Investigation of convective heat transfer phenomena in differentially-heated vertical closed cavity: Whole field experiments and numerical simulations

Abstract

Natural convection in closed cavities has been a subject of intensive research in the past. Compared to numerical studies, the number of experimental works reported in this area have been relatively scarce. Of the limited number of experimental studies available in the literature, a majority have made use of invasive probes, which inherently disturb the flow. In the present work, real-time experimental measurements are carried out using one of the non-invasive techniques (Mach Zehnder interferometry), that provides the whole field temperature distribution of the fluid layer. Experiments are conducted in a differentially-heated vertical closed cavity of aspect ratio three with air as the working fluid. The vertical side walls of the cavity have been subjected to three temperature differences (ΔT = 10, 20 and 30 °C) (Ra = 9.7 × 105, 1.8 × 106 and 2.5 × 106). Transient numerical simulations have also been performed using COMSOL Multiphysics 5.2 and a detailed comparison of experimental and simulation results has been presented in the form of temperature contours, spatial distribution of Nusselt number and spatially-averaged heat transfer rates as a function of Rayleigh number. The interferometric measurements highlighted the importance of corner flows which affect the heat transfer rates between the two thermally active walls of the cavity. Buoyancy-induced flow patterns inside the cavity, as interpreted through interferometric measurements, have further been corroborated through smoke-based visualization technique as well as through the results of numerical simulations. Maximum heat transfer rates have been observed in the corners of the differentially heated cavity. Possible flow transitions have been captured by performing the spectral analysis of the interferometry-based transient data. Based on this analysis, Ra = 1.8 × 106 was found to be greater than the critical Rayleigh number wherein the flow instabilities with two dominant frequencies were to be clearly seen.