Early Flame Research Articles

Increasing the Resistance of Archival Cardboards against Thermal Destruction

Abstract Valuable books and archival documents are stored in protective packaging, most frequently made of cardboard, in the depositories of archives, libraries, museums or galleries. As part of preventive conservation, the cardboard could theoretically be treated during production with suitable substances delaying ignition or decelerating flaming combustion. This work describes a study of the short-term and long-term effectiveness of selected inorganic additive flame retardants and their influence on the properties of cardboards. Substances containing silicon, boron or hydroxide anions were tested on Prolux solid cardboard and Klug corrugated cardboard. Parameters such as the linear burning rate, oxygen index, heat release rate, weight loss during heating, pH of an aqueous extract, and total colour difference were evaluated. A mixture of mica and bentonite Cloisite-Na+ in a solution of Tylose MH6000 was found to be most useful for this purpose. This mixture doubled the time to ignition of Klug cardboard; in measurements of the linear burning rate, it caused early flame self-extinction for both kinds of cardboard and, simultaneously, had no observable detrimental effect on the stability of the treated cardboard.

Ammonia and hydrogen blending effects on combustion stabilities in optical SI engines

Ammonia is a promising energy carrier and carbon-free fuel, which is getting more attention today when fossil energy is increasingly exhausted. However, ammonia combustion in the IC engine suffers from combustion instabilities and misfiring issues. Hydrogen is also carbon-free energy and can be directly obtained by ammonia catalytic reforming. The addition of a small amount of hydrogen can effectively alleviate the weakness of ammonia combustion, while ammonia and hydrogen blending effects on combustion stabilities and their critical conditions remain unclear. In this work, the effects of hydrogen addition on ammonia combustion characteristics were experimentally investigated in an optical SI engine with high compression ratios. The results show that the indicated thermal efficiency first increases and then decreases with the elevation of hydrogen-ammonia energy ratios (χ), with an optimal value at χ≈7.5 %. Further hydrogen addition can aggravate indicated thermal efficiency due to the large heat transfer loss caused by high-temperature hydrogen combustion. Combustion visualizations show that there is a critical threshold of hydrogen-ammonia energy ratio (χ=10.0–12.5 %), around which engine combustion characteristics are changed significantly. When χ < 10 %, the effect of hydrogen addition on the ammonia flame speed becomes more pronounced since the hydrogen substantially improves the stability of the early-stage combustion. When χ > 12.5 %, hydrogen shows an obvious influence on the initial flame formation of ammonia, which is attributed to the low flame stretch sensitivity, facilitating early flame development. In addition, flame propagation when χ < 10 % is more sensitive to temperature than flame propagation with χ > 12.5 %. The current research demonstrates that proper ammonia-hydrogen energy ratios are important for stable combustion and thermal efficiency improvement.

The Synergy Effect of Ignition Energy and Spark Plug Gap on Methane Lean Combustion with Addressing Initial Flame Formation and Cyclic Variation.

Controlling carbon emissions could be a win-win for both the environment and humans, and the use of low-carbon fuels is the key to being carbon-neutral in traffic transportation. Natural gas can achieve low carbon emissions and obtain high efficiency, but the poor lean combustion performance may result in large cycle-by-cycle variations. In this study, the synergy effect of high ignition energy and spark plug gap on methane lean combustion was optically studied under low-load and low-EGR conditions. High-speed direct photography combined with simultaneous pressure acquisition was used to analyze early flame characteristics and engine performance. The results show that high ignition energy can improve the methane engine's combustion stability, especially under high excess air coefficient conditions, and the main reason is that the initial flame formation is improved. However, the promoting effect may become marginal when the ignition energy increases above a critical value. As for the effect of spark plug gap, it varies with the ignition energy, and there exists an optimal spark plug gap for a given ignition energy. In another word, high ignition energy must combine with a large spark plug gap; thus, the promoting effect on combustion stability can be maximized and the lean limit can be extended. The statistical analysis of the flame area shows that the speed of initial flame formation is more important in determining combustion stability. As a result, a large spark plug gap (1.20 mm) can extend the lean limit to 1.4 under high ignition energy conditions. The current study shall give some insights into the spark strategies for natural gas engines.

Experimental and numerical study of the effect of initial temperature on the combustion characteristics of premixed syngas/air flame

The effects of different initial temperatures (T = 300–500 K) and different hydrogen volume fractions (5%–20%) on the combustion characteristics of premixed syngas/air flames in rectangular tubes were investigated experimentally. A high-speed camera and pressure sensor were used to obtain flame propagation images and overpressure dynamics. The CHEMKIN-PRO model and GRI Mech 3.0 mechanism were used for simulation. The results show that the flame propagation speed increases with the initial temperature before the flame touches the wall, while the opposite is true after the flame touches the wall. The increase in initial temperature leads to the increase in overpressure rise rate in the early flame propagation process, but the peak overpressure is reduced. The laminar burning velocity (LBV) and adiabatic flame temperature (AFT) increase with increasing initial temperature. The increase in initial temperature makes the peaks of H, O, and OH radicals increase.

Large-Eddy Simulation Analysis of Flow and Combustion in a Pre-Chamber Ignited Gasoline Engine

ABSTRACT Multi-cycle large eddy simulations were conducted to resolve the turbulent flow field spatially and temporally in a pre-chamber ignited gasoline engine. The numerical models were validated by comparing with both the passive and active pre-chamber experimental data. Correlation analysis results of the two pressure peaks during combustion inside the pre-chamber showed that the combustion inside the pre-chamber directly influences the timing and strength of the turbulent jets, which would ignite the mixture inside the main chamber. Then, the flow field evolution and fuel/air mixture formation inside both the main chamber and pre-chamber during intake and compression strokes were investigated numerically. Results of the passive pre-chamber showed that mixture distribution near the orifices during the compression stroke affects the averaged lambda of the gas inside the pre-chamber. By using the orifices with a tilted orientation, the fuel inside the pre-chamber could be stratified. When the mixture near electrodes is rich, the development of ignition kernel and the consequent flame propagation would benefit. In the case of active pre-chamber, although the lambda distribution patterns of mixture before the spark plug ignition are similar between different cycles, the magnitude of velocity would affect the early flame propagation inside the pre-chamber.

Numerical and experimental investigation of the flame kernel growth in a methane/air mixture near the lean flammability limit

Lean combustion has the potential to improve the thermal efficiency of spark-ignition engines, but it faces the significant challenge of increased cycle-to-cycle variation due to low mixture reactivity and unstable flame dynamics. Computational fluid dynamics (CFD) employing predictive models can guide engine design and optimize operating strategies for lean combustion. However, ignition and combustion models have rarely been validated at fuel-lean conditions, and a fundamental understanding of the early flame kernel growth process is also lacking for a successful sub-model development. The present study develops a numerical simulation framework used to investigate early flame kernel growth in methane/air mixtures. A nanosecond-pulsed discharge (NPD) approach is employed to effectively decouple the flame kernel growth from the electrical discharge due to their difference in timescales, and equivalence ratios near the experimentally measured lean flammability limit (LFL) are selected to focus on challenging mixture conditions. Three numerical investigations, such as the choice of turbulence modeling, grid size, and grid control strategies, are examined to match both LFL and flame kernel structure measured from experiments. It is demonstrated that a quasi-direct numerical simulation (QDNS) with a fixed grid embedding of 10 µm can predict the LFL as ϕCFD=0.61 and match the displacement speed of the kernel's boundary marked in schlieren images. To predict the LFL and flame kernel shape, a fine grid (Δ≤12.5 µm) is needed to capture the consumption of formaldehyde (CH2O) in kernel's reaction branches attached to the anode, and adaptive mesh refinement is replaced with the fixed embedding due to loss of simulation accuracy. Also, it is found that a large-eddy simulation (LES) using the Dynamic Structure model is not suitable for the NPD-induced flame kernel simulation because artificial sub-grid turbulent kinetic energy induced by shock dynamics alters the flow velocity calculation, resulting in divergence of LES from QDNS. Lastly, the simulation well matches the experimental data for the flame kernel evolution in three mixture conditions (ϕ = 0.7, 0.61, 0.55), showing toroidal flame kernel expansion and flame kernel growth/extinction.

Study of Flame Kernel Development at High EGR and High Flow Speed Using Conventional Spark Igniter and Non-Thermal Plasma Under Gasoline Engine Relevant Conditions

ABSTRACT As a novel ignition approach, nanosecond repetitively pulsed discharge (NRPD)-based non-thermal plasma offers significant benefits such as low energy consumption, and lean operating conditions. However, there is no investigation conducted on the early flame kernel formation and development induced by non-thermal plasma. Therefore, in this paper, experiments of conventional spark and non-thermal plasma ignition systems in constant volume combustion chamber (CVCC) are conducted under gasoline engine relevant conditions: wide initial ambient pressures (6.5, 8.3, 11.3 bar), a range of equivalence ratios (0.7–1.0), EGR rates (10−25%), and cross flow speeds (0–30 m/s). The discharge energy of non-thermal plasma is of around 210 mJ, while a single spark event can generate energy from 65 mJ to 83 mJ depending on ambient conditions. A consecutive spark strategy is adopted to guarantee comparable total input energy to non-thermal plasma. The ignition delay and combustion phase obtained from the chamber pressure history are calculated. In the meanwhile, flame kernel radius, flame propagation rate, and flame front length ratio via schlieren images are quantified and analyzed. Results showed that the flame initiated by non-thermal plasma can maintain a robust flame kernel and propagates fast. Under high-speed cross flow and high EGR rate conditions, non-thermal plasma can maintain a robust flame kernel at early stage supporting the initial flame kernel to survive and improve its ignition probability. Considering the effects of the EGR ratio and high-speed cross flow on flame kernel development, non-thermal plasma ignition can efficiently enhance the flame propagation and ignition probability. It is also concluded that non-thermal plasma can successfully ignite under lean (equivalence ratio between 0.7–1.0), high-diluted mixture (25% EGR), and high-speed cross flow of 30 m/s within the range of current study.

Investigation of the effects of passive pre-chamber nozzle pattern and ignition system on engine performance and emissions

The impact of passive pre-chamber (PC) internal volume, nozzle hole pattern (i.e. with and without a central axial nozzle), and PC igniter plug type on performance and emissions was investigated in an optically accessible, single-cylinder, gasoline research engine. The two PC igniter plugs investigated were a conventional inductive coil spark plug and a nanosecond repetitively pulsed (NRP) plasma discharge system previously demonstrated to accelerate early flame propagation. The baseline PC design featured a funnel shaped internal volume with a PC tip that contained six radial nozzles and a larger central axial nozzle. Two additional PC tip geometries were evaluated where either the baseline internal volume was increased or the axial nozzle was removed and the radial nozzle diameters were increased. A sweep of charge equivalence ratios ( ϕ) from nearly stoichiometric to the lean limit was performed for a fixed engine speed (1300 revolutions per minute), and engine load (3.5 bar gross indicated mean effective pressure). Time-resolved PC and main chamber (MC) pressure data as well as MC emissions data were collected to analyze engine performance and emissions characteristics. Combustion in the MC was further investigated using high-speed excited methylidyne radical (CH*) chemiluminescence imaging. Collected results highlighted that while all PC tips and ignition systems exhibited similar performance and emissions down to ϕ = 0.8, relevant differences in thermal efficiency and emissions for leaner charge mixtures were observed, with the results highly dependent on the nozzle pattern and ignition system. Major deviations were correlated to preferential de-pressurization of the PC through the axial nozzle for lean conditions that was not observed for mixture conditions closer to stoichiometric. Results show that a combination of radial and axial nozzle patterns in the PC extended lean-stability limits at the low-load condition evaluated. Further benefits were observed with the use of NRP ignition systems due to faster combustion within the PC volume provided that the volume was sufficiently large.

Energy deposition and plasma propulsion during the time window of microwave-assisted CH4-O2 spark ignition with Ar/N2/CO2 dilution

Microwave-assisted spark ignition (MAI) is a promising technology to achieve ultra-lean combustion and thus satisfy the requirements in the low-carbon age. However, the coupling process between microwave and the early flame kernel is still a complex and critical question before its wide application and further optimization. Therefore, this work explores the energy deposition and plasma propulsion effect during the MAI through the ignition experiments fueled by the methane-air mixture under a range of equivalence ratios with argon (Ar), nitrogen (N2), and carbon dioxide (CO2) as the carrier gas. Both the flame and plasma characteristics are recorded and analyzed through the high-speed shadowgraph method. The assessment of the time window and the microwave energy depositing process is based on the voltage of the spark electrode and the detected voltage of reflected microwave power. Results show that in Ar and N2 cases, the microwave enhancing effect on early flame development decreases as the equivalence ratio approaches the stoichiometric ratio, but this tendency is less obvious in the CO2 case. During the MAI, there is a time window for the microwave energy deposition, and once it ends, the following microwave pulses will no longer be absorbed. In successfully ignited cases, the time window in MAI mode is 1.8 ms in the N2 case and 1.5 ms in the CO2 case, while it is extended to 2.4 ms when adopting Ar as the carrier gas causing one more microwave pulse absorbed. Generally, changes in equivalence ratio show no obvious influence on the energy-absorbing process. Microwave pulses during the time window periodically generate a plasma jet that originates from the electrodes gap region towards the flame front. This plasma jet acts as the medium for the microwave influencing the flame development, and thus the relative position between the plasma jet and the flame front determines the final microwave enhancing effect.

Improved Real-Time Fire Warning System Based on Advanced Technologies for Visually Impaired People.

Early fire detection and notification techniques provide fire prevention and safety information to blind and visually impaired (BVI) people within a short period of time in emergency situations when fires occur in indoor environments. Given its direct impact on human safety and the environment, fire detection is a difficult but crucial problem. To prevent injuries and property damage, advanced technology requires appropriate methods for detecting fires as quickly as possible. In this study, to reduce the loss of human lives and property damage, we introduce the development of the vision-based early flame recognition and notification approach using artificial intelligence for assisting BVI people. The proposed fire alarm control system for indoor buildings can provide accurate information on fire scenes. In our proposed method, all the processes performed manually were automated, and the performance efficiency and quality of fire classification were improved. To perform real-time monitoring and enhance the detection accuracy of indoor fire disasters, the proposed system uses the YOLOv5m model, which is an updated version of the traditional YOLOv5. The experimental results show that the proposed system successfully detected and notified the occurrence of catastrophic fires with high speed and accuracy at any time of day or night, regardless of the shape or size of the fire. Finally, we compared the competitiveness level of our method with that of other conventional fire-detection methods to confirm the seamless classification results achieved using performance evaluation matrices.

Cyclic Variations in the Flame Propagation in an Spark-Ignited Engine: Multi Cycle Large Eddy Simulation Supported by Imaging Diagnostics

Experimental measurements and multi-cycle large eddy simulation (LES) are performed in an optically accessible four-stroke spark-ignition engine to investigate cycle-to-cycle variations (CCV). High-speed combustion imaging is used to measure the early flame propagation and obtain the flame radius and centroids. Large Eddy Simulation generates data-bases for the flame propagation as well as the kinetic energy in the cylinder and confirms the observations from the two-dimensional fields by three-dimensional simulation results. Experiment and simulation are compared with respect to the strength and distribution of CCV. Both approaches reveal CCV causing similar statistics of maximum pressures and combustion speeds. The cycles are categorized as slow and fast cycles using the crank angle of ten percent burnt fuel-mixture. Analysis of the flame centroids shows that slow cycles move further towards the intake-side of the engine compared to fast cycles. The kinetic energy during combustion is averaged for the slow and fast cycles based on the samples being in unburnt and burnt mixture. Studying the kinetic energy level in the unburnt and burnt mixture reveals higher turbulent kinetic energy for the fast cycles as well as larger separation between the global kinetic and the turbulent kinetic energy for the slow cycles, providing evidence for a source of the CCV variations observed in this engine.

Effects of differential diffusion on hydrogen flame kernel development under engine conditions

The early flame kernel development in spark ignition engines is crucial for engine performance. For non-unity-Lewis-number mixtures, it can be significantly influenced by differential diffusion due to the large curvature of the small kernel. Differential diffusion can lead to thermodiffusive instabilities for lean hydrogen/air flames, which are enhanced for high pressures but diminish for increasing temperatures. In this study, direct numerical simulations of lean hydrogen flame kernels under engine conditions have been performed to investigate how differential diffusion affects their growth with an effective Lewis number far smaller than unity and if thermodiffusive instabilities appear under realistic engine conditions with elevated in-cylinder pressure and high unburned temperature. It is found that turbulence triggers the instabilities for flame kernel sizes far smaller than the critical radius of the onset of cellular instabilities for laminar flames. The strong thermodiffusive instabilities significantly facilitate the flame kernel growth. The normalized fuel consumption rate is increased by a factor of up to four, due to an enhanced propagation speed. This is remarkable as it was found in earlier studies that for laminar flames the effects of instabilities become much weaker under these conditions. Thermodiffusive instabilities also lead to large variations of the local fuel/air equivalence ratio resulting in temperatures up to 500 K above the adiabatic temperature, which impacts NOx formation. In addition, thermodiffusive instabilities alter the mechanisms of flame surface area formation. The production and destruction of the surface area by flame propagation are significantly increased. A transition phase can be identified during the formation of the negative curvature regions from the initial spherical kernel.

A Combined Numerical and Experimental Investigation of Cycle-to-Cycle Variations in an Optically Accessible Spark-Ignition Engine

A combined numerical and experimental investigation is carried out to analyze the cycle-to-cycle variations (CCV) in an optically accessible spark-ignition engine with port fuel injection. A stable and an unstable operating condition is considered. Well-established turbulence, combustion, and ignition models are employed in the large-eddy simulations (LES). High-speed measurements of the velocity field via particle image velocimetry and flame imaging in the tumble plane are conducted in the experiments. A detailed comparison between LES and experiments is carried out, including the in-cylinder pressure, the flow fields, the spatial flame distribution, and the fields conditioned on fast and slow cycles. Good agreement is achieved for the variables considering all cycles; yet, some discrepancies are observed for the conditionally averaged quantities. A systematic quantitative correlation analysis between the selected influencing variables and the CCV is presented, in which the influencing variables are extracted from different length scales (r = 3 mm, 12 mm, and 43 mm) and the CCV are distinguished between the early flame kernel development and later flame propagation. Even though the most relevant influencing parameters are different for the two operating conditions, the location of the coherent vortex structure is found to be important for the CCV of both cases.

Optical Investigation of the Influence of In-cylinder Flow and Mixture Inhomogeneity on Cyclic Variability in a Direct-Injection Spark Ignition Engine

In this study, a single-cylinder direct-injection spark-ignition research engine with full optical access was used to investigate the influence of the flow field and fuel/air mixing on cyclic variability, in particular in the early flame propagation. The engine was operated under lean-burn conditions at 1500 rpm. Two different injection strategies were compared, port-fuel injection (PFI) and direct injection (DI), the latter with early and late injection split about 2:1 in fuel mass. High-speed particle image velocimetry captured the flow in the tumble plane in the compression stroke. The velocity fields and the movement of the tumble vortex are analyzed. Simultaneously, a second camera detected the chemiluminescence of the flame, and the projected area of the line-of-sight-integrated flame luminosity was extracted through morphological image processing. By combining pressure-based combustion analysis and high-speed optical diagnostics, the early flame propagation and the flow field are correlated. In separate experiments the equivalence ratio was imaged for the DI at selected crank angles and correlated with CA10 to learn about the influence of mixture inhomogeneity on early flame propagation. With PFI, the flow near the spark plug just before ignition is closely related to the subsequent speed of combustion. The combustion-relevant flow features can be traced back in time to about –90 °CA. In contrast, the chosen DI scheme results in a decorrelation of flow and flame, and the equivalence ratio distribution at ignition becomes more important. For both flow and mixture fields, regions of high correlation with early-combustion metrics are typically associated with gradients in the multi-cycle average fields.

Modelling Study of Cycle-To-Cycle Variations (CCV) in Spark Ignition (SI)-Controlled Auto-Ignition (CAI) Hybrid Combustion Engine by Using Reynolds-Averaged Navier–Stokes (RANS) and Large Eddy Simulation (LES)

The spark ignition (SI)-controlled auto-ignition (CAI) hybrid combustion is characterized by early flame propagation combustion and subsequent auto-ignition combustion. The application of combined SI–CAI hybrid combustion can be used to effectively extend the operating range of CAI combustion and achieve smooth transitions between SI and CAI combustion modes. However, SI–CAI hybrid combustion can produce significant cycle-to-cycle variations (CCV). In order to better understand the sources of CCV and minimize its occurrence, the large eddy simulation (LES) and Reynolds-averaged Navier–Stokes (RANS) approaches were employed in this study to model and understand the cyclic phenomenon of SI–CAI hybrid combustion. Both the multi-cycle LES and RANS simulations were analyzed against the experimental measurements in a single cylinder engine at 1500 rpm and a 5.43 bar average indicated the mean effective pressure (IMEP). The detailed analysis of the in-cylinder pressure traces, IMEP, in-cylinder peak pressure (PP), peak pressure rise rate (PPRR) and the crank angles with fuel mass burned fraction at 10%, 50%, 90% and mode transition was performed. The results indicate that overall, the adopted LES simulations could effectively predict the cyclic variations in the hybrid combustion observed in the experiments, while the RANS simulations failed to reproduce the cyclic characteristics at the chosen engine operating conditions. Based on the LES results, the correlation and visualization studies indicate that the cyclic variations in the local velocity around the spark plug lead to the variations in the early flame propagation, which in turn produce temperature fluctuations among the cycles and result in greater variations in the subsequent auto-ignition combustion events.

Combined effects of intake oxygen enrichment, intake pressure and temperature on combustion behavior and emission characteristics of a small-scaled rotary engine

Intake oxygen enrichment has a promising potential to improve the combustion performance of a small-scaled rotary engine (RE) operating at high altitudes. However, the process is complicated because the intake temperature and pressure are sensitive to altitudes as well as the elongated rotor chamber. In this study, a three-dimensional CFD simulation model coupling with a suitable turbulent model and a reduced chemical kinetic mechanism was established and validated. The combined effects of intake oxygen enrichment as well as intake pressure and temperature under different altitudes on early flame growth, combustion behavior, and emission characteristics of small-scaled REs were numerically investigated. Results showed that intake oxygen enrichment promotes the formation of OH, O, H radicals, which leaded to the significant improvements of the early flame growth, especially for the flame near leading spark plug. The promotion effect due to intake oxygen enrichment became weak with increasing altitude. Moreover, the early flame growth duration at high altitudes was more dramatically shortened by intake oxygen enrichment than that at low altitudes. It is interesting to note that when oxygen concentration of 25% and 29% were applied to the small-scaled RE working at 2000 m and 3000 m altitude, respectively, the engine performance could be restored to the level of that working at sea level at the price of a smaller increment of NO emissions. Under high intake oxygen concentration conditions, the effect of altitude on indicated thermal efficiency of the engine could be ignored. Besides, increasing oxygen concentration significantly reduces HC and CO emissions, especially more significant reduction for HC emissions at higher altitudes. • Early flame growth of the rotary engine was analyzed numerically. • Early flame growth near LSP was more significantly improved by oxygen enrichment. • Effect of intake oxygen enrichment on rotary engine performance was quantified. • Oxygen enrichment significantly reduced HC and CO emissions.

Influence of the spark plug mounting angle on spark ignition process at different flow velocities

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.

Effect of discharge current and discharge duration on the discharge channel behavior and early flame formation

Effect of discharge current and discharge duration on the discharge channel behavior and early flame formation

Impact of ignition energy on the combustion performance of an SI heavy-duty stoichiometric operation natural gas engine

Nowadays, natural gas engine has been brought opportunities because of its low C/H ratio. For the stoichiometric combustion natural gas engine, improving its combustion stability is the main challenge under the condition of high EGR rate. To better understand the effect of the ignition energy on combustion stability, a detailed study was conducted by experiment. A 6-cylinder turbocharged intercooler natural gas heavy-duty engine was used in this study, and the ignition energy was controlled over a wide range under different working conditions. The results show that the ignition energy has an obvious effect on the ignition process of combustion including flame inception and early development when exceeds 100 mJ. Under heavy-load conditions, the elevating of early combustion speed can shorten the duration of combustion (DOC), manifesting more concentrated heat release, and improve the combustion stability and economy of the engine. However, under light-load conditions, the increase of early flame speed has little impact on the subsequent combustion process, and engine performance is not significantly improved. For a specific ignition energy under 1200r/min-80% load working condition, with the advancing of ignition timing (IT), there is a little effect on ignition delay time (IDT), but has more obvious effect on DOC. What’s more, the improvement amplitude of Pmax and indicated mean effective pressure (IMEP) gradually decrease with the advancing of IT at 105 mJ, but they are still larger than at 150 mJ. Overall, better engine performance and thermal efficiency can be achieved by organizing ignition energy.

Propagation of premixed hydrogen-air flame initiated by a planar ignition in a closed tube

Numerical simulations were used to study the dynamics of premixed flames propagating after planar ignition in a closed tube filled with stoichiometric hydrogen-air mixture. The two-dimensional fully compressible reactive Navier–Stokes equations coupled to a calibrated chemical-diffusive model were solved using a high-order numerical method and adaptive mesh refinement. The results show that the flame evolves from an initially planar flame to a double-cusped tulip flame, subsequently to a multi-cusped tulip flame, and finally to a series of distorted tulip flames (DTFs). The DTF forms one after another until the end of combustion. The initial flame lips of the double-cusped tulip flame are produced due to the stretching effect of nonuniform flow caused by the wall friction. The multi-cusped tulip flame forms as secondary cusps are created on the leading flame tips near the sidewalls. The formation of DTFs here is thought to be closely connected to pressure waves generated in the flame propagation process. The first DTF is caused by the combined effects of the vortex motions and the Rayleigh–Taylor (RT) instability driven by pressure waves, while the subsequent DTFs form due to reverse flows and RT instability. Nevertheless, both the vortex motions and reverse flows are essentially induced by the interactions between pressure waves and flow fields. Furthermore, the numerical results were compared to that in the case with a semicircular ignition. It was found that although there are significant differences in the early flame acceleration and tulip formation stages between the two differently shaped ignitions, the dynamics of DTFs are substantially consistent.