First-stage Ignition Research Articles

Effects of oxygen concentrations on the ignition and quasi-steady processes of n-heptane spray flames using large eddy simulation

Exhaust Gas Recirculation (EGR) is a frequently used technique to reduce the production of NOx. The effect of EGR on the early flame evolution, two-stage ignition process and spray flame structures for n-heptane spray flames are investigated using large eddy simulation. The two-stage ignition process is identified based on the formation of key species and early heat release process. Results demonstrate that a longer ignition delay (ID) and flame lift-off length (LOL) under lower oxygen concentration conditions could increase the mixing time for fuel and air. However, the first-stage ignition still initiates in fuel-richer regions for the cases with higher EGR rates due to the lack of oxygen. In contrast, compared to the case with the same initial oxygen content but at a higher gas temperature of 1000 K, the first-stage ignition moves to stoichiometric mixture fraction regions at 900 K. The combustion mode analysis based on hydroxyl and formaldehyde is conducted to distinguish between the low- and high-temperature combustion regions. Most importantly, to study the stabilization mechanism, the chemical explosive mode analysis (CEMA) is conducted based on analysis on the local flow time scale and the chemical time scale. During the early stage of ignition, a balance between reaction and mixing implies that cool flame propagates from the ignition spots through the entire flow field. And during the quasi-steady state, autoignition plays a dominant role.

Cavity-enhanced absorption sensor for carbon monoxide in a rapid compression machine

Abstract A sensor based on cavity-enhanced absorption spectroscopy (CEAS) was implemented for the first time in a rapid compression machine (RCM) for carbon monoxide concentration measurements. The sensor consisted of a pulsed quantum cascade laser (QCL) coupled to a low-finesse cavity in the RCM using an off-axis alignment. The QCL was tuned near 4.89 µm to probe the P(23) ro-vibrational line of CO. The pulsed mode operation resulted in rapid frequency down-chirp (6.52 cm−1/µs) within the pulse as well as a high time resolution (10 µs). The combination of rapid frequency down-chirp and off-axis cavity alignment enabled a near complete suppression of the cavity coupling noise. A CEAS gain factor of 133 was demonstrated in experiments, resulting in a much lower noise-equivalent detection limit than a single-pass arrangement. The sensor thus presents many opportunities for measuring CO formation at low temperatures and for studying kinetics using dilute reactive environments; one such application is demonstrated in this work using dilute n-heptane/air mixtures in the RCM. The formation of CO during first-stage ignition of n-heptane was measured over 802–899 K at a nominal pressure of 10 bar. These conditions correspond to the NTC region of n-heptane and such results provide useful metrics to test and compare the predictions of low-temperature heat release by different kinetic models.

Shock tube study of normal heptane first-stage ignition near 3.5 atm

Shock tube ignition delay times and species time history measurements for Primary Reference Fuels (PRFs) such as normal heptane provide targets for the validation of combustion models, which in turn are used to develop more fuel-efficient engines that have smaller environmental footprints. However, a review of the literature has revealed that most of the shock tube ignition delay time and species measurement data for normal heptane have been obtained at elevated pressures, rather than at relatively low pressures where many other important experimental techniques such as jet-stirred reactors and flow reactors can provide corroborating results. One central problem preventing previous shock tube studies from examining first-stage ignition at these lower pressures was that ignition times were too long under these conditions to be measured within the available shock tube test times. To address this issue, recent advances in shock tube techniques for achieving long uniform test times have been applied in order to measure low-pressure first-stage ignition times of normal heptane together with normal heptane fuel time-history records at times up to about 30 ms. These measurements were performed in the Negative Temperature Coefficient (NTC) region (T=664−792 K) in lean mixtures (21%O2/Ar, equivalence ratio ϕ=0.5) at pressures of roughly P=3.5 atm using both the conventional and Constrained Reaction Volume (CRV) shock tube filling strategies. The data have been used to evaluate the performance of several combustion models, have been compared with other higher-pressure shock tube first-stage ignition times in n-heptane/20–21%O2 mixtures found in the literature, and have been fit using a two-zone Arrhenius model for first-stage ignition delay times. The results showed that some combustion models yield ignition delay time predictions that differ from the experimental results by as much as an order of magnitude, that under these conditions n-heptane first-stage ignition times scale by roughly P−0.71 but are insensitive to ϕ, and that the fraction of fuel remaining after first-stage ignition increases with increasing initial experimental temperature.

Auto-ignition study of FACE gasoline and its surrogates at advanced IC engine conditions

Robust surrogate formulation for gasoline fuels is challenging, especially in mimicking auto-ignition behavior observed under advanced combustion strategies including boosted spark-ignition and advanced compression ignition. This work experimentally quantifies the auto-ignition behavior of bi- and multi-component surrogates formulated to represent a mid-octane (Anti-Knock Index 91.5), full boiling-range, research grade gasoline (Fuels for Advanced Combustion Engines, FACE-F). A twin-piston rapid compression machine is used to achieve temperature and pressure conditions representative of in-cylinder engine operation. Changes in low- and intermediate-temperature behavior, including first-stage and main ignition times, are quantified for the surrogates and compared to the gasoline. This study identifies significant discrepancies in the first-stage ignition behavior, the influence of pressure for the bi- to ternary blends, and highlights that better agreement is achieved with multi-component surrogates, particularly at lower temperature regimes. A recently-updated detailed kinetic model for gasoline surrogates is also used to simulate the measurements. Sensitivity analysis is employed to interpret the kinetic pathways responsible for reactivity trends in each gasoline surrogate.

An experimental and modeling study of dimethyl ether/methanol blends autoignition at low temperature

New rapid compression machine (RCM) ignition delay data for dimethyl ether (DME), methanol (MeOH), and their blends are acquired at engine-relevant conditions (T = 600 K–890 K, P = 15 bar and 30 bar, and equivalence ratios of ϕ = 0.5, 1.0, and 2.0 in synthetic dry air). The data are then used to validate a detailed DME/MeOH model in conjunction with literature RCM and shock tube data for DME and MeOH. This detailed DME/MeOH model, constructed by systematically merging literature models for the combustion of the individual fuel constituents, is capable of accurately predicting the experimental ignition delay data at a wide range of temperatures and pressures. The experiments and simulations both show a non-linear promoting effect of DME addition on MeOH autoignition. Additional analyses are performed using the merged DME/MeOH model to gain deeper insight into the binary fuel blend autoignition, especially the promoting effect of DME on MeOH. It is found that the unimolecular decomposition of HO2CH2OCHO plays an essential role in low temperature DME/MeOH blend autoignition. The accumulation of HO2CH2OCHO before the first-stage ignition and later quick consumption not only triggers the first-stage ignition, but also causes the non-linear promoting effect by accumulating to higher levels at higher DME blending ratios. These analyses suggest the rate parameters of HO2CH2OCHO unimolecular decomposition are critical to accurately predict the first-stage and overall ignition delay times as well as the first-stage heat release profile for low temperature DME/MeOH oxidation.

A study on the low-to-intermediate temperature ignition delays of long chain branched paraffin: Iso-cetane

Abstract Iso-cetane (2, 2, 4, 4, 6, 8, 8-heptamethylnonane) is known as a primary reference fuel for cetane number rating and is regarded as an applicable component for surrogate diesel fuel. In this study, ignition delays for iso-cetane homogeneous mixture were measured at equivalence ratios varying from 0.5 to 2.0, compressed pressures of 10, 15 and 20 bar, and compressed temperatures of 620–880 K in a heated rapid compression machine (RCM). Two-stage ignition characteristic of iso-cetane was observed for all mixtures. Negative temperature coefficient (NTC) behavior of iso-cetane ignition delay appears in the temperature range of 670–730 K, which is significantly lower than that of other large hydrocarbons. Influences of compressed temperature, compressed pressure, and mixture composition on iso-cetane ignition delays were also investigated. It is found that the total ignition delays shorten with the increase of compressed pressure, equivalence ratio and oxygen mole fraction. The first-stage ignition delays exhibit Arrhenius-like dependence on compressed temperature and are relatively insensitive to the change of other parameters. In addition, modeling study was conducted using an updated iso-cetane kinetic model developed from a literature iso-cetane mechanism. Simulation results show that the total ignition delays are underestimated while the temperature range of the NTC behavior is overestimated. Rate of production (ROP) analysis prior to the first-stage ignition was also conducted to identify the controlling reactions generating and consuming OH and HO2 radicals in low-temperature (low-T) reaction pathways.

Effect of methane on pilot-fuel auto-ignition in dual-fuel engines

The ignition behavior of n-dodecane micro-pilot spray in a lean-premixed methane/air charge was investigated in an optically accessible Rapid Compression-Expansion Machine at dual-fuel engine-like pressure/temperature conditions. The pilot fuel was admitted using a coaxial single-hole 100 µm injector mounted on the cylinder periphery. Optical diagnostics include combined high-speed CH2O-PLIF (10 kHz) and Schlieren (80 kHz) imaging for detection of the first-stage ignition, and simultaneous high-speed OH* chemiluminescence (40 kHz) imaging for high-temperature ignition. The aim of this study is to enhance the fundamental understanding of the interaction of methane with the auto-ignition process of short pilot-fuel injections. Addition of methane into the air charge considerably prolongs ignition delay of the pilot spray with an increasing effect at lower temperatures and with higher methane/air equivalence ratios. The temporal separation of the first CH2O detection and high-temperature ignition was found almost constant regardless of methane content. This was interpreted as methane mostly deferring the cool-flame reactivity. In order to understand the underlying mechanisms of this interaction, experimental investigations were complemented with 1D-flamelet simulations using detailed chemistry, confirming the chemical influence of methane deferring the reactivity in the pilot-fuel lean mixtures. This shifts the onset of first-stage reactivity towards the fuel-richer conditions. Consequently, the onset of the turbulent cool-flame is delayed, leading to an overall increased high-temperature ignition delay. Overall, the study reveals a complex interplay between entrainment, low T and high T chemistry and micro-mixing for dual-fuel auto-ignition processes for which the governing processes were identified.

Low-temperature chemistry in n-heptane/air premixed turbulent flames

The effects of low-temperature chemistry (LTC) on n-heptane/air premixed turbulent flames in the thin reaction zones regime are investigated using direct numerical simulations (DNS) with reduced multi-step chemistry (129-species, 1234-reaction mechanism reduced from CaltechMech). An initial mixture of n-heptane/air at an equivalence ratio of 0.7, unburnt temperature of 650 K, and atmospheric pressure, which is in the negative temperature coefficient (NTC) region, is considered. The focus is put on three separate aspects: 1) LTC in turbulent hot flames propagating in this unburnt (fresh) mixture, 2) turbulent hot flames (with LTC) propagating in a mixture that has undergone first-stage ignition, and 3) turbulent cool flames. These types of flames can all be encountered in modern gasoline compression ignition and diesel engines for example. For the first aspect, it is found that LTC has negligible effect for the conditions considered. For the second aspect, at constant Karlovitz number, the increase in turbulent flame speed (relative to that of turbulent hot flames propagating in the unburnt mixture) due to partial ignition of the reactants is attributed to the increase in laminar flame speed, as opposed to turbulence–LTC interaction. Furthermore, the reaction zone is affected by turbulence in the same way as hot flames propagating in an unburnt mixture. For the third aspect, the first DNS of turbulent cool premixed n-heptane/air flames are presented. Under the current conditions, the initial laminar cool flames are strongly affected by auto-ignition, which is expected to occur under engine conditions, and has an ignition front structure. As the turbulent flames develop, turbulent diffusion becomes sufficiently large to initiate self-propagation of the cool flames. The flames are observed to propagate upstream steadily until they reach the inlet. The steady-state turbulent flames are found to have a highly distributed reaction zone. Nevertheless, their reaction zone structure is found to approach that of the reference (self-propagating) laminar flame (which is significantly different than that of the initial ignition fronts). In addition, this strong turbulence does not affect the global chemical pathways compared to those in the reference laminar flame. Finally, their normalized turbulent flame speed is comparable to that of hot flames at similar Karlovitz numbers.

Experimental and modeling study of the autoignition characteristics of commercial diesel under engine-relevant conditions

Knowledge of the autoignition characteristics of diesel fuels is of great importance for understanding the combustion performance in engines and developing surrogate fuels. Here ignition delays of China's stage 6 diesel, a commercial fuel, were measured in a heated rapid compression machine (RCM) under engine-relevant conditions. Gas-phase autoignition experiments were carried out at equivalence ratios ranging from 0.37 to 1.0, under compressed pressures of 10, 15, and 20 bar, and within a temperature range of 685–865 K. In all investigated conditions, negative temperature coefficient (NTC) behavior of the total ignition delays is observed. The autoignition of the diesel fuel exhibits pronounced two-stage characteristics with strong low-temperature reactivity. Experimental results indicate that the total ignition delays shorten with increasing compressed pressure, oxygen mole fraction and fuel mole fraction. The first-stage ignition delays are mainly controlled by compressed temperature and also affected by oxygen mole fraction and compressed pressure but show a very weak dependence on fuel mole fraction. Correlations describing the first-stage ignition delay and the total ignition delay were proposed to further clarify the ignition delay dependence on the multiple factors. Additionally, it is found that the newly measured ignition delays well coincide with and complement the diesel ignition data in the literature. A recently developed diesel mechanism was used to simulate the diesel autoignition on the RCM. The simulation results are found to agree well the experimental measurements over the whole temperature ranges. Species concentration analysis and brute force sensitivity analysis were also conducted to identify the crucial species and reactions controlling the autoignition of the diesel fuel.

Role of low-temperature chemistry in detonation of n-heptane/oxygen/diluent mixtures

The ZND structure of mixtures of n-heptane, oxygen and diluent has been investigated using a reduced reaction scheme that includes the low-temperature chemistry (LTC) pathways. It is shown that for high CO2 contents (XCO2 > 0.82) such that the reaction temperature is relatively low, the structure is affected by LTC and exhibits two distinct stages of energy release caused by low- and high-temperature chemistry, respectively. Based on the ZND structures, the dynamic parameters such as cell size, direct initiation energy and critical tube diameter of detonation within the LTC affected regime have been evaluated using various semi-empirical models. Such detonation structures exhibiting two distinct length scales lead to two distinct values for each of these dynamic parameters. For the evolution of the induction length and cell size, although the total length scales do not show negative response with increasing temperature, the length scale of the first-stage ignition demonstrates negative response when the post-shock temperature decreases within the LTC controlled regime. For the evolution of direct initiation energy, the models based on critical curvature and critical decay rate show negative temperature response, but the model based on cell size predicts continuous increase of critical initiation energy with increasing dilution. For the critical tube diameter, the model based on the critical decay rate approach also exhibits the negative temperature response.

Experimental and kinetic study on ignition of DME/n-butane mixtures under high pressures on a rapid compression machine

Dimethyl ether (DME)/n-butane mixtures are ideal fuel for homogeneous charge compression ignition (HCCI) engine. Fundamental ignition delay data under engine-related conditions are essential to better understand HCCI combustion and to validate DME/n-butane models. However, there is a significant lack of ignition delay data of DME/n-butane mixtures under high-pressure and low-temperature conditions. Therefore, ignition delays of DME/n-butane mixtures at equivalence ratio of 0.5 were measured at different DME blending ratios (0, 20, 40, 60, 80 and 100%), pressures (12, 16, 24 and 30 bar), temperatures (622–929 K) and fuel concentrations (1.6 and 2.2%) using a rapid compression machine (RCM). A numerical analysis was performed using the NUI Aramco Mech 2.0, and a good agreement to experimental data was obtained. The results well indicate the two-stage ignition and negative temperature coefficient (NTC) behavior of DME/n-butane mixtures. With DME addition, system reactivity is enhanced and air-fuel mixtures can be ignited at lower temperatures. Meanwhile, DME addition promotes both the first-stage and total ignition, and the total ignition delay as a function of DME blending ratio decreases nonlinearly, especially at low pressures. In addition, the DME/n-butane mixtures ignite faster at higher fuel concentration and pressure conditions, which is more prominent in the NTC region. Kinetic analysis indicates that compared with slower internal H-atom isomerization rate of n-butane, DME has a more active chain branching path, which promotes the low-temperature oxidation of n-butane and overall fuel, thus faster development of total radical pool, increased concentration of radicals and decreased ignition delay. The addition of 20% DME results in a significant increase of free radicals and heat release rate while the DME chemistry becomes the most ignition-sensitive one in DME/n-butane mixtures when the DME blending ratio reaches 60%.

Characteristics of auto-ignition in gasoline–biodiesel blended fuel under engine-like conditions

The purpose of this study is to demonstrate the effects of biodiesel fraction on auto-ignition for gasoline–biodiesel blended fuel, which combines two fuels with widely different auto-ignition characteristics. First, gasoline was blended with biodiesel at 5%, 10%, 15%, and 20% by volume, and then tested in a rapid compression expansion machine at a compression ratio of 11 and a temperature range of 720–850 K to observe the auto-ignition delay phenomenon under engine-like conditions. The experimental conditions are focused on improving the auto-ignition characteristic of gasoline direct-injection compression ignition combustion strategies under low load and cold start. The heat release rate of the blended fuels was calculated from the pressure trace and displacement history of the piston in order to identify first-stage ignition and second-stage (auto-ignition) ignition delay. Second, a gasoline–biodiesel reaction mechanism was developed to predict the chemical ignition delay of the blended fuels. The reaction mechanism with 4285 species and 15,246 reactions was validated and implemented using the CHEMKIN PRO software. Finally, the chemical ignition delay was predicted by the simulation which was further compared to the experimental measured results. These results revealed that a higher biodiesel fraction helps to obtain shorter ignition delay, which reduces the requirement of intake temperature. The blended fuel with 20% biodiesel showed the lowest ambient temperature at the injection timing requirement and was 80 K lower than gasoline. Each blended fuel exhibited two-stage ignitions in the measured temperature range. The combustion duration and pressure peak of every blended fuel were similar to each other after increasing the biodiesel fraction.

Large eddy simulation of the low temperature ignition and combustion processes on spray flame with the linear eddy model

Large eddy simulation coupled with the linear eddy model (LEM) is employed for the simulation of n-heptane spray flames to investigate the low temperature ignition and combustion process in a constant-volume combustion vessel under diesel-engine relevant conditions. Parametric studies are performed to give a comprehensive understanding of the ignition processes. The non-reacting case is firstly carried out to validate the present model by comparing the predicted results with the experimental data from the Engine Combustion Network (ECN). Good agreements are observed in terms of liquid and vapour penetration length, as well as the mixture fraction distributions at different times and different axial locations. For the reacting cases, the flame index was introduced to distinguish between the premixed and non-premixed combustion. A reaction region (RR) parameter is used to investigate the ignition and combustion characteristics, and to distinguish the different combustion stages. Results show that the two-stage combustion process can be identified in spray flames, and different ignition positions in the mixture fraction versus RR space are well described at low and high initial ambient temperatures. At an initial condition of 850 K, the first-stage ignition is initiated at the fuel-lean region, followed by the reactions in fuel-rich regions. Then high-temperature reaction occurs mainly at the places with mixture concentration around stoichiometric mixture fraction. While at an initial temperature of 1000 K, the first-stage ignition occurs at the fuel-rich region first, then it moves towards fuel-richer region. Afterwards, the high-temperature reactions move back to the stoichiometric mixture fraction region. For all of the initial temperatures considered, high-temperature ignition kernels are initiated at the regions richer than stoichiometric mixture fraction. By increasing the initial ambient temperature, the high-temperature ignition kernels move towards richer mixture regions. And after the spray flames gets quasi-steady, most heat is released at the stoichiometric mixture fraction regions. In addition, combustion mode analysis based on key intermediate species illustrates three-mode combustion processes in diesel spray flames.

Theory of first-stage ignition delay in hydrocarbon NTC chemistry

The first-stage ignition delay of n-heptane/air mixtures is computationally studied using detailed mechanism and theoretically studied using eigenvalue analysis of simplified systems. Results show that the delay has a turnover behavior as temperature increases, being dominated by the competition of low-temperature branching and termination channels as well as the competition of forward and reverse reaction channels. As temperature increases to the intermediate range, the termination and reverse pathways result in a minimum in the delay, the state of which is theoretically derived. Simple analytical solutions for the delay as well as the species evolutions are presented to identify the rate constants that control the first-stage ignition and quantify the influence of the mixture composition, initial temperature and system pressure. It is further demonstrated that the above results also hold for n-octane/air and iso-octane/air mixtures.

Chemical kinetic insights into the ignition dynamics of n-hexane

Normal alkanes constitute a significant fraction of transportation fuels, and are the primary drivers of ignition processes in gasoline and diesel fuels. Low temperature ignition of n-alkanes is driven by a complex sequence of oxidation reactions, for which detailed mechanisms are still being developed. The current study explores the dynamics of low-temperature ignition of n-hexane/air mixtures, and identifies chemical pathways that characterize the combustion process. Two chemical kinetic mechanisms were selected as a comparative study in order to better understand the role of specific reaction sequences in ignition dynamics: one mechanism including a new third sequential O2 addition reaction pathways (recently proposed by Wang et al. 2017), while the other without (Zhang et al. 2015). The analysis is conducted by applying tools generated from the computational singular perturbation (CSP) approach to two distinct ignition phenomena: constant volume and compression ignition. In both cases, the role of the third sequential O2 addition reactions proves to be significant, although it is found to be much more pronounced in the constant volume cases compared to the HCCI. In particular, in the constant volume ignition case, reactions present in the third sequential O2 addition reaction pathways (e.g., KDHP → products + OH) contribute significantly to the explosivity of the mixture; when accounted for along with reactions P(OOH)2 + O2 → OOP(OOH)2 and OOP(OOH)2 → KDHP + OH, they decrease ignition delay time of the mixture by up to 40%. Under HCCI conditions, in the first-stage ignition, the third-O2 addition reactions contribute to the process, although their role decays with time and becomes negligible at the end of the first stage. The second ignition stage is dominated almost exclusively by hydrogen-related chemistry.

Effect of improved accelerating method on efficient chemistry calculations in diesel engine

With detailed chemical kinetics being employed in combustion simulations, its major computational challenge is the time-intensive nature of chemical kinetics integration due to the large number of chemical species and wide range of chemical timescales involved. In this work, an extended tabulated dynamic chemistry approach with dynamic pruning method is carried out to simulate complex spray combustion for non-premixed combustion process. The thought of extended tabulated dynamic chemistry approach with dynamic pruning is achieved by selecting the optimum acceleration method as well as its error tolerances at different combustion stages depending on combustion characteristics involving the low-temperature combustion. The present method is applied to realistically complex combustion systems involving spray flame of n-heptane fuel and non-premixed combustion engine. Computation efficiency of the proposed method is compared with the results using different accelerating methods, including dynamical adaptive chemistry, in situ adaptive tabulation, and coupled method of tabulated dynamical adaptive chemistry. The results show that transient computational cost will decrease for low-temperature combustion by reducing ambient oxygen concentration clearly in spray flame. Meanwhile, very low computational efficiency is presented once the autoignition occurs, especially at the initial oxygen concentration of 21%. Based on the feature, extended tabulated dynamic chemistry approach with dynamic pruning with different dynamic adaptive chemistry error tolerances is proposed to improve computational efficiency. Extended tabulated dynamic chemistry approach with dynamic pruning with larger error tolerance [Formula: see text] improves around two times for decreased amplitude of transient computational cost at high-temperature combustion stage, and at the same time, the computational accuracy is also improved by comparing the important intermediate species obtained by direct integration. For applications in diesel engine, the results show that extended tabulated dynamic chemistry approach with dynamic pruning can accurately capture the first-stage ignition feature that determines the high-temperature combustion stage. In addition, extended tabulated dynamic chemistry approach with dynamic pruning with the smaller in situ adaptive tabulation error tolerance of 0.001 only used at the high-temperature combustion stage significantly improves the performance on diesel engine simulation with a larger chemistry mechanism. The present method further significantly improves computational efficiency with an overall speedup factor of 10 with high-accuracy compared with result using direct integration.

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.

Shock tube measurement and simulation of DME/n-butane/air mixtures: Effect of blending in the NTC region

In this study, the ignition delay times of lean and stoichiometric DME/n-butane/air mixtures were measured behind reflected shock waves at pressures of 2 and 10atm, temperatures from 650 to 1400K. The performances of several chemical kinetic models were evaluated. Numerical simulation and chemical kinetic analysis were conducted in a broad temperature range and at various DME blending ratios to understand the interactions between DME and n-butane. It is found that DME addition can promote both the first-stage and overall ignition delay of n-butane in the NTC region.

Turbulence effects on the autoignition of DME in a turbulent co-flowing jet

Dimethyl ether (DME) autoignition in turbulent co-flowing jets with preheated air is studied using the one-dimensional turbulence (ODT) model. We investigate the effects of molecular and turbulent transport on the autoignition process at different jet Reynolds numbers and two air preheat conditions. Statistics for the cases considered show that the overall effects of turbulence and molecular transport can serve to delay or accelerate autoignition depending upon where ignition starts, the presence of 2-stage or single-stage ignition and the variations in ignition delay times in mixture fraction space. For the higher temperature air preheat cases, the classical view that autoignition is delayed by turbulence is established. For the lower preheat air temperature cases, we show that low-temperature chemistry associated with first-stage ignition can help accelerate the autoignition process and the transition to high-temperature chemistry. This acceleration can reduce the ignition delay time by as much as a factor of 2. Given this work and previous work by the authors based on a different fuel, n-heptane, we find that the ignition delay map based on homogeneous ignition for different mixture fractions can provide a preview of the ignition scenarios for the co-flowing jet configuration regardless of the choice of fuel considered.

Autoignition of DME/H2 mixtures in a rapid compression machine under low-to-medium temperature ranges

Ignition delays of dimethyl ether and hydrogen (DME/H2) mixtures (hydrogen blending ratio ranging from 0% to 60%) were measured in a rapid compression machine at compressed temperatures ranging from 655 to 810K and compressed pressures from 16.9 to 24.5bar with an equivalence ratio range of 0.8–1.6. Experimental measurements are compared with results from numerical simulations using the NUIG Mech_56.54 mechanism and, in general, a good agreement is obtained. The first-stage ignition delay of DME/H2 mixtures proves to be insensitive to compressed pressure and equivalence ratio, whereas the total ignition delay decreases distinctly with the increase of compressed pressure and equivalence ratio, especially in the negative temperature coefficient (NTC) region. Moreover, the simulation results show that the NTC region shifts to a higher temperature when compressed pressure is increased. Increasing the equivalence ratio enhances the overall system reactivity but does not significantly change the temperature range of the NTC region. Both first-stage and total ignition delay increase with the increase of hydrogen blending ratio. The total ignition delay as a function of hydrogen blending ratio increases nonlinearly, and the effect of hydrogen addition becomes more prominent in the NTC region and at 60% hydrogen blending ratio. Kinetic analysis indicates that hydrogen addition consumes OH radical and inhibits DME low-temperature oxidation. More fuel molecules undergo chain propagation reaction subsequently, leading to the decrease of heat release and radicals generated during the first-stage ignition and longer total ignition delay.