Search

• CFD and exergy analysis of a solar air heater with semi-cylindrical sidewalls is proposed. • Semi-cylindrical sidewalls enhance the strength of secondary flow in a ribbed SAH. • A significant enhancement in the effectiveness parameter of 2.27 is obtained. • Lower exergy destruction leads to an enhancement of 95% in exergetic efficiency. • The proposed design outperforms ribbed rectangular SAHs for a wide range of Reynolds number. Solar air heater (SAH) with semi-cylindrical sidewalls and W-baffles is analyzed for energy and exergy efficiency in the turbulent flow regime. Computational fluid dynamics (CFD) analysis is carried out for a fixed baffle inclination (β) and varying the relative baffle height (R h = e/D) and relative baffle pitch (R p = P/D) in the range 0.046–0.115 and 0.46–1.15, respectively. For Reynolds number (5000 < Re < 17,500), the numerical methodology is substantiated using experimental and theoretical correlations obtained from the literature. Smaller vortices near the sharp corners are removed by rounding the sharp edges, allowing the flow of fluid from inside and horizontal walls of the duct towards the semi-cylindrical sidewalls. This increases the overall turbulent kinetic energy. A peak augmentation of 3.24 and 4.03 times is obtained for Nusselt number (Nu) and friction factor (f), respectively, in contrast to conventional SAH. With a maximum enhancement of 127% in the effectiveness parameter relative to smooth SAH, this novel SAH design is evidently energy efficient. Based on CFD results, new correlations are developed in terms of R h and R p , which predicts the values with an absolute deviation of 4% and 7.4%, respectively. With lower exergy destruction, maximum enhancement in thermal and exergetic efficiency is obtained as 40.7% and 95.4%, respectively, for the proposed SAH relative to conventional SAH. Upon comparison with ribbed rectangular duct SAH configurations, the present design with semi-cylindrical side walls outperforms at all flow Re. [ABSTRACT FROM AUTHOR]

Microchannel heatsinks with porous media, wavy and multi-layered configurations introduce compact designs and high efficiencies in cooling applications. Current Computational Fluid Dynamics (CFD) based study bridges the concept by introducing a novel double layered microchannel heatsink with wavy up-down and porous ribs alongside each other. Since Artificial Neural Network (ANN) trained by CFD results reveals designs with better thermal and hydraulic performance with an insignificant computational time, ANN is employed for finding the best performers and effects of waviness on Nusselt number, pressure drop, and maximum temperature difference at the bottom of the microchannel for a Reynold number range of 100 < Re < 800 by defining a thermal efficiency factor. Even though the pressure drop increases in the wavy design, the thermal performance is affected positively. Nusselt number increases more than 55% for Re = 800. The thermal performance increases with the introduction of Dean vortices, especially at higher Re. The best design provides maximum bottom temperature difference of 1.2 °C, with a Thermal Efficiency Factor of 1.34. However, it is observed that better thermal efficiency factors are attained by using a wavy channel at the top layer and keeping the bottom layer straight for the final design. [ABSTRACT FROM AUTHOR]

Abstract: A computational fluid dynamics methodology is developed to study steady laminar flow and heat transfer in periodic zigzag channels with square cross-sections, considering the effect of chaotic advection. The computational domain consists of up to 14 repeating zigzag units with smoothly joined inlet and outlet sections. The results show fully-developed periodic flow fields can be achieved for Re <200; however, a spatially periodic flow field is no longer seen for higher Reynolds number due to the enhanced effect of chaotic advection with increasing Reynolds number; for Re >400 the flow field becomes transient. The flow field shows that the secondary flow pattern varies significantly at different axial locations. Highly enhanced heat transfer appears in the regions where strong Dean vortices occur, as indicated by more pronounced twisting of flow streaklines. For Re >200, the unit-averaged heat transfer enhancement and pressure-drop penalty (relative to straight passages) are no longer uniform from unit to unit but mean values of both of these performance metrics increase with increasing Reynolds number over the whole range of 50< Re <400. [Copyright &y& Elsevier]

Computational fluid dynamics (CFD) and exergy analysis are conducted to investigate the impact of secondary flow produced by V-ribs on the overall performance of a triangular solar air heater (SAH) duct. For a fixed relative rib pitch (R p = 10) and relative rib height (R h = 0.05), the effect of rib inclination (α) is studied using CFD technique for varying Reynolds number (5000 ≤ Re ≤ 20000). Based on the CFD simulation results, empirical correlations capable of predicting Nu and f with an absolute variance of 8.7%, and 4.7%, respectively, are developed. Employing these correlations, exergetic performance analysis is carried out. Maximum effectiveness parameter (ε) of 2.01 is obtained for α = 45° at Re = 7500. The exergy analysis reveals that the entropy generated is lower for the ribbed triangular duct compared to the smooth duct with maximum enhancement in exergetic efficiency (η ex) as 23% for α = 45°. The study is extended for the rectangular duct to compare the performance with the ribbed triangular duct SAH (α = 45°). Results show that ribbed triangular duct SAH (α = 45°) is superior over various configurations of the ribbed rectangular duct SAH at higher mass flow rates. • CFD and exergy analysis of a triangular duct solar air heater with V-rib is proposed. • Secondary flow in triangular duct significantly enhances the Nusselt number. • A significant enhancement in the effectiveness parameter of 2.01 is obtained. • Empirical correlations are developed for Nusselt number and friction factor. • The proposed design exhibits superior performance at high mass flow rates. [ABSTRACT FROM AUTHOR]