Laminar Counterflow Diffusion Flames Research Articles

Numerical study on soot formation in ammonia/ethylene laminar counterflow diffusion flame

The ammonia addition can significantly inhibit soot formation in hydrocarbon fuel flames. However, due to the complexity of the interaction between nitrogen and hydrocarbon, the inhibiting effect of NH3 addition on soot formation has not been fully understood yet. In this study, a mechanism is constructed through mechanism comparison, reduction, and merging. The constructed mechanism includes ammonia, polycyclic aromatic hydrocarbons (PAHs), and nitrogen-containing aromatic compounds (NACs) sub-chemistry. Then the constructed mechanism is coupled with a soot model to conduct a series of simulations of ammonia/ethylene laminar counterflow flames with various ammonia doping ratios. The results show that the constructed model can reproduce the suppression effect of ammonia addition on soot formation well. To explain the chemical inhibiting effect, the rate of production (ROP) and reaction pathway are analyzed. It can be concluded that the main formation pathway of benzene is C2H4 → C2H3 → C2H2 → C3H3 → A1. The decomposition of ammonia consumes H radical which is necessary for the conversion of C2H4 to C2H3, leading to a decrease in the reaction rates along the benzene formation pathway. Moreover, the N-containing species react with the small hydrocarbon molecules/radicals to form hydrogen cyanide (HCN), in which the reaction between C2H2 and N radical is the largest contributor. Compared to the reactions between C1-C2 species and nitrogen-containing species, the reactions between C3 species and nitrogen-containing species contribute less to the chemical effect of NH3 addition. All these reactions reduce the carbon flux of the benzene formation path. In addition, the impact of reactions between HCN and aromatic hydrocarbons which produce NACs is also investigated. The reactions of HCN addition to benzene and naphthalene compete with the C2H2 addition reactions, resulting in a slightly enhanced inhibiting effect of ammonia addition on soot formation.

Large-Eddy-Simulation of Turbulent Non-Premixed Hydrogen Combustion Using the Filtered Tabulated Chemistry Approach

Abstract With air traffic expected to grow 5% annually until the year 2030, alternative fuels such as hydrogen are being investigated in order to tackle the current environmental crisis. Due to safety concerns, future hydrogen combustion chambers will require new designs of injection systems and are expected to operate under multimode combustion regimes. From a large-eddy-simulation (LES) perspective, a prerequisite for the shift toward new hydrogen combustion chamber technologies is a robust turbulent combustion model capable of functioning in non-premixed conditions. Turbulent combustion modeling using flame front filtering is a well-developed strategy in premixed combustion (filtered-tabulated chemistry for large-Eddy-simulation (F-TACLES)). This approach has been extended to non-premixed flames however, it suffers from high flame filter size sensitivity. Moreover, thin hydrogen flame fronts will result in lower resolution on the LES grid, potentially amplifying this issue. In order to address the feasibility of the non-premixed F-TACLES model applied to hydrogen fuel, simple one-dimensional and two-dimensional laminar counterflow diffusion flames are computed. The model is then tested on the three-dimensional Sandia hydrogen jet flame with a Reynolds number of 10,000. Simulations and a priori tests show that tabulated subgrid-scale correction terms are stiff and can result in nonphysical results, however the model is capable of correctly reproducing non-premixed flame structures for small filter sizes.

A kinetic study on the blending behavior of sustainable and conventional aviation fuels: Soot formation processes

Together with electrification, the introduction of short-term carbon cycle alternatives to fossil fuels represents one of the most promising strategies towards a significative reduction in GHG emissions. For sectors such as air transport this option is particularly appealing, as aviation strongly depends on the availability of high energy-density carriers. These needs justify the current interest in sustainable aviation fuels (SAF). In particular, paraffinic kerosenes from Fischer-Tropsch (FT) processes, hydro-processed esters and fatty acids (HEFA) and alcohol-to-jet (ATJ) are within the SAF that are attracting major interests. Their introduction as drop-in fuels, pure or in blends with conventional jet-fuels from fossils, may also have a positive impact on non-CO2 emissions such as soot, whose formation is known to strongly correlate with fuel composition. This study addresses a set of three standard (Jet-A, JP5 and JP8) and three sustainable jet fuels (HEFA-Camelina, FT-SPK, ATJ) unraveling the mechanisms behind their soot emission tendency and that of their blends. Suitable surrogate fuels are formulated from a palette of 11 components using an in-house code targeting physical and chemical properties of the real fuels. The sooting behavior of selected jet-fuels and blends (HEFA/Jet-A, FT-SPK/Jet-A, ATJ/Jet-A) is then calculated and compared with soot volume fraction data in laminar counterflow diffusion flames. A targeted kinetic analysis highlights kinetic pathways governing soot inception in standard jet-fuels in comparison to their sustainable alternatives supporting explanation of the observed trends. Morphological aspects such as the average particle diameters and particles size distribution from HEFA, Jet-A and HEFA/Jet-A flames are also discussed, identifying characteristic behaviors informing effective blending strategies and motivating targeted experimental studies.

Experimental and numerical investigation on soot formation and evolution of particle size distribution in laminar counterflow ethylene flames

A detailed investigation of the process of soot formation in ethylene-fueled laminar counterflow diffusion flames is conducted using dedicated experiments and numerical simulations. Two different strategies based on the Discrete Sectional Method (DSM) and the Split-based Quadrature Method of Moments (S-EQMOM) are considered to model the evolution of soot particle size distributions, and their comparative assessment is carried out for soot formation prediction and particle growth. A consistent chemical reaction mechanism describing the oxidation of hydrocarbon fuels and the prediction of soot precursors with the growth of polycyclic aromatic hydrocarbons (PAHs) up to pyrene ( ▪ ) is examined. Experiments for various strain rates and fuel compositions are performed to assess the sensitivity of soot production to these two parameters. The results show that both modeling strategies captured well the qualitative trends of soot volume fraction under variations in strain rate and mixture composition, with slight over-prediction of the peak values. For both soot models, a higher sensitivity of soot formation is noticed by changes in mixture composition compared to those of strain rate variation. Additionally, the soot models demonstrated promising performance in capturing the experimentally observed evolution of the soot particle size distribution (PSD).

Numerical analysis of hydrogen-oxygen hydrothermal combustion: Laminar counterflow diffusion flames

Hydrogen hydrothermal combustion is a promising technology in the field of energy conversion. The H2–O2 hydrothermal combustion is comprehensively investigated using a one-dimensional laminar counterflow diffusion flame model. The introduction of the real-fluid thermodynamic and transport properties approach corrects the temperature and location of the flame. Effects of strain rate, pressure, inlet temperature, and fuel concentration on flame structure and extinction limit are examined. The increase in strain rate and pressure reduces the maximum temperature and flame thickness, while the inlet temperature and fuel concentration have the opposite effect. The extinction strain rate ranges from 30 s−1 to 5200 s−1 at inlet temperatures of 773 K–973 K and fuel concentrations of 10 mol%–50 mol%, indicating that the hydrothermal flame stability is much worse than gas-phase combustion. The unity Lewis number assumption underpredicts the maximum flame temperature by nearly 100 K at any strain rate.

Quantitative analyses of NO formations from different reaction pathways in methane-air combustion at various pressures and temperatures

Numerical calculations of the laminar counterflow diffusion flames of methane-air are conducted to quantitatively study NO formations from different reaction pathways (mechanisms) at various pressures and/or initial air temperatures. A modified method, which deactivates the initiating reactions in a specific reaction pathway only for NO production while maintaining interactions with other pathways, is developed to calculate NO formation in each mechanism. Results indicate that in the thermal, prompt, N2O, and NNH reaction mechanisms, the NO production rate largely increases as the pressure or the initial air temperature increases, and in particular, as these two parameters are increased together as in a practical combustion system. In the thermal pathway, as the pressure is increased to 15 atm, NO is produced almost entirely on the fuel-lean side. In the prompt pathway, NO production generally occurs in regions with the progress variable ranging from 0.8 to 0.95; the maximum NO production rate moves from the fuel-rich towards the stoichiometric region as the pressure is increased. NO formation from the N2O or NNH mechanism occurs mainly at high pressures, but the corresponding production rate is an order of magnitude smaller than that from the thermal or prompt mechanism, respectively. In all the reaction mechanisms, NO consumption occurs through the reburn mechanism on the fuel-rich side, and the consumption rate significantly increases as the pressure and initial air temperature increase. Results herein provide a quantitative understanding of the effects of pressure and temperature on NO formations from different reaction mechanisms.

In-situ laser diagnostic and numerical investigations of soot formation characteristics in ethylene and acetylene counterflow diffusion flames blended with dimethyl carbonate and methyl formate

Oxygenated fuels have emerged as promising energy carriers for storing renewable energy given their compatibility with current energy systems and their potential of reducing pollutant emissions when burning in blends with traditional fossil fuels. In particular, dimethyl carbonate (DMC) and methyl formate (MeFo) have raised increasing interest for their use as gasoline substitutes in recent years. However, the effect of fuel blending on soot formation remains unclear, which prevents the utilization of the full potential of these fuels. In this work, soot formation of ethylene-based and acetylene-based fuel blended with DMC and MeFo in laminar counterflow diffusion flames was investigated by laser-induced incandescence (LII) in combination with laser-induced breakdown spectroscopy (LIBS) and spontaneous Raman scattering (RS). When substituting the base fuels with DMC or MeFo, the flame temperature profiles remain unchanged, allowing to exclusively focus on the chemical kinetic effect of fuel blending on soot formation. The soot formation in the ethylene-based flames can be enhanced by blending a small amount of DMC, while the soot volume fraction begins to decline at high substitution rates of DMC. Blending DMC with acetylene shows no soot enhancement effect. In comparison, the blending of ethylene with MeFo leads to a direct reduction of soot production. This soot formation behavior was further examined using a reaction pathway analysis based on chemical kinetic modeling. The propargyl recombination pathway plays a more important role than the HACA pathway in the formation of benzene and naphthalene, especially for ethylene-based flames. Different from MeFo, DMC can decompose to generate CH3 radicals and subsequently enhance propargyl formation. Therefore, blending DMC means a reduction of C2H2 and an increase of CH3, which results in the soot formation characteristics of DMC blends. Our kinetic analysis also implies the need for improving the decomposition kinetics of C2H4 to C2H2.

Machine learning techniques to predict the flame state, temperature and species concentrations in counter-flow diffusion flames operated with CH4/CO/H2-air mixtures

The usage of artificial intelligence (AI) is increasing in many fields of research, since complex physical problems can be ‘learned’ and reproduced by AI methods. Thus, instead of numerically solving partial differential equations, describing the physical processes in detail, appropriate AI methods can be used to decrease the calculation time significantly. In the present study, artificial neural networks (ANNs) were used to predict temperature and species concentrations in a laminar counter-flow diffusion flame. To improve the accuracy of the ANNs, a support vector machine (SVM) was used to subdivide the wide range of operating conditions (air–fuel ratio, strain rate, fuel mixture) into ‘flame’ and ‘no flame’ cases. Due to classification with the SVM the prediction performance of the ANNs was optimized and an average error to the reference values (GRI3.0) below 10 K for all cases was detected, whereas the calculation time was decreased by a factor of about 4,800 (solving the transport equations with GRI3.0).

Numerical Study of PAHs and Soot Emissions from Gasoline–Methanol, Gasoline–Ethanol, and Gasoline–n-Butanol Blend Surrogates

Soot formation is an intricate phenomenon, and soot propensity of a fuel is interwoven with the fuel composition, physical and chemical properties, and combustion environment. The present study examines the hypothesis that in addition to the chemical composition of the fuel, the sooting nature of the fuel is closely coupled with its chemical property known as octane sensitivity (S). With this motivation, the present study numerically investigates the effects of gasoline surrogate composition and its property, octane sensitivity (S), on polycyclic aromatic hydrocarbons (PAHs) and soot emissions. Four-component toluene primary reference fuel (TPRF)–alcohol blends, comprising iso-octane, n-heptane, toluene, and one of the three different alcohols- methanol, ethanol, and n-butanol, are used as gasoline surrogates. A total of 320 TPRF–alcohol mixtures, with S in the range of 1–10, are examined under laminar counterflow diffusion flame conditions. A detailed chemical mechanism coupled with a comprehensive soot model, which includes reactions for soot inception, surface growth, PAH condensation, and oxidation, is adopted. The analysis indicates that the toluene content in the fuel mixture has a prominent effect, while the alcohol content and octane sensitivity of the fuel have a weak correlation with the PAHs and soot. Thus, it is not clear if any of these three variables, namely, toluene content in the fuel, alcohol content in the fuel, and S, are individually sufficient to characterize the PAHs and soot across various blends. For this reason, a new variable (XCHO) based on the elemental composition of the fuel mixture is identified and it is shown that XCHO along with S of the fuel characterize soot emissions satisfactorily. Further, a reaction path analysis indicates that the efficacy of alcohols in reducing soot emissions follows the order: methanol > ethanol > n-butanol.

Experimental investigation of the effect of hydrogen addition on the sooting limit and structure of methane/air laminar counterflow diffusion flames

The influence of hydrogen (H2) addition on the sooting limit and structure of methane/air laminar counterflow diffusion flames has been studied. Soot limits have been obtained using the laser light scattering technique for CH4/H2 mixtures with H2 concentration varying from 0–20% by volume in the fuel mixture for different strain rates. The resulting soot limit map shows a significant increase in the fuel concentration at the soot limit with increasing amounts of H2. To understand this trend, temperature and species concentration profiles have been measured and compared under the influence of H2 addition in (a) an incipiently sooting and (b) in a sooting CH4/air flame. Flame temperatures have been measured using a thin-wire S-type thermocouple and species profiles have been measured using the gas chromatography technique (GC). The results indicate a significant decrease in the amounts of higher hydrocarbons such as benzene and naphthalene (which are considered important soot precursors) with an increase in H2 concentration for the sooting flame. To explain this decrease in the concentration of higher hydrocarbons, pathway analysis has been performed to elucidate the influence of H2 on the formation and consumption of soot forming species.

Modeling soot particles as stable radicals: a chemical kinetic study on formation and oxidation. Part I. Soot formation in ethylene laminar premixed and counterflow diffusion flames

Based on recent theoretical and experimental evidences, this work proposes a detailed discrete sectional soot model considering particles and aggregates behaving as stable radicals, paving the way for the implementation of a game-changing assumption in soot kinetic models. The concentration of large closed-shell and open-shell polycyclic aromatic hydrocarbons (PAHs) levels out with the increase of their aromatic structure and, contrary to conventional assumptions in existing soot models, their reactivity approaches that of persistent radical species. These findings were implemented in the model here proposed to describe soot formation (Part I) and oxidation (Part II). While the number of lumped pseudo-species involved is significantly reduced compared to previous discrete sectional soot models since no distinction between the kinetics of closed-shell and open-shell particles is taken into account, the physical meaning and consistency with recent evidences improves together with model performances. Rate parameters of gas-phase reactions involving resonantly stabilized radical PAHs were used as a reference for soot particle kinetics. The model was validated against 65 target ethylene laminar premixed flames and counterflow diffusion flames, with a general good agreement between experimental data and numerical simulations at different pressures (P = 1–8 atm), maximum flame temperatures (Tmax∼1500–2400 K) and soot yields (fv∼0.01 to ∼170 ppm). Parity diagrams allow to identify systematic deviations of the present model from the experiments, while the comparison between all the flames included in the database highlights the relative contribution to soot formation and growth of the main reaction classes constituting the soot model, spotting analogies and peculiarities of the two flame configurations considered. Similar kinetic patterns characterize most of the flames within the database, independently of flame configuration and sooting tendency of the specific case. Large PAH condensation dominates the formation of the first soot nuclei, while the condensation of smaller PAHs up to four aromatic rings is primarily responsible for the growth of larger soot particles and aggregates. Finally, the contribution of the different fluid dynamics in premixed and counterflow flames to particle evolution, resulting in a higher hydrogenation level of soot particles in the latter, is discussed.

Exploring the combustion chemistry of anisole in laminar counterflow diffusion-flames under oxy-fuel conditions

Biomass combustion under oxy-fuel conditions, i.e., burning biomass by using CO2/O2 mixtures as oxidizer instead of air, is a promising approach to mitigate climate change by recycling CO2 from the exhaust gas. Understanding oxy-fuel biomass combustion chemistry can further help to achieve higher efficiency and lower emissions in future design concepts. In this work, we investigated the combustion chemistry of anisole, a potential component of surrogates for biomass and volatiles, in two selected laminar counterflow diffusion flames under oxy-fuel conditions. A time-of-flight molecular-beam mass-spectrometer (ToF-MBMS) and a gas chromatograph with a mass spectrometer (GC–MS) were used to analyze the chemical compositions of the gaseous samples at various flame positions. The combined measurements allowed us to identify and quantify over 40 species, including many polycyclic aromatic hydrocarbons (PAH) and oxygenated polycyclic aromatic hydrocarbons (OPAH). Comparing the experimental results to numerical simulations using the latest kinetic model from Yuan et al. (Combust. Flame 2019, 201, 187–199), we found that some of the aromatic hydrocarbons (e.g., phenylacetylene, styrene, and ethylbenzene), PAH molecules, (e.g., naphthalene and acenaphthylene), and OPAH molecules (e.g., cresol, benzofuran, and dibenzofuran) were overpredicted by the kinetic model. The model analysis indicated that the reaction rate coefficients in PAH and OPAH chemistry may be responsible. Tentative kinetic model updates showed improvement on the predictions of benzofuran, naphthalene, and acenaphthylene. In summary, this work provides comprehensive speciation datasets from two measurement techniques, examines the latest anisole kinetic model in laminar counterflow diffusion-flames, and provides novel insights for biomass combustion chemistry under oxy-fuel conditions.

Chemical speciation and soot measurements in laminar counterflow diffusion flames of ethylene and ammonia mixtures

Ammonia is considered as one of the most promising alternative fuels due to its carbon neutrality and the existing infrastructure for its mass production and delivery. However, burning neat ammonia has the issue of poor flame stability and high NOx emissions, making co-firing ammonia with conventional fuel a more feasible approach. The present work investigated the sooting characteristics of counterflow diffusion flames of ethylene/ammonia mixtures. Experimentally, soot volume fraction (SVF) and average soot particle diameter in the neat ethylene, ammonia- and nitrogen- doped flames were non-intrusively measured. Both SVF and average soot particles diameter were found to decrease with the addition of ammonia. Flame temperature were measured with tunable diode laser absorption spectroscopy and the results suggested that the inhibiting effect of ammonia on soot formation was chemical instead of thermal. For further kinetic insights, numerical simulation with newly-constructed reaction mechanisms were performed and the results were compared against chemical speciation data from gas chromatography (GC) measurements; the results showed that ammonia doping would lead to more significant reduction of benzene concentration than nitrogen doping. Kinetic pathways of the chemical suppressing effect of ammonia addition on soot and its precursor formation were then explained based on numerical results. The major contribution of the present work can be summarized in the following aspects: 1) New comprehensive experimental data on sooting characteristics, important intermediate species concentrations of diffusion counterflow flames of ethylene/ammonia mixtures were provided for model validation; 2) One coupled mechanism with detailed hydrocarbon-nitrogen interactions was established to predict PAHs formation and soot formation; 3) Detailed chemical kinetics insight of ammonia effect on soot formation was presented in the counterflow flame.

Evaluation of the Flamelet/Progress-Variable Approach and Flamelet-Generated Manifolds Method in Laminar Counter-Flow Diffusion Flame

To evaluate characteristics of the Flamelet/Progress-Variable approach (FPV) and Flamelet-Generated Manifolds method which can consider a detailed chemical reaction mechanism, a combustion simulation was performed in a laminar counter-flow diffusion flame. While the numerical solutions of the FPV reproduced the measurements and almost completely agreed with those of the detailed chemical reaction mechanism, the numerical solutions of the FGM method overpredicted the measurements of CO mole fraction and underpredicted the ones of CO2 especially in fuel-rich region, and differed from those of the detailed chemical reaction mechanism. This is because the flamelet table of the FGM indicates the state close to chemical equilibrium and overpredicts dissociation of CO2 when the combustion reaction sufficiently progresses.

Deep-learning accelerated calculation of real-fluid properties in numerical simulation of complex flowfields

A deep-learning based approach is developed for efficient evaluation of thermophysical properties in numerical simulation of complex real-fluid flows. The work enables a significant improvement of computational efficiency by replacing direct calculation of the equation of state with a deep feedforward neural network with appropriate boundary information (DFNN-BC). The proposed method can be coupled to a flow solver in a robust manner. Depending on the numerical formulation of the flow solver, the neural network takes in either the primitive or conservative variables, including the chemical composition of the system, and calculates all relevant fluid properties for the subsequent routines in the solver. Two test problems are employed to validate the proposed methodology. The first uses a preconditioning scheme with dual-time integration for the simulation of swirl rocket injector flow dynamics under supercritical conditions. The second uses a conservative-variable based formulation for the simulation of laminar counterflow diffusion flames for cryogenic combustion. A parametric analysis is performed to optimize the numbers of hidden layers and neurons per hidden layer. The computational accuracy, efficiency, and memory requirements of the neural network are examined. The DFNN-BC model accelerates the evaluation of real-fluid properties by a factor of 2.43 and 3.7 for the two test problems, respectively, and the overall flowfield simulation by 1.5 and 2.3, respectively. In addition, the memory usage is reduced by up to five orders of magnitude in comparison with the table look-up method.

The Effects of CO2 Additional on Flame Characteristics in the CH4/N2/O2 Counterflow Diffusion Flame.

The effects (chemical, thermal, transport, and radiative) of CO2 added to the fuel side and oxidizer side on the flame temperature and the position of the flame front in a one-dimensional laminar counterflow diffusion flame of methane/N2/O2 were studied. Overall CO2 resulted in a decrease in flame temperature whether on the fuel side or on the oxidizer side, with the negative effect being more obvious on the latter side. The prominent effects of CO2 on the flame temperature were derived from its thermal properties on the fuel side and its radiative properties on the oxidizer side. The results also highlighted the differences in the four effects of CO2 on the position of the flame front on different sides. In addition, an analysis of OH and H radicals and the heat release rate of the main reactions illustrated how CO2 affects the flame temperature.

Experimental and numerical investigations on extinction strain rates in non-premixed counterflow methane and propane flames in an oxygen reduced environment

The aim of this work is to contribute to the better understanding of the oxidation process of light hydrocarbons, low calorific value gas and components of natural gas in the presence of inert gases and the influence of oxygen reduction in exhaust gas recirculation situations. The extinction behavior of laminar counterflow diffusion flames of methane (CH4), ethene (C2H4) and propane (C3H8) under nitrogen diluted condition has been investigated experimentally and numerically. In the experimental set-up, a counterflow burner was employed to investigate the extinction behavior under atmospheric conditions (1 bar and 298 K). The present study highlights the non-linear influence on the decrease in the extinction strain rate (ESR) that were found with decreasing fuel content and decreasing oxygen content. The oxygen content on the oxide side was stepwise diluted with nitrogen in order to distinctly show the possible oxygen-depleted influence in the recirculation areas. Additionally, it has been shown that the distance between the nozzles has an impact on the ESR but not on the general rising trend with the increasing fuel content in the non-premixed flames. Moreover, to clarify the influence, the flame zones were visualized using a system of high-speed OH* chemiluminescence camera. It was possible to investigate the width, intensity and position of the flame front from the radical detection. This provided a comprehensive data base for the subsequent numerical validation. In order to complement the experimental investigations, extensive uncertainty analyses were conducted and a new laser-based approach for the detection of flame extinction were demonstrated for the low-visible flames. Besides the experimental study, ESRs of CH4 and C3H8 combustion obtained from the experimental measurements were compared with those predicted by the numerical simulations. The San Diego-2014 reaction mechanism was used to represent the detailed chemical reaction, in which the predicted ESRs agree well with the experimental data was shown. Additionally, the effects of radiative heat loss, molecular transport model and chemical reactions on the ESRs were studied numerically. The present results suggest that the radiative heat loss plays a minor role on the ESRs, the molecular transport model is more significant and the ESRs are quite sensitive to several key reactions.

A study of the sooting behavior of hydro-processed renewable jet and petroleum jet fuels in a laminar counter-flow flame

In this study, the soot formation behavior of hydro-processed renewable jet and petroleum jet fuels (Jet-A1, and JP-5) was investigated using a laminar counter-flow diffusion flame. Laser-induced incandescence was utilized to measure the soot volume fraction. The soot particle morphology of the fuels was characterized using transmission electron microscopy (TEM). It was found that the highest soot volume fraction was obtained in the following order: JP-5 > Jet-A1 > HRJ. Modifications to the reactant concentrations were observed to strongly influence the soot volume fraction by as much as 81%, 60%, and 64% for HRJ, JP-5, and Jet A1, respectively, and led to an increase in the soot particle diameter by as much as 26%, 15% and 10% for HRJ, JP-5, and Jet-A1, respectively. Both petroleum jet fuels reported soot particle diameters that were double those of the HRJ fuels. The fuel with higher H/C ratio (i.e. HRJ) had the lowest soot volume fraction. The smoke point results were found to have a positive correlation with the soot volume fraction for all of the tested fuels.

Experimental Investigation on the Effect of Hydrogen Addition on the Sooting Limit and Structure of Methane/Air Laminar Counterflow Diffusion Flames

Experimental Investigation on the Effect of Hydrogen Addition on the Sooting Limit and Structure of Methane/Air Laminar Counterflow Diffusion Flames

Effects of strain rate and Lewis number on forced ignition of laminar counterflow diffusion flames

Forced ignition is one of the most fundamental and important combustion problems. It is used in advanced engines and closely related to fire safety and accidental explosion. In this study, forced ignition of laminar counterflow diffusion flames with one-step chemistry is studied through two-dimensional simulations. The initial ignition kernel is simply modelled as a hot spot of unburned gas with a specified radius and uniform high temperature inside it. We focus on the subsequent ignition kernel development, during which an expanding axisymmetric triple flame appears under certain conditions. Non-monotonic change of the flame kernel radius with time is observed near the critical ignition conditions. The effects of strain rate and Lewis number on the ignition kernel development and critical ignition conditions are assessed. For the same initial hot spot, successful ignition can be achieved at relatively low strain rate and fuel Lewis number; while the ignition kernel quenches at relatively high strain rate or fuel Lewis number. The strain rate and Lewis number are shown to have great impact on the critical hot spot radius and on the minimum ignition energy. At relatively low strain rates, the minimum ignition energy increases linearly with the strain rate. This is consistent with previous theory on flame ball in a mixing layer. Besides, the optimum ignition position and the influence of the hot spot shape are investigated. It is demonstrated that the optimum ignition position is close to the stoichiometric surface and it depends on the Lewis number and global equivalence ratio.