Extrapolation of Compressor Characteristics to the Low-Speed Region for Sub-Idle Performance Modelling

In order to model the performance of a gas-turbine engine in the sub-idle region, particularly for starting and windmilling, it is necessary to use compressor characteristics which describe their operation and low speeds. As most compressor tests are conducted only in the range of operating speeds normally encountered, the resultant characteristics must be extrapolated in order to define the low speed compressor maps. In this paper, a variety of techniques for the extrapolation of axial-flow compressor characteristics are presented and evaluated. These include those presented by De-You & Zhong-Fan [4], Agrawal & Yunis [1] and Converse & Giffen [3]. The ease and reliability of the extrapolation methods are compared. Problems associated with the prediction of losses within the turbomachinery are highlighted, particularly in respect of compressor operation at high flows and low rotational speeds. This is due to the creation of highly off-design values of incidence which are not encountered during above-idle running, but which may be found during windmilling.