Direct numerical simulation of forced thermal convection in asymmetrically heated channels*

We carry out direct numerical simulation (DNS) of turbulent channel flows at different Prandtl numbers and up to friction Reynolds number Re_τ≈2000, considering flow cases with both symmetric and asymmetric heating[1-2]. The latter configuration has been studied considerably less, although it is certainly the most relevant for engineering applications, where heating is often concentrated on one wall. We show that one-sided heating modifies the temperature fluctuations in the outer layer, where we observe large-scale eddies extending well beyond the channel half height. We show that also in the case of one-sided heating the temperature profile exhibits a logarithmic layer and a wake region, which can be approximated using a parabolic profile. We use these universal features of turbulence to derive analytical approximations for the Stanton number, and to quantify the thermal efficiency of one-sided-heated channels compared the idealized symmetric cases. We find that the thermal efficiency is reduced by up to 30% at low Prandtl number, whereas the increasing relevance of turbulent convection tends to level off the differences at higher Prandtl number, with a heat transfer reduction of about 10% at Pr=4.