Numerical analysis of a solar air heater with offset transverse ribs placed near the absorber plate

Abstract

Solar air heater (SAH) is used to convert readily available solar energy into useful thermal energy. Several studies have been conducted with different roughness elements on the absorber plate. However, study with ribs which are placed slightly above the heated plate, termed as offset transverse ribs, are still scant in the literature. Such study would explain the effect of flow between the ribs and the absorber plate on heat transfer augmentation in a SAH. With this objective a 2-D numerical study is performed here by solving steady RANS equations with RNG k−ɛ turbulence model. The effect of offset transverse ribs on heat transfer, pressure drop and thermal enhancement factor (TEF) is investigated. The geometrical parameters, such as, relative gap (α), relative height (β) and relative pitch (λ) are varied in the range of 0.0–0.045, 0.015 to 0.09, and 0.0333 to 0.1, respectively. The performance of SAH is evaluated on the basis of highest enhancement in relative Nusselt number (NuR/Nu) and thermal enhancement factor (TEF). A slight gap between the ribs and the absorber plate (α = 0.0075) is observed to yield better thermal performance than that for the attached ribs (α = 0). The maximum enhancement in NuR/Nu is found to be 1.52 for α = 0.0075. However, increasing the gap (i.e., α) prevents the flow reattachment with the absorber plate. Increase in rib height, on the other hand, is found to be more effective in heat transfer enhancement, but with a price of a significant enhancement in pressure drop. This leads to a reduction in the overall performance of SAH. Increment in pitch distance, however, reduces the enhancement in both Nusselt number and friction factor. By considering these three parameters, the highest TEF is found to be 1.042 for α=0.0075, β=0.030, and λ=0.0333 at Re equal to 13000. Finally, correlations for NuR and fR are derived as a function of the parameters investigated for helping the design of SAHs.