Flame Front Propagation Research Articles

Imaging diagnostics and PIV measurements of flame propagation and flow structure during ignition process in a three-sector RQL combustor

The ignition of kerosene spray in an aero-engine combustor is complex due to the fuel injection, mixing, and swirl flow. To explore the influence of ignition position on the ignition characteristics of the aero-engine combustor, a three-sector rich-quench-lean (RQL) combustor experiment system was designed. The experimental study on the ignition process of different ignition positions was carried out. Two different positions of igniter, located respectively in the left liner and middle liner, were considered. Experimental results show that observations revealed a consistent ignition propagation pattern across various axial cross-sections, demonstrating distinct roles for combustor structures, such as the outer recirculation zone (ORZ), recirculation vortex, and inner recirculation zone (IRZ) during the process. The upper recirculation vortex and ORZ create favorable conditions for flame kernel formation, which then spreads to ignite the IRZ and lower recirculation vortex, aiding primarily in heat transfer, before propagating downstream to light up additional vortex structures. Furthermore, the ignition position significantly influences the flame development in two schemes, showing variations in flame intensity under different equivalence ratios and resulting in divergent propagation states. Additionally, during circumferential ignition, variances in ignition positions and flow field structures elongate the flame path for the middle ignition scheme, introducing both reverse and co-rotating motions between flame and airflow. This interaction accelerates the clockwise flame front propagation while simultaneously damping the counterclockwise movement, showcasing the complex dynamics of flame behavior in response to ignition and flow field variations.

On dynamics of gasless combustion in slowly varying periodic media

In this paper we consider a classical model of gasless combustion in a one dimensional formulation under the assumption of ignition temperature kinetics. We study the propagation of flame fronts in this model when the initial distribution of the solid fuel is a spatially periodic function that varies on a large scale. It is shown that in certain parametric regimes the model supports periodic traveling fronts. An accurate asymptotic formula for the velocity of the flame front is derived and studied. The stability of periodic fronts is also explored, and a critical condition in terms of parameters of the problem is derived. It is also shown that the instability of periodic fronts, in certain parametric regimes, results in a propagation-extinction-conduction-reignition pattern which is studied numerically.Novelty and significance statement: This work provides a closed form asymptotic description of periodic traveling fronts in a gasless combustion model with step-wise ignition temperature kinetics with a slowly varying concentration field. The stability analysis is performed, and the range of applicability of asymptotic formulas is given. A new propagation-extinction-conduction-reignition regime is identified. This regime emerges exclusively due to periodicity of the concentration field.

The effect of pressure evolution on flame propagation dynamics of H2/CH4 mixtures explosion in a horizontal closed duct

To investigate the effect of explosion pressure on flame propagation of H2/CH4 mixtures, the pressure dynamics of H2/CH4 mixtures explosion with different volume fractions of hydrogen (0, 0.2, 0.4, 0.6, 0.8, 1 (×100/%)) and equivalence ratios (0.8, 1, 1.2, 1.4) is measured using 2 pressure sensors in a horizontal duct with ignition electrode position not at the end. The flame propagation characteristic is studied using images recorded by a high-speed camera. Through the analysis and discussion of the experimental results, the interaction among explosion pressure, flame front, and flow field are discussed. The results show that when the volume fraction of hydrogen is greater than 0.6, the rate of explosion pressure rises, the flame front position, and the flame front speed are greatly improved. At higher and lower equivalence ratios, it is easier to form distorted “tulip” flames. As the volume fraction of hydrogen increases, the inclination angle of the flame front decreases, and becomes a typical “tulip” flame gradually. The convection of the flow field is an important cause of pressure oscillation, which makes the pressure distribution uneven, and its influence on Pmax, (dP/dt)max, and tc is weak. As the volume fraction of hydrogen increases, the duration of pressure oscillation decreases, and the intensity of pressure oscillation increases. The pressure wave has a substantial impact on the flame front propagation of distorted “tulip” flames and the pressure history of typical “tulip” flames.

Combustion Diagnosis in a Spark-Ignition Engine Fueled with Syngas at Different CO/H2 and Diluent Ratios

The gasification of residues into syngas offers a versatile gaseous fuel that can be used to produce heat and power in various applications. However, the application of syngas in engines presents several challenges due to the changes in its composition. Such variations can significantly alter the optimal operational conditions of the engines that are fueled with syngas, resulting in combustion instability, high engine variability, and misfires. In this context, this work presents an experimental investigation conducted on a port-fuel injection spark-ignition optical research engine using three different syngas mixtures, with a particular focus on the effects of CO/H2 and diluent ratios. A comparative analysis is made against methane, considered as the baseline fuel. The in-cylinder pressure and related parameters are examined as indicators of combustion behavior. Additionally, 2D cycle-resolved digital visualization is employed to trace flame front propagation. Custom image processing techniques are applied to estimate flame speed, displacement, and morphological parameters. The engine runs at a constant speed (900 rpm) and with full throttle like stationary engine applications. The excess air–fuel ratios vary from 1.0 to 1.4 by adjusting the injection time and the spark timing according to the maximum brake torque of the baseline fuel. A thermodynamic analysis revealed notable trends in in-cylinder pressure traces, indicative of differences in combustion evolution and peak pressures among the syngas mixtures and methane. Moreover, the study quantified parameters such as the mass fraction burned, combustion stability (COVIMEP), and fuel conversion efficiency. The analysis provided insights into flame morphology, propagation speed, and distortion under varying conditions, shedding light on the influence of fuel composition and air dilution. Overall, the results contribute to advancing the understanding of syngas combustion behavior in SI engines and hold implications for optimizing engine performance and developing numerical models.

Experimental study on the inhibition of hydrogen deflagration by flame-retardant compounded ultrafine dry powder fire extinguishing media containing zinc hydroxystannate

With the rapid development of global hydrogen energy industry, the issue of hydrogen energy safety has emerged, it is prone to cause a serious explosion accident once hydrogen leaks. Therefore, it is urgent to develop efficient suppression media to reduce the severity of hydrogen explosion accidents. In this paper, a new compounded powder composed of ordinary ultra-fine dry powder extinguishing agent (ODPEA) and zinc hydroxystannate (ZHS) is prepared. On the basis of material preparation, an experimental system of hydrogen explosive suppression is established to explore the inhibitory effect of ODPEA-ZHS. In terms of explosion overpressure, the results show that the peak overpressure suppression rates of ODPEA-ZHS are 90.12 %, 92.31 %, and 93.04 % for hydrogen release pressures of 2.0, 4.0, and 6.0 MPa, respectively. Besides, the flame front propagation velocity under the action of ODPEA-ZHS reaches a suppression rate of 40.55 %. Furthermore, the overpressure suppression efficiency of ODPEA-ZHS exceeds 90 % when ignites 1.0 m axially from leak port. Ultimately, the physicochemical synergistic inhibition mechanism of ODPEA-ZHS is revealed by combining material characterization and pyrolysis results. This study can provide effective countermeasures for accident prevention in hydrogen energy industry, and also contribute to the establishment of hydrogen energy safety standards and norms.

Dynamic laser ignition characteristics of solid fuel and oxygen for hybrid rocket system

The transient laser ignition process in a slab burner that is similar to hybrid rocket motors is investigated in this study. The coupling characteristics between solid fuel properties, pyrolysis rate, and laser power were analysed. The results show that the laser ignition process can be divided into four stages: preheating, pyrolysis, ignition, and combustion, with the preheating and pyrolysis stages primarily controlled by the solid heating rate and the mixing characteristics in the fuel-rich combustion zone. Meanwhile, the ignition and combustion stages were characterized by ignition kernel growth and flame front propagation. The ignition delay time and the establishment of steady-state combustion were significantly affected by the ignition energy and fuel properties, i.e., The ignition delay time increases exponentially with decreasing laser energy. An increase in oxidizer flow flux reduces the demand for mixing during ignition, but the corresponding increase in combustion pressure poses challenges to the ignition process. A two-dimensional transient numerical model was established based on optical-thermal coupling theory to further understand the dynamic pyrolysis process of solid fuel particles. The model was validated through fire experiments under different laser ignition energy conditions. The results indicate that the deviation of the maximum penetration depth and ignition delay time between the experiment and the simulation is within 7 %.

Thenovel numerical solutions for conformable fractional Kuramoto-Sivashinsky equations by using Cq-HATM and CHPETM

The present study used the novel conformable fractional methods to examine the nonlinear Kuramoto-Sivashinsky equations of the conformable fractional derivative. The Kuramoto-Sivashinsky equation is a mathematical model employed to elucidate a diverse range of chemical and physical phenomena, encompassing chemical reaction-diffusion processes, plasma instabilities, viscous flow issues, flame front propagation, and magnetized plasmas. The present inquiry provides an examination of convergence and error for the future scheme. The conformable q-homotopy analysis transform method (Cq-HATM) provides h-curves that illustrate the convergence range of the series solution achieved. In order to validate the effectiveness and suitability of the Cq-HATM, we examine separate instances. In this study, we introduce an application that demonstrates the potential benefits and effectiveness of the proposed approach. Furthermore, an error analysis is performed in order to verify the accuracy of the scheme. Numerical simulations are conducted to validate the precision of the forthcoming approach. The findings obtained from the numerical and graphical analyses are presented in this study. The method provided in this study showcases a notable degree of computational precision and ease in examining and resolving intricate phenomena related to conformable fractional nonlinear partial differential equations within the domains of science and technology.

OME[formula omitted]/air mixtures: The structure and propagation mechanism of their laminar premixed flames

Oxymethylene dimethyl ethers (CH3O(CH2O)xCH3 or OMEx) are synthetic substitutes of diesel and jet fuel. They can achieve soot-less combustion and have been shown to exhibit favourable ignition characteristics. With the aim to explore their combustion physics, the structure of their flames is analysed using the algorithmic tools of the Computational Singular Perturbation (CSP) method. It is demonstrated that the flame structure and the mechanism for its propagation is encapsulated in the fastest explosive mode, introduced by chemical dynamics in a narrow region of the flame front. This narrow region extends around the point where the maximum heat release rate (MHRR) is recorded. CSP diagnostics identify (i) reactions H + O2→ O + OH and CO + OH → CO2 + H beyond MHRR as initiating an upstream diffusion of heat and H radicals, both of which are generated by the exothermic reaction H2 + OH → H2O + H, (ii) a downstream convective motion that ensues and (iii) chemical activity that is initiated ahead of MHRR and sustained by the convective transport of fuel and oxygen. CSP also identifies differences in the action of the upstream diffusion of heat and H radicals and of the downstream transport of fuel and oxidiser. It is also shown that the fastest explosive mode far from the neighbourhood of MHRR has no influence on the structure and propagation of the flame front. Although OME2−4/air mixtures exhibit drastically different dynamics in the context of homogeneous autoignition, very small differences are recorded in terms of flame structure and propagation. The findings suggests that, since practically all major processes driving the flame front propagation are similar to those of hydrocarbons, OMEx fuels can be used as “drop-in” fuels in currently employed burners.

Flame evolution and pressure dynamics of premixed stoichiometric ammonia/hydrogen/air in a closed duct

Ammonia is regarded as a promising energy carrier due to its zero-carbon emissions and its suitability for long-distance, large-scale storage, and transportation. Ammonia/hydrogen mixed combustion is an important way to solve the problem of high ignition temperature and low flame speed in the process of ammonia combustion. This study systematically investigates the evolution of flames and pressure dynamics in a closed duct for ammonia/hydrogen/air mixtures with varying hydrogen blending ratio (0–100 %). Simultaneously, the study assesses the impact of flame instability and heat release on the evolution of ammonia/hydrogen/air flames and pressure dynamics. The results show that the flame tip speed and overpressure increase with the increase of hydrogen blending ratio and initial pressure, and when the hydrogen blending ratio is more than 25 %, the growth range of flame tip speed peak and overpressure peak value increases significantly. At the same time, the flame thickness decreases after hydrogen blending, the hydrodynamic instability increases, and the degree of wrinkles on the flame surface increases, which promotes flame acceleration. The Froude number increases significantly, the influence of buoyancy instability gradually weakens, and the flame core does not undergo significant displacement. In the later stage of flame propagation, both the flame front propagation velocity and pressure show significant pulsation phenomena. The heat release calculation results show that hydrogen blending significantly increases the adiabatic flame temperature. Consistent with the evolution law of overpressure, when the hydrogen blending ratio is greater than 25 %, the net heat release increases significantly. The elementary reaction with the highest heat release rate changes from the chain termination reaction R41 to the chain termination reaction R3. The research results can provide theoretical basis and experimental data support for the formulation of safety standards during the regulation and utilization of the combustion structure of ammonia/hydrogen/air mixtures.

Accelerated ignition-shock coupling and deflagration to detonation transition by ozone kinetic enhancement of dimethyl ether mixture

This study aims to understand the effect of kinetic enhancement by ozone addition on the Deflagration to Detonation Transition (DDT) and ignition-shock coupling in a microchannel using dimethyl ether (DME). Simultaneous high-speed chemiluminescence and shadowgraph imaging are carried out to analyze the flame front propagation, shock wave structures, autoignition, and ignition-shock coupling during the DDT process. The experimental results show that for all cases, with and without ozone kinetic enhancement, autoignition and DDT always occur in a reactivity gradient region with oblique shock cluster between the flame front and the flame acceleration-induced precursor shock. The observed DDT occurs via the Zeldovich hot spot gradient mechanism through the following stages: (1) Oblique shock cluster formation and acoustic compression between the flame front and the precursor shock, (2) autoignition initiation in the oblique shock cluster region, (3) spontaneous ignition wave propagation, and (4) ignition-shock coupling. It is found that autoignition location, oblique shock strength, induction zone length, ignition-shock coupling, and DDT onset flame velocity are very different from ozone kinetic enhancement. As the ozone addition is higher, the autoignition is advanced and the ignition location moves farther ahead of the flame front at a lower flame front propagation speed and weaker oblique shock clusters. Two different ignition-shock coupling modes are identified depending on ozone concentration. Without ozone kinetic enhancement, DDT occurs via ignition wave coupling with the precursor shock. However, ozone addition can accelerate the autoignition and initiate a direct ignition-shock coupling for DDT without interaction with the precursor shock. The present results show that kinetic enhancement in autoignition using an ignition enhancer such as ozone or plasma is critical and effective in accelerating and controlling DDT.

Combustion Of Methane With Microdroplets Of Water In A Weak Electric Field

The effect of a weak electric field on the flame of a Bunsen burner, including the case of an aerosol consisting of water microdroplets in an air-methane mixture has been studied in detail. The propagation velocities of the laminar flame have been determined. If the stabilization area (burner edge) and the flame tip are excluded from consideration, the average propagation velocity of a laminar flame in the absence of an electric field is constant. For a stoichiometric mixture of methane with air and aerosol combustion, it is equal to Su≈0.35 and 0.3 m/s, respectively. When an electric field is applied, there is a common trend for cases of combustion of methane and a gas-droplet mixture, i.e., maintaining the flame front propagation velocity from the anode side and increasing by about 8-10% from the cathode side.

The performance of reaction mechanism in prediction of the characteristics of the diffusive-thermal oscillatory instability of methane–hydrogen–air burner-stabilized flames

In this work the performance of various detailed reaction mechanisms of combustion of methane–hydrogen fuel mixtures is studied. The investigation is focused on the burner stabilized flame configuration and is undertaken for the normal pressure conditions, which is a starting point for the analysis of the combustion chemistry at elevated pressures met in rocket engine chambers. The study presents the experimental setup, computational approach and results of comparison, which allow us to access the properties of complex combustion systems in variety of dynamical regimes. The comparison of the predicted and measured characteristics of these regimes is used to test the performance of the reaction models. In particular, the critical behavior, e.g. for blow-up, onset of pulsations and quasi-steady flame front propagation and bifurcation between these regimes may be used to characterize and to study properties of combustion systems with mixed fuels. A neutral stability boundary for onset of pulsations is suggested to be used in this study. A method for automatic identification of the boundary is proposed and implemented with several detailed mechanisms. Although for pure methane the results look acceptable, they show gradual divergence with the increase of percentage of hydrogen addition. Significant quantitative differences between experiments and modeling with respects to frequencies of oscillations are reported even for 2:1 ratio of hydrogen to methane in the unburned mixture. The results of the work indicate that current reaction mechanisms need to be improved in order to gain the quantitative predictability of methane–hydrogen–air combustion at normal pressure before proceeding to the conditions fuel-oxygen high pressure combustion more relevant for rocket chamber.

The Formation of a Flame Front in a Hydrogen–Air Mixture during Spark Ignition in a Semi-Open Channel with a Porous Coating

An experimental study of ignition and flame front propagation during spark initiation in a hydrogen–air mixture in a semi-open channel with a porous coating is reported. The bottom surface of the channel was covered with a porous layer made of porous polyurethane or steel wool. The measurements were carried out for a stoichiometric mixture (equivalence ratio ER = 1.0) and for a lean mixture (ER = 0.4) of hydrogen with air, where ER is the molar excess of hydrogen. The flame front was recorded with a high-speed camera using the shadow method. Depending on the pore size, the velocity of the flame front and the sizes of disturbances generated on the surface of the flame front were determined. Qualitative features of the deflagration flame front at ER = 0.4, consisting of disturbances resembling small balls of flame, were discovered. The sizes of these disturbances significantly exceed the analytical values for the Darrieus–Landau instability. The effect of coatings made of porous polyurethane or steel wool is compared with the results obtained for an empty smooth channel. Depending on the hydrogen concentration in the hydrogen–air mixture, the velocity of the flame front compared to a smooth channel was three times higher when the channel was covered with steel wool and five times higher when the channel was covered with porous polyurethane.

Resonant collisions among X-type, multi-lump, generalized breathers, N-solitons and rogue waves in plasma

This paper studies the (1+1)-dimensional Kuramoto–Sivashinsky equation (1D-KSE) also known as canonical evolution equation within the context of flame front propagation, plasma instabilities, and phase turbulence in reaction–diffusion systems. We use Hirota’s derivatives to formulate bilinear equations and subsequently compute various types of solitons. We obtain lump solution, lump one strip, lump two strip, X-type, lump periodic solution, generalized breather and rogue wave solutions for our governing model with the help of Hirota bilinear method (HBM) and the ansatz approach. Breathers solutions may be used to improve the efficiency of fiber optic communication systems and solitons in plasma waves. Rogue wave solutions may be used for the safety of oil platforms and ships while lump wave solutions can be utilized in manipulating and controlling laser beams for materials processing or laser surgery. We also discuss one, two and other soliton interactions for 1D-KSE under some constraint conditions. These interactions may be discussed in plasma stability and confinement, which can control fusion as a future energy source.

Effect of Plasma-Enhanced Low-Temperature Chemistry on Deflagration-to-Detonation Transition in a Microchannel

This study examines low-temperature chemistry (LTC) enhancement by nanosecond dielectric barrier discharge (ns-DBD) plasma on a dimethyl ether (DME)/oxygen (Ar) premixture for deflagration-to-detonation transition (DDT) in a microchannel. It is found that non-equilibrium plasma generates active species and kinetically accelerates LTC of DME and DDT. In situ laser diagnostics and computational modeling examine the influence of the ns-DBDs on the LTC of DME and DDT using formaldehyde () laser-induced fluorescence (LIF) and high-speed imaging. Firstly, high-speed imaging in combination with LIF is used to trace the presence of LTC throughout the flame front propagation and DDT. Then, competition between plasma-enhanced LTC of ignition and reduced heat release rate of combustion due to plasma-assisted partial fuel oxidation is studied with LIF. Observations of plasma-enhanced LTC effects on DDT are interpreted with the aid of detailed kinetic simulations. The results show that an appropriate number of ns-DBDs enhances LTC of DME and increases formation and low-temperature ignition, accelerating DDT. Moreover, it is found that, with many ns-DBDs, concentration decreases, indicating that excessive discharges may accelerate fuel oxidation in the premixture, reducing heat release and weakening shock–ignition coupling, inhibiting DDT.

Optimizing RCCI combustion for improved engine performance under low load conditions: Impact of low-reactivity fuel and direct injection timing

The objective of this study is to explore the challenges associated with the RCCI (Reactivity Controlled Compression Ignition) concept under low loads, where there may be insufficient combustion or misfires caused by a lean fuel mixture in the combustion chamber. To address this issue, optical diagnostics and post-processing techniques are utilized to investigate the effects of low-reactivity fuel and direct injection timing on combustion and flame characteristics in a dual-fuel engine under such conditions. The study tested three different low-reactivity fuels and five different direct injection (DI) timings while maintaining hydrogenated catalytic biodiesel (HCB) as a high-reactivity fuel. The research findings reveal that an increase in ethanol content results in longer ignition delay and a displacement of the ignition kernels towards the downstream of the fuel stream. Consequently, a more homogeneous combustion process and slower flame propagation occur, ultimately reducing the combustion intensity, cylinder pressure, and peak apparent heat release rate. Premixing E10 allows for a higher indicated mean efficient pressure (IMEP) of 0.20 MPa, exceeding E0 by 0.02 MPa and E100 by 0.05 MPa. The research also highlights that delaying the DI timing of HCB reduces ignition delay time and creates a higher concentration gradient of the combustible mixture, resulting in a higher apparent heat release rate and shorter combustion duration. Additionally, natural combustion luminosity images indicate that delaying the DI timing substantially impacts the characteristics of flame kernels generation and propagation. The ignition kernels formed shifted gradually from the near-wall region to upstream region of the spray, as the blue flame area and front propagation speed showed a significant increase. Retarded DI timing can achieve more favorable flame propagation and a higher IMEP under low loads conditions with a higher premixed energy ratio, while effectively reducing the impact of spray-wall impingement and pool burning phenomena. These findings provide insights into optimizing RCCI combustion for improved engine performance under low load conditions.

Study on the safety of modified aluminum powder in 3D printing process

The application of 3D printing technology in the field of explosives has been a research hotspot in recent years. As the most common explosive additive, how to improve the reactivity of aluminum powder has been widely concerned by many scholars. In order to explore the influence of coating modification process on the safety of aluminum powder in 3D printing process, Hartmann tube and Ignition temperature test system were used to study the sensitivity of dust samples to energy and temperature. The results show that the sensitivity of coated aluminum powder to electrostatic energy is significantly reduced, and the sensitivity to temperature is slightly increased. Better modification technology improves the safety of aluminum powder in 3D printing process. And the flame front propagation law is studied, the results show that the modified aluminum powder has more excellent reactivity. It provides basic theoretical data support for the safe application of the modification technology of aluminum powder in the 3D printing process, which has important reference value for the future application research of 3D printing technology in the field of explosives, and is also one of the key development directions in the future. in constructing both.

Energy output characteristics and safety design of Al-AlH3 composite dust for energetic material additive

Aluminum (Al) is the most widely used metal additive in energetic materials, but its application is limited due to its oxidizable nature. However, aluminum hydride (AlH3) could disrupt the oxide film structure and improve applications of the Al additive in energetic materials by releasing hydrogen. Therefore, this paper explored the explosion characteristics of the Al-AlH3 composite dust by conducting composite dust explosion venting experiments. The influences of composite dust concentrations and mass ratios on the explosion pressure and flame were evaluated. The consequences indicated that when the composite dust concentration was constant, the explosion pressure and the reduction rate of explosion pressure gradually decreased as increasing the mass ratio, which confirmed that the explosion intensity of AlH3 was greater than that of Al. When the composite dust mass ratio was the same, the increase in dust concentration caused the explosion pressure, the pressure rise rate, the flame front propagation distance, and velocity to increase first and then attenuate. 750 g/m3 was the best dust explosion concentration. On this basis, the safety design applicability of composite dust was conducted based on the NFPA 68 explosion venting design standard, which was verified to be suitable for Al dust. Notably, the safety margin should be taken into account when carrying out a safety design for composite dust using NFPA 68. In the paper, the composite dust explosion characteristics were obtained through experiments and mechanism analysis, which provided data support for the research of Al-AlH3 composite dust for energetic material additives.

Numerical analysis of performance, combustion, and emission characteristics of PFI gasoline, PFI CNG, and DI CNG engine

Compressed natural gas (CNG) has gained widespread acceptance in the automobile industry due to its clean-burning properties, lower emissions, and superior anti-knock properties. Typically, CNG is used in bi-fuel mode, where it partially replaces gasoline in a conventional gasoline engine. However, because of its gaseous nature, CNG displaces intake air, which reduces the engine's overall power output. One possible solution is to directly inject CNG into the cylinder during the late intake stroke or at the beginning of the compression stroke, which increases the engine's volumetric efficiency and power output. In a previous study, we examined the impact of injection timing on the performance of direct injection (DI) CNG engines under various engine load and speed conditions. However, the intrinsic combustion process, such as flame speed and flame front propagation, could not be investigated. In this study, we attempted to understand DI CNG engines' in-cylinder flame speed and propagation behavior. We developed a combustion model for the DI CNG engine using computational fluid dynamics. We used methane as the surrogate fuel and six cylindrical nozzles with inlet velocity boundary conditions for CNG direct injection. We studied the CNG combustion process and flame speed propagation using a validated chemical mechanism and a look-up table. We made comparisons to evaluate the numerical model against experimental reference data. We also conducted a comparative simulation of DI CNG, port fuel injection (PFI) CNG, and gasoline engines to assess combustion behavior. The combustion duration was longer for the PFI CNG engine than the gasoline engine, but it was reduced for the DI CNG engine. This was due to the DI CNG engine's 40% higher turbulent flame speed than the PFI CNG engine. The combustion of the air-fuel charge was completed within 40° CA after spark ignition. As a result, DI CNG requires a retard in ignition timing. The DI CNG engine showed a 7% higher power output with a 10% increase in volumetric efficiency compared to the PFI CNG engine. However, the power output was still lower than that of the PFI gasoline engine. The DI CNG engine emitted lower NOx and CO emissions than the PFI CNG engine. This study demonstrates the higher turbulent flame speed and rapid flame front propagation characteristics of DI CNG engines over PFI CNG engines, increasing engine efficiency.

Effect of flash-boiling spray on ignition and flame-holding characteristics under sub-atmospheric pressure

To prevent the performance degradation and flameout of an afterburner/ramjet combustor under sub-atmospheric pressure, the flash-boiling spray by twin-orifice nozzle is attempted to incorporate with the pilot flameholder for reliable ignition and flame stability. Herein, the effects of flash boiling spray by twin-orifice nozzle on the ignition and flame-holding characteristics of evaporating V-gutter flameholder under sub-atmospheric pressure are investigated. According to the analysis of flow patterns and fuel concentrate distribution, the flame stabilization mechanisms of evaporating V-gutter flameholder under lean-state and rich-state are concluded. Compared with subcooled spray, the significant improvement of flash boiling spray on fuel atomization and evaporation under sub-atmospheric pressure can effectively broaden the maximum ignitable velocity of the evaporating V-gutter flameholder, and the lean ignition and blowout fuel-to-air ratios can be reduced by 68.2% and 71.3%, respectively. Furthermore, the flash-boiling spray can promote the establishment of initial flame kernel and accelerate the flame-front propagation, and make the flame stabilization mechanism transform from the rich-flame stabilization mechanism under subcooled spray to the lean-flame stabilization mechanism, resulting in a better performance of flame stability. The results provide a theoretical and technical basis for the practical application of flash-boiling spray in engineering afterburner/ramjet combustors under extreme conditions.