Influence of texture on thermal transport in streamwise-aligned superhydrophobic turbulent channels

Abstract

A series of Direct Numerical Simulations (DNS) is performed for a systematic study of thermal transport in a periodic turbulent channel bounded by superhydrophobic surface (SHS) walls at a friction Reynolds number of Reτ=180. The SHS examined here is comprised of interspersed ridges and cavities aligned along the mean streamwise flow direction. The SHS is modelled as a planar surface consisting of spanwise-alternating regions of free-shear and no-slip boundary conditions. Water is considered as the bulk fluid where the SHS surface is assumed to be in a Cassie-Baxter state with non-wetting cavities containing air. The ridges are maintained at a constant temperature while heat transfer through the air/water interface supported by the ridges is assumed negligible and is modelled as adiabatic. Phase averaged statistics were obtained for a range of relative periodicity widths. Reorganisation of secondary flow structures owing to an increase in SHS feature width and its effect on thermal transport are analyzed with a particular focus on phase-averaged statistics. The relative feature width or periodicity length is found to influence strongly on thermal performance, or the average Nusselt number. A reduction in the turbulent Prandtl number is observed in the buffer region due to an increase in thermal eddy diffusivity relative to the momentum eddy diffusivity. The amount of heat transfer and mixing depend upon the periodicity length, and there is a reduction in the heat transfer along with the drag-reduction. A substantial decrease in the temperature fluctuations is observed in the vicinity of the wall as the production of temperature variance is diminished with an increase in the periodicity length, primarily due to a reduction in the mean temperature gradient in the wall-normal direction.