Dynamics of the thermal boundary layer in low Prandtl number Rayleigh–Bénard convection

The thermal and viscous boundary layers in Rayleigh–Bénard convection significantly influence global heat transport and large-scale flow structure. At low Prandtl numbers, the thermal boundary layer is thicker than the viscous layer due to differing diffusion rates. This study investigates the dynamics of the thermal boundary layer under low Pr conditions using a cylindrical cell filled with the eutectic alloy GaInSn, characterized by Pr ≈ 10^-2, and Rayleigh numbers up to 10⁹. Temperature profiles near the heating plate were measured using thermocouple arrays at the cell's centre and near the sidewall, representing shear and plume ejection regions. We reveal that the thermal boundary layer remains in a transient state, not fully transitioning to turbulence. Central profiles align with the Prandtl-Blasius profile, while sidewall profiles deviate. The shape factor of the temperature profile is consistently smaller than the Prandtl-Blasius profile across the entire range of Rayleigh numbers. Furthermore, thermal boundary layer thickness decreases with Ra, showing exponents of 0.26 in the plume ejection region and 0.2 in the shear region for a cell with an aspect ratio of 0.5. These findings deepen the understanding of thermal convection in liquid metals at low Prandtl numbers.