@article{watkins_chilamkurti_gould_2020, title={Analytic Modeling of Heat Transfer to Vertical Dense Granular Flows}, volume={142}, ISSN={["1528-8943"]}, DOI={10.1115/1.4045311}, abstractNote={Abstract}, number={2}, journal={JOURNAL OF HEAT TRANSFER-TRANSACTIONS OF THE ASME}, author={Watkins, Megan F. and Chilamkurti, Yesaswi N. and Gould, Richard D.}, year={2020}, month={Feb} }
 @article{watkins_gould_2019, title={Experimental Characterization of Heat Transfer to Vertical Dense Granular Flows Across Wide Temperature Range}, volume={141}, ISSN={["1528-8943"]}, DOI={10.1115/1.4042333}, abstractNote={Particle-based heat transfer fluids for concentrated solar power (CSP) tower applications offer a unique advantage over traditional fluids, as they have the potential to reach very high operating temperatures. Gravity-driven dense granular flows through cylindrical tubes demonstrate potential for CSP applications and are the focus of the present study. The heat transfer capabilities of such a flow system were experimentally studied using a bench-scale apparatus. The effect of the flow rate and other system parameters on the heat transfer to the flow was studied at low operating temperatures (<200 °C), using the convective heat transfer coefficient and Nusselt number to quantify the behavior. For flows ranging from 0.015 to 0.09 m/s, the flow rate appeared to have negligible effect on the heat transfer. The effect of temperature on the flow's heat transfer capabilities was also studied, examining the flows at temperatures up to 1000 °C. As expected, the heat transfer coefficient increased with the increasing temperature due to enhanced thermal properties. Radiation did not appear to be a key contributor for the small particle diameters tested (approximately 300 μm in diameter) but may play a bigger role for larger particle diameters. The experimental results from all trials corroborate the observations of other researchers; namely, that particulate flows demonstrate inferior heat transfer as compared with a continuum flow due to an increased thermal resistance adjacent to the tube wall resulting from the discrete nature of the flow.}, number={3}, journal={JOURNAL OF HEAT TRANSFER-TRANSACTIONS OF THE ASME}, author={Watkins, Megan F. and Gould, Richard D.}, year={2019}, month={Mar} }
 @inproceedings{watkins_gould_2017, title={Heat transfer to vertical dense granular flows at high operating temperatures}, DOI={10.1115/es2017-3272}, abstractNote={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.}, booktitle={Proceedings of the asme 11th international conference on energy sustainability, 2017}, author={Watkins, M. F. and Gould, R. D.}, year={2017} }
 @inproceedings{watkins_gould_2016, title={Dense granular flows as a new heat transfer fluid for concentrated solar power}, DOI={10.1115/imece2015-51069}, abstractNote={The increasing interest in concentrated solar power as a new form of renewable energy necessitates an improvement in overall system efficiency. Current heat transfer fluids employed to capture the concentrated heat demonstrate limited working temperature ranges. This study sought to investigate the use of dense granular flows as a possible new heat transfer fluid, as ceramic particles present virtually no restriction on working temperature. A bench-scale system simulating a single tube of a concentrated solar power central receiver was constructed and used to evaluate the heat transfer properties of the flow at low temperatures. Ceramic particles, 270μm in diameter, were gravity-fed through a vertical tube, resulting in granular flows with particle packing fractions of approximately 60%. Radial temperature profiles were measured and used to calculate the mean temperature of the fluid at different axial tube locations. The heat transfer coefficient was then calculated based on the input heat flux and measured wall and mean temperatures. The effect of the mass flow rate on the heat transfer coefficient was examined by using different orifices at the tube exit. As expected, the heat transfer coefficient increased with increasing flow rate. Heat transfer coefficients ranging from 330 to 380 W/m2-K were obtained for bulk temperatures ranging from 40 to 70°C. Previous works demonstrated comparable heat transfer coefficients at higher bulk temperatures. Thus, our preliminary heat transfer coefficient results demonstrate the potential of dense flows of ceramic particles for obtaining beneficial heat transfer properties at extremely high operating temperatures.}, booktitle={Proceedings of the ASME International Mechanical Engineering Congress and Exposition, 2015, vol 8b}, author={Watkins, M. F. and Gould, R. D.}, year={2016} }
 @inproceedings{watkins_gould_2016, title={Effect of flow rate and particle size on heat transfer to dense granular flows}, DOI={10.1115/es2016-59258}, abstractNote={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.}, booktitle={Proceedings of the ASME 10th International Conference on Energy Sustainability, 2016, vol 1}, author={Watkins, M. F. and Gould, R. D.}, year={2016} }