Airflow and thermal analysis of flat and corrugated unglazed transpired solar collectors

Abstract

The flow structure and convective heat transfer mechanisms in Unglazed Transpired solar Collectors (UTCs) are crucial to their performance. High-resolution, 3-dimensional, steady, Reynolds-Averaged Navier–Stokes (RANS), Computational Fluid Dynamics (CFD) simulations have been used to analyze the convective heat transfer processes for both flat and corrugated UTCs. The performance of five potentially suitable turbulence closure models: Standard k–ε, Renormalization Normal Group k–ε (RNG k–ε), Realizable k–ε, Shear Stress Transport k–ω (SST k–ω) and Reynolds Stress Model (RSM), has been evaluated. Two scenarios have been considered: a flat UTC under free stream approaching flow and a corrugated UTC subjected to a plane wall jet flow. The results were compared against experimental data from the literature and those obtained using a full-scale experimental set-up in a solar simulator, in terms of the velocity and turbulent kinetic energy of the airflow as well as the plate surface and air temperature. The validation study showed that the RNG k–ε model has the best overall performance for both UTC geometries at reasonable accuracy and computing cost. A parametric analysis has been conducted using the RNG k–ε model at 3m/s approaching flow velocity, for different suction flow rates (0.01 and 0.06m/s) and free stream turbulence intensities (0.1% and 20%). The results showed that in UTCs, it is the suction velocity, rather than the suction ratio of Vs/U∞, that has the most profound impact on the boundary layer development and thermal efficiency. For flat UTCs, the presence of perforations is more significant than the level of turbulence intensity in the approach flow. However, for corrugated UTCs, the incident turbulence intensity plays a more important role in the system performance than the perforation dimensions due to the turbulent interaction between the corrugated geometry and the incoming flow. Although a high suction rate can help maintain the asymptotic boundary layer profiles and, hence simplify the energy analysis of UTCs, it is not applicable in practice due to the requirement of large fan power. These outcomes will not only further advance the development of UTCs and optimize their performance but also provide insights for the development of simplified thermal analysis models for use in building energy simulation.