Local Flow and Heat Transfer Behaviours in V-L-S Fluidized Bed Evaporator

Abstract

The fluidized bed evaporator is a subject of considerable interest and can be employed in the process industries. However, the V-L-S flow behavior and heat transfer characteristics are still not well understood. In this work, a vertical fluidized bed evaporator with external natural-circulation flow boiling was established to investigate the local V-L-S flow behaviour and heat transfer characteristics and to reveal the influence of flow on heat transfer performance. It consists of a heated tube with inner diameter of 3.8 × 10 −2 m and height of 1.4 m and a circulating tube with the same inner diameter and height of 0.92 m. Both tubes are made of quartz glass plated with transparent electrically heating film. The solid particles added to the evaporator are glass, ceramic, Teflon™ and poly-formaldehyde beads or cylinders with the diameter ranging from 1.8 × 10 −3 to 4 × 10 −3 m. The particle volume changes from 0 to 1 × 10 −3 m 3 and heat flux varies from 5 × 10 3 to 1.2 × 10 4 W m −2. The local velocity and holdup of solid particles, flow region transition, length of the V-L-S flow boiling, fluid circulating velocity and pressure drop in the V-L-S fluidized bed evaporator were visually investigated with CCD measuring technique. The main results on flow are as following. Axial solid holdup in heated tube of the evaporator decreases obviously for middle-density particle systems against the direction of the gravity. Three flow regions can be found in heated tube of the evaporator: L-S region, transition region and V-L-S region, and in transition region, the radial profile of solid holdups is relatively uniform. The increase of particle volume enlarges the length of V-L-S region and pressure drop, while decreases the circulating velocity of fluid mixture. The average heat transfer film coefficients of V-L-S flow boiling were estimated and a dimensionless correlation was obtained based on 120 sets of experimental data with a maximum relative deviation of 10.8%. The axial variation of the heat transfer coefficients has close relation to the axial distribution of the solid holdups.