Exploring the impact of inclination angle on heat transfer and pressure drop characteristics in solar air heaters equipped with rectangular flap barrier at inlet

Abstract

The upgrading of existing systems in industries, along with the limited space for modern equipment, often requires engineers to arrange thermal systems in inclined or vertical positions. Changing the orientation of heat exchangers can have significant consequences, such as higher pumping costs and reduced performance. The present experimental assessment focuses on the influence of inclination angles, heat flux, and geometrical parameters of inlet flaps on heat transfer and fluid flow characteristics inside a solar air heater. The Reynolds number varies from 494 to 6965. Experiments are carried out with two heat fluxes, 0.5 and 1 kW/m2. The geometric configuration of a rectangular inlet flap is characterized by two nondimensional parameters: height ratio (h = 0.75, 0.50, and 0.25) and width ratio (w = 0.25, 0.20, and 0.15). For a plain tube with a 15° inclination and 1 kW/m2 of applied heat flux, the transition initiates at a Reynolds number of 2666 and terminates at a Reynolds number of 4025. For a plain tube with a 30° inclination, the range of critical Reynolds numbers is between 2764 and 4139. With the inlet flap barrier under similar operating conditions, the transition starts and ends earlier than in the plain tube. Additionally, a significant increase in the Nusselt number and friction factor is observed when the inlet flap is placed in the channel. At 15°, the transition begins earlier, while at 30° inclination, the transition is slightly delayed. Novel correlations for the Nusselt number and friction factor are developed to predict heat transfer and pressure drop for unknown parameters.