Ignition Enhancement Research Articles

Single- and dual-channel nanosecond repetitively pulsed discharges at small and large spark gaps for turbulent premixed spherical flame initiation

This paper reports laminar/turbulent ignition probability (Pig,L/Pig,T) of single- and dual-channel nanosecond-discharge (SN and DN) at small and large inter-electrode gaps (dgap = 0.8 mm and 2.0 mm) using a lean n-butane/air mixture in a double-chamber fan-stirred explosion facility capable of generating near-isotropic turbulence over wide ranges of pulsed repetitive frequency (PRF = 1–100 kHz) and r.m.s. turbulence fluctuation velocity (urms = 0–4.9 m/s). The new DN configuration consists of two parallel anode electrodes with 2.6 mm apart and one central cathode electrode on the same vertical plane, which can generate two spatially separated and temporally synchronized discharges having an almost twice larger initial kernel size than the SN configuration. We apply the same total ignition energy Etot≈18 mJ via a train of 9 pulses/6 pulses for SN/DN experiments, each pulse having Eig≈2.2 mJ (SN) and/or 3.3 mJ (DN) except for the first pulse with the same 0.8 mJ regardless of PRF and dgap. Unlike the common notion that the larger ignition kernel leads to higher Pig,L, it is found that the DN configuration at small dgap = 0.8 mm is totally detrimental for spark ignition (SI) with Pig,L = 0% for all PRFs = 1–100 kHz, while the SN configuration at small dgap = 0.8 mm has its maximum Pig,L = 58% at PRF = 40 kHz showing the synergy effect. But the reverse is found at large dgap = 2.0 mm with Pig,L(DN) > Pig,L(SN). Moreover, when 1 m/s < urms < 4 m/s at PRF = 40 kHz and dgap = 2.0 mm, although both Pig,T(DN) and Pig,T(SN) decrease with increasing urms, Pig,T(DN) is noticeably higher than Pig,T(SN) showing ignition enhancement of dual-channel sparks at large dgap, Pig,T(DN) = Pig,T(SN) = 100% when urms < 1 m/s, and Pig,T(DN) = Pig,T(SN) = 0 % when urms ≥ 4.2 m/s. Finally, high-speed schlieren images of SN/DN kernel evolutions are used to comprehend these results that may be useful for the ignition strategy in lean-burn SI engines.

Potential of ozone addition on dethrottling of a gasoline/ethanol blend-fueled direct injection spark ignition engine in part load

Several efforts are required to improve engine efficiency and decrease global carbon dioxide (CO2) emissions and local pollutant products. In countries like Brazil, with a four-decade long experience with ethanol, the use of fossil fuels, although widespread, is being put under intense scrutiny. Governmental programs like ROTA 2030 stimulate engine research and development focused on environment-friendly fuels such as bioethanol-gasoline blends. Brazilian gasoline has about a quarter of ethanol, reducing the carbon footprint while maintaining suitable energy density. Regardless of the fuel, the load control method in part load operation of spark ignition engine causes a considerable penalty in fuel conversion efficiency. For these reasons, ozone addition was tested as an ignition enhancer that would allow higher de-throttling to reduce part-load pumping losses. The residual gas was used to dilute the mixture due to the possibility of keeping the three-way catalyst working properly with the stoichiometric mixture. An experimental investigation on efficiency, emissions and combustion related parameters was carried out in a downsized 1.0 l turbocharged direct-injected engine. Experimental tests were performed at 3 bar of indicated mean effective pressure (IMEP), 1500 rpm and stoichiometric air-fuel ratio. Spark timing (ST) was adjusted to achieve maximum indicated efficiency and the fuel used was Brazilian gasoline. Different ozone concentrations were used as a mixture with the intake air to overcome unstable engine operation by enhancing ignition. The results showed that it is possible to increase the gas exchange efficiency with ozone addition by promoting de-throttling operation with residual gases. Furthermore, the ozone addition exhibits the potential to promote autoignition of the end gas with spark assistance, even with a low compression ratio and residual gas fraction higher than 30%. However, the combustion efficiency is impaired by the higher residual gas fraction in some operation points that leave room for improvements.

Effect of microwave pulse parameters on energy coupling and enhancement of microwave assisted ignition

Microwave Assisted Ignition (MAI) is a promising technology to optimize the lean combustion characteristics of internal combustion engines. This research investigated the effects of microwave pulse waveform and delay time on energy coupling and ignition enhancement by using a power diagnostic with high time-resolution. The pulse width ranged from 80 to 200 μs and the corresponding peak power was adjusted between 1000 and 400 W to keep the incident energy of microwave pulse constant. Results showed that a high power and short width pulse waveform was conducive to the microwave energy absorption. The coupled energy generally decreased with delay time. There was a significant ignition enhancement with only 6∼7 mJ energy coupled into the flame kernel, which was approximately 1/5 of total spark energy. Moreover, the ignition enhancement was well correlated with the coupled energy, suggesting that ignition enhancement was determined by coupled energy via electron collision reactions. Finally, the main influencing factor of coupled energy was the energy coupling pattern which changed from none-coupling to saturated coupling. A high pulse power and short delay time facilitated this change. This study revealed the saturated coupling pattern independent of waveform and delay time, which is caused by a stable electron number density due to the cut-off effect.

Effect of plasma-flow coupling on the ignition enhancement with non-equilibrium plasma in a sequential combustor

The effect of the regime of nanosecond repetitively pulsed discharges (NRPDs) on ignition and stabilization of a natural-gas/hydrogen/air flame in the sequential stage of a lab-scale atmospheric pressure sequential combustor is investigated experimentally. Electrical parameters of the NRPDs are characterized by measuring voltage, current, and deposited energy. Fast gas heating (FGH) of the nanosecond discharges is measured in a single pulse regime and validated by means of 0D kinetic modelling. It was found that the conventional scheme for energy release from internal degrees of freedom adequately describes the dynamics of FGH in vitiated hot environment diluted with air. Short-gated ICCD imaging and spatially-resolved emission spectroscopy are used to identify the coupling between the NRPDs and the vitiated hot flow. The effectiveness of the NRPDs actuation is assessed through the chemiluminescence images of the sequential flame. The distance of the center of gravity of the sequential flame to the outlet of the mixing channel is evaluated, with and without plasma actuation. The effect of fuel reactivity on plasma effectiveness is also studied by varying the fraction of hydrogen in the fuel blend of the second stage of the combustor. The results show that the glow NRPDs regime allows strengthening the flame anchoring for the most reactive blends considered in this work, while the spark NRPDs is required for the ignition and prevention of lean blow-out of the flame for the least reactive fuel blends which exhibit low fractions of hydrogen.

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 thresholds and flame propagation of methane-air mixture: detailed kinetic study coupled with electrical measurements of the nanosecond repetitively pulsed plasma discharges

This study investigated the effects of nanopulsed plasma discharge on combustion enhancement. A disk-ring plasma actuator was developed to be applied to a rectangular cross-sectional burner. Experiments tests were performed with different applied voltages to estimate reduced electric field (E N), mean power and the energy input (E i) to be coupled with numerical analysis. The validation of the plasma chemistry model was carried out using ZDPlaskin numerical solver by comparing the experimental data of mole fraction of O atoms available in the literature. Plasma chemistry computations based on an extended kinetic mechanism of methane/air mixtures were performed by ZDPlasKin. Then, the results obtained by the ZDPlasKin in the form of kinetic effects (concentrations of O, OH, O3, CH, CH3, and H) and thermal effects (joule heating) were used in CHEMKIN to evaluate the combustion enhancement. Flame speed considerably increased with plasma discharge and maximum enhancement has been observed at about 20.9%. The flame temperature corresponding to the peak of heat release rate gets lower with the increase of the E i value. It has been observed that in a clean case, a lower flammability limit was retrieved at φ = 0.6, but in presence of plasma, it was reduced to φ = 0.5. The plasma actuation significantly reduced the time necessary to reach the ignition temperature with respect to autoignition (clean case). The maximum reduction of ignition delay timing was noticed at E i = 10.11 mJ cm−3 and E N = 200 Td, with a value between 10−8 and 10−7 s, significantly lower than the autoignition case is between 10−3 and 10−2 s. Adding to this, ignition analysis was also performed at low inlet temperatures. Results demonstrated that it was possible to reach ignition of CH4/air mixture at a lower inlet temperature than the one required for self-ignition (without plasma), meaning that ignition is even possible in cold flow conditions using plasma. Finally, thermal and kinetic effects of plasma discharge on the ignition enhancement of methane–air mixture have been predicted. It concluded the ignition improvement is mainly due to O, H, and OH species, while the thermal effect was negligible.

Modeling of the effects of non-equilibrium excitation and electrode geometry on H2/air ignition in a nanosecond plasma discharge

This work presents the results of two-dimensional modeling of the effects of non-equilibrium excitation and electrode geometry on H2/air ignition in a nanosecond plasma discharge. A multiscale adaptive reduced chemistry solver for plasma assisted combustion (MARCS-PAC) based on PASSKEy discharge modeling package and compressible multi-component reactive flow solver ASURF+ is developed and validated. This model is applied to simulate the impact of non-equilibrium plasma excitation and electrode geometry and heat loss on the dynamics of the discharge from streamer to spark and ignition kernel development in a H2/air mixture with a pair of cylindrical electrodes. The results show that the plasma-generated species (N2(A), N2(B), N2(a′), N2(C), O(1D), O and H) in the spark and afterglow significantly accelerate the ignition kernel development. The increase of discharge voltage at the same total discharge energy promotes the non-equilibrium active species production. It is found that the production of electronically excited species at higher reduced electric field strength is more efficient in enhancing ignition in comparison to the vibrational excitation and heating. Moreover, the 2D simulation clearly reveals that the electric field and active species distribution are highly non-uniform. The streamers are initiated at the sharp outer edges of the negative and positive electrodes by a strong electric field while the electric field is much weaker at the centerline of the electrodes. Furthermore, the simulations reveal that the ignition enhancement is sensitive to the variation of electrode shape, diameter, and gap size due to the changes of electric field distribution and location of streamer formation. A cylindrical electrode produces a larger discharge volume and ignition kernel than the parabolic and spherical electrodes, when the discharge is localized near the axis of the gap. It is found that there is a non-monotonic dependence of ignition kernel size on the electrode diameter and inter-electrode distance. The increase of electrode diameter and gap size above the optimal conditions leads to the reduction of ignition kernel volume, due to the decrease of active species concentration and gas temperature. At a larger electrode surface area and electrode diameter as well as smaller electrode gap size, the heat loss to electrode plays a greater role in reducing the ignition kernel size and slowing ignition kernel development. This work provides insights and guidance to understand the kinetic enhancement of non-equilibrium plasma and the effects of electrode geometries on ignition for the optimization ignitors in advanced engines.

Evaluating the outcomes of a single-cylinder CRDI engine operated by lemon peel oil under the influence of DTBP, rice husk nano additive and water injection

In the search for an alternative energy source with lesser pollution for transportation needs, bio-oil, a denser and viscous fuel that needs a transesterification process, have been widely considered for diesel engines. However, these problems are solved by using low viscous biofuel, but this improvement also significantly leads to increased NOx emission. Hence this present study investigates the usage of a low viscous biofuel in the CRDI engine with measures to reduce NOx emission through water injection technique. The low viscous bio-oil was used in this study along with an ignition enhancer (di-tert-butyl-peroxide), non-metallic nano additive (rice husk). They were tested in a constant speed, single-cylinder, diesel engine for various loads. Considering the brake thermal efficiency (BTE), 2% and 150 ppm were selected as the optimum value after testing five ratios (1%, 1.5%, 2%, 2.5% and 3%) of di tert butyl peroxide (DTBP) and four ratios (50, 100, 150 and 200 ppm) of rice husk (RH). The lemon peel oil (LPO) with the optimum additive ratio produced 30.69% BTE, which was 4.7% lesser than diesel fuel. A considerable decrease in fuel consumption and emissions except for nitrogen oxides (NOx) is recorded. NOx emission increased by 17.3% for the biofuel blend containing RH and DTBP. To control NOx emission, 2% of water was injected into the intake manifold with the fresh intake air. Two percent by vol. was finalised after experimenting four ratios (1%, 2%, 3% and 4%) of water addition. This 2% water reduces 11% of NOx emission and affects the other outputs, denoted with the 8.9% reduced BTE value compared with diesel fuel. Thus, the LPOC combination proved to operate well in the CRDI engine and produces lower NOx emissions than other LPO blends.

Methane-Air Plasma-Assisted Ignition Excited by Nanosecond Repetitively Pulsed Discharge: Numerical Modeling and Effect of Inert Gas.

Plasma-assisted ignition and combustion are promising approaches for controlling ignition enhancement and flame stabilization. The global loosely coupled plasma-assisted combustion kinetic model has been established by combining the ZDPlasKin and ChemKin codes, which is employed to numerically investigate the effects of the inert gas-diluted methane–air nanosecond repetitively pulsed (NRP) plasma on the ignition process. The results indicate that addition of the inert gas is conducive to increasing the chemical reactive species densities in the methane–air NRP discharge plasma. The addition of inert gases affects the generation pathways of plasma species and their corresponding contribution rates. Compared with the methane–air plasma, the dilution of inert gases shows obvious effects on reducing ignition delays, and the dilution of He and Ar decreases the ignition delays by 58.0 and 84.0%, respectively. CH3 + O2 = CH3O + O and H + O2 = O + OH are the dominant conducive reactions in the methane–air ignition chemistry. Moreover, the dilution of inert gases has considerable influences on the normalization sensitivity coefficients, especially for the reaction of H + O2 = O + OH.

An experimental investigation into the fuel oil-free start-up of a coal-fired boiler by the main solid fossil fuel with additives of brown coal, biomass and charcoal for ignition enhancement

The research is focused on the ignition of solid fuels in a heated air flow. The experimental conditions correspond closely to solid fuel combustion in coal-fired boiler furnaces. Charcoal, wood, brown and bituminous coals were used as independent solid fuels and as fuel mixture components. The particle sizes of these fuels were less than 40 μm, 40–140 μm, 140–250 μm, 250–375 μm and 375–800 μm. They were ignited under the conditions of radiant-convective heating as they were moving in a heated air flow with a temperature of 500–800 °C and at a velocity of 5 m/s. The ignition delay time was the main investigated characteristic, recorded by a high-speed video camera. The induction period of bituminous coal shortens by 25–35% when 25–50% brown coal or wood in an air-dry state is added. The lowest ignition delay times were established for the fuel mixtures of bituminous coal with dry charcoal. The experimental curves of the ignition delay times of fuels and their mixtures versus the heated air temperature were presented as approximation equations. Based on the conducted research findings, a muffle burner design with a vortex structure of the fuel-air flow was proposed for the fuel oil-free start-up of coal-fired boilers.

Ignition enhancement of lean kerosene air mixture by multichannel jet enhanced plasma igniter

Reliable and fast ignition is one of the biggest challenges for the development of advanced propulsion systems. In this study, a novel multichannel jet enhanced plasma ignition strategy is proposed. Based on the flow control strategy, the plasma synthetic jet is used to enhance the spark discharge, to enlarge the spark size, and to increase the ignition energy. The multichannel jet enhanced plasma igniter (MJEPI) discharge characteristics, spark evolution, and the ignition process in a single-head swirl combustor were studied and compared with those of the traditional spark igniter. The discharge efficiency can be improved from 33.5% to 42.7% compared with the traditional spark igniter. The heating ratio has a great influence on the performance of MJEPI. A positive return can be obtained only when the heating ratio is greater than about 13.6. The MJEPI significantly enlarges the initial flame kernel which maintains its strength throughout the ignition process. Due to the enhancement of heat exchange and mass transport of the larger initial flame kernel, the lean ignition limit can be extended by 26%-32% at different air flow and the ignition time is obviously reduced by 29.7%. It can be concluded that the MJEPI is worthy to be further studied.

Experimental Investigation on High-Altitude Ignition and Ignition Enhancement by Multi-Channel Plasma Igniter

Experimental Investigation on High-Altitude Ignition and Ignition Enhancement by Multi-Channel Plasma Igniter

Ignition enhancement and deterioration by nanosecond repetitively pulsed discharges in a randomly-stirred lean n-butane/air mixture at various inter-electrode gaps

At 38th Symposium, Shy and co-workers investigated a turbulence-facilitated-ignition (TFI) phenomenon via both conventional-single-spark-discharge (CSSD) and nanosecond-repetitively-pulsed-discharge (NRPD) at various pulsed-repetitive-frequency (PRF=5–70 kHz) using a fixed inter-electrode gap (dgap=0.8 mm) in a randomly-stirred lean n-butane/air mixture with Lewis number Le≈2.2 ≫ 1 over a range of r.m.s. turbulent fluctuating velocity (u′=0–2.1 m/s). It was found that the ignition probability (Pig) decreases significantly with increasing u′ for most PRFs, except at PRF=60 kHz having turbulent Pig,T = 37% at u′=0.5 m/s > laminar Pig,L = 34%, showing possible TFI. Using the same methodology, this paper presents two new Pig datasets at dgap=0.6 mm and 2.0 mm over wider ranges of PRF=1–90 kHz and u′=0–2.8 m/s. All NRPD experiments apply the same total ignition energy Etot≈23 mJ via a train of 11 pulses, each pulse having about 2.2 mJ except for the first pulse having about 1 mJ when using sufficiently high open-circuit-voltage of 28 kV. Note that Etot≈MIEL≈23 mJ at dgap=0.8 mm as a baseline for comparison; MIEL is the laminar minimum ignition energy measured by CSSD at 50% ignitability and MIEL=26.3 mJ/3.4 mJ at dgap=0.6 mm/2.0 mm. While TFI is noticeably observed at dgap=0.6 mm when PRF=60 kHz with Pig,T = 92% at u′=0.5 m/s > Pig,L = 78%, turbulence reclaims its dominating role when u′ > 1.4 m/s with Pig,T << Pig,L. At dgap=2.0 mm, there is no TFI; a Pig,T transition is found where the decrease of Pig,T with u′ changes from gradually to rapidly when u′ > u′c (critical u′ depending on PRF). We also discover that the curves of Pig,T versus PRF are highly non-monotonic with peaks around PRF=40 kHz for u′ > 1.4 m/s; too high PRFs are detrimental for spherical flame ignition. These results are explained by coupling effects of dgap, PRF, u′, and differential diffusion.

Enhancement of Arc Ignition on Tungsten in Helium Plasmas with Impurity Gases

Enhancement of Arc Ignition on Tungsten in Helium Plasmas with Impurity Gases

Numerical modeling of ignition enhancement using repetitive nanosecond discharge in a hydrogen/air mixture I: calculations assuming homogeneous ignition

Ignition enhancement using repetitive nanosecond discharge (NSD) is studied in a stoichiometric hydrogen/air mixture. Numerical simulations are conducted for the homogeneous ignition process using code incorporating ZDPlasKin and CHEMKIN. The objective is to examine how the characteristics of the NSD affects the ignition delay time and why the NSD promotes a homogeneous ignition process. The influence of pulse number, discharge frequency, reduced electric field, total input energy and input energy per pulse on the ignition process is investigated. It is found that the characteristics of NSD have a significant impact on the ignition delay time. The ignition delay time changes non-monotonically with the reduced electric field, and it depends on both the total input energy and the input energy per pulse. Furthermore, it is shown that the ignition enhancement by NSD is mainly due to the kinetic effects while the thermal effects (Joule heat) are negligible. The ignition enhancement is mainly caused by radicals, especially H and O, produced by NSD. A reaction pathway analysis is conducted to identify the key elementary reactions involved in the ignition enhancement using NSD. The electron impact reactions and quenching reactions of excited species are found to help to produce H and O radicals and thereby promote the homogeneous ignition process.

Numerical modeling of ignition enhancement by repetitive nanosecond discharge in a hydrogen/air mixture II: forced ignition

Non-equilibrium plasma has promising applications in combustion enhancement. In this study, 1D simulations are conducted for the forced ignition process using non-equilibrium plasma generated by repetitively pulsed nanosecond discharge (NSD). The ignition kernel is induced by a discharge area with NSD and it develops and propagates in a static stoichiometric hydrogen/air mixture. The objective is to assess how the characteristics of NSD affect the ignition delay time and ignition kernel development during the forced ignition process. Similar to the homogeneous ignition considered in part I, the forced ignition process is found to be strongly affected by the pulse number, discharge frequency, discharge radius, total input energy, and input energy per pulse of NSD. For a fixed input energy per pulse, the ignition delay time decreases and the propagation speed of the initial ignition kernel increases with the pulse number, discharge frequency and discharge radius. However, for a fixed total input energy, the ignition delay time increases and the propagation speed of the initial ignition kernel decreases with the pulse number. Furthermore, it is found that the promotion of ignition kernel development and propagation in the forced ignition by NSD are mainly due to the kinetic effect rather than the thermal effect.

Reduced reaction mechanism for natural gas combustion in novel power cycles

Abstract We study the auto-ignition behavior of several natural gas surrogates; in particular, the influence of ethane and propane on the ignition delay time of methane is studied. A rapid compression machine was used to obtain experimental measurements of ignition delay times at two different compression pressures (10 and 15 bar) and a wide range of compression temperatures (904–1151 K), for both stoichiometric and fuel-rich ( ϕ = 2 ) mixtures. The experimental results are compared to homogeneous reactor model simulations (HOMREA). A first set of simulations treats chemical reaction in detail, using the AramcoMech 3.0 reaction mechanism. It was observed that the mechanism predicted the observed ignition delay times well. Experimental results indicate that both ethane and propane have an ignition enhancing effect on the mixture, shortening the ignition delay time. Propane, in particular, appears to have a higher influence compared to ethane at low temperatures. In general, fuel rich mixtures show shorter ignition delay times. These trends are well captured by the AramcoMech 3.0 mechanism showing great agreement with experimental data. To make the chemistry underlying the AramcoMech 3.0 available in a concise form, e.g., for CFD applications or similar, a reduced chemistry scheme was developed (reaction mechanism with 49 species and 332 reactions). Simulations using the reduced model showed a similarly good agreement to which was developed based on the detailed AramcoMech 3.0 reaction mechanism. The experimental results are predicted well by the reduced model, in a similar fashion like the detailed scheme. Furthermore, the reduced reaction mechanism is validated against experimental data found in literature, covering a wide range of conditions. This establishes the reduced model as a useful, computationally efficient substitute for describing the effect of ethane and propane onto the auto-ignition of methane.

Numerical investigation on plasma assisted ignition of methane/air mixture excited by the synergistic nanosecond repetitive pulsed and DC discharge

Plasma assisted combustion provides possibilities for reducing ignition delays and controlling pollutant emissions. The zero-dimensional plasma and combustion models have been built up to numerically investigate the effects of the synergistic nanosecond repetitive pulsed (NRP) and DC discharge on the methane/air plasma assisted ignition. The synergistic discharge means exerting the low voltage DC discharge after the NRP discharge in one period of the discharge plasma. The simulation results indicate that the selective excitation of the vibrationally excited species N2(v), O2(v) and CH4(v) as well as the electronically excited species O2(a1Δg) and O2(b1Σg +) by the synergistic discharge is superior to that by the NRP discharge when the electron energy has been deposited into different molecular degrees of freedom. The plasma kinetic effect on the ignition enhancement is highly efficient since it can break though the threshold of the thermal effect. Both the kinetic effect and the thermal effect of the NRP discharge on ignition enhancement are relatively weaker than those of the synergistic discharge. Besides, reactions involved the N2 electronically excited species produce abundant O and H, which is conducive to the formation of the methane oxidation intermediates. e + O2 = e + O + O(1D) in the NRP discharge and e + O2 = e + O2(a1Δg) in the synergistic discharge play the crucial roles in the methane/air plasma enhance ignition, respectively.

An insight into the reaction kinetics of CH3 + O2(a1Δg) and its enhancement effect on methane ignition

The reaction between CH3 and O2(a1∆g) is crucial to understand the effects of electronically excited oxygen in plasma-assisted combustion of methane and other hydrocarbons. In the present work, multireference quantum chemical methods were used to investigate the potential energy surface of CH3 + O2(a1∆g). The RRKM/master equation simulation was employed to compute the rate coefficients of various pathways to this reaction over the temperature range of 300–2000 K and a pressure range of 0.1–100 atm. Special attention has been paid to the nonadiabatic transition between the excited state and ground state, which directly leads to a quenching channel from CH3 + O2(a1∆g) to CH3 + O2(X3∑g−). This quenching reaction has been overlooked by previous theoretical and kinetic modeling studies. We also conducted kinetic modeling to examine the effect of this reaction on the ignition enhancement of methane oxidation. Although the channel of CH3 + O2(a1∆g) quenching to CH3 + O2(X3∑g−) has nonnegligible rate constants comparing with other reaction channels, modeling result with the inclusion of 5% O2(a1∆g) in molecular oxygen shows that the titled reactions shorten the ignition delay time of methane by more than twenty times at 900 K, 1 atm. The ignition enhancement is mainly from the chain branching channels to CH2O + OH and CH3O + O which has been greatly promoted by excess energy from O2(a1∆g). The present study uncovers the kinetic mechanism of this nonadiabatic reaction and provides reasonable rate coefficients for further kinetic modeling of plasma-assisted combustion of methane and other hydrocarbons.