Experimental study on heat transfer between particles and molten salt in gas-solid fluidized bed with immersed tubes

The fluidized bed heat exchanger with immersed tubes plays a key role in heat transfer in the next-generation concentrated solar power system where low-cost inert particles with outstanding thermal stability serve as the heat-absorbing media. However, due to the lack of heat transfer experiments under real conditions, the heat transfer mechanism of such device is still not fully understood, especially in the case where continuous flow of particles is involved. In this study, a series of experiments was conducted based on our self-developed 20 kW_h level fluidized bed heat exchanger and the supporting test system to investigate the fluidizing heat transfer characteristics between inert particles and molten salt for the first time. The variation range of the overall heat transfer coefficient U [17.8-315.8 W/(m²·K)] and the bed-to-tube heat transfer coefficient h_o [18.6-439.1 W/(m²·K)] was obtained by experiments with varying particle flow rate m ̇ p (230-680 kg/h), molten salt flow rate m ̇ s (1300-3000 kg/h) and fluidization number n, defined as the ratio of the actual fluidization velocity to the minimum fluidization velocity, (1.6-3.3). It was found that 2D numerical simulation may seriously overestimate h_o. It was also proved that there exists an optimal fluidization number (n = 1.8) to maximize U and h_o, simultaneously. Finally, a modified correlation for predicting h_o under continuous feed and discharge conditions was proposed and compared with existing models.