordinate model and integrated into the CFD framework. Additionally, the effects of wall emissivity model, absorbing gas content, and ash particle volume fraction on deposition, non-gray radiation characteristics, and heat transfer properties are considered. Results show that the combination of the gas mixture radiation model and particle gray model performs well in deposition calculations. Compared to the dynamic emissivity model, the constant model provides higher heat flux and adhesion efficiency on the tube wall, leading to an overestimation of total deposition mass. The deposition mass and absorption coefficient of the flue gas-particle mixture increase with the absorbing gas content and the particle volume fraction. Besides, due to the dominance of particle radiation in overall radiation, an increase in the volume fraction of ash particles leads to the transformation of the absorbed radiation distribution from “sun-shaped” to a smoother distribution. The analyses indicate that the non-gray behavior of gas-particle mixtures under different deposition conditions needs to be considered, and the results can provide more reliable guidance for the design and optimization of waste heat recovery devices.