In this paper, heat transfer experiments were conducted in order to evaluate the contributions of particle convection, gas convection, cluster convection and also radiation from cluster and dispersed phases to the overall heat transfer coefficient in a furnace chamber of a large-scale supercritical CFB reactor. Heat transfer behavior in a circulating fluidized bed between the water membrane wall and bed inventory has been analyzed for Geldart B particles. Bed inventory used in the tests had particle density in the range of 2650-2750 kg/m(3). The experimental CFB facility has the dimension of 27.6 x 5.3 m in bed rectangular cross-section and 48 m in height of the furnace. The performance tests have been carried out at three different sizes of the bed material with Sauter mean particle diameters of 0.246, 0.272 and 0.444 mm. Above heat transfer studies have been conducted for the operating parameters covering the range of 1082-1183 K for bed temperature, 2.92-5.25 m/s for superficial velocity, 6.86-8.25 kPa for pressure drop, 23-25.6 kg/(m s(2)) for the circulation rate of solids and 0.4-0.44 for the bed voidage at minimum fluidization velocity. In order to predict the overall heat transfer coefficient, a cluster renewal approach was used. The bed-to-wall heat transfer coefficients obtained in these operating ranges were about 78-226 W/(m(2) K). The contributions of convective and radiative heat transfer to the overall heat transfer coefficient were estimated. The variation in contributions depended on solid suspension density and bed temperature. The average contribution of convective and radiative heat transfer components varied between 13.5-54% and 46-86.5%, respectively. (C) 2015 Elsevier Ltd. All rights reserved.