HEAT TRANSFER ENHANCEMENT DUE TO SWIRL EFFECTS IN OVAL TUBES TWISTED ABOUT THEIR LONGITUDINAL AXIS

Abstract

Periodically fully developed swirl flow and heat transfer in axially helically twisted tubes with elliptical cross sections are computationally modeled. The tubular geometry is described by its twist ratio y (ratio of 180°wist-pitch H to hydraulic diameter dh), and flow cross-section aspect ratio a (ratio of minor to major axis of ellipse). Constant-property flows of water (Prandtl number ~3.0) in the laminar Reynolds number regime (10 ≤ Re ≤ 1000) are considered for several different duct geometries (3.0 ≤ y ≤ 6.0 and 0.3 ≤ α ≤ 0.7). The parametric study delineates the effect of swirl on the velocity distribution, isothermal Fanning friction factor f, the temperature field, and Nusselt number Nu for a tube maintained at a uniform wall temperature (T condition). The dominant vortex structure has a spiral formation with a rotating fluid nucleus and two counter-rotating cells as spiral arms in the flatter section of the oval cross section. Both f and Nu are found to be highest for more tightly twisted tubes (y = 3.0) with a flatter elliptical cross section (α = 0.3). Moreover, relative to an equivalent straight oval tube, up to 2.5 times higher heat transfer rates can be achieved for fixed pumping power to render a more compact heat exchanger with a smaller volume or high surface-area density.