Considerations for the Use of Variable Geometry in Gas Turbines

Abstract

The loss of performance of a gas turbine at off-design is primarily due to the rapid drop of the major cycle performance variables with decrease in output, and this may be aggravated by poor component performance. Postulated propulsion demands require that future engines attain performances much more advanced than those of the present day. The specific nature of the improvement in performance will depend on engine duty, but it is expected that the improvement will include higher power loadings and better response, in addition to better fuel burn characteristics. Innovative design and advanced materials and structures will play key roles in bringing about a revolution in gas turbine technology, but these will have to be accompanied by novel control methods which can influence the position of the engine operating point. The latter may require the use of variable geometry in one or more gas path components to improve the internal matching of a component or to re-match the engine cycle such that the component operating point and/or the primary cycle performance variables are re-optimized or controlled with changing operating conditions. This paper examines how variable geometry could be used to improve the off-design performance of gas turbines.

Some Aspects of Variable Geometry Gas Turbine Operation

“…Turbine Stage [Rahnke, 1969] vane position, there is also a loss of efficiency due to changes in pressure ratio and changes in rotational speed, and at certain operating conditions the level of pressure ratio and the position of the vanes may combine to erode turbine efficiency drastically, which in turn might cause a severe loss of engine performance, or it might limit the operation of the engine. This was observed by Roy-Aikins (1988) during his study of the performance of a variable bypass ratio turbofan suitable for an interceptor aircraft. Figure 9 shows the effect of turbine pressure ratio on efficiency at different vane settings at the design speed; the point circled is the operating point for a given turbine work output.…”

“…As was argued by Roy-Aikins (1990), novel control methods must be applied also, in addition to the use of advanced technologies, if future gas turbine engines are to attain significant improvements in off-design performance. Such improvements can be realised mainly by the extensive use of variable geometry components, which are required if the position of the engine operating point is to be influenced actively, in order that a marked deterioration in the primary cycle performance variables does not occur as the engine is throttled or as the flight conditions change.…”

The Off‐Design Performance Simulation of Marine Gas Turbine Based on Optimum Scheduling of Variable Stator Vanes

“…As one of the surge-preventing measures of the compressor, variable stator vane (VSV) can be used to change the installation angle of the cascade with the working conditions changing to improve the compressor performance in the off-design conditions [7][8][9][10][11].…”

Variable geometry gas turbines for improving the part-load performance of marine combined cycles – Combined cycle performance

“…In contrast, also lower load conditions are of importance when considering plants to be used for marine propulsion. The use of VGVs and/or VAN has also been suggested for achieving higher part-load efficiencies for gas turbines [21][22][23][24].…”