| In this study, we examine the convective heat transfer scaling of moderately pressurized Circulating Fluidized Beds (CFB) using a single cold laboratory facility. By recycling fluidization gas mixtures of adjustable density, the experiments simulate the hydrodynamics of a combustor burning coal under a pressure of 0.64 MPa. While matching the hydrodynamics in the upper riser using Glicksman's (1993) reduced set of dimensionless numbers, we vary thermal properties of the flow and measure the effect on the convective heat transfer coefficient at the wall.; In CFB flows, the suspension partly condenses into denser clusters separated from the wall by a thin gas film of order the mean particle diameter (Glicksman, 1997). The clusters generally dominate the convective heat exchange with the wall because of their relatively high solid volume fractions and heat capacity. Accordingly, Lints and Glicksman (1993) suggested that the rate of heat transfer to the wall scales with the mean time spent by individual clusters there.; For this reason, we focus on the hydrodynamics of the cluster motion at the wall. We present a scaling of cluster velocity based on a comparison of measurements from other investigators. A novel thermal marking technique records the residence length of clusters at the wall and leads to a suggested scaling of cluster residence length.; A unique, non-intrusive probe positioned at the wall of our facility measures simultaneously the solid concentration and instantaneous heat transfer coefficient. The heat transfer sensor has a 25 ms response that is at least twice as rapid as those described by previous investigators. The fraction of the wall covered by clusters and the cluster solid volume fraction are also extracted from solid concentration measurements. Guard heaters reduce conduction losses from the probe and assist thermal development of the flow, allowing a measurement of convective heat transfer coefficient that is more representative of large surface-area combustors. Finally, we present a scaling of the Nusselt Number as a function of relevant thermal and hydrodynamic parameters of the flow. |
