Heat transfer in directly irradiated fluidized beds

Directly-irradiated fluidized bed solar reactors are very promising in the context of solar chemistry and concentrated solar power (CSP) applications. With proper choice of the bed solids, fluidized bed reactors can be operated at fairly high process temperatures that enable thermochemical storage with high energy density and production of solar chemicals and fuels. Bed surface overheating upon irradiation is one key to the efficiency of the fluidized bed as thermal receiver and may be responsible for sintering and/or degradation of the fluidized particles. Tailoring the hydrodynamics of the bed close to the region where the incident power is concentrated may disclose effective measures to improve the interaction between the incident radiative flux and the bed and mitigate bed surface overheating. In the present study radiative heat transfer from a concentrated simulated solar radiation source to a fluidized bed is investigated by time-resolved infrared mapping of the bed surface temperature. A fluidized bed of silicon carbide particles (0.127 mm), whose cross-sectional area is 0.78 × 0.78 m, was directly irradiated by highly concentrated simulated solar radiation, emitted by a 4 kWel short-arc Xe lamp coupled with an elliptical reflector. The experimental apparatus is also equipped with a movable nozzle coupled with a bubble generation system located coaxially to the concentrated simulated solar beam. The interaction of the concentrated radiative flux with the fluidized particles moving under the action of bubble bursting was assessed by characterizing the time-resolved bed surface temperature as the fluidization gas velocity was varied. The effect of localized generation of bubbles was also investigated by injecting chains of multiple bubbles from the nozzle located at variable distance from the bed surface.