Preliminary Analysis on the Impact of Electric Power Transfer on Turbofan Windmill Starts.

The drive to reduce the fuel consumption of civil aviation has led to the development of more-electric aircraft with the aim to increase the efficiency of the on-board power systems.  To meet the higher demands for electrical power production, it has been proposed that generators are fitted to both the high-pressure and low-pressure spools (HPS and LPS) within the gas turbine engine. By having the generator on the HPS also coupling as a motor, it allows for power to be transferred from the HPS to the LPS. The ability to transfer power within a gas turbine engine can increase the efficiency during various stages of flight, but also offer improvements in situations such as windmilling starts. The issue with the current method of windmilling starts is that it relies solely on air entering the core of the engine to create the pressure required for relight. However, the most modern turbofan engines under development have Bypass Ratios (BPR’s) exceeding 10, resulting in smaller cores in which less air can enter. The application of power transfer in this specific scenario can harness the power generated by the large fan and use it to accelerate the HPS which should improve the performance of this method of relight. 

The current work being undertaken by myself and the project supervisors at the University of Nottingham is to conduct a preliminary study of the effects that power transfer has on the performance of a windmill start. The methodology behind the research can be broken up into three parts. First is the use of two-dimensional computation fluid dynamics (CFD) to approximate the power generated by the main fan and find the maximum power conditions. The P_MAX output is used as it will provide the HPS with the most acceleration during a windmill start. It is a condition where the product of the rotational speed of the fan and the torque exerted on the fan by the flow is maximum. The subsequent dynamic response of the HPS will be approximated by the P_MAX value supplied to the generator/motor which drives the HPS. The final step involves the use of compressor maps, interpolated to show estimates for the sub-idle region, to analyse the effects of the HPS acceleration on the performance of the relight. The purpose of this study is to provide an initial analysis of the effects of applying the concept of power transfer in this specific scenario and to act as an indicator to whether this application should be studied further. The overall aim of this research is to increase the safety of civil passenger aircraft by improving the reliability of a windmilling start, decreasing the time it takes to relight, and to increase the flight envelope in which it can take place.