Influence of bubble bursting on heat transfer phenomena in directly irradiated fluidized beds

Concentrating Solar Power (CSP) is a fast-growing technology in which several hundreds/thousands of optical mirrors concentrate the solar energy onto a receiver. The received energy can be used to produce electricity through thermodynamic cycles or to drive an endothermic chemical reaction for chemicals production (e.g. solar fuels) or for chemical storage. The receiver is a crucial part of the whole system, as it owns the severe task of collecting and transferring the concentrated solar energy minimizing the heat losses. Dense fluidized beds have been proposed as CSP receivers thanks to their large heat-transfer and thermal diffusivity coefficients, and their use is currently under investigation (1-2). Directly-irradiated fluidized bed receivers are very promising in the context of solar chemistry and CSP applications, but they can undergo to extensive bed surface overheating. Tailoring bed hydrodynamics 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 in order to maximize the receiver efficiency and mitigate the bed surface overheating. The present work addresses the study of the interaction between a concentrated solar radiation and bed surface. The experimental apparatus schematically reported in figure 1a mainly consists of: i) a fluidized bed reactor (square 0.78 x 0.78 m bed column, 0.6 m tall); ii) a simulated solar radiation source, consisting of a short-arc Xe lamp coupled with an elliptical reflector, whose spatial flux distribution map is shown in Figure 1b; iii) a Bubble Generation System (BGS), able to produce bubbles with a minimum diameter of 0.045 m at a maximum frequency of 2 Hz, connected to a submerged nozzle aligned with the focal point of the simulated solar beam. SiC particles (127 m) were used as solid bed material. The main diagnostic tool is represented by an infrared camera used to map the bed surface temperature. Tests were performed at incipient fluidization condition injecting through the nozzle bubbles with different diameters and at different frequencies keeping constant the gas flow rate. Two snapshot sequences of the bubble eruption phenomena are reported in Figure 2. It can be observed that increasing bubble diameter the lateral dispersion heat at bed surface is more efficient, as the hot particles are shifted towards a larger annular region. Nevertheless, the long delay between two successive bubble eruption events brings to a larger bed surface overheating, which could result into a fluidized particles degradation or into higher heat losses due to re-irradiation. A trade-off between these two-fold results has to be found to optimize bed hydrodynamics in CSP applications.