Experimental investigation and prediction of heat transfer in a swirling fluidized-bed combustor

Highlights: • Heat transfer coefficients were investigated in radial and axial locations. • Difference of heat transfer coefficients between bed and freeboard zones was 5–12%. • Swirl number and primary air apparently influenced on heat transfer. • Average Nusselt number can be predicted by empirical models within ±20% error. -- Abstract: In this experimental study, the heat transfer coefficient in a twin-cyclonic swirling fluidized-bed combustor (T-FBC) with a conical shape bed was investigated to verify the effects of combustor design feature and operating parameters. The combustor was operated at a ratio of secondary and tertiary air to primary air ranging from 0 to 0.5 at primary air velocity (u_p) near minimum swirling fluidization condition (u_ms), to about 3u_ms by using silica sand with particle size diameters (d_p) of 300–500, 600–710, and 710–1000 µm as bed material, for swirl number of an annular spiral air distributor of 2.76, and 2.98. The local heat transfer coefficients were measured in radial positions at the levels of 60, 125, 223, and 288 cm above the primary air distributor and the axial profiles of average heat transfer coefficient were characterized. The experimental results revealed that the radial profile of heat transfer coefficient was maximum at the center of the combustor for most of the test trials. Within the range of the tests, the heat transfer coefficient noticeably increased with the increase of the primary air flow rate and swirl number of the air distributor, and reached the almost constant value at 2.5–3u_ms. With design features of this combustor, the heat transfer coefficient in the bed region was comparable to the value in freeboard region though about 5–12% higher. Based on the test results, semi-empirical models for prediction of average Nusselt numbers were developed in the dense bed and freeboard zones for the different swirl numbers of 2.76 and 2.98, and the proposed models were in close agreement with the experimental data within ±20% error for both regions.