OSCILLATIONS OF HEAT TRANSFER AND LARGE-SCALE CIRCULATION IN TURBULENT MIXED CONVECTION

Abstract

Mixed convection (MC) describes the transport of heat in fluids when forced convection (FC) and thermal convection (TC) coexists. It is a very often occurring flow condition e.g. in the oceans, the atmosphere, indoor climatisation or in many industrial processes and applications. MC can be characterised by the dimensionless parameters Rayleigh number $Ra \equiv \Delta T \beta g H^3 / \kappa \nu$, Reynolds number $Re \equiv U H / \nu$, Prandtl number $Pr \equiv \nu / \kappa$ and Archimedes number $Ar = Ra / (Re^2 \times Pr)$ which is the ratio of buoyancy to inertia forces. In this study we investigated the influence of torsional oscilations of the large-scale circulations (LSC) on the heat transfer at MC in a rectangular cavity. To cover a large range of $600 < Re < 3 \times 10^6$ and $1 \times 10^5 < Ra < 1 \times 10^{11}$ two convection cells with an aspect ratio of 1:1:5 (height:width:length) have been constructed using air as working fluid. As a characteristic length the height $H$ of the cell is chosen and the spatially averaged inflow velocity as characteristic velocity $U$. The convection cells consist of a rectangular container with an air inlet at the top and an air outlet at the bottom. Inlet and outlet are located at the same side of the cell. They span the whole length of the cell and are constituted by rectangular channels. The bottom is equipped with a heated copper plate and the top with an aluminium heat exchanger with cooling fins. The small cell with the dimensions $H = 100$ mm, $W = 100$ mm and $L = 500$ mm was designed to be operated under high pressure conditions up to 100 bar. The large convection cell has been designed to work under ambient pressure with the same aspect ratio. However, its dimensions are scaled by a factor of 5.