Hydrodynamics and Heat Transfer during Fluidization of Fine Particles

Recently, new approaches were taken towards the use of dense upward-flowing suspensions as heat transfer fluids and in thermal energy storage systems. Gas/solid two-phase flows are employed as the heat-transfer fluid with the objective of maximizing the energy capture, and enhance the power conversion efficiency using fluidized-bed technology for heat exchange. In most dense gas-solid fluidization systems, particle circulation (e.g., that induced by the bubbles) is the primary cause of particle convective heat transfer. This work concerns the measurement and analysis of the heat transfer coefficient in dense suspensions of fine particles and the dependence on the hydrodynamics of the system. Results show that the heat transfer coefficient first increases as the gas flowrate increases (because the rate of particle renewal at the wall increases to a maximum), after which it decreases (because the density of particles at the wall decreases and this effect becomes dominant). Further experiments establish also the influence of temperature on the flowability and mechanical properties of the suspensions. This was achieved with a designed heat transfer probe that allows the measurement of the heat-transfer coefficient and using a quartz fluidized bed oven-enclosed. A detailed review of heat-transfer mechanisms will also point out the heat transfer mechanisms in this distinctive kind of suspensions.