Design and Aerodynamic Analysis of a Novel Medium Bypass Turbofan Engine Exhaust System

Abstract

This thesis presents computational simulations of lobed mixers used in medium bypass turbofan engine exhaust systems. The aim was to investigate performance and re-design the system to reduce overall mass. The computational domain consisted of one \ang{30} sector of a model exhaust system which encapsulated one lobe wavelength of the lobed mixer and three swirling vanes used to model the flow downstream of a low-pressure turbine stage.

An examination of lobed mixer performance at incompressible and compressible boundary conditions found that the low-speed compressible experiments that were used previously to simulate the aft-end ducting of the engine could not accurately model the true flow features produced during actual engine operating conditions. Further investigation was conducted on scalloping of the baseline mixer geometry and concluded that a moderate scallop produced the best mixing of the core and bypass flow within the common nozzle. The lobed mixer was also shown to be tolerant of core flow swirl up to and including \ang{10} before incurring penalties. The tests on swirl tolerance were performed in service of determining the necessity of Turbine Exhaust Casing struts which, until now, have necessitated a large portion of aft-end ducting axial length to accommodate their aerodynamic effects. Low-pressure turbines are most effective when turning the core flow to \ang{30} swirl, and so the TEC struts are still required for optimal lobed mixer and common nozzle performance. However, it has been shown that the TEC struts and lobed mixer can be fully integrated without penalty to performance and with great impact on total aft-end duct length and therefore engine mass.

Simulations were also performed to investigate the relative mixing rates of temperature and momentum downstream of lobed mixers. Past literature has reported that momentum mixing always lags temperature. However, those discoveries were made downstream of confluent mixers, not lobed mixers. This data showed that the relative mixing rates of temperature and momentum downstream of lobed mixers was inconsistent.