Experimental study on the flow behavior and heat transfer of ceramic balls in a vertical descendant tube

The velocity field distribution of ceramic balls flow was measured with Particle Image Velocimetry (PIV) and the convective heat exchange experiments between ceramic balls of 90℃ and the air of room temperature were conducted in a verticle descendant tube (Diameter 60 mm, lengh 1.6 m) for discovering the ceramic balls flow behavior and heat transfer principles of the biomass pyrolysis descendant tube reactor. The residence time of the ceramic balls in the vertical descendant tube and the heat exchange coefficient were analyzed. The results of the PIV experiments show that the axial velocity of ceramic balls increases along the tube and at any position of the tube, the axial velocity near the wall of 15% radius goes up gradually until reaching the maximum velocity. The radial velocity was zero m/s except near the wall of the tube. The heat transfer experiments indicated that the heat exchange coefficients were identical at each ceramic balls mass flow. The convective heat exchange coefficients worked out with Ranz－Marshall function were less than those by theroetical methods. According to the data obtained with theroetical methods and expriments, the relationship bewteen the Nusselt number and Reyonlds number was regressed: . The coefficients obtained with the regression function increased with the increase of the air temperture and the relationship between the coefficients and air temperature was linear.