Heat transfer to horizontal tubes immersed in a fluidized-bed combustor

Abstract

Experiments are carried out to measure the heat transfer rates to water-cooled horizontal tubes (D T = 25.4 mm) immersed in an atmospheric fluidized-bed combustor burning North Dakota lignite. Silica sand, with average diameter ranging from 0.544 mm to 2.335 mm, is used as bed material. The tests are conducted at average bed temperatures ranging from 587 to 1205 K. For bed temperatures less than 950 K, the bed is heated by a propane gas heater. The superficial velocity is varied from 0.73 to 2.58 m/s. Thus, the effect of bed temperature (T B = 587 – 1205 K), particle size (d̄ p = 0.544 – 2.335 mm), and fluidizing velocity (U = 0.73 – 2.58 m/s) on the heat transfer rate to horizontal tube immersed in a fluidized-bed combustor (0.45 m × 0.45 m) is investigated. Among the existing correlations, those correlations proposed by Glicksman and Decker, Zabrodsky et al., Catipovic et al., Grewal, and Bansal et al. are found to predict the present data quite well, when the contribution due to radiation is included. The radiative heat transfer is estimated as the difference between the heat transfer to oxidized boiler tube and gold-plated tube. The relative contribution of radiation is found to be 13% for a bed of sand particles (d̄ p = 0.9 mm) operating at 1087 K.