Mixed convection in a narrow rectangular cavity with bottom inlet and outlet

Abstract

Abstract A model describing the heat transfer in horizontal mantle heat exchangers used in solar water heaters has been developed by approximating a narrow horizontal annulus to an equivalent rectangular cavity. Particle tracking velocimetry (PTV) and computational fluid dynamics (CFD) simulations were used to determine the flow field. The flow is in the mixed-convection regime, where buoyancy locally affects the flow field and heat flux. The effect of the heat-transfer wall temperature distribution on the net heat-transfer rate has been quantified by the introduction of a stratification correction parameter. A mean Nusselt number, defined by using the difference between the inlet temperature and mean heat-transfer wall temperature, is shown to be independent of buoyancy forces when the stratification correction parameter is taken into account. A correlation has been developed for the mean Nusselt number for a range of geometries and boundary conditions. This correlation predicts the net heat-transfer rate to within 10%.