Numerical Investigations on Film Cooling Performances

This paper presents a numerical investigation on film cooling of gas turbine endwall using commercial CFD software. The cooling effect is widely estimated with adiabatic wall assumption. The adiabatic effectiveness largely depends on film hole geometry. In this paper, three geometries of film hole with the same inclined angle and expansion angle have been investigated with the adiabatic effectiveness. Firstly, a fanshaped hole with laidback and sharp-angle exit has been put forward to compare with traditional fanshaped hole with round exit and fanshaped hole with laidback and round exit on the cooling performance. Through numerical results obtained by the ideal adiabatic film effectiveness, the comparison suggests that the new design can provide larger and more uniform coolant coverage. Considering two coupled effects in film cooling process, i.e. hot mainstream mixing with cooling air, and heat exchange between fluid and solid, the practicality of ideal adiabatic effectiveness and real film cooling effectiveness based on the coupled effect of fluid with solid were analyzed by two materials, with one being steel wall and the other a thermal insulator. The analysis indicates that it is not reasonable to estimate film effect only through consideration of the mixing effect, i.e. fluid flow performances under adiabatic assumption. To describe the second effect, the heat exchange between fluid and solid, the influence of material’s property on the film cooling effectiveness was quantitatively discussed at a given mainstream and coolant flow condition using seven thermal conductivities, and the relation of the film cooling effectiveness with the Biot numbers was exhibited along the center line at five points (x = D, 5D, 10D, 15D, 20D) behind film hole. Through the discussion, it can be found that there is possibility to describe the film cooling effectiveness at the given mainstream and coolant flow conditions using an exponential expression, in which the Biot number is an independent variable. This approach using an expression to describe the second effect may be more practical than using two parameters, adiabatic film effectiveness and heat transfer coefficient.