Colburn j-factor in the transitional flow regime in a plain circular tube with twisted tape insert and square-edge entry

Abstract

The flow regime at which a fluid flows is very important in heat transfer engineering and also analogous to the rate of energy consumption in the domestic or industrial applications of heat exchangers. In this article, the influence of twisted tape insert on the heat transfer coefficients in the transitional flow regime was experimentally investigated and reported. A thin typical twisted tape of twist ratio of 5 was inserted into a plain circular copper tube with water as working fluid, square-edge entry and at a constant heat flux boundary condition of 2 kW/m2 over a Reynolds number range of 500 to 10 394 and Prandtl number range of 4.32 to 6.72. The transitional flow regime was easily identified and differentiated from laminar and turbulent flow regime by plotting the local heat transfer coefficients in terms of local Colburn j-factors against local Reynolds numbers in the fully developed flow region. The local station considered in this study had a length-to-diameter ratio of 141. The results showed that the transitional flow regime commenced at a local Reynolds number of 981 and ended at 1 447 for the tube with twisted tape insert. In the plain tube, which was used for validation, the critical Reynolds number was attained at a local Reynolds number of 3 005 and the transitional flow regime ended at a local Reynolds number of 3 318.