To meet net-zero targets, reducing reliance on fossil fuels and increasing the deployment of renewable energy technologies is essential. In concentrated solar power (CSP) systems, thermal efficiency is constrained by the maximum operating temperature of the heat transfer medium. Particle-based fluidised beds offer a promising alternative, due to their enhanced heat transfer characteristics and ability to operate at substantially higher temperatures than conventional molten salts.

In this study, an indirectly heated tubular gas-solid fluidised bed was investigated to quantify the effects of particle properties and air flow rate on hydrodynamic and thermal behaviour under controlled boundary conditions. Three powders, including silicon carbide (SiC), silica sand (SiO₂), and ordinary sand, were examined at two mean particle diameters. Experiments were conducted at different initial powder bed aspect ratios and fluidisation numbers between 1 and 3. Heating was applied indirectly using three constant external wall temperatures.

The results showed that increasing particle density and size increased the minimum fluidisation velocity and reduced bed expansion. Lower aspect ratios resulted in greater bed expansion due to reduced resistance to bubble growth and gas motion. Increasing fluidisation number reduced outlet air temperature because of shorter gas residence times, while simultaneously increasing thermal efficiency through enhanced gas-solid heat transfer. A dimensionless correlation was developed to predict bed expansion as a function of fluidisation number, particle-to-gas density ratio and particle-to-bed diameter ratio, with an average relative error of 10.1%. These findings provide fundamental insight into the interaction between hydrodynamics, bed expansion and heat transfer in indirectly heated gas-solid fluidised beds and offer design guidance for particle-based thermal systems.