Vincent Larsson presents his master’s thesis

Student: Vincent Larsson
Main Supervisor: Patrick Nilsson, Anna Nyhlén
Examiner: Tomas Grönstedt
Opponent: Oscar Hellgren

Abstract of thesis

An important area of improvement in aircraft engines is fuel efficiency for economic as well as environmental reasons. While civil turbofan engines are primarily designed with fuel efficiency in mind, military engines need to uphold high specific thrust requirements in order to reach supersonic speeds as well as perform the maneuvers necessary in combat. Modern military aircraft engines are therefore designed as low-bypass turbofans capable of producing significant specific thrust, but at the cost of having a high specific fuel consumption at lower speeds.

The variable cycle engine is an engine concept for which the engine cycle can be modified in-flight to fulfil different requirements. The engine type of interest for this thesis is a so called three stream, mixed-flow variable cycle engine, in which a low-bypass turbofan engine has had an additional outer bypass duct added to it. This third stream can be modulated to vary the bypass-ratio of the engine mid-flight, increasing the bypass-ratio when cruising for better fuel efficiency and decreasing the same when requiring higher thrust.

For the thesis, the NPSS software was used to model and simulate two different variable cycle engines. In order to evaluate the models, a base-line model representing a standard low-bypass turbofan engine was modelled for comparison. The primary area of study for the models was the effect of bypass ratio on thrust-specific fuel consumption. The results showed that overall, the variable cycle engines had an advantage over the standard engine regarding fuel consumption at the expense of lower maximum thrust.