Qili Liu

Ethylene Flame Dynamics and Inlet Unstart in a Model Scramjet

Ethylene flame dynamics in a model scramjet installed in an open test section of an arc-heated hypersonic wind tunnel are experimentally investigated with freestreams of Mach 4.5–9. An ethylene fuel jet is injected into the model scramjet that is autoignited by high-enthalpy flows compressed and decelerated by a train of shock waves and boundary layers developing in the scramjet isolator/combustor. The ethylene flame behavior is captured by a high-speed movie camera through optical access windows on the model and characterized by surface pressure/temperature traces recorded at five different locations along the flowpath through the model scramjet. Downstream pressure buildup due to combustion heat release, shock-induced compression, and mass loading via the fuel jet triggers the flame propagation toward upstream, stabilizes the flame at a location in the combustor, or pushes the flame further upstream to cause inlet unstart. In particular, flame dynamics while the scramjet undergoes the inlet unstart with...

Development and testing of the ACT-1 experimental facility for hypersonic combustion research

A new pulsed-arc-heated hypersonic wind tunnel facility, designated as ACT-1 (Arc-heated Combustion Test-rig 1), has been developed and built at the University of Notre Dame in collaboration with the University of Illinois at Urbana-Champaign and Alta S.p.A. The aim of the design is to provide a suitable test platform for experimental studies on supersonic and hypersonic turbulent combustion phenomena. ACT-1 is composed of a high temperature gas-generator system and a model scramjet combustor that is installed in an open-type vacuum test section of the wind tunnel facility. The gas-generator is designed to produce high-enthalpy (stagnation temperature = 2000 K–3500 K) hypersonic flows for a run time up to 1 s. The supersonic combustor section is composed of a compression ramp (scramjet inlet), an internal flow channel of constant cross-section, a fuel jet nozzle, and a flame holder (wall cavity). The facility allows three-way optical accesses (top and sides) into the supersonic combustor to enable various advanced optical and laser diagnostics. In particular, planar laser Rayleigh scattering (PLRS), high-speed schlieren imaging and OH-planar laser induced fluorescence (OH-PLIF) have successfully been implemented to visualize the turbulent flows and flame structures at high speed flight conditions.

Ethylene flame dynamics in an arc-heated hypersonic wind tunnel

Ethylene flame dynamics in a model scramjet is investigated using a pulsed-arc-heated hypersonic wind tunnel simulating supersonic/hypersonic flight conditions (Mach 4.5, 6 and 9 flows of up to 3,500K stagnation temperature). A partially-premixed ethylene flame is auto-ignited with Mach 4.5, 6 and 9 freestream flows in a region downstream of a fuel jet where the high enthalpy flow is compressed and decelerated by incident/reflected shockwaves and development of boundary layers. This flame propagates upstream and anchores at a location depending on flow conditions. In the case of Mach 4.5 freestream flows, inlet unstart phenomenon is observed by excessive heat release from ethylene combustion reactions over a range of fuel concentration (overall equivalence ratio = 1.5 – 2.3). Flame evolution during a test time of hundreds milliseconds is optically resolved by a fast framing camera simultaneously with surface temperature/pressure measurement in the model scramjet. The pressure/temperature traces along with time-sequential flame images (300 Hz) describe the flame dynamics, e.g., flame propagation and flame holding near a wall cavity.

Comparison of Unstart Induced by Mass Addition and Heat Release

The unstart phenomena in a model scramjet with the freestream of both low and high enthalpy Mach 4.5 flow conditions at an arc-heated hypersonic wind tunnel are investigated. Then, the unstart phenomena induced by a nitrogen or ethylene jet at low and high enthalpy conditions are compared. The nitrogen or ethylene jet pressurize downstream by mass addition and heat release from combustion. High-speed schlieren at the jet and the lip of the scramjet model inlet and high resonance frequency surface pressure measurements are used to capture flow features during an unstart process. In both conditions, the similar transient behavior of unstart shockwave system spawned by the flow choking and quasi-steady state of the unstart shockwave system at localized favorable pressure gradient are observed. In combustion driven unstart process, severe oscillatory flow motions of the jet and the unstart shockwave at the lip of the model inlet are observed. On the other hand, the unstarted flow motions triggered by mass addition remains relatively steady after the completion of the unstart process. The discrepancies between the unstart processes induced by the nitrogen and the ethylene jet are explained by flow choking analysis.