Clarification of an improved cluster renewal model (ICRM) for heat transfer characteristics in a rolling circulating fluidized bed (RCFB)

Abstract

This paper presents a novel method to predict the gas layer thickness δ′ in an improved cluster renewal model (ICRM) to calculate the heat transfer coefficient h′c for a rolling circulating fluidized bed (RCFB). Eulerian-Eulerian two-fluid model (TFM) with kinetic theory of granular flow is used to perform a numerical simulation. After comparing the pressure gradient −△p/△z′ and the simulated heat transfer coefficient between multiphase and the wall hgs with previously published experiment data, the correctness and reliability of the simulation are able to be verified. In conclusion, firstly, the variation of h′c calculated from δ′ in the ICRM is almost the same with hgs in the case that the RCFB undergoes the rolling motion. In order to quantitatively evaluate the proposed novel method, an error percentage α between h′c calculated from δ′ and hgs is 2.043% which is less than 5%. This certified that the novel method to predict δ′ in the ICRM has higher accuracy to calculate h′c. Secondly, h′c calculated from δ′ is mainly influenced by rolling amplitude Θ rather than by rolling period T. Specifically, with the increase of Θ, amplitudes of Ts are decreased, which is caused by the increased heat transfer and the decreased δ′ between cluster and wall. The decreased amplitudes of Ts are able to increase heat transfer efficiency, which eventually increases h′c. Thirdly, in the case that normalized rolling period t/T is changed from 0 to 0.5, higher h′c calculated from δ′ in Region Ⅰ results from the large local cross-sectional particle volume fraction clocal in Region Ⅰ with a smaller δ′, while lower h′c calculated from δ′ in Region III results from the smaller clocal in Region III with a larger δ′. Therefore, it is concluded that h′c calculated from δ′ is able to well predict the bed-to-wall heat transfer coefficient of the RCFB even if the rolling motion is considered.