Phase-Locked PIV Investigation of the Effects of the Blowing Ratio of a Pulsed Vortex Generator Jet in a Low-pressure Turbine

At very high altitudes the Reynolds number flow through the low pressure turbine section of the gas turbine engine can drop below 25,000. At these low Reynolds numbers the flow is laminar and extremely susceptible to separation which can lead to increased losses and reduced lift. Small jets of air injected through the suction surface of the airfoil, called Vortex Generator Jets (VGJs), have been shown successful in suppressing separation and maintaining attached flow. Pulsing of these jets has been shown to be as effective as steady jets while reducing the amount of mass flow needed. An experiment using Particle Image Velocimetry (PIV) was set up to study the interaction of the VGJ flow with the main flow. A cascade of Pratt and Whitney Pack-B turbine blades were mounted in the test section of a low speed wind tunnel at Wright Patterson Air Force Base. On the middle six blades were rows of 1mm VGJ holes. The VGJ holes were oriented with a 30o pitch angle and 90o skew angle. The pitch angle is the angle the jet makes with the surface of the turbine blade while the skew angle is the angle the jet makes with the cross-flow. Blowing ratios, a ratio of the jet velocity to the cross-flow velocity, of 0.5, 1, and 2 were examined. These three blowing ratios were selected because they represent when the cross-flow momentum dominates the fluid interaction (B=0.5); when the momentums of the jet flow and cross-flow are equal (B=1); and when the momentum of the jet flow dominates the interaction. Blowing ratios of 0.5 and 1 were studied for pulsing frequencies of 10Hz and 0.4Hz while the blow ratio of 2 was studied only with 10Hz pulsing. A duty cycle of 50% was used for both pulsing frequencies. The two pulsing frequencies allowed data to be taken to show how the pulsed VGJ maintains attached flow (10Hz) and how the pulsed VGJ suppresses the separation bubble (0.4Hz). Results show that jets interacting with separated flow are able to suppress the separation bubble almost immediately for a blowing ratio of 1 and 0.5. The results for suppression and separation growth show the response of the crossflow is very similar in magnitude and timing between the two blowing ratios. The results for the 10Hz pulsing frequency show blowing ratios of 0.5, 1, and 2 are effective. A blowing ratio of 2 is undesirable because it carries more momentum than is needed and would therefore use more massflow than the B=1 or 0.5 case. Results from the B=0.5 case suggest that a blowing ratio of 0.5 is near the minimum effective blowing ratio