Heat transfer investigation in a solid-gas counterflow heat exchanger for thermochemical energy storage applications

This study experimentally investigates the overall heat transfer coefficient between a heated tube wall and a dilute cloud of inert particles falling under gravity through a counterflowing gas, approximating conditions in a reactive heat exchanger using metal oxide particles. Quantifying heat transfer enhancement in solid–gas mixtures is crucial for optimizing thermal systems in next-generation concentrated solar thermal receivers and energy storage systems. Previous studies have not explored the effects of particle size and Reynolds numbers in counter-current flow at low Reynolds numbers (Re_g < 8000). Thus, this study introduces a novel experimental apparatus to measure the overall heat transfer coefficient as a function of key variables, providing guidance for the thermal design of dilute particle solar receivers, heat exchangers, or reactors. Two sizes of high-performance ceramic CARBOBEAD particles were used: HSP-40/70 (d_p50 = 324 µm) and HSP-20/40 (d_p50 = 755 µm). The study found that heat gained by the gas in a solid–gas mixture was negligible for small particles at particle-to-gas mass flow rate ratios greater than 7. Larger particles exhibited 35 to 45 % lower heat transfer compared to smaller particles at Reynolds number of 1450, but a 50 to 55 % enhancement when Reynolds number was increased to 3150. The overall heat transfer coefficient (h_o) of mixture was approximately a linear function of the Reynolds number of the counterflow gas. The impact of particle feed rate on heat transfer was greater for small particles due to their higher particle-to-wall surface area ratio. The h_o increased by 1.5 times for small particle and by 1.3 times for large particle when particle feed was increased from 4 to 15 kg s⁻¹ m⁻². Existing correlation for Nusselt number in co-flow configuration were not reliable for predicting heat transfer in counterflow orientation. These findings provided valuable insights into optimizing particle size and feed rates for enhanced thermal performance in solar energy storage systems.