Laminar Flame Calculations Research Articles

Experimental and numerical investigations of the laminar burning velocities of premixed fuel-rich methane oxy-fuel and oxygen-enhanced flames

In the context of Oxy-fuel combustion, there is a strong industrial interest in highly fuel-rich flames, characterized by an equivalence ratio (Φ) exceeding 2.5, for the production of H2-CO-rich synthesis gas. The validation of existing chemical reaction mechanisms for this specific regime pose a significant challenge due to the scarcity of experimental data for this particular range of equivalence ratio.The scope of this study was therefore the determination of the laminar burning velocity sL of ultra-rich CH4-O2 flames under varying conditions, such as high equivalence ratios and preheating temperatures using the heat flux burner method. The laminar burning velocity was determined for equivalence ratio within the range of 2.3 < Φ < 3.6 and preheating temperature ranging from 300 K to 455 K. Additionally, the impact of the of reducing the oxygen content in the oxidizer on sL was investigated by argon addition in molar percentages ranging from 40 % to 100 %.The results of our study reveal laminar burning velocities ranging from 5 cm/s to 45 cm/s for pure oxy-fuel CH4 flames. Preheating is shown to increase sL and this effect can be represented by a power law correlation sL = sL,0(T/T0)α with α = 1.53. Furthermore, the dilution of the oxidizer with argon leads to a significant decrease in sL of approximately 50 % compared to pure oxy-fuel flames. It is worth noting that detailed reaction mechanisms in combination with molecular transport data are typically not validated within the selected equivalence ratio range.To assess the performance of such reaction schemes in predicting laminar burning velocity under the specified selected conditions, we evaluated 13 different reaction mechanisms sourced from the literature. We compared the laminar burning velocities obtained through calculations of laminar premixed 1D flames and, additionally compared these with experimental data. A sensitivity analysis of the laminar burning velocity were carried out for selected mechanisms. Among the mechanisms, the CalTech2.3 mechanism exhibited the most consistent and accurate performance, particularly at high equivalence ratios.

Data-driven simulation of ammonia combustion using neural ordinary differential equations (NODE)

The direct use of detailed chemical kinetics in combustion simulations is limited by the extremely high computational costs. Recently, Owoyele and Pal (Energy and AI, 2022), proposed the neural ordinary differential equations (NODE) method to accelerate calculations of chemical kinetics and proved its effectiveness in zero-dimensional calculations of hydrogen combustion considering 9 species and 21 reactions. However, its performance for more realistic high-dimensional calculations and more complex kinetic systems remains unexplored. Therefore, this study further applies the method for more complex chemical kinetics of ammonia combustion, especially with optimizations in the data sampling, model training strategies, and model application methods that remedy the problems of versatility and application to more practical simulations. The newly developed NODE models are comprehensively validated in the zero-dimensional calculations of ammonia auto-ignition, one-dimensional calculations of laminar freely-propagating ammonia-premixed flames, and two-dimensional direct numerical simulation (DNS) of ammonia-premixed flames in a temporally evolving jet. Present NODE models focus on seven chemical species, namely NH3, O2, H2, OH, H2O, N2, and NO, and the results show that, compared with the results obtained by using detailed chemical kinetics, this method is able to reduce the computational costs of the zero-dimensional auto-ignition reaction to 1/24 while reproducing the ignition delay time for a wide range of initial temperatures and equivalence ratios with relatively good accuracy. Additionally, the method is able to reduce the computational costs of the one-dimensional freely propagating flame and two-dimensional jet flame to 1/4 and 1/38 respectively, while acceptable reproduction of the laminar flame speed and temporal evolution of the gas temperature and mass fractions of the interested species can be achieved.

Humidification Towards Flashback Prevention in a Classical Micro Gas Turbine: Thermodynamic Performance Assessment

Combustion air humidification has proven to be effective to stabilize hydrogen combustion and to avoid flashback apparition in a typical micro Gas Turbine (mGT). However, both the fuel alteration and combustion air dilution will impact the cycle performance. A complete characterization of this thermodynamic impact is essential to ensure that the mGTs become cleaner, and fully flexible to fit with the expectation of future small-scale decentralized power production. Therefore, the objective of this work is twofold: the determination of the necessary dilution for combustion stabilization, depending on the type of fuel, as well as the impact assessment on the cycle performance. In this framework, a hybrid model of the Turbec T100 mGT combustor, combining a 0D Chemical Reactor Network and 1D Laminar flame calculations, is used to first assess the flashback limits. The laminar flame speed is evaluated to predetermine the necessary minimal water dilution of the combustion air to avoid flashback for several CH4/H2blends. Second, a thermodynamic analysis is performed to assess the impact of the flame stabilization measures on the cycle performance of the mGT using Aspen Plus. The 0D/1D simulation results show that the combustor of the Turbec T100 can operate with fuels containing up to 100% hydrogen. However, the thermodynamic analysis shows that the water dilution leads to a decreased electrical performance. Future work consists in the iterative coupling of both 0D/1D and the Aspen model to correctly predict the flashback limits, considering the altering operating conditions. To conclude, with this work, we provide a framework for future mGT operations with alternative fuels.

An analytic probability density function for partially premixed flames with detailed chemistry

Laminar premixed flame profiles of methane/air free flames and strained flames at different fuel/air ratios and strain rates are analyzed using detailed chemistry with Lewis numbers equal to one. It is shown that the detailed chemistry flame profiles of progress variables CO2 + CO and H2O + H2 in canonically stretched coordinates can be fitted accurately by a slight generalization of recently proposed analytical presumed flame profiles over a wide range of fuel/air ratios through adaptation of a single model parameter. Strained flame profiles can be reproduced using an additional linear coordinate transformation, emulating the compression of the preheat zone by strain as predicted by premixed flame theory. The model parameter can alternatively be determined using only the laminar flame speeds and the fully burnt temperatures from the laminar flame calculations. The stretch factor of the coordinate transformation is proportional to cp/lambda, which drops by a factor up to 4 across the laminar flame. It is shown how the non-constant cp/lambda modifies the laminar flame probability density function (pdf) and a polynomial fit to cp/lambda as a function of the progress variable allows analytical results for the laminar flame pdf and the mean value of the progress variable and of the reaction source term. An analytic pdf for partially premixed flames is proposed based on Bayes's theorem as a combination of a beta pdf for the mixture fraction and the laminar flame pdf's evaluated at the respective fuel/air ratio.

A method to convert stand-alone OH fluorescence images into OH mole fraction

Due to the accessibility of the planar laser-induced fluorescence technique, images of OH fluorescence intensity are often used to study the structure of turbulent flames. However, there are differences between the measured OH fluorescence intensity and the actual OH mole fraction. These are often neglected because accurate conversion from fluorescence to mole fraction requires the combined knowledge of all major species mole fractions and temperature, which was rarely achieved in 2-D. Here, a new method to convert images of OH fluorescence intensity into OH mole fraction is proposed. This model relies only on inexpensive 1-D laminar flame calculations and does not require information on major species or temperature. The primary assumption behind the applicability of this model is the local approximation of multi-dimensional flames with 1-D counterflow flames. The method utilizes the fact that both OH mole fraction and OH fluorescence intensity profiles are self-similar with pressure and scalar dissipation rate. Only two empirical constants need to be calibrated using 1-D laminar flame calculations. The model was validated using computed 2-D axisymmetric laminar flames and 3-D turbulent flames computed with LES. The accuracy of the conversion model was estimated to about 8% (for Reynolds number up to Re =66,800), which includes errors due to the 3-D effects that are not included in this method relying on 2-D images. As a proof of concept, the conversion model was finally applied to one single-shot image of OH fluorescence intensity measured with OH-PLIF for syngas at P=4bar and Re =16,700, demonstrating potential applications of this new method. The method was tested for hydrogen, syngas and methane fuels but, for brevity, only syngas results are reported in detail.

Experimental assessment of the progress variable space structure of premixed flames subjected to extreme turbulence

Flamelet models of turbulent premixed combustion assume that (a) turbulent-transport and combustion processes can be decoupled and treated independently, suggesting that preheat and reaction zones remain layer-like and do not become highly fragmented and/or distributed. By further assuming (b) that the scalar-structure (e.g. the distribution of thermochemical quantities, like species mass fraction, vs. a control variable, such as a reaction progress variable) of a turbulent flame is akin to that in an associated laminar flame, detailed chemical properties can be introduce into a simulation at low computational cost via pretabulated flamelet tables derived from laminar flame calculations. The authors previously quantified conditions when assumption (a) remains valid. The present work assesses assumption (b) for turbulent Reynolds numbers that exceed those of previous studies by ∼ 7 × . Namely, planar laser-induced fluorescence (PLIF) of formaldehyde (CH2O), hydroxyl (OH), and methylidyne (CH) acquired jointly with Rayleigh scattering are used to produce joint PDFs (scatter plots) of the former vs. a progress variable (CR) derived from the latter. Regardless of the turbulence level the piloted Bunsen flames considered here were subjected to, peak-normalized conditional mean (CM) profiles obtained from such joint PDFs agree well with profiles derived from laminar flame calculations. Additionally, within the near field of modestly turbulent flames, two-term β-PDFs are found to accurately describe CR-distributions. However, agreement between β-PDFs and CR-distributions worsens with increased turbulence and axial distance from the burner. Small discrepancies between the measured CM profiles and those obtained from laminar calculations also emerged at highly turbulent conditions. For instance, under such conditions, laminar profiles of CH over predict the measurements where 0.6 < CR < 0.9. Nevertheless, the overall good qualitative agreement between the measured and simulated results supports the use of flamelet-based models for premixed flames subjected to extreme turbulence.

Fuel Effects in Turbulent Premixed Pre-vaporised Alcohol/Air Jet Flames

To study combustion fundamentals of complex fuels under well-defined boundary conditions, a novel Temperature Controlled Jet Burner (TCJB) system is designed that can stabilise both gaseous or pre-vaporised liquid fuels. In a first experimental exploratory study, piloted turbulent jet flames of pre-vaporised methanol, ethanol, 2-propanol and 2-butanol mixtures are compared to methane/air as a reference fuel. Complementary one-dimensional laminar flame calculations are used to provide flame parameters for comparison. Blow-off and flame length as global flame characteristics are measured over a wide range of equivalence ratios. For fuel rich conditions, blow-off limits correlate well with extinction strain rate calculations. Differing flame lengths from lean to rich conditions are explained partly by different flame wrinkling that is assessed using planar laser-induced fluorescence imaging of the hydroxyl radical (OH-PLIF). A study of Lewis-number effects indicates that they have substantial influence on flame wrinkling. Lean alcohol/air flames, opposed to methane/air, have a Lewis-number greater than unity. This impedes curvature development, which promotes relatively large flame lengths. In contrast, across stoichiometric conditions, all alcohol/air mixture Lewis-numbers decrease significantly. At such conditions, alcohol/air flames show alike or even larger wrinkling compared to methane/air flames. However, quantitatively, the differences in flame length and wrinkling observed among the flames can neither be explained alone by Lewis-number differences, nor other global mixture parameters available from 1D laminar flame calculations. This study shall therefore emphasise the need for more detailed experimental analyses of the full thermochemical state of laminar and turbulent flames fuelled with complex fuels.

Assessment of experimental observables for local extinction through unsteady laminar flame calculations

Unsteady premixed and non-premixed counterflow laminar flame simulations were conducted in order to investigate extinction effects on observables commonly used in turbulent combustion. CH4 and n-C12H26 were the fuels studied, with air as the oxidizer at pressures of 1, 5, and 10 bar. It was determined that CH2O persists, compared to all other reactive species, during the extinction transient for both fuels and at all conditions, as the loss of OH concentration removes the dominant CH2O consumption pathway. The persistence of CH2O concentration is duplicated similarly in CH4 and n-C12H26 premixed flames. For non-premixed flames, the results indicate that the peak CH2O concentration reduction for n-C12H26 flames is milder compared to CH4 flames. Increasing the pressure causes an extension of reactivity, resulting in greater CH2O production and thus a delayed decay during the extinction transient. In addition, a change in the magnitude of the applied scalar dissipation rate for the non-premixed flames did not alter the trends of CH2O during extinction. Thus, caution is suggested when using CH2O in turbulent combustion experiments as a marker of the preheat zone thickness, given that increased levels of CH2O could be a result of multiple local extinction events. In addition, the product of OH and CH2O was found to scale well with the heat release rate for CH4 and n-C12H26 flames at multiple pressures. Finally, the CH* and OH* chemiluminescence was examined. CH* was found to extinguish slightly before the other species and more importantly, that once its concentration is reduced to a negligible level, the flame is on its way to extinction with no chance of recovery. OH* was determined to scale well with heat release at both 1 and 10 bar for both fuels and type of flames.

Analysis of chemical pathways and flame structure for n-dodecane/air turbulent premixed flames

This paper analyzes turbulence-chemistry interactions of an n-dodecane-air flame, focusing on the degree to which flame structure and fuel oxidation pathways change in turbulent flames relative to their corresponding laminar flames. This work is based on a lean (ϕ = 0.7) n-dodecane-air flame DNS database from Aspden et al. (2017). The relative roles of dominant reactions that release heat and produce/consume radicals are examined at various turbulence intensities and compared with stretched flame calculations from counterflow flames and perfectly stirred reactors. These results show that spatially integrated chemical pathways are relatively insensitive to turbulence intensity and mimic the behavior of stretched flames. In other words, the contribution of a given reaction to heat release or radical production, integrated over the entire flame, is insensitive to turbulence. Localized analysis conditioned on topological feature of the flame and on temperature is also performed. The former analysis reveals that larger alteration of pathways occurs in the positively-curved regions of the flame. Most significantly, it shows that the thermal structure of the flame is altered, as peak reaction rates and heat release in the low temperature (i.e., below 1200 K) region shift towards higher temperatures with increases in Karlovitz number. This result is particularly interesting given that prior work with lighter fuels (e.g., hydrogen) showed the opposite behavior. Various stretched, laminar flame calculations with altered transport effects were performed for reference. These calculations, using mixture-averaged and Le = 1 transport model, can capture similar shifts towards higher temperatures in laminar flames with increasing stretch. However, the stretch rate and transport values must be tuned to match these shifts in thermal structure differently, depending upon reactions, species, and Ka value. This effect is particularly prominent for low temperature species. Thus, these results show that flames do not simply shift to a Le = 1 thermal structure with high turbulence intensity, as previously suggested, but there is interplay between altered scalar diffusivity and stretch effects.

Wall heat fluxes and CO formation/oxidation during laminar and turbulent side-wall quenching of methane and DME flames

This study is focused on the characterization of wall heat fluxes and its influence upon CO formation/oxidation within atmospheric flames in a side-wall quenching geometry. The influence of different wall temperatures ranging between 330 K and 670 K is compared for stoichiometric methane and dimethyl ether (DME) flames. Coherent anti-Stokes Raman spectroscopy (CARS) and two-photon laser induced fluorescence (LIF) of the CO molecule are used to determine pointwise gas phase temperatures and CO concentrations. Simultaneously, wall temperatures are measured using one-dimensional phosphor thermometry and flame front positions are identified by planar OH-LIF imaging. Wall heat fluxes are estimated from measured gas and wall temperatures. For increasing wall temperatures, quenching distances decrease significantly and the maximum wall heat fluxes rise in the quenching region. Additionally, thermochemical states are analysed using CO/T scatter plots. Compared to one-dimensional unbounded laminar flame calculations, the CO/T dependencies are altered significantly by the presence of a wall. Very close to the wall, for methane/air flames and to a lesser extent for DME/air flames at y = 100 µm, the CO formation branch is shifted towards lower temperatures. In contrast, in the entire near-wall region the CO oxidation branch is shifted to lower temperatures for both fuels. One-dimensional premixed flame calculations accounting for enthalpy losses indicate that the heat loss to the wall is the most likely cause rather than different chemical reaction pathways. Studying the impact of turbulence, both the CO formation and oxidation branch are shifted to lower temperatures in state space. Additionally, an increasing number of intermediate CO mole fractions is observed filling the state space in between both branches. The analysis of turbulent integral time scale derived from PIV data indicates that this phenomenon is dominated by heat transfer, which is enhanced by turbulence.

Scalar dissipation rates in a turbulent partially-premixed dimethyl ether/air jet flame

This paper presents the gradient structure of a turbulent partially premixed dimethyl ether (DME)/air jet flame operating at a jet Reynolds number of 29,300. Temperature and mixture fraction profiles from Raman/Rayleigh/CO-LIF line measurements are used to determine one-dimensional scalar dissipation rates at six axial locations. A major focus of the current work is to assess the effects of experimental artifacts, including spatial resolution, noise, and dimensionality, on the accuracy of the derived scalar dissipation rate. Two-dimensional probability density functions (PDFs) of the mixture fraction gradients are used to determine possible clipping effects due to insufficient spatial resolution. This resolution limit is compared to values determined from one-dimensional dissipation spectra and scaling laws. Spatial resolution also is investigated using laminar flame calculations in conjunction with optical-blur filters representing the experimental setup. The impact of noise is treated by error propagation methods. Monte Carlo simulations and experimental data from laminar flames are used to verify and validate the models used to predict noise propagation for the measurements of the absolute gradients, squared gradients, and scalar dissipation rates. Gradient and scalar dissipation rate detection limits and contribution from apparent dissipation (due to noise effects) are presented as functions of measurement signal-to-noise ratios. A noised lognormal function is introduced to investigate the impact of noise on derived PDFs and corresponding statistical moments of the measured scalar gradients and the scalar dissipation rate within the turbulent flame. Results from the turbulent flame measurements are presented in the form of scatter plots and conditional statistics to examine turbulence-chemistry interaction and develop a database for model assessment. Specifically, the results are compared to laminar flame calculations over a broad range of strain rates with multi-component and unity Lewis number transport assumptions. This comparison is used to assess the relevance of differential diffusion effects on scalar dissipation rates in the turbulent flame.

Raman/LIPF data of temperature and species concentrations in swirling hydrogen jet diffusion flames: Conditional analysis and comparison to laminar flamelets

Simultaneous point measurements of temperature, mixture fraction, major species (H2, H2O, O2, N2) concentrations from KrF laser-induced spontaneous Raman scattering and minor species (OH) concentrations from KrF laser-induced predissociative fluorescence (LIPF) in unswirled (Sg = 0), low swirl (Sg = 0.12), and high swirl (Sg = 0.5) lifted turbulent hydrogen jet diffusion flames into still air are reprocessed to obtain profiles of the Favre-averaged scalars and conditional moments. Large discrepancies between the Favre-averaged and ensemble-averaged temperature, H2O, and OH mole fractions are found at the lifted flame region, due to density weighting of fairly large samples of unreacted mixtures. Conditional statistics are used to reveal the reaction zone structure in mixture fraction coordinates. The cross-stream dependence of conditional means of temperature and species concentration is found to be significant in the lifted flame region of the swirled flames and decreases to negligible levels with increasing streamwise position. Comparison of the measured conditional mean variation of OH vs. H2O with a series of stretched laminar partially premixed flame and diffusion flame calculations reveals that for the unswirled flame, the differential molecular diffusion and radial dependence of conditional means are minor at x/D = 6.4 for stretch rates from a = 14,000 to 400 s-1. For the low and high swirling flames, however, the measured OH vs. H2O conditional means at the lifted flame region are not consistent with stretched laminar flame calculations. The level of partial premixing and the stretch rate decrease with increasing downstream locations. The estimated stretch rate at x/D = 53.5 is about 50–10 s-1, while at x/D = 107 the stretch rate is about 10 s-1 with some measurements at adiabatic equilibrium.

Sidewall quenching of atmospheric laminar premixed flames studied by laser-based diagnostics

Flame-wall interactions (FWI) of laminar premixed methane-air flames at atmospheric pressure are studied using various laser diagnostic methods. Velocity fields and flame front locations are measured simultaneously by two-component particle image velocimetry (PIV) and planar laser induced fluorescence (LIF) of the OH-radical. Coherent anti-Stokes Raman spectroscopy (CARS) and two-photon LIF of the CO molecule are used to determine temperatures and CO concentrations. The FWI process is investigated using a generic burner setup with well-defined boundary conditions, where one branch of a V-shaped flame interacts with a water-cooled stainless steel wall, corresponding to a sidewall quenching (SWQ) geometry. FWI is studied for equivalence ratios of ϕ =0.83, 1.0 and 1.2. The quenching distance of the flames is determined using two different methods. Additionally, the near wall behavior of the flame consumption speed is analyzed and compared with that of a freely propagating laminar flame. Thermochemical properties are analyzed using CO/T-state diagrams. Comparison to one-dimensional laminar flame calculations undisturbed by the presence of a wall highlights the severe impact upon thermochemical states. Comparing characteristic time scales of heat transfer processes to chemical processes indicates that diffusion rather than chemical reaction processes is the reason for these observations.

On defining progress variable for Raman/Rayleigh experiments in partially-premixed methane flames

Progress variable is a central concept in the theory and modeling of partially-premixed flames. However, there has been no consensus in the literature on its definition, particularly in the context of experiments. Several possible definitions of progress variable are considered, based on mass fractions of the seven major species that can be measured by combined Raman/Rayleigh scattering methods applied to turbulent methane flames. The behavior of three candidate progress variables is evaluated using laminar opposed-flow flame calculations. The effects of normalizing by the fully-burnt state rather than the more commonly used equilibrium state are considered. A progress variable, cO, defined as the mass of oxygen bound in the products CO2, CO and H2O, divided by that which would be bound if the sample were taken to its fully-burnt state, is argued to offer some advantages over the other definitions considered. It is shown that inclusion of a larger number of species in calculating cO (18 versus 7) makes little difference, such that the seven major species accessible to Raman/Rayleigh laser diagnostics are sufficient to quantify reaction progress in partially-premixed methane flames at atmospheric pressure. The measured joint statistics of mixture fraction, Z, and progress variable, cO, are presented for two turbulent piloted partially-premixed methane–air jet flames that have significantly different mixture fraction profiles at the jet exit. Favre average radial profiles of first and second moments, as well as the correlation coefficient, RZ, c, highlight differences in near-field scalar structure between the two flames. Examples of joint Z, cO histograms, sampled from 0.3-mm segments centered at the radial location where Z˜ = 0.065 in each profile, are presented. This mixture fraction value corresponds roughly to the location of peak heat release rate on the fuel-rich side in the laminar flame calculations. A clear conclusion is that the common modeling assumption of statistical independence of mixture fraction and progress variable is contradicted by the turbulent flame measurements. The proposed progress variable can be easily calculated from simulations as a post-processing step, independent of the modeling approach, allowing for consistent comparison of measured and modeled turbulent flame results.

Effect of turbulence–chemistry interactions on chemical pathways for turbulent hydrogen–air premixed flames

This paper considers the kinetic pathways of hydrogen oxidation in turbulent, premixed H2–air flames. It assesses the relative roles of different reaction steps in H2 oxidation relative to laminar flames, and the degree to which turbulence–chemistry interactions alters the well understood oxidation pathway that exist in laminar flames. This is done by analyzing the turbulent, lean (ϕ= 0.4), H2–air flame DNS database from Aspden et al. [17]. The relative roles of dominant reaction steps in heat release and radical formation/consumption are analyzed at different Karlovitz numbers and compared with laminar stretched flame calculations from counterflow flames and perfectly stirred reactors. It is found that both the progress variable conditioned and spatially integrated contributions of the dominant reactions remain qualitatively similar between a highly turbulent and a laminar unstretched flame. Larger changes, up to a factor of about two, occur in the relative roles of reactions with secondary influences on heat release and radical production/consumption. These results suggest that the kinetic routes through which H2 is oxidized remain essentially constant between laminar, unstretched flames and high Karlovitz number flames.

Fuel effects on the stability of turbulent flames with compositionally inhomogeneous inlets

This paper reports an analysis of the influence of fuels on the stabilization of turbulent piloted jet flames with inhomogeneous inlets. The burner is identical to that used earlier by the Sydney Group and employs two concentric tubes within the pilot stream. The inner tube, carrying fuel, can be recessed, leading to a varying degree of inhomogeneity in mixing with the outer air stream. Three fuels are tested: dimethyl ether (DME), liquefied petroleum gas (LPG), and compressed natural gas (CNG). It is found that improvement in flame stability at the optimal compositional inhomogeneity is highest for CNG and lowest for DME. Three possible reasons for this different enhancement in stability are investigated: mixing patterns, pilot effects, and fuel chemistry. Numerical simulations realized in the injection tube highlight similarities and differences in the mixing patterns for all three fuels and demonstrate that mixing cannot explain the different stability gains. Changing the heat release rates from the pilot affects the three fuels in similar ways and this also implies that the pilot stream is unlikely to be responsible for the observed differences. Fuel reactivity is identified as a key factor in enhancing stability at some optimal compositional inhomogeneity. This is confirmed by inference from joint images of PLIF-OH and PLIF-CH2O, collected at a repetition rate of 10kHz in turbulent flames of DME, and from one-dimensional calculations of laminar flames using detailed chemistry for DME, CNG, and LPG.

Laminar Flame Calculations for Analyzing Trends in Autoignitive Jet Flames in a Hot and Vitiated Coflow

Experiments of autoignitive jet flames in a hot and vitiated coflow have previously shown various flame behaviors, spanning lifted flames to moderate or intense low oxygen dilution (MILD) combustion. For better understanding the behavior of flames in this configuration, regime diagrams and ignition delay results are presented from well-stirred reactor calculations across a wide range of operating conditions for methane and ethylene fuels. In conjunction with two-dimensional calculations, the importance of flame precursors and oxygen penetration across the reaction zone is revealed. It is found that widely accepted definitions and regime diagrams are inadequate to classify and reconcile the different flame behaviors that are observed experimentally. For accurate prediction of the ignition process, it is necessary to incorporate boundary conditions that capture minor species in the oxidizer. The role of fuel type also has a major impact on the ignition process and flame appearance.

Characterization of extinction/reignition events in turbulent premixed counterflow flames using strain-rate analysis

We investigate extinction/reignition events in two contrasting turbulent premixed flames of the Yale turbulent counterflow flame (TCF) burner, that are both qualitatively and quantitatively different. One of the two chosen flames is a high-burning (HB) flame with a low probability of local extinction while the other flame is a low-burning (LB) flame with a high probability of local extinction. In recent work, we successfully studied the turbulent premixed flames of the Yale TCF burner using the large-eddy simulation/probability density function (LES/PDF) methods. In this present study, the main motivation is to investigate how the compositional structure of the two turbulent flames are related to that of laminar flames. To this end, steady, one-dimensional, strained, opposed-jet laminar flame calculations are performed to investigate the effect of strain rates K on the laminar counterparts of HB and LB, and to evaluate the extinction strain rates Sext for the two flames. Subsequently, a normalized distance Z is defined in terms of the mixture fraction ξ, which is calculated based on the mass fraction of N2. The scatter plots from the particle data of (a) CH2O mass fraction YCH2O*vs. progress variable p* and (b) temperature T*vs. the normalized distance Z* are quite different for the two flames with more samples close to the extinguished laminar profile for the LB flame than for the HB flame. The cell-mean profiles of T vs. Z resemble the laminar profiles at different strain rates even though the LES/PDF are non-trivially 3D and unsteady. These cell-mean profiles are used to evaluate the instantaneous equivalent steady strain rate (ESSR) S for the two flames. The cumulative distribution function (CDF) of S, conditional on S < Sext (i.e., burning samples), is somewhat similar, with the distributions being broad without a peak close to the bulk strain rate. However, comparatively, more samples of the LB flame have the ESSR values above the extinction strain rate, i.e., S > Sext. The scatter plots of the ESSR S vs. the fresh product layer thickness Δf quantify the thinning of the product layer as the strain rate S increases. The scatter for the HB flame follow the corresponding laminar profile quite closely, whereas, the thicknesses observed for the LB flame are higher compared to its laminar prediction.

The transition of ethanol flames from conventional to MILD combustion

The present paper is focused on prevaporised ethanol flames in the transition from conventional combustion to the MILD combustion regime. Photographs and imaging of OH* chemiluminescence reveal a distinctive flame structure when ethanol carried by air burns in a 3% O2 coflow (typical of MILD combustion), differing from that at higher oxygen levels. In comparison to flames carried by air in a 9% O2 coflow, the spatial gradient and the peak in the OH* signal profile are significantly reduced in the 3% O2 coflow, indicating a more uniform distribution of heat release and temperature. The use of N2 as a carrier gas renders the OH* profile for the 6% O2 coflow case similar to that of flames carried by air in the 3% O2 coflow. The experimental results indicate a transition from conventional combustion to MILD combustion with the decrease of coflow O2 level and/or the use of N2 as a carrier gas. Calculations reveal that a substantial drop in the peak heat release rate and/or overall net heat release rate might contribute to the lack of luminosity of flames in the 3% O2 coflow, suggesting a need for threshold values of these two in defining MILD combustion. A series of laminar flame calculations are performed to identify the MILD combustion regime based on the absence of a negative heat release region. The absence of a negative heat release region is found to be strain rate dependent at a given temperature and O2 level of the oxidant stream. This is mainly a result of the enhanced transportation of O2 across the reaction zone at a higher strain rate. At low strain rates, the negative heat release region is more likely to disappear in a 3% O2 oxidant flow due to a combination of low flame temperature and high availability of O2.

Topology and burning rates of turbulent, lean, H2/air flames

Improved understanding of turbulent flames characterized by negative consumption speed-based Markstein lengths is necessary to develop better models for turbulent lean combustion of high hydrogen content fuels. In this paper we investigate the topology and burning rates of turbulent, lean (ϕ = 0.31), H2/air flames obtained from a recently published DNS database (Aspden et al., 2011). We calculate local flame front curvatures, strain rates, thicknesses, and burning velocities and compare these values to reference quantities obtained from stretched laminar flames computed numerically in three model geometrical configurations—a counterflow twin flame, a tubular counterflow flame and an expanding cylindrical flame. We compare and contrast the DNS with these model laminar flame calculations, and show both where they closely correlate with each other, as well as where they do not. These results in the latter case are shown to result from non-flamelet behaviors, unsteady effects, and curvature-strain correlations. These insights are derived from comparisons conditioned on different topological features, such as portions of the flame front with a spherical/cylindrical shape, the leading edge of the flame, and portions of the flame front with low mean curvature. We also show that reference time scales vary appreciably over the flame, and characterizing the relative values of fluid mechanic and kinetic time scales by a single value leads to erroneous conclusions. For example, there is a two order of magnitude decrease in chemical time scales at the leading edge of the front relative to its unstretched value. For this reason, the leading edge of the front quite closely tracks quasi-steady calculations, even in the lowest Damköhler number case, DaF∼0.005.