Development of a Facility for Combustion Stability Experiments at Supercritical Pressure

Abstract

Combustion instability in liquid rocket engines can have severe consequences including degraded performance, accelerated component wear, and potentially catastrophic failure. High-frequency instabilities, which are generally the most harmful in liquid rocket engines, can be driven by interactions between disturbances associated with transverse acoustic resonances and the combustion process. The combustion response to acoustic perturbation is a critical component of the instabililty mechanism, and is in general not well understood. The current paper describes an experimental facility at the Air Force Research Laboratory (AFRL) at Edwards Air Force Base that is intended to investigate the coupling between transverse acoustic resonances and single/multiple liquid rocket engine injector flames.Critical aspects of the facility will be described, including the capability to operate at supercritical pressures that are relevant to high-performance liquid rocket engines, accurately-controlled and cryogenically-conditioned propellants, and optical access to facilitate the use of advanced diagnostics. The transverse acoustic resonance is induced through the use of carefully-controlled piezoelectric sirens, allowing monochromatic excitation across a range of amplitudes at a number of discrete frequencies. The location of the flame within the acoustic resonance mode shape can also be varied through relative phase control of the two acoustic sources. The operating space of the facility, for oxygen and hydrogen operation, will be described. Preliminary nonreacting and reacting data will also be presented to demonstrate the quality of operation of this facility. It is anticipated that future results generated using this facility will provide both fundamental insight into the acoustic-flame interactions as well as provide a database useful for validating combustion instability models. NomenclatureA = area c = speed of sound 2 D 1 = inner injector inner diameter D 2 = inner injector outer diameter D 3 = outer injector inner diameter D 4 = outer injector outer diameter dt = time step f F = acoustic frequency m = waveguide flare constant p c = mean chamber pressure pʹ = acoustic pressure perturbation amplitude t = inner post thickness uʹ = acoustic velocity perturbation amplitude x = streamwise (longitudinal) coordinate axis y = spanwise (transverse) coordinate axis z = depthwise coordinate axis

A Study of Acoustic Forcing on Gas-Centered Swirl-Coaxial Reacting Flows

“…The GCSC injector was tested in the Combustion Stability Lab (EC-4) at Edwards AFRL CA, using RP-2 and gaseous oxygen. A detailed review of the complete system can be found in the work of Wegener et al 15 . The system has since been upgraded to accommodate liquid hydrocarbon propellants.…”

The Response of Cryogenic H2/O2 Coaxial Jet Flames to Acoustic Disturbances

“…Ignition is achieved via a novel ignition system that will be described in future publications due to patent-related issues. Further background on the facility is described by Wegener et al 7 A variety of coaxial jet injector models have been investigated at AFRL under nonreacting flow conditions. 9,23 The coaxial jet injector geometry and near field flow dynamics are illustrated in Fig.…”

“…Previous work at AFRL has made major contributions towards understanding the coupling between acoustic waves and rocket injector flows in a nonreacting regime. [2][3][4][5][6][7][8][9] These experiments employed a high-pressure test facility that allowed for investigation of coaxial jets flowing inert surrogates, generally gaseous,liquid, and supercritical nitrogen, and exposing this shear flow to a variety of acoustic conditions that are representative of a transverse resonance that would occur in a rocket engine. In subcritical conditions, the center flow of the coaxial jet is cooled to a liquid condition, while the outer annular flow is chilled but maintained as a gas.…”

Receptivity of a Cryogenic Coaxial Gas-Liquid Jet to Acoustic Disturbances

“…The facility is equipped with heat exchangers to allow cooling of the inner and outer injector flows. Further details on the experimental facility can be found in Wegener et al 7 A single coaxial jet injector was used for the experiments. The dimensions of the injector are summarized in Table 1.…”

“…2, 3 These flows have also been exposed to a range of transverse acoustic conditions. 4-6 A new experimental facility for reacting coaxial jet flows has been developed as described by Wegener et al 7 The present paper describes preliminary nonreacting data that was collected using this new facility. Although a large quantity of data has been collected in the past for nonreacting conditions, the current data considers a new framework in which to explore the forcing response of coaxial jets.…”