Design Concepts for Low Aspect Ratio High Pressure Turbines for High Bypass Ratio Turbofans

The proposal is to identify cycle improvements and verify structural feasibility of shrouding a low aspect ratio High Pressure Turbine(HPT) rotor designed to use ceramic blades. When the clearance-to-span ratio between the rotating blades and the stationary casing is the same as the clearance-to-span ratio between the rotating shroud and the stationary casing, stage efficiency improves. However, shrouding rotor blades increases centrifugal stresses, and metallic HPT rotor blades are typically unshrouded in order to maximize stage output. Ceramic Matrix Composite(CMC) blades weigh much less than metallic blades. Shrouded CMC blades have lower centrifugal stresses than unshrouded metallic blades. The fuel burn reduction from an increase in stage efficiency due to shrouded HPT blades will be determined. The fuel burn reduction due to the higher temperature capability of CMC blades will also be determined. Cycle efficiency improvements from shrouding HPT rotor blades will increase for future engines. The HPT blade aspect ratio will decrease as engine Overall Pressure Ratio(OPR) increases. Future HPT blade aspect ratios may be less than half of current aspect ratios. While the absolute clearance may decrease in future engines, the relative clearance is likely to increase. Aerothermal analyses will determine the improvement in fuel burn from shrouding cooled HPT rotor blades. Structural analyses will determine stresses for unshrouded metallic and CMC rotor blades, and for shrouded CMC blades.