Thermal fluctuations and boundary layer properties of turbulent natural convection inside open cavities of different dimensions heated from below

Abstract

We report the experimental measurement of the temperature fluctuation in the vicinity of different zones of the thermal boundary layer in water-filled open cubic cavities heated from below and open at the top. The experiments are performed on the cubic cavity of aspect ratio 1 and lateral dimension 30 mm; the results of our previously reported open cubic cavities of aspect ratio 1 and lateral dimensions (120 mm and 240 mm) are also considered here. The transient nature of the temperature has been measured from the temperature–time series recorded across the central axis of the cavity at different vertical positions z from the heated bottom plate. The Prandtl number and Rayleigh number ranges reported in this paper are 4 ≤ Pr ≤ 6 and 105 ≤ Ra ≤ 109, respectively. The different basic statistical properties, of temperature fluctuation such as mean temperature, root mean square, and probability density function, are studied and discussed. The power-law of power spectral density of the temperature fluctuations at different regions of the thermal boundary layer is studied, and the different roles of rate are compared with the previously established theories and models. The validity criteria for the Oberbeck–Boussinesq approximation are fulfilled. The trend of the dimensionless Nusselt number (Nu) representing the global convective heat transfer is obtained and discussed. We also study the variation in Raδth−1 3 for the heat transfer representation in the range of 0.04–0.24, where δth is the boundary layer thickness.