Justin Hardi

Injector-Driven Combustion Instabilities in a Hydrogen/Oxygen Rocket Combustor

Self-excited combustion instabilities of the first tangential mode have been found in a research combustor operated with the cryogenic propellant combination of hydrogen/oxygen. In a series of consecutive test campaigns, the influence of operating conditions on these self-excited combustion instabilities was examined. This included a variation of the combustion chamber pressure, the mixture ratio, and the propellant temperatures. It has been shown how these operating parameters influence the resonance frequencies of the combustion chamber. The analysis of the influence of operating conditions on the oscillation amplitude of the first tangential mode indicated that the instability occurred when the frequency of the first tangential mode of the combustion chamber was shifted into the frequency of the second longitudinal mode of the liquid oxygen injector. With a variation of the injector length, and therefore its longitudinal resonance frequencies, this hypothesis has been tested. Based on the experimental ...

LOx Jet Atomization Under Transverse Acoustic Oscillations

Testing has been conducted with the BKH rocket combustor at the European Research and Technology Test Facility P8 for cryogenic rocket engines at DLR Lampoldshausen. BKH has multiple shear coaxial injectors and an exhaust modulation system for forcing excitation of acoustic resonances in the combustion chamber. Optical access windows allow the application of parallel high-speed shadowgraph and flame emission imaging of the near-injector region. This paper reports measurements of the intact liquid oxygen core during forced excitation of the first transverse acoustic mode. High-speed shadowgraph images show that the mechanism of core breakup and atomization differs between off-resonance and first transverse excitation conditions. The core length is found to decrease with increasing amplitude of acoustic pressure, or equivalently with transverse acoustic velocity, with a core length reduction of up to 70% for conditions approaching those of naturally occurring high frequency combustion instabilities. This de...

Acoustic characterisation of a rectangular rocket combustor with liquid oxygen and hydrogen propellants

High frequency combustion instabilities have plagued the development of liquid propellant rocket engines since their invention. Continuing research efforts aim to understand the mechanisms by which the oscillating combustion chamber pressure of self-sustaining combustion instabilities is driven. To this end, a rectangular combustor with acoustic forcing, designated ‘BKH’, was developed to study flame-acoustic interaction under conditions which are representative of real rocket engines. This article describes the acoustic characterisation of the BKH combustor using high frequency dynamic pressure measurements from the first hot-fire tests using liquid oxygen and ambient temperature or cryogenic hydrogen injectants. Analysis of the resulting high frequency pressure measurements shows excellent agreement with the predicted acoustic properties of the system which were calculated using finite element methods.

Approaches for Comparing Numerical Simulation of Combustion Instability and Flame Imaging

Combustion dynamics are controlled by the coupling between heat addition and gas dynamic modes, and their direct measurement and comparisons with prediction are key to improving computational tools, as well as our fundamental understanding of the problem. This paper demonstrates several methods for characterizing and comparing dynamic combustion response from experiment and simulation. A model rocket combustor that exhibits self-excited instabilities is used for the study. Comparisons are made for two configurations: one stable and one unstable. High-speed chemiluminescence imaging from the experiment was first phase averaged, then treated with an Abel inversion routine to produce a dynamic two-dimensional distribution of heat addition. The distribution before inversion could be compared with line-integrated calculations of the heat release rate from three-dimensional large-eddy simulations, and after inversion to azimuthally averaged cross sections from simulations. Modal decomposition of the two-dimensi...

Study of LOx/H2 Spray Flame Response to Acoustic Excitation in a Rectangular Rocket Combustor

This work is focused on understanding how intrinsic processes may be involved in driving high frequency combustion instabilities. The response of an experimental rocket combustor to both unperturbed and acoustically excited conditions at four different operating points was studied. The combustor has a rectangular cross-section with acoustic forcing, five shear coaxial injection elements running cryogenic oxygen-hydrogen (LOx/H2), and optical access. The operating points comprise two different chamber pressures and two different hydrogen injection temperatures. The chamber pressures of 40 and 60 bar allow the influence of sub- and supercritical LOx, respectively, to be studied. The H2 injection temperatures of ~290 K and ~50 K were selected since low H2 temperature has long been thought to influence the stability of LOx/H2 engines. Combustor response was compared between the four operating points under both undisturbed conditions, and forced resonance of the 1L and 1T modes. The measures used to compare combustor response were acoustic spectra, modal acoustic energy distribution, LOx jet atomization behavior, flame displacement, and chemiluminescent emission intensity. None of the parameters compared revealed a significant difference in response between the four operating points. There were two exceptions of interest. The first was in the distribution of unperturbed acoustic energy to the 1L and 1T modes. The content of both modes was lower at 60 bar, regardless of H2 temperature, except the 1T mode was more energetic at 60 bar when using low temperature H2. The second was the greater decrease in intact LOx core length under 1T-mode excitation with low temperature H2. Both of these observations may be related to the historic tendency for transverse mode instability with decreasing H2 temperature.

Coupling Behaviour of LOX/H2 Flames to Longitudinal and Transverse Acoustic Instabilities

A rectangular combustor with acoustic forcing was used to study flame-acoustic interaction under injection conditions which are representative of industrial rocket engines. Hot-fire tests using liquid oxygen and gaseous hydrogen were conducted at pressures of 40 and 60 bar, which are sub-and supercritical conditions respectively for oxygen. To our knowledge, acoustic forcing has never before been conducted at pressures this high in an oxygen-hydrogen system. Examined samples of hydroxyl-radical emission imaging, collected using a high-speed camera during periods of forced acoustic resonance, show significant response in the multi-injection element flame. Transverse acoustic velocity causes shortening of the flame, concentrating heat release near the injection plane. Fluctuating acoustic pressure causes in-phase fluctuation of the emission intensity. Based on these observations, a theorized flame-acoustic coupling mechanism is offered as an explanation for how naturally occurring high frequency combustion instabilities are sustained in real rocket engines.

Development of validation approaches for numerical simulation of combustion instability using flame imaging

Combustion dynamics are controlled by the coupling between heat addition and gas dynamic modes, hence their direct measurement and comparisons with prediction are key to improving computational tools as well as our fundamental understanding of the problem. Whereas gas dynamic modes are relatively easy to measure and compare, measurement and characterization of heat addition are much more difficult due to its complexity and the lack of any direct means of measurement. This paper demonstrates several methods for characterizing and comparing heat release from experiment and simulation. A model rocket combustor is used for the study. Comparisons are made for two configurations, one stable and one unstable. High-speed, line-of-sight imaging of OH* emission from the experiment was first phase averaged, and then treated with an Abel inversion routine to produce a dynamic, two-dimensional field of heat addition. The field before inversion could be compared to line-integrated calculations of heat release rate from three-dimensional large eddy simulations, and after inversion to azimuthally averaged cross-sections from simulations. Modal decomposition analysis of the two-dimensional fields were performed. The applicability and limitations of each comparison approach are assessed.