An Experimental and Numerical Study of Flow and Convective Heat Transfer in a Freely Falling Curtain of Particles

The flow characteristics and convective heat transfer in a freely falling curtain of spherical particles with an average diameter of 650 μm has been studied experimentally and numerically. Both heated and unheated particle flows have been considered. This work is part of a larger study to determine the feasibility of using particles to directly absorb the insolation in a solar central receiver for high temperature applications. The particles of interest are Norton Master Beads™ which are primarily aluminum oxide. Measurements have been made of particle velocity in heated and unheated particle flows, and particle temperature and air temperature in heated particle flows. Comparison of the measurements with calculations has been made for two particle mass flow rates at room temperature and at two initial elevated particle temperatures. Excellent agreement between numerical and experimental results is obtained for particle velocity in the unheated flow. For the heated particles, both data and predictions show the same trends with regard to particle velocity, particle temperature, and air temperature. However, the calculations of these quantities overpredict the data. The results suggest that the drag coefficient in flows where the particles are hot compared to the air is larger than predicted using conventional methods to account for nonisothermal effects. The prediction of particle temperature and air temperature attained with a drag coefficient that is larger than the standard drag coefficient agrees well with the data.