Temperature field of non-Oberbeck–Boussinesq Rayleigh–Bénard convection in a low aspect ratio cell

Abstract

A time-resolved experimental investigation was undertaken on the temperature evolution of Rayleigh–Bénard convection (RBC) in a slender convection cell with aspect ratio of Γ=0.1. Experiments were conducted for Rayleigh numbers of Ra=5.3×107, 7.6×107, and 9.5×107 and Prandtl number of Pr≈6 within the non-Oberbeck–Boussinesq (NOB) condition with a temperature difference variation in the range of 30 °C≤ΔT≤40 °C. Measurement of the temperature was by applying time-resolved two-color planar laser-induced fluorescence over the initial 2400 s. Experimental observations showed that the lateral confinement of the convection cell leads to the development of a single large-scale thermal plume instead of multiple plumes. Results showed that contrary to expectations, lateral confinement was found to be ineffective in suppressing temperature oscillations near thermal boundaries. Results also indicated that for Ra=5.3×107, 7.6×107, the temperature oscillations had a frequency of f≈0.028 Hz similar to the frequency of the oscillations in Oberbeck–Boussinesq (OB) RBC. For Ra=9.5×107, however, it was found that the frequency of the oscillations was much lower than the OB RBC with a relatively wide range of the oscillations in the vicinity of f≈0.006 Hz. It is also found that the lateral confinement and formation of singular high-energy thermal plumes leads to an increase in the nonsymmetrical temperature distribution of NOB RBC with a bimodal distribution of the temperature field, deviating significantly from the Gaussian distribution temperature field found in OB RBC.