Mechanism of the influence of solids holdup on the heat transfer rate of high-density particle flows in cyclone pyrolyzer

Introducing centrifugal force into a free-fall downer (FFD) creates a cyclone pyrolyzer (CP), which can achieve a synergistic effect of high gas velocity and localized high solids holdup (>0.1) to enhance heat transfer in high-density particle flows. However, the mechanism of inter-particle heat transfer governed by solids holdup under such high-density conditions remains unclear. To isolate the influence of solids holdup from other operational parameters, this study employs the Computational Particle Fluid Dynamics (CPFD) method to investigate the impact of initial solids holdup on the inter-particle heat transfer rate by altering particle distribution at the inlet. This approach contrasts with the conventional method of increasing solids circulation flux. Simulation results show that convective heat transfer is the dominant mechanism within the CP, and an optimized particle spatial distribution at the inlet can improve the heat transfer-reaction performance within the CP. Furthermore, mechanistic analysis reveals that as the solids holdup increases, radiative heat transfer weakens, interparticle heat conduction strengthens, while convective heat transfer is subject to complex influences from the solids holdup. Solids holdup affects convective heat transfer efficiency by adjusting the mixing mode of hot and cold particle groups, the gas-solid velocity difference, and the relative distribution of cold particles and hot airflow. The key to enhancing convective heat transfer efficiency lies in controlling the gas-solid distribution state (i.e., the combined effect of the above three factors), rather than simply pursuing high solids holdup.