Analytical Determination of Nusselt Numbers for Convective Heat Transfer Coefficients in Macroporous Absorbers of High-Temperature Solar Receivers

Abstract

The article presents a new analytical equation for calculating the Nu number in a macroporous structure of channel absorbers for high- temperature solar receivers. This equation is characterized by including the processes of heat and mass transfer in the boundary layers when the flow passes through a macroporous absorber channel of complex shape. The necessity of considering the length of the thermodynamic boundary layer in a macroporous medium of channel type, when calculating the heat transfer coefficient according to the Nusselt equation, is shown. Including the proposed indicators of porosity and flow characteristics in a macroporous channel significantly increases the accuracy of calculations of the heat transfer coefficient for macroporous channel media. The lack of consideration of certain of the proposed values in the Nusselt number for macroporous media explains the discrepancies in the existing calculation relationships and experiments. The empirical coefficients for the Nusselt number are proposed and obtained by the statistical method. The new semi-empirical equation obtained for macroporous channel media is applied to macroporous absorbers of solar receivers. The obtained results will help to predict the characteristics of future macroporous absorbers much more accurately than before to make them more efficient. The obtained equation for calculating the Nusselt number for channel-like macroporous absorbers has been successfully tested for the receiver of concentrated solar power plants with different geometric structure of the absorber.