High Stage Loading Low Pressure Turbines: A New Proposal for an Efficiency Chart

Abstract

In 1999, ITP (Industria de Turbopropulsores, S.A) launched a wide on-going research and development program to advance the state of the art in Low Pressure Turbines for use in future high bypass ratio aeroengines for long range civil transport. The objectives of the program were very aggressive in order to satisfy the market demands. Enormous cost and weight savings (about 30% off) were required, equalling or even improving the efficiency and noise emissions. Through that program, ITP has continuously reduced the components of the turbine, as the most adequate way to achieve the previous targets. As result of that reduction, High Stage Loading Turbine technology has being developed. High stage loading turbines have less number of stages to produce the same work output (high stage loading), fewer numbers of blades to perform the same duty (high and ultra high lift blades) and low rotational velocities. An overview of the two options of high stage loading turbines, high through flow and low through flow, is introduced and how both approaches allow an important reduction in weight and number of components (stages and blades). It is also shown that keeping the same levels of efficiency and noise than conventional turbines, is a technical challenge that demands improved aerodynamics or/and higher rotational velocity. This paper describes the current technology limits that are exceeded by high stage loading turbines and the special aerodynamic and geometrical features that come up. Special attention must be highlighted when the stage loading is increased over 50%. In those cases the Mach number limit is exceeded and the pressure ratio per stage is risen as much than transonic blades can not be avoided. Even on those extreme cases the pressure ratio levels and referred work are still far from typical values of high-pressure turbines and geared low-pressure turbines, and considering the high efficiencies achieved for those last two types of turbines, someone could wonder whether a specific research program is required. To conclude the paper, a new efficiency chart, alternative to Smith Chart, is presented to support the answer to the previous reasonable question.