Heat transfer of transitional regime with helical turbulence in annular flow

Direct numerical simulations of the passive heat transfer in pressure-driven flows through concentric annuli were performed in the subcritical Reynolds-number regime. In this regime, the intermittent-turbulent velocity field would exhibit large-scale patterns of the laminar-turbulent coexistence. The main emphasis was placed on the influence of transition structures on the heat transfer at two radius ratios, r_in/r_out=0.5 and 0.8, at which point a helically-shaped, turbulent transition structure was present. An incompressible Newtonian fluid was considered with fixed fluid properties and a Prandtl number of 0.71. We investigated the dependence of this heat-transfer on the Reynolds number, radius ratio, and thermal bounding condition. The state of helical turbulence was found to provide high heat-transfer rates close to those estimated by the turbulence empirical function even in the transitional regime. The present results showed that the wall-normal turbulent heat flux occurred in both longitudinal-vortex clusters around the turbulent bands as well as inside of the localized turbulent region. The non-turbulent vortex cluster provided a promotion of heat transfer that occurs in intermittent turbulent states.