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BIRS Workshop Lecture Videos

Solar receivers - polydispersity in particle-fluid systems Rahmani, Mona


Talk: Regular Abstract: The effects of polydispersity of particles on thermodynamic and hydrodynamic behavior of solid particle solar receivers (SPR) are studied. SPRs are alternative designs for thermal solar receivers, in which solid particles, laden in a carrier gas, are used to directly absorb concentrated solar radiation. For energy conversion, the heated gas drives a turbine and/or the heated solid particles are used in chemical reaction processes. The main advantages compared to the conventional thermal solar receivers are volumetric heat absorption and transfer by particles, more uniform heat transfer in the gas, and significantly less heat loss to the ambient. However, the challenge is to maximize the efficiency of radiation absorption and heat transfer by particles given their short residence time and interaction with turbulence. The flow in SPRs involves a turbulent mixture of gas and particles in a radiation environment. Investigating the performance of SPRs requires understanding the complex three-way coupling between the gas flow, particle transport and interfacial heat transfer. Here, we use direct numerical simulations of turbulent flows interacting with point particles subject to a heating source to study physical processes in SPRs. The particles are polydisperse, i.e. they have a specific cumulative distribution function (CDF) for particle size. A thermodynamically equivalent monodisperse particle size is calculated for this CDF so that the mass loading ratio of particles to gas and the total frontal area of particles available for radiation absorption are matched between the polydisperse and monodisperse particles. Our results show that the energy balance of the gas and particle phase are significantly different between monodisprese and polydisprse particles. Polydisperse particles are more efficient in transferring heat to the gas phase and generating a uniform gas temperature field. The spatial inhomogeneity of particle concentration due to particle clustering adversely impacts the heat transfer from particles to gas. By quantifying the inhomogeneity in particle concentration by radial distribution functions (RDF), we show that polydisperse particles are more uniformly distributed compared to their counterpart monodisperse particles. We show this is mainly because of the different preferential concentration patterns in different particle size classes in polydisperse particles. However, the heat transfer in polydisperse particles cannot be represented as linear superposition of different classes of monodisperse particles.

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