Mixed and forced convection heat transfer and pressure drop in inclined tube with twisted tape in transition flow.

Understanding the influence of angular orientation on mixed convection heat transfer and pressure drop in circular tubes with swirl generators is crucial for optimizing thermal performance in various engineering applications, including heat exchangers and energy systems. This study aims to investigate the impact of angular orientation on heat transfer enhancement and pressure drop characteristics in a uniformly heated circular tube equipped with a twisted-tape longitudinal swirl generator. An experimental approach was employed, varying key parameters such as Reynolds number (435-10,130), heat flux (2, 3, and 4 kW/m²), twist ratio (P/D = 3, 4, and 5), and angular orientation (15° and 30°). The setup was validated against established correlations for Nusselt number and friction factor, demonstrating strong agreement. The results reveal that angular orientation significantly affects heat transfer and pressure drop at Reynolds numbers up to ~ 1000, where mixed convection plays a dominant role. Beyond this, forced convection prevails. In a plain channel, the transition from laminar to transitional flow occurs at Reynolds numbers of 2924-4088 for a 15° angle of inclination (AoI) and 3001-4274 for a 30° AoI, with the transition occurring slightly earlier at the lower angle. The Richardson number varied from 2.5 in the low laminar regime to 0.0087 in the turbulent regime, with additional variations observed when turbulators were introduced.