Heat transfer between a packed bed and a larger immersed spherical particle

In this paper, heat transfer coefficients between the hot packed beds of particles and the larger cold immersed sphere were experimentally determined. The packed beds consisted of mono-sized spherical glass particles of dp = 1.2, 1.94 and 2.98 mm. The aluminum test spheres of Dp = 6, 12 and 20 mm with K-type (Ni/Al) thermocouples inserted in them were immersed into the bed. The temperatures of the test spheres were recorded until the thermal equilibrium was reached. From these recordings, heat transfer coefficients were determined. The experiments were performed in the range of gas superficial velocity of ∼0.3–0.8 m/s and the bed temperature from 90 to 320 °C. It was found that the measured heat transfer coefficients increased with the increase in gas superficial velocity, while only a slight increase with temperature was observed for test spheres Dp = 6 and 12 mm in the investigated temperature interval. The heat transfer coefficients were generally larger for smaller test spheres, while they did not show significant dependence on the size of the bed particles.The literature correlations for heat transfer coefficients in packed beds in form of Nusselt number and heat transfer factor jH were compared to experimental data from this work. The correlations proposed by Collier et al. (2004) [18] and Handley and Heggs (1968) [2] fit our experimental data best. The mean error between the experimentally determined heat transfer coefficients and the ones calculated from Collier et al. (2004) [18] correlation was 12.7%.