Reducing the Demand of Coolant at the Sidewall of a High Pressure Turbine Cascade by Means of Slot Width Modulation

Within the framework of the German research project AG-Turbo the influence of slot flows associated with cooling configurations were investigated experimentally. One interesting result of this work was the severe enhancement of the horse shoe vortex by the coolant dependent on the slot’s position and the ejected mass. Based on these results, further promising film cooling configurations with slot ejection were investigated numerically. Setting up the numerical studies was a challenging task since a very detailed model of the slot, the supply cavity, and the supply tubes had to be generated. Tests proved that smallest deviations against the experiment resulted in completely different flow structures. Nevertheless, the secondary flow structure as well as the aerodynamic parameters could be modeled excellently. According to the velocity field in front of a plane turbine cascade, higher wall pressure arises upstream of the leading edge region of the blades while the pressure drops to a minimum in the center of the passage. To avoid excessive ejection of coolant in the low pressure region the reduction of the local slot area is an appropriate method to decrease the slot mass flow in this particular area. An iterative method is introduced considering the desired mass flow rate and the prevailing total pressure in order to calculate the slot area and to avoid reverse flow. The variation of the film cooling effectiveness indicates a very uniform distribution of cooling air behind the slot which persists within the passage.