Corbett, M., "Shaft Power Extraction and Waste Heat Rejection using a Three Stream Variable Cycle Engine," SAE Int. J. Aerosp. 5(2):371-385, 2012, doi:10.4271/2012-01-2167.
Variable cycle engines offer the potential to operate a turbine engine more like a high-bypass turbofan during subsonic cruise and more like a turbojet or low-bypass turbofan for high-performance maneuvers or when supercruising. Variable geometry within the engine enables flow holding, allowing it to ingest the maximum amount of air that the inlet can capture even at reduced throttle settings. This approach reduces spillage drag compared to the conventional approach which cuts back engine airflow by reducing fan speed. To achieve the desired thrust, airflow is modulated between the core, inner bypass, and outer bypass. The air in the outer bypass duct, known as the 3rd stream, has been proposed as a heat sink for various engine and aircraft heat loads since it is at a comparatively low temperature, having only passed through the fan portion of the engine's compression system. The objective of this research was to further develop an understanding of the feasibility of extracting large amounts of transient shaft power from a variable cycle engine and rejecting the resultant waste heat into the 3rd stream bypass duct. The engine was found to be capable of providing the required shaft power and rejecting the waste heat to the 3rd stream, but the impact on engine performance is non-negligible. Transient pulses can significantly increase engine temperatures in certain cases, bringing into question the proper approach to modulating the cooled cooling air which is also cooled in the 3rd stream. Furthermore, the 3rd stream itself can become an unattractively hot sink at certain flight conditions.