Kernel Propagation Research Articles

Roles of fuel composition on the ignition process of endothermic hydrocarbons

Due to the characteristics of heat absorption and decomposition, endothermic hydrocarbon fuels (EHFs) have been widely used in scramjets for thermal protection and heat recirculation. The understanding of ignition characteristics of EHFs is of great importance for their safe and efficient utilization. In this paper, the ignition processes of EHFs were numerically simulated at atmospheric pressure and with an initial temperature of 500 K. Three different ignition stages were identified based on the chemical heat release and flame kernel propagation. A 3-component kerosene surrogate model composed of n-dodecane, methyl cyclohexane and m-xylene was adopted, as well as the corresponding chemical kinetic model with 369 species and 2691 reactions. Results showed that the discrepant decomposition characteristics of n-alkanes and cycloalkanes affected the chemical heat release and propagation during the ignition process. Two-stage exothermic characteristic was observed in the time evolutions of chemical heat release rate and fuel decomposition. The mass production of molecules and accumulation of radicals dominated the first and second exothermic peaks, respectively. Furthermore, the minimum ignition energies (MIEs) of EHFs with various methyl cyclohexane were determined to quantify the effect of fuel composition on ignition performance. Characteristically, the MIE dramatically decreased from 10.2 to 2.15 mJ when 20% n-dodecane was replaced by methyl cyclohexane. However, it was slightly increased as methyl cyclohexane continued to increase. Analyses from both physical and chemical aspects were conducted to elaborate the dependence of MIE on fuel composition. The dominant effects of flame-dynamic and chemical effects on different ignition stages were analysed. The faster propagation speed and stronger endothermic ability of methyl cyclohexane led to the nonlinear variation of MIEs. The results in this study provide useful guidance for composition optimization and safety evaluation of EHFs.

Forced ignition of premixed cool and hot DME/air flames in a laminar counterflow

Recently, low-temperature chemistry (LTC) and LTC-induced cool flames have attracted extensive interest since they can substantially influence the ignition process and accelerate the subsequent hot flame propagation. However, it is unclear how the flow affects the forced ignition of a cool flame and its subsequent transition to a hot flame. In this study, we conduct transient 2D simulations for the forced ignition processes of premixed cool and hot flames in a laminar, axisymmetric, counterflow configuration with well-defined flow field. Different ignition energies and strain rates are used to ignite and stabilize cool and/or hot flames in a counterflow. The critical conditions for the ignition of cool and hot flames are identified, and the cool flame kernel quenching and its transition to hot flame are discussed. It is found that the ignition energy determines the highest temperature and thereby controls the thermal runaway process. The strain rate influences the flame kernel propagation and the transition from cool flame to hot flame. A large strain rate may quench the cool flame, while a small strain rate may lead to the transition from cool flame to hot flame due to the sufficiently long residence time. Therefore, cool flame ignition and stabilization can only be achieved within a certain range of strain rates. Furthermore, an ignition regime diagram in terms of ignition energy and strain rate is proposed, and four regimes are identified: (I) only cool flame, (II) only hot flame, (III) ignition failure, and (IV) transition from cool flame to hot flame (with appearance of double flame structure). These results provide new insights into how the flow affects the cool flame ignition and transition.

An investigation on the laminar flame propagation and auto-ignition characteristics of a coal-derived rocket kerosene-Part I: Experimental study

The energy structure of coal-rich and oil-poor in China, makes the conversion of coal to liquid fuels an attractive option to reduce petroleum dependence. In this study, the combustion characteristics of a newly developed coal-derived rocket kerosene were investigated to provide fundamental parameters for evaluating the feasibility and compatibility of the new fuel in potential applications. The laminar flame propagation properties of the fuel were studied at pressures of 1, 2 and 5 bar over a temperature range of 423–483 K for equivalence ratio of 0.7–1.8 in a constant volume combustion bomb, and the auto-ignition characteristics of the target fuel were then experimentally studied in a heated shock tube. Flame propagation process was studied based on the recorded flame morphology firstly. The unsteady transition from spark assisted ignition kernel propagation to normal flame was analyzed and was found to become less conspicuous in flames with stronger reaction intensity (higher initial pressure or temperature) and smaller Lewis number (higher equivalence ratio). The Markstein length was extracted by extrapolating the flame trajectories and used to investigate the flame instability. The results shows that the increase of pressure, equivalence ratio enhance the flame instability, but the impact of initial temperature shows no prominent pattern. The laminar burning velocity and ignition delay times were reported over a wide range to strengthen the understanding of combustion characteristics of the new fuel. A kinetic model was proposed to simulate the combustion chemistry of the fuel and kinetic analysis was performed to identify the key reactions driving the flame propagation and auto-ignition. The present study is hoped to provide insights into the evaluation of fuel application in the future and database support for further combustion reaction kinetic mechanism development.

On the ignition kernel formation and propagation: an experimental and modeling approach

The next generation of advanced combustion devices is being developed to operate under ultra-high-pressure conditions. However, under such extreme conditions, flame tends to become unstable and measurement of fundamental properties such as the laminar flame speed becomes challenging. One potential method to resolve this issue is measuring the ignition-affected region during spherically expanding flame experiments. The flame in this region is more resistant to perturbations and remains smooth due to the high stretch rates (i.e. small radii). Stable flame propagation allows for improved flame measurement, however, the experimentally observed kernel propagation is a function of both inflammation and ignition plasma. Therefore, the goal of the present study is to better understand the plasma formation and propagation during the ignition process, which would allow for reliable laminar flame speed measurements. To accomplish this goal, thermal plasma operating at high pressures is studied with emphasis on the spark energy effects on the formation of the ignition kernel. The thermal effect of the plasma is experimentally observed using a high-speed Schlieren imaging system. The energy dissipated within the plasma is measured with the use of voltage and current probes with a measurement of plasma sheath voltage drop as an input to numerical modeling. The measured kernel propagation rate is used to assess the accuracy of the model. The experiments and modeling are conducted in dry air at 1, 3, and 5 atm as well as in CH4-N2 mixtures at 1 atm, and kernel radius, temperature, and mass are reported. The voltage-drop (as a non-thermal loss) is measured to be approximately 330 5 V (dry air at 1 atm) for glow plasma with a large dependency on pressure, gas composition, electrode surface quality, electrode geometry, electrode shape, and current density. The same loss within the arc plasma is measured to be 15 5 V, however the arc phase loss which agrees with arc propagation is significantly higher (∼45 V) which suggest additional unaccounted for phenomena occurring during the arc phase. With these losses, the modeling results are shown to predict the final kernel radius within 10%–20% of the observed kernel size. The difference found between the modeling and experimental results is determined to be a result of assuming that the primary loss mechanism (voltage drop across sheath formation) remains constant for the duration of glow discharge. The discrepancy for arc discharge is discussed with several potential sources, however, additional studies are required to better understand how the arc formation affects the kernel propagation.

The effect and mechanism of the flow deflector on ignition performance of the centrally staged combustor

Lean premixed prevaporized combustors with a centrally staged scheme are capable to reduce NOx emissions. Ignition is one of the key performances of the centrally staged combustor. The present study proposes a novel method to improve ignition performance by using a flow deflector. The effects of various flow deflector lengths and pressure drops on ignition performance and flame kernel propagation are investigated in this work. It is found that ignition performance is significantly improved by the flow deflector. The ignition process is obtained using a high-speed camera under different operating conditions. The timescale of the successful ignition process is analyzed using a statistical method, revealing the effects of the flow deflector length and pressure drop on the timescale of each phase of ignition. The flame kernel propagation trajectory is extracted and analyzed by combining the flow and spray fields. The mechanism of the flow deflector is analyzed by numerical simulation. It is found that with the flow deflector, the local fuel/air ratio and droplet diameter are both improved, which benefits ignition performance. This work proves that the flow deflector is a potential method to improve ignition.

Study on ignition process and flame expansion and propagation characteristics in jet-cooled pilot flameholders using image processing techniques

Wall-type pilot flameholder has been used in aero-engines to effectively create a stable ignition source and achieve an excellent ignition performance in high-velocity flows. The ignition delay, flame propagation, and flame boundary are crucial characteristics to evaluate the performance of pilot flameholders. Previous work displayed the significantly changed ignition performance and flow features of the wall flameholder when jet cooling schemes were adopted, indicating that the ignition process and the flame features strongly depend on the cooling jet and the optimal design of jet cooling schemes needs further investigation. Therefore, in this study, two practical image processing algorithms are developed to characterize the flame development of the ignition process, reveal the effect of cooling schemes on the ignition delay time, and quantitatively analyze the flame area and expansion ratio of the wall flameholder during stable combustion. Two image processing methods proposed based on MATLAB algorithms exhibit effectiveness in studying the ignition process and flame expansion characteristics. The ignition process is divided into four phases: kernel generation, kernel propagation, flame growth, and stable combustion. Meanwhile, the ignition delay time is identified by counting the total pixels of the flame projected region during the ignition process. The edge detection and pixel statistics of the flame projected region in the binary luminosity image show the flame boundary and expansion ratio under various working conditions. It was found that the cooling jet angle has a significant impact on the ignition process, ignition delay time, flame propagation, and steady flame features. The pressure-driven gas jet cooling scheme with a smaller jet angle and the external-inhaled air cooling scheme with a larger jet angle present a better capability of ignition and flame expansion and propagation.

Effect of the ignition location on lean light-off limits for a gas turbine combustor

The ignition performance of the spray flame is an important safety parameter in a gas turbine combustor. Previous studies have shown that the spark axial location has a significant influence on the ignition performance and flame propagation process, and further research is required. The ignition performance and flame kernel propagation at different spark radial locations in a gas turbine model combustor are studied experimentally in this paper to summarize the influence of spark location on ignition performance from the perspective of kernel propagations, and to find the optimal ignition location. The flame propagation trajectory of each ignition location is obtained through high-speed imaging tests. Four conclusions are obtained as follows: 1) In a successful ignition event, flame kernels from different ignition positions will eventually propagate to the same position, which is defined as the flame holding position (FHP). After the flame residence stage, the kernel will begin to grow until the entire combustor is ignited. The FHP is with low axial velocity, small turbulent fluctuation and appropriate fuel concentration to effectively maintain the flame. 2) The ignition performance is the worst when the ignitor is near the wall, and is the best when the ignitor is near the FHP, that is, the inner boundary of the recirculation zone. 3) The ignition performance deteriorates if the flame has to cover greater distances before reaching the FHP where it can ignite the flame stabilization zone. A spark at the FHP can greatly reduce the propagation distance of the flame kernel, thereby avoiding the obstruction of the propagation process. 4) The propagation trajectory of the flame kernel is related to the ignition location and is mainly controlled by the non-reacting flow field and fuel concentration.

Effects of inter-pulse coupling on nanosecond pulsed high frequency discharge ignition in a flowing mixture

This work numerically investigates the effects of non-equilibrium nanosecond plasma discharge pulse repetition frequency, pulse number, and flow velocity on the critical ignition volume, minimum ignition energy, and chemistry in a plasma-assisted H2/air flow at 300 K and 1 atm using a multi-scale adaptive reduced chemistry solver for plasma assisted combustion (MARCS-PAC). The interactions between discharges/ignition kernels spanning decoupled, partially-coupled and fully-coupled regimes in a pulse train are studied. For a single pulse discharge, increased flow velocity increases the minimum ignition energy required due to the increase of convective heat loss and flame stretch. The results show that the minimum ignition kernel propagation speed at the critical ignition kernel volume increases with the flow velocity. The minimum critical ignition volume decreases with the increase of plasma discharge energy. For sequential two-pulse discharges, ignition fails at both decoupled and partially-coupled regimes even when the total discharge energy is above the minimum ignition energy, but succeeds only in the fully-coupled regime at a shorter inter-pulse time. Overlap of the OH radical pool between the sequential two-pulse discharges and the increase of the chemistry effect due to the increase of reduced electric field in the fully-coupled regime contribute to the ignition enhancement. In addition, for two-pulse discharges in the fully-coupled discharge regime, the mixture can be ignited at a total energy below the minimum ignition energy of a single pulse with the same flow conditions. Moreover, for a given total discharge energy with multiple pulsed discharges, the enhancement of the ignition kernel volume has a non-monotonic dependence on discharge frequency and pulse number. The effective ignition enhancement can be achieved with an optimal pulse repetition frequency and pulse number. This work provides a new understanding of the mechanism for repetitive plasma ignition and insights for the optimization of plasma ignition in a reactive flow.

Theoretical analysis on the forced ignition of a quiescent mixture by repetitive heating pulse

Recently, forced ignition by nanosecond-repetitive-pulsed-discharge (NRPD) has received great attention since it can greatly promote ignition. However, there is no theoretical analysis on ignition induced by multiple heating pulses such as NRPD. Therefore, this work attempts to provide a theoretical interpretation on the ignition of a quiescent, flammable mixture by multiple discharges and to assess the effects of repetitive pulse on ignition characteristics. Based on fully transient formulation, analytical expressions describing ignition kernel propagation induced by multiple pulses are derived. The key parameters of multiple pulse heating, such as energy distribution among individual pulse, intermittent duration between neighboring pulse and total pulse numbers are appropriately incorporated, and their effects on ignition characteristics are assessed. It is found that because of memory effect, the flame kernel continues to propagate after switching off the external heating source. Sequentially introducing identical heating pulses at appropriate intermittent duration repetitively exploits the memory effect during ignition kernel development and thus extends propagating distance of the flame front. The repetitive pulse heating can promote ignition capability due to the flame revitalization effect, i.e., ignition reinforcement to the flame front thanks to additional thermal energy supplied by subsequent pulse. This is consistent with the synergistic effect of NRPD observed in previous experimental studies. In particular, as the pulse number increases, the minimum ignition energy decreases and approaches to an asymptotic value. In the limit of large pulse number, it is found that the integration of the implicit expressions describing the ignition kernel evolution does not depend on the pulse number explicitly. This substantiates the invariance of minimum ignition energy with heating pulse number. The present theory explains the effects of multiple discharges on ignition and provides insights on ignition enhancement.

Ignition measurement of non-premixed propane with varying co-flowing AIR through high-speed schlieren stereoscopic colour imaging

The flame kernel propagation characteristics stemmed from the spark ignition of low-Re, non-premixed propane (C3H8) under varying air co-flow conditions were studied using DFCD (Digital Flame Colour Discrimination) incorporated stereoscopic image processing technique. The current work focused on the viability of obtaining the combustion initiating kernel propagation speed (SP), equivalence ratio (Φ) and kernel depth (z) characteristics simultaneously as these parameters are important in the fundamental combustion theories and highly desired in the field of practical burner monitoring and diagnostics. Through the superimposition of schlieren and DFCD-enhanced colour images, the observed spatial location of kernel spearhead was found to conform to the disturbance front observed from the schlieren visualisation. Flame stretch principle was applied to experimentally determine the un-stretched kernel propagation speeds (SP,O). Concurrently, the estimation of the temporal state of reactant mixture equivalence ratio (Φe) variation was made using DFCD-segregated spatial colour signal characteristics. Further comparison were made against the CHEMKIN simulated results of un-stretched laminar flame speed (SL,O) at different Φ. The experimentally determined SP,O values were found to overlap the SL,O range corresponding to the measured Φe. The steady occurrence of digital colour fidelity observed for all considered cases suggest that the most reactive mixture fraction (ξMR) path followed by the kernel occurred in the fuel-rich premixed Φ condition (approximately Φe = 1.34 to 1.40). Furthermore, stereoscopic reconstruction of DFCD-enhanced flame colour images provided additional dimensional linkage to correlate the dimensional-spectral variation along with changes in combustion flow conditions. The dimensional-spectral property, through the computation of local colour ratio and stereoscopically reconstructed z, illustrated linearity in both the time domain and in temporal cross-correlation with Φe. The observed shift of the cross-correlated cluster towards the fuel-richer Φe condition is coherent to the corresponding increase in Re and exemplifies the observed conformance of the derived SL,O and Φe to properties of premixed combustion along the propagation front potentially denoting the path of ξMR.

Numerical Study of Hydrogen Auto-Ignition Process in an Isotropic and Anisotropic Turbulent Field

The physical mechanisms underlying the dynamics of the flame kernel in stationary isotropic and anisotropic turbulent field are studied using large eddy simulations (LES) combined with a pdf approach method for the combustion model closure. Special attention is given to the ignition scenario, ignition delay, size and shape of the flame kernel among different turbulent regimes. Different stages of ignition are analysed for various levels of the initial velocity fluctuations and turbulence length scales. Impact of these parameters is found small for the ignition delay time but turns out to be significant during the flame kernel propagation phase and persists up to the stabilisation stage. In general, it is found that in the isotropic conditions, the flame growth and the rise of the maximum temperature in the domain are more dependent on the initial fluctuations level and the length scales. In the anisotropic regimes, these parameters have a substantial influence on the flame only during the initial phase of its development.

Node classification using kernel propagation in graph neural networks

In this work, we introduce a kernel propagation method that enables graph neural networks (GNNs) to leverage higher-order network structural information without increasing the complexity of the networks. Recent studies have introduced GNNs that include higher-order neighborhood features containing global network information by propagating node features using a higher-order feature propagation rule. Though these GNNs have shown to improve node classification performance, they fail to include local connectivity information. Alternatively, GNNs also concatenate increasing orders of adjacency matrix in deeper layers in order to include higher-order structural information. In addition to global network information, GNNs also make use of node features which are network and node dependent features that serve to distinguish structurally isomorphic sub-structures within graphs. However, such node features may not always be available or depending on the network, may lead to deteriorating classification performance. Hence, to resolve these limitations, we propose a kernel propagation method that introduces a pre-processing step for GNNs to leverage higher-order structural features. The higher-order structural features are computed using a weighted random walk matrix which is node independent while using the first-order spectral propagation rule which explicitly considers local connectivity. Through our benchmark experiments, we find that the computed higher-order structural features are capable of replacing node dependent features while performing node classification task with performance on par with the state of the art approaches. Further, we also find that including both node features and higher-order structural features increases the performance of GNNs on large scale benchmark networks considered in this study. Our results show that considering local and global structural information as input to GNNs lead to an improvement in node classification performance in the absence/presence of node features without loss of performance.

Semi-supervised kernel matrix learning using adaptive constraint-based seed propagation

In this paper, we propose semi-supervised kernel matrix learning (SS-KML) using adaptive constraint-based seed propagation (ACSP). Conventional SS-KML methods such as pairwise constraint propagation (PCP) and kernel propagation (KP) have achieved outstanding performance in data classification. However, they are likely to distort the global data structure because of using hard constraints in their semi-definite problems (SDPs) for constraint propagation. Moreover, given a large number of pairwise constraints and a large amount of samples, they tend to be incredibly complex, thus being hard to be applied to real-life complex problems such as internet-scale image categorization. To address this problem, we utilize adaptive constraints to effectively maintain the inherent coherence of samples and successfully propagate constraint information into all samples. Moreover, we adopt seed propagation to remarkably reduce the computational complexity of SS-KML. Experimental results demonstrate that ACSP achieves a significant improvement in performance over PCP and KP in terms of both effectiveness and efficiency.

Experimental study of the flame propagation of gaseous fuel based mixture using optical visualisation

At this article presents experimental investigations carried out to determine the flame kernel propagation of gaseous fuel-air mixture under spark-ignition engine-like conditions. Various combustion processes taking place in an experimental combustion chamber are discussed. Consecutive shots of the development of the fuel flame front in a constant volume combustion chamber under different initial conditions are presented. Direct visual recording of an image of the combustion process is performed or Schlieren photography techniques are used. Modern types of spark plugs with different gaps in the entire price range are examined. CNG and LPG are used as gaseous fuels, since they are the main alternative fuels used in internal combustion engines for vehicles. Some of the results, such as pressure, maximum pressure and electric spark indicators, are shown as graphics. To calculate the speed of the flame propagation, specialised software processing the areas of the individual photos was used. The conclusions emphasize the results of relative air/fuel ratio, most favourable type of spark plugs and their gaps and location in the combustion chamber.

Minimum ignition energy and propagation dynamics of laminar premixed cool flames

Laminar premixed cool flames, induced by the coupling of low-temperature chemistry and convective-diffusive transport process, have recently attracted extensive interest in combustion and engine research. In this work, numerical simulations have been conducted using a recently developed open-source reacting flow platform reactingFOAM-SCT, to investigate the minimum ignition energy (MIE) and propagation dynamics of premixed cool flames in a 1D spherical coordinate. Results have shown that when ignition energy is below the MIE of regular hot flames, a class of cool flames could be initiated, which allow much wider flammability limits, both lean and rich, compared to hot flames. Furthermore, the overall cool flame propagation dynamics exhibit intrinsic similarity to those of hot flames, in that, they begin with an ignition kernel propagation regime, followed by two transition regimes, and eventually reach a normal flame propagation regime. However, a spherical expanding cool flame responds completely differently to stretch. Specifically, a regular outwardly propagating hot spherical flame accelerates with increasing stretch rate when the mixture Le < 1 and decelerates when Le > 1. However, it is found that a cool flame always tends to decelerate with increasing stretch rate regardless of mixture composition, exhibiting unique flame aerodynamic characteristic. This research discovers novel features of premixed cool flame initiation and propagation dynamics and sheds light on flame transition, spark-ignition system design, and advanced engine combustion control.

Experimental investigation of the ignition process in a separated dual-swirl spray flame

Centrally staged lean premixed pre-evaporation combustion technology has been used in aero engines for its capability in effectively reducing NOx emissions while still providing stable flames. Previous work has shown the “ignition delay” phenomenon in the forced ignition process, revealing that the burner ignition process is complex and needs further exploration. In this work, the spark ignition and flame kernel propagation in a centrally staged optical model combustor are experimentally investigated to further illustrate the interaction between random kernel motion and flow statistics from the perspective of timescales and kernel propagations. Stochasticity involved in each individual phases of the whole ignition is obtained via repeated high-speed imaging tests. Timescales of the successful ignition processes are analyzed in a statistical method, revealing the time randomness of ignition phases. Spatial probability of the flame kernel propagation in turbulent sprays is calculated and analyzed with the combination of time-averaged flow and spray distribution. The flame projected area evolutions show that the burner ignition phase can be further divided into three phases: flame propagation, flame residence and flame growth. Three modes of ignition failure are identified during the burner ignition phase. The flame residence phase features a special period of spark ignition with low light emissions from the highly wrinkled flame kernel in the center, which is the “ignition delay” before the growth of flame. Timescales of ignition stages involve stochasticity and the flame growth phase has a higher proportion in the whole process. Statistical analysis of the flame kernel spatial position shows that in turbulent spray the flame kernel has a greater probability of propagating to the region with low axial velocity, low shear strain rate, and appropriate spray concentration.

Onset of intermittency in stochastic Burgers hydrodynamics.

We study the onset of intermittency in stochastic Burgers hydrodynamics, as characterized by the statistical behavior of negative velocity gradient fluctuations. The analysis is based on the response functional formalism, where specific velocity configurations-the viscous instantons-are assumed to play a dominant role in modeling the left tails of velocity gradient probability distribution functions. We find, as expected on general grounds, that the field-theoretical approach becomes meaningful in practice only if the effects of fluctuations around instantons are taken into account. Working with a systematic cumulant expansion, it turns out that the integration of fluctuations yields, in leading perturbative order, to an effective description of the Burgers stochastic dynamics given by the renormalization of its associated heat kernel propagator and the external force-force correlation function.

Research on characteristics and effects of combustion performance by amplified ignition energy in CVCC system

This study was conducted to analyze the flame characteristics, which is generated through the combustion process according to air/propane mixture ratio and initial pressure as well as increasing the area of flame kernel generation by connecting high capacity capacitor to a standard ignition device. The experiment method involved constant volume combustion chamber system featuring a standard ignition device connected to amplified ignition device and the flame kernel and flame propagation were observed through the spark generated between the electrodes of spark plug. Also, ANSYS Fluent program was used to suggest interpretive results in order to analyze the characteristics of chemical reaction generated within the flame. Experiment conditions included initial pressure being configured at 2, 3, and 4 bar along with air/propane mixture ratio configured at excess air factor (λ) = 1.0, 1.2, and 1.4. Experimental results showed that amplified ignition increased the flame kernel generation compared to standard ignition and was able to decrease the rate of loss for flame propagation speed compared to that of standard ignition as initial pressure and excess air factor increased. In conclusion, the amplified flame can reduce flame instability by increasing the reaction zone rather than the standard flame even if the initial pressure increases. In addition, it is considered that the thermal expansion on the flame front can be increased by the amplification of ignition energy.

Effects of fuel stratification on ignition kernel development and minimum ignition energy of n-decane/air mixtures

Fuel-stratified combustion has broad application due to its promising advantages in extension of lean flammability limit, improvement of flame stabilization, enhancement of lean combustion, etc. In the literature, there are many studies on flame propagation in fuel-stratified mixtures. However, there is little attention on ignition in fuel-stratified mixtures. In this study, one-dimensional numerical simulation is conducted to investigate the ignition and spherical flame kernel propagation in fuel-stratified n-decane/air mixtures. The emphasis is placed on assessing the effects of fuel stratification on the ignition kernel propagation and critical ignition condition. First, ignition and flame kernel propagation in homogeneous n-decane/air mixture are studied and different flame regimes are identified. The minimum ignition energy (MIE) of the homogeneous n-decane/air mixture is obtained and it is found to be very sensitive to the equivalence ratio under fuel-lean conditions. Then, ignition and flame kernel propagation in fuel-stratified n-decane/air mixture are investigated. The inner equivalence ratio and stratification radius are found to have great impact on ignition kernel propagation. The MIEs at different fuel-stratification conditions are calculated. The results indicate that for fuel-lean n-decane/air mixture, fuel stratification can greatly promote ignition and reduce the MIE. Six distinct flame regimes are observed for successful ignition in fuel-stratified mixture. It is shown that the ignition kernel propagation can be induced by not only the ignition energy deposition but also the fuel-stratification. Moreover, it is found that to achieve effective ignition enhancement though fuel stratification, one needs properly choose the values of stratification radius and inner equivalence ratio.

Numerical Study of Ignition Process in Turbulent Shear-less Methane-air Mixing Layer

In this work, ignition process in a turbulent shear-less methane-air mixing layer is numerically investigated. A compressible large eddy simulation method with Smagorinsky sub-grid scale model is used to solve the flow field. Also, a thickened flame combustion model and DRM-19 reduced mechanism are used to compute species distribution and the heat release. Non-reacting mean and RMS axial velocity profiles and mean mixture fraction are validated against experimental data. Instantaneous mixture fraction contours show that the large bursts penetrate from the fuel stream into that of the oxidizer and vice versa and a random behaviour in the cross-stream direction. Flame kernel initiation, growth and propagation are analysed and compared with the experimental data. The ignition results show that the flame is not stable and blow-off occurs, but a more detailed investigation shows that local and short time flame stabilization exist during blow-off. During these local stabilization, heat release increased at the upstream edge of the flame. Most_upstream flame edge scalar analysis shows that the methane mass fraction has a dominant role in the local flame stabilization. OH, HO2, CH2O and heat release contours demonstration reveal that HO2 and CH2O mass fraction as well as the heat release reach a maximum on the border of the flame, but the maximum OH concentration is located in the middle of flame kernel.