Performance improvement of a flat-plate solar collector by inserting intermittent porous blocks

Abstract

Numerical analysis of the thermal performance of a flat plate solar collector (FPSC) is presented. The FPSC is inserted with porous metal foam blocks intermittently for promoting thermal mixing. Based on the presence of blocks at the inlet and the outlet, four different arrangements are used namely NN, NP, PN and PP, wherein N means absent, and P means present. Also, four different cases based on the increasing number of porous blocks as per respective arrangement are considered viz., Case 1 with 1 or 2, Case 2 with 3 or 4, Case 3 with 5 or 6, and Case 4 with 7 or 8 porous blocks. Influence of height of porous blocks (S = 0–1), and permeability of the porous medium (Darcy number, Da = 10−4 - 10−1) on the collector outlet temperature, i.e. overall heating of the working fluid (Prandtl number, Pr = 7), has been studied. Numerical experiments are performed by modifying a generic code (SimpleFOAM) from the OpenFOAM® repository with the extended Darcy-Brinkman-Forchheimer model for realising porous medium. Results indicate that significant augmentation in heat transfer can be achieved by increasing the number of blocks due to improved thermal mixing. The increment was prominent for higher values of the height of porous blocks. However, the pressure drop penalty has to be spent in such cases. The performance of the FPSC channel improves when the number of porous blocks is minimal, along with lesser height. The value is higher even than the case of a channel filled with a continuous porous layer of varying thickness. Overall, a better performance evaluation criteria value is reported for the insertion of the porous block in comparison to both, empty and filled porous FPSC channel. Detailed insights are further provided on the inclusion of the Forchheimer term while modelling the porous medium. For lower values of permeability, wherein the porous resistances are higher in amplitude, the results vary significantly for Darcy-Brinkman model in comparison to Extended Darcy-Brinkman-Forchheimer model. The manuscript provides an impetus for further experimental work on the present case, and assists to explore the performance improvement in an FPSC channel by insertion of the porous medium. © 2019 Elsevier Ltd