Effect of Flow Rate and Particle Size on Heat Transfer to Dense Granular Flows

The increasing demand for renewable energy sources necessitates the development of more efficient technologies. Concentrated solar power (CSP) towers exhibit promising qualities, as temperatures greater than 1000°C are possible. The heat transfer fluid implemented to capture the sun’s energy significantly impacts the overall performance of a CSP system. Current fluids, such as molten nitrate salts and steam, have limitations; molten salts are limited by their small operational temperature range while steam requires high pressures and is unable to act as an effective storage medium. As a result, a new heat transfer fluid composed of ceramic particles is being investigated, as ceramic particles demonstrate no practical limit on operation temperature and have the ability to act as a storage medium. This study sought to further investigate the use of dense granular flows as a new heat transfer fluid. Previous work validated the use of such flows as a heat transfer fluid; the present work examined the effect of flow rate, as well as the particle size and type on the heat transfer to the particle fluid. Three different types of particles were tested, along with two different diameter particles. Of the three materials tested, the particle type did not appear to effect the heat transfer. Particle diameter, however, did effect the heat transfer, as a smaller diameter particle yielded slightly higher heat transfer to the fluid. Flow rates ranging from 30 to 200 kg/m²-s were tested. Initially, the heat transfer to the flow, characterized by the convective heat transfer coefficient, decreased with increasing flow rate. However, at approximately 80 kg/m²-s, the heat transfer coefficient began to increase with increasing flow rate. These results indicate that a dense granular flow consisting of small diameter particles and traveling at very slow or fast flow rates yields the best wall to “fluid” heat transfer.