Effects of macroscopic hydrodynamics on heat transfer in a three-phase fluidized bed

The macroscopic hydrodynamics and heat transfer of a two-dimensional (2D) three-phase fluidized bed have been quantitatively studied using particle image analyzer (PIA) and heat transfer probe, respectively. Three flow regimes transiting with increasing gas velocity are investigated, including dispersed and coalesced bubbling regimes characterized by two flow conditions (4- and 3-region flow). These flow regimes are comprised of different macroscopic structures, which are profoundly dominate the local heat transfer distribution. Generally, the bubble stream can enhance the heat transfer due to inducing the turbulent intensity, while the vortex acts like a barrier to inhibit the heat transfer. Different solid holdups are introduced into the flow to examine the effect on the heat transfer. It is found that the solid particles can initially increase the collision frequency between the solid particles and heating object, which leads to the increase of heat transfer. However, increasing the solid holdup over 8% becomes a negative effect on the heat transfer. The average heat transfer coefficient increases with the liquid and gas velocities in the beginning, and then levels off with further increasing the liquid and gas velocities.