Enhancing Heat Transfer in Turbulent Channel Flows: A Numerical Study on the Geometric and Orientational Effects of Delta, Trapezoidal, and Rectangular Winglet Vortex Generators

This paper presents a numerical study on the enhancement of heat transfer in a solar air heater (SAH) duct using winglet-type longitudinal vortex generators (WLVGs). Delta (DW), trapezoidal (TW1, TW2), and rectangular (RW) winglets are examined in pointing-up (PU) and pointing-down (PD) orientations, with the span-wise spacing ratio (S/H) varied to determine the optimal layout (S/H = 1.43, b/H = 0.50). Computational Fluid Dynamics (CFD) simulations using the GEKO turbulence model in ANSYS Fluent are performed for Reynolds numbers (Re) ranging from 5,000 to 22,500. Flow structures are analysed via Q-criterion isosurfaces, Nusselt number distributions, and streamwise vorticity contours. Results show that PU orientations generally outperform PD due to closer vortex–wall interaction. The RW configuration exhibits the highest Nusselt number, achieving a maximum Nusselt number of 114.37 at Re = 22,500, primarily attributed to the persistence of its generated vortices. However, it also results in the greatest frictional penalty, with a maximum friction factor of 0.1562 at Re = 5,000 and a corresponding normalized value of 𝑓/𝑓₀ = 5.21 at Re = 22,500. Consequently, the RW configuration yields the lowest thermal enhancement factor (TEF) at high Reynolds numbers, reaching a minimum value of 1.050 at Re = 22,500, despite its strong heat transfer rate. In contrast, the highest TEF is achieved with the PU TW1 configuration (TEF = 1.473 at Re = 5,000), which offers the most favourable balance between enhanced heat transfer and acceptable frictional losses. These results provide design-oriented implications for solar air heater (SAH) systems, identifying PU TW1 as the most energy-efficient configuration, whereas RW may be more suitable for applications where maximizing heat transfer rate is prioritised over minimising frictional losses.