Local laminarization at negligible light gas injection into the compressible weakly accelerated boundary layer

Abstract

The paper presents a numerical modeling of a compressible xenon boundary layer over a permeable wall with various concentrations of helium in a helium-xenon mixture. We set zero intensity of the gas injection at the wall and a mild streamwise pressure gradient with acceleration parameter K = 4 × 10−7. A local laminarization may occur near the wall under such conditions. This phenomenon is not well-studied, especially in the helium-xenon mixture flow and has a noticeable effect on heat and mass transfer coefficients at the wall. The numerical model consists of the Prandtl equations, which describe the dynamic, thermal and diffusion processes within the boundary layer, and the k-ω-γ turbulence model, which is able to reasonably simulate a laminar-turbulent transition as well as a suppression of turbulence. In some simulated cases we blocked the turbulence model to obtain parameters of exclusively laminar flow. Also, the numerical model allows blocking the term of viscous dissipation in the energy equation and setting the constant temperature at the outer border of the boundary layer in order to evaluate a compressibility effect on the flow. Throughout, we applied finite-difference methods for solving the partial differential equations. The simulation demonstrated that a local laminarized flow may occur near the wall under the considered conditions, while an outer part of the boundary layer keeps a turbulent regime. Such character of the stream leads to levels of friction and mass transfer intermediate to laminar and turbulent flows at the same conditions. We obtained that the laminarized area is placed at y+<30, and this value does not depend on the helium concentration at the wall.