Cool Flame Ignition Research Articles

Spatial distribution and temporal evolution of wall-stabilized DME/O2 premixed cool flames

The low-temperature oxidation of dimethyl ether (DME) was investigated in premixed wall-stabilized cool flames at two equivalence ratios (ϕ) of 0.2 and 0.5. Using a time-of-flight mass spectrometry (TOF-MS) coupled with gas chromatography (GC), the spatial distributions of major intermediate species, including DME, CH2O (formaldehyde), CO, CO2, and CH3OCHO (methyl formate), were quantified under well-controlled boundary conditions. Moreover, the temporal evolutions of multiple intermediate species in the wall-stabilized cool flame ignition process were measured via TOF-MS, while the wall temperature was gradually ramped up from 550 K to 730 K. Several kinetic models were examined herein to assess the estimated low-temperature reactivity of DME by comparing the one-dimensional axisymmetric simulation results with the experimental data. Wall-stabilized cool flame structures at equivalence ratios ϕ of 0.2 and 0.5 were quantitatively examined with the major intermediate species. It is found that the kinetic models reasonably predict the onset of the reaction zone near the wall. Among these models, Kurimoto et al.’s model gives better predictions for the distributions of CH2O and CO, which are characteristic species of cool flames. In addition, time-resolved measurements of the unsteady cool flames identified the negative temperature coefficient (NTC) turnover points for different species across various temperature regions. It is also found that the Kurimoto et al. model still indicates a slightly higher reactivity of DME in the low-temperature range, resulting in earlier DME consumption and a shift of NTC window to lower temperatures at ϕ = 0.2.

Numerical studies on the ignition and propagation for spherically expanding premixed cool flames under gravitational conditions

Cool flame speed is a fundamental parameter, which requires special consideration in terrestrial measurement due to the slow propagation of the cool flame, the intrusion of gravity, and multiple flame regimes (cool, hot, and double flames). The current study employs multiscale numerical simulations with and without gravity to investigate premixed spherical cool flame ignition and propagation. One-dimensional simulations without gravity are adopted to study the effects of initial ambient temperature T0, pressure P0, equivalence ratio ϕ, and ignition kernel temperature TH on feasible conditions for an isolated cool flame measurement, emphasizing the intrinsic couplings among the cool flame, hot flame, and autoignition. Besides considering the minimum and maximum ignition energies for low-temperature ignition (LTI), the results highlight the impacts of the auto-ignition upstream and hot flame downstream of the cool flame on speed measurement. Comprehensive effects of T0, P0, and ϕ are evaluated in order to obtain accurate flame speed versus stretch rate data for extrapolation. Two-dimensional simulations focus on the cool flame deformation with buoyancy. Buoyancy induces a toroidal vortex, causing the spherical cool flame to be mushroom-like at high Richardson numbers, a non-dimensional metric used to evaluate the relative importance of buoyancy. The flame deformation at different ambient temperatures and equivalence ratios are observed. Detailed analyses of the flow field, the deformed flame structure, and the effects of stretch and diffusion are performed. In addition, three equivalent flame radius definitions are proposed and compared to extrapolate cool flame speed. Different from the previous studies on the gravitational hot flames, the current results identify the flame volume-based radius (i.e., Req,V) to be the best one for extrapolating non-spherical cool flame speed, with which cool flames faster than 8 cm/s can be measured within 5% uncertainty. Finally, a systematic strategy is proposed to guide the experimental design. The current work reveals unique cool flame dynamics under gravity conditions and helps to measure the cool flame speed in a terrestrial environment.

Forced ignition of premixed cool and hot DME/air flames in a laminar counterflow

Recently, low-temperature chemistry (LTC) and LTC-induced cool flames have attracted extensive interest since they can substantially influence the ignition process and accelerate the subsequent hot flame propagation. However, it is unclear how the flow affects the forced ignition of a cool flame and its subsequent transition to a hot flame. In this study, we conduct transient 2D simulations for the forced ignition processes of premixed cool and hot flames in a laminar, axisymmetric, counterflow configuration with well-defined flow field. Different ignition energies and strain rates are used to ignite and stabilize cool and/or hot flames in a counterflow. The critical conditions for the ignition of cool and hot flames are identified, and the cool flame kernel quenching and its transition to hot flame are discussed. It is found that the ignition energy determines the highest temperature and thereby controls the thermal runaway process. The strain rate influences the flame kernel propagation and the transition from cool flame to hot flame. A large strain rate may quench the cool flame, while a small strain rate may lead to the transition from cool flame to hot flame due to the sufficiently long residence time. Therefore, cool flame ignition and stabilization can only be achieved within a certain range of strain rates. Furthermore, an ignition regime diagram in terms of ignition energy and strain rate is proposed, and four regimes are identified: (I) only cool flame, (II) only hot flame, (III) ignition failure, and (IV) transition from cool flame to hot flame (with appearance of double flame structure). These results provide new insights into how the flow affects the cool flame ignition and transition.

Ignition and extinction characteristics of n-alkane cool flames on a heated wall with different surface reactivity

Cool flames play essential roles in the ignition process of hydrocarbon fuels, so that understanding their dynamics is highly crucial to develop advanced engines with high thermal efficiency. There have been extensive studies to elucidate the combustion characteristics of cool flame and the low-temperature chemistry governing its kinetics. However, wall chemical effects on the cool flame ignition and extinction are still not fully clarified, although our recent related works have shown their significant impact on the cool flame ignition temperatures of DME.In this study, ignition and extinction characteristics of n-alkanes with carbon numbers from 7 to 10 are examined systematically, with particular attention paid to the effects of fuel carbon number, oxygen concentration, and surface reactivity. An experimental setup enabling the ignition and extinction of cool flames in the vicinity of a wall is newly built. The surface reactivity is controlled by depositing thin films of specific materials, namely SiO2, Fe, and Ru, on the wall surface. The cool flame ignition and extinction behaviors of each fuel on each wall are identified by measuring HCHO molecules with laser-induced fluorescence (LIF). In addition, numerical simulations assuming one-dimensional axisymmetric impinging flows with detailed gas-phase reactions are performed in order to further investigate the effect of fuel carbon number on the cool flame ignition and extinction. The results indicate that the cool flame ignition and extinction behaviors are highly dependent on the surface reactivity, showing completely different aspects according to the fuel.

Development of Forced Cool-Flame Ignition and Detection Device for a Fuel Droplet

Development of Forced Cool-Flame Ignition and Detection Device for a Fuel Droplet

Ignition characteristics of premixed cool flames on a heated wall

This work examined the cool flame ignition and stabilization on heated walls with different surface reactivities. A premixed mixture of dimethyl ether (DME) and O2 was impinged on a heated wall and the wall temperature was linearly ramped to determine the cool flame ignition temperature. Numerical analyses identify a hysteresis phenomenon in the cool flame ignition and extinction processes, which appears to be primarily governed by the low-temperature reaction kinetics. The formation of HCHO during the cool flame ignition process was monitored using the planar laser-induced fluorescence (PLIF) technique synchronized with instantaneous wall temperature measurements. The data indicated that the HCHO concentration near the wall surface abruptly increased at the ignition of the premixed cool flame. The predicted hysteresis behaviors for cool flame ignition and extinction with a nonlinear folded S-curve and a smoothly stretched S-curve were successfully observed experimentally for the first time ever for lean mixtures. Moreover, the results show that compared with the ignition limits on an inert SiO2 surface, noticeable increases and decreases of the cool flame ignition temperature were observed on Fe and Pt surfaces, respectively. These findings suggest that significant wall chemical effects occur during the cool flame ignition process, based on interactions between the gas phase and surface reactions.

A cell model analysis for droplets inside non-dilute n-heptane droplet clouds near autoignition limit

Vaporization and two-stage autoignition of single droplets inside the droplet clouds were numerically studied using a spherical cell model. We aimed to investigate the behaviors of individual droplets in a moderately hot ambient (600–800 K) under the influence of surrounding droplets at atmospheric pressure. For a given ratio of distance between droplets L and the droplet diameter D0, autoignition only occurs when D0 is above certain critical value. Exemplar cases with L/D0 = 10 were used for detailed discussion as they are found to be near the autoignition limit. For droplets smaller than 400 μm, no ignition occurs and the surrounding gas phase is saturated by fuel vapor before the droplet completely vaporizes. A simple model of final saturation state was established. With this model, it was observed that when the gas phase in the droplet cloud is saturated, the temperature and fuel concentration distributions are insensitive to the distance between droplets. For droplets larger than 500 μm, a variety of autoignition processes including first stage ignition, cool flame burning, second stage ignition, and diffusional extinction are observed. Unlike single isolated droplets, such behaviors are primarily due to heat accumulation and limited supply of oxidizer that pertains to the scenario of non-dilute droplet cloud. Autoignition of individual droplets in a cloud only happens in the region where chemical time scale is smaller than the physical time scale for vaporization and diffusion. The temperature range of autoignition is broadened with increasing initial droplet diameter. Two potential mechanisms were identified for such broadening. Analyses of ‘probable droplet cloud autoignition (PDCA)’ region were found to qualitatively predict the state switch of vaporization, cool flame ignition, and second stage ignition.

Two-dimensional laboratory-scale DNS for knocking experiment using n-heptane at engine-like condition

A two-dimensional laboratory-scale DNS of a knocking experiment using a stoichiometric n-C7H16/O2/Ar mixture at engine-like condition with the latest reduced SIP isooctane kinetics was conducted. A compressible reactive flow solver PeleC combined with an in-house efficient chemical kinetics solver MACKS was used. The results of DNS were compared with a “knocking experiment by Kyushu University” conducted in a 14 × 14 × 80 mm rectangular constant volume chamber for the same mixture at an initial temperature of 480 K and pressure of 0.33 MPa. The comparisons showed the present DNS with the latest, validated reduced chemical kinetics successfully reproduced the knock onset timing within an error of 2% as well as the overall features of the experimental observations seen in the Schlieren images such as flame shape transitions, pressure history, and the characteristic faint increase in pressure due to the flame front attachment with the chamber wall. In addition, the present DNS captured that the cool flame ignition occurred homogeneously but accompanied by the rapid formation of density gradients vertical to the side walls in the unburned gas region.

Ignition and extinction of strained nonpremixed cool flames at elevated pressures

Cool flames, governed by low-temperature chemistry, are closely related to engine knock. Since the low-temperature chemical kinetics is promoted at elevated pressures, the ignition and extinction of nonpremixed cool flame at elevated pressures were experimentally and computationally investigated herein in the counterflow. Specifically, the hysteretic ignition and extinction behavior of the nonpremixed cool flame was for the first time observed and quantified. S-curve analysis was conducted to demonstrate the thermal and chemical structure of the cool flame and to elucidate the dominant chemical pathways during the ignition and extinction processes. The dominant low-temperature chemical reactions shift from those responsible for radical runaway to exothermic reactions that sustain the cool flame. Increasing the ambient pressure and/or the oxygen concentration in the oxidizer stream promote the heat release from the cool flame, and hence, result in an extended hysteresis temperature window between ignition and extinction. It is further noted that while the observed cool flame ignition temperatures were well predicted by computation, significant discrepancies existed for the extinction temperatures based on the well-adopted reaction mechanism used. Possible reasons were discussed to facilitate further cool flame studies and the development of the low-temperature chemistry.

Autoignition of n-decane Droplets in the Low-, Intermediate-, and High-temperature Regimes from a Mixture Fraction Viewpoint

Detailed numerical simulations of isolated n-decane droplets autoignition are presented for different values of the ambient pressure and temperature. The ignition modes considered included single-stage ignition, two-stage ignition and cool-flame ignition. The analysis was conducted from a mixture fraction perspective. Two characteristic chemical time scales were identified for two-stage ignition: one for cool-flame ignition, and another for hot-flame ignition. The appearance and subsequent spatial propagation of a cool flame at lean compositions was found to play an important role in the ignition process, since it created the conditions for activating the high-temperature reactions pathway in regions with locally rich composition. Single-stage ignition was characterized by a single chemical time scale, corresponding to hot-flame ignition. Low-temperature reactions were negligible for this case, and spatial diffusion of heat and chemical species mainly affected the duration of the ignition transient, but not the location in mixture fraction space at which ignition first occurs. Finally, ignition of several cool flames of decreasing strength was observed in the cool-flame ignition case, which eventually lead to a plateau in the maximum gas-phase temperature. The first cool flame ignited in a region where the fuel / air mixture was locally lean, whereas ignition of the remaining cool flames occurred at rich mixture compositions.

Ignition delay times of single kerosene droplets based on formaldehyde LIF detection

Single droplet ignition experiments were performed with GTL kerosene, Jet A-1, and Exxsol D80 (reduced aromatics) in the temperature range from 550 to 900K in air at 3bar. The laser-induced fluorescence (LIF) intensities of liquid GTL kerosene and Exxsol D80 are on a lower level than that of Jet A-1. Formaldehyde LIF and background emission were measured with pulse-to-pulse wavelength switching at closely spaced excitation wavelengths (353.373 and 353.386nm). Typically, formaldehyde LIF and background emission were observed simultaneously. However, in some droplet ignition sequences background emission was observed first followed by formaldehyde LIF after a time delay. Therefore, in the case of kerosene background subtraction has to be performed carefully to derive correct ignition delay times based on formaldehyde LIF (cool flame). A pulsating combustion was observed in several kerosene droplet ignition experiments. The cool flame ignition was found to appear significantly earlier for the synthetic GTL kerosene compared to Exxsol D80 and conventional Jet A-1 kerosenes. For GTL kerosene we found a cool flame ignition delay time of 624ms, while corresponding values for Exxsol D80 and Jet A-1 amount to 1467 and 1699ms, respectively. The differences for the starting time of hot ignition (around 1050ms) and ignition temperatures (near 700K) are comparatively small under the conditions of our experiments.

Hydrogen addition effect on a reaction front propagation in NTC-affected auto-igniting mixture

Synergic effect of fuels with different ignition properties on engine combustion process has been followed with interest recently. To further reveal their interaction mechanism during combustion, numerical simulations have been performed in this study, addressing hydrogen addition effect on a reaction front propagation in negative temperature coefficient (NTC) –affected auto-igniting dimethyl ether (DME) and air mixture. Firstly, homogeneous auto-ignition responses show that auto-ignition delay is obviously retarded in NTC regime while advanced beyond the regime as hydrogen addition due to the decreasing heat release during cool flame ignition and increasing heat release during final hot ignition. Chemical analysis indicates that the variation in this polarized heat release distribution is attributed to NTC-affected chemistry and total equivalence ratio. Secondly, the results of a reaction front propagation in non-reacting mixture show that the NTC-affected reaction front is also influenced by hydrogen addition, characterized as the delayed cool flame ignition and increasing maximum temperature gradient of a reaction front as well as linearly increasing reaction front speed. Finally, when the reaction front propagates in NTC-affected partially auto-igniting mixture, it shows that both reaction progress and hydrogen addition significantly increase transport-controlled deflagration front speed. However, contrary to the reaction progress, hydrogen addition depresses NTC-affected cool flame and combustion regime transitions from transport-controlled deflagration towards reaction-controlled spontaneous ignition. And meanwhile, slight flame acceleration is also observed when the reaction progress crosses over the corresponding temperature region dominated by NTC chemistry.

Discharge effect on the negative temperature coefficient behaviour and multistage ignition in C3H8-air mixture

It has been first shown by modelling that the discharge does not suppress but stimulates the development and increases the intensity of cool flame. It sharply reduces the delay time of cool flame ignition (isothermal stage) by more than 2 orders of magnitude and diminishes the total induction time by more than 1 order of magnitude due to radicals resulting from dissociation of fuel and oxygen molecules by electron impact. Under the discharge impact, the negative temperature coefficient (NTC) of the overall oxidation rate of the combustible mixture disappears or its maximum is largely decreased at the temperature range 640–760 K. A diminution of NTC maximum depends on the specific deposited energy and is provided by a replacement of the chemical kinetic mechanism at the isothermal stage in the NTC region. The relation between the specific input energy, concentrations of primary radicals resulting from a discharge and key component C3H7O2 which determines the NTC behaviour has been obtained.

저온연소조건에서 n-heptane/alcohol 혼합연료의 냉염과 열염에 대한 착화지연 관찰

The ignition delay time is an important factor to understand the combustion characteristics of internal com- bustion engine. In this study, ignition delay times of cool and thermal flame were observed separately in homogeneous charge compression ignition(HCCI) engine. This study presents numerical investigation of ignition delay time of n-heptane and alcohol(ethanol and n-butanol) binary fuel. The O2 concentration in the mixture was set 9-10% to simulate high exhaust gas recirculation(EGR) rate condition. The numerical study on the ignition delay time was performed using CHEMKIN codes with various blending ratios and EGR rates. The results revealed that the ignition delay time increased with increasing the alcohol fraction in the mixture due to a decrease of oxidation of n-heptane at the low temperature. From the numerical analysis, ethanol needed more radical and higher temperature than n-butanol for oxidation. In addition, thermal ignition delay time is sharply increasing with decreasing O2 fraction, but cool flame ignition delay time changes negligibly for both binary fuels. Also, in high temperature regime, the ignition delay time showed similar tendency with both blends regardless of blending ratio and EGR rate.

An optimization-based approach to detailed chemistry tabulation: Automated progress variable definition

Detailed chemistry tabulations built from canonical combustion problems, such as premixed or diffusion flamelets, have been the subject of multiple studies. Chemical look-up table construction strategies may rely on the definition of progress variables, which are usually obtained from combination of reactants and final products mass fractions. Finding a progress variable such that all relevant species can be retrieved from knowledge of it is not always straightforward; especially for fuel mixtures composed of more than a single hydrocarbon, or for simpler fuels but under conditions where the global reaction progress is not continuous, as in the case of cool-flame ignition. To overcome these difficulties, automated methods are discussed for defining progress by variables in which all species of a chemical scheme, even minor ones, are involved. This is done by formulating constraints applied to progress variable definition: they should evolve in a monotonic manner from fresh to burnt gases and the species derivative in progress variable space should stay moderate for tabulation accuracy. This set of constraints is discretized along chemical trajectories observed in laminar canonical flames, to be formulated in terms of an ensemble of inequalities, which are solved using optimization tools. The outcome is a set of weighting coefficients to be applied to every species of the detailed chemical scheme, in order to construct the progress -variable space. The methods are successfully applied to methane and kerosene premixed flamelets and to n-heptane self-ignition, under conditions with cool-flame effects.

Effects of droplet interaction on spontaneous ignition of an n-decane droplet pair

As a fundamental study on the droplet-interaction effect in the fuel-spray ignition, spontaneous ignition of an n-decane droplet pair rapidly inserted into hot air was experimentally studied in the ambient temperature range where the low-temperature oxidation reactions are active. Two droplets suspended on 14μm SiC fibers initially at room temperature were inserted into a hot furnace. Droplet diameter was 1mm. Three hot junctions of K-type thermocouples with a diameter of 25μm were located near the droplet pair, and cool-flame and hot-flame appearances were detected. The experiments were performed in microgravity to exclude the effect of buoyancy. First, at atmospheric pressure, where only cool flame appears, cool-flame ignition delay and cool-flame temperature increased with decreasing inter-droplet distance. The former is supposed to be mainly caused by the mutual cooling effect, and the latter is by the enhanced fuel supply through duplicated fuel sources. Next, at 0.3MPa, where two-stage ignition occurs, cool-flame ignition delay increased with decreasing inter-droplet distance in the same way as at atmospheric pressure. On the other hand, the duration between cool-flame appearance and hot-flame appearance (second induction time) decreased with decreasing inter-droplet distance. This is supposedly caused by higher cool-flame temperature through higher fuel concentration in the reaction zone at the moment of cool-flame appearance. Thus the mutual cooling effect was dominant before cool-flame appearance, while the effect of duplicated fuel sources was dominant after cool-flame appearance.

A Physics and Tabulated Chemistry Based Compression Ignition Combustion Model: from Chemistry Limited to Mixing Limited Combustion Modes

This paper presents a new 0D phenomenological approach to predict the combustion process in multi injection Diesel engines operated under a large range of running conditions. The aim of this work is to develop a physical approach in order to improve the prediction of in-cylinder pressure and heat release. Main contributions of this study are the modeling of the premixed part of the Diesel combustion with a further extension of the model for multi-injection strategies. In the present model, the rate of heat release due to the combustion for the premixed phase is related to the mean reaction rate of fuel which is evaluated by an approach based on tabulated local reaction rate of fuel and on the determination of the Probability Density Function (PDF) of the mixture fraction (Z), in order to take into consideration the local variations of the fuel-air ratio. The shape of the PDF is presumed as a standardized <i>β<i/>-function. Mixture fraction fluctuations are described by using a transport equation for the variance of Z. The standard mixture fraction concept established in the case of diffusion flames is here adapted to premixed combustion to describe inhomogeneity of the fuel-air ratio in the control volume. The detailed chemistry is described using a tabulated database for reaction rates and cool flame ignition delay as a function of the progress variable c. The mixing-controlled combustion model is based on the calculation of a characteristic mixing frequency which is a function of the turbulence density, and on the evolution of the available fuel vapor mass in the control volume. The developed combustion model is one sub-model of a thermodynamic model based on the mathematical formulation of the conventional two-zone approach. In addition, an extended sub-model for multi injection is developed to take into account interactions between each spray by describing their impact on the mixture formation. Numerical results from simulations are compared with experimental measurements carried out on a 2 liter Renault Diesel engine and good agreements are found.

Spontaneous Ignition of a Droplet Pair in Microgravity

As a fundamental study on the droplet-interaction effect in the fuel-spray ignition, spontaneous ignition of an n-decane droplet pair in hot air was experimentally studied. Two droplets initially at room temperature were brought into a hot furnace at atmospheric pressure. Droplet diameter was 1 mm. Three hot junctions of K-type thermocouples with a diameter of 25 μm were located near the droplet pair, and the cool-flame appearance was detected. The experiments were performed in microgravity in order to get rid of the effects of natural convection. Ignition delays and temperatures of cool flame were evaluated from the thermocouple outputs. Backlit droplet images were also recorded, and the vaporization rate was evaluated. Inter-droplet distance was varied, and ambient temperature (hot-air temperature) was fixed at either 600 K or 720 K, where only cool flame appears for n-decane droplets. Generally both cool-flame ignition delay and cool-flame temperature increased with decreasing inter-droplet distance, while cool-flame ignition delay took a minimal value at a certain inter-droplet distance.

Phenomenological characteristics of cool-flame oxidation of cyclohexane

The phenomenological characteristics of cool flames of cyclohexane are studied. It is shown that cyclohexane flames arise at lower temperatures (beginning at T ≈ 230°C) and lower pressures (lower than 10 torr at T = 290°C) than alkane and alkene flames. The temperature dependence of the lower pressure limit of the cool flame is determined. It is established that, depending on the conditions (p, T, and C6H12/O2 ratio), cool-flame ignition is characterized by a rise and drop of the temperature ΔT or by several peaks imposed on each other. The dependence of the cool-flame structure (for ΔT) on the oxygen content in the reaction mixture is studied. It is shown that the cool flame resulting from cyclohexane oxidation is accompanied by ring opening and formation of products with a smaller number of carbon atoms. It is established that cool flames of cyclohexane are characterized by a negative temperature coefficient.

Detailed chemistry-based auto-ignition model including low temperature phenomena applied to 3-D engine calculations

A model for the auto-ignition of hydrocarbons applicable to 3D internal combustion engine calculations is proposed in this paper. The limits of classical methods using an auto-ignition delay are investigated when cool flame phenomena are present. A method based on tabulated reaction rates is presented to capture the early heat release induced by low temperature combustion. Cool flame ignition delay when present and cool flame fuel consumption are also tabulated. The reaction rate, fuel consumption, and cool flame ignition delay tables are built a priori from complex chemistry calculations. The reaction rates, which directly depend on instantaneous changes of thermodynamic conditions, are then integrated during the 3D engine calculation. The model is first validated through comparisons with complex chemistry calculations in constant and variable volume configurations where good agreement has been found. The model is then applied to both a Diesel computation with spray injection and residual gases, and a Diesel HCCI configuration. Comparisons with experimental results show that the auto-ignition essential features are well reproduced in these cases.