Experimental and numerical study of fluid flow and heat transfer in an annulus of inner twisted square duct and outer circular pipe

Abstract

Heat transfer and friction factor characteristics of air flow in an annulus formed by an inner twisted square duct and an outer circular pipe is studied experimentally for Reynolds number range of 400–60,000. Experiments were conducted with air as working fluid. A uniform wall temperature at the inner wall of annulus was maintained while the outer pipe was kept insulated. Twist ratios (ratio of the pitch to outer width of twisted square duct) of 10.6 and 15 were used in the experiments. The transitional Reynolds number for laminar flow to turbulent flow was identified as 3000. Results were compared with the flow inside the annulus of an equivalent straight square duct in a circular pipe for identical pumping power. Results showed considerable enhancement in the heat transfer and pressure drop in both the laminar and turbulent flow regimes. The influence of the annulus parameter on the friction factor and heat transfer was also studied by varying the outer pipe diameter. At a given twist ratio, higher values of the friction factor and the Nusselt number were observed for smaller annulus parameters (ratio of cross section area of square twisted duct to cross section area of circular pipe). In order to extend the experimental results to a larger range of fluids, a numerical analysis was undertaken for fully developed flow inside the annulus for the experimental twist ratios. The parameters range covered in the simulations were Reynolds numbers of 100–100,000 and Prandtl numbers of 0.7–20. The local velocity distribution across the annulus cross-section were analysed. Empirical correlations for friction factor and Nusselt number are provided separately for laminar and turbulent regimes. Guidelines for selection of twisted square duct are provided in terms of Reynolds number and enhancement factor. The results are significant because it will contribute to the development of compact double pipe heat exchangers.