A methodology for capturing the impacts of bleed flow extraction on compressor performance and operability in engine conceptual design

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Brooks, Joshua Daniel
Mavris, Dimitri N.
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The commercial aviation industry continually faces the challenge of reducing fuel consumption for the next generation of aircraft. This challenge rests largely on the shoulders of engine design teams, who push the boundaries of the traditional design paradigm in pursuit of more fuel efficient, cost effective, and environmentally clean engines. In order to realize these gains, there is a heightened requirement of accounting for engine system and subsystem level impacts from a wide range of variables, earlier in the design process than ever before. One of these variables, bleed flow extraction, or simply bleed, plays an especially greater role; due to the approach engine designers are taking to combat the current state of fuel efficiency. For this reason, this research examined the current state of bleed handling performed during the engine conceptual design process, questioned its adequacy with regards to properly capturing the impacts of this mechanism, and developed a bleed handling methodology designed to replace the existing method. The traditional method of handling bleed in the engine cycle design stage relies on a variety of engine level impacts stemming from zero dimensional thermodynamic analysis, as well as the utilization of a static performance characterization of the engine compression component, the axial flow compressor. The traditional method operates under the assumption that the introduction of additional bleed to the compressor system has created no additional compressor level impact. The methodology developed in this work challenges this assumption in two parts, first by creating a way to evaluate the compressor level impacts caused by the introduction of bleed, and second by implementing the knowledge gained from this compressor level evaluation into the engine cycle design, where the engine level impacts could be compared to those predicted by the traditional method of bleed handling. The compressor level impacts from the addition of bleed were quantified using a low fidelity, multi-stream, meanline analysis. Here, an innovative approach was developed which cross pollinated existing methods used elsewhere in the analysis environment, to account for the bleed impact in the object oriented modeling environment. Implementation of this approach revealed that the addition of bleed negatively and significantly impacts the compressor level performance and operability. With the completion of the above analyses, this newly acquired capability to quantify, or at least qualify, the compressor level bleed impacts was tied into the engine level cycle analysis. This form of component zooming, allows the user to update the bleed flow rate from a number of locations along the compressor, as well as the compressor variable stator vain orientation, within the existing cycle analysis. Utilization of this ability provided engine level performance and operability analyses which revealed a disparity between the traditional and herein developed bleed handling methodology’s predictions. The found results reveal a need for more stringent handling of bleed during the engine conceptual design than the traditional method provides, and suggests that the developed methodology provides a positive step to the realization of this need.
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