Swirling hot air is a promising heat transfer enhancement technology for anti-icing applications in aircrafts, where the swirling flow is accompanied by pretty high turbulence and a quite thin boundary layer. It is of interest to investigate the compound heat transfer characteristics of the swirling air configuration combined with surface structures on the internal wall. This paper carries out a series of numerical computations to obtain the Nusselt number and pressure loss data in such a swirling air heat transfer system with four kinds of surface structures (trenches, ribs, dimples and bulges) on the wall and with different tangential inlet jets placed along the tube. At a tube Reynolds number from 10,000 to 50,000, the results show that the surface dimples and bulges are conducive to improving the Nusselt number, but the surface trenches and ribs show a Nusselt number deterioration relative to the smooth swirl tube. Among the four investigated surface structures, the surface bulges perform best, which can enhance the Nusselt number by up to 15.0%, increase the total heat transfer quantity by up to 17.3% and reduce the hot air pressure loss by up to 15.6%. Furthermore, the circumferential velocity distribution and swirl number are introduced to describe the flow fields. The surface trenches and ribs lead to less of a reduction in the circumferential velocity and swirl intensity, while the surface dimples and bulges could significantly suppress the in-tube swirl intensity.

Heat transfer and pressure loss characteristics of a novel Vortex rib in a cooling channel have been studied both experimentally and numerically under the Reynolds numbers from 10,000 to 80,000 and rotation numbers from 0 to 0.385. The Vortex rib is proposed by combining W-rib and W-patterned oblong dimple. Compared with the W-rib, the Vortex rib enhances heat transfer by up to 11.4% (Re = 10,000 and Ro = 0.31) while reducing the pressure loss by up to 18.7% (Re = 32,000 and Ro = 0) under non-rotating and rotating conditions. Compared to the W-patterned oblong dimple, the heat transfer of the Vortex rib increases by up to 47.9%. The heat transfer dead zones are significantly improved by near-wall vortex induced by Vortex rib. The Vortex rib can appreciably reduce the turbulent mixing intensity in the mainstream of the cooling fluid but keep the near-wall high turbulent mixing intensity, guaranteeing the turbulent heat transport capacity. Thus, the heat transfer can be improved while maintaining the pressure loss in the channel.

The paper presents a numerical study of the heat transfer, pressure loss and flow characteristics of swirl cooling in elliptical tubes, which are compared to the counterpart of swirl cooling in a circular tube with a diameter of D=50.0 mm under equal passage Reynolds numbers and equal jet Reynolds numbers. The swirl tubes with two kinds of fixed tube length of 12D and 20D are compared, where there are sequentially arranged three tangential jet inlets over the leading tube length of 12D. The numerical results show that the swirl tubes with the tube length of 12D has a much better heat transfer performance. Under equal passage Reynolds numbers, the elliptical swirl tubes with the tube length of 12D show appreciably higher Nusselt numbers by up to 22.8% and lower pressure loss coefficients by up to 69.0% than the circular tube. Under equal jet Reynolds numbers, the elliptical tubes can reduce the global heat transfer performance modestly by up to 25.6%, but reduce the pressure loss much significantly by up to 70.6%. Mostly due to much less pressure loss, the elliptical tubes have remarkably higher thermal performance in terms of the obtained heat transfer coefficient per unit pumping power for both L₁=12D and L₂=20D. The numerical simulations indicate that the suppression of elliptical tubes on the swirling flow development reduces the heat transfer on the wall between the jet inlets, and decreases the wall shear force and the pressure loss in the tube.