Transitional mixed convection in inclined solar air heater tube with wavy inserts

Abstract

This study experimentally explores the influence of inclination angle and wavy tape inserts on mixed convection heat transfer and pressure drop in a solar air heater tube. Experiments at 15° and 30° inclinations, with Reynolds numbers from 455 to 6059 reveal that increasing inclination reduces heat transfer and pressure drop. Transition Reynolds numbers vary with heat flux levels: 3 kW/m2 (2528-3793), 2 kW/m2 (2520-3801), and 1 kW/m2 (2627-3848). The Colburn factors in the laminar and transition flow regimes shows distinct characteristics but converge in turbulent flow. The wavy tape with a width ratio of 0.8 and a wave ratio of 0.75 exhibits the highest Colburn factor and friction factor, while a width ratio of 0.6 and a wave ratio of 2.25 lead to minima of these values. With increase in the inclination, the effect of buoyancy forces starts decreasing. This leads to reduced disturbance compared to a lower angular position. Novel correlations were also developed for predicting the Colburn and friction factors.