Investigation of Rayleigh-Bénard convection in heat transfer oils in a miniature enclosure with laser-induced fluorescence

A Rayleigh-Bénard (R-B) Convection (Rayleigh number Ra = 2.20⋅107, Prandtl number Pr = 29.9) of a synthetic heat transfer oil Marlotherm LH (benzyltoluene) in a miniature enclosure is characterized with a two-color laser-induced fluorescence measurement technique (2c-LIF). Therefore, a miniature convection cell with unity aspect ratio was developed, which enables temperature differences up to 120 K. The fluorescence signal is generated by dissolving the fluorophore Nile red in the heat transfer oil and exciting the mixture at 532 nm with a pulsed Nd:YAG laser. First, the 2C-LIF technique is calibrated within the convection cell under homogeneous temperature conditions. The calibration reveals a decreasing relative temperature sensitivity with increasing liquid temperatures. Second, the motion of thermal plumes within the convection cell was analyzed, while the bottom wall was heated to 360 K and the top wall was cooled to 240 K. This results in a respective temperature field of the mixture in the range of 300 K to 345 K. The time-resolved 2C-LIF measurements enable a characterization of the buoyancy driven flow in terms of 2D temperature distribution, heat flux density, thermal plume shape and plume velocity. The investigated plumes are characterized by large temperature stratification velocities and velocities in the range of 15 mm/s. Stationary large recirculation zones were not detected, which are typical for such thermal R-B convection conditions.