Supersonic combustion mechanisms were investigated experimentally in a transversely injected ethylene-fueled scramjet model combustor with a cavity flame holder. Laser-Induced Breakdown Spectroscopy (LIBS) was employed to quantify local equivalence ratios, with simultaneous CH* chemiluminescence measurements to identify heat-release regions. The LIBS measurement locations were scanned precisely using a 3-axis stage controller actuated by stepping motors. Experiments were conducted in a Mach 2.0 airflow at a stagnation temperature of 1900 K and a stagnation pressure of 0.37 MPa. Four global equivalence ratios, 0.071, 0.154, 0.194, and 0.239, were examined, corresponding to cavity shear-layer stabilized, jet-wake stabilized, oscillation between jet-wake and ram, and ram combustion modes, respectively. The results revealed a common flamestabilization structure across all combustion modes. The flame tip was located near a fuel-rich region at an equivalence ratio of 2-3, whereas the primary reaction zone, indicated by strong CH* chemiluminescence, appeared farther downstream in the near-stoichiometric region. Despite substantial changes in the overall flame structure, this feature persisted for all combustion modes. Together with our previous results obtained using a different fuel and injection configuration, the same flame stabilization structure was observed, indicating that this characteristic behavior is consistent in cavity-stabilized supersonic combustion. Moreover, the time-averaged LIBS results provided insight into the instability mechanism between the jet-wake and ram combustion modes. Relative to the jet-wake mode, the oscillation mode exhibited a broader near-stoichiometric mean mixture field, whereas the ram mode showed a lower-penetration and richer distribution closer to the wall and cavity shear layer. Combined with previous image-based instability analyses, these support an interpretation in which vortexinduced fuel transport promotes a mean mixture distribution favorable for heat release and drives the transition between jet-wake and ram combustion states. Novelty and significance statement This study provides a quantitative characterization of local equivalence-ratio distributions associated with combustion-mode transition and instability in a cavity-stabilized dual-mode scramjet combustor under realistic high-enthalpy supersonic flow conditions. By applying laser-induced breakdown spectroscopy together with CH* chemiluminescence, the work directly links localized mixture structure to flame stabilization across cavity shearlayer, jet-wake, oscillatory, and ram combustion modes. A common flame-stabilization structure is identified in all modes: a fuel-rich flame-tip region and a downstream near-stoichiometric main reaction zone. The measurements further reveal that the oscillatory mode corresponds to an intermediate mixture state between the high-penetration jet-wake mode and the lower-penetration ram mode, providing new experimental evidence for the role of localized fuel transport in mode transition. These findings advance the physical understanding of supersonic combustion instability and provide a useful diagnostic and design basis for low-instability, highperformance scramjet combustors.

Transient concurrent flame spread over solid surfaces involves multiple interacting physical processes, where simplified modelling with effective parameter identification is still challenging. This study presents a modelling framework that combines scaling-based formulation with Bayesian parameter inference to quantitatively capture the transient behaviors of concurrent flame spread. Experimental data under varying airflow speeds and temperatures are used to train the model. Results show that the scale model, using the inferred parameters, can accurately predict the time evolution of key quantities, including the positions of flame tip, flame drag front, pyrolysis front, pyrolysis end, and mass loss rate, confirming the validity of the approach. Most inferred parameters are consistent with values reported in the literature, while the Markov Chain Monte Carlo (MCMC) framework can also identify the influence of the experimental boundary conditions and reflects it in the inferred flow-related parameters. The mechanisms behind the increase in flame spread rate with airflow speed and temperature are also analysed. Flow speed accelerates flame spread by increasing flame temperature, reducing flame stand-off distance, enhancing surface heat flux and mass loss, and extending the preheating zone. In contrast, the flow temperature has a milder effect, primarily by increasing flame temperature and reducing re-radiative heat loss from the solid surface. This study demonstrates the potential of combining scaling models with Bayesian inference to model complex transient flame spread phenomena.

The flammability limit of cylindrical polymethyl-methacrylate (PMMA) samples in an opposed-flow was observed experimentally under various gravity levels at 1G or more utilizing a centrifuge. Flame spread on the sample was observed, and the limiting oxygen concentration (LOC) was measured in various opposed-flow velocities ranging from 5 to 30 cm/s. Two rotating radii of the centrifuge were tested to assess the effects of Coriolis force on the LOC. A scale analysis model, based on an energy balance equation, was evaluated for thermally thick cylindrical samples. The buoyancy-induced flow was modeled in the analytical model. Model parameters were evaluated with a Bayesian approach: The Metropolis Hasting (a Markov Chain Monte Carlo) method. The results indicated that flames were tilted by the Coriolis force when the oxygen concentration was sufficiently rich from the LOC in 4G. However, the effects were negligible under the oxygen concentration conditions near LOC. No significant difference in the LOC was also observed even when the rotating radius of the centrifuge changed. At 1G or higher, the LOC approached to a certain value asymptotically with decreasing the opposed-flow velocity. The LOC also increased with increasing the gravity level and sample diameter. The measured LOCs were used to sample model parameters using MCMC methods. Consequently, a suitable fitting curve of the model to the experimental LOC was obtained. Using the predicted model parameters, the LOC under microgravity, Moon, and Mars conditions were predicted as a function of the opposed-flow velocity. In microgravity, the LOC increased greatly with decreasing the forced flow velocity (the radiative extinction regime), and the minimum LOC (MLOC) was recognized. On Moon, the credible interval of LOC was rather large in the low-velocity region, and the MLOC was also recognized. Moreover, it was possible that the cylindrical PMMA sample exhibited the highest flammability not under microgravity but a partial gravity condition.Novelty and SignificanceFrom observing the limiting oxygen concentration of the flammability limit for cylindrical polymethyl-methacrylate (PMMA) samples in opposed-flows under various gravity conditions, we developed a model of limiting oxygen concentration (LOC) for thick cylindrical samples with the buoyancy induced flow was developed, and we estimated difficult-to-predict parameters using Bayesian statistical methods. Using this model, we can estimate the flammability limits under various gravity conditions of manned space exploration, including Moon and Mars. In addition, the present study shows that the minimum limiting oxygen concentration (MLOC) is the lowest in partial gravity. These results open a new research field for partial gravity combustions, apart from the normal gravity and microgravity and contribute to the understanding on the fire safety on manned space explorations to Moon and Mars.

Near-limit downward flame spread over cylindrical thermoplastic materials under various gravities, oxygen concentrations and opposed flow velocities were investigated experimentally. Black low-density polyethylene tubes with stainless-steel (SS) and copper (Cu) cores were tested. Key parameters, including flame spread rate (FSR) and limiting oxygen concentration (LOC), were measured, and flame oscillations were analyzed using continuous wavelet transforms. Results show that, for SS-core samples, gravity-induced stretch effects reduce the flame temperature, weakening the dripping effect. Dripping seldom occurs near LOC. The trends of FSR and LOC are consistent with blow-off theory. The dominant frequency for oscillatory flame front increases with gravity, while the amplitude decreases owing to shorter dripping lengths. For Cu-core samples, far from extinction, the metal core acts as a heat source, enhancing dripping and FSR, especially in hypergravity. Near the LOC, the Cucore shifts to a heat sink, thus the flame-flow interaction begins to dominate the FSR. LOC for Cu-core sample is higher than that for SS-core sample, and increases more mildly with gravity and opposed flow. Flame oscillations are more intense in hypergravity due to a stronger dripping flow, but it becomes less important near LOC. This study provides insights into the flammability of thermoplastic material, contributing to fire safety in spacecraft.

In this study, the combustion mechanism of turbulent jet ignition (TJI) with a super-rich hydrogen pre-chamber under globally lean conditions to achieve efficient combustion with low NOx emissions is investigated. Experiments were conducted in a rapid compression machine using a diaphragm-isolated pre-chamber filled with uniformly rich mixtures. Time-resolved measurements of OH* chemiluminescence images, near-infrared emission images from water molecules, and pressure in both chambers were performed to analyze the ignition and subsequent flame propagation. Furthermore, large-eddy simulations (LES) were conducted to investigate the ignition characteristics and flame structures in more detail. Results indicated that peak pressures for TJI were slightly lower than for conventional spark ignition (SI) at the same global equivalence ratio, likely due to additional heat losses. However, TJI significantly increased the pressure rise rate, enabling more efficient combustion in ultra-lean conditions by enhancing the degree of constant-volume heat release. Distinct flame structures were observed experimentally, consisting of a bright core plume surrounded by lower-intensity zones. During the TJI process, a large initial pressure difference between the pre-chamber and main chamber resulted in flame lift-off, characterizing the jet-ignition mode. As the pressure gap decreased, the flame transitioned to the flame-ignition mode, characterized by the attachment of the flame to the nozzle. Statistical analysis concluded that the ignition mode transition duration and lift-off height level during the transition exhibited an inverse correlation with pre-chamber richness and global equivalence ratio. The 2 mm orifice reduced pressure gaps and facilitated a faster mode transition compared to the 1 mm orifice. LES results showed good agreement with the experiments and exhibited the flame mechanism consisting of an outer lean premixed zone and an inner non-premixed core. Although stoichiometric combustion in the non-premixed region was observed H2O from pre-chamber products likely contributed to dilution, which may have helped limit the temperature rise and reduce NOx formation.Novelty and Significance StatementThe novelty of this study lies in the investigation of lean combustion mechanisms using turbulent jet ignition (TJI) with a super-rich hydrogen pre-chamber as a staged combustion technology with low NOx emissions. A unique combustion mechanism consisting of a non-premixed core diluted by H2O and surrounding lean flame front was identified. The significance of this work is its potential to stabilize ignition under globally lean conditions while effectively controlling combustion temperatures to reduce NOx emissions. These findings offer valuable insights for the practical application of TJI in hydrogen internal combustion engines, paving the way for high-efficiency, low-emission designs in future hydrogen-powered systems.

Scaling analysis offers a promising approach for predicting the flammability of electrical wires in manned space missions, yet determining the high-dimensional model parameter vector remains challenging, particularly when extrapolating to unknown gravities. This study introduces an effective method for predicting the limiting oxygen concentration (LOC) of electrical wires across different gravities, by integrating physical flame spread modeling with the data-driven technique and ground-based experimental data. A flame spread model for thin wires was developed based on the energy conservation law, and the LOC is determined by the existence of a stable-physical solution to the resulting nonlinear equation. Model parameters were estimated using a Bayesian approach, specifically the Markov Chain Monte Carlo (MCMC) method, trained on experimental LOC data under various hypergravity and opposed flow conditions. The results demonstrate the model's capability to predict LOC curves accurately across gravity levels, providing fire safety margins based on uncertainties in model parameters and experimental measurements. The physics underlying the inferred parameter values was discussed, showing the important role of boundary layer effect and structure of the flame front. The interdependence of model parameters governing the blowoff or quenching branches allows for their values to be inferred via posterior distributions or inherent correlations. Interestingly, despite notable variations in quenching and blowoff branches, the predicted minimum LOC (MLOC) remains largely unchanged from 0 g to 4 g, thus it appears feasible to consider MLOC as a quasi-intrinsic property of the tested wire itself. Stable flame spread cannot be sustained in low-speed, low-gravity environments, even in oxygen-enriched atmospheres, due to the significant radiative heat loss. This "physics-informed data-driven" modelling approach bridges the gap between uncertainties in scaling analysis and practical flammability evaluations, thereby offering a more reliable methodology for screening spacecraft materials using ground-based facilities that support fire safety for space missions to Moon and Mars.

The supersonic combustion mechanism was experimentally investigated using a Mach 2.5 n-dodecane fueled ramjet/scramjet dual-mode combustor with a cavity flame holder at stagnation pressure of 1021 kPa and a stagnation temperature of 1847 K. Heated n-dodecane was injected from the cavity closeout ramp. Optical diagnostics, including CH* and OH* chemiluminescence and Laser induced breakdown spectroscopy (LIBS), were employed. During steady combustion, the cavity shear-layer stabilized combustion was established autonomously without any assistance of the torch igniter. The local equivalence ratio was measured by moving the breakdown points using a 3-axis high precision motorized stage with measurements of CH* chemiluminescence in the cavity flame holders. Additionally, measurements were performed at fixed positions at 30 Hz as the fuel injection rate increased gradually. The results indicated the intensive OH* chemiluminescence emissions near the lower wall near the closeout ramp during the stable cavity shear-layer combustion. The equivalence ratio in the lower region of the flame was sufficiently rich, exceeding 2, while the flame region with high OH* and CH* signals was stoichiometric or lean. A steep gradient in the equivalence ratio was observed across the flame. During the transient combustion, as the fuel injection rate increased gradually, the equivalence ratio within the cavity increased. Additionally, the equivalence ratio within the cavity was high on the step side and decreases toward the ramp side. Near the flame regions exhibiting strong CH* chemiluminescence signals, the equivalence ratio approached unity. When the flame left the measurement point, the equivalence ratio exceeded 2, showing a substantial gradient across the flame. The flame tip was located near the fuel rich region exceeding 2. The findings of this study provide valuable insights for the design of kerosene-fueled scramjet combustors. Furthermore, the steady cavity flame demonstrated here has potential applications as a piloted flame for staged combustions.

Dynamic mode decomposition (DMD), based on Koopman analysis, is a tool capable of spatiotemporal analysis for various spatial resolutions, from one-dimensional signals to three-dimensional computational fluid dynamics (CFD) and experimental data. Outputs of the DMD consist of amplitudes, frequencies, decaying rates, and spatial modes. However, the effects of spatial resolution (time-series data in one-dimensional signal and spatial grid in two-dimensional data) and quantitative analysis of DMD are limited to one-dimensional signal data. In this study, the effects of spatial resolution with a fixed time scale of data and correction using scaling factors 2/A on DMD amplitudes and A on DMD spatial mode strengths are investigated, where A is the number of the time-series data in one-dimensional signal or the number of the spatial grid in two-dimensional data. First, proofs of the scaling factors for one-dimensional(line layout) and two-dimensional(grid layout) data are presented. Second, the effect of spatial resolution on the amplitudes and spatial mode strengths and their scaled results are confirmed using one-dimensional artificial signal data, two-dimensional artificial signal field data, two-dimensional vortex shedding simulation data, and two-dimensional pulsating flow experimental data with various data resolutions. The results show that the amplitude increases proportionally with the spatial resolution, and the spatial mode strength is inversely proportional to the time series or spatial resolution of the data in all cases. As a result of applying the scaling factors to one-dimensional artificial signal and two-dimensional artificial signal field data, the amplitudes and spatial modes contain the same values regardless of the change in spatial resolutions. The scaled amplitudes and spatial mode strengths on vortex shedding simulation and two-dimensional laminar pulsating jet show good agreements with slight differences, regardless of the spatial resolution change. The proposed scaling factor can be applied to compare data quantitatively obtained with different spatial resolutions.

The present study investigates the effects of fuel penetration height on combustion instabilities in an ethylenefueled scramjet model combustor with a cavity flameholder. Experiments were performed at the stagnation temperature of 1900 K, the stagnation pressure of 0.37 MPa, and the Mach number of 2. Three fuel injection orifice diameters:2.3, 2.4, and 2.5 mm, were tested to elucidate the effects of ethylene penetration height on combustion instabilities. Furthermore, high-speed measurements of CH* chemiluminescence and shadowgraphs were performed with high-speed video cameras. To examine the dynamics of the combustion instabilities, timeresolved CH* chemiluminescence images and shock parameters, extracted from snapshots of shadowgraphs, were analyzed using a non-linear dimensionality reduction algorithm: Gaussian Process Dynamical Model (GPDM). Thereafter, further analyses on the acquired latent variables were conducted with Recurrence Plot (RP). The experimental results showed that the fuel equivalence ratio (phi) range for cavity shear-layer combustion mode expanded as d decreased. Furthermore, for phi = 0.18, the instability behavior remarkably changed at around d = 2.4 mm. Therefore, the instability dynamics for phi = 0.18 were investigated using GPDM and RP including results from our previous study (d = 2, 3, and 4 mm), revealing differences in the instability behaviors. For d = 3 and 4 mm, jet-wake combustion and ram combustion modes were established alternately with a frequency of about 1600 Hz. In contrast, at d = 2.4 and 2.5 mm, although a similar instability in the d = 4 mm case was present at almost the same oscillation frequency, a different instability behavior was also confirmed. This additional instability exhibited an intermediate state between jet-wake combustion and ram combustion modes. These two instabilities emerged aperiodically. For an unstable combustion in shear layer observed for d = 2 and 2.3 mm, corresponding RPs exhibited black patches, indicating that the oscillation amplitude diminished substantially.

To clarify the effect of oxidative degradation of vegetable oils on their droplet combustion behaviors, a single -droplet combustion experiment was carried out. Refined palm oil (RPO) and Jatropha curcas oil (JCO) were used as representative vegetable oils. First, these vegetable oils were oxidatively degraded for 12 h by the Rancimat method to produce degraded RPO (RPO12) and JCO (JCO12), respectively. The effects of degradation on fuel properties were evaluated in terms of the kinematic viscosity and fatty acid composition, which were measured with an Ubbelohde viscometer and GC-MS, respectively. The results revealed that JCO was more susceptible to oxidative degradation and its kinematic viscosity significantly increased after degradation. This was due to its high content of unsaturated fatty acids, such as oleic and linoleic acids. Then, single-droplet combustion experiments were performed at 0.1 MPa and 1023 K. A droplet with a diameter of approximately 0.5 mm was inserted into an electric furnace and spontaneously ignited. Puffings were observed under all conditions, regardless of fresh or degraded RPO and JCO. The puffing periods of RPO12 and JCO12 were longer than those of RPO and JCO due to the longer time from bubble expansion to droplet disruption. Furthermore, the change in the droplet diameter caused by droplet disruption was suppressed for JCO12 and RPO12, which was probably attributed to the increased kinematic viscosity. Finally, the diameter of secondary droplet released by the droplet disruption was explored. The average diameters of secondary droplet of RPO12 and JCO12 were larger than those of RPO and JCO, and this was probably because of the increase in surface tension. Novelty and Significance Statement: The novelty of this study is that a series of experiment, i.e., production of oxidatively degraded vegetable oils, fuel property analysis (viscosity measurement and fatty acid composition analysis), and droplet combustion experiment, were simultaneously carried out. Thus, effects of oxidative degradation on combustion behavior were systematically clarified for the first time. This study is significant in that it provides the knowledge on treating vegetable oils in internal combustion engines, which is necessary for realizing a carbon neutral society.

The excitation mechanism of the thermoacoustic combustion instability (TCI) in a hydrogen-rich ram combustor was investigated experientially, focusing on the effects of jet-to-crossflow momentum ratios on combustion instabilities. The momentum ratios were varied using two orifice diameters: 1.0 mm and 1.4 mm, while maintaining the constant equivalence ratio. Combustion experiments were performed using a high-enthalpy wind tunnel. To capture the transition from combustion noise to the TCI, the equivalence ratio was increased gradually from 1.5 to 2.7 for each orifice diameter. To understand the unsteady initiation process of TCI, the time-resolved OH* chemiluminescence images and combustion pressure were analyzed using a neural network inspired by Koopman theory (Koopman network). The experimental results showed that TCI was excited at higher equivalence ratio for the larger injection orifice. Prior to the initiation of TCI, a low frequency oscillation at less than 400 Hz was observed. The frequency increased with the momentum ratios and finally reached the frequency of the longitudinal oscillation mode of the combustor (440 Hz). Koopman network analysis implied that the low frequency oscillation mode was induced by the oscillation of the shear layer of the hydrogen jets impinging on the wall, which was characterized by the Strouhal number based on the shear layer length. As the momentum ratios increased, shear layer length decreased due to the increase in the penetration height of hydrogen jets. This led to the increase in the frequency of the impinging shear layer oscillation. When the frequency reached that of the longitudinal oscillation mode, the local flame blowout was observed, involving the oscillation of the impinging shear layer. Koopman network analysis also indicated that the oscillation of the impinging shear layer and the local flame blowout induced the TCI.

The effects of the burnt-to-unburnt temperature ratio (Tb/Tu) (reciprocal density ratio) on thermoacoustic combustion instability were investigated in a hydrogen-rich ram combustor by controlling the inlet air temperature under cruising conditions. Steady and unsteady combustion dynamics were investigated with the simultaneous measurement of combustion pressure and near-infrared (NIR) emissions. Time-resolved NIR images were analyzed using dynamic mode decomposition (DMD) to extract modes of the combustion instability, which show two oscillation modes: longitudinal (500 Hz) and flame-vortex interaction modes (2000 Hz) with a Strouhal number of approximately 0.5. The thermoacoustic combustion instability was not excited at Tb/Tu = 3.3, but was excited at Tb/Tu= 2.6 and 2.9. The DMD modes show that the structure of the vortex mode is similar to that of the fourth longitudinal mode at Tb/Tu = 2.6 and 2.9, indicating that the coupling of the vortex and the fourth longitudinal modes excited the thermoacoustic combustion instability.

Abstract The dynamic mode decomposition(DMD), based on the Koopman analysis, is a tool capable of spatiotemporal analysis of one-dimensional signals to three-dimensional CFD data. Outputs of the DMD consist of amplitudes, frequencies, decaying rates, and spatial modes. However, the effects of data resolution (size) and the quantitative analysis of the DMD are limited to one-dimensional signal data. In this study, the effects of the data resolution and quantitative analysis using scaling factors 2/√M on the DMD amplitudes and √M on the DMD spatial mode strengths are investigated, with M being the data size. Firstly, proofs of the scaling factors for one-dimensional and two-dimensional data are presented. Second, the effect of data size on amplitudes and spatial mode strengths and their scaled results are confirmed using one-dimensional artificial signal data, two-dimensional artificial signal field, two-dimensional vortex shedding simulation, and a two-dimensional pulsating flow experiment data with various data resolutions. The results show that the amplitude increase proportionally to the size of the data, and the spatial mode strength is inversely proportional to the size of the data in all cases. As a result of applying the scaling factors in one-dimensional artificial signal and two-dimensional artificial signal field data, the amplitudes and spatial modes contain the same values regardless of the change in resolutions. The scaled amplitudes and spatial mode strengths on vortex shedding simulation and two-dimensional laminar pulsating jet show good agreements with slight differences regardless of the resolution change. The proposed scaling factor can be applied to compare data quantitatively obtained with different resolutions.

Effects of fuel jet penetration height on supersonic combustion behaviors were investigated experimentally in a supersonic combustion ramjet model combustor at a Mach speed of 2 and at a stagnation temperature of 1900 K. The jet-to-crossflow momentum flux ratio was varied to control the fuel-jet penetration height, using several injectors with different orifice diameters: 2, 3, and 4 mm. First, transverse nitrogen jets were observed to identify a relationship between the fuel jet penetration height and the momentum flux ratio by focusing Schlieren photography. Then, supersonic combustion behaviors of ethylene were investigated through com-bustion pressure measurements. Simultaneously, time-resolved images of CH * chemiluminescence and shad-owgraphs were recorded with high-speed video cameras. Furthermore, a morphology of supersonic combus-tion modes was investigated for various equivalence ratios and fuel penetration heights in a two-dimensional latent space trained by the shared Gaussian process latent variable models (SGPLVM), considering CH * chemiluminescence images and the shock parameters. The results indicated that the penetration height of ni-trogen jets was a function of the jet momentum flux ratio; this function was expressed by a fitting curve. Five typical combustion modes were identified based on time-resolved CH * chemiluminescence images, shadow -graphs, and pressure profiles. Even for a given equivalence ratio, different combustion modes were observed depending on the fuel penetration height. For an injection diameter of 3 and 4 mm, cavity shear-layer and jet-wake stabilized combustions were observed as the scram modes. On the other hand, although the cavity shear-layer and lifted-shear-layer stabilized combustions were observed, no jet-wake stabilized combustion was observed for an orifice diameter of 2 mm. Fuel penetration heights above the cavity aft wall were ex-pected to affect the combustion behavior. Finally, a morphology of the supersonic combustion modes was clearly shown in the two-dimensional latent space of the SGPVLM. & COPY; 2022 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

Influences of the spark-plug mounting angle on the spark ignition of gasoline-air mixtures in high velocity flows were investigated in a combustion chamber connected at the end of a rapid compression machine. Gasoline-air mixture with an air excess ratio of 1.9 was combusted. A spark plug was mounted on the combustor wall at angles of −30°, −15°, 0°, 15°, and 30°. Flow velocity at the spark plug was varied from 5 to 20 m/s by adjusting the driving pressure of the piston. Time resolved spark-discharge voltage and current were measured with a voltage and a current probe, and the discharge energy was calculated using them. Behaviors of the discharge channel were observed optically with a high-speed video camera in addition to simultaneous measurements of OH* chemiluminescence from the flame kernel with the other high-speed video camera. The results showed that the averaged induction time of flame kernel initiation increased monotonously with an increase in the plug mounting angle from −30° to 30° in flows with velocities of 5 and 10 m/s. However, the increasing trend became less obvious with velocities of 15 and 20 m/s, involving with larger variation in induction time. Discharge duration became shorter with an increase in the plug mounting angle regardless of the same setup. Negative mounting angle resulted in frequent short-circuits/restrikes at the early phase. On the other hand, positive mounting angles resulted in a longer discharge channel length, higher energy deposition before the first restrike and longer induction time of the first restrike. Although the discharge channel at positive mounting angles elongates along with the flow, the early flame kernel was developed toward the center of the swirl. Thereafter, flame propagated outward from the central region, where the flow speed was lower than the downstream area. Negative mounting angle led to a more oval flame kernel shape and the propagation closer to combustion chamber wall, which showed a shorter induction time of flame kernel initiation in the cases of 5 and 10 m/s. However, fragments of the flame kernel tended to be formed due to frequent short-circuits with the velocities of 15 and 20 m/s. This resulted in the large variation of the induction time.

In order to improve the ignition stability and reduce the cycle-to-cycle variation, it is necessary to understand the mechanism of the flame kernel development and the local quenching effect during the spark ignition process. In this study, experiments for the spark ignition process in a high-speed lean gasoline-air mixture turbulent flow field were conducted. OH* chemiluminescence measurement and focusing Schlieren photography was applied to observe the development of flame kernel and discharge channel behaviors simultaneously. Results indicated that flame kernel fragments, generated by the restrike and short- circuit of discharge channels, quenched due to the local turbulence, which led to slow flame propagation or misfire. In that cases, the initial flame kernels showed stretched behaviors, along with high curvatures.

The pre-cooled turbojet (PCTJ) engine has been studied in the Japan Aerospace Exploration Agency (JAXA). Although conventional jet engines operate under fuel lean conditions, the PCTJ engine is designed to operate with an equivalence ratio beyond 2 in the afterburner. The jet noise intensity can increase with increasing the equivalence ratio due to the increase in the exhaust velocity. Combustion instabilities have also been a major concern in terms of acoustic radiation under rich conditions, which involve intensive pressure oscillations resulting in combustion noise. In this study, effects of equivalence ratio on the acoustic characteristics of the afterburner are investigated using a high enthalpy wind tunnel. As a result, combustion noise is observed at lean and rich equivalence ratios but has a little contribution to the overall acoustic radiation. Regarding the jet noise, the sound intensity is almost the same regardless of the equivalence ratio since the convective velocity does not change so much.

Flame kernel formations of close dual-point laser induced sparks were investigated experimentally, fo-cusing on the hydrodynamic effects induced by an interaction of shock waves produced by the laser induced sparks. Dual sparks were produced near the center of the combustion chamber by splitting of a ray emitted by a 532 nm Nd:YAG laser. Methane/air mixtures were ignited under a quiescent condition in a constant volume chamber with detailed measurements of the ignition energy and the pressure history. The minimum ignition energy was derived as an ignition energy having an ignitability of 50% using the logistic regression method. The flame kernel initiation process was also observed by Schlieren photography using a high-speed video camera. The offset of laser induced sparks were adjusted by tuning angles of mirrors and lenses. The ignition performance of single-and close dual-point laser breakdown induced sparks was investigated in de-tail in terms of the minimum ignition energy and the combustion induction time. Time resolved Schlieren photographs indicated that two hump shaped kernels grew rapidly during the initial stage in the vicinity of the plane of symmetry defined by the laser sparks under certain conditions. Their formation was due to the hy-drodynamic effects induced by Mach shock waves, which resulted from interactions of the dual shock waves. The minimum ignition energy of the close dual-point laser induced sparks near the lean limit at 1.0 MPa was much lower than that of single-point laser induced sparks, although it was greater than that of the single ones at 0.1 MPa. The combustion induction time, which was defined as the time corresponding to the maximum pressure increase rate, was shortened for close dual-point laser induced sparks, especially for lean mixtures at high pressure. Robust flame kernels were formed by close dual-point laser induced sparks with Mach shock wave formation, and improved ignition performance for lean mixtures at high pressure was observed.(c) 2023 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

An experimental study on dielectric barrier discharge (DBD) plasma-assisted ignition was performed through the use of a plug-type DBD electrode at various pressures p up to 500 kPa. The ignition behaviors of lean methane-air mixtures with high-frequency DBD were investigated near the lean limit in a constant volume chamber. Using a high-speed video camera, the plasma formations of the DBD were observed for air and a mixture of the equivalence ratio phi = 0.3. Plasma spectroscopy was also conducted with an echelle grating and imaging spectrometers. The time-averaged detailed plasma emission spectra from 400 nm to 920 nm and the transient spectrograms in the vicinity of the N 2 second positive system (372-382 nm) for phi = 0.5 were measured. The minimum ignition energy (MIE), minimum discharge duration, and minimum amplitude of the applied voltage were obtained through logistic regression. The results indicated that a plasma emission lifetime became much longer than the discharge time with an application of the high-frequency sinusoidal voltage at high pressure. The longest plasma lifetime was observed at a discharge frequency of 1,180 kHz. The plasma lifetime showed the maximum at a specific pressure. The detailed spectroscopy also showed that continuum spectra at 500 kPa could be fitted to the Planck's curve with temperature above 30 0 0 K at 50 0 kPa. The profiles of the MIE as a function of pressure exhibited the minimum in the vicinity of 100 kPa. The existence of the optimal frequency for high-efficiency ignition was shown. At the fixed frequency, an increase in the applied voltage amplitude resulted in a decrease in the MIE. In contrast, the effects were saturated over a certain threshold. At 500 kPa, time-resolved plasma spectroscopy showed a rapid local gas heating to the flame temperature over a specific frequency of the applied voltage. This indicated that there was the optimal frequency where the MIE was the minimum. (c) 2021 The Combustion Institute. Published by Elsevier Inc. All rights reserved.