Off-design performance of various gas-turbine cycle and shaft configurations

The design-point performance of various gas turbine cycles such as simple, regenerative, and intercooled-regenerative, is well understood. It is also understood that more elaborate shaft arrangements such as one, two, or three concentric or nonconcentric shafts, and a separate power turbine shaft, provide better starting and operational flexibility, and wider plateaus of high off-design performance. However, the types of different off-design performance one can obtain with these different shaft arrangements has not been previously reported. In this paper we use a computer program to investigate the design-point and off-design-point performance of engines with the following: one single shaft joining the compressor, turbine and load; one shaft joining compressor and turbine, and one shaft for the power turbine; two shafts for compressor and turbine, and one shaft for the power turbine; and three shafts joining the compressor and turbine, and one shaft for the power turbine. This is done by specifying typical compressor and turbine maps, and computing various aspects of off-design performance. The advantages and disadvantages of the various arrangements are investigated and discussed.