Heat Transfer and Pressure Drop Studies Through a Square Duct Fitted With Increasing and Decreasing Order of Twisted Tape

Experimental investigation of heat transfer and friction factor characteristics of a square duct fitted with an increasing and decreasing order of twist ratio set have been studied under a nearly uniform wall temperature condition for laminar flow using water as test liquid. The Reynolds number was varied from 100 to 2100. Cold water flows through the inner square duct, and hot water passes through the circular annulus, in countercurrent fashion. There is not much change in the magnitude of heat transfer coefficient enhancement with increasing twist ratio and with decreasing twist ratio set, as the intensity of swirl generated at the inlet or at the outlet in the order of increasing twist ratio or decreasing twist ratio is the same in both the cases. The experimental results show that Nusselt numbers were found to be 1.33-4.27 and 0.9-3.46times the plain square duct forced convection values based on constant flow rate and constant pumping power criteria, for increasing and decreasing order of twist ratio sets, respectively.