Experimental study of turbulent gas–solid heat transfer at different particles temperature

Abstract

Thermal interaction between a turbulent vertical gas flow and particles injected at two different temperatures is investigated experimentally and numerically. Two-phase k– τ and k θ – τ θ numerical model in four-way simulation is used in a Lagrangian–Eulerian framework. In agreement with the numerical results, the experiments show that addition of hot particles to suspension can cause increasing/or decreasing heat transfer coefficient. This effect depends on the particle size and gas Reynolds number. The particle diameter is 600–1200 μm in bed of 11, 15 and 21 m/s gas velocities. Hot particles are injected at gas temperature (80 °C) while cold particles initial temperature is −20 °C. Nusselt number is shown to be increased by adding the hot particles to the bed with large particles and low gas velocities, while it can be decreased when the particles are small or the gas velocity is high. Increasing or decreasing heat transfer coefficient by adding hot particles can be mainly due to (1) increasing the suspension heat capacity, (2) reducing the particle Reynolds number and (3) changes in thermal turbulence intensity. The results show that the trend of the variation of Nusselt number can be explained by the variation of the thermal turbulence intensity. More significance thermal turbulence production due the particles causes increasing heat transfer coefficient. While when the productive role of particles is weak, the heat transfer coefficient would be lower by hot particle addition.