Effects of Freestream Turbulence on Air-Mist Film Cooling: Two-Phase Flow Simulations

Abstract

Air-mist film cooling is a potential technique to protect the surface of turbine vanes operating at high temperatures for improved thermal efficiency. The variation in the performance of air-mist film coolant is evaluated here for a wide range of freestream turbulence (0.2 to 10%) like the operating condition of the gas turbine. The investigated domain consists of a flat plate with a series of discrete holes of 35° streamwise orientation and connected to a common delivery plenum chamber via a pipe of diameter D = 12.7mm. A two-phase mist consisting of finely dispersed water droplets of 10.0μm in an airstream at a mist concentration of 3.0% is introduced as a secondary flow. The blowing ratio and density ratio are 0.5 and 1.2, respectively, where the Reynolds number based on the diameter of the hole is 1.0 × 104. The Reynolds Averaged Navier Stokes equation in the Eulerian-Lagrangian frame is used to simulate the two-phase flow by ANSYS Fluent 15.0 with the k-ε realizable model. The simulation resolves the mean thermal-flow field and dynamics of droplets. The injected droplets in the crossflow behave like a small heat sink as they evaporate while advecting downstream and are expected to provide improved protection of the heated surface. High turbulence intensity enhances the mixing of droplets with the crossflow, thereby improving the spanwise diffusion of droplets. Reduction of the strength of the counter-rotating vortex pair is also evident. The area-averaged film cooling effectiveness increases by 21.5%, with an increase of turbulence intensity from 0.2 to 10%. However, the increase in aerodynamic losses is almost as high as 39%.