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The Sydney Flame provides an excellent framework for systematic analysis of premixed, partially premixed and non-premixed methane/air combustion. It represents (depending on the axial position from the fuel inlet) non-premixed, partially premixed and fully premixed combustion. We simulate the test case FJ200-5GP-Lr75-57, which represents the partially premixed mode. Large-eddy simulations (LES) are conducted using a flame surface density combustion model (FSD) using one-step chemistry and an artificially thickened flame model (ATF) with a global two-step and an analytically reduced multi-step chemistry reaction mechanism. Unity Lewis numbers are assumed ( L e = 1 ) at first. The FSD and ATF models are compared using the same computational mesh, numerical scheme and boundary conditions. Both models will be analysed regarding their accuracy and computational efficiency in the regime of partially premixed combustion. A comparison of the results points out the strengths and weaknesses of the FSD models. The FSD model with inclusion of flame stretch effects yields good agreement with mean experimental temperature, CO2 and H2O mass fraction distributions in contrast to the ATF model using the global two-step mechanism, which overestimates downstream temperature, CO2 and H2O mass fractions. The FSD model performs well in all regions, which are dominated first by premixed, then partially premixed and finally non-premixed combustion along the flame. The ATF multi-step chemistry shows good results only in the premixed mode region, while mean temperature, CO2 and mass fractions are overestimated in the non-premixed mode regions at higher mixture fraction values. Including differential diffusion into the transport equations improved the ATF model results in comparison with experiments. A mesh study revealed, that the ATF model performs only after mesh refinement. In particular results are improved in the non-premixed combustion region. In contrast, the FSD model is less resolution sensitive and performs very well for both meshes. An evaluation of the Wasserstein metric provides a quantitative assessment of different ATF model setups on simulation accuracy. Finally, computational times for all simulation setups are compared. [ABSTRACT FROM AUTHOR]

A combined experimental and numerical investigation of partially premixed laminar methane-air flames undergoing side-wall quenching (SWQ) is performed. A well-established SWQ burner is adapted to allow the seeding of the main flow with additional gaseous products issued from a (secondary) wall inlet close to the flame's quenching point. First, the characteristics of the partially premixed flame that quenches at the wall are assessed using planar laser-induced fluorescence measurements of the OH radical, and a corresponding numerical simulation with fully-resolved transport and chemistry is conducted. A boundary layer of enriched mixture is formed at the wall, leading to a reaction zone parallel to the wall for high injection rates from the wall inlet. Subsequently, in a numerical study, the wall inflow is mixed with dimethylmethylphosphonat (DMMP), a phosphor-based flame retardant. The DMMP addition allows the assessment of the combined effects of heat loss and flame retardants on the flame structure during flame-wall interaction. With an increasing amount of DMMP in the injected mixture, the flame stabilizes further away from the wall and shows a decrease in the local heat-release rate. Thereby, the maximum wall heat flux is significantly reduced. That results in a lower thermal load on the quenching wall. The flame structure analysis shows an accumulation of the intermediate species HOPO at the wall similar to the CO accumulation during the quenching of premixed flames without flame retardant addition. The study shows how the structure of a partially premixed flame is influenced by a wall that releases either additional fuel or a mixture of fuel and flame retardant. The insights gained from the canonical configuration can lead to a better understanding of the combined effects of flame retardants and heat losses in near-wall flames. [ABSTRACT FROM AUTHOR]

Simulations of two cases in a novel multi-regime burner configuration are undertaken using a presumed joint probability density function (PDF) approach with tabulated chemistry. The flame conditions are varied by changing the central jet equivalence ratio, which produces different multi-regime combustion modes in the non-premixed inner flame. An outer premixed flame and recirculation zone behind a bluff body are present to supply heat and combustion products to stabilise the inner flame. A two-progress variable approach is tested to improve predictions of carbon monoxide (CO) in the post-flame regions, where CO oxidation occurs. The large eddy simulation set-up and sub-grid combustion model are assessed through comparisons with time-averaged measurements for radial profiles at different streamwise locations. The jet break-up length, the shear layers and the mixture fraction distribution are well captured in both cases. The temperature distribution is well captured for the inner flame in each case but the temperature and mixture fraction are over predicted in the downstream regions of the outer premixed flame, which is due to increased dilatation that suppresses air entrainment. Improved predictions of the CO mass fraction are obtained for the outer premixed flames with the two-progress variable approach. Over predictions are seen in the upstream regions of the inner flame when the CO mass fraction is obtained from a look-up table, suggesting that the CO mass fraction should be transported to include the convection/diffusion balance in regions where there is no flame. Furthermore, transporting the CO mass fraction with a one-progress variable approach produces over predictions in the burnt regions, suggesting a two-progress variable model is needed to capture the consumption region of CO. The multi-regime combustion characteristics are observed to be stronger in flame MRB26b, where non-premixed and rich premixed combustion is present. For flame MRB18b, the non-premixed contribution is smaller and weak stratified combustion is observed. [ABSTRACT FROM AUTHOR]

• The effect of ozone addition on soot formation in flames has been investigated. • A reduction in the number and size of soot particles has been observed. • AFM shows particles flattening with O 3 , as due to changes in aromatic crosslinking. • Ozone promotes larger aromatic soot constituents, altering chemical pathways. • Ozone addition exhibits a minor contribution in the soot oxidation flame region. The effect of ozone addition on soot formation in partially premixed laminar methane flames at two equivalence ratios, Φ = 11.9 and Φ = 7.6 (with 500 ppm and 570 ppm of ozone, respectively), has been investigated. Soot particles collected in the centerline of the flames at several heights above the burner have been examined in terms of size, morphology, and chemical/structural characteristics, by differential mobility analysis, atomic force microscopy, and Raman spectroscopy respectively. The results show that adding ozone to the flame reduces the number of soot particles, as well as their average size. The typical height profiles of the collected particles on substrates are shown to be also flatter as compared to the pristine flames, possibly indicating a lower level of cross-linking in the aromatic network forming the particles. Also, the Raman spectroscopy analysis indicates that ozone addition promotes the formation of larger aromatic soot constituents. Moreover, within the selected experimental conditions and ozone concentration of several hundred parts per million, the flame temperature is unaffected regardless of the presence or absence of ozone. Consequently, it can be inferred that the observed modifications in soot characteristics are predominantly attributed to chemical factors. The observed effects are all consistent with a possible chemical interaction of atomic oxygen, resulting from the decomposition of ozone in the post-flame zone, with aromatic π-radicals, precursors of the soot particles. [ABSTRACT FROM AUTHOR]

The higher pollutant level in non-premixed combustion and safety issues pertaining to premixed combustion can be counteracted by partially-premixed mode of combustion. The partially premixed flames exhibit the benefits of both premixed and non-premixed flames. Partially premixed flames enhances complete combustion leading to reduced soot formation and hence lower emission. However, the equivalence ratio plays an important role in the stability of such flames. This paper reports the experimental investigation on the flame characteristics and stability of partially premixed LPG-air flames in tubular burner. The stability curve obtained for the base case without any secondary flow shows that the velocity at lift-off, drop-back, and blowout increases with increasing equivalence ratio. In the presence of secondary co-flow air, the lift-off and blow off velocity decreases compared to base case indicating poor stability due to extensive flame stretch leading to aerodynamic quenching. The experimental results show that the velocity of flow at lift-off, blowout, and drop-back are higher in the presence of secondary swirl air than the base case. Co-swirl air increases the stability due to better mixing at the flame base with increased residence time. Flame stability deteriorates with co-flow air as co-flow strains the flame boundary due to flame stretch. [ABSTRACT FROM AUTHOR]

Combustion instability is investigated computationally for a ten-injector rocket engine using a compressible flamelet progress variable (FPV) model and detached eddy simulation (DES). A C++ code is developed based on OpenFOAM 4.1 to apply the combustion model. Flamelet tables are generated for methane/oxygen combustion at the pressure of 200 bar using a 12-species chemical mechanism. The flames at this high pressure level have similar structures as those at much lower pressures. A power law is determined to rescale the reaction rate for the progress variable to address the pressure effect. The combustion is also simulated by a one-step-kinetics (OSK) model for comparison with the FPV model. Premixed and diffusion flames are identified locally for both the FPV and OSK models. Study of combustion instability shows that a combined first-longitudinal and first-tangential mode of 3200 Hz is dominant for the FPV model while the OSK model favors a pure first-tangential mode of 2600 Hz. The coupling among pressure oscillation, unsteady transverse flow and helicity fluctuation is discussed. A preliminary study of the resonance in the injectors, which is driven by the acoustic oscillation in the combustion chamber, is also presented. Novelty and significance In this paper, we demonstrate one of the first successful applications of a compressible flamelet-progress-variable (FPV) combustion model to numerical study of the methane/oxygen combustion dynamics and instability in a realistic multi-injector liquid rocket engine. A relatively detailed 12-species high-pressure chemical mechanism, which was originally developed by Professor Hai Wang's group at Stanford University, is applied for the first time to combustion simulation at chamber pressures around 200 bar. Compared with a one-step global kinetics model, the FPV model predicts realistic chamber temperature and favors a dominant combined 1L+1T acoustic mode at 3200 Hz rather than a pure 1T mode at 2600 Hz. The predicted dominant frequencies agree well with theoretical analysis. Strong quarter-wave-length resonance is found to occur inside injectors and coexist with the chamber acoustic oscillations. [ABSTRACT FROM AUTHOR]

An accurate prediction of the probability density function (PDF) of the mixture fraction is crucial to the prediction of combustion since mixing plays an important role in turbulent non-premixed and partially premixed flames. This work provides an assessment of the large-eddy simulation (LES)/filtered density function (FDF) method for the prediction of the PDF of the mixture fraction. The advantage of the LES/FDF method is that it provides the full predictions of the statistical distribution of scalars including the mixture fraction. The predictive accuracy of the method for the PDF is yet to be fully validated. Assessing the prediction of the PDF of the mixture fraction, a conserved scalar, is an important starting point. The Sydney/Sandia inhomogeneous inlet jet flame is used as a test case. A quick comparison shows that the LES/FDF predicted PDF shapes of the mixture fraction deviate significantly from the commonly presumed Beta-PDF as well as from the experimental data in the flame. To examine the source of the discrepancy, we clarify the different PDF definitions used in the comparison among the predictions, measurements, and presumed shape PDFs. The discrepancy observed from the comparison is largely reconciled by clarifying the difference between the PDFs that are examined. The PDF of the resolved mixture fraction is shown to be close to the Beta-PDF in both the measurements and predictions, while the PDF directly deduced from the LES/FDF particles deviates significantly from the Beta-PDF. A multimodal PDF analysis and a pseudo convergence analysis are conducted to provide plausible evidence to support the predicted multimodal PDF shapes. The sub-filter scale FDF is shown to be close to the Beta-PDF too through the construction of a synthesised PDF, which supports the common presumed Beta-PDF assumption used in the presumed PDF methods when combined with LES. [ABSTRACT FROM AUTHOR]

The effects of different baffled configurations of acoustic dampers placed downstream of the dump plane of a combustor on the mean flowfield, coherent structures, acoustics, stability and dynamics of partially premixed methane flames were experimentally investigated. Particle image velocimetry (PIV) was used to capture the instantaneous flowfield downstream of the dump plane or the baffles and proper orthogonal decomposition (POD) was used to analyze coherent structures. High-speed imaging was used to capture flame dynamics, and a Bruel and Kjaer microphone was used to perform acoustics measurements. Two interchangeable baffle configurations were tested; one consists of eight-blade radial baffles attached to a circumferential baffle and placed underneath of a circumferentially-slotted cup (AD#1), and another consists of eight-blade radial baffles attached to a circumferential baffle (AD#2). The results revealed significant improvement in flame stability when using baffles. This is attributed to several factors. First, the burner baffled configurations caused a significant reduction in the amplitude of coherent structures and acoustics. Second, the presence of baffles promoted either a very narrow CRZ (in case of AD#2) or no ORZ with a weak CRZ (in case of AD#1). This positive axial velocity convected vortical structures farther downstream of AD#1. Third, both flame–flame interaction in AD#1 configuration (resulting from the axial flow off the circumferential slots and that from the swirling flow in the core flow region) and flame-baffles interaction in AD#2 configuration (resulting from the interplay between baffles' tips and the swirling flame in the core region) helped reducing the azimuthal mean velocity and its rms component in the flow core region. This consequently resulted in a significant change in the azimuthal acoustic waves' behavior within the combustor/confinement. The results revealed that the baffled burner configurations enhanced mixing as witnessed by the more flame bluish color and consequently its stability. [ABSTRACT FROM AUTHOR]

This paper presents an experimental study of the effect of fuel nozzle geometry on swirling partially premixed methane flames, where the focus is put on the ensuing flowfield and its role on coherent structures' suppression. The burner consists of a central interchangeable fuel nozzle surrounded by a swirling co-airflow where both discharge into a short mixing tube. The nozzle geometry is classified into two groups, namely, single- and multi-orifice nozzles. The swirling motion of the co-airflow is produced using a radial-type swirl generator with a swirl number of 1.15. The flowfield characteristics and coherent structures are documented using particle image velocimetry (PIV). Flame front dynamics are captured using Mie scattering technique. Quantitative laser sheet (QLS) is used to qualitatively shed light on the mixing characteristics downstream of the mixing tube exit, and laser Doppler velocimetry (LDV) is used to extract the coherent structures' peak frequency from the power spectra. The results revealed that the fuel nozzle geometry significantly affects the mean flowfield, mean, and root-mean-square (RMS) of the flame front location, flame front asymmetry, and coherent structures' amplitude. Higher spread rate and faster decay caused by single-orifice nozzles inside the mixing tube result in divergent flames with higher degree of flame front asymmetry downstream of the mixing tube exit. On the other hand, multi-orifice nozzles mitigate coherent structures, enhance mixing, and hence, promote the most appropriate conditions for coherent structures' suppression. [ABSTRACT FROM AUTHOR]

Multi-regime turbulent combustion modelling remains challenging and is explored with occurrence of local extinction in this study. A partially premixed model based on unstrained premixed flamelets is used in this work to investigate a piloted jet flame configuration with inhomogeneous inlets. Three different cases are simulated, which differ in the bulk mean velocity that amounts respectively to about 50%, 70% and 90% of the blow off velocity measured experimentally. As the jet velocity approaches the blow off limit, local extinctions start to occur along the flame surface and thus these flames are challenging from a modelling prospective. Two different numerical approaches, involving scaled and unscaled progress variable respectively, are compared to elucidate their abilities and limitations to predict local extinctions and to deal with the three-stream problem at the pilot/coflow interface. The key modelling details for such predictions are indicated and discussed. LES results are systematically compared to two sets of experimental measurements available in the literature for the three flames. The differences observed in the two experimental datasets are also discussed with the help of LES results. Although both approaches show promising agreement for the flame statistics, the scaled progress variable approach better predicts the local extinctions. The unscaled approach shows to naturally handle the three-stream problem without additional treatment for the pilot/coflow interface, which is required for the scaled approach. Furthermore, computed scalar dissipation rate of mixture fraction is compared with the measurements showing good agreement for the conditions investigated. This further suggests that local extinctions can be predicted using unstrained flamelets if the correct scalar mixing and its dissipation are captured. [ABSTRACT FROM AUTHOR]

• Turbulent planar natural gas flames are created in a concentric flow slot burner. • The flame stability is controlled by varying the mixture inhomogeneity. • The flame front curvature and flame surface density are extracted using OH-LIF images. • The mixing field was measured using Ryleigh scattering technique at the nozzle exit. • The effects of turbulence and mixture inhomogeneity on the flame are investigated. The present study is an extension of our earlier mixing field investigation in partially premixed flames. In the current work, the link between the mixing field and flame structure, and hence flame curvature and surface density are experimentally studied. The flame structure is captured using a high-speed OH-PLIF technique, whereas the crosswise mixing fields were previously presented in our earlier study. A concentric flow slot burner is employed for producing turbulent planar partially premixed natural gas flames at different levels of mixture inhomogeneity. The effects of the level mixture inhomogeneity, the equivalence ratio, the air to fuel stream velocity ratio, and Reynolds number "Re" on the flame characteristics are investigated. The link between the previously reported data on the mixing field and the flame structure are examined. The correlations between the mixing field and the flame structure were clearly observed in the current study. That is, as the mixture becomes highly inhomogeneous, in rich input jet conditions cases, altering Reynolds number and the mixing length led to a pronounced change in both flame structure and flame curvature while altering L/D influences the flame structure. On the other hand, the effects of the mixing length and Reynolds number on the flame structure and curvature in a more homogeneous mixture were not significant. The data showed that the flame surface density was inversely proportional to the level of the mixture inhomogeneity and directly proportional to the Reynolds number. The significant effect of the jet equivalence ratio on the level of mixture inhomogeneity leads to a significant change in the flame structure and flame curvature. In addition, if the mixture inhomogeneity is less pronounced the variation s of the mixing length and Reynolds number lead to minimal effects on the flame structure and flame curvature. [ABSTRACT FROM AUTHOR]

The stability and the mean structure of methane partially premixed conical burner flames was investigated numerically using ANSYS Fluent. The study presents and discusses the stability curves of the partially premixed flame and maps the mean flame structure based on contours of mass fraction of O2, CO and temperature. From the data obtained, it can be concluded that both premixed and non-premixed flames are less stable than the partially premixed flames. An optimum level of partially premixing was found and the flames beyond this threshold were found to be less stable. This optimum level was found, when the ratio of the mixing length to the nozzle diameter is equal to 5. At this specific degree of partially premixing, the flame exhibited triple interaction reaction zones. It was found that with an increase of the angle of the cone of the burner, the air entrainment increases which, in turns breaks the stabilization core and hence cause a reduction in the flame stability limit. The main role of the cone is to provide a protection from the surrounding environment at early phase of the reaction near the jet exit where turbulence with high intensity was observed. [ABSTRACT FROM AUTHOR]

Abstract The mixture fraction and reaction progress variables are key for presumed probability density function (PDF) based flamelet models for partially premixed flames. The importance of the joint statistics of these two variables are evaluated using a joint composition-enthalpy transported PDF/Finite Volume method applied to established databases for inhomogeneous jet flames. The accuracy of the approach is first examined for three flames with different mixture fraction inlet profiles and departures from blow-off. The (joint-) statistics of mixture fraction and reaction progress variable are subsequently analysed with the covariance of the two variables used to evaluate the assumption of statistical independence. Results show that the current approach is able to reproduce the near-field features of both homogeneous and inhomogeneous jet flames. The latter leads to a stratified premixed flame that evolves to diffusion-dominated combustion as the influence of the pilot fades. In agreement with measured data, the mixture fraction variance is strongly affected by the near-field combustion mode and the misalignment with the reaction progress variable variance is obvious in the stratified premixed flame. It is shown that the covariance of the mixture fraction and reaction progress variable is influenced by turbulence-chemistry interactions and that, generally, the two parameters remain strongly correlated with the consequence that the assumption of statistical independence is implausible. [ABSTRACT FROM AUTHOR]

A large eddy simulation study is undertaken with the objective of improving carbon monoxide (CO) estimations compared to measurements of enclosed turbulent flames in laboratory scale burners. The sub-grid combustion is modelled using a presumed joint probability density function approach with tabulated chemistry based on unstrained flamelets. The thermochemical conditions are mapped using the mixture fraction and reaction progress variable. A second progress variable is used to represent the post-flame oxidation of CO. The CO mass fraction is transported, and the CO production and consumption rates are stored in separate look-up tables using each progress variable. The two-progress variable model is assessed using enclosed flames that are stabilised behind a bluff body and in a gas turbine model combustor with swirling flows. The performance of the two-progress variable model is assessed by using standard practice approaches to compute the CO mass fraction with a single progress variable, which include obtaining the CO mass fraction from a look-up table or from its transport equation. Transporting the CO mass fraction with a single progress variable gives significant overestimations in the burnt regions of the bluff body stabilised flame. The two-progress variable model gives significant improvements in the burnt regions, since the disparate time scales of CO production and consumption are captured by this model. The look-up approach also significantly overestimates the CO mass fraction in the flame stabilisation region in both configurations, where convection and diffusion processes are dominant. Therefore, the CO mass fraction needs to be transported with two progress variables to capture the intricate convection-diffusion-reaction balance in the flame stabilisation region and within the flame brush. Furthermore, the two-progress variable model gives improvements in the post-flame oxidation regions towards the chamber exit in the gas turbine model combustor, whereas the look-up approach gives significant overestimates. [ABSTRACT FROM AUTHOR]

In this study, a new set of direct numerical simulations is generated and used to examine the influence of mixture composition heterogeneities on the propagation of a premixed iso-octane/air spherical turbulent flame, with a representative chemical description. The dynamic effects of both turbulence and combustion heterogeneities are considered, and their competition is assessed. The results of the turbulent homogeneous case are compared with those of heterogeneous cases which are characterized by multiple stratification length scales and segregation rates in the regime of a wrinkled flame. The comparison reveals that stratification does not alter turbulent flame behaviors such as the preferential alignment of the convex flame front with the direction of the compression. However, we find that the overall flame front propagation is slower in the presence of heterogeneities because of the differential on speed propagation. Furthermore, analysis of different displacement speed components is performed by taking multi-species formalism into account. This analysis shows that the global flame propagation front slows down due to the heterogeneities caused by the reaction mechanism and the differential diffusion accompanied by flame surface density variations. Quantification of the effects of each of these mechanisms shows that their intensity increases with the increase in stratification’s length scale and segregation rate.

Highlights • Influence of exit flow conditions on coherent structures and flashback has been investigated in detail. • Flame outside of the mixing tube experiences symmetric vortex shedding at high equivalence ratio, and PVC at low equivalence ratio near blowout conditions. • The frequency of coherent structures is found depend on the swirling airflow Reynolds number, swirl number, and fuel nozzle geometry. • Flashback’s mean region inside the mixing tube is found directly proportional to the strength and frequency of the coherent structures. Abstract The effect of fuel nozzle geometry and swirling airflow on the flashback and its relationship with the coherent structures of partially premixed methane flame is investigated experimentally. Particle image velocimetry (PIV) and laser Doppler velocimetry (LDV) are used to document flow characteristics, and Schlieren imaging technique is used to study flame appearance and vortex shedding frequency downstream of the burner exit. Proper orthogonal decomposition (POD) technique is applied to capture the coherent structures, along with phase averaging of the linear superposition of the first four POD modes. Seven different fuel nozzle geometries and two swirl number (S = 0.79 and 1.15) are tested. The nozzles are categorized into three groups, with each has similar equivalent diameter; namely, a symmetric nozzle (used as a reference), nozzles with polygonal orifices (group A), and angled multi-orifice nozzles (group B). The results of the flow field inside the mixing tube show that the strength of coherent structures and flashback propensity increase with the swirling airflow Reynolds number, swirl number, nozzle bluff body area, and the number of the peripheral angled orifices of the fuel (central) nozzle. On the other hand, the results of flame appearance outside of the mixing tube indicate that methane flame experiences symmetric vortex shedding at high swirl number and low Reynolds number, while it experiences PVC near blowout conditions at low swirl number and high Reynolds number. Furthermore, the frequency of coherent structures is found to depend on the swirling airflow Reynolds number, swirl number, and fuel nozzle geometry. Additionally, the flashback’s mean region inside the mixing tube is found directly proportional to the strength and frequency of the coherent structures. [ABSTRACT FROM AUTHOR]

We investigate the onset of thermoacoustic instabilities in a turbulent combustor terminated with an area contraction. Flow speed is varied in a swirl-stabilized, partially premixed combustor and the system is observed to undergo a dynamical transition from combustion noise to instability via intermittency. We find that the frequency of thermoacoustic oscillations does not lock-on to any of the acoustic modes. Instead, we observe that the dominant mode in the dynamics of combustion noise, intermittency and thermoacoustic instability is a function of the flow speed. We also find that the observed mode is insensitive to the changes in acoustic field of the combustor, but it varies as a function of upstream flow time scale. This new kind of thermoacoustic instability was independently discovered in the recent theoretical analysis of premixed flames. They are known as intrinsic thermoacoustic modes. In this paper, we report the experimental observation and the route to flame intrinsic thermoacoustic instabilities in partially premixed flame combustors. A simplified low-order network model analysis is performed to examine the driving mechanism. Frequencies predicted by the network model analysis match well with the experimentally observed dominant frequencies. Intrinsic flame-acoustic coupling between the unsteady heat release rate and equivalence ratio fluctuations occurring at the location of fuel injection is found to play a key role. Further, we observe intrinsic thermoacoustic modes to occur only when the acoustic reflection co-efficients at the exit are low. This result indicates that thermoacoustic systems with increased acoustic losses at the boundaries have to consider the possibility of flame intrinsic thermoacoustic oscillations. [ABSTRACT FROM AUTHOR]

We present in this paper a numerical simulation to the problem of triple flame propagation in a porous-walls channel in the presence of a temperature gradient across the channel. The problem has been formulated using the thermo-diffusive approximation and then solved numerically using finite elements method. The study showed that temperature gradient plays a crucial role on the existence and propagation of triple flames. More precisely, the effect of temperature gradient on the flame propagation was found to: (i) cause the flame to exist only for a limited range of values of the temperature gradient parameter, (ii) establish multiplicity of solutions each of them characterizes unique combustion regimes, (iii) modifies the flame shape from the usual triple flame shape when the temperature gradient is large, and finally (iv) enhance the reactivity of the underlying mixture, but on the other hand will have serious implications on the safety of the combustion chamber. [ABSTRACT FROM AUTHOR]

We study asymptotically in this paper the influence of the temperature gradient across the mixing layer on the propagation triple flames formed inside a porous wall channel. The study begins by formulating the problem mathematically using the thermo-diffusive model and then presents a thorough asymptotic analysis of the problem in the limit of large activation energy and thin flames. Analytical formulae for the local burning speed, the flame shape and the propagation speed in terms of the temperature gradient parameter have been derived. It was shown that varying the feed temperatures can significantly enhance the burning of the reactants up to a critical threshold, beyond which no solutions can be obtained. In addition, the study showed that increasing the temperature at the boundaries will modify the usual triple structure of the flame by inverting the upper premixed branch and extending it to the boundary, which may have great implications on the safety of the adopted combustion chambers. [ABSTRACT FROM AUTHOR]

This paper examines the influence of mixing a fuel with an oxidizer-rich reactant on the existence and propagation of partially premixed flames formed in a porous-wall channel. The amount of the injected fuel has been taken as a parameter to generate different states of the combustion mixtures. The problem has been mathematically formulated in the framework of the thermodiffusive model under a single-step chemical reaction. Numerical simulations based on finite elements were then employed to solve the governing partial differential equations. The study has shown that the degree of fuel premixing in the combustionmixtures as well as the value of the Lewis number of fuel and oxidizer plays a great role on the existence and structure of triple flames. It is found also that flame extinguishes for larger values of the premixing parameter. Flame structures were also found to exhibit several changes such as the shift of the leading edge toward the fuel supply side and the formation of twin flames. It has been predicted that the premixing can substantially enhance the reactivity of the combustion mixture, but on the other hand it tends to shift the flame to the boundary which may destroy the underlying combustion chamber. For nonunit Lewis numbers, fuel premixing is found to shift triple flames from the stoichiometric surface and attach the flame to the channel walls. It is predicted also that the maximum reaction rate reduces with increasing the Lewis numbers. [ABSTRACT FROM AUTHOR]

The work presents a comparative study of the fuel-air mixing and flame stabilization characteristics of two, high velocity ratio, coaxial jet configurations comprising either a plain circular arrangement or a convoluted trailing edge nozzle. The high velocity ratio, coaxial stream interaction results in a stable central recirculation zone at a distance downstream of the nozzle exit where a range of flame configurations can be successfully stabilized under suitable flow and mixing conditions. In this configuration, the reacting front region is supplied with both a radial and an axial propane-air mixture gradient which are formulated through a central upstream premixer duct and the coannular jet interaction downstream of the exit plane. Depending on the nozzle shape, plain circular or convoluted, a variety of partially premixed turbulent flame configurations can be obtained at a position detached farther from the nozzle exit. Measurements of mixture concentrations, mean temperatures, and flame chemiluminescence images of OH* and CH* provided information for the interpretation of the relative variations in flame structure and burner performance. Preliminary supporting computational investigations for the cold flow fuel-air mixing patterns and the accompanying mixing wake axial velocities have also been conducted to give an insight into the predominant aerodynamic phenomena involved. The combined methodology helped to elucidate some aspects of the influence of the variable inlet fuel-air mixture distributions and their interaction with the high velocity ratio coaxial jet mixing topologies on the diverse flame configurations studied. [ABSTRACT FROM AUTHOR]

Highly resolved measurements of scalar dissipation rates in turbulent piloted CH 4 /air flames with compositionally inhomogeneous inlets are presented. These were performed using Sandia's Raman–Rayleigh–LIF system but with data acquisition and processing strategies that result in enhanced spatial resolution and reduced noise. They complement earlier measurements with coarser resolution. The burner stabilising these flames enables variability in the mixture fraction profile at the exit plane. Earlier studies have shown enhanced stability at an optimal compositional inlet profile that leads to multiple modes of combustion, with premixed-stratified flames close to the jet exit but transitioning to diffusion-dominated burning downstream. It is found that at upstream locations, for jets with homogeneous inlets as well as for the high-temperature regions of flames with inhomogeneous inlets, both fine and coarse measurements of scalar dissipation rates yield similar results, giving confidence that measurements resolve the local dissipation scales. Downstream locations in homogeneous mixtures also show similar results for both coarse and fine measurements across all mixtures. Differences arise in the rich, inner regions of turbulent flames with inhomogeneous inlets, where the fine resolution measurements are more reliable due to the existence of steep gradients in composition. Both data sets provide a comprehensive platform to enhance the modelling of turbulent flames in the presence of multi-modes of combustion. [ABSTRACT FROM AUTHOR]

The current work investigates experimentally the combined effects of oxy-combustion and partial premixing of the oxidizer (O 2 plus CO 2 ) and the fuel (compressed natural gas, CNG) on flame stability considering new burner configuration, namely perforated plate burner. The work explains and quantifies the limits of flammability, visual flame appearance as well as the extinction mechanisms and the visual flame length on ranges of operating conditions. The work investigates the flame stability under two different sets of experiments. The first set was conducted over a range of equivalence ratio to obtain the upper and lower flammability limits in terms of oxygen fraction (percentage of O 2 in the oxidizer mixture). The second set was conducted to quantify the range of equivalence ratio within which stable flames can be obtained at constant oxygen fraction of 36%. Visual flame appearance and extinction mechanisms are also investigated. The results showed that operation with an oxygen fraction of less than 29% is not possible over the considered range of equivalence ratio. Extinction at the upper flammability limit occurs by flashback when oxygen fraction exceeds 42% However, at the lower flammability limit, extinction occurs by blow-off when oxygen fraction drops beyond 29%. [ABSTRACT FROM AUTHOR]

We studied the effect of pressure on the flame structure of incipiently sooting partially premixed counterflow ethylene flames. Oxygen was added to the fuel stream of a purely diffusion flame and removed from the oxidizer side to lower the overall equivalence ratio at constant stoichiometric value of the mixture fraction. These rich conditions lead to a two-stage combustion mode, with substantial amounts of CO and H 2 in the post-premixed flame region burning in the diffusion flame part. Soot was formed in the region straddling the gas stagnation plane and sandwiched between the two flame components. We quantified profiles of species as large as 3-ring aromatics. Using C 6 H 6 as a qualitative marker of the flame soot load, we observed that under partially premixing an eightfold pressure increase lead to a substantial increase in the concentration of C 6 H 6 despite the fact that the flame peak temperature was deliberately decreased by hundreds of degrees to preserve conditions of incipient sooting compatible with gas microsampling. Part of the increased soot load with pressure was attributed to a diminished back diffusion of oxidizing radicals like OH from the diffusion flame side towards the gas stagnation region where the soot was formed. Benzene mole fraction and visible soot luminosity increased with the lowering of the equivalence ratio regardless of the pressure because of an increase in temperature in the region between the two flame components of the double flame structure, with attending enhancement of pyrolytic growth reactions. A comparison of the experimental results with one-dimensional modeling of the flames, revealed good agreement for major species, some key intermediates and benzene, in the latter case for just one of the two tested chemical kinetic mechanisms. Reaction path analysis revealed an increased role of the C2/C4 path in the formation of C 6 H 6 as the pressure increased. [ABSTRACT FROM AUTHOR]

Soot and its gaseous precursors are quantified in detail (precursors up to 166 amu, volume fraction, particle size, number concentration, and light emissivity dispersion exponent) in a laminar partially premixed counterflow flame of ethylene. The investigated flame has an equivalence ratio Φ = 2.43 and a mixture fraction Z st = 0.4, resulting in a distinct double-flame structure consisting of a rich premixed flame component and a diffusion flame component, both stabilized on the fuel side of the stagnation plane. The value of the equivalence ratio makes the premixed flame the dominant contributor to soot production, with soot being oxidized completely by OH from the diffusion flame component. Particle size is measured to increase quasi-monotonically, but remains within a few nanometers throughout the soot forming region. Aromatic species are primarily formed in the post flame region of the premixed flame. Their mole fractions peak close to the premixed flame and decrease as the diffusion flame is approached. The experimentally measured gaseous species are captured well by kinetic models, with the exception of two critical species in soot chemistry: benzene and naphthalene. [ABSTRACT FROM AUTHOR]

The paper presents Large Eddy Simulations (LESs) for the Sydney ethanol piloted turbulent dilute spray flames ETF2, ETF6, and ETF7. The Flamelet Generated Manifold (FGM) approach is employed to predict mixing and burning of the evaporating fuel droplets. A methodology to match the experimental inflow sprayprofilesis presented. The spray statistical time-averaged results show reasonable agreement with mean and RMS data. The Particle Size Distribution (PSD) shows a good match downstream of the nozzle exit and up tox/D= 10. Atx/D= 20 and 30 the PSD is under-predicted for droplets with mean diameterD10> 20μm and over-predicted for the smaller size droplets. The simulations reasonably predict the reported mean flame structure and length. The effect of increasing the carrier velocity (ETF2–ETF7) or decreasing the liquid fuel injection mass flow rate (ETF2–ETF6) is found to result in a leaner, shorter flame and stronger spray–flow interactions. Higher tendency to local extinction is observed for ETF7 which is closer to blow-off compared to ETF2 and has higher scalar dissipation rates, higher range of Stokes number, and faster droplet response. The possible sources of LES-FGM deviations from the measurements are discussed and highlighted. In particular, the spray time-averaged statistical error contribution is quantified and the impact of the inflow uncertainty is studied. Sensitivity analysis to the pre-vaporized nozzle fuel mass fraction show that such small inflow perturbations (by ± 2% for the ETF2 flame) have a strong impact on the flame structure, and the droplets’ dynamics. Conditional scatter plots show that the flame exhibits wide range of mixing conditions and bimodal mixing lines particularly at upstream locations (x/D < 20), where the injected droplets are still penetrating the centerline. This is relaxed further downstream as droplets gradually evaporate and burn in a diffusion like mode. [ABSTRACT FROM AUTHOR]

In this work, two sets of experiments were performed including air and oxy-combustion premixed flames stabilized over a perforated-plate burner. A set of experiments was performed considering air–fuel combustion in order to identify a range of equivalence ratio for stable flame operation at fixed oxidizer Reynolds number and over a range of premixing ratio (namely 7, 25, 45, 67 and 128). Also, effects of premixing ratio ( L / D ) on lower and upper flammability limits over a range of oxidizer Reynolds number were investigated. Another set of experiments was performed considering oxy-fuel combustion in order to identify a range of equivalence ratio for stable flame operation at fixed oxidizer Reynolds number. The study documents the visual flame length, appearance and color and identifies the extinction mechanism outside the flammability limits for both air and oxy-combustion flames over wide ranges of operating equivalence and premixing ratios. For air combustion flames, the results showed wide ranges of flammability limits at lower premixing ratios and tight ranges of flammability limits at higher premixing ratios. This can be attributed to the increase in the degree of flame diffusivity while reducing the premixing ratio and, as a result, more stable flame is obtained. Reductions in the emissions of NO and CO were observed while increasing the premixing ratio for the air combustion case. For the oxy-combustion flames, the results showed stable flame operation at oxygen fraction of 36%; however, the flammability limits were approximately 20% lower as compared to those of air combustion flames. Flash back was observed when operating oxygen fraction exceeded 40% and flame stability was affected badly with the decrease of oxygen fraction. The visual flame length was longer in cases of oxy-combustion flames as compared to those of air combustion. [ABSTRACT FROM AUTHOR]

This work addresses some fundamental questions in the area of partially premixed combustion—what parameters control the fraction of flamelets that are premixed (versus non-premixed), and what are the locations of high probability of premixed (versus non-premixed) combustion? To answer these questions there is a need to measure the flame index ( ξ ) and its statistical properties, and this information previously has not been available. Flame index is + 1 where a premixed flamelet exists and is - 1 at the location of a non-premixed flamelet. A new method to measure flame index was developed that adds NO 2 to the air; acetone is used as one component of the fuel. Laser-induced fluorescence images indicate the locations of flamelets and whether the gradients of the fuel and O 2 are in the same direction or not. Flame index was measured in a gas turbine model combustor that was designed at DLR that is a good example of partially premixed combustion. Measurements show how the mean flame index varies in space; near the fuel injector the combustion is 50% non-premixed and 50% premixed while downstream the flamelets are mostly premixed. This trend is consistent with two numerical simulations of swirl flames; however for simple lifted jet flames the premixed flamelets do not extend so far downstream. It was found that one parameter that controls the fraction of flamelets that are premixed is the ratio of the fuel injection velocity to the air velocity. Increasing this ratio increases the fraction of flamelets that are premixed because it increases the distance that the fuel stream penetrates into the more intense mixing region. Good signal-to-noise ratios of 24 (for acetone) and 13 (for NO 2 ) were achieved and an uncertainty analysis is presented that is based on calibration experiments. [ABSTRACT FROM AUTHOR]

Chemical energy carriers synthesized from renewable energy sources such as ethanol or oxymethylene ethers (OME) will become increasingly important for CO 2 -neutral thermochemical energy conversion processes. Therefore, it is important to make these processes efficient and clean. This needs more predictive numerical simulation tools and an improved understanding of the combustion process. For this purpose, spatially resolved measurements of local thermochemical states in reaction zones are required, for which combined Raman- and Rayleigh spectroscopy is suitable. Since a large number of intermediate hydrocarbons occur in the reaction zones of ethanol and OME flames, Raman spectroscopy must be evolved for quantitative measurement of these species over a wide temperature range. Against this background, this study pursues the goal of creating the instrumental and apparatus-related pre-requisites. The setup of a new dual-dispersion spectrometer and its main specifications are presented. The usability of the spectrometer is demonstrated on the example of premixed and partially-premixed ethanol/air and OME-3/air flames. For this purpose, a new counterflow burner is presented, which enables laminar, single-phase combustion processes of pre-vaporized fuels. [ABSTRACT FROM AUTHOR]

Soot formation is quantified in detail (volume fraction, particle size, number concentration, and light emissivity dispersion exponent) in a series of partially premixed counterflow flames of ethylene at equivalence ratios, Φ, equal to 6.5, 5.0, and 4.0, and with maximum temperature spanning approximately 200 K. The focus is to investigate the effect of peak temperature and equivalence ratio on soot formation while maintaining constant global strain and stoichiometric mixture fraction. Oxygen is progressively displaced from the oxidizer to the fuel stream of a diffusion flame to stabilize partially premixed flames of decreasing Φ, showing a double-flame structure consisting of a rich premixed flame component stabilized on the fuel side of the stagnation plane and a diffusion flame component stabilized on the oxidizer side. Soot is detected in the region sandwiched between the two flame components, is formed in both of them, and is convected away radially at the Particle Stagnation Plane (PSP). At fixed Φ, raising the peak temperature invariably raises the soot volume fraction throughout the probed region. Vice versa, at fixed peak temperature, lowering the equivalence ratio causes the premixed flame component to shift away from the diffusion flame component, with the consequent broadening of the soot forming region and an increase in both soot volume fraction as well as soot particle sizes through an enhancement of surface growth. Detailed probing of the region in the vicinity of the PSP offers evidence of soot oxidation from molecular oxygen. Furthermore, when the maximum temperature is sufficiently low, the net soot production rate turns negative because surface oxidation overwhelms surface growth. Comparing the soot number production rate inferred from experiments to the dimerization rate of benzene (C 6 H 6), naphthalene (C 10 H 8), and pyrene (C 16 H 10) reveals that only the smallest aromatics are present in flames at sufficiently large concentrations to account for soot nucleation. This observation applies to both the diffusion flame and the premixed flame components and confirms previous findings in strictly diffusion flames. [ABSTRACT FROM AUTHOR]

Large eddy simulations of turbulent swirl-stabilised flames gradually approaching blow-off conditions in a gas turbine model combustor are undertaken. The global equivalence ratio of the flames is reduced by increasing and decreasing the air and fuel flow rates respectively. The filtered reaction rate for partially premixed combustion is modelled using a presumed joint probability density function flamelet model. The average position of the flame is closer to the fuel nozzle when the global equivalence ratio is decreased, contrary to what is expected. Comparisons are made with simultaneous particle image velocimetry, and acetone and OH planar laser induced fluorescence imaging for the corresponding simulated case. It is demonstrated that the mechanisms leading to local extinction are well captured in the simulation and insufficient mixing induced by the precessing vortex core leads to local extinction. Further analysis of the simulations shows that including heat loss effects within the modelling is important for flames near or approaching blow-off. The non-adiabatic simulation influences the re-stabilisation of the flame after lift-off and shows that the flame leading edge follows the rotation of the PVC. A blow-off correlation based on the Damköhler number is proposed using the PVC rotation frequency and a chemical time scale. Analysis shows that the simulated flames respond to the correlation and including non-adiabatic flamelets is important for flames approaching blow-off. [ABSTRACT FROM AUTHOR]