Numerical analysis of the performance enhancement of a flat–plate solar collector using porous insertions

Abstract

Transport phenomenon in porous medium has always been a topic of academic and industrial interest as it offers rich flow physics and has several practical applications. It beholds interconnected tortuous paths that offer better flow control and the extra surface needed for heat transfer enhancement in limited volume applications. The numerical modelling of porous media theory also provides computational economy in cases where multiple identical elements are arranged uniformly. Hence, computational endeavours involving flow and heat transfer within and around a porous object is appealing. In the first chapter of this thesis, the onset of vortex shedding in the flow across a two-dimensional porous square cylinder, placed in a uniform flow, has been numerically analysed. The exercise is carried out for a wide range of Reynolds number (Re = 1 – 150), and Darcy number (Da = 10􀀀6 - 10􀀀2), but particular attention is given to the transitional flow regime, i.e. intermediate Re andDa values. Details on the critical Reynolds number for the various values ofDa have been highlighted. At intermediate values of permeability, a jump in the flow parameters is seen, which pre-pones vortex shedding initiation. Proper reasoning for this occurrence is presented, along with a discussion on flow physics in the two-dimensional unsteady flow regime up to Re = 150. In the second chapter, while studying the heat transport from the hot porous square cylinder, the effects of Prandtl number on forced convection heat transfer is presented for Pr = 0.71 - 100, Re = 1 - 40 and Da = 10􀀀6 - 10􀀀2. Significant augmentation in heat transfer is reported at higher Prandtl and Darcy number values. The jump experienced at intermediate flow properties did not affect the thermal transport phenomenon. Moreover, at the extreme parameter values considered in the present study, a stalling in the enhancement ratio of Nusselt number Nu has been reported. After understanding the role of porous foam on flow control and heat transfer augmentation, its usage in solar energy-related equipment to improve the thermal performance is studied. As the depletion of fossil fuel and its adverse impact is forcing the humanity to look for viable alternatives for energy supply, a low performing flat plate solar collector (FPSC) can be used with porous insertions for domestic/industrial water heating and space ventilation. In the third chapter of this thesis, two different combinations of porous insertions are tested well. Multiple porous blocks are laid out near the top absorber plate of the FPSC channel that is exposed to the solar radiation. The different arrangements of these blocks are tested along with various aspect ratios and permeability values (Da = 10􀀀4 - 10􀀀1). In another layout, two rectangular porous are conjugated together by an intermediate trapezoidal porous block, and these are placed near the bottom insulator plate of the FPSC channel. Again, variation in performance enhancement due to the aspect ratio of these foam blocks is tested along with permeability levels (Da = 10􀀀4 - 10􀀀1). Both the analyses are carried out using steady-state assumption for a fair comparison, but with practical experimental physical conditions. When the porous foam is placed near the absorber plate, maximum thermal performance is achieved with a thin and long porous layer. On the other hand, when the porous foam is placed in the bottom insulator plate, the maximum performance is achieved at intermediate values of aspect ratio for lower permeability. Details of both the configurations are given, along with recommendations for future numerical and physical experiments. In the fourth chapter of the thesis, the optimum angle for the maximum thermal performance of the FPSC channel is presented. The collector channel is filled with fully saturated porous metal foam, and the influence of permeability (Darcy number, Da = 10􀀀4 - 10􀀀1), radiation insolation parameter (Rd = 0 - 5), buoyancy parameter (Richardson number, Ri = 0 - 5), and collector channel inclination angle ( = 0o - 45o) on the collector channel outlet temperature i.e., effective heating achieved has been studied. Results suggest that the flow and thermal fields change significantly with buoyancy and radiation parameter variation. An increment in performance is assured using porous metal foam for better thermal mixing, along with buoyancy parameter and Radiation parameter. Intermediate inclination angles provide a higher heat transfer rate with a meagre compromise with thermal performance. In the final chapter of the thesis, the local thermal non-equilibrium assumption has been tested thoroughly for various parameters, including the permeability, porosity, variable porosity models, and Radiation parameter values. Both exponential and linear variable porosity models have been tested extensively for the parameters considered in the present study, along with the consideration of buoyancy force for the channel inclined at an optimum inclination for maximum performance. The results from the numerical experiments suggest that for the present design of the FPSC channel, the LTE assumption holds as maximum LTNE value calculated is lesser than 5% for lower particle Nusselt number and thermal conductivity ratio factor. At higher values of these variables, LTNE modelling is necessary. Variable porosity model is noticed to predict higher thermal performance values in comparison to the constant porosity model. Further, a vivid comparison between Rosseland and P1 radiation models is also given. The results predicted by these models are found almost similar for the various conditions tested for modelling the FPSC channel. A comment on the arrangement of the variable porosity and the channelling effect is given. The open - source tool OpenFOAM®has been used in the present thesis, and the generic codes from its repository have been modified to model the superficial velocity, local non-thermal equilibrium, radiation heat transfer in optically thick medium, and variable porosity in the porous medium. Exhaustive details on the code modifications have been presented in the thesis.