Heat Transfer and Pressure Drop of Periodically Fully Developed Swirling Laminar Flows in Twisted Tubes With Elliptical Cross Sections

Abstract

Periodically fully-developed swirling laminar flows in twisted tubes with elliptical cross sections are computationally simulated. The tubes are helically twisted and their geometry is described by the 180° twist ratios y of 3.0, 4.5 and 6.0, and ellipse cross-section aspect ratio of 0.7. Constant-property flow of water (nominal Pr = 3.0) with a Reynolds number range of 10 to 1200 is considered. The analysis quantifies the improvement in the Nusselt number as well as the increase in friction factor in order to map the effective heat transfer enhancement due to the twisted-tube-geometry-induced swirl flows. To this effect, the numerical results are compared with the baseline cases having a twist ratio of y = infinity, or straight elliptical cross-section tubes for which well established correlations are available. Numerical results show that the friction factor and the Nusselt number are a strong function of the twist ratio and the Reynolds number. The increase in fRe and Nu is higher as the twist becomes tighter (lower values of y). For Reynolds numbers below about 100, the heat transfer results do not deviate significantly from the straight-tube values, but at higher values of Re, significant enhancement in heat transfer is evident for all twist ratios considered here. The friction factor and Nusselt number results provided in this paper will help practicing engineers in integrating twisted elliptical tubes in various heat transfer applications.