Measurement of Surface Heat Transfer Due to Particle Impact

Heat transfer coefficients for a surface continuously impacted by a stream of falling particles in air and in helium were measured as functions of particle flux and particle velocity. The purpose was to provide well-controlled data to clarify the mechanisms of heat transfer in particle suspension flows. The particles were spherical glass beads with mean diameters of 0.5, 1.13, and 2.6 mm. The distribution of the particle impact flux on the surface was determined by deconvolution from the measurement of the total solid masses collected at both sides of a movable splitter plate. The particle velocity was calculated from a simple, well-established model. The experimental results showed that in air, the heat transfer coefficient increases approximately linearly with particle impact flux. At high impact fluxes, the heat transfer coefficient decreases with particle impact velocity, and at low impact fluxes, it increases with particle impact velocity. Furthermore, the heat transfer coefficient decreases drastically with the particle size. In helium gas, it was found that at low particle impact fluxes, the difference between the coefficients in helium and in air is small, whereas at high fluxes, the difference becomes large. A length scale, V/n˙d_p², was used to correlate the data. At low particle Reynolds numbers, gas-mediated heat conduction was identified as the dominant particle/surface heat transfer mechanism, whereas at high particle Reynolds numbers, induced gas convection was the dominant mechanism.