Heat transfer enhancement in rectangular ducts with wire coil inserts: Applications in building-integrated CPVT systems

This study evaluates the thermal and hydraulic performance of rectangular ducts equipped with wire coil inserts under non-uniform heat flux conditions, replicating realistic operating environments in building-integrated Concentrated Photovoltaic Thermal (CPVT) systems. The aim is to mitigate localized overheating by improving heat removal from the photovoltaic cells. Experiments were carried out on a 2.5  m long duct with a 23 × 8 mm² cross-section, exposed to concentrated solar fluxes of 16,667 and 25,000  W/m². Water was used as the working fluid, and three wire coil configurations, varying in pitch (7.5–19.3  mm), thickness (0.6–1.4  mm), and diameter (7–7.5  mm), were tested. Performance metrics including PV surface temperature reduction, Nusselt number, and Fanning friction factor were assessed over a wide Reynolds number range (700–20,000). Results show that heat transfer was enhanced by up to 77 %, with a corresponding wall temperature reduction of up to 6 °C. Throughout the entire Reynolds number range, the inserts promoted early turbulence, effectively suppressing the laminar-to-turbulent transition regime observed in the smooth duct. This led to more stable turbulent flow conditions. Among the tested configurations, an optimal insert geometry was identified, offering the best compromise between enhanced heat transfer and pressure drop penalty. Overall, the findings confirm the effectiveness of passive techniques for improving thermal regulation in CPVT systems and address a notable research gap by combining non-circular duct geometry with non-uniform heating, an operating condition rarely explored in existing literature.