Wake-separation bubble interactions in low Reynolds number turbomachinery

• Main Author: Howell, R. J.
• Published: University of Cambridge 1999
• Subjects: 629.13
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.604667

There is a continual demand for a reduction in the cost and weight of aero-engines. Suppliers of engines always need to reduce manufacturing costs and airlines want to carry more passengers or cargo. This thesis will show that reducing the number of blades in the low pressure turbine is a potent way of achieving both of these goals. Through a program of experimental and numerical work, it is shown how it is possible to reduce the number of blades in the turbine by approximately 15% relative to the first generation of high lift blading employed in the BMW Rolls-Royce BR715 low pressure turbine. A series of measurements from surface mounted hot films are shown from two full scale BMW Rolls-Royce LP turbines. These measurements define the current state of the art of LP turbine blade design. It is demonstrated that the suction side boundary layer flow is entirely dominated by the passage of wakes from upstream blade rows. Low speed measurements were carried out on a linear cascade of highly loaded low pressure turbine blades (designated TL10) similar in style to those used for the BMW Roll-Royce tests. The rig used for these tests incorporated a moving bar wake generator to simulate the presence of a single upstream blade row. Reductions in loss were measured when wakes were present compared to the case with steady inflow at low Reynolds numbers. A novel technique was used to increase the blade loading to above the levels of the TL10 profile. The loss reduction of a number of suction side pressure distributions were then investigated for cases with and without incoming wakes. Loss reductions were again observed when wakes were present and this reduction in loss increased as the lift of the profiles increased. Hot film measurements indicated that moving the position of boundary layer separation aft will decrease the losses generated. Further measurements proved that aft loaded profiles performed better than forward loaded profiles with unsteady inflow. The key to the loss reductions was the interaction of the wakes (and the turbulent spots that they form) with the separation bubble.