NUMERICAL SIMULATION TO INVESTIGATE EFFECT OF DOWNSTREAM GROOVES ON FILM COOLING EFFECTIVENESS OF GAS TURBINE BLADES

This paper presents a new design concept to increase the adiabatic effectiveness of cylindrical holes used for film cooling in gas turbine. Rectangular grooves are created at the downstream of each cylindrical hole. The grooves are incorporated into the film cooling system to reduce the adverse effects of the kidney vortices since the kidney vortices reduce the effectiveness of circular cross-section film cooling holes at moderate to high blowing ratios by inducing jet lift-off. The groove shape is defined by two geometric parameters viz. width and depth. A single row of five discrete film cooling holes on a flat plate with an inclination angle of 30° along streamwise direction and pitch to diameter ratio of 2 was chosen as the baseline test case. In this study, cooling effectiveness curves obtained by four grooved plates having different groove configuration is compared amongst themselves and with the simple plate. Numerical simulations have been performed at three different blowing ratios of 0.5, 1.0 and 1.5 for each case of the rectangular groove and simple plate by using CFD technique (ANSYS Fluent) and the flow field is solved by using k-ε realizable turbulence model. The results showed that the lateral averaged cooling effectiveness is increased remarkably when the downstream rectangular grooves are present. This increase is because of the fact that, in grooved plates, majority volume of the coolant is flowing within the grooves, so it is properly guided and protected by the grooves. This reduced the turbulent mixing between mainstream and coolant flow. Another reason is that side walls of the groove do not allow hot mainstream gasses to enter underneath coolant jet from the sideways. This reduced the jet lift-off and improved the cooling effectiveness. Apart from this, the effects of each geometrical parameter of the groove on the film cooling effectiveness were studied in detail and observed that average cooling effectiveness distribution is higher for grooves with least aspect ratio for low blowing ratio.