High-temperature Ignition Research Articles

Study of Exhaust Gas Composition and Dilution Effects on Diesel Spray Ignition Characteristics

ABSTRACT Exhaust gas retained in a cylinder is unavoidable, affecting diesel spray ignition. However, its effects on diesel spray ignition characteristics are not completely clear so far, due to the coupled and complicated function of exhaust gas compositions and dilution effects. To reveal the effect mechanism, three-dimensional simulations and chemical kinetics calculations were performed. The results illustrate that with the rise of exhaust gas rate, the initial position of the flame moves downward due to the longer ignition delay. The introduction of exhaust gas compositions decreases both the low-temperature and high-temperature ignition delay while dilution on oxygen mass fraction primarily affects and prolongs the high-temperature ignition delay. Nitric oxide (NO) significantly decreases the ignition delay compared to other exhaust gas compositions, with the high-temperature ignition delay experiencing a substantial decrease of 33.33% under 500 ppm NO addition at the ambient temperature of 820K. With the NO addition in the fresh intake air, conversion reactions occur between NO and nitrogen dioxide, and the reactions of NO with hydroperoxyl (HO2) and nitrogen dioxide with hydrogen radical lead to the generation of hydroxyl radical throughout the low- and high-temperature ignition stages, which enhances system reactivity and contributes to a decrease of ignition delay. The decrease in the oxygen mass fraction significantly reduces the rate of production of HO2 and hydrogen peroxide, which results in a decrease in the rate of production of hydrogen peroxide decomposition and HO2 deoxidation into hydroxyl radicals, prolonging the high-temperature ignition. The revealed mechanism may be applicable to the diagnosis of exhaust gas residual fault phenomenon.

Reduced Kinetic Mechanism for Modeling High-Temperature Ignition and Flames for n-Pentanol.

n-Pentanol has emerged as a promising substitute for fossil fuels due to its potential in reducing greenhouse gas emissions. This work presents a reduced combustion mechanism for modeling high-temperature ignition (T > 1000 K) and flames for n-pentanol/air mixtures. The model comprises only 64 species and 282 chemical reactions. As we know, this is the first comprehensive study to include all available experimental data for these phenomena and this specific fuel/oxidizer mixture, reported in the literature. Our approach involves coupling a detailed submechanism for n-pentanol to the San Diego mechanism, a reduced scheme for C1-C4 hydrocarbons, followed by a systematic reduction process, mainly using sensitivity analysis and steady-state approximation. We quantitatively assessed the agreement between experimental data and simulation results using deviation or error indicators along with graphical comparisons. Remarkably, the ignition delay times and flame speeds calculated using the reduced kinetic model exhibit a high agreement with the experimental data reported in the literature.

Synergistic enhancement of oil-water separation and flame retardant properties of melamine sponges based on 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) and perfluorinated compounds

Facing the escalating issue of industrial wastewater discharge and its profound environmental repercussions, the creation of novel sponge materials that excel in oil-water separation while incorporating flame retardancy becomes critically urgent. Despite their superior absorption capabilities, conventional oil-water separation sponges typically overlook flame retardant features, a limitation that curtails their applicability. To address this gap, we have developed a multifunctional hydrophobic sponge through a sophisticated multi-step chemical self-assembly process. This method grafts 9,10-Dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) and perfluorinated compounds onto melamine foam (MF), yielding a composite material designated as DOPO-PF@MF. DOPO-PF@MF boasts a dual functionality of hydrophobicity and flame retardance. Its modified surface exhibits a pronounced hydrophobic-lipophilic balance, with a static water contact angle of 134°. It also displays favorable wetting characteristics towards various organic solvents, such as methanol, ethanol, dimethylformamide, dichloromethane, and hexane. This material demonstrates an impressive adsorption capacity for these solvents, with the adsorption-desorption capacity for ethanol remaining consistent over ten cycles. Furthermore, DOPO-PF@MF efficiently separates emulsions of methylene chloride and water, showcasing its versatility. Moreover, DOPO-PF@MF exhibits commendable chemical resilience under extreme temperatures, ranging from 0 °C to 100 °C, and withstands both acidic and alkaline conditions (pH 1–14), preserving its structural integrity against corrosive substances. Most notably, it surpasses unmodified melamine foam in flame retardancy. The incorporation of DOPO fortifies the material's resistance to high-temperature ignition, effectively inhibiting flame propagation and reducing burn rate, enhancing safety measures.

Facile synthesis and magneto-alignment effect on the microwave absorption property of NiCo2O4/NiO/Co3O4 composites

A simple method of high-temperature ignition to metalorganics was successfully used to synthesise composites of NiCo2O4, NiO and Co3O4. A detailed investigation of the composite’s microwave absorption properties revealed the magneto-alignment effect. By closely arranging NiCo2O4, NiO and Co3O4 nano-particles in epoxy resin, the magnetic exchange is strengthened under the influence of a magnetic field. The sample without magnetic alignment, S0, exhibited poor absorption efficiency with reflection loss (RL) values hardly reaching − 10 dB across a frequency range of fixed and different thicknesses (2–18 GHz). In comparison, the oriented composite exhibited higher permittivity and permeability, particularly in the high-frequency range. Considering the improved impedance matching and reasonable adjustment of dielectric and magnetic losses, the microwave absorbing performance of the oriented composite substantially improved in the 10–14 GHz range. The minimum RL is reduced to − 45.11 dB with an effective bandwidth of 4 GHz. Herein, we present a straightforward method to increase the microwave absorption performance of NiCo2O4 by modifying the macroscopic order and degree.

Mechanism insights into low-temperature oxidation of n-heptane on CeO2 surface: A DFT study

The low-temperature oxidation (LTO) mechanism of n-heptane on CeO2 surfaces was investigated in this study employing density functional theory (DFT). Firstly, the adsorption conformations of relevant species were optimized, and the main reaction steps' energy barriers and reaction-rate coefficients were calculated. The results show that the hydroxyl groups of fuel radicals strengthen the adsorption of fuel radicals on CeO2 surfaces. Furthermore, the energy barriers associated with the low-temperature (LT) chain branching pathway of n-heptane on the CeO2 surface exhibit lower values compared to those observed in the gas phase. The presence of CeO2 is also observed to mitigate the detrimental impact of competing reactions on the low-temperature branching pathway of n-heptane. It should be noted that the energy barrier for the decomposition of H2O2 on the CeO2 surface is 32.575 kcal/mol, 33.2 % lower than that in the gas phase, which allows H2O2 to be decomposed much easier at low temperatures. The resultant OH radicals desorb to the intimated gas phase, accelerating the oxidation of fuel molecules in the gas phase. Therefore, n-heptane transitioned to high-temperature ignition more easily and rapidly in the presence of CeO2. The calculation's findings adequately explain the experimental finding that adding CeO2 improves diesel fuel combustion.

Analysis of Thermomechanical Stress of High-Temperature Ignition Surface Caused by Drop–Wall Interaction at Engine Conditions

Abstract Drop–wall interaction is a complex phenomenon encountered in diverse industrial applications. An important example is fuel droplets impinging on a high-temperature ignition plug in a direct-injection compression-ignition engine. The ignition plug, comprised of heat-resistant materials, will experience thermal shock due to abrupt temperature changes. The ensuing temperature fluctuation in the solid wall induces thermal stress, and if this stress surpasses the material's strength in that mode, failure can occur. Therefore, it is imperative to analyze the temperature dynamics on the high-temperature surface to enhance material durability. This study focuses on drop–wall interactions in the engine environment. Utilizing the Smoothed Particle Hydrodynamics (SPH) method, this research simulates fuel droplet impingement on an ignition plug with various materials to characterize heat transfer, thermal penetration, and temperature distributions in the heated wall. The investigation also delves into the behavior of ceramic material, specifically silicon nitride, assessing its thermomechanical stress and durability based on the stress–number of cycles (S-N) curve. Thermal stress is computed by considering temperature gradients and material properties, while mechanical stress is evaluated based on the bending momentum and momentum flux induced by the spray. A parametric study explores diverse materials such as tungsten carbide, iron, stainless steel, carbon steel, and aluminum. Results indicate that thermal stress outweighs bending and spray-induced stress. Moreover, the analysis reveals that silicon nitride exhibits the lowest thermal stress distribution and superior durability, potentially capable of operating for infinite cycles under engine-relevant conditions.

A Phenomenological Thermal Spray Wall Interaction Modeling Framework Applied to a High-Temperature Ignition Assistant Device

Abstract Airborne compression ignition engines must operate with reliable ignition systems to achieve proper ignition at every cycle, particularly at high altitudes. Glow-plug-based ignition-assistant (IA) devices can provide the necessary energy to preheat the fuel and ensure ignitability of the fuel-air mixture. Ignitability of liquid sprays can be facilitated via direct impingement onto the hot IA surface, however this comes with adverse effects on the IA durability. Therefore, optimizing an IA's design requires detailed understanding of the physics of fuel spray impingement of superheated surfaces. While spray impingement on relatively low wall temperatures has been extensively studied and appropriate numerical models have been proposed through the years, fundamental understanding of high-speed liquid spray impingement on superheated walls is still elusive. This work aims to formulate a phenomenological thermal spray-wall interaction framework for modeling the film-boiling-induced heat transfer, atomization, and dispersion of fuel spray droplets impinging onto a superheated IA device. A qualitative comparison of the new phenomenological model is performed against optical experiments from the literature of an F-24 fuel spray injected onto an IA device located 12 mm away from the injector tip. The temperature of the IA was set at 1400 K. The fuel injection pressure was 400 bar, while the ambient gas pressure and temperature were 30 bar and 800 K, respectively. The performance of the phenomenological model is evaluated in comparison with two other state-of-art models from the literature. A qualitative analysis of the different spray and fuel-air mixture characteristics is performed to outline the differences in the predictions offered by the new phenomenological model and the two state-of-art spray-wall interaction models.

The high-temperature ignition kinetics of nitroethane: A shock-tube experimental and kinetic modeling study

Nitroethane is a representative nitro-containing energetic molecule, and is considered to be a potential propellant and a fuel additive. Thus, understanding its combustion characteristics and chemistry is critical in the development of hierarchical kinetic models of nitro-containing energetic materials. Herein, we investigate the high-temperature ignition kinetics of nitroethane via experimental measurements and kinetic modeling. Ignition delay times of nitroethane are measured using a high-pressure shock tube in the temperature range 900–1150 K, at pressures of 5 and 10 bar, for equivalence ratios of 0.5, 1.0 and 2.0 in diluted air. A detailed kinetic mechanism of nitroethane is updated by incorporating new reaction rate coefficients for hydrogen abstraction reactions that are studied using quantum chemistry calculations at the G4//M06–2X/6–311++G(d, p) level of theory and transition state theory. The kinetic model simulations using the updated mechanism are in good agreement with the experimental results at all conditions. Reaction path and sensitivity analyses were conducted to understand the ignition kinetics of nitroethane, and the results reveal that abstraction reactions play a critical role in the oxidation of nitroethane in addition to the C–N bond dissociation and HONO elimination reactions.

Numerical study of ignition process of ternary hydrogenated catalytic biodiesel/methanol/n-octanol blends under high-temperature and high-pressure conditions

This article presents a numerical simulation study of the spray combustion process of methanol/n-octanol/hydrogenated catalytic biodiesel (HCB) blended fuel in a high-temperature and high-pressure constant volume combustion chamber environment, using the Reynolds-averaged Navier-Stokes (RANS) method. This study focuses on the effects of the fuel properties on two-stage ignition processes. The blended fuel, denoted as M15, is composed of methanol, octanol, and HCB in volume ratios of 15 %:17 %:68 %. The pure HCB (M0) was also calculated under the same operating condition as references. The results indicate that M15 exhibits significant delays in both high-temperature and low-temperature ignition compared to M0. At low ambient temperatures, methanol demonstrates a strong inhibitory effect on spray ignition in part due to its competition with alkanes for OH radicals. Additionally, high-temperature ignition of M0 primarily occurs at the periphery of the spray, whereas the ignition of M15 transitions toward the fuel-rich region near the nozzle exit. This transition is attributed to the ignition delay, which provides better-mixing characteristics in the M15 spray, along with the oxygen content introduced by methanol.

Alkylperoxy radicals are responsible for the formation of oxygenated primary organic aerosol.

Organic aerosol (OA) is an air pollutant ubiquitous in urban atmospheres. Urban OA is usually apportioned into primary OA (POA), mostly emitted by mobile sources, and secondary OA (SOA), which forms in the atmosphere due to oxidation of gas-phase precursors from anthropogenic and biogenic sources. By performing coordinated measurements in the particle phase and the gas phase, we show that the alkylperoxy radical chemistry that is responsible for low-temperature ignition also leads to the formation of oxygenated POA (OxyPOA). OxyPOA is distinct from POA emitted during high-temperature ignition and is chemically similar to SOA. We present evidence for the prevalence of OxyPOA in emissions of a spark-ignition engine and a next-generation advanced compression-ignition engine, highlighting the importance of understanding OxyPOA for predicting urban air pollution patterns in current and future atmospheres.

Embedded direct numerical simulation of ignition kernel evolution and flame initiation in dual-fuel spray assisted combustion

The flame initiation process in dual-fuel spray assisted combustion is presently not fully understood. Here, diesel spray assisted combustion of premixed methane/oxidizer/EGR is explored in the post-ignition phase by scale-resolved simulations. The modified dual-fuel ECN Spray A forms the baseline configuration. An extensive local grid refinement (approaching DNS limit) around one of the first high-temperature ignition kernels is carried out in order to examine the validity of hypothesized flame initiation and deflagration. A high quality LES is used to solve the spray dynamics, while the embedded quasi-DNS (eq-DNS) region offers detailed information on the ignition kernel evolution. The finite-rate chemistry is directly integrated, utilizing 54 species and 269 reactions. Local combustion modes are investigated for the ignition kernel development toward spontaneous ignition and premixed flame propagation using various approaches, including the reaction front displacement speed, energy transport budget, and chemical explosive mode analysis. Furthermore, a new criterion based on reaction flux analysis is introduced, which is compatible with dual-fuel combustion. The spatial and temporal scales associated with the ambient methane consumption and consequent flame initiation are characterized. For the first time in dual-fuel spray assisted simulations, numerical evidence is provided on the initiation of premixed flames, and the corresponding timescale is reported. Particularly, there is a transient mixed-mode combustion phase of approximately 0.2 ms after the spray second stage ignition wherein extinction, ignition fronts, and quasi-deflagrative structures co-exist. After such a transient period, the combustion mode becomes essentially deflagrative. Finally, interactions between turbulence and premixed flame front are characterized mostly in the corrugated regime.

Investigation of the injection pressure impact on non-monotonic two-stage ignition delay of diesel engines under cold-start

Poor startability under cold-start conditions is a long-term obstacle of the compression ignition engine and adjustment of the injection pressure is an effective way to improve the startability. Therefore, a detailed investigation of the effect of injection pressure (Pi) on the ignition characteristics of the fuel spray under cold-start conditions is needed. Though numerous studies have been conducted on the effect of Pi on the ignition characteristics of the fuel spray, the two-stage ignition process of the fuel spray under cold-start conditions is still an unknown field. In this paper, the effect of Pi on the two-stage ignition process of the fuel spray under cold-start conditions is investigated in a constant volume combustion chamber through simultaneous Shadowgraphy and high-speed natural luminosity method an analytical model for two-stage ignition delay is established accordingly for further investigation. Based on the experiment results, a monotonic decreasing trend on low-temperature ignition delay time LTI (i.e., form 4.5 ms to 2.5 ms under the ambient temperature of 760 K) and an increasing trend on the cool flame propagation time τ (i.e., form 0.4 ms to 1.6 ms under the ambient temperature of 760 K) are found and finally results in a non-monotonic trend of high-temperature ignition delay (HTI) as the injection pressure raises from 40 MPa to 160 MPa. According to the established model, increased Pi leads to promoted atomization and intensified dilution effect of the fuel spray, which is concluded as the reason for decreased LTI and increased τ, respectively. In this way, the non-monotonic trend of HTI under engine cold-start as Pi increases is quantitatively explained through the promoted model.

Controlled ignition of low-carbon gas-engine fuels based on natural gas and hydrogen: process kinetics

The potential wide use of environmentally friendly low-carbon gas fuel based on natural gas, hydrogen, their mixtures, and syngas in power generation and transport requires detailed information about the kinetics of ignition of these gases at temperatures below 1000 K, at which fuel ignition occurs in internal combustion engines (ICEs) and gas turbines. The same temperature range is also important for monitoring the storage and transportation conditions of these fuels. Although there are quite a few studies addressing the ignition of classical gas fuels such as methane or hydrogen, there is an obvious lack of works dealing with real natural gases and gas mixtures. Furthermore, even for methane and hydrogen, data on the ignition at high temperatures (T > 1000 K), which have been mainly gained by the shock-wave method for highly diluted mixtures, are at variance with the kinetic estimations for real conditions of operating with them or their use in ICEs. Considering the ignition characteristics at T < 1000 K is also important for syngas, the largest-scale base product of gas chemistry and the main industrial source of hydrogen. The pronounced discrepancies between the extrapolation of the results obtained for high-temperature ignition to lower temperatures and the results of kinetic modelling of these processes make it necessary to analyze their causes. This review addresses new experimental results on the ignition of methane–alkane and methane–hydrogen mixtures (real gas fuels) and kinetic modelling of these processes, which reveal significant changes in the ignition behaviour at T < 1000 K. These changes in the ignition process upon the variation of the temperature, pressure, and composition of the mixture are related to significant changes in the methane and hydrogen oxidation mechanisms in this temperature range. They are mainly caused by changes in the kinetics and, hence, the role of peroxide compounds and radicals in methane and hydrogen oxidation following temperature and pressure variation. The established features bring about the question of the adequacy of the existing criteria for assessing the knock resistance of gas engine fuels, primarily those containing hydrogen, when they are used in ICEs, and for assessing their explosiveness and measures taken for their safe handling. The review considers the possible methods for improving the knock characteristics of natural and associated gases to meet the requirements of power equipment manufacturers.<br> The bibliography includes 128 references.

Large eddy simulation of combustion recession: Effects of ambient temperature and injection pressure

In the present study, large eddy simulation is used to investigate combustion recession for the Engine Combustion Network Spray A flame at two ambient temperatures (850K and 800K) and two injection pressures (100MPa and 50MPa). The present numerical results are able to capture different combustion recession phenomena after the end-of-injection (AEOI). With an injection pressure of 100MPa, the model predicts a “separated” combustion recession at the ambient temperature of 850K and no combustion recession at the ambient temperature of 800K, in which both predictions correspond to the measurements. The combustion recession is mainly controlled by the auto-ignition process at the ambient temperature of 850K. At the ambient temperature of 800K, the local temperature within the fuel-rich region is not high enough to promote the high-temperature ignition process. As time progresses, the mixture within the fuel-rich region rapidly transitions to become an overly fuel-lean mixture, which further hinders high-temperature ignition to occur. Nonetheless, it is shown that lowering the injection pressure to 50MPa causes the combustion recession to occur at the ambient temperature of 800 K. This is likely attributed to the low injection case having a lower air entrainment rate AEOI, which causes the mixtures upstream of the lift-off position to transition slower from fuel-rich to fuel-lean mixtures.

High-speed formaldehyde planar laser-induced fluorescence and schlieren to assess influences of injection pressure and oxygen concentration on Spray A flames

The present study investigates the effect of injection pressure and ambient oxygen concentration variations on the Engine Combustion Network Spray A flame. The two-stage ignition and combustion processes for individual injections are characterized using a high-speed planar laser-induced fluorescence (PLIF) and schlieren imaging diagnostic at high pressure (near 6 MPa). For PLIF measurements, we use a pulse-burst-mode Nd:YAG laser capable of producing a 6 ms long 355-nm pulse-train with 300 pulses at 70 mJ/pulse, separated by 20 µs. The PLIF imaging at high-acquisition rate offers unique insights into understanding the spatiotemporal distribution of formaldehyde (CH2O) and polycyclic aromatic hydrocarbons (PAH). High-speed line-of-sight schlieren with a pulsed infrared light emitting diode (LED) is applied simultaneously with PLIF to visualize the spray penetration, low- and high-temperature ignition, and turbulent flame dynamics, while in-vessel pressure is used in concert with the optical diagnostics to assess the ignition delay time of the high-temperature ignition events. The PLIF and line-of-sight measurements, documenting the position and timing for formation and consumption of CH2O and the inception and distribution of PAH downstream with changes in ambient oxygen concentration and injection pressure, provide a unique database for detailed evaluation of single-event (e.g. LES) or ensemble-average CFD.

An experimental and kinetic modeling study on the high-temperature ignition and pyrolysis characteristics of cyclohexylamine

Amine compounds are an important class of nitro-containing fuels and represent as promising renewable fuels. However, the structural effect from the alkyl group on the combustion characteristics of amines remains an urgent problem to promote their usage in practical engines. To probe and replenish the combustion properties of amine fuels, this work reported an experimental and kinetic modeling study on the high-temperature ignition and pyrolysis characteristics of cyclohexylamine over a wide range of conditions. Specifically, a high-pressure shock tube is used to measure the ignition delay times under the temperature range from 1100 K to 1700 K with pressure of 2 and 5 bar and the equivalence ratios of 0.5, 1.0 and 2.0. The pyrolysis characteristics is studied using a single-pulse shock tube at fuel concentration of 0.5% diluted by argon in the temperature range 950–1400 K at 5 and 10 bar with the residence time around 1.6 microsecond. A detailed kinetic mechanism is developed to simulate the experimental results. To improve the accuracy of the detailed mechanism, primary initial reactions of cyclohexylamine including the hydrogen abstraction reactions and the CN decomposition reaction are theoretically studied at G4//M06–2X/6–311++G(d, p) level of theory with transition state theory and RRKM/mater equation. The detailed kinetic mechanism shows good prediction accuracy of the ignition delay times and pyrolysis product distributions under different temperature, pressure and equivalence ratio conditions. Sensitivity analysis results indicate that the most sensitive reaction is O2 + H = O + OH with the most negative sensitivity coefficient and the abstraction reaction of vinylamine with OH radical, C2H3NH2 + OH = C2H3NH + H2O also exhibits large sensitivity coefficients. Reaction path analysis reveals that the abstraction reactions control the initial oxidation of cyclohexylamine and cyclohexen-1-amine, cyclohexen-2-amine, and cyclohexen-3-amine are also major intermediates during the oxidation of cyclohexylamine besides small pyrolysis products.

Characteristics of ethylene–air continuous rotating detonation in the cavity-based annular combustor

Characteristics of continuous rotating detonation (CRD) in a cavity-based annular combustor are studied through three-dimensional simulations, which are solved in OpenFOAM with a skeletal chemical kinetics mechanism. The results show that the continuous rotating detonation wave (CRDW) attaches to the outer wall and detaches from the inner wall in the cavity-based annular combustor. A high-temperature recirculation zone is found in the cavity with reactive intermediate species in the cavity-based annular combustor. The cavity is identified as a high-temperature ignition source, which provides heat and active intermediate radicals enhancing the detonability of the combustible gases in front of the CRDW. Consequently, the propagation performance, the propulsive performance, and the detonation combustion intensity are promoted in the cavity-based annular combustor. The propagation mode of the CRDW transforms from an unstable mode to a single-wave mode with the application of cavity. As the cavity deepens from 0 to 15 mm, the propagation velocity significantly increases from 923.21 to 1662.81 m/s and the fuel-based specific impulse increases from 941.91 to 1044.48 s as the cavity depth varies from 0 to 15 mm. Furthermore, the detonation-dominant combustion fraction remarkably improves from 27.21% to 62.29%.

High temperature ignition of ammonia/di-isopropyl ketone: A detailed kinetic model and a shock tube experiment

Ignition delay times (IDTs) of NH3/di-isopropyl ketone (DIPK) mixtures with DIPK fractions of 0%, 5%, 10%, and 50% were measured in a shock tube at pressures around 1.75 and 10bar, temperatures from 1300 to 2100 K, and an equivalence ratio of 0.5. A DIPK-NH3 model was proposed including the DIPK sub-model, NH3 sub-model, and reactions between nitrogen-containing species and hydrocarbon species. The proposed DIPK-NH3 model well predicts the IDTs measured in this study, and the IDT and laminar flame speed data of pure NH3 and pure DIPK reported in the literature. The reactions between nitrogen-containing species and hydrocarbon species consist of four reaction classes: (1) prompt NO and reburn mechanism; (2) recombination reactions and small amines mechanisms; (3) H-abstraction reactions; (4) disproportionation reactions. Reaction classes 1–4 were successively added into a combined NH3 and DIPK model. Comparison of the model predictions shows that the reaction class 1 and 2 have almost no influence on the ignition, while class 3 inhibits the ignition and class 4 significantly inhibits the ignition. The ignition inhibiting effects of class 3 mainly come from C3H6+NH2=C3H5-A+NH3 (R3032) at DIPK blending ratio of 5% and 10%, and C2H4+NO=C2H3+HNO (R3058) at DIPK blending ratio of 50% and 10 bar. The ignition inhibiting effects of class 4 mainly come from HCO+NH2=CO+NH3 (R3050), C2H3+NH2=C2H2+NH3 (R3047), and NH2+C3H5-A=C3H4-A+NH3 (R3065) at DIPK blending ratio of 5% and 10%, and C3H5-A+NO=C3H4-A+HNO (R3067) at DIPK blending ratio of 50% and 10 bar. The NH3/DIPK oxidation pathway and effects of DIPK blending ratio on the IDT of NH3 were also analyzed in detail.

Ignition and stabilization of laminar premixed n-heptane/air flames under engine-like conditions

The multi-stage ignition process of a laminar premixed n-heptane/air mixture is investigated under engine-relevant conditions. The cold fuel/air mixture is injected into a hot environment through an inflow boundary at different inlet velocities. Results show that a double-flame structure for stoichiometric mixture is observed and it transitions to a single flame at an inlet temperature (TFuel) of 700 K. The cool flame is stabilized near the inlet for the case with TFuel = 900 K before the hot flame merging with it. However, both the cool and hot flames move upstream for TFuel = 800 K due to the negative temperature coefficient (NTC) behavior. In addition, the effect of inlet velocity is also studied. By increasing the inlet velocity, the hot flame becomes lifted off. Comparisons of the ignition Damkӧhler number (Daign) defined by the ratio of the residence time to the cool- or hot-flame IDTs illustrate the different controlling mechanisms. At a low temperature of 700 K, both low-temperature combustion (LTC) and high-temperature combustion (HTC) have a Daign value of unity, indicating that the stabilization process is mainly controlled by autoignition rather than diffusion. The heating effect from the hot environment enhances the NTC characteristics of the premixed flame, thus, shortening the IDT. Whereas at 800 K, the value of Daign for the LTC is of unity, but that of HTC decreases, especially at a low inlet velocity, indicating that diffusion plays a more important role for HTC and the flame is stabilized by propagation, which can also be confirmed from the comparisons of the 0-D and 1-D ignition delay times (IDTs). Diffusion of intermediate radicals and heat suppresses the appearance of the high-temperature ignition at an inlet velocity of 1 m/s. At 900 K, high inlet velocity can increase the Daign to unity, indicating a transition in the combustion regime from propagation to autoignition.

Coupling effect of thermal conducting and low-temperature reaction process on ignition characteristics under diesel-like conditions

The typical behavior of misfiring after low-temperature ignition (LTI) in the cold-start of a diesel engine is mainly caused by thermo-kinetic imbalance and is related to the coupling effect of thermal conducting and low-temperature chemistry. This paper abstractly constructed a two-zone model to characterize such a diesel-like operating condition and investigated the coupling effect of thermal conducting and low-temperature chemistry on ignition characteristics mechanistically. The results show that at relatively low temperature/equivalence ratio environments, thermal conducting has a stronger effect on the ignition process and acts mainly on the chemical reaction rate rather than the reaction pathway. At lower temperature environments (600 K–700 K), the two zones' temperature trajectories almost overlap before LTI. At a higher temperature (700 K–800 K), thermal conducting and low-temperature reactions together affect the ignition process, causing the lower-temperature zone to earlier translate from LTI to high-temperature ignition (HTI), in contrast to the situation without thermal conducting. Thermal conducting induced by fuel-concentration gradient has an effect after the LTI of richer zones, which causes the HTI to delay, but for the leaner zone, it increases the low-temperature reaction rates sharply and advances the ignition. This study provides theoretical support to overcome the unstable ignition problem of cold-start.