Heat Transfer to Vertical Dense Granular Flows at High Operating Temperatures

Ceramic particles as a heat transfer fluid for concentrated solar power towers offers a variety of advantages over traditional heat transfer fluids. Ceramic particles permit the use of very high operating temperatures, being limited only by the working temperatures of the receiver components, as well as demonstrate the potential to be used for thermal energy storage. A variety of system configurations utilizing ceramic particles are currently being studied, including upward circulating beds of particles, falling particle curtains, and flows of particles over an array of absorber tubes. The present work investigates the use of gravity-driven dense granular flows through cylindrical tubes, which demonstrate solid packing fractions of approximately 60%. Previous work demonstrated encouraging results for the use of dense flows for heat transfer applications and examined the effect of various parameters on the overall heat transfer for low temperatures. The present work examined the heat transfer to dense flows at high operating temperatures more characteristic of concentrated solar power tower applications. For a given flow rate, the heat transfer coefficient was examined as a function of the mean flow temperature by steadily increasing the input heat flux over a series of trials. The heat transfer coefficient increased almost linearly with temperature below approximately 600°C. Above 600°C, the heat transfer coefficient increased at a faster rate, suggesting an increased radiation heat transfer contribution.