Particle dispersion in fluidized bed heat exchangers from Eulerian-Lagrangian simulations

High-temperature particle-based thermal energy storage (TES) at temperatures > 700°C is an effective method to provide around-the-clock heat from next-generation concentrating solar power (CSP) plants. Narrow-channel counter-flow fluidized bed heat exchangers offer a promising approach to transferring heat out of hot particles because fluidization results in stronger mixing and higher heat transfer coefficients. However, while experiments show that particle dispersion across the dept of the bed channel increases particle-wall heat transfer, particle dispersion over the longer length scales in the vertical direction redistributes the thermal energy over the fluidized bed height and flattens the temperature. This reduces the particle heat transfer by lowering the effective log mean temperature difference between the particles and the power cycle fluid. Recent studies on finned walls have explored the effectiveness of extended surfaces' effectiveness in suppressing vertical dispersion and significantly improving particle-wall heat transfer. A richer understanding of the particle-wall heat transfer mechanisms through detailed simulation can support better design of heat exchanger surfaces for increased particle-wall heat fluxes.

We perform Eulerian-Lagrangian simulations to investigate the effects of extended surfaces in narrow-channel fluidized bed heat exchangers on particle dispersion. To demonstrate the effectiveness of extended surfaces, we compare the overall particle-based dispersion coefficient without extended surfaces against aligned and staggered extended surface configurations. We also quantify the degree of mixing in the three dimensions using the Lacey index. Our simulations show that the extended surfaces pose a significant obstacle to the flow of particles, reducing the size and the age of bubbles in the bed. This mechanism reduces vertical mixing, thus mitigating vertical temperature flattening in fluidized bed heat exchangers.