First-stage Ignition Research Articles

High spatiotemporal resolution optical measurements of two-stage ignition and combustion in Engine Combustion Network Spray D flames

This study explores flame structures and combustion dynamics in high-pressure n-dodecane fuel sprays, focusing on the formation and consumption of formaldehyde (CH2O) during autoignition and the development of poly-aromatic hydrocarbons (PAH) as soot precursors. These processes are crucial for optimizing combustion efficiency and reducing emissions. However, traditional approaches, which rely on single-shot measurements or ensemble-averaged visualizations, often overlook critical early-stage processes during low-temperature ignition. To overcome these challenges, we employed an innovative high-speed planar laser-induced fluorescence (PLIF) technique at 50 kHz using a pulse-burst Nd:YAG laser system with an excitation wavelength of 355 nm. This approach, applied for the first time to Engine Combustion Network (ECN) Spray D flames, provides unprecedented insights into the combustion processes at varying ambient temperatures and oxygen concentrations. Additionally, simultaneous high-speed schlieren imaging at 100 kHz was used to visualize spray penetration, first-stage ignition, and thermal expansion zones. Our findings reveal that, similar to Spray A flames, CH2O forms in cold, fuel-rich zones well upstream of the combustion zone. However, in Spray D flames, the schlieren signal softening observed in the jet's head does not lead to complete disappearance, and the CH2O signal is absent from the full head of the spray. During the second-stage ignition, CH2O consumption accelerates due to high-temperature reactions, leading to a significant reduction in its signal. Unlike the mushroom-shaped structure seen in Spray A flames, Spray D flames exhibit a quasi-steady PAH phase structure, with lean peripheral mixtures insufficient for soot precursor formation. Notably, reducing ambient oxygen concentration to 13 % while maintaining or increasing temperature prolongs the presence of CH2O, highlighting its influence on ignition dynamics and oxidation processes in dodecane spray flames. This study provides new insights into the combustion mechanisms of high-pressure sprays and offers valuable data for developing next-generation combustion technologies, including models, aimed at improving efficiency and reducing emissions.

Exploring the first-stage ignition and model optimization in the comprehensive study of n-dodecane oxidation

n-Dodecane is commonly employed as a surrogate for investigating the combustion characteristics of jet and diesel fuels. Enhancing comprehension of its combustion behavior and developing accurate chemical kinetics models for simulating combustion is of paramount importance in engine development. This study focuses on a detailed exploration of n-dodecane oxidation kinetics under low-temperature conditions and presents a novel dataset concerning the first-stage ignition delay time. A broad spectrum of experimental conditions is investigated, encompassing a range of temperature (600 ∼ 1350 K), pressure (5 ∼ 20 atm), equivalence ratios (0.5 ∼ 1.0), and dilution gases (N2 and Ar). Additionally, combustion experiments in a pure oxygen environment are performed, contributing valuable data to existing research. To enhance the precision of the chemical reaction kinetics model of n-dodecane, this study integrates updated rate coefficients obtained from the latest theoretical calculations for specific reaction classes. The improved rate rule provides a more accurate reference for the construction of the chemical reaction kinetics model of long straight alkane. The resulting improved model excels in accurately predicting both the first-stage ignition delay time and the total ignition delay time under a wide range of operational conditions. Additionally, the model performance is rigorously evaluated through a comprehensive assessment against a diverse array of datasets gathered from various literature references. The results show that, in contrast to the previously proposed model, this enhanced model provides highly reliable predictions over a broad range of parameters.

Effect of Ethanol and Iso-Octane Blends on Isolated Low-Temperature Heat Release in a Spark Ignition Engine

<div>Low-temperature heat release (LTHR) is of interest for its potential to help control autoignition in advanced compression ignition (ACI) engines and mitigate knock in spark ignition (SI) engines. Previous studies have identified and investigated LTHR in both ACI and SI engines before the main high-temperature heat release (HTHR) event and, more recently, LTHR in isolation has been demonstrated in SI engines by appropriately curating the in-cylinder thermal state during compression and disabling the spark discharge. Ethanol is an increasingly common component of market fuel blends, owing to its renewable sources. In this work, the effect of adding ethanol to iso-octane (2,2,4-trimethylpentane) blends on their LTHR behavior is demonstrated. Tests were run on a motored single-cylinder engine elevated inlet air temperatures and pressures were adjusted to realize LTHR from blends of iso-octane and ethanol without entering the HTHR regime. The blends were tested with inlet temperatures of 40°C–140°C at equivalence ratios of 0.5, 0.67, and 1.0 with boosted (1.5 barA) conditions. The measured LTHR decreased with increasing ethanol content for all conditions tested; iso-octane–ethanol blends with above 20% ethanol content (by volume) showed minimal LTHR under engine conditions. These net effects resulted from the combination of thermal effects (charge cooling) and chemical effects (reactivity changes at low temperatures). The effect of temperature, pressure, fuel composition, and equivalence ratio on ignition delay times calculated from chemical kinetic modeling are presented alongside pressure–temperature trajectories of the in-cylinder gases to explain the trends. The underlying cause of the trends is explained by using a sensitivity analysis to determine the contribution of each reaction within the chemical kinetic mechanism to first-stage ignition, revealing the effect of introducing ethanol on the OH radical pool and resulting LTHR intensity.</div>

Direct numerical simulation of ammonia/n-heptane dual-fuel combustion under high pressure conditions

Ammonia (NH3) combustion has received increasing attention for its carbon-free nature. However, the utilization of ammonia in engines is constrained by its relatively low reactivity. To overcome this shortcoming, ammonia blending with high-reactivity fuels has been used. In the present work, direct numerical simulation (DNS) of ammonia/n-heptane dual-fuel combustion was performed to understand the effects of ammonia addition on the combustion behavior of n-heptane sprays. Three DNS cases with NH3 energy ratios of 0%, 20%, and 40% were considered. Two-stage combustion was observed in all cases. It was found that both the first-stage and second-stage ignition delay times increase with increasing NH3 energy ratio, and the peak of mean heat release rate (HRR) reduces with increasing NH3 energy ratio. For low-temperature combustion, ignition kernels were observed in the case with pure n-heptane while ignition occurs almost volumetrically in the cases with NH3 addition. The reaction zone of the cases with NH3 addition is thicker compared to that of the case with pure n-heptane. The reactions of NH3 involve radicals, such as OH, which affects the low-temperature combustion of n-heptane. In the stage of high-temperature combustion, individual ignition kernels with significant heat release rate were observed in the cases with NH3 addition, where the reaction zones are thin. In contrast, for the case with pure n-heptane, large heat release rate occurs simultaneously over the entire domain with thick reaction zones. Ignition occurs in regions characterized by low scalar dissipation rate for both low-temperature and high-temperature combustion in all cases. It was found that, though the contributions of NH3-related reactions to the total HRR are noticeable, the contributions of important elementary reactions to the total HRR in pure n-heptane combustion still prevail in the cases with NH3 addition, which indicates that n-heptane chemistry dominates that of NH3 in this work.

Mechanism development for larger alkanes by auto-generation and rate rule optimization: A case study of the pentane isomers

The core chemistry and thermodynamic data of large alkanes in NUIGMech1.3 were recently updated. In the present work, the set of rate rules for large alkanes is optimized against experimental data to improve the predictive capability of the mechanism. As an initial step in developing a consistent set of rate rules for any larger alkane, we optimized the mechanism of the pentane isomers, whose mechanisms were generated based on 185 rate rules in 24 reaction classes using the MAMOX++ code. The final combined mechanism contains 1427 species and 6676 reactions. For the efficient optimization of such a large mechanism, the Optima++ code was extended to rate rules and was linked with the Zero-RK simulation code. As reference data, first-stage and total ignition delay times measured in shock tubes and rapid compression machines, and species concentrations measured in jet-stirred reactors were collected in wide ranges of conditions. The prior uncertainties of the Arrhenius equations of the 185 rate rules were determined based on a review of alkane fuel rate constant studies. The PCA-SUE method was used for the selection of the influential rate rules. This method identified 94 important rate rules, whose Arrhenius parameters were subsequently optimized within their prior uncertainty ranges using Optima++ against a representative subset of the data collection with a moderate computational effort. The optimization notably improved the accuracy of the mechanism, which now performs significantly better than the Bugler et al. mechanism (PROCI, 2017). The present study demonstrates the effectiveness of the proposed methodology, thereby paving the way to the optimization of a complete set of rate rules that can be used for the generation of a reliable combustion mechanism for any larger alkane, and with some extensions even for unsaturated fuels or oxygenated fuels such as biodiesels.

Effects of unburnt reaction progress on stretch flame dynamics under elevated temperatures

Flames in practical combustors and engines are inevitably subject to the effects of upstream chemical reaction progress and stretch, induced by elevated thermodynamic conditions and flow non-uniformities, respectively. Recent shock tube experiments and simulation studies on flame propagation have shown that flame speed under engine-relevant conditions can be enhanced with non-negligible upstream chemical reaction progress, especially when low-temperature heat release is involved in the unburnt mixture. On the other hand, depending on the mixture equivalence ratio and diluents, nonequidiffusion (including the non-unity Lewis number effect and the preferential diffusion effect) can couple with the flame stretch to fundamentally affect the flame propagation, which can manifest as either facilitation or suppression. Depending on the transport property of a reacting mixture, there hence can be either inhibition or promotion from unburnt reaction progress and stretch effects on flame propagation. In the current work, through one-dimensional numerical simulations of transient planar and spherical flames of n-heptane/air under elevated thermodynamic conditions, the combined effects of upstream chemical reaction progress and stretch on flame propagation are investigated. Results show that for both lean and rich n-heptane/air mixtures, flame speed can be substantially promoted with reaction progress, while the rich mixtures can exhibit opposite stretch dependence after first-stage ignition. Different definitions of effective Lewis number are adopted to explain the change in Markstein length for spherical flame in lean and rich mixtures reformed by the low temperature chemistry. This work fills an important gap in laminar premixed flame research relevant to practical combustion systems and can provide useful insight into local turbulent flame behaviors and phi-sensitivity in engine combustion.

Effect of first-stage ignition on the onset of knocking in SI engines

Knocking limits the performance and efficiency of highly compressed, downsized spark ignition combustion engines. In this paper, the effect of the first-stage ignition on knock is analyzed using a quasi-dimensional engine model. The standard coherent flame model is used for turbulent combustion, and five available reduced chemical kinetics mechanisms are applied to thermal ignition of the end-gas. Measurements in a modified CFR engine operating with two PRF mixtures are used to test the predictive ability of the models and to identify the conditions that lead to knocking. Although the chemical kinetics models used predict similar autoignition delay curves, they do not result in the same knock predictions. The results show that the chemistry must correctly capture the NTC range to predict the observed onset of knocking. In addition, the chemistry models are used to determine the first- and second-stage ignition delay times and their dependence on engine speed and compression ratio. Results show that the autoignition stages occur at relatively fixed temperatures, independent of engine speed and compression ratio. A sensitivity analysis shows that only one set of reactions in the low-temperature chemistry contributes to first-stage ignition, which in turn determines the onset of knocking. This suggests that fuel additives that inhibit or delay the inflection in the NTC region from low- to high-temperature could prevent knock in SI engines.

Chemical insights into the two-stage ignition behavior of NH3/H2 mixtures in an RCM

This study investigated the two-stage ignition behavior of NH3/H2 mixtures using a rapid compression machine (RCM). The ignition delay times and first-stage ignition delay times of NH3/H2 blends were measured at temperatures ranging from 881 to 1127 K, pressures of 15 and 25 bar, and equivalence ratios (ϕ) of 1.0 and 1.5. Experimental results showed that H2 had a significant promotional effect on the ignition of NH3, and two-stage ignition behavior was observed in some test mixtures. Time-resolved species concentrations were recorded using the gas chromatography (GC) method during the single- and two-stage ignition process. Species evolution suggested that the consumption of NH3 and H2 separated during the two-stage ignition process. The oxidation of H2 primarily occurred at the first-stage ignition point, while NH3 oxidation occurred at the total ignition point. A kinetic model was developed to predict the two-stage ignition behavior of NH3/H2 and the species profiles. Model analysis showed that H2 played a key role in initiating the oxidation process and contributed to the early heat release. When H2 content was high (e.g. 50%), its oxidation led to the simultaneous total oxidation of NH3, resulting in single-stage ignition characteristics. However, when a small amount of H2 was present (e.g. 10%), its oxidation only partially consumed NH3, leading to the two-stage ignition behavior. Sensitivity analyses indicated that the co-oxidation of fuels during the first-stage ignition was primarily dominated by H2 oxidation chemistry, while NH3 oxidation became dominant during the following total ignition stage as H2 was completely consumed. Additional model simulations revealed that the ignition behavior of NH3/H2 mixtures is strongly influenced by temperature and pressure. A distinct threshold for temperature and pressure was identified, demarcating the transition between single-stage and two-stage ignition phenomena. Moreover, the fraction of H2 in the mixture had a significant impact on the ignition behavior, with higher fractions leading to increased intensity of the first-stage ignition and closer proximity to the total ignition point. However, when the H2 fraction exceeds a certain threshold, two-stage ignition behavior disappears.

High-Resolution Two-Dimensional Numerical Simulation of n-Heptane/Air Autoignition Under the Influence of Temperature Fluctuation

ABSTRACT High-resolution two-dimensional numerical simulations are performed to investigate n-heptane/air autoignition process under different initial temperatures with varied levels of temperature fluctuations. It is found that temperature fluctuation has an important impact on the autoignition process, and its effects depend on the initial mean temperature. When is located at the upper turning point of the negative temperature coefficient (NTC) regime, as the temperature fluctuation increases, the autoignition mode evolves from a single-stage ignition mode controlled by high-temperature chemistry (HTC) to a mode characterized by the coexisting of multi-stage and single-stage ignition. Both the first-stage heat release and ignition delay increase, but the overall heat release and the total ignition delay decrease. In the presence of a large temperature fluctuation, the low-temperature chemistry (LTC), and HTC reactions, single-stage and two-stage ignitions could coexist in the domain. Chemical explosive mode and sensitivity analyses are performed to identify the reaction mode and the key element reactions in the mutli-stage autoignition process. The temperature fluctuation has a significant promotion effect on the overall ignition except when is located at the lower turning point of the NTC regime. Overall, the NTC phenomenon is weakened under the influence of temperature fluctuation, approaching to zero temperature coefficient (ZTC) phenomenon.

Large eddy simulation of a premixed dual-fuel combustion: Effects of inhomogeneity level on auto-ignition of micro-pilot fuel

In a premixed dual-fuel (DF) methane-diesel engine, the ignition of the lean premixed methane/air mixture starts with the assistance of a pilot diesel injection. Auto-ignition of pilot fuel is important as it triggers the subsequent combustion processes. A delay in the auto-ignition process may lead to misfiring, incomplete combustion, and thus higher greenhouse emissions due to methane slip. Hence, a better understanding of the auto-ignition process for the pilot fuel can help to improve the overall engine performance, combustion efficiency, and to lower exhaust emission levels. In the present study, large eddy simulation (LES) is used to investigate the auto-ignition process of micro-pilot diesel in premixed DF combustion in a constant volume combustion chamber (CVCC). The entire DF combustion processes including methane gas injection, methane/air mixing, pilot diesel injection, and ignition are simulated. The numerical model is validated against experimental data. The present numerical model is able to capture the ignition delay time (IDT) within a maximum relative difference of 7% to the measurements. A higher relative difference of 38% is obtained when methane gas injection and mixing are omitted in the simulation and the methane/air is assumed homogeneous. This demonstrates the importance of inhomogeneity pockets. To study the effects of temperature and methane inhomogeneities separately, different idealized inhomogeneities in temperature and methane distribution are considered inside the CVCC. The inhomogeneity in the temperature is observed to have a more profound influence on the IDT than the methane inhomogeneity. The inhomogeneity pockets of temperature advance the first-stage ignition and, subsequently, the second-stage ignition. A sensitivity analysis on the effect of inhomogeneity wavelength reveals that the larger wavelengths enhance the combustion due to the presence of pilot diesel jets in the desirable regions for a longer time duration.

On tert-butyl hydroperoxide in ignition intensification of n-heptane under engine-relevant condition

The higher reactivity of hydrocarbon fuels is necessary to match supersonic airflow in scramjet engines and micro-scale effect in micro-internal combustion engines. Tert-butyl hydroperoxide (TBHP) was selected in this study as an additive reactively enhancer to explore its capacity to intensify the auto-ignition of a typical hydrocarbon (n-heptane). Ignition delay times (IDTs) of n-heptane doped with TBHP were thus measured at engine-relevant conditions covering pressure of 20 atm, temperatures of 650 – 1250 K, and equivalence ratios of 0.5 and 1.0. A nonlinear promotion effect of TBHP on the auto-ignition of n-heptane was observed. Detonation caused by TBHP in region 2 has an impact on actual reflected shock temperature and pressure at high beginning temperatures (> 1000 K). With the addition of TBHP, the negative temperature coefficient (NTC) behavior of first-stage ignition eventually vanishes accompanied with a weakened main NTC of IDT. Analyses of exothermic reactions, reaction pathways, and species evolution indicated that the quick production of ȮH and ĊH3 radicals from TBHP enhances the reactivity of n-heptane and affects the main exothermic reaction in the early period. This study reveals the potential of TBHP to be used in the ignition regulation of the combustion of hydrocarbon fuels and provides a methodological approach for the efficient utilization of hydrocarbon fuel in scramjet engines and micro internal combustion engines.

Low-temperature oxidation of n-octane and n-decane in shock tubes: Differences in time histories of key intermediates

The impact of chain length on the time histories of key intermediate species that form upon first-stage ignition was studied experimentally in a shock tube using multi-color laser absorption diagnostics. CO, CH2O, OH, and composite time histories of heavier carbonyls were measured simultaneously during the low temperature oxidation of n-octane & n-decane in the temperature range of 720–860 K, and pressure range of 4.1–5.8 atm. The test mixtures were 0.64% & 0.6% fuel in oxygen, respectively. A two-color diagnostic was also developed and used in tandem with other diagnostics to quantify the evolution of temperature during oxidation. To our knowledge, this work reports the first measurement of CH2O, OH, and temperature during oxidation of C8 or heavier hydrocarbon fuels at low temperatures. The measured time histories were also compared against the predictions of two recent detailed kinetic models. Significant differences in the measured and predicted first-stage ignition delay times, absolute concentration of species post-ignition and the maximum rate of formation of species were observed. Our measurements will serve as targets for further refinement of these detailed kinetic models and in the development of rate rules for similar classes of fuels. The measurements also revealed significant differences in the relative reactivity of n-octane and n-decane. Taken together with previous speciation studies conducted in n-heptane, we observe a strong correlation between the species time histories, reactivity, and chain length of straight chain alkanes. This observation lends further support to the selection of CO and CH2O as target species for development of Low-Temperature Hybrid Chemistry (HyChem) models for real transportation and aviation fuels in future.

Considerations for the temperature stratification in a pre-burn constant-volume combustion chamber

In recent years, the Engine Combustion Network (ECN) has developed as a worldwide reference for understanding and describing engine combustion processes, successfully bringing together experimental and numerical efforts. Since experiments and numerical simulations both target the same boundary conditions, an accurate characterization of the stratified environment that is inevitably present in experimental facilities is required. The difference between the core-, and pressure-derived bulk-temperature of pre-burn combustion vessels has been addressed in various previous publications. Additionally, thermocouple measurements have provided initial data on the boundary layer close to the injector nozzle, showing a transition to reduced ambient temperatures. The conditions at the start of fuel injection influence physicochemical properties of a fuel spray, including near nozzle mixing, heat release computations, and combustion parameters. To address the temperature stratification in more detail, thermocouple measurements at larger distances from the spray axis have been conducted. Both the temperature field prior to the pre-combustion event that preconditions the high-temperature, high-pressure ambient, as well as the stratification at the moment of fuel injection were studied. To reveal the cold boundary layer near the injector with a better spatial resolution, Rayleigh scattering experiments and thermocouple measurements at various distances close to the nozzle have been carried out. The impact of the boundary layers and temperature stratification are illustrated and quantified using numerical simulations at Spray A conditions. Next to a reference simulation with a uniform temperature field, six different stratified temperature distributions have been generated. These distributions were based on the mean experimental temperature superimposed by a randomized variance, again derived from the experiments. The results showed that an asymmetric flame structure arises in the computed results when the temperature stratification input is used. In these predictions, first-stage ignition is advanced by 24μs, while second-stage ignition is delayed by 11μs. At the same time a lift-off length difference between the top and the bottom of up to 1.1 mm is observed. Furthermore, the lift-off length is less stable over time. Given the shown dependency, the temperature data is made available along with the vessel geometry data as a recommended basis for future numerical simulations.

An experimental and modeling study on auto-ignition of ammonia in an RCM with N2O and H2 addition

Deep insights into the combustion kinetics of ammonia (NH3) can facilitate its application as a promising carbon-free fuel. Due to the low reactivity of NH3, experimental data of NH3 combustion can only be obtained within a limited range. In this work, nitrous oxide (N2O) and hydrogen (H2) were used as additives to investigate NH3 auto-ignition in a rapid compression machine (RCM). Ignition delay times for NH3, NH3/N2O blends, and NH3/H2 blends were measured at 30 bar, temperatures from 950 to 1437 K. The addition of N2O and H2 ranged from 0 to 50% and 0 to 25% of NH3 mole fraction, respectively. Time-resolved species profiles were recorded during the auto-ignition process using a fast sampling system combined with a gas chromatograph (GC). An NH3 combustion model was developed, in which the rate constants of key reactions were constrained by current experimental data. The addition of N2O affected the ignition of NH3 primarily through the decomposition of N2O (N2O (+M) = N2 + O (+M), R1) and direct reaction between N2O and NH2 (N2H2 + NO = NH2 + N2O, R2). The rate constant of R2 was constrained effectively by experimental data of NH3/N2O mixtures. Two-stage ignition behaviors were observed for NH3/H2 mixtures, and the corresponding first-stage ignition delay times were reported for the first time. Experimental species profiles suggested the first-stage ignition resulted from the consumption of H2. The oxidation of H2 provided extra HO2 radicals, which promoted the production of OH radicals and initiated first-stage ignition. Reactions between HO2 radicals and NH3/NH2 dominated the first-ignition delay times of NH3/H2 mixtures. Moreover, the first-stage ignition led to the fast production of NO2, which acted as a key intermediate and affected the following total ignition. Consequently, the reaction NH2 + NO2 = H2NO + NO (R3) was constrained by total ignition delay times.

The ignition characteristics of dual-fuel spray at different ambient methane concentrations under engine-like conditions

In the present study, dual-fuel ignition process of pilot diesel spray injection into a natural gas-air mixture under engine-like conditions is numerically investigated via large-eddy simulation (LES). Methane and n-dodecane are considered as surrogates for natural gas and diesel fuels, respectively. This study aims to provide understandings of kinetic interactions for dual-fuel spray during the ignition process. The effects of ambient methane concentration on the ignition delay times (IDTs), product distributions, and heat release rates (HRRs) during dual-fuel ignition process are emphasized. The results show that methane prolongs the IDT of n-dodecane, especially for the first-stage IDT. Kinetic analyses reveal that the competition for OH between the dehydrogenation reactions of methane (CH4 + OH <=> CH3 + H2O) and n-dodecane (RH + OH <=> R + H2O) results in the increasing first-stage IDT of pilot spray. Moreover, it is found that the retarding effect positively correlates with the methane concentration. For product distributions, it is observed that the transition of CH2O from fuel-lean to fuel-rich regions is inhibited in the first-stage ignition. In the second-stage ignition, the concentration of CO is decreased as the methane concentration increases and the area with high CO concentration is gradually shifted to the downstream of the spray. In addition, HRRs of low-temperature exothermic reactions appear at the radial periphery as well as the head of the spray front; HRRs of high-temperature exothermic reactions are gradually shifted from the head to the center of the spray with the increase of ambient methane concentration. The HRRs of low- and high-temperature reactions are decreased with increasing ambient methane concentration, and the major reactions for heat release at various ambient methane concentrations are different.

Facilitated ignition and detonation initiation of hydrogen-oxygen mixtures by ozone doping

This work computationally studies homogeneous and inhomogeneous ozone addition in hydrogen-oxygen ignition and transition to detonation, with emphasis on the ozone-induced two-stage behaviors in the chemical kinetics and wave dynamics. In 0D homogenous ignitions, ozone addition induces an additional first-stage reaction and substantially reduces the ignition delay time (IDT). At intermediate initial temperatures, e.g. T0 = 600 K, a critical value, αcr, is identified to delineate different regimes for ozone fraction in the total oxidizer, α. Specifically, the first-stage ignition occurs several orders earlier than IDT for α<αcr, while the IDTs for two stages are comparable for α>αcr. Mechanistically, low-to-intermediate temperature HO2 chemistry introduces a quasi-steady intermediate state in the former regime, whereas ozone reactions directly accelerate high-temperature H2O2 reactions in the latter one. Furthermore, according to the Zel'dovich mechanism, ozone concentration non-uniformity can establish reactivity gradient and hence further initiate double spontaneous waves with multiple dynamics. Coherent coupling between the second-stage heat release and pressure (shock) wave can further evolve into detonation, whereas such synchronized enhancement is not observed in the first-stage ignition due to its large excitation time. Corresponding to the multi-timescales in 0D ignition, when the initial α interval are respectively below, across and above αcr, the low- and high- temperature reaction fronts always separate, first separate then merge, and always merge, respectively. It is also noted that at higher initial temperatures αcr decreases, and that the first-stage reaction becomes less concentrated. Consequently, detonation can be initiated with smaller amounts of ozone and is minimally influenced by the first-stage reaction.

Mars Ascent Trajectory Optimization Based on Convex Optimization

In this paper, a Mars ascent trajectory optimization algorithm based on convex optimization is designed for the Mars ascent trajectory optimization problem. The main accomplishment of this paper is the trajectory optimization of the second-stage ignition based on the convex optimization. For the first-stage ignition, open-loop guidance is used. In this stage, the ascender flies using the maximum thrust to track the attitude profile, thus completing the ascent flight control. In the presence of errors in the first-stage ignition, the experiment generates multiple sets of first-stage shutdown point data and completes simulation verification experiments based on them. The simulation verifies that the second-stage ignition trajectory optimization based on convex optimization algorithm can achieve high accuracy into orbit despite the large starting point deviation.

Investigating the kinetic effect of prenol on iso-octane auto-ignition by means of an experimental and modeling study

• An experimental study of iso -octane/prenol blends is performed inside a RCM. • Validated kinetic models of the oxidation of the neat fuels are merged. • IDTs reflect the strong inhibiting behavior of prenol on low-temperature reactivity. • This effect is suggested to originate from combined gas-phase and catalytic effects. A detailed experimental and kinetic modeling study was dedicated to understand the reported octane hyperboosting effect of prenol, by means of the measurement of the ignition delay times of its blends with iso -octane, and measurement of the mole fraction profiles of the fuels and intermediates inside the ULille rapid compression machine. These results show that prenol addition leads to a reduction of the first-stage ignition phenomena and negative temperature coefficient behavior, which is only qualitatively captured by the model and is consistent with knock resistance improvement. It is suggested that this behavior is caused by two different factors. The first originates from gas-phase reactivity of prenol, and spans from the formation of unreactive unsaturated species through resonance-stabilized radicals, thereby constituting a competitive pathway for the radical pool generated by iso -octane. The second is of catalytic nature and cannot be captured by means of gas-phase kinetic modeling, but could also play an important role in the behavior of prenol in internal combustion engines.

Experimental investigation of low-temperature autoignition in turbulent premixed swirling flames

The role of low-temperature chemistry/ignition (LTC/LTI) in aero engine-like combustors that feature confined bluff-body and swirling flows inside has been studied in the present work. A series of CH4/DME/air mixtures were experimentally investigated at ϕ=0.65 with the volume fraction of DME in the fuel blend (αDME) ranging from 0% to 100% to take LTI into account. To this end, OH-PLIF and CH2O-PLIF measurements combined with PIV and thermocouple methods are used to capture high-temperature and low-temperature flames (HTFs and LTFs) and the flow field and temperature in the outer recirculation zone (ORZ). It is found that adding DME into the lean CH4/air mixture has the potential to introduce LTF in the ORZ under certain conditions. There are three flame regimes in the bluff-body swirl burner in terms of DME enrichment (αDME): (1) When αDME<50%, neither of LTF and HTF exist in the ORZ, and only a V-shape HTF is observed between the inner shear layer (ISL) and inner recirculation zone (IRZ), denoted as regime I. (2) When 50%≤αDME≤70%, a stable LTF in the ORZ can co-exist with the above V-shape HTF, denoted as regime II. (3) When αDME>70%, besides the V-shape HTF, the LTF in the ORZ and a new HTF front between the ORZ and outer shear layer (OSL) can exist intermittently, i.e., the LTF first occurs in the ORZ and then transitions to the new HTF between the ORZ and OSL, denoted as regime III. Furthermore, using an ignition Damköhler number (Daig), defined as the ratio of maximum fluid residence time and evaluated shortest first-stage or second-stage ignition delay time in the ORZ, is able to reasonably classify the flame regimes. These results open up the possibility of employing LTI to extend the stability margin of lean swirling flames.

Unraveling the role of EGR olefins at advanced combustion conditions in the presence of nitric oxide: Ethylene, propene and isobutene

The role of EGR (exhaust gas recirculation) olefinic constituents at advanced combustion conditions in the presence of nitric oxide is unraveled in this study through experimental and modeling efforts using a twin-piston rapid compression machine operating at a stochiometric fuel loading with 20% EGR by mass, pressures of 20 and 40 bar, and temperatures from 680 to 950 K. Five different levels of olefin addition, focusing on ethylene, propene and isobutene, with a fixed amount of NO at 70 ppm are doped into test mixtures of PACE-20, a multi-component gasoline surrogate, where olefin addition effects are characterized through changes in ignition times and heat release rates. Experiments indicate that all three EGR olefins inhibit autoignition reactivity and low-temperature heat release at Tc < 850 K, with isobutene exhibiting the greatest impact, while at Tc > 850 K, low ethylene and propene additions promote reactivity. A recently updated chemical kinetic model, with detailed gasoline/NOx interacting and olefin/NOx interacting chemistry incorporated, is adopted to simulate the experiments. Simulation results are somewhat inconsistent with the experiments, where the model captures the inhibiting effects of all olefins on first-stage ignition reactivity, while consistently predicting a promoting effect on main ignition reactivity. Sensitivity and rate of production analyses reveal that adding olefins greatly alters the role of the consuming pathways for the olefins and their primary derivatives at the initial stage of the oxidation process, particularly with the presence of NO, where the olefins and their derivatives interact with both NOx species such as NO2 and other species such as OH and HO2. The olefin/NOx interactions are particularly pronounced with propene and isobutene addition, where these lead to increased ignition reactivity by facilitating NO production and are mostly responsible for the disagreement between the model and experiments. Further investigations of olefin interacting chemistry, particularly those with NOx species, are needed for chemistry models to accurately predict the complicated effects of EGR.