A rationale for the level of model fidelity required to provide the most representative flow field information to ascertain the feasibility of using a Variable Pitch Fan (VPF) in a modern high bypass ratio aero engine to generate reverse thrust is described in this paper. This is done by comparing the 3D RANS flow field solution for a newly developed reverse flow VPF design from two research models: i) isolated engine model in which the bypass duct, guide vanes, splitter and VPF are wrapped in an axisymmetric nacelle and placed in a generic far-field domain and b) integrated model in which the engine is installed to an airframe in landing configuration through a pylon and placed in a far-field domain bound by a rolling runway. The flow field solution obtained at an aircraft landing speed of 80 knots indicates that even though both models can predict the general flow patterns, there are substantial differences in parameters such as the amount of reverse stream, circumferential distribution of flow properties and flow development downstream of the engine. These differences impact the levels of reverse thrust generated, flow distortion entering the core engine and resultant airframe forces. This study makes the case that it is necessary to use an integrated model that includes a full engine nacelle installed on an airframe, to answer design questions for engineering the VPF system to generate reverse thrust.
The constant and increasing demand to obtain more accurate turbomachinery performance prediction in the design and analysis process has led to the development of higher fidelity flow field models. Despite extensive flow field information can be collected from 3-D RANS numerical simulations, the computational cost is expensive in terms of time and resources, especially if they are used as solvers within a design-optimisation framework. In contrast, 2-D throughflow methods, such as streamline curvature (SLC), provide an acceptable flow solution in minutes. The use of modern and advanced-design transonic axial-flow compressors and fans has been expanding due to their high shock-induced single-stage pressure ratios while being light, compact and robust. Transonic-flow analysis in blading is complex due to the shock structures involved and associated phenomena. Previous 2-D SLC tools have failed to replicate the real compressible-flow physics, assuming and oversimplifying the shock-system shape and location. The situation aggravates, when the assumed overall shock configuration applies only for design point at unstarted operations, requiring of empirical correlations to estimate the shock-loss coefficient for off-design operations. The overall compressor performance prediction is thence highly-dependent on the shock modelling quality. For this reason, a physics-based shock -structure and -loss model was developed and implemented into an existing in-house 2-D SLC compressor performance simulator to enhance the aerodynamic prediction in transonic axial-flow compressors. The novel shock-loss model is fully coupled to the 2-D SLC software, for which a blade-element-layout method was adapted to obtain the profile geometry definition. The analytical shock-loss model possesses the capability to operate at started and unstarted passages utilizing an iterative-solution method to position the choke-induced passage-shock. A significant contribution of the new shock-loss model is the solution of the relative total-pressure loss for the entire blade span, comprising the inlet relative subsonic supercritical and supersonic regions. In this manner, shock losses were determined throughout the blade span and for various off-design operating conditions, including those at choking. 2-D SLC simulations were conducted for the NASA Rotor 67 Fan to validate the models accordingly against test-rig data and verify against previous model estimations and 3-D CFD results. The analytical shock - structure and -loss model improved the shock-loss prediction between 40–50% with respect of the state-of-the-art models and showed satisfactory agreement against measured data within 0.6% at the blade tip and 0.3% at mid-span sections.
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.
It is commonly accepted that fouling degrades severely axial compressor performance. Deposits build up as operating hours sum up, causing a decrease in the compressor's delivery pressure, efficiency and flow capacity. Researchers have also concluded that compressor susceptibility to fouling depends on many factors, such as atmospheric conditions, air quality, filtration system, the size and design of the compressor, etc. The current study identifies four basic operating scenarios which refer to the same compressor, in order to put forward a comparative assessment as to how incoming air quality would affect compressor performance. SOCRATES, an in-house, streamline curvature-based through-flow tool, in conjunction with a detailed, fully customizable fouling empirical model, conceived based on flow physics and relevant experimental data, is used to qualify and quantify, compressor degradation with time.
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