Experimental measurements of fuel and flame temperatures in a methane/air co-flow diffusion flame are performed using coherent Stokes Raman scattering. A three-beam hybrid fs/ps configuration is employed such that the excitation bandwidth of the ultrafast pulses includes the rovibrational transitions in the Fermi-dyad of CO and the mode of CH. Time-domain spectroscopic models were developed and employed to simultaneously extract the CH and CO temperatures separately from the same CSRS spectra. Axial and radial temperature profiles of CH and CO in the diffusion flame are presented. Methane temperatures ranged from 300 to 900 K while CO temperatures ranged from 500 to 2500 K. At probe volume locations higher above the burner, the fuel and product temperatures were in strong agreement. Measurement locations near the methane injector revealed inhomogeneities between fuel and flame temperatures before significant mixing by diffusion. For the first time, a three-beam ultrafast CSRS configuration is demonstrated to simultaneously measure differing CH and CO temperatures.

This work describes the characterization of flame stabilization mechanisms present in a recirculation-stabilized ethylene-air flame in supersonic coflow. OH-PLIF measurements are presented to highlight the influence of fuel jet momentum on the combustion-stabilizing radical pool at the center of the recirculation zone. The time-averaged shape of the jet interaction region and its proximity to areas of high heat release in the shear layer are also described as a function of fuel jet momentum. Decomposition of high-speed chemiluminescence images of the electronically excited methylidyne (CH*) radical indicates the presence of a large-scale instability. The flame base position extracted from the chemiluminescence data and the fuel jet penetration height extracted from high-speed schlieren measurements are analyzed in the time domain to reveal a temporal coupling. This coupling highlights the sensitivity of flame stabilization to the dynamics of the fuel injector.

Laser ignition of rocket propellants offers significant advantages over conventional ignition systems, including reduced weight and improved reliability. However, the complex multi-physics processes governing ignition remain poorly understood, particularly regarding the role of thermal expansion effects. This study combines high-fidelity large-eddy simulations (LES) with experimental validation to investigate the mechanisms controlling laser ignition success in a subscale model rocket combustor using gaseous methane and oxygen as propellants. Novel numerical schlieren and chemiluminescence imaging techniques based on physics-based principles are developed to enable direct comparison with experimental data. Four ignition scenarios with varying fuel mass flow rates are examined, ranging from immediate flame quenching to rapid flame anchoring. The LES accurately reproduces experimental pressure traces, schlieren imaging, and chemiluminescence patterns, validating the computational approach. Results demonstrate that successful ignition depends critically on thermal expansion effects rather than traditional turbulent flame propagation mechanisms. When sufficient fuel is present in the chamber, thermal expansion generates upstream flow velocities that enable flame kernel propagation against the high-velocity jet flow and subsequent anchoring at the combustor inlet. The study reveals an optimal fuel concentration range that ensures reliable ignition while limiting structural overpressure, providing practical guidelines for rocket engine design. These findings challenge conventional understanding of rocket ignition mechanisms and offer new pathways for developing lighter, safer, and more reliable propulsion systems.Novelty and significanceThis study presents a paradigm shift in understanding rocket ignition mechanisms by demonstrating that thermal expansion effects arising from sudden consumption of reactants, rather than traditional turbulent flame propagation, can govern successful laser ignition in gas rocket combustors. The study challenges the conventional understanding that requires flammable mixtures in recirculation zones for ignition success. Consequently, lower fuel amounts inside the combustor can be used to reduce the overpressure without jeopardizing ignition outcome. These findings offer new design pathways for propulsion systems by leveraging thermal expansion mechanisms rather than relying on excessive reactant buildup, providing practical guidelines for developing lighter and safer rocket engines.

Laser induced spark ignition was investigated at sub-atmospheric nozzle exit pressure in a model rocket combustor. An oxygen-centered shear-coaxial injector with methane as the fuel was used in the experiment. Particle image velocimetry (PIV) and schlieren imaging were used to characterize the oblique shock waves and expansion fans that form within the supersonic underexpanded pre-ignition reactant flow. The flow field included regions of reduced velocity and higher pressure (downstream of oblique shocks) and regions of increased velocity and low pressure (downstream of expansion fans). The axial velocities in the jet spanned from 350 to 480 m/s while radial velocities spanned from-25 to 25 m/s, with increases and decreases commensurate with the oblique shocks and expansion fans. The changes in velocities directly correlated with variations in laser energy deposition and consequently ignition probability. An experimental curve fit relating laser energy deposition to local velocity magnitude is generated. Fractional changes in flow velocity were shown to correspond with order of magnitude changes in deposited laser energy. Low velocity regions after oblique shock waves resulted in a larger fraction of laser energy deposited and highest probability of ignition, high velocity regions after expansion fans resulted in significantly less laser energy deposited and less favorable or zero probability of ignition. Novelty and significance statement: Miniaturized laser ignition systems carry immense potential for robust ignition capabilities for in-space propulsion systems where multiple restarts are necessary. Laser ignition was investigated with supersonic co-axial reactant injection at sub-atmospheric conditions representative of in-space propulsion devices using particle imaging velocimetry, schlieren, and chemiluminescence measurements. Highspeed measurements of local-velocity, pressure, and density fields allowed characterization of the mechanics of CH4-O2 ignition and consequent flame stabilization. Probability of ignition success was spatially mapped and correlated with laser energy deposition which was directly influenced by local velocity within the underexpanded jet structure. The coupled behavior of local supersonic flow velocity, pressure, and input energy dictated the resulting energy deposited and outcome of laser ignition. This work provides quantitative evidence of the criticality of laser focal location with respect to local flow features in the ignition of a supersonic flow

ABSTRACT Experimental measurements of fuel and flame temperatures in a methane/air co‐flow diffusion flame are performed using coherent Stokes Raman scattering. A three‐beam hybrid fs/ps configuration is employed such that the excitation bandwidth of the ultrafast pulses includes the rovibrational transitions in the Fermi‐dyad of CO and the mode of CH. Time‐domain spectroscopic models were developed and employed to simultaneously extract the CH and CO temperatures separately from the same CSRS spectra. Axial and radial temperature profiles of CH and CO in the diffusion flame are presented. Methane temperatures ranged from 300 to 900 K while CO temperatures ranged from 500 to 2500 K. At probe volume locations higher above the burner, the fuel and product temperatures were in strong agreement. Measurement locations near the methane injector revealed inhomogeneities between fuel and flame temperatures before significant mixing by diffusion. For the first time, a three‐beam ultrafast CSRS configuration is demonstrated to simultaneously measure differing CH and CO temperatures.

Time-resolved thermometry is vital for quantitative characterization of reacting turbulent flows but remains largely limited to point measurements, making higher-dimensional approaches attractive for capturing spatiotemporal thermal gradients. In this work, a one-dimensional chirped-probe pulse coherent anti-Stokes Raman scattering (CARS) instrument has been developed for gas-phase thermometry. Calibration and characterization of the instrument are detailed, and its capability is demonstrated through measurements in a laboratory flame. The ro-vibrational transitions of N2 are targeted in a uniform temperature flow field to calibrate the instrument and extract necessary laser parameters. The best-fit modeled laser parameters are examined in detail. Variations in laser parameters along the measurement line are captured and show dependence on non-uniform beam propagation and probe volume number density. After calibration, the instrument is successfully applied to a thermal gradient generated with a cooled-wall located perpendicular to the flow of the burner combustion products.

Abstract The impact of fuel temperature and composition on flame structure and in situ soot formation is investigated at 1.0 MPa using spatially resolved laser-induced incandescence (LII) measurements. Concurrent hydroxyl chemiluminescence (OH* CL) measurements are used to correlate soot formation and retention with flame heat release. Experiments were conducted over a range of fuel temperatures spanning from 300 K to 575 K, and two liquid hydrocarbon fuels (Jet A and hydroprocessed esters and fatty acids synthetic paraffinic kerosene (HEFA-SPK)). Operating conditions are selected to correspond to mission-relevant low-power pilot-only operation for taxiing and approach, and high-power pilot-and-main operation for cruise. During pilot-and-main operation, fuel aromatic content is vital for the production of soot regardless of fuel temperature. Without aromatics in the fuel, soot production is drastically reduced. For fuels with aromatic content, increasing fuel temperature results in a shift in soot formation structure from small, high-intensity pockets to more frequent, less intense dispersed fields. Pilot-only operation operates locally at an equivalence ratio well above critical sooting limits, resulting in soot formation for Jet A as well as HEFA-SPK. LII signal intensity is ∼3.5 times more intense with aromatics present in the fuel at this condition, indicating their importance in soot formation. The inclusion of aromatics in the fuel results in more rapid soot formation and shifts the location of peak signal intensity upstream. Elevated fuel temperatures for pilot-only operating conditions result in more rapid soot consumption in the downstream region of the combustor only when aromatics are present.

A thermally perfect, one-dimensional model that is capable of evaluating flow properties along the isolator and combustor of a dual-mode ramjet operating in either a ramjet or scramjet mode is described. The model is incorporated into a multistage optimizer that computes an estimated fuel-burning profile for a given experimentally measured static pressure profile, and this solution is used to infer information about additional flow properties and the combustion process. Measurements obtained in a 1X-scale, ethylene-fueled, axisymmetric dual-mode ramjet are used with this optimizer to determine how combustion efficiency and heat addition vary with respect to different parameters. It is determined that there is a statistically significant difference in the heat release distribution between ramjet and scramjet operating modes, with the rate of heat release near the entrance of the combustor being larger for ramjet modes. Combustion efficiency in ramjet operating modes is found to be dependent on combustor residence time and velocity at the combustor entrance, whereas combustion efficiency in scramjet operating modes is largely independent of combustor flow properties and is instead primarily dependent on flameholder geometry.

Atmospheric soot from aircraft emissions negatively affects the environment and human health and indicates an inefficiency in combustion. A diagnostic set-up to inform the design and operation of aircraft engines was developed, leveraging laser diagnostic methods, to reduce harmful soot emissions. Laser-induced incandescence (LII) is a diagnostic that allows for non-intrusive measurements of soot volume fraction and primary particle size in combustion environments. International Sooting Flame (ISF) target conditions were used for a canonical laminar diffusion flame. Soot volume fraction measurements were conducted and compared with previous experimental values to validate the accuracy of the experimental setup and techniques used. Time-resolved laser-induced incandescence was conducted at 10 MHz to determine the primary particle size of soot particles. The validated LII experimental setup was applied to a liquid-fueled swirl-stabilized flame at aircraft engine-relevant conditions. LII signal was measured for both conventional and sustainable aviation fuels. A comparison of soot quantity and location was made for a range of equivalence ratios and fuel composition.

The influence of fuel composition on self-excited combustion instabilities in a high-pressure combustor operated with ammonia, hydrogen, and natural gas fuels is characterized with high-frequency pressure measurements and imaging of the flame structure. A micromix multi-stage injector with 19 elements is used to introduce the fuel blend premixed with heated air into an optically accessible combustor operated at-1.1 MPa. As hydrogen is substituted for natural gas, longitudinal thermoacoustic instabilities are observed in the combustor with pressure fluctuation amplitudes as large as 8% of the mean chamber value. In the absence of natural gas, limit-cycle instability magnitude is largely insensitive to ammonia addition. However, fuel compositions with > 20% natural gas result in positive correlation between instability amplitude and ammonia concentration. Spatial distribution of heat release in the combustor evaluated from OH* chemiluminescence imaging reveals axial growth in the regions of heat release fluctuation as hydrogen decreases, thus correlating flame length to instability amplitude. Distinct transitions from the first harmonic of the fundamental longitudinal acoustic mode (-1100 Hz) to the fundamental mode (-550 Hz) are observed with root mean square pressure fluctuations exceeding 7% of the mean chamber value. These cases correspond to hydrogen mole fractions <= 50%. Analysis of the phase relationship between pressure fluctuations upstream and downstream of the flame zone indicates acoustic coupling of the injector as a key contributor to instability growth.

Flow-flame interactions in a solid fuel ramjet combustor operating under conditions representative of supersonic flight are investigated in an optically accessible 2-dimensional combustor. Experiments were conducted with two exit nozzle configurations. The average cross-flow velocity is affected with a change in nozzle dimensions and its effects on flame structure and turbulent flow characteristics are studied. High-speed CH* chemiluminescence imaging was employed to capture heat release dynamics, while particle image velocimetry resolved the velocity field at the flow reattachment region. Concomitant high-frequency pressure measurements complemented these diagnostics. The results demonstrate that an increase in cross-flow velocity leads to a more compact flame and a smaller recirculation zone. Enhanced velocity gradients and higher root mean square velocity fluctuations were observed near the fuel grain, promoting turbulent mixing and transport. Spectral proper orthogonal decomposition analysis revealed dominant frequencies associated with flame and velocity fluctuations, which varied significantly between nozzle configurations.

One-dimensional chirped-probe pulse coherent anti-Stokes Raman scattering measurements are demonstrated at elevated temperatures up to 1060 K in an atmospheric pressure flame. Sufficient signal-to-noise ratios are observed for 50 shot averaged measurements at 20 Hz and single-shot measurements at 1 kHz. The probe volume region has a spatial dimension of 2 mm parallel to the bulk flow direction. The signal image is binned into 18-pixel regions with an effective spatial resolution of 135 mu m in the 1 kHz measurements. The variation in laser parameters across the measurement line are investigated and reasonable variations in input beam dispersion characteristics are reported. Some distortion of the wavefront is observed, indicated by variations in pump/Stokes delay and probe time delay. The fitted laser parameters are applied to measurements acquired in a thermal boundary layer to demonstrate temperature sensitivity along the line measurement.

The impact of fuel composition on flame dynamics is investigated at 10 bar using 100 kHz hydroxyl planar laser induced fluorescence (OH-PLIF) measurements. The PLIF images are synchronized with high-frequency pressure measurements at distinct locations in the chamber to investigate mechanisms for thermo-acoustic feedback with the flame heat release and resonant acoustic modes of the combustor. Experiments were conducted over a range of fuel compositions spanning a hydrogen fuel fraction ( chi ) of 30% to 90%, and an ammonia decomposition efficiency ( eta ) from 40% to 100%. In predominantly hydrogen-rich fuel mixtures ( chi , eta > 0.8), flame stabilization occurs along the shear layer, and flame holding is observed closer to the burner face. As methane composition increases ( chi < 0.8), significant flame corrugation and commensurate high heat release rate are observed as chemical kinetic rates and thermodiffusive effects decrease. In these cases, OH radical production and transport between neighboring injector elements are observed. The flame heat release and transport of combustion products in this region occur during the compression phase of the acoustic cycle and are responsible for ignition of fresh reactants. These periodic ignition, heat release, and flame displacement at the resonant acoustic mode frequency of the combustor result in self-excited longitudinal combustion instabilities ( p (RMS ') / p (CC )similar to 1-7%). With significant concentrations of methane and ammonia in the fuel blend ( chi < 0.9, eta < 0.6), the amplitude of this instability increases ( p (RMS )' / p( CC) > 7%), and periodic global extinction and re-ignition of flames are observed with acoustic expansion and compression.

The reactivity of transverse waves in detonations of methane, oxygen and nitrogen are experimentally assessed using MHz rate schlieren and chemiluminescence imaging. In these highly unstable mixtures, the mode of wave propagation is more complex than what is described by the cellular instability model that is conventionally used for weakly unstable mixtures. Behind the low-speed leading shock in unstable waves, the processed gas remains essentially unreacted until transverse waves reach this region. In highly unstable waves, the transverse waves have a range of reactivity, that is rates of reaction in the flow immediately behind the wave. In this study, we present examples of transverse waves for near-limit detonations and analyse four cases in detail. In some cases, these waves appear to be essentially non-reactive or cause very slow reaction. In other cases, the transverse waves can be highly reactive. In the most extreme example, the transverse wave is propagating at the Chapman-Jouguet speed with a small reaction zone, i.e. a transverse detonation. A reactive oblique shock model is used to approximate the triple-point configuration of this case as a double-Mach reflection, which shows good agreement with the images. The reaction evolution along path lines is analysed using detailed reaction mechanisms and considerations about flow-field unsteadiness. Length scales of the energy release and expansion processes within the reaction zone region are used to explain the observed modes of wave propagation and interaction.