A Method for the Conceptual Design of Integrated Variable Cycle Engines and Aircraft Thermal Management Systems
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Clark, Robert Arthur
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Abstract
Development efforts for current and future military fighter aircraft are tasked with fulfilling strenuous requirements, many of which are at odds with each other. An increased demand for high power electronics and weapons systems has put the need for auxiliary power generation and heat dissipation on par with the more traditional military aircraft requirements of extended range and speed, stealthiness, and enhanced maneuverability. All of these requirements can be traced back in some way to the propulsion system, which is arguably the single most important subsystem on any aircraft, military or commercial. For decades, the low bypass ratio mixed flow turbofan (MFTF) has been the architecture of choice for the propulsion systems that power military fighter aircraft. However, the competing nature of modern aircraft requirements has begun to highlight the drawbacks of the fixed cycle MFTF, and has led to the development of variable cycle engines (VCEs). Variable cycle engines show promise in increasing thrust, reducing fuel consumption, and improving heat dissipation capability, all of which are critical requirements for military aircraft. There has been a further recognition that thermal management requirements need to be assessed earlier in the conceptual design phase in concert with the propulsion system, given that current aircraft such as the F-35 struggle to meet heat dissipation requirements. Unfortunately, the existing conceptual cycle design methods used to select engine cycles were not developed with variable cycle engines in mind. The objective of this research is to enhance conceptual design-level modeling methods for integrated design of variable cycle engines and thermal management systems in order to better achieve aircraft-level mission requirements.
The key difference between a variable cycle engine and a traditional fixed cycle engine is the presence of variable geometry features whose positions are modulated specifically to move air between the different streams in the engine. A method of variable cycle engine design is presented that accounts for these variable geometry as a means of aiding the propulsion system designer during the conceptual design phase of the propulsion system. A series of research questions, hypotheses, and experiments that build on each other are posed in order to address the need for conceptual cycle designers to better understand how variable cycle engines impact the cycle design process, especially in the context of integrated propulsion and thermal management systems.
The first research question and experiment address the need to determine optimum variable geometry positions for off-design analysis of a variable cycle engine throughout the complete flight envelope of a fighter aircraft. Existing design methods require an optimizer to determine variable geometry position targets at every off-design operating condition used during aircraft mission analysis, which, for refined mission analysis methods can be hundreds or thousands of off-design points. This results in significant cost due to the repeated use of the optimizer. This thesis develops a method for determining variable geometry schedules, which can be generated cheaply with only a small number of optimizer calls, and then used in place of the optimizer during off-design evaluation of the variable cycle engine. The use of variable geometry schedules during the off-design process is shown to significantly reduce the computational cost of off-design analysis of variable cycle engines.
The second research question and experiment examine the design process for variable cycle engines and incorporate the use of the variable geometry schedules directly into the engine design process. Current design methods in the literature utilize nested optimization techniques in order to determine the optimum positions of variable geometry features during the design process. The method in this thesis takes the variable geometry schedules, shown in the first experiment to be useful for off-design analysis, and incorporates them directly into an engine design loop. The use of variable geometry schedules during the design process is shown to reduce the overall number of required engine design iterations by two orders of magnitude, relative to current design methods in the literature.
The third research question and experiment address the need to assess the impact of integrating a thermal management system into the design of the variable cycle engine. The literature is sparse on how incorporating the design of a thermal management system directly into the engine design process impacts the selection of the design cycle for a variable cycle engine. This thesis demonstrates how design integration of the engine and thermal management system shifts the location of the optimal cycle within the cycle design space of a variable cycle engine. Furthermore, the utility of variable geometry schedules is demonstrated through a cycle design scenario, where schedules that minimize fuel burn or maximize heat dissipation capability for the aircraft are shown to lead the cycle designer to different locations in the optimized cycle design space.
The design methods utilized for each of these experiments are synthesized into an overall conceptual design method called PREHEAT-V (Preliminary/Conceptual Design Method for Handling Heat and Aircraft Thermal Management in Variable Cycle Engines), which incorporates variable geometry optimization techniques directly into a multiple design point cycle design process. The PREHEAT-V design method allows cycle designers to evaluate large candidate variable cycle engine design spaces in a computationally efficient manner, and assess the impact of heat dissipation requirements on the optimum design cycle. The PREHEAT-V method emphasizes evaluating aircraft-level mission requirements, rather than engine-level requirements, since the ultimate barometer of success for military aircraft is mission capability, not engine capability.
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Date
2023-11-28
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Dissertation