Cool Flame Temperature Research Articles

Effect of NO2 addition on flame dynamics and chemical kinetics of near-limit n-dodecane cool and warm diffusion flames

The present study is a continuation of the previous study by Zhou et al. (2021), in which kinetic effects of NO addition on n-dodecane cool and warm diffusion flames are investigated via counterflow flames and a skeletal n-dodecane/NOx model is developed by performing jet-stirred reactor experiments. Here, effects of NO2 addition on the flame dynamics and kinetics of n-dodecane diffusion flames are studied. The focus is on understanding the kinetic effects of NO2 on cool flame extinction limits and the transitions between cool, warm, and hot flames and elucidating the differentiated effects of NO and NO2 on low- and intermediate-temperature chemistry of n-dodecane. NO2 also plays different roles in cool and warm flames. Specifically, NO2 addition reduces the cool flame temperature and strain rates at cool flame extinction, indicating that NO2 inhibits the low-temperature chemistry of n-dodecane. NO2 addition increases the warm flame temperature, delays the warm flames extinction to cool flames, and promotes the warm flames re-ignition to hot flames, demonstrating NO2 enhances the intermediate-temperature oxidation of n-dodecane. However, reaction pathways and sensitivity analyses reveal that reactions responsible for NO2 and NO affecting the low- and intermediate-temperature oxidation of n-dodecane diffusion flames are differentiated. For cool flames, reaction R + NO2 <=> RO + NO for NO2 addition and reaction RO2 + NO <=> RO + NO2 for NO addition are the major corresponding reactions, respectively, which compete with the low-temperature chain-branching reaction pathway. For warm flames, the kinetic effects of NO on the intermediate-temperature oxidation are due to NO converting the relative inactive HO2 to reactive OH radicals via the reaction NO + HO2 <=> OH + NO2, which simultaneously accelerates radical production of QOOH => QO + OH to the increase of warm flame temperature, while the effect of NO2 is mainly induced by the conservation of NO2 to NO. Moreover, comparisons of the effects of NO and NO2 addition on cool flame extinction limits are performed, and results show that the kinetic effects of NO2 addition are weaker than those of NO addition since the effects of NO2 are partially contributed by NO from the conservation of NO2.

Extinction characteristics of isolated n-alkane fuel droplets during low temperature cool flame burning in air

Large carbon number n-alkanes are a notable component in all real transportation fuels, and their chemical structure fosters substantial low temperature kinetic reactivity. Normal alkanes have been studied in various canonical configurations but rarely in systems with strong coupling between low temperature chemistry and transport for pure as well as for multi-component n-alkane mixtures. The Flame Extinguishment (FLEX) experiments onboard the International Space Station provided a unique platform for investigating low temperature multi-phase n-alkane and iso-alkane combustion. Among the many interesting phenomena experimentally observed, cool flame extinction can occur, accompanied by the concurrent formation of a surrounding cloud of condensed vapor. In this work we conduct numerical simulations of high and low temperature combustion of large, initially single-component n-heptane, n-decane and n-dodecane droplets. The role of initial droplet diameter, operating pressure, and n-alkyl carbon number on the extinction of hot and low temperature flames is investigated and compared against the available experimental data. While all three fuels exhibit similar hot flame behavior, cool flame activity increases with the carbon number, resulting in an increased cool flame temperature and decreased extinction diameter. Multi-cyclic “hot/cool flame transitions” are found in air as pressure is slightly increased above one atmosphere. The cyclic behaviors correspond to continuously varying hot and cool flame transitions across the high, low, and negative temperature coefficient (NTC) kinetic regimes. Further increase in pressure results in a second stage steady “Warm flame” transition. The extinction of hot and cool flame has a strong non-linear dependence on ambient pressure but as the hot flame extinction diameter increases with pressure the extinction diameter of the cool flame decreases. The computational results are compared with a recent asymptotic analysis of FLEX n-alkane cool flames.

Forced turbulence affected auto-ignition and combustion modes under engine-relevant conditions

Auto-ignition phenomena are paid more attention due to their close relation with abnormal combustions in advanced internal combustion engines. Previous investigations on intensive auto-ignition and knocking phenomena have been mainly conducted under standard flow conditions, whereas the role of turbulence is not fully clarified under engine-relevant conditions. In this study, forced turbulence affected auto-ignition and combustion modes were investigated in a novel rapid compression facility fueled with isooctane/air mixtures. In combination with instantaneous pressure acquisition, high-speed photography and infrared imaging were employed for auto-ignition initiation and reaction wave propagation. The combustion modes were comparatively investigated under standard flow and forced turbulence conditions. Experimental results show that standard flow scenarios are dominated by a thermo-chemical mechanism. With the elevation of intake pressure and thereby energy density, sequential auto-ignition induced normal combustion, end-gas auto-ignition induced mild knocking, and super-knock are observed in sequence, with obvious cool flame temperature rise and two-stage auto-ignition. Under forced turbulence conditions, the adiabatic core starts to be destroyed by the colder fluid from wall boundary layers, leading to lower temperatures and larger thermal stratifications. Such that prolonged auto-ignition timing, reduced burning rate, and attenuated cool flame are observed. Energy density plays an important role in strong knocking initiation, and there are some increases in the demarcation under forced turbulence conditions. However, mixture reactivity still plays the first-order significance in intensive auto-ignition and knocking formation. The current work helps to provide useful insights into the nature of auto-ignition and knocking combustion and the gaps between fundamental platforms and practical engines.

An alternative approach to accommodate detailed ignition chemistry in combustion simulation

In the current work, we utilize a recently developed staged Livengood–Wu (L–W) correlation to evaluate and predict the two-stage ignition behaviors of different primary reference fuels (PRFs) under homogeneous reciprocating engine conditions, to demonstrate its capability in capturing detailed ignition kinetics in combustion simulation. Based on the understandings of the global and detailed kinetics of low-temperature chemistry, simplified Arrhenius-based global expressions were developed for both ignition stages, and a linear correlation was applied for the cool flame temperature rise at the end of the first stage, for PRFs with various octane number (ON) 0, 20, 60 and 87 under constant volume conditions. For their performance under homogeneous charge compression ignition (HCCI) conditions, it is shown that for the same engine speed, the inlet temperature range allowing for two-stage ignition shrinks with increased ON numbers, and eventually two-stage ignition turns into single-stage ignition with sufficiently high ON numbers. Based on the ignition database, the combustion phasing for all PRFs under a wide HCCI operation range are predicted with the staged L–W integral method with satisfactory accuracy, including the first- and second-stage ignition timing as well as the cool flame temperature rise. The sensitivities of each of the engine operation parameters are successfully captured, further demonstrating the applicability of the staged L–W method for realistic fuels with low temperature chemistry and its potential applications for engine combustion control. This study also provides an alternative approach of accommodating low temperature ignition kinetics in combustion simulation, especially useful for those under variable thermodynamic conditions.

A predictive Livengood–Wu correlation for two-stage ignition

The Livengood–Wu correlation has been widely used to predict the state of auto-ignition in internal combustion engines, although its application to two-stage ignition processes remains unresolved. In this study, the original Livengood–Wu integral is extended to such two-stage ignition process and applied to simulations of typical operations within homogeneous charge compression ignition engines. Specifically, based on recent understanding of the global and detailed kinetics of low-temperature chemistry leading to ignition, simplified Arrhenius-based global reaction expressions were developed for both stages of constant-state auto-ignition. It is shown that the original Livengood–Wu integral works well for the first-stage ignition delay, as demonstrated in previous studies. Furthermore, by also accurately describing the cool flame temperature and pressure increment at the end of the first-stage ignition, the second-stage ignition delay can in addition be coupled with the first-stage ignition and predicted satisfactorily with a second integral. This formulation is then applied to extensive homogeneous charge compression ignition engine operation conditions, showing satisfactory predictive capability.

On the crossover temperature and lower turnover state in the NTC regime

This work investigates the thermodynamic and chemical kinetic behavior of the lower stationary point, denoted as the lower turnover state, of the NTC curve near the low temperature regime, where the ignition delay exhibits a local minimum. It is found that the ignition delay varies Arrheniusly with the turnover temperature under various pressures, and that the turnover temperature, Tlower, and the initial pressure, P0, are related and can be correlated by another Arrhenius dependence, as P0 ∼ exp(−Ea/Tlower). These two behaviors have been systematically investigated using detailed mechanisms for typical n-alkanes exhibiting NTC, namely n-butane and n-heptane. It is found that the first and second stage ignition delays are approximately equal to each other at the lower turnover states under various pressures, such that the total ignition delay shares similar temperature dependence with the first stage ignition delay. To further characterize the second ignition stage, the cool flame temperature rise at the end of the first stage is investigated and shown to decrease linearly with increasing initial temperature. A crossover temperature is then defined at the state where the cool flame temperature rise disappears. This state can be correlated with a constant α, which denotes the ratio between OH production from the low-temperature chemistry and the total fuel consumption. It is shown that the second stage ignition delay correlates well with the reaction rate constant for the H2O2 decomposition. Analytical expressions are then derived based on these kinetic insights on the two stages, which well reproduce the lower turnover temperature under different pressures and provide further insights into the two-stage ignition at the turnover states. Comparison of the present results with the literature experimental data of the n-heptane ignition delay time shows good agreement.

Initiation and propagation of laminar premixed cool flames

Cool flames, being essential features of chemical kinetics of large hydrocarbon fuels, are closely related to the negative temperature coefficient (NTC) phenomenon and engine knock. In this work, the coupling of cool flame chemistry and convective–diffusive transport is computationally and experimentally investigated. A 1-D planar premixed cool flame induced by a hot pocket is first simulated for DME/O2/N2 mixtures with detailed chemistry and transport, demonstrating the existence of a residence time window for quasi-steady propagation. Then with residence time limited by aerodynamic straining, a steady-state premixed cool flame is simulated in a counterflow of heated N2 against a DME/O2/N2 mixture. It is found that with a high strain rate, corresponding to short residence time, low-temperature heat release is suppressed, resulting in a stretched low-temperature S-curve system response; and that with a sufficiently low strain rate, corresponding to long residence time, ignition induced by low-temperature chemistry would transition to a high-temperature, intensely burning flame. Consequently, a steady-state premixed cool flame exists only for residence time in a strain rate window. A symmetric counterflow configuration is then simulated to determine the cool flame temperature and flame speed at a fixed local strain rate, showing very different controlling chemistry and characteristics as compared to the normal laminar flames governed by high-temperature chemistry. In a companion experimental investigation, premixed cool flames in the counterflow were observed with a high-sensitivity CCD camera in the UV spectrum, with/without a bandpass filter corresponding to the characteristic wavelength of excited HCHO. The chemiluminescence from the cool flame is found to become more intense with increasing equivalence ratio, even for rich mixtures, while the position of the cool flame is insensitive to variation in the equivalence ratio at the same strain rate. These observations qualitatively agree with the numerical simulations, demonstrating the essential features of premixed cool flames.

An empirical modeling approach for the ignition delay of fuel blends based on the molar fractions of fuel components

An empirical ignition delay model has been developed for the fuel blends of isooctane, n-heptane, toluene and ethanol based on their molar fractions in the stoichiometric condition. The model employs traditional Arrhenius type correlation and cool-flame temperature rise correlations to describe the negative temperature coefficient (NTC) region for the fuel blends. The overall ignition delay of a fuel blend is correlated with individual ignition delay information based on the molar fraction of the fuel component in the mixture. The proposed model is successfully validated against the published experimental ignition delay data using various binary, ternary and quaternary fuel blends of isooctane, n-heptane, toluene and ethanol.

Development and Parametric Evaluation of a Tabulated Chemistry Tool for the Simulation of n-Heptane Low-Temperature Oxidation and Autoignition Phenomena

Accurate modelling of preignition chemical phenomena requires a detailed description of the respective low-temperature oxidative reactions. Motivated by the need to simulate a diesel oil spray evaporation device operating in the “stabilized” cool flame regime, a “tabulated chemistry” tool is formulated and evaluated. The tool is constructed by performing a large number of kinetic simulations, using the perfectly stirred reactor assumption. n-Heptane is used as a surrogate fuel for diesel oil and a detailed n-heptane mechanism is utilized. Three independent parameters (temperature, fuel concentration, and residence time) are used, spanning both the low-temperature oxidation and the autoignition regimes. Simulation results for heat release rates, fuel consumption and stable or intermediate species production are used to assess the impact of the independent parameters on the system’s thermochemical behaviour. Results provide the physical and chemical insight needed to evaluate the performance of the tool when incorporated in a CFD code. Multidimensional thermochemical behaviour “maps” are created, demonstrating that cool flame activity is favoured under fuel-rich conditions and that cool flame temperature boundaries are extended with increasing fuel concentration or residence time.

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.

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.

Cool-Flame Behaviors of an n-Decane/Ethanol Binary-Fuel Droplet in Microgravity

Ignition delays and temperatures of cool flame of isolated n-decane/ethanol binary-fuel droplets in hot air were experimentally measured under microgravity and normal gravity by thermocouples. The droplet diameter was about 1 mm. Ambient pressure was atmospheric pressure, and ambi ent temperature was 620 K or 660 K under microgravity and varied between 580 K and 780 K under normal gravity, where only cool flame appears for pure n-decane droplets. Mole fraction of ethanol of the binary-fuel was varied. Near the droplet were placed three hot junctions of K-type thermocouples with a diameter of 25 mm. The droplet center and the three hot junctions were arrayed in a straight line with a constant distance of 2 mm. Both under normal gravity and microgravity, ignition delay decreased with increasing ambient temperature and increased with more addition of ethanol to n-decane. Fuel composition or ambient temperature affected cool-flame temperature very little under microgravity.

Numerical analysis of the cool flame behavior of igniting n-Heptane droplets

For many technical combustion applications like Lean Prevaporized Premixed (LPP) Systems the knowledge of droplet self-ignition processes as the basic element of spray ignition is necessary. Especially the two-stage ignition behavior due to a cool flame and therefore the Zero Temperature Coefficient Behavior (ZTC) are hereby important. Detailed Numerical Simulations give the opportunity to analyze these processes leading to droplet self-ignition, to get a deeper insight into the existing phenomena. The present study is carried out with a former validated, detailed numerical model including a substantial chemical reaction mechanism for n-Heptane with 437 reactions and 92 species. The Temperature Profiles as a function of time and as a function of radius as well as the profiles of the temperature gradient as a function of time and as a function of radius clarify the characteristics of the ignition process. The mixture fraction due to evaporation and diffusion and therefore the pool on the for the cool flame appearance important species OQ’OOH determines the activity of the low temperature reactions and thus the maximum possible cool flame temperature. The first local temperature rise, which is produced by the low temperature reactions, takes place below ambient temperature at very low fuel / air mixture ratios. It can be found, that the place of the maximum temperature and of the maximum temperature gradient as well, do not coincide during the first stage of ignition. The spatial in-homogeneity of the temperature and species concentration field around the droplet allows for a parallel burning of a cool flame caused by low temperature reactions, even when hot flame ignition has already occurred.

SINGULARITY SPECTRUM OF NONPERIODIC TIME SERIES: SURROGATE DATA AND WAVELET TRANSFORM

We study the singularities of time series giving the evolution of the cool flame temperature of a hydrocarbon. Several tests using surrogate data and wavelet transform are applied to the original signals. The results suggest that the hypothesis of deterministic chaos cannot be ruled out in the original signals. The multifractal organization of their local singularities is proven.

Detailed numerical simulations of the multistage self-ignition process of n-heptane, isolated droplets and their verification by comparison with microgravity experiments

Detailed understanding of the basic physical and chemical processes of self-ignition phenomena for technical fuel sprays is required for many technical combustion applications. Because single droplets as the basic elements of fuel sprays allow the study of fundamental ignition behavior, this work focuses on the multistage self-ignition behavior of Φ 0.7 mm single n-heptane droplets in air. The investigated ambient conditions are temperatures from 580 to 1000 K and pressures from 0.3 to 1 MPa. A substantial physical and chemical model is developed for the detailed numerical simulation. The chemical reaction mechanism consists of a 62-step kinetic model with special consideration of the low-temperature reaction branch. The program was validated by comparison with results from microgravity experiments at Drop Tower Bremen, which allow clarification of the ignition process free from natural convection. Cool flame and hot flame appearances were obtained from non-intrusive interferometric measurement in a well-tested experimental setup. The calculated ignition delays resulting from temperature and temperature gradient criteria, respectively, were compared with these experimental results. Furthermore, the cool flame temperature was measured with a K-type thermocouple of Φ 25 μm at the place of its appearance and was also compared with the numerical simulations. A quantitative good agreement for first and total ignition delays as well as for the cool flame temperature could be achieved. With this detailed numerical model, the multistage ignition behavior was analyzed, and the ignition criteria employed in interferometric measurement, which are temperature and temperature gradient, respectively, were confirmed.

Pressure dependence of the auto-ignition temperature of methane/air mixtures

The results of experiments conducted to determine the auto-ignition temperature (AIT) and the cool flame temperature (CFT) of methane/air mixtures at elevated pressures (200–4700 kPa) and for concentrations from 30 up to 83 vol.% are reported. The experiments were performed in a closed spherical vessel with a volume of 8 dm 3. It is shown that methane/air mixtures can react spontaneously for methane concentrations far outside the flammability limits. Cool flames are observed for methane concentrations higher than 40 vol.%. The AIT and CFT are strongly pressure-dependent and decrease with increasing pressures. A Semenov correlation is developed for the CFT as a function of the pressure. The upper explosion limit (UEL) of methane/air mixtures at elevated pressure and temperature is linked with the CFT. It is shown that for certain conditions of pressure and temperature, the flammability range increases considerably even for a small temperature rise.

Wavelets Transform of a Multifractal Measure in a Cool Flame Experiment

In order to characterize the chaotic variations of the cool flame temperature observed during the oxidation of a hydrocarbon at low temperature and under non-homogeneous conditions, we perform multifractal analyses of different measures. In the cool flame localization domain, the singularity spectrum obtained for the visited temperature histogram is comparable to the spectrum deduced from a wavelet transform. In the cool flame propagation domain where the temperature histogram is too narrow to be analyzed, the wavelet transform allows us to prove the multifractal character of the chaos observed. The choice of the parameter value retained to perform the wavelet transform is discussed in detail.

Spontaneous ignition of liquid droplets from a view of non-homogeneous mixture formation and transient chemical reactions

Spontaneous ignition of isolated single fuel droplets in air is investigated. Special attention is given to the transient behavior of chemical reactions. Single-stage and two-stage ignition or cool flame behavior is investigated. A fuel droplet of n-heptane, n-dodecane, or iso-octane is suddenly exposed in a high-temperature, high-pressure air to ignite. The succeeding process is observed by a Michelson interferometer that visualizes the instantaneous temperature field around a droplet. Experiments are done with air of 500–1100 K in temperature and 0.1–2.0 MPa in pressure. Ignition regions are mapped on a temperature-pressure plane and the roles of the low-and high-temperature branches of chemical reactions are described. Induction times and their temperature or pressure dependence are analyzed. The diameter dependence of the induction times shows influences of physical process only on first induction time. Pressure dependence of the second induction time indicates that the dominating factor for the induction time is the nature of a cool flame during the first stage. Mixture conditions at the occurrence of a cool flame and the relation between mixture condition and second induction times are examined. Fuel and temperature dependence of second induction time and TC (zero-temperature coefficient) behavior of total induction time are explained by the dependence of the mixture condition on those parameters through cool flame temperature.

Mechanism of cool-flame propane oxidation

Direct data on free radicals in cool flames of the propane-oxygen mixture have been obtained. The concentrations of peroxy radicals in cool flame is approximately 15 times higher than in the preflame region at the same temperature (330°C). Acetaldehyde additive extends the cool-flame temperature region.

The effect of additives on the cool flame combustion of n-heptane

The effect of a large number of compounds on the cool flame combustion of n -heptane at around 250°C in a static system has been investigated. The organometallic, halogen, nitrogen, and sulfur compounds used (except dimethylamine and α -toluene thiol) had no effect. Alcohols and olefins, however, inhibited the oxidation, increasing the cool flame limits and the “induction period” ( τ ) before a cool flame when this occurred. The relative inhibiting efficiency is explained on the basis of the reaction of alkylperoxy radicals with these inhibitors and of the fate of the radicals formed. The temperature rise in the center of the reaction vessel increased exponentially during τ : the rise just before a cool flame occurred remained constant as the total pressure was increased at constant ambient temperature ( T 0 ) but decreased markedly as T 0 increased from the cool flame temperature limit. The consumption of n -heptane at the end of the induction period and the concentration of higher aldehydes were much greater when isobutanol or cyclohexene was added; and when good inhibitors are present, the flames may, in fact, be due to these higher aldehydes.