MEASUREMENT OF SURFACE HEAT-TRANSFER DUE TO PARTICLE IMPACT

Heat transfer coefficients for a surface continuously impacted by a st ream of falling particles in air and in helium were measured as functi ons of particle flux and particle velocity. The purpose was to provide well-controlled data to clarify the mechanisms of heat transfer in pa rticle suspension flow's. The particles were spherical glass beads wit h mean diameters of 0.5, 1.13, and 2.6 mm. The distribution of the par ticle 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 s imple, well-established model. The experimental results showed that in air; the heat transfer coefficient increases approximately lineal ear ly with particle impact flux. At high impact fluxes, the heat transfer coefficient decreases with particle impact velocity, and at low impac t fluxes, it increases with particle impact velocity. Furthermore, the heat transfer coefficient decreases drastically with the particle siz e. In helium gas, it was found that at low particle impact fluxes, the difference between the coefficients in helium and in air is small, wh ereas at high fluxes, the difference becomes large. A length scale, V/ nd(p)(2), was used to correlate the data. At low particle Reynolds num bers, gas-mediated hear conduction was identified as the dominant part icle/surface heat transfer mechanism, whereas at high particle Reynold s numbers, induced gas convection was the dominant mechanism.