Augmenting the Thermal Performance of Solar Air Heater with Absorber Plate Roughened with Pin-Fins

Abstract

Solar air heaters (SAH) are one of the most promising and economical solar thermal collection appliances which can cater mainly to the needs of space heating and drying of agricultural products. They are easy to operate requiring minimum maintenance owing to their simple design without any moving parts. In the current work, effort is made to improve the thermal performance of a solar air heater by incorporating pins-fins on the absorber plate. The RNG $\mathbf{k}-\boldsymbol{\varepsilon}$ turbulence model is applied in the study, by utilizing the finite volume-based solver ANSYS FLUENT 19.2. The length of absorber plate is considered to be 1100 mm, while taking three values of pin fin diameter (d) as 6 mm, 8 mm and 10 mm corresponding to height ratio (HR) of 0.17, 0.25 and 0.34 respectively, pitch ratio (PR) of 8, 10 and 15, with the variation of Reynolds number from 3,500-16,000. To get an insight of the flow physics and heat transfer characteristics, velocity streamlines, contours of velocity, temperature, heat transfer coefficient are provided. The highest value of Nusselt number (Nu) is found to be 162.31 at $\boldsymbol{Re}=\mathbf{16,000},\mathbf{H}_{\mathbf{R}} =\mathbf{0.17}$ and $\mathbf{P}_{\mathbf{R}} =\mathbf{8}$. Thermo-hydraulic augmentation index (THAI) of the pin fins absorber plate is reported to improved performance to that of the other type of absorber plate. At $\boldsymbol{Re}$ = 10,000, $\mathbf{H}_{\mathbf{R}} =\mathbf{0.25}$ and $\mathbf{P}_{\mathbf{R}} =\mathbf{8}$ has the highest value of THAI of 2.212, the minimum value of THAI is 1.666 found at $\mathbf{Re}=\mathbf{13,000},\mathbf{H}_{\mathbf{R}} =\mathbf{0.34}$ and $\mathbf{P}_{\mathbf{R}} =\mathbf{10}$. The results indicate that for the considered range of $\boldsymbol{Re}$, the new absorber plate design led an improved performance of the SAH in terms of heat transport from the absorber plate to the air stream, with minimal concomitant pressure drop penalty.