Total Ignition Delay Times Research Articles

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.

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.

Experimental study on microwave-induced puffing, micro-explosion, and combustion characteristics of ammonium dinitramide-based liquid propellant droplets

Microwave ignition technology has the advantages of high ignition energy, stable ignition, and spatial multi-point ignition. These advantages make this technology promising for future application in green single-component propellants. In this paper, the ignition characteristics of ammonium dinitramide (ADN)-based liquid propellant droplets under the influence of microwaves at room temperature are investigated using experimental methods. The effects of microwave power on puffing, micro-explosion, and combustion behavior of ADN-based liquid propellant droplets were studied. The droplet and flame diameters were statistically related to time, and the microwave-assisted droplet ignition mechanism was analyzed. A new rectangular waveguide resonant cavity was designed in which the droplet is placed at the maximum electric field strength of the device. The droplet morphology and flame profile inside the resonant cavity were photographed with a high-speed camera. The experimental results showed that the microwave positively influenced the puffing, micro-explosion, and combustion behavior of droplets. When the microwave power was increased from 200 to 280 W, the total droplet evaporation time and ignition delay time were reduced by 56.5% and 35.2%, respectively. The positive effects of microwaves on combustion have been summarized as the thermal effect of microwaves on polar molecules and the promotion of fuel oxidation reactions by microwave-induced plasma. The plasma was found to control the development of the initial flame propagation front and to influence the temperature during the combustion reaction process. In this paper, we propose the mode of droplet combustion under microwave induction as a plasma discharge and several stages of the droplet combustion process. This research provides novel insight into the study of the microwave ignition mechanism of liquid fuels.

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.

An experimental and detailed kinetic modeling study of the auto-ignition of NH3/diesel mixtures: Part 1- NH3 substitution ratio from 20% to 90%

Ammonia (NH3) is considered as a good carbon-free hydrogen-carrier fuel and has gained extensive attention as a promising alternative fuel for internal combustion engine in recent years. To explore the application potential of the NH3 burned with diesel fuel in internal combustion engine, the auto-ignition delay times of NH3/diesel fuel blends were measured in a rapid compression machine at a wide NH3 energy fraction of 20%, 40%, 60%, 70%, 80%, and 90%, temperature range of 675–995 K, pressures of 20–50 bar, and equivalence ratios of 0.5–1.5. According to the variation of ignition delay times with temperature, three different combustion regimes for NH3/diesel mixtures can be determined. It is found that the typical NTC behavior and two-stage ignition process were only observed at the regime where diesel chemistry dominates. Both the first-stage and the total ignition delay times increase with rising NH3 energy fraction, but decrease with the increase of equivalence ratio. Then, an updated NH3/diesel kinetic mechanism was proposed by adding new cross reactions between diesel and NH3. Results show that the current mechanism exhibits an obvious improvement for ignition delay time prediction compared to the original mechanism, though it still fails to reproduce the ignition delay times of NTC range for the mixture containing 20% NH3. Further sensitivity analysis and the OH radical rate of production analysis indicate that the overestimation of the reactivity for the NH3/diesel fuel blends at NTC range is closely related to reaction NO+HO2NO2+OH. The promoting effect of NO species on the conversion of inactive HO2 radical into active OH radical at low temperature greatly accelerates the autoignition. In addition, the current mechanism does not include the cross reaction between the large NTC-related species (QOOH, OOQOOH, OQOOH, ROO) and the N-containing species (NO, NO2, NH2), especially for the NO related reactions, which may have a great impact on the IDT simulation of the NH3/diesel mixtures. Consequently, this work presents new ignition delay time measurements and mechanism optimization for NH3/diesel mixtures. Continuous works should be emphasized on these unclear reactions to further understand the combustion behavior of NH3/diesel fuel blends.

Numerical Investigation of the Ignition Delay Time of Kerosene Premixed Combustion in an SI Engine

SI engines are installed widely in small aircrafts as they have good fuel economy. Currently, these SI engines are fueled with gasoline, although their safety can be improved if kerosene is used. However, the combustion performance of kerosene cannot fulfil the requirements due to the differences in physicochemical properties. This study investigates the ignition delay time of kerosene at a pressure range of 15–35 bar and a temperature range of 600–1000 K. A detailed chemical reaction mechanism is employed for the premixed combustion process. Under the initial conditions of 1000 K and 35 bar, with an equivalence ratio of 1, the total ignition delay time of kerosene is 0.401 ms. The NTC range of kerosene is determined as roughly 750–920 K. Subsequently, the chemical reaction paths with an equivalence ratio of 0.8, 1, and 1.2 and an initial pressure of 15, 20, and 25 bar were analyzed. The rate-determined elementary reactions were obtained based on a sensitivity analysis. The difference between kerosene and gasoline are also compared, and the rate-determining reactions that affect the ignition of kerosene and gasoline are discussed. The results of this study can provide a reference for the combustion performance improvement and knock suppression of SI engines fueled with kerosene.

Experimental and modeling study of the autoignition characteristics of gasoline/hydrogenated catalytic biodiesel blends over low-to-intermediate temperature

Gasoline/Hydrogenated catalytic biodiesel blending fuel is a prospective alternative fuel for gasoline compression ignition (GCI) engine which has the potential to solve the ignition difficulties of GCI especially at low load conditions. However, the autoignition behavior of gasoline/hydrogenated catalytic biodiesel blends is unknown. In this study, the ignition delay times of gasoline/hydrogenated catalytic biodiesel (HCB) blends were measured in a heated rapid compression machine over a wide range of conditions (the temperature range of 655–870 K, pressures of 10 and 15 bar, equivalence ratios of 0.5–1.5). Experimental results show that the measured ignition delay time of the gasoline/HCB blends exhibits negative-temperature coefficient behaviors under the investigated conditions. The total ignition delay times diminish with rising pressure, equivalence ratio and HCB ratio in the blends. The simulation results of ignition delay times for gasoline/HCB blends using a detailed mechanism are compared with experimental data. The detailed model can predict the trends of ignition delay time well during low-to-intermediate temperature region. Finally, kinetic analyses were performed under different compressed pressure, temperature, equivalence ratios and blending ratios. The dominating reactions and decisive factors of ignition are explored to gain a deeper understanding of the ignition characteristic of gasoline/HCB blends. It is found that the autoignition of fuel is completely caused by HCB related reactions at low temperature, which explains why the blending of HCB can expand the working condition downward from the perspective of chemical kinetic.

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.

Oxidation of 2,6-dimethylheptane at low temperature: Kinetic modeling and experimental study

Branched alkanes represent an important class of compounds in conventional fuels and some bio-derived fuels. This study is dedicated to the investigation of the low-temperature oxidation chemistry of 2,6-dimethylheptane using a combination of experimental and computational methods. All the reactants, transition states, and products in the first oxidation stage, which are crucial to the initiation reactions in the low-temperature reaction chain, were optimized through the B3LYP/CBSB7 level of theory and a kinetic mechanism that included the new reaction pathways was assembled. Ignition delay time measurements were carried out in a rapid compression machine and the results were compared with modeling predictions. The kinetic mechanism is able to capture both the first and total ignition delay times with a root-mean-square deviation of 39.6%. In addition, sensitivity analysis is performed to quantify the impact of newly developed chemistry of 2,6-dimethylheptane on ignition delay time. Rate parameters found in this study may be applicable to other branched alkanes with similar molecular structure.

Autoignition characteristics of bio-based fuels, farnesane and TPGME, in comparison with fuels of similar cetane rating

Bio-based alternative fuels have received increasing attention with growing concerns about depletion of fossil reserves and environmental deterioration. The development of new combustion concepts in internal combustion engines requires a better understanding of autoignition characteristics of the bio-based alternative fuels. This study investigates two cases of alternative fuels, namely, a kerosene-type fuel farnesane and an oxygenated fuel, TPGME, and compares those fuels with full-boiling range of fuels with similar cetane number. The homogeneous autoignition and spray ignition characteristics of the selected fuels are studied using a modified CFR octane rating engine and a cetane rating instrument, respectively. When comparing farnesane with a full-boiling range counterpart (HRJ8), their similar cetane ratings result in comparable combustion heat release, but the overall ignition reactivity of farnesane is stronger than HRJ8 during the pre-ignition process. Results from a constant volume spray combustion chamber indicate that the spray process of farnesane and HRJ8 strongly influences the overall ignition delay of each fuel. Despite the similar cetane ratings of TPGME and n-heptane, TPGME shows greater apparent low-temperature oxidation reactivity at low compression ratios in the range from CR 4.0-5.5 than n-heptane. A simplified model focused on the key reaction pathways of low-temperature oxidation of TPGME has been applied to account for the stronger low-temperature reactivity of TPGME, supported by density functional theory (DFT) calculations. Regardless of the similar cetane ratings of the fuels, n-heptane and JP-8/SPK lead to similar total ignition delay times, while TPGME shows the shortest overall ignition delay times in the constant volume combustion chamber.

Autoignition of CRC diesel surrogates at low temperature combustion conditions: Rapid compression machine experiments and modeling

As federal programs require increasingly stringent engine emissions and fuel economy standards, these ambitions can only be met if next-generation combustion technology is developed focusing on high-efficiency and low-emissions engines. Recent research has indicated the need to operate engines at higher compression ratios and with low temperature combustion (LTC) to achieve the needed gains in engine efficiency and reductions in emissions. Because there is a lack of understanding of the chemistry of diesel fuel components and their mixtures at these LTC conditions, this limits the ability to develop predictive chemical kinetic models that can be used to optimize engine combustion. The current study aims to fill in gaps in fundamental combustion data on surrogate fuel mixtures relevant to diesel fuels. Specifically, four multicomponent diesel surrogates formulated by the Coordinating Research Council (CRC) to emulate an ultra-low-sulfur research-grade #2 certification diesel fuel (CFA), namely V0a (4 components), V0b (5 components), V1 (8 components), and V2 (9 components), have been investigated in a rapid compression machine (RCM) through determination of total and first-stage ignition delay times. Autoignition characteristics of lean to rich fuel/O2/N2 mixtures, for the four CRC surrogates and CFA, have been measured using an RCM at LTC relevant pressures and temperatures, in the ranges of 10–20 bar and 650–1000 K, respectively. The equivalence ratios have been varied by independently changing the oxygen mole fraction and the fuel mole fraction in the test mixtures, thereby illustrating the individual effects of oxygen concentration and fuel loading on diesel autoignition. Autoignition results of these four CRC surrogates are compared among them and with those of CFA. Some degree of agreement in autoignition response between each CRC surrogate and CFA is observed, while discrepancies are also identified and discussed. In addition, a detailed chemical kinetic model for diesel surrogates has been developed and validated against these newly-acquired RCM data. This model shows reasonable agreement with the overall ignition delay time results of the current RCM experiments. Chemical kinetic analyses of the developed model were further conducted to help identify the reactions controlling the autoignition processes and the consumption of fuel components in CRC surrogates.

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.

Effects of hydrogen peroxide addition on two-stage ignition characteristics of n-heptane/air mixtures

Rising fuel cost and environmental concerns of greenhouse emissions have driven the development of advanced engine technology with optimal fuel strategy that can simultaneously yield high thermal efficiency and low emissions. Due to its strong reactivity and extra oxygen atom serving as an oxidizer, hydrogen peroxide (H2O2) has been used along with other hydrocarbons to promote overall combustion process. To explore the potential benefits of H2O2 in clean combustion technology, a numerical study with detailed chemistry is conducted to investigate the effects of H2O2 addition on the two-stage ignition characteristics of n-heptane/air mixtures at low-to-intermediate temperatures (below 1000 K), with due emphasis on how the negative temperature coefficient (NTC) behavior is affected. The results show that H2O2 addition shortens both the first-stage and total ignition delay times of n-heptane/air mixtures and suppresses the NTC behavior by reducing the upper turnover temperature. With increasing H2O2 addition, the lower turnover temperature, corresponding to the first-stage ignition delay minimum, is found to increase first and then decrease. Chemical kinetic analyses show that the addition of H2O2 promotes both first- and second-stage ignition reactivity by enhancing OH production through H2O2 decomposition. Furthermore, low-temperature chemistry controls the first-stage ignition, while H2O2 chemistry dominates the second-stage ignition.

Numerical simulation of the effects of evaporation on the n-heptane/air auto-ignition process under different initial air temperatures

Numerical simulations are carried out to study the effects of fuel evaporation and mixing on autoignition process of n-heptane/air with complex chemistry. By assuming that n-heptane spray evaporation and mixing are infinitely fast under initial air temperature Tair of 740–1200 K and pressure of 1–24 atm, homogeneous gaseous mixture of n-heptane/air with different equivalence ratio can be obtained. Three types of the most reactive mixture fraction ηMR are identified in mixture fraction space: ηMR,LTand ηMR,ig are associated with the shortest first-stage ignition by low temperature chemistry (LTC) and the total two-stage ignition delay time respectively, while ηMR,HT corresponds to the shortest single-stage ignition delay time by the high temperature chemistry (HTC). With the increasing initial air temperature Tair, ηMR,ig and ηMR,LT will increase, but the corresponding ignition delay times decrease. When Tair is high enough to make the temperature of gaseous mixture higher than the upper turnover temperature of NTC region in very lean mixture, ηMR,HT will appear with shortest ignition delay time. Moreover, the ignition delay time at ηMR,HT could be less than that at ηMR,LT in the spray autoignition process under high initial air temperature Tair=1200K and low initial pressure, 1 atm and 6 atm. This finding suggests that in spray autoignition process, due to the cooling effect of evaporation, LTC reaction and HTC reaction may coexist if the interval between the ignition delay times of ηMR,HT and ηMR,LT is quite small, and also that the high temperature ignition may precede the low temperature ignition. In addition, sensitivity and reaction pathway analysis are performed to disclose the controlling chemical kinetics at the three types of the most reactive mixture fraction ηMR of the case with high initial air temperature 1200 K and pressure 6 atm.

Experimental and modeling validation of a large diesel surrogate: Autoignition in heated rapid compression machine and oxidation in flow reactor

Autoignition and oxidation characteristics of a three-component diesel surrogate fuel were studied in a heated rapid compression machine (RCM) and a flow reactor (FR) over a wide range of conditions. The surrogate components, n-cetane, 2,2,4,4,6,8,8-heptamethylnonane (HMN), and 1-methylnaphthalene (1-MN), all fall within the C10–C20 range of real diesel fuel. The RCM-measured ignition delay times (IDTs) and FR oxidation data of the surrogate fuel were then compared with those of the target diesel. It is found that the diesel surrogate fuel is in good agreement with the target diesel in total ignition delay time and FR speciation data over the whole temperature range, reflecting the success of the surrogate fuel construction. A semi-detailed kinetic model was used to predict the autoignition and oxidation behavior of the surrogate fuel in RCM and FR experiments. Results show that the kinetic model captures well the measured total IDTs and the mole fractions of O2, H2, CO2, and small hydrocarbons under all investigated conditions. To deepen the understanding of low-temperature reaction scheme, rate of production (ROP) analysis of OH and HO2 radicals was carried out during the first-stage ignition to explore the causes of low-temperature and negative temperature coefficient (NTC) reactivity. A brute force sensitivity analysis of first-stage ignition delay time was also conducted to further identify the controlling reactions in the first-stage ignition of the surrogate fuel.

Impact of exhaust gas recirculation on ignition delay times of gasoline fuel: An experimental and modeling study

The ignition delay times of a research grade gasoline, RON95E10, are measured in a rapid compression machine and in a shock tube. The experiments are carried out for fuel/O2/Ar/N2 mixtures with two equivalence ratios of 0.77 and 1.18 at pressures between 20 and 40 bar over the temperature range of 700–1250 K. In particular, data are reported for two exhaust gas recirculation (EGR) loadings of 0 and 25% to demonstrate the impact of EGR on gasoline ignition delay times. The presented data are, to the authors’ knowledge, the first set of ignition delay times of real gasoline with EGR and at engine relevant high pressures covering the entire intermediate temperature range. A published gasoline mechanism (Cai and Pitsch, 2015) is modified as part of this study following the latest kinetic knowledge and is further used to compute the ignition delay times at the conditions experimentally investigated. The simulations employ a four-component surrogate consisting of n-heptane, iso-octane, toluene, and ethanol. It is shown that the proposed model predicts the total ignition delay times of gasoline accurately under various conditions and reflects correctly the influences of pressure, EGR, and equivalence ratio on ignition delay times, while it fails to predict the first stage ignition delay times with very high accuracy due to the neglect of the olefin and naphthene content in the surrogate formulation. Furthermore, the influence of exhaust gas addition on ignition delay times is analyzed and discussed in more detail using results from numerical simulations. The chemical impact of EGR is minor but not negligible and varies at different conditions depending on the EGR composition.

The impact of physicochemical property interactions of iso-octane/ethanol blends on ignition timescales

This work presents new measurements of liquid fuel ignition delay times of iso-octane and ethanol fuel blends obtained from an ignition quality tester at the National Renewable Energy Laboratory (NREL IQT), which are compared to previous ignition delay data from the University of Michigan rapid compression facility (UM RCF), at the same experimental conditions. Pressure-time histories were used to determine liquid fuel ignition delays at global stoichiometric non-premixed conditions for iso-octane, ethanol and iso-octane/ethanol blends of 25, 50, 75% by volume in mixtures of 10% oxygen diluted in nitrogen. Temperatures ranging from 880 to 970 K were studied at a pressure of 10 atm. By comparing total ignition delay times from the NREL IQT with chemical ignition delay times from the UM RCF, the contributions of physical phenomena were quantified as representative time scales for spray injection, breakup and evaporation processes, and for gas-phase turbulent mixing. Regression analyses were developed for ignition time scales as function of blend level and charge temperature. Non-dimensional analyses were also carried out to determine the relative effects of physical time scales with respect to chemical ignition delay times.

A chemical kinetic mechanism for the low- and intermediate-temperature combustion of Polyoxymethylene Dimethyl Ether 3 (PODE3)

Polyoxymethylene Dimethyl Ether (PODEn, n = 3–5) is a potential alternative fuel or a green fuel additive for diesel engines, owing to the unique C–O alternating chain structure (CH3O[CH2O]nCH3) and high cetane number. In this paper, the rate constants for five reaction classes about the model molecule PODE1, including hydrogen abstraction, beta-scission of corresponding R radicals, isomerization of RO2 radicals, and decomposition of QOOH radicals, were calculated with canonical transition state theory (CTST) at the CCSD(T)-F12a/aug-cc-pVTZ//M06-2X/def2-TZVPP or CBS-QB3//B3LYP/CBSB7 level of theory. Based on the quantum chemistry and chemical kinetic study of the model molecule PODE1, the first detailed reaction mechanism (225 species, 1082 reactions) which can describe the ignition characteristics of PODE3 at low and intermediate temperature was developed. To validate this mechanism, rapid compression machine (RCM) was used to conduct the quasi-homogeneous experiments to measure the total ignition delay time at various effective temperatures (640–865 K) for three different PODE3/O2/N2 mixtures (ϕ = 0.5, O2:N2 = 1:8; ϕ = 1.0, O2:N2 = 1:15; ϕ = 1.5, O2:N2 = 1:20) and two different effective pressures (10 bar, 15 bar). Homogeneous Charge Compression Ignition (HCCI) experiments fueled with PODEn (n = 1–4) mixture, in which the mass fraction of PODE3 is 88.9% were also conducted in a naturally aspirated single-cylinder HCCI research engine at 1600 r/min, two exhaust gas recirculation (EGR) rates (0%, 42%), and three different charge-mass equivalence ratios for each EGR rate to take the real engine working condition into consideration. Good agreement was achieved in the comparison of the experimental data and the simulation results utilizing our newly developed mechanism for PODE3. This surrogate model will contribute to the design of fuels blended with PODEn, and to the prediction of the combustion and emission characteristics of engines using PODEn.

A comparison of sensitivity metrics for two-stage ignition behavior in rapid compression machines

A rapid compression machine (RCM) multi-zone model is used to simulate the ignition of primary reference fuel (PRF) mixtures that exhibit two-stage ignition behavior. Sensitivity coefficients for each reaction in the PRF mechanism are calculated from four different metrics: (1) first-stage energy release, (2) first-stage pressure rise, (3) first-stage ignition delay time, and (4) total ignition delay time. The sensitivity coefficients are used to provide four unique rankings, and the rankings are compared using Spearman’s rank correlation coefficient. Special emphasis is given to comparing the rankings based on first-stage energy release and total ignition delay time. The level of agreement between these two rankings is shown to depend on the reaction conditions. Simulation cases with high peak heat release rates during the first stage of ignition tend to exhibit disagreement in the rankings, indicating that new kinetic information can be obtained by studying first stage energy release in addition to total ignition delay time. Simulations show that the high peak heat release rates are associated with energy release across a broad range of temperatures (range can be in excess of 100K even for lean conditions). This distribution leads to a discrepancy between sensitivity coefficients calculated for the total ignition delay time and the first-stage energy release. Sensitivity coefficients for the total ignition delay time are characterized by reactivity at the highest temperatures in the RCM, while sensitivity coefficients for the first-stage energy release are characterized by reactivity across the full range of temperatures in the RCM.

Transient analysis on ignition process of catalytic hybrid rocket motor

This paper is aimed to analyze the transient process and ignition delay time of catalytic hybrid rocket motor through two-dimensional axisymmetric transient simulations and lab-scale firing tests. The overall catalytic ignition process is divided into five stages by analyzing typical firing traces. The total ignition delay time includes valve response time, pipeline filling time, catalytic bed startup time, monopropellant pressure buildup time and fuel grain heating time. The numerical simulations mainly focus on the fuel grain heating stage of catalytic hybrid rocket motor. The ignition transient flow fields of the motor with 90% hydrogen peroxide (90HP) and polyethylene (PE) propellants are obtained. Simulation results also reveal that the fuel grain heating time exponentially decreases with the increase of the oxidizer mass flow flux. Several firing tests of 90HP-PE lab-scale catalytic hybrid rocket motor are conducted to analyze the delay time of different ignition stages. The fuel grain heating time trend summarized by the experimental data accords well with that by the simulation results. Besides, the experimental data also show that the pipeline filling time, the catalytic bed startup time, the monopropellant pressure buildup time and the fuel grain heating time all decrease when the oxidizer mass flow rate increases. Therefore, the total ignition delay time almost linearly decreases with the increase of the oxidizer mass flow rate.