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Title:Mixing and sustained combustion in a cavity flameholder for scramjet propulsion
Author(s):Cisneros-Garibay, Esteban
Director of Research:Freund, Jonathan B; Pantano, Carlos A
Doctoral Committee Chair(s):Freund, Jonathan B
Doctoral Committee Member(s):Stephani, Kelly A; Lee, Tonghun
Department / Program:Mechanical Sci & Engineering
Discipline:Theoretical & Applied Mechans
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Supersonic combustion
Cavity flame holders
Turbulent combustion
Chemical kinetics
Abstract:Hypersonic flight remains a technological frontier in great part due to challenges associated with corresponding high-speed propulsion systems. Supersonic air-breathing engines—scramjets— provide attractive theoretical performance in the hypersonic flight corridor (with flight Mach number roughly greater than 5), though these are not fully realized yet. A central challenge in scramjet design is combustion stabilization in supersonic flows: mixing and combustion time scales can exceed flow residence time by over a factor of 10. Cavities provide means for fuel—oxidizer mixing and flame stabilization in low-speed recirculation zones. Understanding how cavity combustion affects mixing, entrainment, and core-flow interactions will guide improved scramjet design. We conduct and analyze detailed simulations of cavity-stabilized combustion and a corresponding inert cavity. The flow configuration is based on the Arc-heated Combustion Tunnel (ACT) II experiments at the University of Illinois. A M = 1, Reynolds-number Re = 29,000 round ethylene jet enters a cavity with length-to-depth ratio L/D = 3.5 from its 45º rear wall. Oxygen-rich oxidizer mixes into the cavity from the M = 3, Re = 77,000 core flow. In the sustained-combustion case, a non-premixed jet flame stabilizes in the cavity. We verify that our simulations reproduce key measurements of the inert and burning ACT II flows by comparing against measured shock angles and time-averaged wall-pressure. Reacting-flow simulations, as we conduct here, require numerical evaluation of chemical kinetics. We develop a software package, Pyrometheus, for the automatic implementation of chemical source terms and Jacobians. Our approach has two principal features that simplify implementation: (i) it is based on a code template; and (ii) it is compatible with automatic differentiation. The Cantera library is used to set model parameterization and co-verification. Examples of the verification are provided, along with comparisons between automatically-differentiated Jacobians and finite-difference approximations. Directly-fueled cavities are imperfect mixers and, with sustained-combustion, unburned fuel can accumulate near the upstream cavity wall. We analyze jet, composition, and transport statistics to understand how such zones arise, and how the jet influences mixing and flame patterns. The cavity is shorter than the length for fuel and oxygen to fully mix and burn, by about a factor of 6, and the resulting jet truncation leads to formation of disparate composition zones: a fuel-rich region in the upstream third of the cavity, with an essentially fuel-free recirculation zone downstream. An upward mean fuel flux results from the jet impinging on the upstream wall, which delivers unburned fuel to the mixing shear layer, where a flame is established. The mixing-layer flame accounts for over 70% fuel consumption and heat release, and does not rely on the cavity jet flame for stabilization. The heat release and expansion of sustained combustion in the cavity lead to complex flow over the cavity. Upstream boundary layers transiently separate, and the shocks over the cavity are highly unsteady with respect to the corresponding inert flow. Furthermore, the shocks reflect off and interact with the shear layer. We analyze these in detail. Their collective effect is the formation of a virtual throat over the cavity, with core flow dropping from M = 3 to M = 2. A quasi-one-dimensional flow model reproduces simulation Mach number to within 10%. We analyze shock-generated entropy as a source of mismatch between quasi-one-dimensional predictions and simulation results. Finally, we provide a Lagrangian description of cavity entrainment. Lagrangian tracers seeded upstream in the isolator allow us to compute the probability that they entrain into the cavity. Sustained cavity combustion suppresses net entrainment, by about a factor of 2. This is a consequence of shear-layer deflection to accommodate for thermal expansion due to combustion.
Issue Date:2021-10-26
Rights Information:2021 by Esteban Cisneros-Garibay. All rights reserved.
Date Available in IDEALS:2022-04-29
Date Deposited:2021-12

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