# Analytical Determination of Nusselt Numbers for Convective Heat Transfer Coefficients in Channel Macroporous Absorbers

## TL;DR

This paper presents a new analytical method to calculate heat transfer in solar absorbers with macroporous structures, improving efficiency predictions.

## Contribution

A novel analytical equation for Nusselt numbers in macroporous absorbers is introduced, incorporating boundary layer effects and flow characteristics.

## Key findings

- The new equation improves accuracy in heat transfer coefficient calculations for macroporous media.
- Empirical coefficients derived using statistical methods validate the equation against numerical data.
- The method enhances predictions of absorber performance in concentrated solar power plants.

## Abstract

This article introduces a novel analytical equation for computing the Nusselt number within the macroporous structures of channel absorbers utilized in high-temperature solar receivers. The equation incorporates heat and mass transfer processes occurring within boundary layers as fluid flows through complex-shaped macroporous absorber channels. The importance of accounting for the length of the thermodynamic boundary layer within channel-type macroporous media when calculating heat transfer coefficients using the Nusselt equation is demonstrated. By incorporating proposed indicators of porosity and flow characteristics, this method significantly enhances the accuracy of heat transfer coefficient calculations for such media. Discrepancies observed in existing calculation relationships and experiments are attributed to the omission of certain proposed values in the Nusselt number for macroporous media. To address this, empirical coefficients for the Nusselt number are derived using statistical methods. The resulting semi-empirical equation is applied to macroporous absorbers in solar receivers. The findings enable more accurate predictions of future absorber characteristics, enhancing their efficiency. The derived equation is successfully validated against numerical data across various geometric structures of absorbers in concentrated solar power plants.