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Title:Experimental study of flow choking and inlet unstart in an axisymmetric model scramjet
Author(s):Baccarella, Damiano
Director of Research:Lee, Tonghun
Doctoral Committee Chair(s):Lee, Tonghun
Doctoral Committee Member(s):Elliott, Gregory S.; Stephani, Kelly; Gazzola, Mattia
Department / Program:Mechanical Sci & Engineering
Discipline:Mechanical Engineering
Degree Granting Institution:University of Illinois at Urbana-Champaign
Abstract:Hypersonic air-breathing propulsion systems, such as supersonic combustion ramjets (scramjets), are a key technology for the future of high-speed civil transportation, low-cost access to space, and national defense. A major problem affecting scramjets at lower Mach numbers is inlet unstart, described as the disgorging of the shock system from the intake of the engine. One of the possible causes of unstart is flow choking, occurring when sonic conditions are reached at some location downstream of the inlet. Downstream flow choking is an extremely complex phenomenon still not completely understood. The present work aims to provide a better understanding of choking-induced unstart by analyzing how this condition develops in an axisymmetric scramjet geometry. A preliminary part of this work has been dedicated to the design and characterization of a small-scale, pulsed, arc-heated hypersonic facility at the University of Illinois at Urbana-Champaign. The new facility, designated ACT-II (Arc-heated Combustion Tunnel), is capable of providing high-enthalpy flow conditions for extended testing time (up to 1 second). ACT-II was revealed to be an ideal facility to study choking-induced unstart that, occurring on a timescale of tens of milliseconds, is beyond the capabilities of conventional impulse facilities (shock tunnels, expansion tunnels, etc.). ACT-II can be operated in both direct-connect and free-jet modes and is optimized for laser diagnostics and flow/flame visualization. The first part of the study on unstart was dedicated to the investigation of the combustion-induced choking mechanism in a circular supersonic combustor. ACT-II was operated in direct-connect mode using ethylene as fuel. The combustion cases were compared with tests at the same conditions in which an inert gas (air) was injected instead of fuel to isolate the effects of combustion heat release. The study revealed that combustion heating has a negligible effect on the supersonic core, and that irreversibilities play a major role in lowering the Mach number of the flow in the combustor. In the second part of the work, the isolator flow of a free-jet scramjet model was studied at low-enthalpy conditions. Simultaneous pressure measurements and condensed CO2 planar laser Rayleigh scattering (PLRS) visualization were used as diagnostic tools. Downstream choking, in this case, was induced by mass injection (nitrogen). This study provided an unprecedentedly detailed characterization of the unstart process. In response to the backpressure rise induced by mass injection, the flow in the isolator separates and a shock train develops. Under certain conditions, the shock-train undergoes an oscillatory motion accompanied by propagation of pressure waves throughout the model scramjet. The final part of the work addressed choking and unstart in the free-jet model scramjet at high-enthalpy conditions. The purpose was to investigate similarities and differences between mass-induced (injecting air) and combustion-induced (injecting ethylene) choking. The results show important differences in the shock-propagation dynamics through the combustor. Nevertheless, the isolator flow was not drastically affected by these differences and exhibited a similar behavior in both cases.
Issue Date:2018-06-20
Rights Information:Copyright 2018 Damiano Baccarella
Date Available in IDEALS:2018-09-27
Date Deposited:2018-08

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