Experimental determination of heat transfer coefficient to horizontal tube submerged in packed-fluidized bed

Abstract

The aim of the thesis was to experimentally determine the heat transfer coefficient to the surface of a horizontal tube submerged in a bubbling fluidized bed containing packing material. This was done for bed temperatures ranging from 400ºC to 900ºC and superficial gas velocities from 0.04 m/s to 0.411 m/s. Six different packing materials were evaluated; two sizes of aluminium silicate balls, two sizes of Raschig rings, an RVT metal saddle ring (RMSR) and an RVT Hiflow® ring. Additionally, the pressure drop of the packed fluidized beds was measured. Using the pressure data, the vertical segregation of the packed fluidized beds was also estimated. The experimental work was carried out in a lab-scale reactor. The determination of the bed-to-tube heat transfer coefficient was done by measuring the increase in temperature of water flowing through a horizontal tube submerged in the packed bed. An electrical furnace provided the heat to the reactor. Air was used as a fluidizing gas and the bed particles consisted of Baskarp B-28 silica sand, sieved to a size of 212-300 μm. The bed-to-tube heat transfer coefficient increased with increasing bed temperature for all evaluated packing materials. In general, the packed-fluidized bed cases showed lower heat transfer coefficients than a reference case of a fluidized bed containing no packing material. The RMSR saddle ring performed the best, with a maximum heat transfer coefficient of 1298 W/(m2K), only slightly lower than the reference case. The Hiflow ring, despite its similarities to the RSMR ring, performed approximately 25% worse at temperatures of 800ºC or higher, at fully fluidized conditions. For all packings, the heat transfer coefficient increased with increasing gas velocity up to a certain maximum value, after which it decreased slowly with increasing gas velocity. Also, for all packings, the heat transfer coefficient was lower than the heat transfer coefficient of the fluidized bed when it contained no packing material. The exception being the RMSR packing at high velocities. Important packing material parameters for limited negative effect on the heat transfer ability and pressure drop of the bed were high packing void ratio, open packing structure and the packing not physically covering heat exchange surfaces to a great degree. The best performing packing, which had the least impact on bed heat transfer and pressure drop, was the RMSR 25-3.