Non-premixed Jet Research Articles

Large Eddy Simulation of soot evolution in turbulent reacting flows: Presumed subfilter PDF model for soot–turbulence–chemistry interactions

In turbulent reacting flows, soot evolution is strongly influenced by small-scale soot–turbulence–chemistry interactions. Specifically, soot is formed during combustion of fuel-rich mixtures and, in non-smoking flames, is rapidly oxidized at slightly fuel-rich mixtures before being transported by turbulence into fuel-lean mixtures. Furthermore, different soot evolution mechanisms are dominant over distinct regions of mixture fraction. For these reasons, a new subfilter PDF is proposed to account for this distribution of soot in mixture fraction space. In this model, the sooting mode of a soot subfilter PDF is locally activated only at fuel-rich mixture fractions where surface growth is locally faster than oxidation. This model is first validated a priori against a recent DNS database of a turbulent nonpremixed jet flame. Compared to a subfilter PDF model without any mixture fraction dependence, a significant decrease in the oxidation source term is observed, while the surface growth source term remains hardly affected. This new model is implemented within a Large Eddy Simulation (LES) framework, applied to two laboratory-scale turbulent nonpremixed sooting jet flames, and validated via comparisons with experimental measurements of time-averaged centerline temperature and soot volume fraction. Both the new model and the previous model predict the temperature fairly well. However, compared to the previous model, the soot volume fraction predicted by the new model is significantly increased and in much better agreement with the experimental measurements. In addition, the sensitivity to the subfilter PDF is significantly greater than the sensitivity to grid resolution and is greater or at least comparable to the sensitivity to chemical mechanism.

Conditional dynamic subfilter modeling

A novel “conditional” variation of the dynamic approach for modeling of large eddy simulation subfilter terms is derived and tested. In contrast to the traditional dynamic closure, which stabilizes “raw” dynamic coefficients by averaging across ensembles of expected statistical homogeneity, the novel variation averages conditionally on some set of scalars whose local values are expected to correlate with the local degree of turbulence. Simulations of a nonpremixed jet flame show that the conditional dynamic model is both tractable and stable and produces predictions which are essentially indistinguishable from the traditional dynamic closure, although both models give suboptimal predictions. Future work could potentially improve the predictions of both models—facilitating a fairer comparison—by considering a more uniform or “pancake-like” grid.

Numerical investigation of embedding some hot obstacles in a low speed reacting flow of the moderate or intense low-oxygen dilution in a jet-in-hot-coflow

Given the daily growth of pollutant emissions in this age of technology, every method that can increase the efficiency of the combustion as well as reduce the pollution is very important to consider. One way to reach this goal is moderate or intense low oxygen dilution. In the present work, this technology of turbulent nonpremixed CH4-H2 jet flames has investigated in various working conditions in Dally burner, numerically. In this regard, the finite volume method in OpenFOAM package has used with k-ε Re-Normalization Group as well as the discrete ordinates radiation model. Furthermore, the eddy dissipation concept for interaction between turbulence and chemistry in combination with the detailed reaction mechanism have been employed. At first, the results are compared with the experimental data and after validating the obtained results of the solution, the moderate or intense low-oxygen dilution combustion is investigated by putting some hot obstacles in the burner. The results show that the hot obstacle has a role similar to the hot spot and helps this condition of combustion to reduce the initial cost in order to preheat the hot oxidizer co-flow. This means that putting the hot obstacle in the burner causes to reduce the preheat temperature and the energy consumption per second up to 300 K and 17.9%, respectively, while it increases the turbulence intensity, reaction zone, outlet CO2 and H2O species while reduces NOx and CO emissions.

Large Eddy Simulation on the Effects of Pressure on Syngas/Air Turbulent Nonpremixed Jet Flames

ABSTRACT The influence of increasing pressure on nonpremixed syngas/air turbulent jet flames is numerically investigated using large eddy simulations in conjunction with a steady laminar flamelet approach. The applicability of the steady flamelet approach is assessed through an extensive parametric study of laminar counterflow flames and tangential stretching rate analysis on target flame structures at different pressures. Two sets of large eddy simulations, exploring pressure values up to atm, are carried out. The first one (series A) is characterized by a constant jet Reynolds number, while the second one (series B) is characterized by a constant jet inlet velocity. Both campaigns show narrower flame brushes and reduced radical concentrations with increasing pressure. While for series A the flame length is not sensitive to pressure, a longer flame brush is noticed for series B, being mainly caused by the increased mass flow rate. The sensitivity of the local flame behavior to pressure, such as the OH layer thickness and position, is compared to the available experimental results, showing similar trends with a satisfactory agreement.

Simultaneous high-speed imaging of temperature, heat-release rate, and multi-species concentrations in turbulent jet flames.

We report the high-speed imaging of multi-species and multi-parameter combustion diagnostics for turbulent non-premixed jet flames using a three-legged burst-mode laser system. Simultaneous OH/CH2O planar laser-induced fluorescence and Rayleigh-scattering imaging measurements at a 10-kHz rate are obtained. OH and CH2O concentrations, flame temperatures, and heat-release rates are simultaneously acquired in two-dimensions at 10 kHz.

Influence of flow topology and scalar structure on flame-tangential diffusion in turbulent non-premixed combustion

Flamelet-based combustion models have been extensively used in the modeling of turbulent non-premixed combustion due to their notable advantages in reducing computational cost. The classical one-dimensional flamelet equations are derived based on an assumption: flame-normal transport of thermochemical state variables in mixture fraction gradient direction dominates over flame-tangential transport along iso-surfaces of mixture fraction. The motivation of this work is to quantify the contribution of tangential diffusion (TD) and seek the source of TD, since several recent studies have shown that TD effects may play an important role in the regions with large curvature and finite-rate chemistry. In this work, the most probable cause and effect chain that elucidates the source of TD in a turbulent non-premixed jet flame is proposed for the first time from a flow topology perspective. To this end, local flow topology and its effects on scalar structure are first investigated. It is found that the local flow structures with unstable focus compressing (UFC) topology have the highest probability of large compressive strain and that the negative correlation between curvature and strain strongly depends on local flow topology, scalar structure and reaction. Then, the influence of flow topology and scalar structure on the relevance of TD is assessed with the angle between reactive scalars and mixture fraction. The results show that TD effects mainly exist in the regions with high curvatures and low strain rates or scalar dissipation rates. These findings suggest that the local flow structures with low probability of UFC topology tend to cause significant flame-tangential diffusion in the turbulent non-premixed flame.

Large Eddy Simulation of a Syngas Jet Flame: Effects of Preferential Diffusion and Turbulence–Chemistry Interaction

A large eddy simulation of the turbulent syngas non-premixed jet flame of Sandia ETH/Zurich B is conducted using an in-house version of FireFOAM, a fire simulation solver within OpenFOAM. Combustio...

A Computational Investigation of Nonpremixed Combustion of Natural Gas Injected Into Mixture of Argon and Oxygen

Natural gas is traditionally considered as a promising fuel in comparison with gasoline due to the potential of lower emissions and significant domestic reserves. These emissions can be further diminished by using noble gases, such as argon, instead of nitrogen as the working fluid in internal combustion engines. Furthermore, the use of argon as the working fluid can increase the thermodynamic efficiency due to its higher specific heat ratio. In comparison with premixed operation, the direct injection of natural gas enables the engine to reach higher compression ratios while avoiding knock. Using argon as the working fluid increases the in-cylinder temperature at top dead center (TDC) and enables the compression ignition (CI) of natural gas. In this numerical study, the combustion quality and ignition behavior of methane injected into a mixture of oxygen and argon have been investigated using a three-dimensional transient model of a constant volume combustion chamber (CVCC). A dynamic structure large eddy simulation (LES) model has been utilized to capture the behavior of the nonpremixed turbulent gaseous jet. A reduced mechanism consists of 22-species, and 104-reactions were coupled with the CFD solver. The simulation results show that the methane jet ignites at engine-relevant conditions when nitrogen is replaced by argon as the working fluid. Ignition delay times are compared across a variety of operating conditions to show how mixing affects jet development and flame characteristics.

Quantitative planar temperature imaging in turbulent non-premixed flames using filtered Rayleigh scattering.

The paper presents results demonstrating the application of filtered Rayleigh scattering (FRS) for quantitative temperature measurements within turbulent non-premixed jet flames. Through targeted fuel tailoring, i.e., the selection of a specific fuel mixture, temperature measurements are made under non-premixed fueling conditions with a single FRS measurement. For this to be feasible, the instantaneous measured FRS signal is uniquely proportional to the local temperature for all thermo-chemical states of the targeted fuel-oxidizer system. Simulated results using laminar, counterflow flame calculations show that for select CH4/H2/Ar fuel mixtures issuing into air, a unique relationship between the local FRS signal and temperature is achieved for all mixture fraction values over a full range of operating conditions from near-equilibrium to near-extinction flames. Furthermore, for the selected FRS-optimized fuel, the local mixture-averaged Rayleigh scattering cross section is nearly constant from fuel to oxidizer to products. Thus, traditional laser Rayleigh scattering (LRS) can be used to determine the temperature as a "standard" to which to compare and assess the FRS-based temperature results. Simultaneous LRS-FRS temperature measurements from Re=10,000, 20,000, and 30,000 turbulent non-premixed jet flames are presented that show good agreement between the two techniques in terms of instantaneous temperature fields and statistical quantities at various spatial locations within the flame. These results provide confidence that the current approach of FRS thermometry allows for accurate temperature measurements within the selected set of turbulent non-premixed flames.

Calculated concentration distributions and time histories of key species in an acoustically forced laminar flame

A numerical study of the fluid-chemical interactions in a steady and time-varying laminar non-premixed jet flame was conducted to advance understanding of the complex interplay between the flame chemistry, fluid dynamics and soot evolution. Modelling of the steady flame is performed with two alternative reduced mechanisms and compared with the significant body of experimental data that are now available to provide confidence in the calculated values of mixture fraction, which was not previously available. A Method-of-Moments soot model with a 47-species mechanism provides much better agreement with the measured soot volume fraction than does a 32-species mechanism, but both mechanisms predict both the temporal and spatial profiles of mixture fraction to agree within 6%. Nevertheless, neither scheme predicts a reduction in temperature that coincides approximately with the location immediately upstream from the measured soot field. The calculations of the unsteady flame also reveal new insights about the cause of the pinch-off point and the neck zone, together with the role of buoyancy at the flame tip.

Examination of errors of table integration in flamelet/progress variable modeling of a turbulent non-premixed jet flame

• Different flamelet tables can be generated by using the same laminar flamelet solutions. • Different flamelet table integration approaches can lead to different predictions. • Effect of table integration on a turbulent combustion flame is investigated. A flamelet model implementation consists of two steps: the generation of a set of laminar flamelet solutions and the integration of the laminar flamelet solutions with presumed shape probability density functions (PDFs) to produce a flamelet table for turbulent flame simulations. Many studies have been done in the past to examine the effect of different flamelet modeling strategies including the effect of employing different laminar flamelet solutions for the modeling. However, little work has been done to examine the effect of different presumed PDF table integration approaches on different flamelet model predictions. This work aims at investigating the source of errors arising from the flamelet table integration. The flamelet/progress variable model is chosen as a representative flamelet model, and three different presumed PDF table integration approaches are compared to examine the effect of table integration on flamelet model predictions. A laboratory-scale turbulent non-premixed jet flame (Sandia flame D) is chosen as a test case for the examination. In general, some evident sensitivity of the modeling results to the different flamelet table integration approaches is observed. The underlying reasons for the performance difference of different approaches are explored, and it is found that a model that preserves the one-dimensional laminar flamelet structure during the presumed-PDF table integration can improve the model prediction accuracy. Different sources of errors involved in flamelet model implementation are investigated, including numerical integration errors, flamelet table errors, and the errors in the predictions of the flamelet independent variables.

An experimental study of turbulent lifted flames at elevated pressures

Studying the lift-off behavior of non-premixed jet flames is important to understanding the flame stabilization mechanisms in practical systems, including gas flares and combustors, and to improving the safety of pressurized fuel tanks in case of fuel leaks. This is typically done with the canonical configuration of an axisymmetric fuel jet issuing into a quiescent or co-flowing oxidizer and abundant data are available in the literature. However, most of these data were collected at normal or sub-atmospheric pressure and little data are available at elevated pressure and high Reynolds numbers, conditions relevant to practical configurations. The present study fills this gap by reporting lift-off height measurements of methane and ethane non-premixed jet flames for pressures up to 7 bar and Re = 57,500 in the presence of an air co-flow. Data are interpreted using Kalghatgi's model for the dimensionless lift-off height, which was previously proven successful for sub-atmospheric to normal pressures, as well as elevated pressure but only for low turbulent Reynolds numbers and propane. In this contribution, Kalghatgi's model has been shown to accurately predict the slope of the lift-off height vs. jet velocity curves at elevated pressure and large Reynolds number for methane and ethane. A new term, a function of the stoichiometric mixture fraction, the laminar burning velocity, and a turbulent Schmidt number, is also introduced to extend the predictive capabilities of Kalghatgi's model to configurations featuring a co-flow.

Observations on the Role of Auto-Ignition in Flame Stabilization in Turbulent Non-Premixed Jet Flames in Vitiated Coflow

Turbulent combustion of non-premixed jets issuing into a vitiated coflow is studied at coflow temperatures that do not significantly exceed the fuel auto-ignition temperatures, with the objective of observing the global features of lifted flames in this operating temperature regime and the role played by auto-ignition in flame stabilization. Three distinct modes of flame base motions are identified, which include a fluctuating lifted flame base (mode A), avalanche downstream motion of the flame base (mode B), and the formation and propagation of auto-ignition kernels (mode C). Reducing the confinement length of the hot coflow serves to highlight the role of auto-ignition in flame stabilization when the flame is subjected to destabilization by ambient air entrainment. The influence of auto-ignition is further assessed by computing ignition delay times for homogeneous CH4/air mixtures using chemical kinetic simulations and comparing them against the flow transit time corresponding to mean flame liftoff height of the bulk flame base. It is inferred from these studies that while auto-ignition is an active flame stabilization mechanism in this regime, the effect of turbulence may be crucial in determining the importance of auto-ignition toward stabilizing the flame at the conditions studied. An experimental investigation of auto-ignition characteristics at various jet Reynolds numbers reveals that turbulence appears to have a suppressing effect on the active role of auto-ignition in flame stabilization.

Nonlinear mode transition mechanisms of a self-excited Jet A-1 spray flame

Low-frequency combustion instabilities arise in an aero-engine gas turbine combustor under idle/sub-idle conditions, particularly when pilot-mode non-premixed combustion is sustained. Despite the fundamental importance of such instabilities, little is known about how they are initiated and grow in the system. Here, we present nonlinear mode transition processes using several analysis methods, including Fourier/Hilbert transforms, phase portraits, spectrograms, low-order analytic modeling, and high-speed flame visualization methods. Our results demonstrate that a non-premixed Jet A-1 spray flame yields an intermediate-amplitude, quasi-periodic L1 mode oscillation at 50 Hz for a low pilot equivalence ratio, and the frequency gradually increases with increasing fuel flowrates. Subsequent to a critical point (103 Hz), the system undergoes a discontinuous mode transition, giving rise to the formation of an extremely large pressure oscillation with an L2 mode structure at 244 Hz. Several key triggers were found to induce the mode shift: (i) generation of a high intensity pulse in the flame's heat release rate due to the spontaneous ignition of unburned reactant mixtures, (ii) emergence of a large-scale vortical structure and its interaction with a partially premixed flame front, and (iii) development of self-sustained limit cycle oscillations driven by periodic convection of a hot spot – a mechanism known as entropy wave propagation. Our study identifies the occurrence of a large-amplitude peak followed by a local minimum intensity, analogous to the activation energy concept, as an essential step for entry into a new state with large-amplitude limit cycle oscillations.

Detachment mechanisms of turbulent non-premixed jet flames at atmospheric and elevated pressures

The stability limits of a turbulent flame in a practical combustor are important characteristics that influence its performance. The mechanisms controlling the stability limits of turbulent non-premixed flames are examined here in the canonical configuration of a fuel jet in co-flow air. This study focuses on the conditions leading to the detachment of flames from the injector nozzle by means of an experimental parametric study in which pressure (1 ≤ P ≤ 10 bar), fuel (methane and ethane), nozzle wall thickness (t = 0.20 mm, 0.58 mm, and 0.89 mm), jet velocity (0.5 ≤ Uj ≤ 16.5 m s−1), and co-flow velocity (Uc = 0.3 m s−1, 0.6 m s−1, and 0.9 m s−1) are varied. It is shown that the mechanism leading to detachment depends on the ratio of the nozzle wall thickness to the laminar flame thickness. If this ratio is smaller than 3, the nozzle is “thin” and type I detachment occurs (flame base stability lifting). In this case, the detachment velocity decreases with pressure and is proportional to the laminar burning velocity. If the ratio is larger than 3, the nozzle is “thick” and type II detachment occurs (local flame extinction lifting). Then, the detachment velocity is controlled by the extinction strain rate. Experiments also show that the Kolmogorov scale of turbulence regulates local flame extinction and type II detachment and a model is proposed to predict detachment for any fuel, pressure, and nozzle wall thickness using the computed extinction strain rate and the Kolmogorov time scale. Finally, the data show that elevating pressure allows stabilizing attached non-premixed jet flames with high Reynolds numbers without the need for complex stabilization strategies such as pilot flames, swirl, or oxygen/hydrogen enrichment. Pressure allows studying flame/turbulence interactions at Reynolds numbers relevant to practical applications while conserving simple configurations amenable to diagnostics and modeling.

Effect of air jet momentum on the topological features of turbulent CNG inverse jet flame

Inverse jet flame (IJF) configuration is a variant of coaxial nonpremixed jet flame which can produce a nonluminous compact blue flame with independent control of air and fuel jet momentum. In the present work, the effect of central air jet velocity on visible topological features such as, visible flame height and base flame height is investigated experimentally. Also the fluid dynamics behind the evolution of base flame in IJF configuration is unraveled in this study. Notably, buoyancy induced oscillations of IJF, which is relatively unexplored in open literature is studied in this work. Moreover, a systematic classification of IJF based on the evolution of its visible flame features with variation in the central air jet and annular fuel jet velocities is reported. In addition, a semi-empirical correlation for visible flame height of IJF with air-fuel momentum ratio is arrived in the present work which can be useful in the design of IJF based burners utilized for impingement heat transfer applications.

Numerical Simulation of Propagation of Unsteady Tribrachial Flames in Laminar Non-Premixed Jets

The characteristics of propagating Tribrachial Flames in non-premixed laminar jets have been investigated computationally, using a two dimensional model in ANSYS FLUENT 16.0. The edge of the propagating flame in a laminar jet regime has a Tribrachial flame structure: a lean premixed flame, a rich premixed flame, and a diffusion flame, all extending from a single location. This is usually observed in non-premixed combustion such occurring in burners. Such flames are important in the interaction of heat and mass transfer with chemical reactions in gas turbines and commercial burners. A computational study has been performed to assess the laminar tribrachial flame propagation in methane jets and to compare the tribrachial flame propagation for various flow rates. With an increase in the fuel jet Reynolds number, the tribrachial point is found to move closer to the nozzle. Additionally the tribrachial point is found to become larger with an increase in the Reynolds number.

Effects of large aromatic precursors on soot formation in turbulent non-premixed sooting jet flames

In this paper, we computationally study the effects of large Polycyclic Aromatic Hydrocarbons (PAH) (more than two aromatic rings) on soot yield and distribution in turbulent flames, when they are considered as nucleating species for soot formation. This is examined in two turbulent non-premixed sooting jet flames using ethylene and a jet fuel (JP-8) surrogate as fuels. For each flame, two Large-Eddy Simulations (LES) are performed with two different soot nucleation strategies. In the first strategy, a range of PAH from naphthalene to cyclopenda[cd]pyrene are considered as nucleating species, while in the second strategy, naphthalene is considered as the only soot nucleating species and the effects of larger PAH are represented entirely by naphthalene. Flamelet-based chemistry-tabulation is used for the major thermochemical quantities, such as density, temperature, and major species mass fractions. Turbulence-chemistry interactions for PAH are accounted for by transporting their mass fractions and using a recently developed PAH relaxation model for their source term closure. The effects of large PAH on soot are highlighted by comparing the PAH profiles, soot nucleation rate, and soot volume fraction distributions obtained from both simulations for each test flame. These results are also compared against experimental measurements, when available. From these comparisons, it is first shown that naphthalene is predominantly formed along the flame centerline, and larger PAH species with more than two aromatic rings are primarily formed away from the centerline. Further, it is found that these larger PAH species only contribute to around of the overall soot nucleation rate along the flame centerline but their contributions are more substantial (up to ) away from the centerline. Finally, it is shown that representing the effects of these large PAH species by naphthalene leads to an under-prediction of around for the soot volume fraction magnitude along the flame centerline. Away from the centerline, this under-prediction can be as much as 100

Numerical Investigation of an OxyfuelNon-Premixed CombustionUsing a Hybrid Eulerian Stochastic Field/Flamelet Progress Variable Approach: Effects of H2/CO2Enrichment and Reynolds Number

In the present paper, the behaviour of an oxy-fuel non-premixed jet flame is numerically investigated by using a novel approach which combines a transported joint scalar probability density function (T-PDF) following the Eulerian Stochastic Field methodology (ESF) and a Flamelet Progress Variable (FPV) turbulent combustion model under consideration of detailed chemical reaction mechanism. This hybrid ESF/FPV approach overcomes the limitations of the presumed- probability density function (P-PDF) based FPV modelling along with the solving of associated additional modelled transport equations while rendering the T-PDF computationally less demanding. In Reynolds Averaged Navier-Stokes (RANS) context, the suggested approach is first validated by assessing its general prediction capability in reproducing the flame and flow properties of a simple piloted jet flame configuration known as Sandia Flame D. Second, its feasibility in capturing CO2addition effect on the flame behaviour is demonstrated while studying a non-premixed oxy-flame configuration. This consists of an oxy-methane flame characterized by a high CO2 amount in the oxidizer and a significant content of H2 in the fuel stream, making it challenging for combustion modelling. Comparisons of numerical results with experimental data show that the complete model reproduces the major properties of the flame cases investigated and allows achieving the best agreement for the temperature and different species mass fractions once compared to the classical presumed PDF approach.

LES/CMC modelling of ignition and flame propagation in a non-premixed methane jet

The Large Eddy Simulation (LES) / Conditional Moment Closure (CMC) model with detailed chemistry is used for modelling spark ignition and flame propagation in a turbulent methane jet in ambient air. Two centerline and one off-axis ignition locations are simulated. We focus on predicting the flame kernel formation, flame edge propagation and stabilization. The current LES/CMC computations capture the three stages reasonably well compared to available experimental data. Regarding the formation of flame kernel, it is found that the convection dominates the propagation of its downstream edge. The simulated initial downstream and radial flame propagation compare well with OH-PLIF images from the experiment. Additionally, when the spark is deposited at off-centerline locations, the flame first propagates downstream and then back upstream from the other side of the stoichiometric iso-surface. At the leading edge location, the chemical source term is larger than others in magnitude, indicating its role in the flame propagation. The time evolution of flame edge position and the final lift-off height are compared with measurements and generally good agreement is observed. The conditional quantities at the stabilization point reflect a balance between chemistry and micro-mixing. This investigation, which focused on model validation for various stages of spark ignition of a turbulent lifted jet flame through comparison with measurements, demonstrates that turbulent edge flame propagation in non-premixed systems can be reasonably well captured by LES/CMC.