EXPERIMENTAL INVESTIGATION OF AXIAL HEAT TRANSFER AND ENTRANCE EFFECT IN RANDOMLY PACKED BEDS BY A NAPHTHALENE SUBLIMATION TECHNIQUE

摘要

Local average heat transfer characteristics along the axial direction in randomly packed beds are investigated experimentally by using a naphthalene sublimation technique and the heat-to-mass transfer analogy. Packed beds of three particle diameters of 12 mm, 10 mm, and 8 mm are investigated, respectively. The relationship between the Nusselt number along the axial direction and the axial porosity distribution is analyzed, and the influence of the entrance effect on the overall heat transfer is explored. In the present paper, the axial porosity distribution is obtained by numerical analysis of a randomly packed bed generated by a discrete element method (DEM). The results of pressure drop show that most experimental points lie within +/- 20% deviations of the Ergun equation and that the pressure distribution along the axial direction decreases almost linearly. Furthermore, it is found that in the axial direction, the Nusselt number increases in the first three layers and stays almost constant after that, where the heat transfer is fully developed. The development of Nusselt number along the axial direction can be attributed to the change of axial porosity distribution and to the alterative turbulence. Besides, the Nusselt number in the first layer is about 60-80% of the average Nusselt number in the bulk of the packed bed in the present Reynolds number range (373-2867). Finally, the entrance effect can have an influence on the overall Nusselt number when the packed bed is shorter than 8-16 particle diameters.