RICERCA ITALIANA     

Trailing edge cooling concepts of high temperature gas turbine blades

Università degli Studi di Firenze

Abstract

During last years gas turbine development brought to the production of both more efficient and lighter aeroengine and heavy duty system with efficiency close to 60% (combined cycle). One of the most important elements of such success has been the constant increase in maximum cycle temperature, consequence of technological progress in casting techniques and materials and especially in improved cooling systems. The aim of this research program is an aero-thermal investigation of the airfoil trailing edge region. The design of this region is very critical because two opposite requirements: trailing edge thickness must be as small as possible in order to reduce aerodynamic losses, but the presence of a cooling system requires to increase such thickness. Thus reaching good performances in these areas is particularly hard: available space is limited and trailing edge temperature has to be strictly controlled, being it often subjected to damages. Additional problems are encountered in rotor blade cooling. Basic studies are commonly able to give significant evaluations of performance of single cooling systems with turbulators or film cooling, whereas actual design process requires the estimation of the global behaviour of several combined systems; moreover, theirapplication in rotor blades obliges to take into account centrifugal forces.
Both aero-thermal and heat transfer phenomena are to be investigated, performing tests in static and rotating models, including the analysis >>>

Principal Investigator

Bruno Facchini Università degli Studi di FIRENZE

Research Objectives

During last years gas turbine development, both in aeronautic and heavy duty field, has increased much more than other combustion
engines based on fossil fuels. Such development allowed the achievement of some important goals:
- the highest energy system efficiency (about 60% in combined cycles)
- very high specific work, and then a reduction in the number of components and in weight of aeroengine
- the lowest exhaust emission with respect to all kind of combustion engines.
One of the most important elements of such success has been the constant increase in maximum cycle temperature, i.e. turbine inlet
temperature (TIT). Such enhancement is the consequence of technological progress due to the research activity carried on, often
together, by industrial world and university. In order to enhance TIT, new casting techniques and materials were set up and
especially improved cooling systems were introduced and developed. Up to now such research activities were carried on, mainly, by
North American and English universities and research centres, while the contribution of Italian research activity is limited, as it can
be seen in the open literature. On the contrary, there are three Italian factories, with international relevance, working in the gas
turbine field. Then it is important for Italian university to develop specific knowledge in cooling systems for gas turbine airfoils, in
order to avoid >>>

First Results

During last years gas turbine development, both in aeronautic and heavy duty field, has increased much more than other combustion engines based on fossil fuels. Such development allowed the achievement of some important goals:
- the highest energy system efficiency (about 60% in combined cycles)
- very high specific work, and then a reduction in the number of components and in weight of aeroengine
- the lowest exhaust emission with respect to all kind of combustion engines.
One of the most important elements of such success has been the constant increase in maximum cycle temperature, i.e. turbine inlet temperature (TIT). Such enhancement is the consequence of technological progress due to the research activity carried on, often together, by industrial world and university. In order to enhance TIT, new casting techniques and materials were set up and
especially improved cooling systems were introduced and developed.
This research program will point out some specific aspects of blade cooling; the aim of this research program is an aero-thermal investigation of the airfoil trailing edge region. The design of this region is very critical because two opposite requirements: cascade aerodynamic and cooling system. Thus reaching good performances in these areas is particularly hard: available space is limited and trailing edge temperature has to be strictly controlled, being it often subjected to damages.
The investigation will be >>>

Timescale

24 months

National and international background

The increase of gas turbine performances, in terms of efficiency and reliability, is the main goal for industry. During last years the usage of gas turbine has increased, especially for electric and mechanic power generation [1-I]. Cycle efficiency increases if pressure ratio and maximum cycle temperature and/or turbine inlet temperature (TIT) increase too [2-1]. Such enhancement is not compatible with materials now used for airfoil manufacturing, even if casting techniques were improved very much during last thirty years [3-I]. So the development of cooling techniques and their application to gas turbine airfoils is one of the most important elements for gas turbine development.
Design of cooling systems is based on the integration between various experimental correlations based on physical models [2-I]. Such information are based on standard cooling geometries (flat plate, constant height ducts, uniform flows, etc.) , investigated with similitude tests [3-I]. Then it is necessary to evaluate performances of more realistic cooling systems also with non uniform flow conditions.
Design of cooling system must be also realized in accordance with the aerodynamic requirements of vanes and blades. In modern transonic gas turbine aerodynamic losses are mainly due to trailing edge thickness; the internal cooling system may bring to an increase of this parameter and then reduce aerodynamic airfoil performance. Now casting techniques are able to create very thin trailing >>>