Experimental study on convective heat transfer and flow resistance characteristics of water flow in twisted elliptical tubes

Heat transfer and flow resistance characteristics of water flow inside the twisted elliptical tubes (TETs) with different structural parameters were experimentally investigated. Effects of tube structural parameters (aspect ratio and twist pitch) on the performance of TETs were analyzed and the overall thermal-hydraulic performance of TETs was evaluated. Experimental results showed that the TETs can provide considerable heat transfer augmentation and also high pressure drop inside tube. Larger tube aspect ratios and smaller twist pitches resulted in higher heat transfer coefficients and friction factors. The best operating regime for TETs is at lower Reynolds numbers. It was also discovered that the experimental Nusselt numbers/friction factors can be expressed with one unified equation for entire Reynolds number range, which confirms the early flow transition from laminar to turbulent in TETs. Experimental results were compared with some existing correlations, and the causes for the differences between them were analyzed. Heat transfer enhancement mechanism of TETs was discussed from the viewpoint of field synergy. The longitudinal vortex induced by the twisted tube wall improves the synergy between the velocity vector and temperature gradient, which in turn results in a better heat transfer performance. (C) 2011 Elsevier Ltd. All rights reserved.