Flame Structure Studies Research Articles

Stability study of flame structure using photodiodes in surface flame burners

Radiation spectroscopy can be effective in identifying pollutants and species. By examining a photodiode, the frequency peaks obtained from the analysis, and the range in which the cell flame is converted to a surface flame, we obtained the value ratio of the cell flame to the surface flame in the range of 0.7–0.74 in different powers. The frequency peak in this range decreases from the maximum value of 9.9823 Hz to its minimum value of 9.058 Hz in different powers. By analyzing the temperature compared to the frequency peak, we found that in the range of equivalence ratio 0.7–0.75, the frequency peak decreases from 9.5 to 9.9 Hz to 8.7–9 Hz. The temperature has an increasing behavior, and in the equivalence ratio, the temperature is in the range of 1400–1500 °C, i.e., at its maximum value. We observed the cell formation process and its conversion to surface flame by flame detection. The flame height in the cellular and superficial regions is in the range of 0.65–0.85, which is the minimum flame height of 3–10 mm, and NO and CO were examined in the ratio of different equations and compared with temperature. In the ratio equivalence ratio of 0.77–0.81 in the temperature range of 1500 °C (maximum), the value of NO is about 16 ppm (maximum), and the value of CO is about 2 ppm (minimum). That is, when the temperature is at its maximum, it becomes CO minimum and NO maximum. This can be used for different applications such as similar and industrial burners.

Study of methanol spray flame structure and combustion stability mechanisms by optical phenomenology and chemical kinetics

Methanol is a very promising clean alternative fuel, and the study on pure methanol spray flames has not been found before to authors' knowledge. In this study, the flame structure, soot formation, and combustion stability of spray flame for methanol and conventional fuels were investigated at the ambient pressure of 4 MPa and the ambient temperature of 950 K by the means of optical phenomenology, theoretical analysis and chemical kinetic simulations. Optical images reveal that the methanol spray auto-ignition location and the established steady lifted flame is away from the injector nozzle compared to that of isooctane and n-heptane. The results of the RGB two-color method show that the KL factor in the methanol spray flame is only 0.04–0.05, which is one twentieth of n-heptane and isooctane. The combustion stability of methanol increases with increasing injection mass. Methanol spray flame is the most unstable which is 13.33% and 151.58% less than that of the spray flame of isooctane and n-heptane respectively. In chemical kinetic simulations, the appropriate region for methanol high temperature reactions is outside the relative dense region. The residence time of the rich mixture is short and may not come into contact with the supporting peripheral hot product pool, which leads to poor combustion stability.

Mixing and flame structure study of inverse swirl diffusion flames

Swirling flames appear in many practical applications to enhance flame stabilization. However, relatively few studies have investigated the inverse swirl diffusion flames (ISDFs) in detail. In ISDFs, a central swirling air jet is surrounded by an annular methane fuel jet. In the first part of this work, the stability map of ISDFs is studied. This map shows two distinctive stability regions: a swirling-jet-like region at low swirl and a compact swirling flame region at high swirl intensity. Regardless of the bulk air jet velocity (U), these two regions are separated by an optimum swirl intensity -leading to the highest flame stability. Then across these two regions, the development of the fuel–air mixing and the flame-flow interaction were investigated via acetone-PLIF and simultaneous PIV/OH-PLIF measurements. At the same U, increasing the swirl intensity leads to different mixing patterns. An annular-rich jet exists in the swirling-jet-like region versus a compact short mixing field in the compact flame stability region. Moreover, the development of the fuel–air mixing field downstream of the burner exit impacts the flame-flow interaction. The fast and early fuel–air mixing enhances the mutual interaction between the lower edge of the flame and the wrinkled flame, and the spots of the high heat released from the downstream regions - hence a highly stable flame is obtained. The lower flame edge is seen to track the instantaneous lower stagnation point of the swirling flow, while the increased heat release spots/flame wrinkling track the shear layer vortices. Beyond the optimum swirl intensity, increasing the swirl intensity retarding the fuel–air mixing and weakens the mixture ignitability, straining the flame in the vicinity of the flame edge or the vortices of the shear layer. These results suggest that suitable modifications of the mixing field close to the burner nozzle of ISDFs may increase flame stability.

Experimental and numerical study of flame structure and emissions in a micro gas turbine combustor

Abstract Experimental and numerical study of flame and emission characteristics in a tubular micro gas turbine combustor is reported. Micro gas turbines are used for distributed power (DP) generation using alternative fuels in rural areas. The combustion and emission characteristics from the combustor have to be studied for proper design using different fuel types. In this study methane, representing fossil natural gas, and biogas, a renewable fuel that is a mixture of methane and carbon-dioxide, are used. Primary air flow (with swirl component) and secondary aeration have been varied. Experiments have been conducted to measure the exit temperatures. Turbulent reactive flow model is used to simulate the methane and biogas flames. Numerical results are validated against the experimental data. Parametric studies to reveal the effects of primary flow, secondary flow and swirl have been conducted and results are systematically presented. An analysis of nitric-oxides emission for different fuels and operating conditions has been presented subsequently.

Quantum cascade laser spectroscopy-based high-sensitive temperature measurement technology

The development of temperature sensors with ultra-fast response, high sensitivity, and wide range of temperature is of important academic value for the study of flame structure and combustion characteristics under extreme conditions. Recently, tunable diode laser absorption spectroscopy (TDLAS) is developed rapidly as an important means of combustion diagnostics due to its outstanding characteristics, such as non-invasiveness, rapid response, and high realizability. The absorption lines of TDLAS are extended to the mid-infrared region. Two spectral lines of H2O at center wavelength of 2136.14 and 1874.31 cm − 1 are used as candidate pair, which provides a wide measuring range (800 to 4000 K) and high-temperature sensitivity (4.55 at 1000 K) sensor for temperature measurement. We used two quantum cascade lasers with wavelength of 4.68 and 5.34 μm to scan the selected line pair of H2O rapidly in the combustion environment and obtained the temperature information of the combustion field. Our work provides a good solution for the temperature measurement of extreme combustion environment with ultra-fast response, high sensitivity, and wide temperature range.

Topological imaging of turbulent premixed, prevaporized liquid fuel jet flames using CH (C-X) band PLIF

New imaging capabilities for topology in prevaporized, liquid-fuel flames are presented with the application of CH-radical planar laser-induced fluorescence (PLIF) using the C2Σ+-X2Π (v′=0, v″=0) band. Imaging of turbulent flame structure for ethanol, n-heptane, n-dodecane, and kerosene (JP-8) fuels has been conducted in jet flames using a piloted McKenna burner with a central jet tube. This work presents the first application of CH PLIF for the study of full flame structure and curvature effects with prevaporized liquid fuels, particularly n-dodecane and kerosene. Liquid fuel structure is also compared to results with methane and ethylene. Results show that good signal-to-noise ratio (SNR) is achieved for all fuels, and the method is feasible in the presence of a dilute droplet field, despite the resonant nature of the technique. Reaction-zone structures clearly depict evidence of nonequidiffusive (Le ≠ 1) effects: strong variation in CH fluorescence is observed in regions of high curvature. Simultaneous excitation of the OH radical is also possible with prevaporized liquid fuels, for combined imaging of the reaction and heat-release zones. In kerosene flames, with aromatic species, fuel fluorescence may also be simultaneously captured, in addition to that from CH and OH, providing information on vapor localization.

On the use of PIV, LII, PAH-PLIF and OH-PLIF for the study of soot formation and flame structure in a swirl stratified premixed ethylene/air flame

Soot formation and oxidation are investigated in swirl stratified premixed ethylene/air flames at atmospheric pressure. The effects of both swirl and stratification are studied to understand the relationship between the flame structure, soot precursors and soot. The topology of the flame is obtained with particle image velocimetry (PIV) and planar laser-induced fluorescence (PLIF) on hydroxyl radical (OH). The production of polycyclic aromatic hydrocarbons (PAHs) is investigated using PLIF by mainly probing the aromatic compounds with two benzene rings (i.e. naphthalene) that are known to actively participate in soot nucleation and growth. Soot production is investigated using laser-induced incandescence (LII), giving quantitative data on the soot volume fraction. Extensive information on the flame structure and the mechanisms of formation/consumption of soot is gathered based on the coupling of these laser diagnostics. An image analysis of velocity, OH, PAHs and soot distributions enables us to propose a scenario that describes the link between the inception, growth, aggregation and oxidation processes. In particular, the data reveal the presence of distinct zones for these processes: a thermal decomposition region in which PAHs contributes to nascent soot formation, organised as filaments along the interface between the PAH region and the inner recirculation zone (IRZ); a mixing region controlled by large moving structures that favour the growth and aggregation of nascent soot into mature soot; and an oxidation region leading to the fast consumption of soot particles. These processes are impacted to varying extents by the intensities of swirl and stratification.

Interpreting diffusion flame structure by simultaneous mixture fraction and temperature measurements using optical and acoustic signals from laser-induced plasmas

The measurement of mixture fraction and temperature is of great importance in non-premixed flame structure studies and combustion optimization. However, obtaining profiles of mixture fraction and temperature simultaneously for a wide range of fuels and compositions has proven to be challenging, especially when suitable spatial and temporal resolutions are desired. In this study, the acoustic and optical signals from laser induced plasmas were simultaneously used for the first time to obtain acoustic-based laser-induced breakdown thermometry (LIBT) combined with the laser induced breakdown spectroscopy (LIBS). The system was first calibrated in an ethylene-air premixed flame. Then the atomic-ratio and temperature distributions along the centerline of an ethylene counterflow diffusion flame were measured. The strong compositional and temperature gradients in diffusion flames represents a potential challenge, but simultaneous measurements were successfully performed within a 1.5 mm wide physical region, where the equivalence ratio ranged from 0.5 to 12 and temperature ranged from 1100 K to 2000 K. The elemental mass fractions and mixture fraction distribution were inferred based on measured atomic ratio distributions. The preferential diffusion of H relative to C was directly observed by the measurement of C/H ratio. Lastly, the physics behind the LIBT technique is discussed and analyzed. This work demonstrates that the combination of LIBT and LIBS holds promise as a simple tool to measure the mixture fraction and temperature in a broader range of combustion conditions; and facilitates a better understanding of flame structure.

Numerical investigation of non-premixed and premixed rotational tubular flame: a study of flame structure and instability

Tubular flames are considered due to their advantages in the geometry of the flame. The major importance of tubular flame which makes it different from other flames is its uniform temperature distribution. Therefore, it reduces the possibility of the formation of thermal fluctuations and hot spots along the furnaces. In this paper, high-speed, non-premixed and premixed tubular flames are numerically investigated using computational fluid dynamics under various operational conditions. Methane/air and CO2-diluted methane/oxygen combustions are considered in both non-premixed and premixed modes. $$k - \omega \, SST$$ model, eddy dissipation concept combustion model, and P1 radiation model have been used as numerical models. Numerical results are validated against available experimental measurements in the non-premixed tubular flame using DRM22 kinetic mechanism for methane/air combustions and GRI-Mech 3.0 for methane/oxygen mixtures. The structure of the tubular flame in the combustion chamber and stability limits of tubular flame in terms of operating conditions have been studied in the present paper. Results show that premixed tubular flames establish more uniform radial temperature distribution and wider stable flame operating conditions. In addition, diluted methane/oxygen tubular flames have been shown broader stable condition limits than methane/air flames.

Combustion of ethyl acetate: the experimental study of flame structure and validation of chemical kinetic mechanisms

Mole fraction profiles for reactants, major products, and main combustion intermediates were measured in near-stoichiometric and fuel-rich premixed burner-stabilized flames of ethyl acetate (AcOEt) under atmospheric pressure. Three recent chemical kinetic mechanisms for the AcOEt combustion were validated against the data reported. Their different predictive capabilities were explained using the results of kinetic analysis.

Effect of CO content on laminar burning velocities of syngas-air premixed flames at elevated temperatures

Dynamic composition changes in the syngas is a challenge for its combustion at elevated temperatures and pressures without accurate knowledge of flame properties. The present work discusses the simultaneous effect of change in CO mole fraction (15, 20, 25% by volume) and N2 mole fraction (55, 50, 45% by volume) on the burning velocities of syngas-air mixtures at elevated temperatures. The laminar burning velocities are measured using an externally heated diverging channel method and numerical simulations using detailed reaction chemistry. The laminar burning velocity is observed to increase with CO. Various experiments at elevated temperatures showed the burning velocities increase with an increase in the unburnt mixture temperature. The comparison of both the burning velocities and temperature exponents for various equivalence ratios with numerical predictions using two kinetic schemes (GRI-Mech 3.0 and Davis) show that the predictions of GRI-Mech 3.0 closely match with the experimental results. Flame structure studies helped reveal that more than 65% of H2 diffuses into the reaction zone while CO remains unburnt. High reactivity and molecular diffusion of H2 molecules into flame zone drives this initial decomposition leading to the formation of various active radicals. The increase in reaction rates between OH and CO (R99 OH + CO = H + CO2) for the cases of higher CO content corresponds to a faster CO oxidation. The rate of production and depletion of H radical speeds up with a decrease in N2 mole fraction in the compositions. The increase in H and OH concentration promotes the total reaction rate of the mixture. The increase in the burning velocity with an increase in CO content can be attributed to this mechanism. However, the reduction in the N2 (dilution) molecules for these compositions also increases the reaction rate, thereby resulting in increased laminar burning velocities of syngas-air mixtures.

A computational study of soot formation and flame structure of coflow laminar methane/air diffusion flames under microgravity and normal gravity

A numerical study is conducted of methane–air coflow diffusion flames at microgravity (μg) and normal gravity (1g), and comparisons are made with experimental data in the literature. The model employed uses a detailed gas phase chemical kinetic mechanism that includes PAH formation and growth, and is coupled to a sectional soot particle dynamics model. The model is able to accurately predict the trends observed experimentally with reduction of gravity without any tuning of the model for different flames. The microgravity sooting flames were found to have lower temperatures and higher volume fraction than their normal gravity counterparts. In the absence of gravity, the flame radii increase due to elimination of buoyance forces and reduction of flow velocity, which is consistent with experimental observations. Soot formation along the wings is seen to be surface growth dominated, while PAH condensation plays a more major role on centreline soot formation. Surface growth and PAH growth increase in microgravity primarily due to increases in the residence time inside the flame. The rate of increase of surface growth is more significant compared to PAH growth, which causes soot distribution to shift from the centreline of the flame to the wings in microgravity.

A direct numerical simulation study of flame structure and stabilization of an experimental high Ka CH4/air premixed jet flame

In the present work, a direct numerical simulation (DNS) of an experimental high Karlovitz number (Ka) CH4/air piloted premixed flame was analyzed to study the inner structure and the stabilization mechanism of the turbulent flame. A reduced chemical mechanism for premixed CH4/air combustion with NOx based on GRI-Mech3.0 was used, including 268 elementary reactions and 28 transported species. The evolution of the stretch factor, I0, indicates that the burning rate per unit flame surface area is considerably reduced in the near field and exhibits a minimum at x/D = 8. Downstream, the burning rate gradually increases. The stretch factor is different between different species, suggesting the quenching of some reactions but not others. Comparison between the turbulent flame and strained laminar flames indicates that certain aspects of the mean flame structure can be represented surprisingly well by flamelets if changes in boundary conditions are accounted for and the strain rate of the mean flow is employed; however, the thickening of the flame due to turbulence is not captured. The spatial development of displacement speeds is studied at higher Ka than previous DNS. In contrast to almost all previous studies, the mean displacement speed conditioned on the flame front is negative in the near field, and the dominant contribution to the displacement speed is normal diffusion with the reaction contribution being secondary. Further downstream, reaction overtakes normal diffusion, contributing to a positive displacement speed. The negative displacement speed in the near field implies that the flame front situates itself in the pilot region where the inner structure of the turbulent flame is affected significantly, and the flame stabilizes in balance with the inward flow. Notably, in the upstream region of the turbulent flame, the main reaction contributing to the production of OH, H+O2⇔O+OH (R35), is weak. Moreover, oxidation reactions, H2+OH⇔H+H2O (R79) and CO+OH⇔CO2+H (R94), are influenced by H2O and CO2 from the pilot and are completely quenched. Hence, the entire radical pool of OH, H and O is affected. However, the fuel consumption layer remains comparably active and generates heat, mainly via the reaction CH4+OH⇔CH3+H2O (R93).

On the flame stability and laminar burning speeds of syngas/O2/He premixed flame

Fundamental properties such as flame structure and laminar burning speed of syngas/O2/He premixed flames have been investigated. Synthetic gas, also known as syngas, is a mixture of hydrogen and carbon monoxide that has been used in the present study. Three different mole fractions of 5%, 10% and 25% of hydrogen in the syngas were used in this research. Experiments were done in a cylindrical and a spherical chambers. Flame structure studies were made in the cylindrical vessel. The cylindrical vessel was coupled with a Z-shape schlieren system, equipped with a high speed CMOS camera, which has the capability of capturing pictures up to 40,000 frames per second, in order to study flame structure and instability. Hydrodynamic and thermo-diffusive instabilities on the flame front during propagation of flames have been observed and studied. Helium’s smaller molar heat capacity and higher thermal conductivity than nitrogen results in a more stable and smooth flames. The increase range of smooth flames can be used to validate chemical kinetics mechanisms at higher pressures and temperatures. Pressure rise data as a function of time during the combustion process was obtained through a pressure transducer in the spherical vessel was the primary input of the multi-shell model used to calculate the laminar burning speed for the smooth flames. Power law correlations over a wide range of pressures (from sub-atmospheric up to 7.3atm), temperatures (298K up to 650K), and equivalence ratios (0.6–3.0) have been developed for laminar burning speeds of smooth H2/CO/O2/He flames. At equivalence ratios corresponding to maximum laminar burning speeds, the experimental laminar burring speeds show a positive relationship with pressure. Experimental laminar burning speeds of H2/CO/O2/He mixtures have been compared with numerical values calculated by free flat flame simulation using two chemical kinetics mechanisms and some discrepancies discovered for equivalence ratios greater than 2. The effect of helium as a diluent on flame morphology and laminar burning speeds of syngas are compared with two diluents, nitrogen as well as nitrogen that is further diluted with synthetic exhaust gas recirculation (SEGR).

Ozone effect on the flammability limit and near-limit combustion of syngas/air flames with N2, CO2, and H2O dilutions

The effect of the presence of ozone on the flammability limit and near-limit combustion of syngas (H2/CO)/O2 flames with N2, CO2, and H2O dilutions was numerically studied. This study involved systematic simulations of 1-D planer laminar premixed flames under near-limit conditions using the PREMIX code considering the detailed description of chemical kinetics and molecular transport, and the intrinsic radiation as the only heat loss mechanism. The flammability limits were determined by solving the singular turning points using the one-point continuation mathematical approach. The results showed that the presence of O3 expanded the flammability limit of syngas as O3 reduced the lower flammability limit and increased the upper flammability limit. With increasing O3, the H2/CO/O2 flames could be sustained at higher N2, CO2 and H2O dilution ratios. The laminar flame speed was also enhanced by the presence of O3, and the enhancement effect of O3 on the laminar flame speed was much more profound under near-limit conditions, being 4–11 times greater than that under near-stoichiometric conditions. Detailed near-limit flame structure and kinetic studies revealed that O3 was consumed at relatively low temperatures (∼700K), producing active radicals, such as H and OH. These radicals interfered with the H2/CO/O2 chemistry and thereby significantly accelerated the overall combustion process under near-limit conditions. For near-stoichiometric flames, the large disparity between the characteristic temperatures of O3 related reactions (∼500K) and those of H2/CO/O2 chemistry (>1250K) depressed the interaction between O3 related reactions and H2/CO/O2 chemistry and thus only limited O3 enhancement effect was observed under near-stoichiometric conditions.

Effect OF CO2/N2 dilution on laminar burning velocity of liquid petroleum gas-air mixtures at elevated temperatures

The present experimental study reports the effect of CO2/N2 dilution on laminar burning velocity of premixed LPG (liquid-petroleum-gas)-air mixtures at elevated temperatures using a preheated mesoscale diverging channel technique. The experiments were carried out for a range of equivalence ratios varying from 0.8 < Φ < 1.3 with percentage dilution of the fuel component by volume (β) for CO2 varying from 10% < β < 30% and N2 varying from 10% < β < 40%. A power–law correlation has been obtained for the present experimental data as a function of percentage dilution, mixture temperature and equivalence ratio. It has been observed that an increase in dilution with CO2/N2 leads to an increase in temperature exponent (α). The increase in temperature exponent due to CO2 dilution is more pronounced as compared to N2 dilution case. A flame structure study has been carried out to understand the effect of mixture temperature and diluent using USC (University of Southern California) Mech II reaction mechanism.

Numerical study of flame structure in the mild combustion regime

In this paper, turbulent non-premixed CH4+H2 jet flame issuing into a hot and diluted co-flow air is studied numerically. This flame is under condition of the moderate or intense low-oxygen dilution (MILD) combustion regime and related to published experimental data. The modelling is carried out using the EDC model to describe turbulence-chemistry interaction. The DRM-22 reduced mechanism and the GRI2.11 full mechanism are used to represent the chemical reactions of H2/methane jet flame. The flame structure for various O2 levels and jet Reynolds numbers are investigated. The results show that the flame entrainment increases by a decrease in O2 concentration at air side or jet Reynolds number. Local extinction is seen in the upstream and close to the fuel injection nozzle at the shear layer. It leads to the higher flame entertainment in MILD regime. The turbulence kinetic energy decay at centre line of jet decreases by an increase in O2 concentration at hot Co-flow. Also, increase in jet Reynolds or O2 level increases the mixing rate and rate of reactions.

Flame pattern, temperatures and stability limits of pulverized oxy-coal combustion

The challenges in maintaining a stable oxy-coal flame have been reported in the open literature. In case of pulverized coal combustion, narrower flammable regimes arise not only of differences in the feed gas composition, but also due to changes in the flow dynamics because of reduced volume flow rate through the burners to match similar temperatures and radiative heat transfer within the furnace. The present work focuses on the study of flame structure and stability of pre-dried lignite fired by a staged feed-gas burner. Experimental runs were carried out in a 0.40MWth laboratory facility, with the objective of investigating the effects of swirl strength and feed gas distribution in the burner registers. To assess the experimental data, a numerical study of the non-reacting flow in the near burner region is performed. Predicted flow pattern and flame images show that the primary fuel jet penetrates the internal recirculation zone, forming an annular doughnut reverse flow region for both firing conditions and resembling a type-1 flame pattern. Temperature measurements indicate that the feed gas distribution among the burner registers control the flame temperature in parallel with the swirl number in both air-fired and oxy-fired environments. The oxy-fuel cases exhibite markedly reduced temperatures on the flame axis. This difference in the core arises not just from the more pronounced penetration depth of the primary jet, but also due to reaction mechanisms including pyrolysis and gasification reactions at the higher concentrations of CO2 and water vapor. Flame stabilization is shown to be strongly dependent on an appropriate distribution of the feed gas and strength of swirling flow in the burner front. The results correlated in terms of swirl number, secondary/tertiary flow ratio and secondary/primary axial momentum point to similarities in the stability limits between air-firing and oxy-firing.

Flame structure and kinetic studies of carbon dioxide-diluted dimethyl ether flames at reduced and elevated pressures

The flame structure and kinetics of dimethyl ether (DME) flames with and without CO2 dilution at reduced and elevated pressures were studied experimentally and computationally. The species distributions of DME oxidation in low-pressure premixed flat flames were measured by using electron-ionization molecular-beam mass spectrometry (EI-MBMS) at an equivalence ratio of 1.63 and 50mbar. High-pressure flame speeds of lean and rich DME flames with and without CO2 dilution were measured in a nearly-constant-pressure vessel between about 1 and 20bar. The experimental results were compared with predictions from four kinetic models: the first was published by Zhao et al. (2008) [9], the second developed by the Lawrence Livermore National Laboratory (LLNL) (Kaiser et al., 2000) [13], and the third has been made available to us as the Aramco mechanism (Metcalfe et al., 2013) [14]; as the fourth, we have used an updated model developed in this study. Good agreement was found between measurements and predictions from all four models for all major and most typical intermediate species with and without CO2 addition in low-pressure flat flame experiments. However, none of the models was able to reliably predict high-pressure flame speeds. Although the updated model improved the prediction of flame speeds for lean mixtures, errors remained for rich conditions at elevated pressure, likely due to uncertainty in the rates of CH3+H(+M)=CH4(+M) and the branching and termination reaction pair of CH3+HO2=CH3O+OH and CH3+HO2=CH4+O2. CO2 addition considerably decreased the flame speed. Kinetic comparisons between inert and chemically active CO2 in DME flames showed that CO2 addition affects rich and lean DME flame kinetics differently. For lean flames, both the inert third-body effect and the kinetic effect of CO2 reduce H-atom production. However, for rich flames, the inert third-body effect increases H-atom production via HCO(+M)=H+CO(+M) and suppression of the kinetic effect of CO2 by shifting the equilibrium of CO+OH=CO2+H.

A direct numerical simulation study of turbulence and flame structure in transverse jets analysed in jet-trajectory based coordinates

AbstractAn${\mathrm{H} }_{2} / {\mathrm{N} }_{2} $jet in cross-flow (JICF) of air is studied using three-dimensional direct numerical simulation with and without chemical reaction in order to investigate the role of the complex JICF turbulent flow field in the mechanism of fast fuel-oxidant mixing and of aerodynamic flame stabilization in the near field of the jet nozzle. Focus is on delineating the flow/mixing/chemistry conditions that are necessary and/or sufficient to achieve flame anchoring that ultimately enables the formulation of more reliable and precise guidelines for design of fuel injection nozzles. A mixture averaged diffusion formulation that includes the effect of thermal diffusion is used along with a detailed chemical kinetics mechanism for hydrogen–air combustion. A new parametrization technique is used to describe the jet trajectory: solution of Laplace’s equation upon, and then within, an opportune scalar surface anchored by Dirichlet boundary conditions at the jet nozzle and plume exit from the domain provides a smoothly varying field along the jet path. The surface is selected to describe the scalar mixing and reaction associated with a transverse jet. The derived field,$j(\mathbi{x})$, is used as a condition to mark the position along the natural jet trajectory when analysing the variation of relevant flow, mixing and reaction quantities in the present direct numerical simulation (DNS) datasets. Results indicate the presence of a correlation between the flame base location in parameter space and a region of low velocity magnitude, high enstrophy, high mixing rate and high equivalence ratio (flame root region). Instantaneously, a variety of vortical structures, well known from the literature as important contributors to fuel-oxidant mixing, are observed in both inert and reactive cases with a considerable span in length scales. Moreover, instantaneous plots from reactive cases illustrate that the most upstream flame tongues propagate close to the trailing edge of the fuel jet potential core near the jet shear layer vortex shedding position. Some degree of asymmetry with respect to the domain mid-plane in the spanwise direction is observed in the averaged fields, both for the inert and reactive cases.