Ethylene-air Flames Research Articles

Soot Measurements and a Simplified Analytical Formulation-Based Modeling in Laminar Counterflow Diffusion Flames

ABSTRACT The paper presents soot volume fraction measurements and a simplified soot formation model. Soot formation is a complex physicochemical process predominantly modeled using detailed chemistry and stochastic methods. The current work presents a simplified analytical function-based approach to calculate the net soot formation rate by coupling the local mixture-fraction and temperature fields. The validation experiments were carried out in a unique large-diameter (40 mm) counterflow burner for ethylene-air flames at various strain rates and normalized separation (L/D) distances. The soot volume fractions (fv) were determined using Laser Induced Incandescence (LII) technique calibrated by the Light Extinction (LE) method. Further model validations were carried out against the experimental measurements in the literature. The modeling work was performed on OpenFOAM®. The model predicted soot distributions for various peak flame temperatures, stoichiometric mixture-fractions, burner dimensions, fuel-oxidizer compositions, and strain rates with good accuracy. The study underscores the importance of considering two soot formation rate peaks, one at high temperature and one at low temperature, for accurate predictions, unlike a single peak employed in common analytical models. Furthermore, the effect of velocity boundary conditions on soot zone locations and the significance of providing realistic velocity conditions for the simulations are also discussed. Particle Image Velocimetry (PIV) measurements were performed to determine the actual nozzle-exit velocity profiles.

Non-periodic shear layer instabilities driving local extinction in premixed ramjet cavity flames

The local extinction characteristics of turbulent premixed ethylene-air flames are experimentally investigated in a high-speed cavity combustor to provide a full characterization of the temporally evolving non-periodic hydrodynamic instabilities that result in local extinction. The local flame extinction phenomenon is captured temporally using high-speed particle image velocimetry (PIV) and broadband chemiluminescence. The evolution of the flame topology, turbulent structures, and flame strain rate are analyzed through the extinction process to distinguish the turbulent shear layer mechanisms contributing to flame instabilities and extinction. A frequency analysis of the extinction event was performed and found the flame extinction to occur at non-periodic temporal intervals. A further wavelet decomposition revealed the presence of a non-periodic hydrodynamic instability in the shear layer causing extinction. The instability mechanism is related to the stability of vortex pairing in the shear layer, which were found to correspond to large-amplitude wave generation on the flame surface. In instances of flame extinction, the vortex pairs exhibit a core deformation from an elliptic instability mechanism, leading eventually to the shredding of the vortical cores, which is the primary mechanism for excessive turbulent flame strain and resulting extinction. Although the local flame extinction mechanism is driven from flame and shear layer interactions, the condition for local flame extinction to occur requires the flame to interact with an elliptical instability in the shear layer. The results can guide strategies to enhance combustion stability, performance, and efficiency.

A Filter-Based Approach for the Measurement of Mass Absorption Coefficient of OC and EC Components in Soot

ABSTRACT There is a general consensus that soot produced by hydrocarbon combustion consists of two components, elemental carbon associated with the graphitic nanostructure and organic carbon. Despite the relevance of evaluating their radiative properties, particularly for organic carbon, the knowledge of the spectral absorption behavior remains limited. In this work, a dual-beam five wavelengths absorption photometer is used to estimate the optical properties of particulate matter generated by a laminar premixed ethylene-air flame at different residence times along the flame. The wavelength dependence of the light absorption of aerosols collected on filters is analyzed in terms of both the optical band gap and the absorption Ångström exponent, AAE. The aerosols are then analyzed in terms of the organic and elemental carbon content by a thermal-optical OC-EC analyzer. Combining the results of light absorption and OC-EC analysis, the spectral mass absorption coefficient of both the organic carbon and elemental carbon components in soot are obtained.

A priori and a posteriori analysis of flamelet modeling for large-eddy simulations of a non-adiabatic backward-facing step

A lean premixed ethylene–air flame in a backstep configuration is simulated on multiple grids using both direct numerical simulations (DNS) with reduced order kinetic mechanism and large eddy simulations (LES) with flamelet-based thermochemistry. The configuration includes preheated reactants and a recirculation zone that provides radicals and high temperature gases to stabilize the flame. Heat losses are present due to the proximity of cooled walls. The reacting flow obtained from DNS at different resolutions is first analyzed to investigate the property of heat transfer within the recirculation region. LES based on adiabatic flamelets with a correction of the heat capacity is then tested, and its ability to account for heat losses is compared to results obtained using a three-dimensional non-adiabatic flamelet approach. Mean fields and subgrid properties are compared to those obtained from DNS to assess the capability of the LES models. The results show that the non-adiabatic flamelet approach can predict recirculation region and temperature fields with good accuracy. The model with heat capacity correction is able to effectively correct the heat capacity behavior as observed by a priori comparisons. However, in the a posteriori context, it is observed to overestimate the temperature field, although the correct size of the recirculation region is predicted. The combined a priori and a posteriori analyses on the same configuration and at different mesh resolutions allow for a precise separation of modeling effects due to heat transfer at the wall and combustion closure, thus providing indications on the LES performance in the context of flamelets.

Studies on flow field, and instabilities in a large diameter opposed jet burner

This work reports studies on flow fields, and instabilities exhibited by opposed jets at equal momenta, for Reynolds numbers (Re) ranging from 178 to 5000 using a large diameter counterflow jet setup. Flow instability investigations were conducted over a range of aspect ratios (α) too. This study identifies the regions of bi-stability and those of oscillatory stagnation plane offsets identified by Re. Experiments on flow field were carried out using particle image velocimetry technique, and the paper presents the axial and radial velocity profiles at various locations as well as their gradients. A decreasing trend in stagnation plane displacements with the Reynolds number was observed. The experiments in comparison with the past literature suggest a possible dependence of the stagnation plane displacements on a nozzle-exit diameter. The trends in maximum stagnation plane displacements (δmax), as well as the critical Reynolds numbers (Recr) with aspect ratios (α), are analyzed and compared. The flow field studies reveal the need for two dimensional axisymmetric simulations with realistic velocity boundary conditions to predict opposed jet flow phenomena accurately. Reacting flow instability studies were also carried out for equal momenta using methane-air and ethylene–air flames at various aspect ratios. The results show an enhanced bistability for ethylene–air flames over methane–air flames.

Soot Size Determination In A Swirl Stratified Premixed Ethylene-Air Flame By A Sampling Probe And Elastic Light Scattering

An experimental study was performed to measure the size and number density distributions of soot particles produced in turbulent, stratified (high spatial equivalence ratio gradient), swirled (rotating flow) premixed ethylene-air flames at atmospheric pressure. Soot particle size and number density measurements are initially performed using a Scanning Mobility Particle Sizer (SMPS). For this purpose, two-stage sampling and dilution device designed for hot gas analysis was used. The dilution rate of the sampling apparatus was evaluated to ensure a most representative probing. In a second step, the ex-situ measurements were replaced by single-shot measurements recorded with a high cadency planar multi-angle light scattering (2D-MALS) laser diagnostic technique, based on the relationship between aggregate size and light scattering angle. In a third step, results obtained by the SMPS as well as by the optical diagnostic were compared. Because a direct confrontation of the obtained results obtained is not feasible due to difference in nature of both techniques, a two-step post-processing methodology as developed. For this purpose, the median electric mobility diameters measured by SMPS were converted into diameters of gyration by means of a semi-empirical conversion tool. Moreover, a time average data processing was applied to the soot size distributions recorded by 2D-MALS, considering that SMPS measurements performed over a relatively large acquisition time converge to a single value. Results obtained in stratified swirled premixed flames by SMPS exhibit an overall good agreement with the optical measurements. This good similarity between the results provides a remarkably high degree of confidence in the quality of the measurements for both measurement systems investigated. Furthermore, the results suggest that a comparison of soot aggregate size and number density measurements between a sampling probe and a light scattering technique is still possible in turbulent flames at atmospheric pressure. While the current work has focused on the performances of SMPS and the 2D-MALS approach on a one-to-one basis, future work will also address the application of these diagnostics to high-pressure two-phase flames for which measurements are made in confined environments.

Particle size distributions in turbulent premixed ethylene flames crossing the soot inception limit

The current study presents novel experimental data on soot particle size distributions (PSDs) with mobility diameters in the size range 4≤Dm[nm]≤230 obtained from four premixed turbulent ethylene-air flames crossing the soot inception limit. The flames are stabilised against hot combustion products from nitrogen diluted hydrogen flames in a back-to-burnt (or fresh-to-burnt) opposed jet configuration. The burner features fractal grid generated turbulence to provide accurate control of the turbulence. A scanning mobility particle sizer (SMPS) equipped with nano- and long–DMA columns coupled with a dual dilution port quartz probe is used and a comprehensive analysis of optimal sampling conditions to minimise particle losses is presented. Spatially resolved data along the stagnation point streamline is obtained to show the evolution of PSDs through the turbulent flame brush. It is shown that turbulent transport distributes soot particles across the mixing layer between the two jets with the maximum median and mean mobility diameters found close to the stagnation point. The impact of the estimated total mean rate of strain (420≤aT[s−1]≤610) and equivalence ratio (1.8≤ϕUN≤2.2) on PSDs is also quantified.

Applying an in-situ calibration method of spectral line shape to determine flame temperature of methane and ethylene spherically expanding flames

Spectral line shape caused by the collision between probing molecules and surrounding species is an important parameter that affects the accuracy of combustion diagnostics. Here, we report an in-situ calibration method of the pressure broadening coefficients using spherically expanding flames over continuously increasing flame pressure. A desirable isothermal condition is produced in the spherical expanding flame with unity Lewis number, in which the thermal and molecular diffusions are balanced during flame propagation. Laminar burning velocities and Markstein length of methane-air and ethylene-air flames are measured to understand their thermodynamic and transport properties. Mid-infrared absorption spectra of carbon dioxide at 4.2 µm are recorded from 0.3 atm to 5 atm in a single run of the experiment. Pressure broadening parameters are extracted using an empirical fitting procedure. After extending to wide ranges of flame conditions, the flame temperatures of the methane-air and ethylene-air flames are correctly determined using the in-situ-determined line shape parameters. The flame temperature deviation from adiabatic flame temperature is imperceptible in the weakly stretched quasi-steady spherically expanding flames. Such unity Lewis number spherical expanding flames are proven to be near-ideal reactors for spectroscopic calibration in combustion diagnostics.

Influence of potassium chloride on PAH concentration during soot formation studied using laser-induced fluorescence

Previous studies have shown that addition of potassium chloride (KCl) reduces soot concentration and primary particle size. To improve the understanding of this phenomenon, this work focuses on investigating the influence of KCl addition on formation of polycyclic aromatic hydrocarbons (PAHs), assumed to be precursors of soot. The method used is laser-induced fluorescence (LIF), for which different size classes of PAHs can be probed using selected combinations of excitation and detection wavelengths. KCl solutions of different concentrations were aspirated into premixed ethylene-air flames, and spectral and 2D imaging PAH-LIF measurements were performed using laser excitation at 266 nm and 532 nm. PAH-LIF spectra using 266 nm excitation showed a peak in the UV region at around 350 nm and a peak at around 550 nm in visible region, where the latter peak intensity increased strongly with flame height. The normalised PAH-LIF spectra obtained using 532 nm excitation showed no spectral change with flame height. The main results from this study showed that KCl addition, firstly, had negligible influence on the formation of smaller PAHs (measured using excitation at 266 nm and detection at ∼ 320 nm), and secondly, decreased the formation of larger PAHs at increasing flame heights (measured using excitation at 532 nm and detection in the visible region).

In situ characterisation of absorbing species in stationary premixed flat flames using UV\u2013Vis absorption spectroscopy

A one-dimensional premixed ethylene–air flame is investigated regarding the presence of various combustion intermediates and products relevant for the formation of carbonaceous particles for various equivalence ratios and spatial positions using in situ UV–Vis absorption spectroscopy. A laser driven light source in combination with a fast spectrometer allow to record absorption spectra at a high rate required for practical combustion devices. The approach is coupled with a least squares regression procedure using a database of several absorbing species in the flame. To account for the high temperature flame conditions, the absorption spectra are convoluted by a simplified Maxwell–Boltzmann distribution model. While the approach is based on several assumptions and a verification requires future detailed intercomparison with other techniques, a first semi-quantitative evaluation can be obtained. This novel approach opens a potential route to the in situ measurement of the evolution of polycyclic aromatic hydrocarbons (PAHs) in flames.

Two-dimensional laser-induced incandescence for soot volume fraction measurements: issues in quantification due to laser beam focusing

Two-dimensional laser-induced incandescence (LII) measurements usually involve the use of a cylindrical lens to illuminate the planar region of interest. This creates a varying laser fluence and sheet width in the imaged flame region which could lead to large uncertainties in the quantification of the 2D LII signals into soot volume fraction distributions. To investigate these effects, 2D LII measurements using a wide range of laser pulse energies were performed on a premixed flat ethylene–air flame while employing a cylindrical lens to focus the laser sheet. Using shorter focal length of the focusing lens resulted in larger variation of the LII signal profiles across the flame. A heat – and – mass – transfer - based LII model was also used to simulate the measurements and good agreement was found. The ratio between focal length (FL) and image length (IL) was introduced as a useful parameter for estimating the bias in estimated soot volume fractions across the flame. The general recommendation is to maximize this FL/IL ratio in an experiment, which in practice means the use of a long focal length lens. Furthermore, the best choices of laser fluence and detection gate width are discussed based on results from these simulations.

Molecular structures in flames: A comparison between SNapS2 and recent AFM results

Models capturing the growth of polycyclic aromatic compounds (PACs) and nanoparticles in combustion have always faced a large degree of uncertainty due to the paucity of detailed direct experimental validation. In particular, data on molecular structures, chemical composition, size, cross-linking, and aliphatic chains is still very limited. In the past few years, we have developed an atomistic code, SNapS2, that models the formation of PACs in combustion conditions, providing information on the chemical and structural evolution of these compounds. In this paper, we present a detailed analysis of the compounds formed in a premixed ethylene-air flame that was previously characterized experimentally by AFM (Commodo et al. 2019). The comparison is based on molecular structures and extends to other properties of PACs. The results demonstrate that SNapS2 is able to capture the vast array of PACs in terms of a large variety of functional groups. The simulations, confirmed to a large extent by the experimental data, show the presence of different types of oxygenated (e.g., phenol, ketone) and cyclic (acenaphthylene-type, acenaphthene-type, and fluorene-type five-membered rings) functional groups. Moreover, the structures that were not observed in the simulations (indane-type five-membered rings or six-membered rings containing oxygen) are missing due to the lack of formation pathways, highlighting the fact that important kinetic mechanisms are still missing in the literature. Finally, we observed that a preponderant number of high molecular mass PACs are curved, which may have an impact on their aggregation and sampling. Overall, while some discrepancies remain due to the inherent limitation of the model and the AFM techniques, this work demonstrates the unique capabilities of the SNapS2 code to provide insights on the structures and chemical pathways of PACs.

Electronic band gap of flame-formed carbon nanoparticles by scanning tunneling spectroscopy

The electronic band gap of flame-formed carbon nanoparticles (CNPs) is determined for the first time using scanning tunneling spectroscopy (STS). CNPs are sampled from a laminar, burner-stabilized stagnation ethylene-air flame through in-situ insertion of gold-coated mica substrates, on which CNPs are collected as thin films. Optical band gap measurements using UV–Vis absorption spectroscopy are also performed to compare with the STS measurements. The suitability of STS in measuring electronic band gaps is first demonstrated through measurements using reference materials: the electronic band gaps of naphthalene, pyrene, and coronene are found to be slightly larger than the respective optical band gaps, as expected, because of the over potential in the presence of a current. Flame-formed CNPs of three different particle size distributions were sampled and tested. The electronic band gap of CNPs shows a clear size-dependency that supports the earlier conclusion (Liu et al. Proc. Natl. Acad. Sci. U.S.A. 116 (2019) 12,692–12,697) that flame-formed CNPs exhibit apparent quantum confinement or quantum dot behaviors. Furthermore, the measured electronic band gaps matches the respective optical results within the experimental uncertainty of 0.1–0.2 eV. The over potential observed in PAH samples is not present in CNP samples, which may indicate stronger intermolecular interactions among the PAHs constituting the CNPs than those of crystalline PAHs.

Constrained-temperature solutions of coflow laminar diffusion flames

We investigate the use of an experimental 2-D temperature profile to constrain detailed numerical solutions of a sooting coflow laminar diffusion flame. Experimentally, four optical diagnostic techniques are used to measure the two-dimensional temperature field in an ethylene-air coflow flame. This experimental temperature field is then used to impose the temperature in the solution process, thus obviating the need to solve the energy equation and, in particular, to incorporate costly models of radiative losses in the flame. Results are presented for a 40% ethylene-air flame on the Yale Coflow Burner. In the unconstrained solution of the complete set of governing equations, the location of maximum temperature is found along the flame wings, whereas the experimental temperature field has its maximum along the centerline. Similarly, the location of peak soot volume fraction migrates from along the flame wings in the unconstrained calculation, where soot surface growth processes dominate, to the centerline in the constrained case, where soot inception is the dominant condensed-phase formation mechanism. The distribution of soot in the constrained solution is much more consistent with experimental observations, and this fact illustrates how the validation of a soot sub-model may be complicated by the necessity of modeling distributed heat losses in the flame.

Characterization of the effects of a plasma injector driven by AC dielectric barrier discharge on ethylene-air diffusion flame structure

AbstractA dielectric barrier discharge plasma controlled diffusion flame experimental system was built based on the designed coaxial swirling plasma injector. The air plasma was generated within the annulus gap of the injector by alternating current dielectric barrier discharge. The discharge characteristics and power of plasma injector under different actuation intensities and air flowrates were measured. Through the measurement techniques, such as schlieren imaging, broadband chemiluminescence image and CH* chemiluminescence, the effect and mechanism of plasma on ethylene-air normal diffusive jet and flame was explored. The results showed that a large number of filamentary discharge channels are formed in air plasma. The increase of air flowrates weakened the intensity of discharge to a certain extent. The induced jet generated by the plasma can short the laminar length of the ethylene-air jet, accelerate the transition of the flow jet, enhance the turbulence and the mixing of the fuel and the oxidizer. The higher the actuation intensity, the shorter distance of the cold jet transition zone, the higher the jet turbulence. Depending on the aerodynamic and kinetic effects, plasma can improve the stability of ethylene-air diffusive flame and reduce the lift height between the flame root and injector. The plasma can also expand the flammability limit of ethylene-air flame and make the flame ignited under some conditions that could not be. In addition, the CH* chemiluminescence shows that, in a certain range of discharge voltage, the heat release distribution can be changed on both sides of the flame, and its representative length are generally reduced as the voltage rises. On the contrary, the overtop voltage could lead to a decrease of flame heat release.

WIDECARS multi-parameter measurements in premixed ethylene-air flames using a wavelength stable ultrabroadband dye laser.

Width-increased dual-pump enhanced coherent anti-Stokes Raman spectroscopy (WIDECARS) measurements were used to determine the temperature and major species mole fractions in laminar, premixed, ethylene-air flames operating at atmospheric pressure. Conventional ultrabroadband dye lasers for WIDECARS, which use Pyrromethene dyes, have historically suffered from day-to-day wavelength shifting. To overcome this problem, a new ultrabroadband dye laser was developed in this study to provide a stable wavelength and power generation. A new dye laser pumping scheme and a mixture of Sulforhodamine 640, Kiton Red 620, and Rhodamine 640, was used to generate the desired FWHM ${\sim}{15}\;{\rm nm}$∼15nm (${410}\;{{\rm cm}^{ - 1}}$410cm-1) bandwidth. The WIDECARS measured mole fraction ratios of ${{\rm CO}_2}$CO2, CO, and ${{\rm H}_2}$H2 with ${{\rm N}_2}$N2 agreed well with chemical equilibrium calculations.

Experimental Investigation of Soot Production and Oxidation in a Lab-Scale Rich\u2013Quench\u2013Lean (RQL) Burner

Swirl-stabilized, turbulent, non-premixed ethylene–air flames at atmospheric pressure with downstream radially-injected dilution air were investigated from the perspective of soot emissions. The velocity and location of the dilution air jets were systematically varied while the global equivalence ratio was kept constant at 0.3. The employed laser diagnostics included 5 kHz planar laser-induced fluorescence (PLIF) of OH, 10 Hz PAH-PLIF, and 10 Hz laser-induced incandescence (LII) imaging of soot particles. OH-PLIF images showed that the reaction zone widens with dilution, and that regions with high OH-LIF signal shift from the shear layer to the axis of the burner as dilution increases. Dilution is effective at mitigating soot formation within the central recirculation zone (CRZ), as evident by the smaller PAH-containing regions and the much weaker LII signal. Dilution is also effective at halting PAH and soot propagation downstream of the dilution air injection point. The high momentum dilution air circulates upstream to the root of the flame and reduces fuel penetration lengths, induces fast mixing, and increases velocities within the CRZ. Soot intermittency increased with high dilution velocities and dilution jet distances up to two bluff body diameters from the burner inlet, with detection probabilities of < 5% compared to 50% without dilution. These results reveal that soot formation and oxidation within the RQL are dependant on the amount and location of dilution air injected. This data can be used to validate turbulent combustion models for soot.

Hybrid Eulerian-Lagrangian method for soot modelling applied to ethylene-air premixed flames

Soot formation has become an important issue in the design of gas turbine combustors due to its environmental impact and its contribution to radiative heat transfer in the combustion chamber. However, efficient and accurate prediction of soot particles formation, growth, oxidation and interaction in gas turbine combustors is still an open field in computational fluid dynamics. The present approach proposes to combine a reduced gas-phase chemistry, a sectional model for polycyclic aromatic hydrocarbons, and a Lagrangian description of soot particles dynamics. The Lagrangian description has been chosen for its ability to simulate the evolution of the particle size distribution. A numerical procedure is proposed to minimise its CPU cost. This approach was successfully applied to the simulation of steady laminar premixed ethylene-air flames at three fuel equivalence ratios, which constitutes a prerequisite towards its use in an aeronautical combustion chamber.

Effect of propagation speed on the quenching of methane, propane and ethylene premixed flames between parallel flat plates

This paper reports on an experimental investigation of the effect of flame speed on the quenching distance. In a rectangular section channel, the quenching distance of methane-air, propane-air and ethylene-air flames, arriving on the quenching element with different speeds, is investigated. The experiments are performed in a constant volume combustion chamber comprising two volumes separated by the quenching section. The quenching tests are performed at pressure close to atmospheric, with an initial temperature of the walls and flammable mixture of 293 K. Flame quenching is assessed from pressure measurements, while the apparent flame speed is determined from fast schlieren imaging. The results show a similar trend for the three fuels considered: the quenching distance for laminar propagating flames is about twice smaller than when the flow becomes turbulent, close to the entrance of the channel. In the case of laminar flames, the quenching distance is almost constant at 1.3–1.5 mm for the conditions considered, while for turbulent flames, depending on the fuel and equivalence ratio, the quenching distance is in the range 2–2.5 mm. Starting from the quenching theory, the results are analyzed and the effects of flame curvature and turbulence are discussed.

Large eddy simulation of CH4-air and C2H4-air combustion in a model annular gas turbine combustor

Combustion instabilities are one of the major challenges in developing and operating propulsion and power generating gas-turbine engines. More specifically, techniques for managing the increasingly stringent emissions regulations and efficiency demands have often given rise to thermo-acoustic instabilities, particularly for annular combustors operating in a lean premixed mode. In this paper, we combine experimental and computational methods to examine unsteady gas turbine combustion in a full annular model gas turbine combustor installed at NTNU, operating both methane- and ethylene-air blends. The experimental data consists of flame images, high-speed OH* chemiluminescence images, as well as pressure and heat-release time-series at discrete locations for the ethylene-air case. The computational set-up consists of the 18 inlet tubes and swirlers, and the full annular combustor placed in a large external domain. The computational model consists of a compressible finite rate chemistry LES model using skeletal methane-air and ethylene-air combustion chemistry. The combustor is simulated in its self-excited state, without external forcing. From the experiments and simulations the methane and ethylene cases are found to behave differently: The ethylene-air flames are much smaller than the methane-air flames, resulting in different interaction between adjacent flames. The LES predictions show good qualitative agreement with the measurements in terms of instantaneous and time-averaged flame structure. Comparing measured and predicted time-series of pressure and heat-release also shows good quantitative agreement with respect to the dynamics and structure for the ethylene-air case. Investigating the predicted combustion dynamics using Proper Orthogonal Decomposition (POD) confirms the importance of the self-excited azimuthal mode on the behavior of the flame: the presence of nodes and anti-nodes of pressure induced fluctuations of the swirler mass-flow, which then, in turn, influence the heat-release. These events occur shifted in time.