Turbulent mixed convection heat transfer for non-uniform heat flux distributions on a horizontal circular tube

Abstract

This study numerically investigated influence of nonuniform circumferential heat flux distributions boundaries on secondary flow, internal heat transfer and friction factor characteristics of a horizontal circular tube in turbulent mixed convection regime. A three dimensional steady-state numerical simulation for inlet Reynolds number of 3 030 to 202 400 was implemented on ANSYS Fluent version 14. The circumferential non-uniform heat flux distribution was simulated as a sinusoidal function of heat flux incident on the tube model. The k-ε model was used to simulate the turbulent flow of the heat transfer fluid through the tube model. A steel tube with wall thickness of 5.2 mm, length to innerdiameter ratio of 160 and thermal conductivity of 16.27 W/mK was used. The tube-wall heat conduction and the external heat flux losses via convection and radiation were also considered. It was found that circumferential spans of non-uniform heat flux distributions boundaries have significant effects on the buoyancy-driven secondary flow for Reynolds number range of 3 030 to 9 100. The Richardson number increased with the circumferential span of the heat flux boundary due to buoyancy-effects and the internal heat transfer coefficient was higher than where buoyancy-effect was neglected. Internal heat transfer coefficients and friction factors for non-uniform heat flux cases were found to be higher than the uniform heat flux cases. These revealed that at Re less than 9 100, secondary flow effects, heat flux intensities and heat flux distributions boundary type must be considered in determining internal heat transfer and friction factors characteristics of the tube. Internal heat transfer coefficients increased with fluid inlet temperatures, while friction factor decreased with an increase in fluid inlet temperatures. For Re above 9 100, internal heat transfer coefficients and friction factors are independent of secondary flow effects, heat flux intensities, circumferential spans of heat flux distributions and heat flux boundary type. This indicates that classical correlations are suitable for higher Reynolds turbulent flow, but in laminar and low Reynolds turbulent flow regimes, classical equations were not suitable for non-uniform heating.