The trend towards increased bypass ratio and reduced core size in civil aero-engines puts a strain on ground-start and relight capability, prompting renewed interest in sub-idle performance modelling. While a number of studies have looked at some of the broad performance modelling issues prevalent in this regime, the effects that bleed can have on sub-idle performance have not been addressed in the literature. During start-up and relight, the unknown variation in bleed flows through open handling bleed valves can have a considerable impact on the compressor’s operating line. This paper combines experimental, numerical and analytical approaches to look at the effect that sub-idle bleed flows have on predicted start-up operating lines, along with their effect on compressor characteristics. Experimental whole-engine data along with a purpose-built core-flow analysis tool are used to assess the effect of bleed model uncertainty on engine performance models. An experimental rig is used to assess the effects of reverse bleed on compressor characteristics and measurements are compared against numerical results. Several strategies for the generation of sub-idle maps including bleed effects are investigated.
The sub-idle operation of turbofan engines for ground start and relight has been of interest since the advent of the turbofan engine architecture and more so in recent years. Apart from being able to start the engine on the ground quickly, a significant challenge in the engine design exercise is to demonstrate the capability of inflight relight under windmill conditions. This becomes more critical for very high bypass ratio (VHBR) turbofans where the core mass flow rates are even lower due to the large bypass ratios. To test the ability of an engine to relight, it is necessary to know the component sub-idle performance characteristics, which are normally unknown. Over the years, substantial and significant work has been published on the generation of meanline methods to predict the axial flow core compressor sub-idle performance; however, there is practically no previous research published on standard tools for sub-idle regime in axialflow fans. In this investigation, the appropriateness of a low-order method based on interpolation is examined for the sub-idle performance characterization of the fan. It is shown that although results for the fan root may lack sufficient accuracy, the obtained solutions for the fan tip match CFD predictions very well. Also, a numerical investigation is carried out to study the effect of reverse flow through the bypass duct during ground start. In general, it is shown that reverse bypass duct flow during ground start has very little effect on core entry engine flow
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