Thermal study of Rayleigh–Bénard convection in heat transfer oils in a small cubic enclosure using laser-induced fluorescence

The present study focuses on the thermal characterization of a Rayleigh–Bénard (R–B) convection (Rayleigh number Ra = 2.20⋅10⁷ and Prandtl number Pr = 29.9) in the synthetic heat transfer oil Marlotherm LH (benzyltoluene) with a two-color laser-induced fluorescence measurement technique (2c-LIF). For this purpose, a compact convection chamber with unity aspect ratio was developed, which enables extreme temperature differences up to 120 K. The fluorescence signal is generated by doping the heat transfer oil with the fluorophore Nile red and its excitation by a pulsed Nd:YAG laser at 532 nm. First, the 2c-LIF technique is calibrated under homogeneous temperature conditions in the cell. Here, the relative thermal sensitivity decreases with increasing liquid temperatures. Second, the detachment and rise or fall of multiple thermal plumes in the R–B cell is analyzed, while the bottom wall was heated to 360 K, and the top wall was cooled to 240 K, resulting in a respective temperature field of the mixture in the range of 300–345 K. The time-resolved LIF measurements enable a characterization of the buoyancy-driven flow in terms of temperature field, heat flux density, thermal plume shape and plume velocity. The local heat flux density (11.5 kW/m²), heat transfer coefficient (311 W/m²⋅K) and Nusselt number (36.4) of the cold boundary were determined from the temperature profile. The highest plume velocities are in the range of 15 mm/s at the studied condition with large temperature stratification. No stationary large recirculation zones were detected in the cell, which are typical for such thermal R–B convection conditions.