Higher temperatures in turbines

• Optimisation of the removal and flow path of cooling medium
• Optimisation of internal blade cooling
• Multidisciplinary design of future blade profiles (aerodynamics, cooling and flow leakage)
• Improved, reliable sealing systems

Currently, the cooling medium most frequently used in gas turbines is air. A portion of the compressed air is removed from the compressor and fed to the gas turbine blades, bypassing the combustion chamber. Around one fifth of the air extracted from the compressor is used for cooling purposes and is thus not used in the actual generation process, which has a negative impact on the efficiency.

An increase in the turbine inlet temperatures improves the efficiency of the gas turbine on the one hand, but also leads to a higher requirement for cooling air on the other, which in turn reduces the gain in efficiency. For this reason, future activities should particularly concentrate on the development of new cooling methods and of suitable materials with high temperature-resistance. A basic assumption behind this approach is that gas turbine inlet temperatures of almost 1,500 °C (17 bar) are necessary in order to achieve a gas and steam efficiency of 65%. In addition, this gas turbine process is to be combined with the steam circuit on the basis of 700 °C steam turbine technology.

The familiar cooling methods that are currently used – such as convection, impingement or film cooling, or a combination of various cooling types – are not sufficient for gas turbines with inlet temperatures that are in this high range. For this reason, new cooling methods and materials of construction that will be compatible with these high temperatures are among the areas being analysed. The integrated and optimal design of all turbomachinery is also of great importance. The so-called effusion cooling method has been developed as an improvement to film cooling. It differs from film cooling in that the cooling air is not fed through drillholes, but instead flows uniformly through a porous material (avoiding local overheating) and is continuously fed to the entire profile surface. Effusion cooling is based on the principle of providing a homogeneous cooling film, while transpiration cooling also aims to achieve a phase change of the cooling fluid, which further increases the effectiveness of cooling. Transpiration cooling is a technology that will be developed in the long term. One of the main challenges here is the ability to control the phase change in a targeted manner.

Work on the area of "cooling" is concentrating on the flow path of the cooling medium, internal blade cooling, and the interaction of cooling, aerodynamics and flow leaks.

Various promising engineering approaches exist that will reduce the cooling medium requirement and further increase the gas temperature in the turbine. These include the development of improved high-temperature materials and thermal insulation layers. Other engineering improvements on the horizon include innovative component cooling and improved protection against heat input achieved by using thin cooling films. The cooling films are protective insulating gas layers that separate the hot working fluid from the gas turbine blades. Innovative design concepts are characterised by an increasing degree of design detail. The combination of impingement and film cooling would appear to be particularly promising alongside transpiration cooling, which is the most efficient method but is also more difficult to implement from an engineering viewpoint. These cooling methods are increasingly being considered as part of wall-integrated configurations in the combustion chamber. The main reason for this is that there have been significant improvements in production technology in this area in recent times. A noteworthy characteristic of the component geometries here is that they reconcile competing demands in certain cases, e.g. the necessity that the components have sufficient structural strength (thicker walls) and the cooling requirement (thin walls). The focuses of the development work include: optimal compatibility of the cooling methods used; the minimisation of manufacturing and maintenance costs; high reliability; and increased functionality, reliability and safety by using particle removal systems.

The overall efficiency of a plant depends not only on the quality of the individual components, but is also strongly determined by the degree to which the individual components and the interfaces between them are designed in an integrated manner. One example here is the interdependency between the flow paths in the combustion chamber, which are strongly characterised by swirling flow, and the first row of stationary blades in the turbine. Swirling flow has a major influence on aerodynamic losses for this row of blades and the creation/spread of the cooling film on the blade surface. If the complex structure of the outlet flow from the combustion chamber is considered during the design process, it is possible to optimise the first row of stationary blades from an aerodynamic viewpoint and to improve film cooling. In this way, the process efficiency can be improved directly and the amount of cooling air reduced.

Improved performance and increased demands as regards the compactness of plants will lead to the development of high-pressure turbines in the future that feature so-called transonic flow regimes – i.e. above the speed of sound – in the stator and the rotor. This increases the aerodynamic loading on the turbine blades and also leads to continuously increasing thermal loads due to increased turbine inlet temperatures. Intensive cooling of the trailing edges of the blades is essential. This zone of the blade often determines the blade's service life. At the same time, the trailing edges must be as thin as possible, as the thickness of the trailing edge has a major influence on the profile losses caused by eddies. This results in competing demands as regards structural strength, ease of manufacture and the resultant aerodynamic losses for the stationary and rotating turbine blades. The optimisation of the trailing edge area is also of great importance.

Modellierung des thermomechanischen Ermüdungsverhaltens einer thermisch hochbelasteten Gasturbinenschaufel
Forschende Organisation: Federal Institute for Materials Research and Testing (BAM) - Division 5.2 - Mechanical Behaviour of Materials

Heat transfer and effectiveness of film cooling on a three-dimensionally contoured side wall (in gas turbines)
Hybrid models (design calculations for turbulence on gas turbine blades)
Organisation carrying out research: Karlsruhe Institute of Technology (KIT) - Fakultät für Maschinenbau - Institut für Thermische Strömungsmaschinen
Project number: 0327719J

Material and process technology for a modular design concept for gas turbine components that are designed for use with hot gases
Organisation carrying out research: Siemens AG - Power Generation - Dep. PE41
ALSTOM Power Systems GmbH
Project numbers: 0327705T, 0327705U

Probabilistic calculation of service life for designs at extreme temperatures
Organisation carrying out research: Siemens Aktiengesellschaft - Energy Sector - Dep. E F PR GT EN 412
Project number: 0327718A

Experimental investigations on innovative designs of rotating cooling systems in turbine blades
Organisation carrying out research: German Aerospace Center (DLR) - Institute of Propulsion Technology
Project number: 0327713S

Influence of rotation on cyclone and impingement cooling in innovative blade cooling systems
Organisation carrying out research: Technische Universität Darmstadt - Fachbereich Maschinenbau - FG Gasturbinen, Luft- und Raumfahrtantriebe
Project number: 0327716G

Project group for cooling medium flow paths & internal blade cooling: Innovative geometries for the blade edge area – 3D platform and blade tip geometry
Organisation carrying out research: Alstom Power Systems GmbH
Project number: 0327716V

Innovative geometries for the blade edge area – 3D platform and blade tip geometry
Organisation carrying out research: Rolls-Royce Deutschland Ltd & Co KG
Project number: 0327716W

Particle removal in the secondary air system
Organisation carrying out research: Karlsruhe Institute of Technology (KIT) - Department of Mechanical Engineering - Institut für Thermische Strömungsmaschinen
Project number: 0327725C

Investigation of wall-integrated impingement and film cooling configurations for turbine blades that are subject to high thermal loads
Organisation carrying out research: Universität Stuttgart - Faculty of aviation and aerospace engineering and geodesy - Institute of Aerospace Thermodynamics
Project number: 0327725F

Investigation and optimisation of the inlet flow and internal circulation in rotating branched cavities
Organisation carrying out research: Technische Universität Dresden - Fakultät Maschinenwesen - Institut für Strömungsmechanik - Lehrstuhl für Magnetofluiddynamik
Project number: 0327725G