Heat transfer coefficient characterization at the solar collector wall-fluid interface

Abstract

Paper presented to the 3rd Southern African Solar Energy Conference, South Africa, 11-13 May, 2015.In this paper, a numerical study is carried out to characterize the transient local heat transfer coefficient at the fluid-solid wall interface of a solar collector. For that purpose, the considered collector wall geometry is a flat plate with nonnegligible thickness which is subjected to a variable solar heat flux. The transient conjugated conduction-convection heat transfer has been taken into account. The heat transfer coefficient is calculated as a function of the plate thickness as well as the position along the plate. A good agreement has been found between the calculated temperatures and other experimental results. The heat transfer coefficient evolutions, as a function of time, have been obtained for various positions along the plate. The results showed that at first, high values of the heat coefficient are reached, and then it decreases and tends to constant values. It has been also noticed that at a fixed value of time, the heat transfer decreases when the position is increased from the beginning of the plate towards its end. The parametric study allowed obtaining a correlation of the transient convective heat transfer coefficient as a function of the steady state coefficient (which depends on the flow velocity and the coordinate of the considered point on the plate), multiplied by a function of time and the plate properties. The results have been used to optimize the heat transfer coefficient measurement technique using the pulse method. The pulse method consists in imposing a heat flux on a wall, and then to calculate, by an inverse method, the heat transfer coefficient from the time evolution of surface temperature (thermo-gram). Measurement of the heat transfer coefficient is based on the introduction into the inverse model of a function that represents the theoretical evolution of this coefficient due to the energy excitation. This function is deduced from the numerical study conducted in this work.