A study of turbulent heat transfer in curved pipes by numerical simulation

Turbulent heat transfer in curved pipes was studied by numerical simulation. Two curvatures delta (pipe radius a / curvature radius c) were considered, 0.1 and 0.3; results were also obtained for a straight pipe (delta=0) for comparison purposes. A tract of pipe 5 diameters in length was chosen as the computational domain and was discretized by finite volume multiblock structured grids of ~5.3x10^6 hexahedral cells. Fully developed conditions were assumed; the friction velocity Reynolds number was 500, corresponding to bulk Reynolds numbers between 12 630 and ~17 350 according to the curvature, while the Prandtl number was 0.86. Simulations were protracted for 20 LETOT’s a/utau; the last 10 LETOT’s were used to compute first and second order time statistics, including rms fluctuating temperature and turbulent heat fluxes. In curved pipes, time averaged results exhibited Dean circulation and a strong velocity and temperature stratification in the radial direction. Turbulence and heat transfer were strongly asymmetric, with higher values near the outer pipe bend. In the outer region, counter-gradient heat transport by turbulent fluxes was observed. For a given friction velocity Reynolds number, overall turbulence levels were lower than in a straight pipe; nevertheless, heat transfer rates were larger due to the curvature-induced modifications of the mean flow and temperature fields.