Influence of Rectangular Ribs on Exergetic Performance in a Triangular Duct Solar Air Heater

Abstract

Several artificial roughness (ribs) configurations have been used in flat plate solar air heaters (SAH) in recent years to improve their overall performance. In the present work, energy and exergy analyses of rectangular ribs in a triangular duct SAH for varying relative rib heights (e/D = 0.02–0.04), relative rib pitches (P/e = 5–15), and rib aspect ratios (e/w = 0.5–4) are evaluated and compared with smooth SAH. The analysis reveals that the entropy generated due to heat transfer is lower for the ribbed triangular duct compared to the smooth duct. It is also observed that the width of the rib plays a crucial role in minimizing heat losses to the environment. A maximum reduction of 43% and 62% in exergy losses to the environment and exergy losses due to heat transfer to the fluid is achieved, respectively, with a rib aspect ratio (e/w) of 4 compared to the smooth plate. It is found that in contrast to the smooth plate, ribs beneath the absorber plate effectively improves thermal and exergetic efficiency. Maximum enhancement of 36% and 17% is obtained in exergetic efficiency (ηex) and thermal efficiency (ηth), respectively, for e/w = 4, P/e = 10 and e/D = 0.04. Results also show the superiority of the ribbed triangular duct over the ribbed rectangular duct for an application requiring compact SAH with a higher flowrate.

Mathematical Model for Energy and Exergy-Based Simulation of Triangular Solar Energy Extractor for Air Heating Applications

“…e terms in equation ( 16) are evaluated using the relations as follows: Problems in Engineering e edge and bottom losses from the system are evaluated using the relation as follows [21]:…”

“…Step 9: e plate efficiency factor F′ is calculated by using the value of convective heat transfer coefficient value h c and the heat loss coefficient value of UL by using the relation shown below [21]:…”

“…Step 10: e outlet temperature based heat gain value (Qu 2 ) is calculated using the equation as mentioned below [21,22]:…”

“…Among these conditions, roughness pitch of 10 and angle of attack of 45 formulate better turbulence with a minimum formulation of eddy viscosity. Nidul et al [21] carried out exergetic performance using an analytical model for straight rectangular ribs and concluded that the enhancement in thermal and exergy efficiency by 36 and 17% for the rib aspect ratio of 4. Kumar et al [22] compared the exergetic performance of semicircular and square straight ribs and resulted that semicircular ribs enhance the performance by 26%.…”

A comprehensive review of experimental investigation procedures and thermal performance enhancement techniques of solar air heaters

“…The convective heat transport among the air and absorber plate is poor due to lower value of thermal conductivity of air, giving rise to higher temperatures of absorber plate and significant thermal losses to the surroundings, which ultimately result in low thermal performance of solar air-heating systems. 6 Moreover, the poor heat transfer coefficient due to the formation of viscous sub-layer (no-slip boundary condition) is another concern that results in low thermal performance. Various studies are carried out to improve the collector performance using different heat transfer enlargement techniques that introduce turbulence and increase heat transfer area, such as fins, 7 ribs, 8 baffles 9 and porous media (wire mesh screen layers, 7,10 limestone, iron scraps, and gravels 11 ) inside the collector.…”