Heat Transfer Coefficient Between Flat Surface and Sand

Thermal energy storage (TES) systems are of interest in solar thermal power applications as an effective means of retaining energy. One of the primary issues with this type system is the exchange of thermal energy coming off the power field. In a heat exchanger, the effective heat transfer coefficient between the exchange mediums plays a crucial factor in determining the sizing of the heat exchange unit. A concept utilizing sand as a cheap particulate thermal medium was recently proposed for an alternative thermal energy storage system. The overall system will be described in some detail; however, the primary focus of this research report will be to present the experimental results measuring the heat transfer coefficient between flowing sand and a representative heat exchanger surface. To measure the heat transfer coefficient a horizontal rotating drum is used to continuously deposit sand over a centrally positioned test article. The heat transfer coefficient in this case was calculated by taking the power input divided by the known area of the test article covered by the sand as well as the measured temperature difference between the article surface and sand temperature. Calibrated thin film thermocouples attached to the test article surface as well as thin film thermocouples suspended into the sand pooling in drum satisfy the needed temperature measurements. Then, by electrically heating a known area of the test article, a heat transfer coefficient between the sand and surface can be determined. Insulation of key end surfaces and errors such as heat leak due to air as well as measurement inaccuracies were also accounted for in the experimental setup and are included in the report’s error propagation analysis. The overall results compare heat transfer coefficients measurements for a range of different sands and sizes, as well as model comparisons with known literature on the subject.