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 kWh 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 ho [18.6–439.1 W/(m2·K)] was obtained by experiments with varying particle flow rate ṁp (230–680 kg/h), molten salt flow rate ṁs (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 ho. It was also proved that there exists an optimal fluidization number (n = 1.8) to maximize U and ho, simultaneously. Finally, a modified correlation for predicting ho under continuous feed and discharge conditions was proposed and compared with existing models.