Project Cluster / 2011-10-04
Higher pressure and lower flow losses in turbines
In this way, compressors increase pressure and compress gases using their blades. In contrast, turbines reduce the pressure of flow media such as steam or gases in order to convert the expansion of these media into power that makes the blades rotate. In both cases, the process is carried out more efficiently when losses in the flow due to eddies and flow separation are reduced. Research work is optimising the power transmission between pressurised gas and blades and continually minimising the flow losses.
Current aerodynamics research is working on optimising the shape – i.e. the three-dimensional profile – of the blades in the various components and stages with the aid of complex calculation procedures. The goal is as follows: As much energy as possible should be converted from the primary flow into useful power; to achieve this, the energy losses from secondary flow effects should be reduced as much as possible. In addition, the various secondary flow phenomena are being investigated in detail. These can be reduced by keeping the pressure loading relatively low at the edges of the blades and selecting an optimised blade geometry, for example. Over the course of recent decades, the results established by individual research projects on turbomachinery have proven useful as inputs into other research projects. As an example: Ideas from the field of aerodynamics have been taken up by materials research and production technology, thus establishing the initial technical basis for the achievement of certain goals.
The enormous progress made with materials (monocrystals, titanium alloys) and in production technology (honeycomb structures, casting and milling techniques, etc.) allows aerodynamics engineers to go close to the limits of what is physically possible when designing blades.
- Aero-thermodynamic optimisation of compressors and turbines
- Extension of operating range with increased efficiencies, particularly in partial-load operation
- Provision of the aerodynamic and aero-mechanical blade stability that is necessary for flexible power plant operation, combined with a sufficiently long service life
- Rig tests to determine the loading limits of compressors
- Improvement of the steam parameters
- Increasing expansion efficiencies
- Improving the flexibility of turbine operation and of plant integration
Aero-thermodynamic optimisation of compressors and turbines
Aerodynamic losses in and the efficiency and stability of modern axial compressors subject to high loads are determined to a large degree by unsteady flow at the edges of the annular zone. In the case of axial compressors subject to high loads in particular, detailed understanding of the dynamic processes taking place at the blades and in cavities is an important basis for achieving designs that have increased efficiency and, as a result, for improved process efficiency of the gas turbine. A further goal is the development of a suitable, improved casing structure for high-pressure compressor stages.
Experiments are to determine whether the surfaces in contact with the flow medium and the geometry can increase the efficiency and also the stability of the compressor.
Extension of operating range with increased efficiencies, particularly in partial-load operation ("partial-load flexibility")
Design measures in the blade tip and the stator gap areas could increase efficiency and improve operating behaviour in partial-load operation. To achieve this, detailed investigations of unsteady flow phenomena around the blade tips of the rotors are necessary. Also important is the improvement of the compressor's surge line, an area where the operation becomes unstable. This can be achieved by improving the design of the compressor stators. This means the compressor's operating point can be raised to higher efficiencies and that improved partial-load behaviour can be achieved. Overall, these measures significantly improve efficiency in the context of applications in low-CO2 power plants. In addition, the operating range of the compressor is extended. In this way, the requirement for improved partial-load stability is fulfilled.
In order to further optimise gas turbines, it is essential that findings from preliminary tests on compressor aerodynamics and structural strength be verified by practical tests on large-scale compressor test rigs. It is equally important that the high reliability of existing gas turbine compressors should also be transferred to new equipment. The gradual development of blade profiles and the design of gas turbine compressors is being driven with this goal in mind. The suitability of the improved components is to be confirmed by experimental investigations on a multi-stage segment that has been selected in the compressor and has a number of rows of blades. Investigations in compressor rigs are essential in order to optimise the downstream stages of multi-stage segments, in particular. Conventional measurement methods are not sufficient here. For example, so-called grid measurements (2D method) are not sufficient as they do not take into account significant 3D flow effects (side wall boundary layers, blade gap). Another complicating factor is that targeted loading and overloading of the downstream stages is not possible in a test gas turbine because of the limiting turbine inlet temperature with established measurement methods.
In steam turbines, steam is the flow medium that drives a rotor, which in turn drives a power generator or compressor. The steam turbine is one of the most important items of power generating equipment in use. Around half of the electricity generated worldwide comes from steam turbines that are fuelled by coal, nuclear energy, petroleum or natural gas, or alternatively by biomass, solar energy or geothermal energy. Thus large amounts of fuels can be saved and CO2 emissions significantly reduced by optimising these key components.
Increasing expansion efficiencies
An important development goal in steam turbine technology is the improvement of efficiency. This can be achieved by increasing the output for the same amount of fuel consumption. In other words, a considerable reduction in CO2 emissions at constant electrical power is possible by increasing the thermal efficiency. Among the focuses of activities in this area are the development and refinement of innovative sealing approaches and the improvement of design procedures for large low-pressure blades. Also being pursued is the optimisation of exhaust steam flow, with the aim of reducing exhaust losses downstream of the final stage. Lower exhaust losses can be achieved by using larger exhaust cross-sectional areas. This then results in an increase in the efficiency. At the same time, larger exhaust cross-sectional areas also mean that larger blades are necessary in the final stage. However, fluid flow and mechanical considerations have placed limits on increasing the size of final-stage blades up to now. Increased Mach numbers for the flow arriving at the rotating blade and increased three-dimensional effects result from the size of the final stages. For this reason, carefully coordinated experimental and numerical investigations are necessary in order to analyse complex flows in the low-pressure range.
Increasing flexibility / improving partial-load behaviour
Up to now, optimal efficiency has been the main consideration in the design philosophy that has been applied to steam turbines. However, the liberalisation of the electricity market and the increasing use of renewable energy sources in electricity generation have resulted in new requirements. A high degree of flexibility and optimal partial-load behaviour have become necessary to an increasing extent.
The following requirements can be identified on this basis:
- Quick start-up and shut-down of steam turbines
- Strong increase in numbers of load cycles
- Reduction of the minimum stable loading point
- High efficiency and lower emissions at the partial-load operating point
It is of central importance that the threshold loadings of multi-stage steam turbines operated at very low partial loads first be described and presented in the form of characteristic diagrams. In this way, the loss mechanisms can be identified in detail. The results will then lead to improved understanding of the flow phenomena and their effects on turbine components. In addition, this data can be used to develop numerical optimisation strategies for the reliable prediction of turbine regimes.
A further goal is the optimisation of blade arrays for the low-pressure final stages. Their damping behaviour must be improved to deal with the increasing dynamic loading. In addition, stochastic analyses of service lives are needed to provide reliable information on fatigue in low-pressure blades under high and low cyclical loadings. An additional engineering challenge is the improvement of the attachment of low-pressure blades, which will have to withstand increased dynamic forces in the future.
A steam turbine should simply harness as much rotational energy as possible from the steam pressure as possible using an intelligent blade layout. However, the compressor and turbine work together in a gas turbine from a technical viewpoint, but perform opposing functions from a physical viewpoint. A compressor acts like a pump. It sucks air in with its blades and compresses it to a high pressure. The compressed gas then flows into the turbine to a lower pressure and, in the process, drives the blades there. It can be said that the gas pressure increases continuously in the compressor due to the input of energy, while it is gradually reduced again in the turbine due to the removal of energy. The combustion chamber, which is situated between the compressor and the turbine, increases the temperature and thus also the energy content of the gas.
The blades in the compressor and the turbine thus carry out opposing tasks: The diagonally positioned blades in the compressor exert force on the gas and "push" it forward; in the case of expansion in the turbine, the gas exerts a force on the blades and drives them, which causes the shaft to which the blades are attached to rotate. Accordingly, the blades in the compressor and those in the turbine have different shapes and layouts.
To achieve the greatest possible output, the compressor and the turbine are constructed with a number of blade wheels. However, a stationary blade wheel must always be positioned between the rotating blade wheels, as the flow is diverted when it meets a rotating blade wheel. The flow must first be corrected again before it reaches another rotating wheel, i.e. it must be redirected into line with the original inlet flow direction again. This redirection is necessary so that the gas/steam stream can be diverted again. This means that it would not be possible to further harness the stream's energy content in the turbine without redirection. For this reason, compressors and turbines always consist of a sequence of rotating blade wheels (also called rotors) and stationary blade wheels (stators); a stator-rotor pair is referred to as a stage. A compressor always has more stages than a turbine.
24 current research projects
Optimisation of the steam turbine inlet flow taking into account structural-mechanical and aerodynamic considerations
Organisation carrying out research: Siemens Aktiengesellschaft - Power Generation
Project number: 0327716B
Interactions between flows in the inlet casing and in the transonic compressor stage of an industrial gas turbine
Organisation carrying out research: Bergische Universität Wuppertal - Fachbereich D - Architektur, Bauingenieurwesen, Maschinenbau, Sicherheitstechnik
Project number: 0327716F
Optimisation of transonic compressor stages with casing inserts
Organisation carrying out research: German Aerospace Center (DLR) - Institute of Propulsion Technology
Project number: 0327717C
High-performance brush seals for large pressure gradients
Organisation carrying out research: Technische Universität Carolo-Wilhelmina zu Braunschweig - Fakultät 4 - Maschinenbau - Pfleiderer-Institut für Strömungsmaschinen
Project number: 0327716P
Influences of sidewall contours and flow leakage interaction on the efficiency of turbines with shrouded blading
Organisation carrying out research: Rheinisch-Westfälische Technische Hochschule Aachen - Fakultät 4 - Maschinenwesen - Lehrstuhl und Institut für Dampf- und Gasturbinen
Project number: 0327716R
Efficiency-optimised stators with and without inner shroud
Organisation carrying out research: Technische Universität Dresden - Fakultät Maschinenwesen - Institut für Strömungsmechanik - Lehrstuhl für Turbomaschinen und Strahlantriebe
Project number: 0327716T
Secondary flow in turbine diffusers
Organisation carrying out research: Leibniz Universität Hannover - Fakultät für Maschinenbau - Institut für Turbomaschinen und Fluiddynamik
Project number: 0327717B
Aero-elastic behaviour in the blade tip area in compressors subject to high loads
Organisation carrying out research: Technische Universität Darmstadt - Fachbereich Maschinenbau - FG Gasturbinen, Luft- und Raumfahrtantriebe
Project number: 0327719D
Investigation and comparison of competing adaptive seal systems in turbomachinery with regard to effectiveness and dynamic behaviour
Organisation carrying out research: Karlsruhe Institute of Technology (KIT) - Department of Mechanical Engineering - Institut für Thermische Strömungsmaschinen
Project number: 0327717G
Aero-elastic investigation of turbine blades using linear simulation methods, taking into account platform and shroud
Organisation carrying out research: German Aerospace Center (DLR) - Institute of Propulsion Technology
Project number: 0327816
Design of compressor blading to investigate instabilities and flow-separation mechanisms at rotating blade peaks
Organisation carrying out research: Rolls-Royce Deutschland Ltd & Co KG
Project numbers: 0327830, 0327838
Optimisation and robust design of coupled rotating blading
Organisation carrying out research: Leibniz Universität Hannover - Fakultät für Maschinenbau - Institut für Dynamik und Schwingungen
Project number: 0327719A
Investigation of aero-elastic loading of final stages of steam turbines under partial loads and measures to avoid vibrations
Organisation carrying out research: Siemens AG - Power Generation - Dep. P11M2
Project number: 0327716K
FlexComp project (optimised compressor blading for more flexible and more environmentally friendly turbine operation)
Organisation carrying out research: Siemens AG - Power Generation - PE211
Project number: 0327805B