What is it about?
A performance model for high bypass ratio turbofan engines is described. Overall system performance comprises three power conversion steps: 1) conversion of fuel heat release power to hot gas power; 2) conversion of hot gas power to shaft power; 3) conversion of shaft power to propulsive power. Physical control volumes within the engine system are associated with each of the power conversion processes, and power conversion efficiencies are defined – thermal, transfer, and propulsive. A shaft power efficiency for the core engine, the net effect of the first two steps, is also defined. The propulsive efficiency, primarily associated with the fan bypass stream, is referenced to the equivalent shaft power produced by the core engine.
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Why is it important?
The association of the power conversion processes with physical control volumes makes it clear what losses occur at each power conversion step. In a high-BPR aircraft gas turbine, most of the power produced by the core engine is in the form of shaft power to drive the big fan, similar to what happens in a turboprop engine. The propulsor, a ducted fan or a propeller, could just as easily be powered by the shaft power output of an electric motor. Aircraft propulsion systems based on hydrogen fuel cell with electric drive train are now being considered, and a consistent basis of comparison of performance between a high-BPR aircraft gas turbine engine and an H2/electric system would be the shaft power efficiency.
Perspectives
My thinking on this subject dates back to the 1980s/early 1990s when we were working on advanced high-speed propeller, or "propfan", engines and then also very high-BPR geared-drive ducted fan engines. I was struck by the difference in how performance was described - propeller performance described in terms of shaft power input and thrust output; fan performance in a high-BPR turbofan described in terms of fan pressure ratio and isentropic efficiency. I came to the conclusion that it made sense to describe high-BPR turbofan performance in terms of a propulsive efficiency referenced to shaft power input, same as the propeller definition.
Wesley Lord
self employed
Read the Original
This page is a summary of: A Performance Model for High-BPR Turbofan Engines, January 2024, American Institute of Aeronautics and Astronautics (AIAA),
DOI: 10.2514/6.2024-1371.
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