Turbulent Lifted Jet Flame Research Articles

The evolution of soot morphology for the maturation of nascent particle in a turbulent lifted jet flame

In order to understand the soot formation in Diesel engine, a turbulent jet flame is used to simulate the combustion in the cylinder. The experimental study is performed to investigate the evolution of soot morphology for the maturation of nascent particle in a turbulent lifted jet flame fueled by n-heptane/toluene mixtures. An ultrasonic atomizer is used to evenly spread the fuel droplets to acquire single primary particles. Transmission electron microscopy is used to study the morphology of the particle sampled from the flame at different heights. Small soot aggregates are acquired from all the samples. Particle maturation such as agglomeration is accelerated under a high temperature by comparing the particle morphology sampled on the centerline and the wings of the flame. The precursors of nascent particles are depicted as dark nucleation dispersed to semitransparent polycyclic aromatic hydrocarbons. The nanostructure of nascent particles transforms from an amorphous state to an onion structure with the maturation of particles. Surface growth initially dominates the maturation of nascent particles in the direction of outside to inside for single particles. Agglomeration begins to emerge with the increased probability of collision at the end of maturation. Surface growth and agglomeration increase the mean particle diameter as the flame height increases. The oxidability of particles that undergo surface growth and agglomeration notably increases. The structure of nascent particles is inclined to be ordered and the mean particle diameter decreases with oxidation dominating the combustion reaction.

On the flame stabilization of turbulent lifted hydrogen jet flames in heated coflows near the autoignition limit: A comparative DNS study

Three-dimensional direct numerical simulations of turbulent lifted hydrogen jet flames in heated coflows are performed with a detailed H2/air chemical mechanism to understand their ignition dynamics and stabilization mechanisms. Turbulent lifted jet flames with four different coflow temperatures, Tc, between 750 K and 1100 K are investigated by examining the instantaneous/time-averaged values and conditional means of heat release rate and species critical to ignition, and by performing a displacement speed analysis and a local combustion mode analysis with an indicator, α. Although Tc at 950 K is higher than the autoignition limit, the flame is primarily stabilized by flame propagation rather than autoignition, while at 1100 K, flame stabilization is found to be highly affected by autoignition. The local combustion mode analysis further reveals that at 950 K, even if a local ignition mode with |α|<1 first appears in the near field of the jet, it develops into a local extinction mode with α<−1 as local temperature decreases due to the excessive mixing of heated coflow and cold H2 within vortical structures, which inhibits the ignition kernel development upstream of the flamebase. At 1100 K, however, a local ignition mode prevails upstream of the flamebase. To further identify the effect of a vortex on the early development of an ignition kernel in a mixing layer between the heated coflow and cold H2, a series of two-dimensional DNSs are performed, varying several vortex parameters and air temperature, as a reference for the more complicated corresponding 3-D turbulent DNS cases. The results substantiate that the development of a vortex in the mixing layer tends to retard the autoignition within the vortex, especially when its temperature is slightly above the autoignition limit.

Assessment of the stabilization mechanisms of turbulent lifted jet flames at elevated pressure using combined 2-D diagnostics

The stabilization mechanisms of turbulent lifted jet flames in a co-flow have been investigated at a pressure of 7 bar. The structure of the flame base was measured with combined OH and CH2O planar laser induced fluorescence (PLIF) and the spatial distribution of equivalence ratio was imaged, simultaneously, with CH4 Raman scattering. The velocity field was also measured with particle imaging velocimetry (PIV). Different bulk jet velocities Uj and co-flow velocities Uc were examined. Data show that flames with Uc = 0.6 m/s stabilize much further away from the nozzle than those with Uc = 0.3 m/s and that their structure does not resemble that of the edge-flames found closer to the nozzle. In addition, for Uc = 0.6 m/s, the measured lift-off height decreases with increasing bulk jet velocity, which is opposite to what is typically observed for lifted flames. Statistical examination of CH4 Raman images shows that the flames with Uc = 0.6 m/s propagate through regions of the flow where the equivalence ratio is not always stoichiometric but, instead, spans the whole flammability range. This is not consistent with edge-flames and is, instead, indicative of premixed burning. This is corroborated by PIV results which show that the flame base velocity exceeds that typically reported for edge-flames. Measurements of relevant flow properties were also conducted in non-reacting jets to predict the turbulent burning velocity of these lifted flames burning in a premixed mode. For Uc = 0.6 m/s and relatively large bulk jet velocities (Uj = 10 and 15 m/s), the predicted turbulent burning velocities are sufficiently high to counter the incoming flow of reactants and, in turn, allow flame stabilization. However, for a lower bulk jet velocity of Uj = 5 m/s, the predicted turbulent burning velocity is much less, leading to blow-out. This explains why the lift-off height decreases with increasing jet velocity for methane at 7 bar and Uc = 0.6 m/s. Data also shows that increasing pressure promotes transition from edge-flames to premixed flames due to reduced laminar burning velocity and enhanced mixing.

Modelling of non-premixed turbulent combustion with Conditional Moment Closure (CMC)⋆.

Conditional Moment Closure (CMC), an advanced turbulent reacting flow method, has been applied to the challenging cases with a varying degree of turbulence-chemistry interactions. The CMC approach may be used either in the RAMS or LES context, this is reviewed in the first part of this paper, while the second part is dedicated to applications on Sandia piloted jet flames D and F and lifted hydrogen jet flame. In case of the Sandia piloted jet flame D, the RANS-CMC simulation results are in agreement with the experimental data. On the other hand, when one comes to the results for the Sandia flame F, extinction is not captured. These discrepancies are attributed to the use of RANS in combination with the boundary conditions set in CMC. However, in case of turbulent lifted jet flame in vitiated co-flow, the LES-CMC model is able to capture the axial and radial profiles of mixture fraction, temperature and major species. The lift-off height is found to be very sensitive to the co-flow temperature as well as the co-flow velocity. The LES-CMC results highlight the potential of the technique to simulate the problems which involve complex turbulence-chemistry interactions.

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.

Physical mechanisms that cause intermittency that presages combustion instability and blowout in a turbulent lifted jet flame combustor

ABSTRACTThe physics of intermittent dynamics that presages the onset of impending combustion instability and blowout in a turbulent lifted jet flame combustor is investigated. It is observed that the transition from combustion noise to combustion instability happens via intermittency while varying the relative location of the burner inside a confinement as a bifurcation parameter. For further change in burner location past the condition of combustion instability, the intermittency before the flame blows out is once again observed. The authors show that the coupled interaction of flow, flame dynamics, and combustion chamber acoustics is the physical reason for the occurrence of intermittency prior to combustion instability. In contrast, intermittent dynamics that presages blowout occurs due to the interplay between flame blowout precursor events and the driving of high-amplitude oscillations as the flame propagates towards the burner.

Stabilisation of turbulent auto-igniting dimethyl ether jet flames issuing into a hot vitiated coflow

Experimental results focusing on understanding the role of autoignition in the stabilisation of turbulent lifted jet flames of dimethyl ether issuing into a hot vitiated coflow are explored in this paper. A stability map of the flame lift-off height as a function of coflow temperature and jet partially premixing ratios is reported, indicating a strong sensitivity of the mean and rms lift-off heights to the coflow temperature. A new experimental setup that utilises high repetition rate (10kHz) joint OH and CH2O PLIF combined with a third camera to track out of plane motion via chemiluminescence emission is employed. This allows the temporal dynamics of the flame base and autoignition kernels to be tracked at high spatial resolution over a spatial window of 20mm (height) × 43mm (width). To capture the temporal statistics of the flame over the full lift-off height range, a complementary, but not simultaneous, large field of view high-speed chemiluminescence imaging experiment was employed. The use of high-speed diagnostics is shown to be invaluable to understand the temporal dynamics of the flame base region, such as identifying and tracking autoignition kernels ahead of, and merging with, the main flame base. For low coflow temperatures, autoignition kernels are found to be fundamentally important to the stabilisation of the flame: small autoignition initiated kernels rapidly grow to form large flame kernels that eventually merge with the main flame base and in doing so rapidly establish a new lift-off height significantly upstream of the original flame base. For high coflow temperatures, autoignition kernels whilst still present, are significantly smaller in mean size and play a reduced role in the overall flame stabilisation. This leads to the conclusion that partially premixed flame propagation plays an increased role in overall flame stabilisation for high coflow temperature flames.

Turbulent lifted flames of H2/N2 fuel issuing into a vitiated coflow investigated using Lagrangian Intermittent Modelling

This paper reports results of numerical simulations of a turbulent lifted jet flame of hydrogen–nitrogen mixtures including the effects of the autoignition. The impact of burned gases on the flame stabilization is analysed under the conditions of a laboratory jet flame in a vitiated coflow. In this study, mass flow rate, temperature and exact chemical composition of hot products mixed with air sent toward the turbulent flame base are fully determined. The effects of both non-infinitely fast chemistry and partially premixed combustion are taken into account within a Lagrangian intermittent framework. Detailed chemistry effects are incorporated through the use of a tabulation delay. The concept of residence time of the particles and the transport equation for the mean scalar dissipation rate are included. Numerical simulation of the turbulent diluted jet flame of H2/N2 studied by Cabra and his co-workers at Berkeley University is performed and satisfactory results are obtained: the flame liftoff height is reasonably captured and the predictions display a reasonable agreement with respect to experimental data.

Modelling of turbulent lifted jet flames using flamelets: a priori assessment and a posteriori validation

This study focuses on the modelling of turbulent lifted jet flames using flamelets and a presumed Probability Density Function (PDF) approach with interest in both flame lift-off height and flame brush structure. First, flamelet models used to capture contributions from premixed and non-premixed modes of the partially premixed combustion in the lifted jet flame are assessed using a Direct Numerical Simulation (DNS) data for a turbulent lifted hydrogen jet flame. The joint PDFs of mixture fraction Z and progress variable c, including their statistical correlation, are obtained using a copula method, which is also validated using the DNS data. The statistically independent PDFs are found to be generally inadequate to represent the joint PDFs from the DNS data. The effects of Z–c correlation and the contribution from the non-premixed combustion mode on the flame lift-off height are studied systematically by including one effect at a time in the simulations used for a posteriori validation. A simple model including the effects of chemical kinetics and scalar dissipation rate is suggested and used for non-premixed combustion contributions. The results clearly show that both Z–c correlation and non-premixed combustion effects are required in the premixed flamelets approach to get good agreement with the measured flame lift-off heights as a function of jet velocity. The flame brush structure reported in earlier experimental studies is also captured reasonably well for various axial positions. It seems that flame stabilisation is influenced by both premixed and non-premixed combustion modes, and their mutual influences.

Tomographic PIV measurements in a turbulent lifted jet flame

Measurements of instantaneous volumetric flow fields are required for an improved understanding of turbulent flames. In non-reacting flows, tomographic particle image velocimetry (TPIV) is an established method for three-dimensional (3D) flow measurements. In flames, the reconstruction of the particles location becomes challenging due to a locally varying index of refraction causing beam-steering. This work presents TPIV measurements within a turbulent lifted non-premixed methane jet flame. Solid seeding particles were used to provide the 3D flow field in the vicinity of the flame base, including unburned and burned regions. Four cameras were arranged in a horizontal plane around the jet flame. Following an iterative volumetric self-calibration procedure, the remaining disparity caused by the flame was less than 0.2 pixels. Comparisons with conventional two-component PIV in terms of mean and rms values provided additional confidence in the TPIV measurements.

Lagrangian Intermittent Modelling of a Turbulent Lifted Methane-Air Jet Flame Stabilized in a Vitiated Air Coflow

The present manuscript reports results of numerical simulations of turbulent lifted jet flames of methane with special emphasis placed on the autoignition effects. The impact of dilution with burned gases on the flame stabilization is analyzed under the conditions of a laboratory jet flame surrounded by a vitiated air coflow. In this geometry, mass flow rates, temperature levels and exact chemical composition of hot products mixed with air are fully determined. The effects of both finite rate chemistry and partially premixed combustion are taken into account within a Lagrangian intermittent framework. Detailed chemistry effects are incorporated through the use of chemical time scales, which are tabulated as functions of the mixture fraction. The concept of residence time (or particle age) and the transport equation for the mean scalar dissipation rate of the mixture fraction fluctuations, i.e., \(\widetilde{\varepsilon}_Z=\overline{\rho \mathcal{D} (\partial Z'' / \partial x_i) (\partial Z'' / \partial x_i) }/\overline{\rho}\), are also considered. This allows to improve the description of turbulent mixing including both the large scales engulfment processes through the consideration of the residence time scale, as well as the small scales molecular mixing processes the intensity of which is set by the integral scalar (mixing) time scale \(\tau_Z=\widetilde{Z''^2}/\widetilde{\varepsilon}_Z\). Numerical simulations of the turbulent diluted jet flame of methane studied by Cabra and his co-workers are performed. The flame liftoff height is reasonably well-captured and the computational results display a fairly satisfactory level of agreement with experimental data. They also confirm that the consideration of an additional transport equation for the mean scalar dissipation rate may significantly improve the computational results. The investigation is supplemented by a sensitivity analysis of the scalar to turbulence time scale ratio Cmix = τZ/τt often referred to as the mixing constant. Finally, the manuscript ends with the application of the proposed closure to experimental conditions which are characterized by variations of (i) the jet exit velocity, (ii) coflow velocity and (ii) coflow temperature.

Analysis of Flame Characteristics in a Laboratory-Scale Turbulent Lifted Jet Flame via DNS

A fully compressible 3D solver for reacting flows has been developed and applied to investigate a turbulent lifted jet flame in a vitiated coflow by means of direct numerical simulation (DNS) to validate the solver and analyze the flame characteristics. An eighth-order central differencing scheme is used for spatial discretization and a fourth-order Runge-Kutta method is employed for time integration. The DNS results agree well with the experimental measurements for the conditional means of reactive scalars. However, the lift-off height is under predicted. The mean axial velocity develops into a self-similar profile after x/D = 6. The normalized flame index is employed to characterize the combustion regime. It is found that at the flame base the gradients of the reactants are opposed and diffusion combustion is dominant. Further downstream, the contribution of premixed combustion increases and peaks at x/D = 8. Finally, the stabilization process is examined. The turbulent lifted flame is proved to stabilize in the lean mixtures and low scalar dissipation rate regions.

DNS analysis of a three-dimensional supersonic turbulent lifted jet flame

A fully compressible solver for direct numerical simulation of supersonic combustion has been developed and applied to investigate a three-dimensional spatially-developing supersonic turbulent jet flame. High-resolution bandwidth-optimized weighted essentially non-oscillatory scheme of spatial discretization and total variation diminishing temporal integration are used to capture the intricate phenomena with an accurate characteristic system and an updated detailed H2/air reaction mechanism. The numerical algorithms are first validated by simulating some canonical problems, and then used to analyze the global features, compressibility effects, and the stabilization mechanism of the supersonic lifted jet flame. It is found that the lifted flame consists of a stable laminar flame base with auto-ignition as the stabilization mechanism, a violent mixing region in which vigorous turbulent combustion occurs with both premixed and diffusion flames, and a far-field outer diffusion flame as well as an inner weaker premixed flame. Although diffusion combustion dominates the flame in general, premixed combustion is significant within the violent mixing region. The heat release rate in the high-speed region can be significantly influenced by the compressibility. In the region of fuel compressing, the heat release rate increases as the compressive power increases. While in the region of fuel expanding, the heat release rate also increases as the expansive power increases albeit with a smaller magnitude. The auto-ignition in a fuel-lean mixture near the most reactive condition is the main stabilization mechanism, which is determined from the statistical analysis of the heat release rate and the Damköhler number as well as the deduction of the lift-off height of the flame.

LES-CMS simulations of a turbulent lifted hydrogen flame in vitiated co-flow

Results from a study of turbulent lifted jet flame with Conditional Moment Closure (CMC) turbulent combustion model are reported. First, a qualitative description of the simulation results is given, by examining the instantaneous resolved species mass fraction fields. The structure of the lifted flame is also assessed by comparing the measured temperature and species mass fraction profiles and the simulation results. The model is able to capture the axial and radial profiles of mixture fraction, temperature and major species. The sensitivity of prediction to boundary conditions is also explored and the lift-off height is found to be very sensitive to the co-flow temperature. Finally, the stabilisation mechanisms, auto-ignition or premixed flame propagation, are addressed. No evidence of premixed flame propagation is found: the diffusion in physical space is negligible for all studied conditions.

Scalar and its dissipation in the near field of turbulent lifted jet flame

In this study various scalar dissipation rates and their modelling in the context of partially premixed flame are investigated. A DNS dataset of the near field of a turbulent hydrogen lifted jet flame is processed to analyse the mixture fraction and progress variable dissipation rates and their cross dissipation rate at several axial positions. It is found that the classical model for the passive scalar dissipation rate ϵ ˜ ZZ gives good agreement with the DNS, while models developed based on premixed flames for the reactive scalar dissipation rate ϵ ˜ cc only qualitatively capture the correct trend. The cross dissipation rate ϵ ˜ cZ is mostly negative and can be reasonably approximated at downstream positions once ϵ ˜ ZZ and ϵ ˜ cc are known, although the sign cannot be determined. This approach gives better results than one employing a constant ratio of turbulent timescale and the scalar covariance c ″ Z ″ ∼ . The statistics of scalar gradients are further examined and lognormal distributions are shown to be very good approximations for the passive scalar and acceptable for the reactive scalar. The correlation between the two gradients increases downstream as the partially premixed flame in the near field evolves ultimately to a diffusion flame in the far field. A bivariate lognormal distribution is tested and found to be a reasonable approximation for the joint PDF of the two scalar gradients.

Simultaneous planar measurements of soot structure and velocity fields in a turbulent lifted jet flame at 3 kHz

We describe a newly developed combustion diagnostic for the simultaneous planar imaging of soot structure and velocity fields in a highly sooting, lifted turbulent jet flame at 3000 frames per second, or two orders of magnitude faster than “conventional” laser imaging systems. This diagnostic uses short pulse duration (8 ns), frequency-doubled, diode-pumped solid state (DPSS) lasers to excite laser-induced incandescence (LII) at 3 kHz, which is then imaged onto a high framerate CMOS camera. A second (dual-cavity) DPSS laser and CMOS camera form the basis of a particle image velocity (PIV) system used to acquire 2-component velocity field in the flame. The LII response curve (measured in a laminar propane diffusion flame) is presented and the combined diagnostics then applied in a heavily sooting lifted turbulent jet flame. The potential challenges and rewards of application of this combined imaging technique at high speeds are discussed.

Flame stabilization in a lifted non-premixed turbulent hydrogen jet with coaxial air

To understand hydrogen jet liftoff height, the stabilization mechanism of turbulent lifted jet flames under non-premixed conditions was studied. The objectives were to determine flame stability mechanisms, to analyze flame structure, and to characterize the lifted jet at the flame stabilization point. Hydrogen flow velocity varied from 100 to 300 m/s. Coaxial air velocity was regulated from 12 to 20 m/s. Simultaneous velocity field and reaction zone measurements used, PIV/OH PLIF techniques with Nd:YAG lasers and CCD/ICCD cameras. Liftoff height decreased with increased fuel velocity. The flame stabilized in a lower velocity region next to the faster fuel jet due to the mixing effects of the coaxial air flow. The non-premixed turbulent lifted hydrogen jet flames had two types of flame structure for both thin and thick flame base. Lifted flame stabilization was related to local principal strain rate and turbulent intensity, assuming that combustion occurs where local flow velocity and turbulent flame propagation velocity are balanced.

부상된 수소 난류확산화염의 화염구조

To understand hydrogen jet liftoff height, the stabilization mechanism of turbulent lifted jet flames under non-premixed conditions was studied. The objectives were to determine flame stability mechanisms, to analyze coexistence of two different flame structure, and to characterize the lifted jet at the flame stabilization point. Hydrogen flow velocity varied from 100 to 300 m/s. Coaxial air velocity was changed from 12 to 20 m/s. Simultaneous velocity field and reaction zone measurements used, PIV/OH PLIF techniques with Nd:YAG lasers and CCD/ICCD cameras. Liftoff height decreased with the increase of fuel velocity. The flame stabilized in a lower velocity region next to the faster fuel jet due to the mixing effects of the coaxial air flow. The flame stabilization was related to turbulent intensity and strain rate assuming that combustion occurs where local flow velocity and turbulent flame propagation velocity are balanced. At the flame base, two different flame structures were found that was the partial premixed flames and premixed flame.

Simultaneous three-component PIV/OH-PLIF measurements of a turbulent lifted, C3H8-Argon jet diffusion flame at 1.5 kHz repetition rate

Planar laser-induced fluorescence (PLIF) and stereoscopic particle image velocimetry (PIV) were simultaneously applied at 1.5kHz repetition-rate to acquire time-resolved, cinematographic planar measurements of the flamebase stabilization region of a turbulent lifted jet flame. The current work focuses on the development and demonstration of the diagnostic system and presents key examples of the new capabilities it provides. OH-PLIF is used to identify and track the spatial position and shape of the flamefront near the flamebase, while the stereoscopic PIV is used to acquire three-component planar velocity measurements over the same region. Initial results indicate that the significant temporal histories and experimental access to the Vz-velocity component afforded by this system are frequently essential for accurate interpretation of planar measurements in the flow field of a lifted turbulent jet flame.

Level‐set based flamelet approach for simulating turbulent lifted jet flame

AbstractThe present study focuses on numerically investigating the flame structure, flame liftoff, and stabilization in a lifted turbulent H2/N2 jet flame with a vitiated coflow. To realistically represent the turbulent partially premixed nature in the flow region between nozzle exit and flame base, the level‐set approach coupled with the conserved scalar flamelet model has been applied. The unstructured‐grid level‐set approach has been developed to allow the geometric flexibility and computational efficiency for the solution of the physically and geometrically complex reacting flows. The pressure–velocity coupling is handled by the multiple pressure‐correction method. The predicted flame pattern is in good conformity with the measured one. In terms of the liftoff height, the agreement between prediction and experiment is quite good. Even if there are noticeable deviations in a certain region, the predicted profiles for the overall flame structure agree reasonably well with the experimental data. These numerical results indicate that the present level‐set‐based flamelet approach in conjunction with the unstructured‐grid finite‐volume method is capable of realistically predicting the essential features and precise structure of the turbulent‐lifted jet flame with computational efficiency. Copyright © 2008 John Wiley &amp; Sons, Ltd.