Study Of Autoignition Research Articles

Effect of the wood species on the fire behavior in vertical orientation

The objective of this study was to investigate the parameters controlling auto-ignition, degradation and auto-extinction of wood. For this purpose an extensive set of experiments was conducted varying extrinsic parameters such as external heat-flux but also the type of wood. Varying the wood species allowed to explore the role of thermal properties and wood composition. Seven wood samples were tested, some light and some heavy, both hardwood and softwood. The experimental setup was based on a double-cone calorimeter, which allowed to accurately change the imposed heat flux at a predefined moment. More than 600 tests were carried out in a vertical orientation, allowing a statistical analysis. For each test, mass loss, surface temperature and in-depth temperature of the samples were measured using a precision scale, an infrared camera and thin wire thermocouples embedded using a special machining, respectively. The auto-ignition study showed that the time to auto-ignition increases linearly with density. Despite a wide range of these times to ignition, the surface temperatures at ignition were in the same order of magnitude for all species considered: between 450 and 700 °C for auto-ignition before 2 min and between 700 and 800 °C for auto-ignition after 2 min. The onset of char oxidation was observed at low heat fluxes. It occurs at different times depending on the wood species, but at similar surface temperatures, between 380 and 400 °C. The sliding double heating cone made it possible to identify the criteria for auto-extinction: the heat flux for auto-extinction can vary from 40 to 55 kW.m−2 depending on the wood species, and a linear correlation was found between the mass loss rate at extinction and the initial density of each sample studied. The study highlights the dominant role of density for auto-ignition and auto-extinction.

Spray autoignition study of bio-oxygenated additives blended with aviation kerosene under engine-like cold-start conditions

Abstract Bio-oxygenated fuels are expected to be used as a clean alternative energy source to improve the ignition behavior and pollutant emissions of RP-3 kerosene in compression ignition engines. In this study, the spray autoignition of PR-3 blended with different types of oxygenated fuels (including n-pentanol (PeOH), methyl propionate (MP), methyl ethyl ketone (MEK), 1,2-dimethoxyethane (1,2-DME), and 2-Ethylhexyl nitrate (EHN)) was measured using a constant-volume combustion chamber. Experiments were performed on three sets of blended fuels with different oxygen contents (2.5 wt%, 5 wt%, 10 wt%) in the temperature range of 723∼863 K and at ambient pressures of 2.2 and 4 MPa. A kinetic analysis utilized a merged RP-3 low-temperature kinetic model containing various oxygenated components. The ignition delay of blended fuels increases with the addition of PeOH, MP, and MEK, particularly PeOH, which demands more energy absorption at low temperatures due to its higher specific heat and latent heat of vaporization. As ambient pressure increased, the ignition delay period shortened for all blended fuels; however, PeOH and MEK demonstrated more significant low-temperature suppression. The combined kinetic model can reasonably predict the trend of the effect of oxygenated additives. 1,2-DME showed significantly more low-temperature reactivity versus PeOH, MP, and MEK. The variability is because the products of secondary O2 addition and isomerization of 1,2-DME undergo low-temperature branched chain reactions, but other oxygenated fuels produce more inert components.

An experimental and kinetic modeling study of auto-ignition and flame propagation of ethyl lactate/air mixtures, a potential octane booster

Spark ignition engines are one of the main technologies in the transport sector. The improvement and optimization of the fuels used to empower these engines are of vital importance, both for economic and environmental reasons. In particular, one of the main issues of spark ignition engines is the knock phenomenon; new formulations of fuels are being studied in order to overcome this problem. In this study, a possible innovative anti-knock, octane booster additive is considered: ethyl lactate. This molecule is almost unknown in combustion literature, as it has been used only as green solvent and food additive. The first experimental results under combustion conditions are presented, together with a kinetic mechanism. Two set-ups have been employed: a rapid compression machine, to measure ignition delay times, and an innovative spherical bomb, OPTIPRIME, to obtain laminar flame speeds. The results are encouraging for the expected application and the mechanism shows good performance. Ignition delay times at all conditions are well predicted by the mechanism and, when compared to ethanol, they are longer, implying a greater anti-knock capability. A rate of production analysis has been performed, where the unimolecular reaction leading to ethylene and lactic acid has been proved to be quite important at high temperatures and lean conditions. For laminar flame speeds, the agreement between model and experiments is good, with some discrepancies at lean conditions and high pressures. Compared to ethanol, at rich and stoichiometric conditions ethyl lactate flame speeds are slightly slower except at lean conditions, indicating that under some conditions this molecule could provide better performances than ethanol as an octane booster additive.

Simultaneous side-wall-schlieren and -emission imaging of autoignition phenomena in conventional and constrained-reaction-volume shock-tube experiments

Simultaneous schlieren and emission imaging through the side wall of a round shock tube is reported for application to experimental autoignition studies. A round, optically accessible shock tube featuring windows designed as aberration-corrected cylindrical lenses provides a large side-wall field of view compatible with both emission and schlieren imaging. Autoignition experiments are reported for non-dilute propane–oxygen–argon mixtures (ϕ=1 C3H8 in 21% O2, 79% Ar) at temperatures (T5) spanning the range 1,203–1,438 K and pressures near 1 atm. Experiments are performed using both conventional and constrained-reaction-volume (CRV) filling, allowing for the most detailed comparison to date of ignition dynamics between these two types of experiments. When T5 exceeds 1,290 K, strong ignition is observed to initiate in the immediate proximity of the end wall regardless of the filling strategy. In lower-T5 experiments, mild ignition is observed to initiate spontaneously at remote locations, which often exceeding the imaging FOV in conventional experiments but are constrained within about 5 cm of the end wall in CRV experiments. Measured ignition delay times (IDTs) are compared to predictions from three kinetic mechanisms. IDTs simulated using the San Diego and USC Mech II mechanisms are of the same magnitude as the measurements but exhibit a lower activation energy; the propane submechanism from NUIG 1.1, meanwhile, systematically over predicts observed IDTs by roughly a factor of two. Comparisons are also made between the experimental measurements and predictions from a recently introduced 1-D axial-temperature-gradient model describing remote ignition in shock tubes. Considering the effect of a supposed dT5/dz=150 K/m gradient, the 1-D model functionally predicts the remote ignition observed in T5< 1,290 K experiments and suggests the use of CRV filling may significantly reduce the error introduced into IDT measurements by remote ignition at low-T5 conditions.

A Rapid Compression Machine Study of 2-Phenylethanol Autoignition at Low-To-Intermediate Temperatures

To meet the increasing anti-knocking quality demand of boosted spark-ignition engines, fuel additives are considered an effective approach to tailor fuel properties for satisfying the performance requirements. Thus, screening/developing bio-derived fuel additives that are best-suited for advanced spark-ignition engines has become a significant task. 2-Phenylethanol (2-PE) is an attractive candidate that features high research octane number, high octane sensitivity, low vapor pressure, and high energy density. Recognizing that the low temperature autoignition chemistry of 2-PE is not well understood and the need for fundamental experimental data at engine-relevant conditions, rapid compression machine (RCM) experiments are therefore conducted herein to measure ignition delay times (IDTs) of 2-PE in air over a wide range of conditions to fill this fundamental void. These newly acquired IDT data at low-to-intermediated temperatures, equivalence ratios of 0.35–1.5, and compressed pressures of 10–40 bar are then used to validate the 2-PE model developed by Shankar et al. (2017). It is found that this literature model greatly overpredicts the current RCM data. The comparison of experimental and simulated results also provides insights into 2-PE autoignition behaviors at varying conditions. Further chemical kinetic analyses demonstrate that the absence of the O2-addition pathway of β-R. radical in the 2-PE model of Shankar et al. (2017) could account for the model discrepancies observed at low-to-intermediated temperatures.

Analysis of Auto-Ignition Chemistry in Aeroderivative Premixers at Engine Conditions

Abstract The minimization of auto-ignition risk is critical to the design of premixers of high power aeroderivative gas turbines as an increased use of highly reactive future fuels (for example, hydrogen or higher hydrocarbons) is anticipated. Safety factors based on ignition delays of homogeneous mixtures are generally used to guide the choice of a residence time for a given premixer. However, auto-ignition chemistry under aeroderivative conditions is fast (0.5–2 ms) and can be initiated within typical premixer residence times. The analysis of what takes place in this short period necessarily involves the study of low-temperature auto-ignition precursor chemistry, but precursors can change with fuel and local reactivity. Chemical explosive modes (CEMs) are a natural alternative to study this as they can provide a measure for auto-ignition risk by considering the whole thermochemical state in the framework of an eigenvalue problem. When transport effects are included by coupling the evolution of the chemical explosive modes to turbulence, it is possible to obtain a measure of spatial auto-ignition risk where both chemical (e.g., ignition delay) and aerodynamic (e.g., local residence time) influences are unified. In this article, we describe a method that couples large eddy simulation (LES) to newly developed, reduced auto-ignition chemical kinetics to study auto-ignition precursors in an example premixer representative of real life geometric complexity. A blend of pure methane and di-methyl ether (DME), a common fuel used for experimental auto-ignition studies, was transported using the reduced mechanism (38 species/238 reactions) under engine conditions at increasing levels of DME concentrations until exothermic auto-ignition kernels were formed. The resolution of species profiles was ensured by using a thickened flame model where dynamic thickening was carried out with a flame sensor modified to work with multistage heat release. This paper is outlined as follows: First, a reduced mechanism is constructed and validated for modeling methane as well as DME auto-ignition. Second, sensitivity analysis is used to show the need for chemical explosive modes. Third, the thickened flame model modifications are described and then applied to an example premixer at 25 bar/890 K preheat. The chemical explosive mode analysis closely follows the large thermochemical changes in the premixer as a function of DME concentrations and identifies where the premixer is sensitive and flame anchoring is likely to occur.

Development of a fast-virtual CFR engine model and its use on autoignition studies

Homogenous charge compression ignition engines have been studied as an alternative to the conventional diesel combustion to attain high efficiency with ultra-low NOx and soot emissions for a wide variety of fuels. However, its usage in real applications has been restricted due to the difficulties regarding combustion control and operating range extension. The modification of the fuel characteristics may be a pathway to solve the previous hurdles. Therefore, this research presents a relevant methodology to assess the fuel response to HCCI boundary conditions based on 0-D and 1-D modelling for detailed chemistry solution and state conditions definition, respectively. The results suggest that the methodology can predict the early stages of the fuel oxidation with good accuracy. For the objective of predicting the start of combustion, the best results are obtained using tabulated chemistry when investigating fuels that have pre reactions and a low temperature heat release. As the oxidation process progresses, the deviation of the pressure-temperature trajectory from non-reactive to reactive conditions after the low temperature heat release decreases the predictive capability to some extent. Nonetheless, the methodology outcomes are still valid as a qualitative metric for reactivity determination as well as the intermediate and high temperature ignition delay.

Theoretical and Experimental Study of 3-Pentanol Autoignition: Ab Initio Calculation, Shock Tube Experiments, and Kinetic Modeling.

3-Pentanol is a potential alternative fuel or a green fuel additive for modern engines. The H-abstraction reactions from 3-pentanol by H, CH3, HO2, and OH radicals are significant in the 3-pentanol oxidation process. However, corresponding rate constants are forced to rely on either analogy from sec-butanol or estimation from alkanes due to a lack of direct experimental and theoretical study. In this work, stationary points on the potential energy surfaces (PESs) were calculated with the high-level DLPNO-CCSD(T)/CBS(T-Q)//M06-2X/cc-pVTZ method, which is further used to benchmark against the CBS-QB3 method. Then, the high-pressure limit rate constants for target reactions, over a broad range of temperature (400-2000 K), were calculated with the phase-space theory and conventional transition state theory. A comparison was made between the calculated rate constants and the values available in Carbonnier et al. [ Proc. Combust. Inst. 2019, 37(1), 477-484]. The rate constants for the above H-abstraction reactions in the Carbonnier model were updated with the calculated results, followed by a modification based on the computed results of 3-pentanol + HO2 to obtain the revised model. Validation against the shock tube (ST) and the jet-stirred reactor (JSR) measurements from the literature proved the revised model an optimal one. Furthermore, using an ST, ignition delay times (IDTs) for the 3-pentanol/air mixtures were measured spanning a temperature range of 920-1450 K, pressures of 6, 10, and 20 bar, and equivalence ratios of 0.5, 1.0, and 1.5. Generally, IDTs decrease with increasing temperature and reflected shock pressure. Improved predictions to present experimental data were obtained by using the revised model as compared with the Carbonnier model. Finally, sensitivity analysis was conducted using the revised model to gain an in-depth comprehension of the 3-pentanol autoignition.

Heat transfer augmentation by recombination reactions in turbulent reacting boundary layers at elevated pressures

A study of a reacting boundary layer flow with heat transfer at conditions typical for configurations at elevated pressures has been performed using a set of direct numerical simulations. Effects of wall temperatures are investigated, representative for cooled walls of gas turbines and sub-scale rocket engines operating with hydrocarbon as fuels. The results show that exothermic chemical reactions induced by the low-enthalpy in the boundary layer take place predominantly in the logarthimic sub-layer. The majority of the heat release is attributed to the exothermic recombination of OH and CO to produce CO2 and H2O. The recombination reactions result in an increase of the wall heat loads by up to 20% compared to the inert flow. The gas composition experiences strong deviations from the chemical equilibrium conditions. In fact, a quenching of the major species is observed within the viscous sub-layer and the transition region. Analysis of chemical time-scales shows that the location of quenched composition coincides with the region where the Damköhler number decreases below unity. Within the viscous sub-layer, a secondary reaction zone is detected, involving the production of formyl and formaldehyde radicals that provide an additional source of energy release. The analysis of the reaction paths showed that reactions with zero activation energy are responsible for this change in gas composition, which also account for the initial branching of hydrocarbon fuels decomposition according to previous auto-ignition studies. The effect of the secondary recombination reactions is more prominent for the lower wall temperature case. Finally, the role of turbulent fluctuations on the species net chemical production rates is evaluated, showing a strong correlation between species and temperature fluctuations. This leads to a pronounced deviation of the mean reaction rates ω˙(Yk,T)¯ from the reaction rates obtained under the assumption of laminar finite rate.

Autoignition study of iso-cetane/tetralin blends at low temperature

iso-Cetane and tetralin are the two fuel components commonly considered in literature diesel surrogates, and are hydrocarbon classes representative of iso-alkanes and naphthoaromatics, respectively. Since both surrogate components are involved in the key reactions controlling the autoignition reactivity of diesel surrogates, autoignition studies of iso-cetane, tetralin, and their blending behavior at low temperatures can be helpful in developing/refining chemical kinetic models of diesel surrogates, in order to better predict diesel ignition response under low temperature combustion conditions. In this investigation, autoignition experiments of iso-cetane, tetralin, and their binary mixtures at varying blending ratios have been conducted in a rapid compression machine (RCM) in the temperature/pressure range of 630–930 K and 10–20 bar. For the neat fuel components, the newly-acquired RCM data have been compared with the literature experimental results at some overlapping conditions. In addition, a chemical kinetic model describing the oxidation of iso-cetane and tetralin compiled/updated from the diesel surrogate model developed by Lawrence Livermore National Laboratory is used to simulate the experimental datasets. It is shown that this model exhibits good agreement with the experimental results of tetralin and also predicts the total ignition delay times of iso-cetane at lower temperatures reasonably well. However, the current model fails to fully capture the increase of total ignition delay time with increasing amount of tetralin in the binary blends observed in the present RCM experiments. Therefore, chemical kinetic analyses of the current model are conducted to help identify possible reasons leading to the discrepancies between simulations and experiments. Furthermore, the chemical kinetic interactions between iso-cetane and tetralin are explored by including additional cross-reactions among iso-cetane, tetralin, and their fuel fragments in the current model to assess their effects on model predictions. Based on the chemical kinetic analysis results, future direction for model refinements is discussed.

On Traveling Front of Ignition in Co-Flow Laminar Reactive Jets

In this paper we derive a simple, still experimentally feasible, model for propagation of traveling fronts of ignition which were recently observed in experimental studies of autoignition of co-flow reactive jets at high pressure. We obtain a closed form analytical solution for this model and present a detailed analysis of this solution. We also present a qualitative comparison of the model predictions with available experimental data.

An experimental and kinetic modeling study of gas-phase autoignition of n-Pentadecane at low-to-high temperatures and elevated pressures

In this study, low-to-high temperature oxidation of n-pentadecane (nC15H32) is investigated in a rapid compression machine and a heated shock tube. The experiments are conducted at equivalence ratios of Φ = 0.5–2.0 with air as an oxidizer for the fuel-lean condition while diluted fuel mixtures for other equivalence ratios. A temperature range of 650–1450 K is investigated at a pressure of 6 and 10 bar in a rapid compression machine and a heated shock tube while a temperature range of 900–1350 K is investigated at a pressure of 15 bar in a heated shock tube. The fuel showed a two-stage ignition behavior which is a typical characteristic of large hydrocarbons and a negative temperature coefficient regime. The dependence of ignition delay times on pressure, equivalence ratio, and oxygen mole fraction are separately presented and discussed. The experimental results revealed the sensitivity of ignition delay times to all these factors. The available experimental data of C10-C16 n-alkanes are compared which showed a small effect of chain length on the oxidation of alkanes. An updated reaction mechanism is presented after revising the rate constants of various reaction classes. The kinetic modeling predictions are compared and noticeable improvements are observed in the performance of the updated mechanism especially at low temperatures. The flux analysis is carried out to observe the fuel consumption along major low-temperature pathways. The sensitivity analysis is then performed to analyze the performance of the updated mechanism. To our knowledge, this is the first study on the autoignition of n-pentadecane.

Measuring the Adiabatic Ignition Delay of n-Pentane Mixture using Rapid Compression Machine

Recent combustion research has focused on low temperature combustion to meet engine emission regulations and to advance the development of low temperature homogenous compression ignition engines. Autoignition studies in this temperature regime are primarily performed by Rapid Compression Machines (RCMs) which are sensitive to the heat transfer characteristics of the experimental device. RCMs are widely used to measure autoignition data such as ignition delay and species concentration. Measured ignition delays from RCMs are typically reported at an adiabatic condition; however, this assumption may produce a systematic error in ignition delay measurement as heat transfer is observed to reduce the pressure and temperature during the autoignition process, e.g., a longer ignition delay has a greater pressure and temperature drops. RCMs are custom built and have unique design characteristics that affect the heat transfer during the autoignition process. In addition, depending on the diluent composition (e.g., helium versus nitrogen or argon), different heat transfer characteristics are expected. As a result, autoignition results at similar conditions may vary from facility to facility or depending on the used diluent. The dependency of the measured data on the used facility or diluent may produce uncertainty in the data which impact the development of high-fidelity combustion mechanisms. In this work, a new method is developed and utilized to eliminate heat transfer from the ignition delay data. To evaluate the new method, the autoignition of n-pentane mixtures in the low temperature regime were investigated using an RCM. To vary the heat transfer, the compression ratio of the RCM was changed and the ignition delays were measured at similar pressure and temperature conditions. The tests were performed at an equivalence ratio of approximately one and nitrogen and argon as diluents. By applying the new method, the effect of heat transfer on the ignition delays were eliminated successfully and ignition delays at adiabatic condition were determined. A detailed kinetic model of n-pentane was used to simulate the measured adiabatic ignition delay, which agreed well with the experimental data.

Autoignition studies of NH3/CH4 mixtures at high pressure

Autoignition delay times of NH3/CH4 mixtures with CH4 fractions of 0%, 5%, 10% and 50% were measured in a rapid compression machine at equivalence ratio φ = 0.5, pressures from 20 to 70 bar and temperatures from 930 to 1140 K. In addition, measurements were performed for NH3 mixtures with 10% CH4 at φ = 1.0 and 2.0. Methane shows a strong ignition-enhancing effect on NH3, which levels off at higher CH4 fractions, as the ignition delay time approaches that of pure methane. Autoignition delay times at 10% CH4 at φ = 0.5 and 1.0 are indistinguishable, while an increase of ignition delay times by factor of 1.5 was observed upon increasing φ to 2.0. The experimental data were used to evaluate six NH3 oxidation mechanisms capable of simulating NH3/CH4 mixtures. The mechanism previously used by the authors shows the best performance: generally, it predicts the measured ignition delay times to better than 30% for all conditions, except for 50% CH4 addition for which the differences increase up to 50% at the highest temperature. Sensitivity analysis based on the mechanism used indicates that under lean conditions the reaction CH4 + NH2 = CH3 + NH3 significantly promotes ignition for modest CH4 addition (5% and 10%), but becomes modestly ignition-inhibiting at 50% CH4. Sensitivity and rate-of-production analyses indicate that the ignition-enhancing effect of 50% CH4 addition is closely related to the formation and decomposition of H2O2. Flux analysis for NH3/CH4 mixtures indicates that CH4 + NH2 = CH3 + NH3 contributes substantially to the decomposition of methane early in the oxidation process, while CH3 + NO2 (+M) = CH3NO2 (+M) is a significant reservoir of NO2 at low temperature. Additionally, an anomalous pre-ignition pressure rise phenomenon, which is not reproduced by the simulations, was observed with high reproducibility for the NH3 mixture with 50% CH4 addition.

Autoignition study of LPG blends with diesel and HVO in a constant-volume combustion chamber

There is an increasing interest in reducing CO2 as far as it is the main responsible of greenhouse gas (GHG) emissions. Since the contribution of heavy-duty (HD) vehicles to GHG emissions of the road transport sector reaches 24% in the European Union, Liquefied Petroleum Gas (LPG) is suggested as a promising alternative fuel to be introduced in HD vehicles. The main drawback when using LPG in diesel engines is its poor autoignition behavior. To overcome this difficulty is possible to mix LPG with Hydrotreated Vegetable Oil (HVO) or diesel fuel, which acts as an ignition improver. This work has focused on LPG-HVO blends up to 75 %m/m LPG content, with lower LPG content admixtures included. Another measure to promote autoignition is using advanced injection strategies, particularly at low thermal conditions in the combustion chamber. This study concluded that 25LPG-75HVO blend (without any injection strategy modification) and 50LPG-50HVO blend (with advanced injection strategy) show better autoignition behavior than diesel fuel at low thermal conditions. At usual combustion conditions, LPG-HVO blends up to 75 %m/m LPG content can be supplied using an advanced injection strategy, and achieving similar autoignition behavior than diesel. LPG and HVO contents in the blend can be balanced depending on the thermal conditions. Therefore, a novel injection system with an in-line mixing valve capable of managing the instantaneous LPG/HVO ratio for compression ignition engines, is suggested. Testing these blends with advanced injection strategies using the in-line mixing system in a real HD engine would confirm these results.

Autoignition study of methyl decanoate using a rapid compression machine

Methyl decanoate (MD), a widely used surrogate of biodiesels, was investigated for its autoignition characteristics using a heated rapid compression machine (RCM). In this study, the ignition delay times (IDTs) of MD were measured at the compressed pressures of 5–20 bar, equivalence ratios varying from 0.53 to 1.60 and compressed temperatures of 633–855 K. An obvious two-stage ignition behavior was observed at low temperatures and typical negative temperature coefficient (NTC) phenomenon of total IDT was experimentally captured. The influences of compressed pressure, fuel and oxygen content, and nitrogen concentration on ignition delay times were systematically studied. The simulation results of Herbinet’s mechanism and Grana’s mechanism under variable volume simulation were compared with the experimental data. It is found that the two mechanisms qualitatively predict the autoignition features of MD but still need further optimization. In addition, reaction pathway analysis and sensitivity analysis were conducted to offer further insight into the low-to-intermediate temperature autoignition chemistry of MD.

The experimental study of autoignition of tetralin at intermediate-to-high temperatures

Tetralin or 1,2,3,4-tetrahydronaphthalene (C10H12) is a naphthenic-aromatic hydrocarbon. It is also adopted as a component for the formulation of transportation fuel surrogates. In this study, the ignition delays of tetralin/air mixtures are measured in a heated shock tube at moderate to high temperatures ranging from 830 to 1380 K, at pressures of 6, 10, 20 bar and under fuel-lean, stoichiometric and fuel-rich conditions. The dependence of ignition delays on temperature, pressure and equivalence ratio is analyzed and a four-parameter correlation is developed. The mechanism in current form showed less reactivity as compared to the experimental data. It is, therefore, modified by adjusting the rate constants of most sensitive reactions at both high and low temperatures. The mechanism showed noticeable improvements at low temperatures and high pressures. The sensitivity analyses are conducted to identify the reactions governing the fuel ignition under different pressures and equivalence ratios. The important reaction pathways are identified at both low and high temperatures to understand the underlying chemical kinetics.

Auto Ignition Study of Methane and Bio Alcohol Fuel Blends

Abstract Methane (CH4) and bio alcohols have different ignition properties. These have been extensively studied and the resulting experimental data have been used to validate chemical kinetic models. Methane is the main component of natural gas, which is of interest because of its relative availability and lower emissions compared to other hydrocarbon fuels. Given growing interest in fuel-flexible systems, there can be situations in which the combustion properties of natural gas need to be modified by adding biofuels such as bio alcohols. This can occur in dual-fuel internal combustion engines or gas turbines with dual-fuel capabilities. The combustion behavior of such blends can be understood by studying the auto ignition properties in fundamental combustion experiments. Studies of the ignition of such blends are very limited in the literature. In this work, the auto ignition of methane and bio alcohol fuel blends is investigated using a shock tube facility. The chosen bio alcohols are ethanol (C2H5OH) and n-propanol (NC3H7OH). Experiments are carried out at 3 atm and 10 atm for stoichiometric and lean mixtures of fuel, oxygen, and argon. The ignition delay times of the pure fuels are first established at conditions of constant oxygen concentration and comparable pressures. The ignition delay times of blends with 50% methane are then measured. The pyrolysis kinetics of the blends is further explored by measuring CO formation during pyrolysis of the alcohol and methane–alcohol blends. The resulting experimental data are compared with the predictions of selected chemical kinetic models to establish the ability of these models to predict the disproportionate enhancement of methane ignition by the added alcohol.

A six-compound, high performance gasoline surrogate for internal combustion engines: Experimental and numerical study of autoignition using high-pressure shock tubes

This paper presents experimental and modeling data for the autoignition of a novel, six-component, high performance gasoline surrogate fuel comprising ethanol, n-heptane, i-octane, 1-hexene, methylcyclohexane, and toluene (AL-P-I-O-N-A). Experimental tests are conducted in two high-pressure shock tubes to determine the ignition delay time as a function of pressure, temperature and equivalence ratio. Ignition delay times were measured at 10 and 30 bar in the temperature range from 749 to 1204 K and equivalence ratios ranging from 0.35 to 1.30. A modified Arrhenius equation is defined to mathematically describe the ignition delay time of the proposed surrogate. For experimental data with temperatures higher than 900 K, a multiple linear regression identified the pressure dependence exponent of 0.72 and stoichiometry dependence exponent of 0.62, as well as a global activation energy of ≈109 kJ/mol. A simplistic approach to mechanism reduction based on the elimination of reactions with no relevant rate of progress was used in order to reduce an extensive detailed kinetics model (hierarchically constructed with more than 17800 reactions). The reduced detailed kinetics model with 4885 elementary reactions among 326 chemical species was used for numerical simulations. Comparisons between the experimental and numerical data are favorable, with the predictions using the reduced kinetics model differing by less than 0.056% when compared to the complete mechanism. It was observed that for low temperatures the proposed reduced kinetics model agrees only qualitatively with the measurements. In order to understand the likely cause of this discrepancy a brute force sensitivity analysis on IDT was performed, elucidating the more influencing reactions on the ignition delay times. The experimental data obtained in this research was compared to available data in the literature in terms of anti-knock index (AKI) and for a scaled pressure of 30 bar (τ30) at a stoichiometric composition. A modified Arrhenius equation was then fitted and an AKI dependence exponent of -1.11 was obtained, inferring that the higher the AKI the higher the IDT, independent of fuel composition at temperatures lower than the NTC region. This trend should be confirmed by further studies.

An experimental and modeling study of n-hexadecane autoignition under low-to-intermediate temperatures

N-hexadecane is a potential candidate of diesel surrogate fuels and is also the largest linear alkane (n-alkanes) with known chemical kinetic models. The objective of this study is to investigate the autoignition characteristics of n-hexadecane in the low-to-intermediate temperature region and to validate the existing kinetic models. In this study, the ignition delay times (IDTs) of n-hexadecane were measured using a heated rapid compression machine (RCM) at two pressures of 7 and 10 bar, and over equivalence ratios ranging from 0.5 to 1.3. Two-stage ignition characteristic and the negative temperature coefficient (NTC) behavior of total ignition delay time were experimentally captured. This study paid special attention to the influence of pressure, equivalence ratio, and oxygen content on the IDTs of n-hexadecane. It is observed that both the total IDTs and the first-stage IDTs decrease with the rise of those parameters. It is worth noting that the first-stage IDT is found to show a greater dependence on temperature but a weaker dependence on other parameters compared to the total IDT. The observed IDT dependence in the low-temperature region (LTR) were quantitatively described by ignition delay time correlations. The newly measured IDTs were then validated against two kinetic models (LLNL and CRECK). Simulation results show that both models underpredict the first-stage IDT but generally capture the temperature dependence. The CRECK model well predicts the total IDTs of n-hexadecane while the LLNL model significantly underpredicts the total IDTs at most investigated conditions. To the best of our knowledge, this study is the first investigation on n-hexadecane autoignition under low-to-intermediate temperatures, which deepens the understanding of large n-alkane oxidation and contributes to the improvement of the existing kinetic models.