Gas-solid mixing and heat transfer performance in alternating spout deflection

Abstract

Spout deflection has long been regarded as a threat to efficient gas-solid interactions in spout fluidised beds, as it may break the stable symmetrical particle circulation in the spout-fluidisation flow pattern. In this paper, the so-called alternating spout deflection (ASD) is evaluated and compared with the spout-fluidisation (SF) flow pattern in terms of gas-solid hydrodynamics, mixing and heat transfer performance at particle scale by means of a Computer Fluid Dynamic-Discrete Element Method (CFD-DEM) model. The comparisons show in the SF over ~40% particles are in a densely packed state, whereas in the ASD the proportion of particles in a packing state is less than 20%; Then their mixing performance are compared using the improved Lacy index, indicating that the mixing speed and final state of the mixing process are similar. Further, the two patterns are compared in terms of heat transfer. It is indicated that in the ASD flow pattern, the mean particle temperature drops quicker; moreover, the homogeneity of the heat transfer is also better in terms of mean square error of particle temperature, compared to the SF. The underlying mechanism is also explored: in the ASD the hot particle cores can be mitigated due to the spout incoherence phenomena, which is however unavoidable in the SF. This paper corrects the negative view that the alternating spout deflection is not as effective as the spout-fluidisation pattern in terms of gas-solid mixing and heat transfer performance.