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Film cooling is one of the preferred methods for effective cooling of a gas turbine that forms a protective layer between hot flue gases and blade surface. This paper investigates the interaction of mist in the secondary flow and physics indicating the upper limit of mist concentration. Numerical simulations are performed on a flat plate having a series of discrete holes with 35 degree streamwise orientation and the holes are connected to a common delivery plenum chamber. The blowing ratio, density ratio and Reynolds number based on freestream and hole diameter (D) are 0.5, 1.2 and 15885 respectively. A two-phase mist consisting of finely dispersed water droplets of 10 micron in an airstream is introduced as the coolant from these holes. The latent heat absorbed by the evaporating droplets significantly reduces the sensible heat of the main stream, providing heat sinks that result in enhanced cooling effectiveness. The coupling between the two-phases is modelled through the interaction terms in the transport equations. Computations are performed by ANSYS Fluent 15.0 using k-ε realizable model. The results illustrate insight of complex transport phenomena associated with the mist of varying concentration from 2% to 7%. It has been observed that the maximum enhancement of cooling effectiveness reaches 43% at X/D = 10 for 2% mist by mass with an average enhancement of 26.5%. For 3% mist, the maximum enhancement becomes 80% at X/D = 16 with the average cooling enhancement of 43%. Mist concentrations 5% and beyond trend to increase average cooling because of more absorption of latent heat by droplets, but its trajectories shift towards wall, detrimental to the blade due to corrosion effect and thermal stresses.

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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%.