Variable geometry exhaust system for low specific thrust turbofans

Abstract

This research was undertaken within the context of the Rolls-Royce University Technology Centre (UTC) in Aero System Design, Integration & Performance at Cranfield University, hosted by the Centre for Propulsion Engineering. Aero-engine designers are moving towards lower specific thrust and higher bypass ratio engines in an attempt to improve mission fuel burn. When going to Ultra-High bypass ratio and very low specific thrust engines there may be a possibility for further mission fuel burn improvement equipping such engines with a ‘Variable Geometry Exhaust System’ (VGES). This system might also be required when looking at the possibility of surge of the compressor components in very low specific thrust engines. The performance improvement of the VGES could arise from the aspect that this system includes a mini-mixer, where part of the bypass flow is mixed with the core flow. The idea behind the mini-mixer is the possibility to gain the benefits of a mixed exhaust turbofan while reducing the losses these systems introduce. This is because the mini-mixer will not mix the complete exhaust flows. Therefore, the mixing chamber will not require the same length as a regular mixed exhaust system. Next to the mini-mixer the VGES presents the possibility to vary four flow areas within the engine’s exhaust system, hence potentially increasing the available ‘handles’ for optimum performance and preventing surge of the compressors. Two of these variable areas are exhaust nozzles and the other two are the inlets to the mixer. The main focus of this PhD programme was the representative simulation of the VGES with a mini-mixer, taking all important pressure losses, as well as mixing efficiency into account. The work reported herein discusses the engine cycle optimisation, weight and mission fuel burn studies undertaken. For this research two different turbofan engine architectures are examined, the ‘direct-drive’ and the ‘geared’ configuration, where the fan is coupled to the Low Pressure Turbine (LPT) via a reduction gearbox. The mission profile investigated is for an A330-size aircraft with a range of 3000 nm. Technology limitations are set for a potential entry into service in 2025-2030. The results of this study showed that regardless of how many areas were allowed to vary of the four variable areas, no significant performance benefit was present. In regards to surge possibility with the current assumption for the very low specific thrust engine configurations there was no need for the variable areas. The results of the study also demonstrate that the mini-mixer itself can improve engine performance and mission fuel burn. With the current assumptions the maximum reduction in mission fuel burn observed is 0.5% at the lower fan diameter range investigated for the geared configuration. For the direct drive the maximum reduction found was 0.1% The reason for the direct-drive configuration having less improvement is due to the limit the LPT lets the fan FPR be reduced, which influenced the thrust gain from the mini-mixer, compared to the geared configuration. Furthermore, for both architectures going to higher fan diameters, there is no benefit anymore present by using the mini-mixer. The reason for this is that the thrust gain is numerically associated with the reduction in FPR and LPT pressure ratios when comparing to an unmixed engine with the same Specific Thrust. When the FPR of an unmixed is already very small the amount of thrust gain when using a mixed exhaust system can achieve is very small at this point the losses start outweighing the benefits. This research programme has made a useful contribution to knowledge in various areas of engineering and scientific relevance. First of all the viability of a novel piece of technology was investigated, with potential benefits as well as practical limitations identified. The influence of mixing efficiency and pressure loss in a complex, mini-mixer arrangement were studied and a useful methodology was established to identify performance trade-offs. Also, the study looked into the effect on high-bypass ratio turbofan engine performance of using multiple variable nozzle areas simultaneously. The results are reported herein for the very first time.