Numerical simulations and experiments on heat transfer around a probe in the downer reactor of a triple-bed combined circulating fluidized bed

Abstract

Due to very short residence time in the downer, heat transfer performance inside it has great significance for coal gasification. In this study heat transfer around a heating probe in the downer of a circulating fluidized bed was studied through both experimental and numerical work. Both experimental results and numerical results revealed that average heat transfer coefficients decreased with the increasing superficial air velocity or decreasing solids mass flux, which could be attributed to the decreasing solids holdup, as particle convection was dominant in the heat transfer mechanism under current cases. Numerical simulation also revealed that heat transfer may deteriorate with the increasing particle size. Finally it was found that the heat transfer performance with constant temperature boundary condition around the probe was much better than that with constant heat flux boundary condition. These results may help us better understand the heat transfer in the downer and design downer with high efficiency.