Non-premixed Ethylene Air Research Articles

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.

Relationship between soot and scalar dissipation rate in the soot-inception region of turbulent non-premixed jet flames

Abstract In previous work we employed the Krypton Planar Laser-Induced Fluorescence technique combined with simultaneous Laser Induced Incandescence to obtain measurements of mixture fraction, temperature and soot volume fraction in the soot inception region of turbulent non-premixed ethylene-air flames (Park et al., 2017). This paper extends that work and reports the first simultaneous measurements in turbulent flames of the scalar dissipation rate and soot-volume fraction. The specific jet composition was 50% C2H4, 46% N2 and 4% Kr and the jet Reynolds number was 8300. The measurements were made 10 cm downstream of the jet exit where the soot first begins to form. The present study uses the same data set to derive the scalar dissipation rate fields and relate the dissipation to mixture fraction, soot volume fraction and temperature. The peak mean soot volume fraction was found to be well inside of the stoichiometric surface near values of mixture fraction from 0.3 to 0.4, which roughly corresponded to the regions of peak mean scalar dissipation. The polycyclic aromatic hydrocarbon field was also imaged, but not simultaneously with the other quantities. Although, the mean soot location is correlated to the mean dissipation, this result is misleading as shown by joint probability density function (PDF) between soot volume fraction and scalar dissipation. The joint PDF shows that soot is in fact no typically spatially coincident with regions of high scalar dissipation; in fact, higher dissipation is correlated with lower values of soot. These latter observations are consistent with results from recent numerical simulations that soot was preferentially found in the areas of low scalar dissipation rate and soot is locally inhibited by high scalar dissipation rates.

Lagrangian tracking of soot particles in LES of gas turbines

Expected stringent legislation on particulate matter (PM) emission by gas turbine combustors is currently motivating considerable efforts to better understand, model and predict soot formation. This complex phenomenon is very difficult to study in detail with experiment, and numerical simulation is an essential complementary tool. Considering that the chemistry of soot particles strongly depends on their size, the numerical prediction of soot formation requires the description of their size distribution. To do so, either Eulerian methods (sectional or moments), or stochastic Lagrangian approaches are reported in the literature. In the present work a far more simple semi-deterministic Lagrangian approach is proposed. Combined to the semi-empirical model of Leung et al. (1991) for soot chemistry, the Lagrangian approach is first validated on a one-dimensional premixed ethylene-air flame. The model is then applied to a gaseous non-premixed ethylene-air burner measured at DLR and computed with Large Eddy Simulation (LES). The gaseous chemistry is described with an Analytically Reduced Chemistry (ARC) to guarantee a good prediction of combustion and gaseous soot precursors. Results are validated against experiment and compared, in terms of accuracy and CPU cost, to an Eulerian semi-empirical model. To the authors knowledge, it is the first time that such Lagrangian particle tracking approach is used for soot. Results obtained in terms of accuracy and computing time are very encouraging.

Coupling an LES approach and a soot sectional model for the study of sooting turbulent non-premixed flames

Due to their negative impacts on environment and human health, future regulations on soot emissions are expected to become stricter, in particular by controlling the size of the emitted particles. Therefore, the development of precise and sophisticated models describing the soot production, such as sectional methods, is an urgent scientific and industrial challenge. In this context, the first objective of this work is to use for the first time a sectional model to perform an LES of a sooting turbulent flames in order to demonstrate its capacities. For this, the whole LES formalism for this approach is developed. It includes state-of-art models for the description of the gaseous phase and an extension of a soot subgrid intermittency model to the sectional approach, originally proposed for the hybrid method of moments. Then, the LES is used to analyze a turbulent non-premixed ethylene–air jet diffusion flame and results are validated by available experimental data. The quality of results for the gaseous phase is satisfactory and results for the solid phase show a reasonable agreement with the experimental results in terms of localization, intermittency and soot volume fraction magnitude. Once the coupled LES-sectional approach validated, having access to the full information on the spatial and temporal evolution of the soot Particle Size Distribution (PSD), the second objective of this work is to provide a new fundamental insight on soot production in turbulent non-premixed flames. First, it is observed that a one-peak and a two-peak PSD shapes are observed at the bottom and downstream of the flame, respectively. Second, high fluctuations of the PSD distribution is observed all along the flame. In particular, a time bimodal behavior is observed with the presence of a zone with regular transitions between one- and two-peak PSD shapes. By analyzing soot particles Lagrangian paths, these high fluctuations are shown to be linked with the wide range of history paths of soot particles, which are mainly driven by turbulence.

Automated determination of size and morphology information from soot transmission electron microscope (TEM)-generated images

The thermophoretic sampling of particulates from hot media, coupled with transmission electron microscope (TEM) imaging, is a combined approach that is widely used to derive morphological information. The identification and the measurement of the particulates, however, can be complex when the TEM images are of low contrast, noisy, and have non-uniform background signal level. The image processing method can also be challenging and time consuming, when the samples collected have large variability in shape and size, or have some degree of overlapping. In this work, a three-stage image processing sequence is presented to facilitate time-efficient automated identification and measurement of particulates from the TEM grids. The proposed processing sequence is first applied to soot samples that were thermophoretically sampled from a laminar non-premixed ethylene-air flame. The parameter values that are required to be set to facilitate the automated process are identified, and sensitivity of the results to these parameters is assessed. The same analysis process is also applied to soot samples that were acquired from an externally irradiated laminar non-premixed ethylene-air flame, which have different geometrical characteristics, to assess the morphological dependence of the proposed image processing sequence. Using the optimized parameter values, statistical assessments of the automated results reveal that the largest discrepancies that are associated with the estimated values of primary particle diameter, fractal dimension, and prefactor values of the aggregates for the tested cases, are approximately 3, 1, and 10 %, respectively, when compared with the manual measurements.

Dynamics of flow–soot interaction in wrinkled non-premixed ethylene–air flames

A two-dimensional simulation of a non-premixed ethylene–air flame was conducted by employing a detailed gas-phase reaction mechanism considering polycyclic aromatic hydrocarbons, an aerosol-dynamics-based soot model using a method of moments with interpolative closure, and a grey gas and soot radiation model using the discrete transfer method. Interaction of the sooting flame with a prescribed decaying random velocity field was investigated, with a primary interest in the effects of velocity fluctuations on the flame structure and the associated soot formation process for a fuel-strip configuration and a composition with mature soot growth. The temporally evolving simulation revealed a multi-layered soot formation process within the flame, at a level of detail not properly described by previous studies based on simplified soot models utilizing acetylene or naphthalene precursors for initial soot inception. The overall effect of the flame topology on the soot formation was found to be consistent with previous experimental studies, while a unique behaviour of localised strong oxidation was also noted. The imposed velocity fluctuations led to an increase of the scalar dissipation rate in the sooting zone, causing a net suppression in the soot production rate. Considering the complex structure of the soot formation layer, the effects of the imposed fluctuations vary depending on the individual soot reactions. For the conditions under study, the soot oxidation reaction was identified as the most sensitive to the fluctuations and was mainly responsible for the local suppression of the net soot production.

Direct numerical simulations of non-premixed ethylene–air flames: Local flame extinction criterion

Direct Numerical Simulations (DNS) of ethylene/air diffusion flame extinctions in decaying two-dimensional turbulence were performed. A Damköhler-number-based flame extinction criterion as provided by classical large activation energy asymptotic (AEA) theory is assessed for its validity in predicting flame extinction and compared to one based on Chemical Explosive Mode Analysis (CEMA) of the detailed chemistry. The DNS code solves compressible flow conservation equations using high order finite difference and explicit time integration schemes. The ethylene/air chemistry is simulated with a reduced mechanism that is generated based on the directed relation graph (DRG) based methods along with stiffness removal. The numerical configuration is an ethylene fuel strip embedded in ambient air and exposed to a prescribed decaying turbulent flow field. The emphasis of this study is on the several flame extinction events observed in contrived parametric simulations. A modified viscosity and changing pressure (MVCP) scheme was adopted in order to artificially manipulate the probability of flame extinction. Using MVCP, pressure was changed from the baseline case of 1atm to 0.1 and 10atm. In the high pressure MVCP case, the simulated flame is extinction-free, whereas in the low pressure MVCP case, the simulated flame features frequent extinction events and is close to global extinction. Results show that, despite its relative simplicity and provided that the global flame activation temperature is correctly calibrated, the AEA-based flame extinction criterion can accurately predict the simulated flame extinction events. It is also found that the AEA-based criterion provides predictions of flame extinction that are consistent with those provided by a CEMA-based criterion. This study supports the validity of a simple Damköhler-number-based criterion to predict flame extinction in engineering-level CFD models.

Soot formation with C1 and C2 fuels using an improved chemical mechanism for PAH growth

Recently, an improved chemical mechanism of PAH growth was developed and tested in soot computations for a laminar co-flow non-premixed ethylene–air diffusion flame. In the present work, the chemical mechanism was enhanced further to accommodate the PAH gas phase growth in methane, ethylene and ethane co-flow flames. The changes in the mechanism were tested on a methane/oxygen and two ethane/oxygen premixed flames to ensure no degradation in its application to C2 fuels. The major soot precursors were predicted in a satisfactory matter. The robustness of the soot solution methodology was tested for different fuels by solving methane/air, ethane/air and ethylene/air co-flow laminar diffusion flames using a single solution algorithm for all three cases. The peak soot volume fractions, which varied by two orders of magnitude between fuels, were predicted within a factor of two for all flames. The computations were also able to reproduce the spatial distributions of soot and to explain the variation in soot formation pathways among the fuels. Despite a similarity in bulk properties of the flame, the soot particles in different flames exhibit significantly different growth modes. Ethylene/air flames tend to form soot earlier than methane/air flames and inception plays a bigger role in the latter.

Numerical investigation of soot formation mechanisms in partially-premixed ethylene–air co-flow flames

Recently, an improved chemical mechanism of PAH growth was developed and tested in soot computations for a laminar co-flow non-premixed ethylene–air diffusion flame [Dworkin et al., Combust. Flame 158(9) (2011) 1682–1695]. With the intention of testing the robustness of the solution methodology on partially-premixed systems, this work used the same algorithm as that in the study of Dworkin et al. for computations of two sets of sooting partially-premixed co-flow laminar ethylene–air flames. The results show very good qualitative and good quantitative agreement with the experimental results for soot volume fractions and soot precursors, without any changes to the parameters of the model. The soot yield was found to initially increase with decreasing primary equivalence ratios, and then to decrease for Φ<24, reaching levels lower than the non-premixed case for Φ<10. On the flame centerline, both PAH and acetylene-related processes were found to be important for soot growth. The initial increase in the soot yield was linked to higher inception rates. On the wings of the flame the dominant soot growth process was found to be HACA growth. The initial increase in the soot yield was mostly due to higher acetylene yield leading to faster surface growth. The primary air was also found to influence the soot oxidation process by increasing OH radicals in both the centerline and the wings region.

Assessment of interferences to nonlinear two-line atomic fluorescence (NTLAF) in sooty flames

Nonlinear excitation regime two-line atomic fluorescence (NTLAF) is a laser-based thermometry technique that has application in turbulent flames with soot. However, no assessment of the various interferences from soot or its precursors in flames with high soot loadings on the technique is available. To examine these issues, both on- and off-wavelength NTLAF measurements are presented and compared for laminar nonpremixed ethylene-air flames. Laser-induced incandescence (LII) measurements were used to determine the corresponding soot concentration and location in the investigated flames. The measurements indicate that interferences, such as spurious scattering and laser-induced incandescence from soot, are not significant for the present set of flame conditions. However, interferences from soot precursors, predominantly condensed species (CS) and perhaps polycyclic aromatic hydrocarbons (PAH), can be significant. Potential detection schemes to correct or circumvent these interference issues are also presented.

Cavity ignition and flameholding of ethylene–air and hydrogen–air flows by a repetitively pulsed nanosecond discharge

Repetitive nanosecond pulse plasma assisted ignition and flameholding of premixed and non-premixed ethylene–air and hydrogen–air flows are studied in a cavity flow at a pressure of 0.2 atm and flow velocities of up to 100 m/s. Ignition occurs via formation of multiple filaments in the fuel–air plasma, although air plasma remains diffuse until the fuel is added. After ignition occurs in the cavity, with ignition delay time of a few milliseconds, the plasma becomes diffuse and the flame couples out to the main flow. The use of a short cavity (length-to-depth ratio L/ D = 1) results in repetitive ignition and flame blow-off, caused by slow mixing between the main flow and the cavity. Increasing the length-to-depth ratio to L/ D = 3, as well as choking inlet air and fuel flows resulted in stable flameholding and nearly complete combustion in both premixed and non-premixed ethylene–air and hydrogen–air flows at u = 35–100 m/s. Air plasma temperature before fuel is added ranges from 70 °C to 200 °C. When the nanosecond pulse discharge is operated in repetitive burst mode, continuous ethylene–air flame is maintained only at a high duty cycle, which increases with the flow velocity. In hydrogen–air, the flame remains stable after the plasma is turned off. Nanosecond pulse discharge ignition of ethylene–air is compared with ignition by DC arc discharge of approximately the same power. DC arc discharge results in sporadic ignition and flame blow-off, much lower burned fuel fraction, and significantly lower flow velocity at which ignition can be achieved. Kinetic modeling is used to identify the reduced mechanism of plasma chemical oxidation and ignition of hydrogen, and to demonstrate the mechanism of energy release low-temperature reactions of radicals generated in the plasma (primarily O and H atoms).

Conditional moment closure prediction of soot formation in turbulent, nonpremixed ethylene flames

Results obtained from incorporating a semiempirical soot model into a first-order conditional moment closure (CMC) approach to modeling turbulent nonpremixed flames of ethylene and air are presented. Soot formation is determined via the solution of two transport equations for soot mass fraction and particle number density, with acetylene and benzene employed as the incipient species responsible for soot nucleation, and the concentrations of these species calculated using a detailed gas-phase kinetic scheme involving 463 reactions and 70 species. The study focuses on the influence of differential diffusion of soot particles on soot volume fraction predictions. The results of calculations are compared with experimental data for three sooting ethylene flames and, in general, predictions of mixing and temperature fields within the three flames show good agreement with data. Soot volume fraction predictions are found to be in significantly better accord with data when differential diffusion is accounted for in the CMC-based soot model, supporting the importance of such effects in sooting flames, as previously noted by Kronenburg et al. in relation to methane combustion. Overall, the study demonstrates that the CMC-based soot model, when used in conjunction with a model of differential diffusion effects, is capable of accurately predicting soot formation in turbulent nonpremixed ethylene–air flames.

Temperature measurement by holographic interferometry for non-premixed ethylene-air flame with a series of state relationships

Abstract This investigation developed a systematic method to measure the temperatures and the mass concentrations of local species in a non-premixed ethylene–air flame using the holographic interferometry (HI) technique. A nearly linear state relationship between the Gladstone–Dale constant and the mixture fraction was deduced. Normalized state relationships between the refractive index and the mixture fraction were established from detailed combustion simulations for the NPFs. The maximum error of temperature caused by the assumption of composition of air was reduced from 48.84% to under 1.6% for the NPFs using the systematic method. The method can overcome the difficulty that the concentrations of local species must be known when measuring the temperature by HI for NPFs.

Implementation of the population balance equation in CFD codes for modelling soot formation in turbulent flames

The simulation of soot formation in turbulent diffusion flames is carried out within a CFD code, by coupling kinetics and fluid dynamics computations with the solution of the population balance equation via the Direct Quadrature Method of Moments, a novel and efficient approach based on a quadrature approximation of the size distribution of soot particles. A turbulent non-premixed ethylene–air flame is used as the test case for validation of the model. Simplified kinetic expressions are employed for modelling nucleation, molecular growth and oxidation of particles, along with a Brownian aggregation kernel. A recently proposed approach for modelling the evolution of fractal dimension is used with a monovariate population balance to predict the morphological properties of aggregates.