High Lift Low Pressure Turbine Blades

Introduction

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 a more passengers or cargo. These web pages show how it was 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 film anemometers were carried out on two full scale BMW Rolls-Royce LP turbines. These measurements defined the current state of the art for LP turbine blade design.

Low speed measurements were carried out on a linear cascade of highly loaded low-pressure turbine blades (designated the DATUM profile) similar in style to those used for the BMW Rolls-Royce tests. The rig used for these tests incorporated a wake generator to simulate the presence of a single upstream blade row. The wakes are shed from moving bars. 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 production 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 where 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 decreased 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.

To further understand the interaction of the separation bubble with wakes and turbulent spots, a series of experiments were carried out in a large scale flat plate rig with an imposed low pressure turbine pressure distribution. Artificial turbulent spots were generated upstream of the separation bubble. The detailed interaction of these spots with the separation bubble were investigated using hot film anemometers and hot wire anemometers.

As a proof of this research, a very highly loaded, low-pressure turbine profile was designed and then tested in cascade. The new profile produced losses only marginally higher than those for the datum profile with unsteady inflow. However, the profile resulted in a 15% reduction in blade numbers. Overall, there is a net benefit as there is a substantial reduction in the number of blades required in the low-pressure turbine.

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Howard Hodson, Robert Howell, Ramesh and Rory Stieger

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