Experimental Investigation of Convective and Radiative Heat Transfer to a Dilute Particle Cloud in a Subatmospheric Environment

Redox-active particles offer significant potential for long-term thermochemical energy storage and solar fuel production. However, high thermal resistance between the particle cloud and heat exchanger wall reduces the efficiency of these systems. Heat transfer between particles and the wall is complex and under-researched, despite its importance in solar thermal energy storage. In this study, we conducted experiments to determine the heat transfer coefficients between a free-falling particle cloud and the heated surface of a tubular furnace under subatmospheric pressure. Key variables explored include particle feed rate from 3.7 to 44 kg s^-1 m^-2, wall temperature from 300 °C to 900 °C, and pressure from 98,000 Pa to 0.2 Pa. Experimental data for the overall heat transfer coefficient were obtained at various temperatures and particle feed rates, maintaining a lower pressure of 100 Pa. Results showed that at constant wall temperature, the overall heat transfer coefficient increased with higher particle feed rates, but this negatively affected particle temperature gain. Additionally, the combined convective heat transfer coefficient became independent of particle feed rates beyond 20 kg s^-1 m^-2 at low pressure. Further tests with constant particle feed rates and wall temperature revealed a significant drop in heat transfer performance between 1000 Pa and 10 Pa, due to reduced particle and wall convection. Convective heat transfer contribution became negligible below 10 Pa.