Experimental Investigations of Heat Transfer in Simultaneously Developing Transitional Regime of Mixed Convection Flows in a Vertical Tube

The study of flow behavior in the simultaneously developing transitional regime of mixed convection flows is rare. It has been believed that the transitional regime will give a good compromise between pressure drop and heat transfer compared to laminar and turbulent flow regime. In this experimental study, the friction factor (f) and Nusselt number (Nu) characteristics for buoyancy-assisted and opposed flows of water in concurrently developing transitional regime of mixed convection through a vertical tube are studied. Experiments were done for Reynolds numbers (Re) varying from 500 to 15,000, Grashof numbers (Gr) from 1.25 × 10⁴ to 5 × 10⁶, Prandtl numbers (Pr) from 3 to 7, and Richardson numbers (Ri) from 0 to 0.1 subjected to uniform heat flux boundary conditions. A flow visualization provision after the test section which confirms an early transition in buoyancy-opposing flow (⁠$Rec$= 2264) compared to buoyancy-aiding flow (⁠$Rec$ = 2468) at a fixed Ri of 0.1. Further, with the increase in Ri from 0 to 0.1, the average f decreases, and the average Nu increases in both aiding and opposing flows. It confirms that the onset of transition gets delayed with the increase of heat flux supplied in both the flows. Based on the present outcomes, an efficient heat exchanging device can be operated either to delay or advance the transition in a vertical pipe flow for optimum heat transfer.