Some Studies on Heat Transfer in Circulating Fluidized Beds

Abstract

Experimental investigations were made to study the effect of fin and vertical probe height on hydrodynamics and heat transfer in circulating fluidized beds (CFB). The experiments were conducted in a 100 mm I.D., 5.15 m tall CFB unit. Air was supplied by a high pressure centrifugal blower. The distributor plate used was straight hole onfice type having 12.4% open area. The test section was located at 2.75 in above the distributor and electric tape heater was used as the source of heat. The temperatures of the inside wall and the bed at about the midpoint in the test section were measured with copper constantan thermocouples. Five plain and five finned test sections have been examined, three of which had rectangular fins and two had pin fins. Local sand of mean diameter 310 μm was used as the bed material. Measurements covered a range of superficial air velocity from 5.6 to 12.5 m/s, suspension density from 18 to 76 kg/m3 and bed temperature from 330 to 365 K. Three bed inventories of 20, 26 and 32 kg and five heat fluxes in the range of 3580 to 7876 W/m2 were used. One empirical model was developed with the help of dimensional analysis to predict heat transfer in a hot CFB to bare tube surfaces. One analytical model has been developed for the prediction of heat transfer to finned surfaces in a CFB both for long and short fins. An empirical equation has been developed correlating the parameters Nup, Rep and Lh/D to estimate heat transfer from the probes of different vertical heights. In addition one expression for calculating particle residence time has also been derived. The results predicted from the models and correlations have been compared with the present experimental results as well as those of other investigators and good agreement is observed. With the use of fins, the heat transfer coefficient was found to decrease by a maximum of 32% but the total heat transfer got enhanced by about 103% due to the additional surface area provided by the fins. Heat transfer coefficient was found to decrease and particle residence time was found to increase with the increase of vertical height of the probe.