Towards a Quieter Low Pressure Turbine: Design Characteristics and Prediction Needs

Abstract

This article examines Low Pressure Turbine (LPT) noise in modern aircraft and shows how difficult it is to design a quiet LPT. LPT noise has long been neglected in terms of research and prediction capability due to it being considered of lesser importance. Recent developments in LPT design have led to what is termed a “turbine noise storm,” in which the LPT is designed to be cut-off but in the end there is significant LPT noise and little or no ability to reduce it. These developments include the trend toward lower blade counts and solidities. The situation is further hurt due to the inability to predict LPT noise, particularly for the new type of designs. The inability to reduce the LPT noise once the engine and airplane is designed is due to there being no available light-weight high-temperature acoustic-lining technology that can be added to fix a LPT noise problem without a significant penalty. In an attempt to understand what makes some engines' LPT quiet while others are very loud, modern turbine designs are characterized into several LPT noise “classes” and important aspects of each class are shown. Finally, the need for a semi-empirical prediction is discussed including what essential elements this prediction needs to have to be relevant to modern aircraft preliminary design.