Multiphase reactant injection in rotating detonation combustors (RDCs) gives rise to additional complexities as compared to gas-gas systems, as liquid breakup and evaporation timescales are driven by the local unsteady flow field. Understanding the rate limiting factors in the mixing field can yield insight into understanding and predicting combustor behavior and performance. In this study, a kerosene-oxygen rotating detonation combustor with a movable innerbody was tested, allowing for variation of the fuel injection location relative to a fixed oxidizer annular slot injector. Tests were conducted with the geometry fixed across a test and with geometry variation through the test. Vacuum specific impulse is quantified, and was observed to change as the fuel injector position was altered. Peak combustor performance was achieved at an injector offset of 10 mm when the combustor geometry was fixed and at an injector offset of 5 mm when the innerbody was translated during the test. Combustor operating frequency, wave number, and wave speed all decreased as injector offset increased. A 1-D model is presented to examine the relationship between injector position and operating frequency. The liquid particle breakup timescale was well correlated with wave arrival time, indicating that breakup could be a rate controlling process for multiphase RDCs.

The spatio-temporal evolution of laser induced spark ignition of non-premixed gaseous methane and oxygen is investigated. The reactants are injected into an optically accessible combustion chamber from an oxidizer centered shear-coaxial injector. High-speed schlieren imaging and measurements of laser energy deposited are used to characterize ignition behavior at various spark locations throughout the chamber. A spatial map of ignition probability is generated from tests at multiple axial and radial locations from the point of propellant injection. The rate of pressure rise from successful tests reveals two modes of ignition dependent on the location of the laser-induced breakdown, direct and indirect, based on laser-induced breakdown occurring within reactant jet or in the recirculation region, respectively. Physical processes occurring over multiple timescales throughout the direct ignition method are first discussed. Ignition outcomes associated with the indirect method are determined by the hydrodynamic ejection protruding from the laser spark, the behavior of which is explored in detail using flow statistics extracted from images taken at a single spark location. The spatial–temporal progression of this jet is found to be dependent on deposited laser energy, leading to a relationship between the amount of energy added to the flow and ignition outcome. General bounds for spark location, deposited laser energy, and ejection behavior are identified to predict ignition outcomes. Cases that are an exception from these bounds are investigated in detail to understand the cause of unique behavior. These cases lie within regions of the variable space that are susceptible to stochastic elements of the flow.Novelty and Significance Statement: This work investigates laser induced spark ignition of O2 and CH4 issuing from a shear-coaxial injector in a model rocket combustor. Probability of ignition as a function of deposited energy and spatial location is studied with high-speed schlieren imaging and pressure measurements. Two distinct ignition modes that have previously not been identified, direct and indirect, are dependent on the laser deposition location with respect to the reactant jet. For indirect cases, where laser deposition is at a distance from the reactants, the hydrodynamic jet ejection generated from the breakdown of the plasma plays a critical role on ignition probability. The behavior of the ejection jet is characterized to provide a phenomenological description of the ignition process. The results of this research show the important interaction between chemical, fluid dynamic, and thermodynamic processes and their impact on laser ignition.

The reaction field within detonations of methane-oxygen-nitrogen and hydrogen-oxygen-nitrogen are imaged with OH planar laser induced fluorescence (PLIF) at a rate of 300 kHz. Simultaneously, 5 MHz schlieren and broadband chemiluminescence measurements are acquired. The OH PLIF and chemiluminescence images are compared and small-scale structures are resolvable with the PLIF that are otherwise integrated over in the chemiluminescence. In the methane fueled mixture, an asymmetric transverse wave collision is analyzed and by overlaying the schlieren and PLIF images, localized reaction is observed within vortices ejected from the symmetry line of the collision. In a hydrogen fueled mixture, a Kelvin-Helmholtz instability is resolved along the shear layer between the high-speed shock and transverse wave. The OH PLIF signal tightly conforms to the vortices on the high-speed shock side of the shear layer. The formation of an unreacted gas pocket is captured and as the pocket convects downstream increased OH PLIF emission around the periphery suggests that it burns deflagratively.

We examine two instances of transverse wave collision in a mixture of methane and oxygen diluted with 45% nitrogen. This mixture is classified near-limit and manifests a highly unstable structure. Simultaneous imaging of schlieren and broadband chemiluminescence was performed at a rate of 5 MHz to study the spatially- and temporally-coupled interactions between gas dynamic and chemical kinetic processes. Distinct modes of combustion are observed following transverse wave collision in each case. An unsteady reaction zone model is applied to understand the reactivity of the lead shock using measured values of the shock speed, acceleration, and curvature. In one case, an explosion behind the front immediately follows the creation of the new high-speed shock, which is an example of what is typically observed in detonation with regular cellular structure. Results of the model show that the high-speed shock directly initiates reaction before decaying and becoming non-reactive. In the other case, a reactive gas jet follows the high-speed shock, and no explosion occurs. The high-speed shock is unable to support reaction due to a combination of rapid deceleration and low shock speed. Analysis of the chemiluminescence field indicates that the reaction within the forward jet is supported by turbulent vortex structures and fresh reactants are provided along the shear layers between the high-speed shock and transverse wave. In each case a reverse jet is also observed that makes contact with unreacted gas pockets behind the front. The role of these jetting structures on wave propagation is discussed.

A thermodynamic model for rotating detonation combustion is developed which quantifies the impacts of deflagrative modes and incomplete combustion in the detonative cycle. Reactant preburning produces a larger loss in specific impulse than afterburning given preburning significantly reduces the Mach number and shock-induced pressure rise of the detonations. Incomplete combustion of reactants is shown to reduce the specific impulse by a larger amount than both preburning and afterburning. Recent experimental stagnation pressure measurements on an RDC enable modeling the relative contributions of preburning, afterburning, detonating, and incomplete combustion to the total heat release of the combustor. The amount of preburning is shown to increase significantly with increasing operating pressure while the heat release in the detonation decreased by similar amounts. The experimental RDC produced vacuum specific impulses up to 97.8% of what a deflagrative combustor could theoretically produce at equivalent operating conditions. The ideal detonation cycle suggests specific impulses 104.7% of the deflagrative maximum are achievable for these conditions. It is shown that the loss in specific impulse as a result of incomplete combustion of the reactants for all conditions tested is more severe than the loss due to deflagrative modes of combustion within the RDC.

An experimental stagnation pressure measurement technique is presented for a rotating detonation combustor (RDC). Schlieren imaging enables rotating detonation wave passage to be correlated with oscillations observed in the under-expanded exhaust plume. By measuring the spatiotemporal variation in exhaust plume divergence angle, stagnation pressure measurements of the RDC were acquired at a rate of 1 MHz. Combustor mass flux was varied between 202 and 783 kg/m2s, producing equivalent available pressures (EAPs) in the range of 3.42–13.5 bar. Time-averaged stagnation pressure measurements gathered using this technique were in agreement with the measured EAP within ±1.5%. Time-resolved stagnation pressure measurements allow for the pressure ratio produced across detonation wave cycles to be determined. For the conditions tested, detonation pressure ratios and wave speeds decreased while increasing the mean operating pressure of the combustor. Numerical modeling of the conditions tested indicates that the decrease in pressure ratio and wave speed is a result of elevated levels of combustion prior to the detonation wave arrival (i.e., “preburning”). Simultaneous OH* chemiluminescence measurements within the combustion chamber show an increase in preburned heat release relative to detonative heat release for increasing operating pressures of the RDC, in agreement with the results of the numerical model. Modeled chemical kinetic timescales decrease by approximately the same magnitude by which the preburning mass fraction increased in the range of operating pressures tested, suggesting that the faster reaction rates associated with higher pressure combustion may be the reason for increased preburning within the combustor.

View Video Presentation: https://doi.org/10.2514/6.2023-2377.vid The mechanics of laser induced spark ignition of non-premixed gaseous methane and oxygen are investigated in a model rocket combustor. The reactants are injected into an optically accessible combustion chamber from an oxidizer centered shear coaxial injector. High-speed schlieren imaging and measurements of deposited laser energy are used to characterize ignition behavior at various locations in the combustion chamber. A spatial map of ignition probability is generated at multiple axial and radial locations from the point of reactant injection at an equivalence ratio (��) of 1.34. As the spark location is moved from the reactant jet to the recirculation region of the combustion chamber, two distinct modes of ignition, direct and indirect, are observed. For direct ignition, ignition is achieved with every ignition attempt and flame kernel formation is achieved within 50 ��s of spark deposition. When the spark is located outside the reactant jet in the recirculation zone, ignition is achieved by interaction between the hot gas convected by hydrodynamic ejection out of the plasma kernel and the fresh reactant core. The ignition probability with indirect ignition is dependent on the spark location and the structure of the hydrodynamic ejection.

The impact of periodic detonation wave impact on a liquid fuel jet is investigated in a linear detonation combustor. The linear detonation combustor operated with gaseous natural gas and oxygen generates sustained, self-excited detonation waves that propagate along its length at approximately 8 kHz, representing a wave propagation frequency in typical rocket rotating detonation engines. The effect of the detonation wave on the dynamic injection and break-up of a single diesel jet injected into the combustion chamber at varying injection pressures is evaluated with chemiluminescence, fuel planar laser induced fluorescence and Mie scattering measurements at 100 kHz. The detonation wave significantly impacts the liquid jet trajectory with its deflection in both windward and leeward direction as the adverse pressure gradient across it changes between wave passages. The maximum recovery height of the liquid jet is observed to be consistent across all operating conditions, and dependent on the detonation wave strength in the chamber.

Megahertz rate schlieren images are captured to visualize the time-varying spatial structure of underexpanded flow exhausting from a Rotating Detonation Combustor (RDC). Periodic oscillations of the jet boundary divergence angle are observed in response to stagnation pressure changes induced by the passage of rotating detonation waves contained within the combustor. The time-averaged divergence angle of the exhaust plume is measured, and the average stagnation pressure of the combustor is reported. The average stagnation pressure obtained through the analysis of schlieren images is shown to converge to a value within ±��% of the Equivalent Available Pressure – an average stagnation pressure obtained from the thrust produced by the RDC. Results indicate that non-intrusive optical analysis of the RDC plume structure provides a means to quantify the stagnation pressure of the combustor.

The occurrence of reactive transverse waves in a narrow channel are studied in stoichiometric mixtures of methane and oxygen with 40-45% nitrogen dilution. Schlieren and CH* chemiluminescence measurements are obtained at MHz rates to study dynamic features of the detonation. Observations are made regarding the different ways that reactive and detonative transverse waves appear. A specific instance of transverse detonation is quantitatively analyzed and it is found that the shocked gas upstream of the transverse detonation builds up until consumed by the transverse detonation. The speeds of the transverse detonation and leading high-speed shock, itself a detonation, are measured to be moving at U_CJ and 1.2U_CJ, respectively.

Hydrocarbon fueled detonations are imaged in a narrow channel with simultaneous schlieren and broadband chemiluminescence at 5 MHz. Mixtures of stoichiometric methane and oxygen are diluted with various levels of nitrogen and argon to alter the detonation stability. Ethane is added in controlled amounts to methane, oxygen, nitrogen mixtures to simulate the effects of high-order hydrocarbons present in natural gas. Sixteen unique mixtures are characterized by performing statistical analysis on data extracted from the images. The leading shock front of the schlieren images is detected and the normal velocity is calculated at all points along the front. Probability distribution functions of the lead shock speed are generated for all cases and the moments of distribution are computed. A strong correlation is found between mixture instability parameters and the variance and skewness of the probability distribution; mixtures with greater instability have larger skewness and variance. This suggests a quantitative alternative to soot foil analysis for experimentally characterizing the extent of detonation instability. The schlieren and chemiluminescence images are used to define an effective chemical length scale as the distance between the shock front and maximum intensity location along the chemiluminescence front. Joint probability distribution functions of shock speed and chemical length scale enable statistical characterization of coupling between the leading shock and following reaction zone. For more stable, argon dilute mixtures, it is found that the joint distributions follow the trend of the quasi-steady reaction zone. For unstable, nitrogen diluted mixtures, the distribution only follows the quasi-steady solution during high-speed portions of the front. The addition of ethane is shown to have a stabilizing effect on the detonation, consistent with computed instability parameters. (c) 2022 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

We present experimental observations of the density field and reaction structure of methane and natural gas detonation waves propagating in a narrow channel. Simultaneous time-resolved schlieren and CH* chemiluminescence images are used to describe the structure of the unstable front. Nitrogen dilution concentration is varied, and the effect of increasing dilution is to increase the instability level and cell size and decrease the chemiluminescence intensity. Comparison is made between methane- and natural gas-fueled detonations. The effect of the higher hydrocarbons present in natural gas, primarily ethane, is to increase the fine-scale structure of the detonation front and create a more continuous reaction front. Utilizing the simultaneous images, observations are made about the formation and dissipation of the material separated across the shear layer behind the front.

Multi MHz schlieren measurements are performed of methane-oxygen 2D planar detonations diluted with between 20% and 33% nitrogen or argon. The 5 MHz temporal resolution provided by these images allows for key structures to be traced as they evolve. An edge tracking algorithm is developed and applied in order to compute the velocity of the leading front. A detailed velocity map is presented that shows the cellular structure of each detonation, similar to a soot foil, but also illustrates the dynamic evolution of the structure. Finally, kernel density estimates of the probability density function of front velocity and the first four moments are used to statistically describe the two-dimensional structure of unstable methane detonations.

Simultaneous 100 kHz particle image velocimetry (PIV) and 10 kHz OH * chemiluminescence (CL) were used to investigate flow and flame dynamics in a liquid-fueled, swirl-stabilized, piloted burner operated at 1.0 MPa. The PIV measurements were focused on the inner recirculation zone (IRZ), which forms downstream of an annular bluff-body feature that separates the pilot and main reactant jets and plays a critical role in flame stabilization. The OH * CL images captured the large-scale flame structure and dynamics. For the operating condition studied in this work, a self-excited combustion instability was present and caused axisymmetric variations in flame length and global heat release at a frequency of 810 Hz. Dynamic Mode Decomposition (DMD) was performed on the measured velocity fields to isolate fluctuations associated with the thermoacoustic mode from other dynamic flow processes. The DMD results showed that the 810 Hz instability causes strong fluctuations of both the axial and radial velocity components within the shear layer between the pilot jet and IRZ, along with concomitant changes in the size and shape of the IRZ. The DMD spatial modes also revealed periodic generation of vortices within the pilot-IRZ shear layer at frequencies ranging from 4-5.5 kHz. Wavelet analysis was used to study the interaction between these smaller-scale flow structures and the large-scale flow-flame dynamics associated with the combustion instability. The frequency magnitude related to vortex shedding was shown to fluctuate with the 810 Hz instability. The peak frequency magnitude followed the maximum radial velocity in the shear layer, which occurs when the IRZ is at its farthest radial extent and the velocity gradients in the pilot-IRZ shear layer are the strongest. The characterization of cross-scale, flow-flame coupling in this highly turbulent, high-pressure flame was enabled by the state-of-the-art 100 kHz PIV measurements. (c) 2020 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

We study the emergence of precessing vortex core (PVC) oscillations in a swirling jet experiment. We vary the swirl intensity while keeping the net mass flow rate fixed using a radial-entry swirler with movable blades upstream of the jet exit. The swirl intensity is quantified in terms of a swirl number S. Time-resolved velocity measurements in a radial-axial plane anchored at the jet exit for various S values are obtained using stereoscopic particle image velocimetry. Spectral proper orthogonal decomposition and spatial cross-spectral analysis reveal the simultaneous emergence of a bubble-type vortex breakdown and a strong helical limit-cycle oscillation in the flow for S > S-c where S-c = 0.61. The oscillation frequency, f(PVC), and the square of the flow oscillation amplitudes vary linearly with S - S-c. A solution for the coherent unsteady field accurate up to O(epsilon(3)) (epsilon similar to O((S - S-c)(1/2)) is determined from the nonlinear Navier-Stokes equations, using the method of multiple scales. We show that onset of bubble type vortex breakdown at Sc, results in a marginally stable, helical linear global hydrodynamic mode. This results in the stable limit-cycle precession of the breakdown bubble. The variation of f(LC) with S - S-c is determined from the Stuart-Landau equation associated with the PVC. Reasonable agreement with the corresponding experimental result is observed, despite the highly turbulent nature of the flow in the present experiment. Further, amplitude saturation results from the time-averaged distortion imposed on the flow by the PVC, suggesting that linear stability analysis may predict PVC characteristics for S > S-c.

OH* chemiluminescence and pressure measurements were acquired during the recommissioning of the Caltech Narrow Channel Facility (NCF) at Purdue. A high degree of versatility was built into the newly designed NCF reactant delivery system to allow for efficient testing of multiple reactant combinations. The ability of the dynamic injection, C2H2-O2 driven branched initiator to produce a planar wave capable of detonating less sensitive mixtures was verified. The detonation wave speed of multiple hydrogen-oxygen-argon and hydrogen-oxygen-nitrogen mixtures, spanning two instability regimes, was studied and agreement with the Chapman-Jouguet velocity was found for each. Ultrahigh-speed chemiluminescence imaging captured 8-10 frames of the detonation wave traveling through the optically-accessible section. These measurements revealed that the structure of the reaction zone increases in complexity as the mixture transitioned from the weakly unstable to moderately unstable regime.

Precessing vortex cores (PVC), arising from a global instability in swirling flows, can dramatically alter the dynamics of swirl-stabilized flames. Previous study of these instabilities has identified their frequencies and potential for interaction with the shear layer instabilities also present in swirling flows. In this work, we investigate the dynamics of precessing vortex cores at a range of swirl numbers and the impact that turbulence, which tends to increase with swirl number due to the increase in mean shear, has on the dynamics of this instability. This is particularly interesting as stability predictions have previously incorporated turbulence effects using an eddy viscosity model, which only captures the impact of turbulence on the base flow, not on the instantaneous dynamics of the PVC itself. Time-resolved experimental measurements of the three-component velocity field at ten swirl numbers show that at lower swirl numbers, the PVC is affected by turbulence through the presence of vortex jitter. With increasing swirl number, the PVC jitter decreases as the PVC strength increases. There is a critical swirl number below which jitter of the PVC vortex monotonically increases with increasing swirl number, and beyond which the jitter decreases, indicating that the strength of the PVC dominates over turbulent fluctuations at higher swirl numbers, despite the fact that the turbulence intensities continue to rise with increasing swirl number. Further, we use a nonlinear van der Pol oscillator model to explain the competition between the random turbulent fluctuations and coherent oscillations of the PVC. The results of this work indicate that while both the strength of the PVC and magnitude of turbulence intensity increase with increasing swirl number, there are defined regimes where each of them hold a stronger influence on the large-scale, coherent dynamics of the flow field.

Many industrial combustion systems, especially power generation gas turbines, use fuel-lean combustion to reduce NOx emissions. However, these systems are highly susceptible to combustion instability, the coupling between combustor acoustics and heat release rate oscillations of the flame. It has been shown in previous work by the authors that a precessing vortex core (PVC) can suppress shear layer receptivity to external perturbations, reducing the potential for thermoacoustic coupling. The goal of this study is to understand the effect of combustor exit boundary condition on the flow structure of a swirling jet to increase fundamental understanding of how combustor design impacts PVC dynamics. The swirling jet is generated with a radial-entry, variable-angle swirler, and a quartz cylinder is fixed on the dump plane for confinement. Combustor exit constriction plates of different diameters are used to determine the impact of exit boundary condition on the flow field. Particle image velocimetry (PIV) is used to capture the velocity field inside the combustor. Spectral proper orthogonal decomposition, a frequency-resolved eigenvalue decomposition that can identify energetic structures in the flow, is implemented to identify the PVC at each condition in both energy and frequency space. We find that exit boundary diameter affects both the structure of the flow and the dynamics of the PVC. Higher levels of constriction (smaller diameters) force the downstream stagnation point of the vortex breakdown bubble upstream, resulting in greater divergence of the swirling jet. Further, as the exit diameter decreases, the PVC becomes less energetic and less spatially defined. Despite these changes in the base flow and PVC coherence, the PVC frequency is not altered by the exit boundary constriction. These trends will help inform our understanding of the impact of boundary conditions on both static and dynamic flame stability.