Counterflow Diffusion Flames Research Articles

Assessing PAHs-based soot inception models in various laminar non-premixed flame configurations

Accurate soot modeling is challenging due to the complex underlying physical and chemical processes. To improve the fidelity of the soot prediction in inverse coflow flame configurations, an empirical reactive soot inception model with radical involvement was developed in our previous work [Guo et al. Combust Flame 254 (2023) 112,853]. In this study, the model is further assessed by comparing the predictions of polycyclic aromatic hydrocarbons (PAHs) and soot parameters, including volume fraction, number density, and particle size, with measurements in various laminar diffusion flame configurations. These configurations encompass counterflow diffusion flames (CDF), normal coflow diffusion flames (NDF), and inverse coflow diffusion flames (IDF). The empirical reactive inception model is also compared to the reported irreversible and reversible inception models. The results show that the empirical reactive inception model with radical involvement most accurately capture axial soot behavior in IDF configuration, while irreversible and reversible models predict a sustained increase in soot mass, inconsistent with measurements. In CDF configurations, the empirical reactive model and the reported reversible model have better performance than the irreversible inception model, regarding the predictions of spatial distributions of soot and PAHs. Moreover, in NDF configuration, the soot peaks predicted by the reactive and reversible models are in reasonable agreement with measurements within a factor of 2–3. For the soot spatial distribution in NDF, the difference in the predictions is mainly reflected in the flame wings, which is caused by the temperature dependences of the inception rate provided by the different models.

Effects of Ammonia Addition on the Sooting Characteristics of Ethylene Counterflow Diffusion Flames with Oscillating Strain Rates

Effects of Ammonia Addition on the Sooting Characteristics of Ethylene Counterflow Diffusion Flames with Oscillating Strain Rates

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).

Enhancement and suppression of counterflow diffusion flame by HFC-125

Efficient fire extinguishing is a safety prerequisite for protecting the environment from fire damage. Understanding the effect of fire extinguishing agent on flame in complex environment is basic work for efficient fire extinguishing. We investigate the impact of HFC-125 on the extinguishment of propane-air highly stretched diffusion flames. Flame thickness is measured using the proportional method. Parameters such as dimensionless flame stand-off distance and flame stretch rate are calculated. Low concentrations of HFC-125 initially increase the flame thickness, but as the concentration increases further, the flame thickness decreases and the flame suppression effect of HFC-125 intensifies. HFC-125 reduces the amount of carbon soot generated and accelerates the transition from yellow to blue flame. Within a specific concentration range, N2 has a minimal effect on flame structural parameters compared to HFC-125. High concentration of N2 forms a high bright blue flame, while HFC-125 reacts with the flame to form a blue-violet flame. Compared to HFC-125, HFC-227ea can significantly reduce flame thickness and increase dimensionless flame stand-off distance, exhibiting a notable suppression effect. Furthermore, high concentrations of HFC-125 substantially reduce the critical flame stretch rate and provide a lower extinguishing concentration in counterflow diffusion flames than in the cup burner.

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.

Combustion studies of MMA/[formula omitted] for a hybrid rocket motor

Poly(methyl methacrylate) (PMMA) is the synthetic polymer of methyl methacrylate (MMA), which is used in a multitude of everyday applications, can also be used as a solid fuel in hybrid rockets. When used as a fuel in a combustion chamber, PMMA undergoes thermal decomposition and phase-change (solid-to-gas), where gaseous MMA (C5H8O2) is the primary product. The gaseous MMA then undergoes combustion with an oxidizer. Experimental studies of this combustion chamber have been performed in literature, and this study helps produce high-fidelity simulations, which can match experimental observations while giving access to more data in the combustion chamber. Simulations of turbulent diffusion flames are performed with gaseous MMA introduced through the combustion chamber walls. Two different diffusion models are used alongside a finite-rate chemistry model to observe the effects of differential diffusion. The rate of inflow of MMA is controlled by the temperature field in the combustion chamber, and the results from the 3D simulation are comparable to 1D counterflow diffusion flame simulations. Simulations are also performed with a tabulated chemistry model developed to improve the computational efficiency and with two different internal diameters to analyze the geometric effects. If one wishes to capture the flow field profiles and fuel regression rate, chemistry effects need to be included. If one wishes to capture the internal chemistry profiles of the combustion products, differential diffusion effects are important. The fuel regression rate and the radial chemistry profiles from the simulations are compared with the experimental results and show good agreement.

Experimental and kinetic study on aromatic formation in counterflow diffusion flames of methane and methane/ethylene mixtures

Aromatics are molecular soot precursors, yet detailed kinetic mechanisms describing their formation in flame environments are insufficient even for the simplest hydrocarbon fuel of methane. Considering the building roles of methane-related chemistry in current hierarchically-structured mechanisms, the present study is devoted to the chemistry of aromatic formation in CH4 flames. We established a series of incipiently sooting methane counterflow diffusion flames (CDF) and characterized their thermochemical structures including the scalar fields of temperature, concentrations of major species, C2–C4 minor intermediates, benzene, large aromatic species, and soot volume fraction. The measurement techniques include tunable diode laser absorption spectroscopy (TDLAS), gas chromatography–mass spectrometer (GC–MS) and laser induced incandescence (LII). The experimental dataset was then used to assess various recently proposed kinetic mechanisms. The results showed that the literature mechanisms tested would significantly overpredict benzene formation in CH4 CDFs (up to a factor of 10). In addition, none of the tested mechanisms could capture the experimentally observed suppressing effects of CH4 addition on the formation of aromatic species in C2H4 CDFs. Potential reactions responsible for the model failures are discussed. The main contributions of this study are to: 1) identify the limitations of existing aromatic mechanisms when applied to methane CDFs; 2) clarify potential reactions/pathways related to model deficiencies; 3) provide novel experimental dataset covering both gas-phase speciation and soot for methane CDFs. It is our hope that the present data and analysis would deepen our understanding on benzene formation chemistry, and contribute to the refinement of aromatic formation predictive models.

Soot nucleation in diffusion flames and the role of aromatic hydrocarbons

We perform spatially resolved measurements of light scattering of soot in atmospheric pressure counterflow diffusion flames to complement previously reported data on soot pyrometry, temperature and gaseous species up to three-ring polycyclic aromatic hydrocarbons (PAHs). We compare two flames: a baseline ethylene flame and a toluene-seeded flame in which an aliquot of ethylene in the feed stream is replaced with 3500 ppm of pre-vaporized toluene. The goal is twofold: directly adding an aromatic fuel to bypass the formation of the first aromatic ring, widely regarded as the main bottleneck to soot formation from aliphatic fuels, and assessing the impact of a common component of surrogates of transportation fuels on soot formation. The composition of the fuel and oxidizer streams are adjusted to ensure invariance of the temperature-time history, thereby decoupling the chemical effects of the fuel substitution from other factors. The doping approach enables the comparison of very similar flames with respect to combustion products, radicals and critical precursors to aromatic formation (C2–C5 species), in addition to the temperature-time history. Doping with toluene boosts the aromatic content and soot volume fraction relative to the baseline ethylene flame, but, surprisingly, the soot number density and nucleation rate are affected modestly. As a result, the observed difference in volume fraction in the toluene-doped flame is reflective of larger initial particles at the onset of soot nucleation. The nucleation rate when soot first appears near the flame is of the same order as the dimerization rate of single-ring aromatics, in contrast with the expectation that the dimerization of larger PAHs initiates the process. Even though in and of itself nucleation contributes modestly to the overall soot loading, nucleation conditions the overall soot loading by affecting the size of the initial particle, which ultimately affects subsequent growth.

Effects of H2 addition on soot formation in counterflow diffusion flames of propane: A comparative analysis with He and N2 addition

Co-firing hydrogen (H2) with hydrocarbon fuels is widely seen as an effective pathway to reduce soot emissions. Nevertheless, the soot-reducing potential of H2 can be strongly dependent on the type of the co-fired hydrocarbon and the combustion conditions. In extreme cases H2 enrichment could even promote soot production in hydrocarbon flames, thus highlighting the necessity of an in-depth understanding on the roles of H2 addition. The present study aims to clarify the effects of H2 addition on soot formation in non-premixed flames of propane. The effects of He and N2 addition are included for a cross-comparison to identify the different effects of H2 (i.e., direct chemical roles, dilution or transport effects associated with fuel stream's physical properties). The technique of laser-induced incandescence (LII) was used for soot volume fraction measurement, and accompanying soot modeling analysis was performed for additional kinetic insights into the experimental results. The measurements and modeling results consistently show that H2 is a more effective soot inhibitor than the chemical inert additive of He. Considering the comparative transport properties between H2 and He, we concluded that H2 has additional chemically-inhibiting effects on soot formation; kinetic analysis indicates that the chemical roles of H2 mainly occur in the stages of soot precursor formation and soot nucleation process. Another interesting finding is that while both He and N2 are chemically inert, He has notably stronger soot-reducing ability than N2, suggesting the favorable physical properties of He (i.e., the high diffusivity) in reducing soot formation. The major contributions of this work include: 1) experimentally identify the different effects of H2 (chemical, dilution and physical) on soot formation in counterflow flames of propane, 2) unravel the chemical roles of H2 in the different sooting stages including soot precursors formation, soot inception and surface growth process, 3) provide new datasets for mechanism validation and refinement related to sooting characteristics of H2/hydrocarbon mixture fuels.

Investigating local growth conditions in the flame synthesis of molybdenum-oxide nanoplates from molybdenum substrates

Molybdenum-oxide nanoplates are grown directly from the surface of molybdenum substrates using a catalyst-free flame synthesis technique. To investigate the effects of oxidizing routes, e.g., from gas-phase O2, H2O, and CO2 as reactants, on the growth and morphology of the molybdenum-oxide nanostructures, quasi-1D counterflow diffusion flames are strategically utilized and probed at specific locations within their flame structures. Moreover, two separate flame structures are compared using different fuels (i.e., methane and hydrogen) and diluted with nitrogen to have near-identical temperature profiles. For example, the H2O oxidizing route is scrutinized by assessing growth at the same elevated temperature locations at the fuel sides of the reaction zones of a CH4 flame (comprising H2O and CO2 as synthesis reactants) and an H2 flame (comprising only H2O as synthesis reactant). The temperature effect on the resultant morphologies are examined at 1280 K, 1500 K, and 1720 K. Single-crystalline MoO2 nanoplates (of thicknesses of 60-80 nm, widths of 200-450 nm, and lengths of 1-2 µm) that are regularly distributed with high-surface-area-exposed packing are obtained at 1720 K on the air side of the CH4 flame (with O2, H2O, and CO2 as synthesis reactants) for 10 min growth durations. Based on the results and Gibbs free energy calculations, the CO2 route is deduced to seed the growth of the nanoplates at the nucleation stage heterogeneously, with subsequent condensation-based growth from volatile intermediate species, such as MoO2(OH)2, playing an important role. With such studies revealing fundamental growth mechanisms, scale-up of the process with different flame configurations is afforded.

On the soot formation characteristics of an ammonia-ethylene counterflow diffusion flame based on the orthogonal decoupling method

Ammonia is regarded to be one of the most competitive fuels for achieving carbon neutral due to its carbon-free properties. In this work, the effects of adding ammonia (NH3) to the fuel stream of an ethylene counterflow diffusion flame on the soot formation characteristics were numerically investigated. The orthogonal decoupling method was used to completely separate the dilution, chemical and thermal effects of the added NH3 by introducing two different virtual NH3. The influence of these effects and their interactions were explored by the range analysis, respectively. The results show that the soot volume fraction (SVF) of the flame decreases under the dilution, chemical and thermal effects of NH3, with the dominated effect being the chemical effect, followed by the dilution effect, and the least is the thermal effect. Besides, the chemical-thermal interaction also has an important impact on the SVF. The increase of SVF is dominated by the HACA surface growth, while the decrease of that is dominated by the OH oxidation. The HACA surface growth reaction is inhibited under the three effects of NH3 addition mainly due to a lower concentrations of C2H2 and H radical. The number density of soot increases under the chemical effect, while decreases under the dilution effect. The mole fractions of polycyclic aromatic hydrocarbons (PAHs) reduces under these three effects, leading to a lower inception rate, which in turn has a significant effect on the number density of soot.

Numerical analysis of a nanosecond repetitively pulsed plasma-assisted counterflow diffusion flame

A computationally efficient model is proposed to analyze plasma-assisted combustion using nanosecond repetitive pulsed (NRP) plasmas. The NRP plasma discharge is placed in the oxidizer stream of a counter-flow diffusion flame. The effect of changing the flow rate and the pulse repetition frequency (PRF) of a continuous NRP plasma discharge on the temperature and species profiles of a counter-flow diffusion flame is investigated numerically. The results confirm that oxygen atom and nitrogen vibrational states are the most important species to enhance combustion. The results also show that kinetic effects are much more significant for higher PRF and lower pulse voltage. In addition, when steady plasma profiles are used instead of unsteady plasma profiles, the extinction strain rates increase by 25.8%, 21.1%, and 10.8% for PRF equal to 1, 2, and 4 kHz, respectively.

Numerical study of the O2/CO2, O2/CO2/N2, and O2/N2-syngas MILD combustion: Effects of oxidant temperature, O2 mole fraction, and fuel blends

Moderate or Intense Low-oxygen Dilution (MILD) combustion of a syngas fuel under air-fuel, oxygen-enhanced, and oxy-fuel condition are numerically studied with using counterflow diffusion flame. Fuel composition, temperature of oxidant (Tox), and oxygen mole fraction (XO2) are selected as the main parameters. Fake species (FCO2) with the same CO2 physical properties is used for separation the physical and chemical effects of replacing CO2 with N2. According to the results, under the high preheating temperatures, the chemical effect of changing the oxidant composition from N2 to CO2 is the main reason of the changes in flame structure, ignition delay time (IDT) and heat release rate (HRR) while physical differences play a more prominent role in the low preheating temperature MILD combustion. In all XO2, the physical and chemical effects of replacing CO2 with N2 have almost the same role on the maximum flame temperature. The results of IDT expressed that chemical discrepancies of CO2 and N2 play a key role on IDT enhancement by increasing CO2 in the oxidant composition. The sensitivity analysis of CH2O for variations of Tox and XO2 shows that reactions R54, R56, R58, and R101 are the main responsible of lower HRR and higher IDT by moving from air-syngas to oxy-fuel MILD combustion.

Oxygen effects on soot formation in H2/n-heptane counterflow flames

Blending of hydrocarbon fuels with hydrogen is a practically feasible way to rapidly integrate a carbon neutral energy vector in the transport sector and to effectively reduce harmful particulate matter emissions. This work investigates the effects of hydrogen (H2) addition on soot formation in n-heptane in low strain rate counterflow diffusion flames with different oxygen mole fractions (XO2=0.3 ∼ 0.45). Flame temperature, soot volume fraction (fv), monocyclic (MAHs), and polycyclic aromatic hydrocarbons (PAHs) are measured by using OH-2C-PLIF/thermocouple, LII/LE, and PAH-LIF methods, respectively. Measurements of n-heptane flames with addition of helium (He), as an inert gas with a specific heat capacity similar to that of H2, are also carried out and compared. Numerical simulations are then performed using a soot discrete sectional model coupled with detailed gas-phase kinetics to interpret the data obtained. Temperature and soot volume fraction profiles are better predicted by 2D simulations, which account for the non-negligible impact of radial diffusion and buoyancy under low strain rate conditions. Then, it is shown that both the fv and PAH yields almost linearly decrease by increasing H2 or helium (He) addition. The reduction of PAHs and soot is larger in n-heptane/H2 flames due to H2 chemical effects, absent in the n-heptane/He flames, hampering the formation of peri‑condensed structures. The effect of O2 concentration in the oxidizer stream is also studied. Experimental and simulation results show that PAHs and soot increase with XO2 for all flames. Finally, the effects of H2/He addition varying XO2 on soot formation are investigated. With the increase of XO2, the difference in the peak flame temperatures of n-heptane/H2 and n-heptane/He flames decreases. Consequently, compared to the neat n-heptane flame, the reduction of peak fv becomes more and more pronounced in the case of hydrogen addition rather than the helium addition since the H2 soot inhibiting chemical effect prevails over the H2 temperature effect which instead promotes soot formation.

Combining flamelet-generated manifold and machine learning models in simulation of a non-premixed diffusion flame

Flamelet Generated Manifold (FGM) is an example of a chemistry tabulation or a flamelet method that is under attention because of its accuracy and speed in predicting combustion characteristics. However, the main problem in applying the model is a large amount of memory required. One way to solve this problem is to apply machine learning (ML) to replace the stored tabulated data. Four different machine learning methods, including two Artificial Neural Networks (ANNs), a Random Forest (RF), and a Gradient Boosted Trees (GBT), are trained, validated, and compared in terms of various performance measures. The progress variable source term and transport properties are replaced with the ML models. Particular attention was paid to the progress variable source term due to its high gradient and wide range of its value in the control variables space. Data preprocessing is shown to play an essential role in improving the performance of the models. Two ensemble models, namely RF and GBT, exhibit high training efficiency and acceptable accuracy. On the other hand, the ANN models have lower training errors and take longer to train. The four models are then combined with a one-dimensional combustion code to simulate a counterflow non-premixed diffusion flame in engine-relevant conditions. The predictions of the ML-FGM models are compared with detailed chemical simulations and the original FGM model for key combustion properties and representative species profiles.

From molecular to sub-μm scale: The interplay of precursor concentrations, primary particle size, and carbon nanostructure during soot formation in counter-flow diffusion flames

The focus of this work is on counter-flow diffusion flames of C2H4/air near the sooting limit (fV < 500 ppb) to shed light on the transition of precursor molecules to primary soot particles and their nanostructure. The applied experimental techniques cover non-intrusive methods based on laser-induced incandescence (LII) for determination of soot volume fractions, primary particle sizes, and ratios of refractive indices for absorption at different wavelengths. The methods are complemented by intrusive methods such as sampling with microprobes and analysis of gas composition by gas chromatography and mass spectrometry, particle sizing by differential mobility analysis, and determining carbon nanostructures of particles by high-resolution transmission electron microscopy (HRTEM). For numerical simulation, the well-known ABF-model is applied.The experimentally determined structures of the counter-flow flames could be well reproduced within the experimental uncertainties by the ABF-model for varying fuel mass fractions and strain rates. The numerical simulations identify the different processes during particle formation and the main variation can be attributed to variation of surface growth rates when applying strain rates and fuel mass fractions leading to higher soot volume fractions. Particle size distributions detected in the fuel flow near and below the stagnation plane appear bimodal with distinct maxima at particles sizes about 5 nm and 20 nm. The nanostructure of the primary particles exhibits basic structural units (BSUs), that increase up to the size of 7 Å to 10 Å at increasing soot volume fractions from about 100 ppb to 300 ppb. This coincides with the increase of the mole fractions of the detectable soot precursors with increasing soot volume fraction. The ratios of refractive index functions for absorption E(m,λ532)/E(m,λ1064) and E(m,λ266)/E(m,λ1064) decrease approximately linearly with increasing soot volume fractions. This is corroborated by the analyses of HRTEM-images that clearly bring about increasing sized BSUs with increasing soot volume fractions causing a shift of the absorption to larger wavelengths.

A Numerical Study of CO2 Decoupled Effects on Extinction Limit and Flame Microstructure in CH4/Air Counterflow Diffusion Flame with Various Pressures

Extinction limits and flame microstructures of CH4 versus air (N2/O2) counterflow diffusion flames with additional CO2 to oxidizer stream under various pressure conditions were numerically investigated by a proposed decoupled strategy. The chemical and thermal effects of CO2 addition were isolated from the synergistic effect on extinction limits and flame microstructures. The results show that the extinction limits were decreased due to the CO2 addition and extended caused by the enhanced pressure, respectively. The changes of flame microstructures from far away from extinction to near extinction mainly are reflected in a narrower combustion zone and thinner flame thickness, while there is no visible displacement of the flame front location. Quantitative analyses manifest the thermal effect of CO2 addition on extinction limit is greater than chemical effect, and elevated pressure can enhance both thermal and chemical effects. An interesting phenomenon occurred between the thermal and chemical effects resulting from increasing CO2 mole fraction in an oxidizer on extinction limit; the former monotonically increase along with CO2 mole fraction, while the latter represents a kind of approximate quadratic-parabola tend along with CO2 mole fraction. The distributions of the production and consumption rate of hydroxyl (OH) radicals were presented to clarify the contribution of the kinetic term in the asymptotic solution to the extinction limits with increasing CO2 molar fraction and various pressures.

Predictions of NOx and SOx in MILD regime based on thermal conversion of solid sewage sludge surrogates

Thermal treatment of sewage sludge using a combination of gasification and combustion of the producer gas is an attractive option to produce sterilized high-notorious bio-char and recover energy from the feedstock. Given the high temperature of the producer gas after gasification, employing the MILD (Moderate or Intense Low Oxygen Dilution) combustion regime in the second stage might benefit the heat balance of the process and result in low emissions. The current work employs multi-component surrogates derived from solid fuel samples of various ultimate elemental compositions and moisture content, a stochastic reactor model for gasification, and a 1D counterflow diffusion flame configuration for combustion with a dedicated, detailed chemical mechanism. Using an improved methodology for emission measurement, we analyzed the combustion emissions of the wet producer gas from gasification, which includes fuel-NO precursors, sulfur components, and tar species up to a size of C11. The analysis confirms that thermal and fuel-NO emissions are reduced in the MILD regime. While high moisture content in the feedstock has been shown to enhance the formation of the pollutant via identified chemical pathways, different representations of fuel samples (ultimate composition) are found to have a significantly more considerable impact on the overall SOx and NOx emissions. The more sulfur is contained in the ultimate composition of the solid surrogate fuel, the more SOX is formed in the combustion stage. This direct relation does not hold for NO. The NO emissions in MILD combustion are the lowest in the performed simulations for the highest amount of fuel-bonded nitrogen. This is caused by the key role of the fuel-NO precursor ammonia (NH3) and its radicals that act in NO formation but also are a driving force in NO reduction. Given this key role of specific fuel-NO precursors, it is further highlighted that there is a significant difference in predicting NO, depending on the complexity of the fuel-NO precursors in the boundary condition of the flame calculation.

Effects of inlet flow non-uniformities on thermochemical structures and quasi-one-dimensional simulation of sooting counterflow diffusion flames

Counterflow flames are routinely used for investigating fundamental flame and fuel properties such as laminar flame speeds, autoignition temperature, extinction strain rate, and chemistries of soot formation. The primary merit of counterflow flame is that the essentially two-dimensional configuration can be mathematically treated as a one-dimensional problem with certain assumptions made; this dimensional reduction is much beneficial for computational costs, which are critical for the investigation of complex chemistries such as those of soot formation. In this work, we performed a comprehensive investigation on the performance of the 1D modeling by comparing the results with experimental measurements and the more rigorous 2D models. We focused on the effects of inlet flow uniformities, which are frequencies assumed in the 1D model but challenging to realize in experiments. Parametric studies on the effects of nozzle flow rates, nozzle separation distances, and curtain flow rates on inlet flow uniformities and the 1D modeling were performed. The results demonstrated the importance to specify actual velocity boundary conditions, either obtained from experiments or from two-dimensional modeling to the 1D model. An additional novel contribution of this work is a quantitative presentation of the fact that the presence of the curtain flow would exert a notable influence on the core counterflow by modifying the radial distribution of the nozzle exit velocity although the effects can be accounted for by using the correct velocity boundaries in the quasi-1D model. This work provides recommendation for various geometry and operational parameters of the counterflow flame to facilitate researchers to select proper burner configuration and flow conditions that are amiable for accurate 1D modeling.

In-situ temperature and major species measurements of sooting flames based on short-gated spontaneous Raman scattering

Spontaneous Raman scattering is a conventional in-situ laser-diagnostic method that has been widely used for measurements of temperature and major species. However, utilization of Raman scattering in sooting flames suffers from strong interference including laser-induced fluorescence, laser-induced incandescence, and flame luminosity, which has been a challenge for a long time. This work introduces an easy-to-implement and calibration-free Raman scattering thermometry in sooting flames based on a 355-nm nanosecond-pulsed laser beam. Several strategies were utilized to increase the signal-to-noise ratio and suppress the interference: (1) nanosecond intensified CCD gate width; (2) optimized intensified CCD gate delay; (3) specially designed focused laser beam; (4) ultraviolet polarizer filter. The temperature was obtained by fitting the spectral profile of Stokes-Raman scattering of N2 molecules without any calibrations. Based on the measured temperature, the mole fraction of major species can be evaluated. This method was applied to measure the temperature and major species profiles in a steady ethylene–air counterflow diffusion flame with a spatial resolution of 1.2 mm × 10.8 mm × 0.13 mm. The experimental results agree well with the simulation results in both sooting and non-sooting regions, demonstrating the feasibility of this method for quantitative diagnostics of temperature and major species in multiphase reacting flows.