Turbulent Jet Flames Research Articles

Relative influence of soot oxidation kinetics and subfilter soot-turbulence interactions on soot evolution in turbulent nonpremixed flames

Oxidation of soot is a critical process in determining the evolution of soot in turbulent reacting flows. On the one hand, the kinetic rates of soot oxidation remain one of the most significant uncertainties in soot models. On the other hand, turbulent transport plays a significant role in the distribution of soot with respect to the flame structure so the location of soot in regions where oxidation can actually occur. The objective of this work is to assess the relative importance of small-scale turbulent transport and oxidation kinetics on the prediction of soot evolution in turbulent nonpremixed flames. A series of Large Eddy Simulations has been performed in a turbulent nonpremixed piloted jet flame of ethylene and air using a series of soot subfilter PDF models (with different assumptions regarding the small-scale turbulent transport of soot) and a series of kinetic rates for soot oxidation (with differences in kinetic rates for both OH and O2 oxidation). Similar to previous work, the results show that accounting for the disappearance of soot in lean mixtures due to soot oxidation is extremely important to avoid excessive “spurious oxidation” of soot otherwise artificially present in lean mixtures. In addition, the sensitivity to soot oxidation kinetics is found to be significantly less important than this effect. However, once this effect is taken into account in the soot subfilter PDF, the sensitivity to soot oxidation kinetics becomes rather significant compared to further refinements to the soot subfilter PDF models. More specifically, the assumption in the soot subfilter PDF models whether soot oxidation is infinitely fast or finite rate compared to small-scale turbulent transport is a less significant model sensitivity compared to the sensitivity to the kinetic rates of soot oxidation. Furthermore, oxidation by OH and O2 are revealed to play fundamentally different roles in soot evolution.

Thermal Runaway Propagation in Li-ion Battery Packs Due to Combustion of Vent Gases

Accurate understanding of propagation of thermal runaway is of much importance for developing safe battery pack designs. Combustion of vent gases emerging from a trigger cell undergoing thermal runaway has not been studied in sufficient detail, even though the additional heat generated during combustion likely plays an important role in thermal runaway propagation. This work presents comprehensive numerical modeling and simulations of thermal runaway propagation in a pack of cylindrical cells. The model accounts for multiple coupled non-linear phenomena, including vent gas flow and combustion, radiation and thermal runaway. Non-premixed combustion of venting gas is modeled using k-ε turbulence model and finite rate chemical kinetics. Simulation results are shown to be in good agreement with experimental data for a benchmark turbulent non-premixed jet flame. Simulations show that hot combustion products are rapidly transported in gaps between cells, potentially leading to self-sustained thermal runaway propagation to adjacent cells. Results demonstrate the critical importance of combustion in determining the nature of propagation of thermal runaway. The vent hole location is identified as an important parameter that influences whether and the extent to which thermal runaway propagation occurs. This work contributes towards the practical understanding of thermal runaway safety of Li-ion battery packs.

Extensive Discussions of the Eddy Dissipation Concept Constants and Numerical Simulations of the Sandia Flame D

The indisputable wide use of the Eddy Dissipation Concept (EDC) implies that the resulting mean reaction rate is reasonably well modeled. To model turbulent combustions, an amount of EDC constants that differ from the original values was proposed. However, most of them were used without following the nature of the model or considering the effects of the modification. Starting with the energy cascade and the EDC models, the exact original primary and secondary constants are deduced in detail in this work. The mean reaction rate is then formulated from the primary constants or the secondary constants. Based on the physical meaning of fine structures, the limits of the EDC constants are presented and can be used to direct the EDC constant modifications. The effects of the secondary constant on the mean reaction rate are presented and the limiting turbulence Reynolds number used for the validity of EDC is discussed. To show the effects of the constants of the EDC model on the mean reaction rate, 20 combinations of the primary constants are used to simulate a laboratory-scale turbulent jet flame, i.e., Sandia Flame D. After a thorough and careful comparison with experiments, case 8, with a secondary constant of 6 and primary constants of 0.1357 and 0.11, can aptly reproduce this flame, except for in the over-predicted mean OH mass fraction.

Experimental study of gasoline engine with EGR dilution based on reactivity controlled turbulent jet ignition (RCTJI)

The exhaust gas recirculation (EGR) method can suppress knock and improve the thermal efficiency of engines, but it also deteriorates the combustion stability and engine power. Turbulent jet ignition (TJI) is a reliable ignition way for improving ignition stability and burning rate. However, the EGR in the pre-chamber will worsen the reactivity of the fuel-air mixture in the pre-chamber and subsequently lower the intensity of the TJI. To this end, the reactivity controlled turbulent jet ignition (RCTJI) based on a newly air-assisted pre-chamber turbulent jet ignition with a scavenging function has been proposed to improve the reactivity in the pre-chamber for the first time. The combustion characteristics, pressure oscillation, and combustion stability of a single-cylinder spark-ignition engine under four ignition modes of spark ignition (SI), passive TJI, active TJI, and scavenged TJI have been investigated. The results show that three TJI modes can obviously shorten the combustion duration due to the fast jet flame. Compared to the SI mode, the spark timing of TJI is not sensitive to the change in EGR rate. In particular, the scavenged TJI could effectively promote the chemical reactivity in the pre-chamber, and subsequently improves the intensity of the turbulent jet flame and shortens the ignition delay and combustion duration. As a result, using the scavenged TJI can effectively reduce cycle variation and achieve the lowest indicated specific fuel consumption (ISFC) at high EGR rates. However, with the increase in EGR rate, the SI combustion gradually becomes unstable. In contrast, the scavenged TJI can effectively extend the EGR tolerance. In addition, the present work further has confirmed the three main combustion stages of jet ignition in terms of the heat release rate.

Evaluation of a Turbulent Jet Flame Flashback Correlation Applied to Annular Flow Configurations With and Without Swirl

Abstract The adaptation of high hydrogen content fuels for low emissions gas turbines represents a potential opportunity to reduce the carbon footprint of these devices. The high flame speed of hydrogen air mixtures combined with the small quenching distances poses a challenge for using these fuels in situations where a significant premixing is desired. In particular, flashback in either the core flow or along the walls (i.e., boundary layer flashback) can be exacerbated with high hydrogen content fuels. In this work, the ability of a flashback correlation previously developed for round jet flames is evaluated for its ability to predict flashback in an annular flow. As a first step, an annular flow is generated with a centerbody located at the centerline of the original round jet flame. Next, various levels of axial swirl is added to that annular flow. Additional flashback data are obtained for various mixtures of hydrogen and methane and hydrogen and carbon monoxide for all the annular flow configurations. Pressures from 3 to 8 bar are tested with mixture temperatures up to 750 K. Flashback is induced by slowly increasing the equivalence ratio. The results obtained show that the same form of the correlation developed for round jet flames can be used to correlate flashback behavior for the annular flow case with and without swirl despite the presence of the centerbody. Adjustments to some of the constants in the original model were made to obtain the best fit, but in general, the correlation is quite similar to that developed for the round jet flame. A significant difference with the annular flow configurations is the determination of the appropriate gradient at the wall, which in the present case is determined using a cold flow computational fluid dynamics simulation.

Conditional analysis of temperature and strain rate effects on dissipation structure in turbulent non-premixed jet flames

This work presents results from simultaneous high-resolution temperature and velocity measurements in a series of turbulent non-premixed jet flames. The filtered Rayleigh scattering (FRS)-based temperature measurements demonstrate sufficient signal-to-noise (SNR) and spatial resolution to estimate the smallest scalar length scales and accurately determine dissipation rate fields. A comprehensive set of conditional statistics are used to characterize the small-scale structure, including the dependence of dissipation layer widths on Reynolds number, temperature, and dissipation magnitude. In general, the dissipation layer thickness decrease with increasing Reynolds number and increase with increasing temperature. However, dissipation layer widths show two distinct behaviors with respect to dissipation magnitude. For small dissipation values, increases in magnitude results in broadening of the dissipation layer, while for larger magnitude values of dissipation, the layer widths are thinned, highlighting the complexity of small-scale turbulent mixing. Additionally, measured ratios of the dissipation layer width to the Batchelor length scale are consistent across all Reynolds numbers and agree with previous studies in non-reacting flows. The unique aspect about the current set of measurements is the ability to examine the interaction of dissipation structure with turbulent flow parameters for the first time in turbulent non-premixed flames. Particularly, the strain rate/dissipation relationship is examined and compared to previous studies in non-reacting flows. It is found that the dissipation layers tend to align normal to the principal compressive strain axis and this tendency increases with increasing Reynolds number. For the lowest Reynolds number case, no dependence of the dissipation layer width nor dissipation rate magnitude on strain rate is found. However, for higher Reynolds numbers, a strong dependence of the dissipation layer width and dissipation rate magnitude on the principal compressive strain rate is observed. These results indicate the direct role of the compressive strain rate field on small-scale mixing structure in reacting flows.

Pump-probe strategy for instantaneous 2D detection of CH3 in flames using a single laser

Visualization of the reaction zone of flames using CH radicals as markers is restricted by the low concentration of CH in fuel-lean conditions. To address this, methyl radicals (CH3) are employed as a substitution of CH in premixed methane/air flames. A pump-probe method was adopted with the pump laser photolyzing CH3 and the probe laser detecting the photolyzed CH (X2Π) fragments. Laser excitation scans were performed to ensure that the fluorescence detected was from CH only. Visualization of the reaction zone of flames was accomplished by a CH3 photofragmentation laser-induced fluorescence technique in fuel-lean conditions (the equivalence ratio of 0.4), where CH planar laser-induced fluorescence did not work in both laminar and turbulent jet flames. The proposed pump-probe method of CH3 can be used to visualize the reaction zone of hydrocarbon combustion under both fuel-lean and fuel-rich conditions with a superior signal-to-noise ratio.

Experimental study of turbulent flame propagation under wall film conditions

The presence of a liquid wall film caused by spray impingement is inevitable in modern direct injection engines and can have a substantial effect on flame dynamics and pollutant emissions. The present study investigated the effects of a wall film on turbulent flame propagation characteristics under different initial conditions. In these trials, methane-air premixed turbulent jet flames were established in a constant volume vessel with a small pre-chamber connecting to the main chamber through a narrow orifice. Various fuel films were prepared on the surface of a side plate installed in the main chamber. The flame morphologies were captured using a high-speed schlieren photographic system and the intensity of pool fire was examined by an image intensifier with UV lens. Meanwhile, temporal pressures in both chambers were recorded by two pressure sensors respectively. Changes in turbulent combustion phases in conjunction with single and multi-component wall films were analyzed based on flame propagation features, pressure data and film pyrolysis characteristics over ranges of equivalence ratios (0.8–1.2) and initial pressures (0.2–0.4 MPa). Gas phase emissions were also evaluated to further explore the effects of wall films on turbulent combustion. The high reactivity of the fuel films was found to increase the turbulent flame speed and provide higher peak pressures, although the heat absorbed by evaporation delayed the pressure rise time. The volatility of the film material affected the propagation speed together with the jet flame area growth and these effects were more obvious under lean combustion conditions. Film pyrolysis was triggered as multiple turbulent flows converged near the film region and produced a pool fire with soot deposition. The peaks of pressure and pressure rise rate observed when using a diesel film were intermediate between those obtained with n-dodecane and n-tetradecane films. A lubricating oil film pool fire also appeared after increasing the initial pressure but its intensity was weaker than those of the fires generated by the fuel films.

Characteristics of turbulent flames in a confined and pressurised jet-in-hot-coflow combustor

Combustion in hot and low oxygen environments—such as those encountered in practical devices including inter-turbine burners and sequential gas turbines—is not yet fully understood at a fundamental level, particularly in terms of the effects of pressure. To meet this gap in understanding, a confined-and-pressurised jet-in-hot-coflow (CP-JHC) combustor has been developed to facilitate optical diagnostics of turbulent flames in hot and vitiated coflows for the studies of flame stabilisation, structure and soot formation at elevated pressures. The CP-JHC burner has been designed for steady operation at 10 bar with internal temperatures of up to 1975 K, with a water-cooled central jet issuing into a hot oxidant stream of combustion products from a non-premixed natural gas/H2 burner. This work describes the key features and operational capabilities of the CP-JHC burner and presents a selection of experimental results showing characteristics not previously available. Specifically, temperature measurements of the hot coflow are used to estimate the enthalpy deficit of the stream, revealing an increase in thermal efficiency with increasing heat input, and a decrease with increasing pressure. Chemiluminescence imaging of OH∗ and CH∗ is performed for turbulent jet flames to study the flame structure under various operating conditions, and true-colour imaging results are also included to highlight the change in soot formation under elevated pressures. The mean images indicate a change in stabilisation behaviour with changes in pressure and jet Reynolds number (Rejet), which is further investigated by a statistical analysis of the short-exposure CH∗ images. This analysis reveals that an increase in Rejet from 10,000 to 15,000 leads to an increase in the mean lift-off height (from the jet exit plane) from approximately 1.5 to 6 jet diameters at atmospheric pressure, while the flames at elevated pressures show significantly less variation and tend to stabilise at the jet exit for P > 3.5 bar(a). The experimental findings are complemented by numerical simulations of laminar opposed flow flames, providing additional insights into the fundamental chemical kinetics effects which influence these flames. In particular, a monotonic reduction in both the maximum and integrated OH∗ and CH∗ mass fractions is observed with increasing pressure. This reduction is particularly pronounced at lower pressures, with a reduction to 10% of the atmospheric-pressure value at 3 bar(a) for the integrated OH∗ mass fraction. Additionally, this behaviour is shown to be related to the combined effects of a shift in the formation pathways and the increased impact of collisional quenching.

Experimental study on the flame radiation fraction of hydrogen and propane gas mixture

Hydrogen added to hydrocarbon fuels can improve the combustion efficiency and reduce soot emissions. However, there are some safety problems with the storage and transportation of the hydrogen and hydrocarbon fuel gas mixture. In order to investigate the combustion characteristics of gas mixture, the propane and hydrogen are used as the fuel gas in this study. A series of experiments have been carried out to investigate the radiation fraction of the turbulent diffusion jet flame under ambient conditions (300 K, 1 bar). The experimental results showed that the dilution effect of hydrogen is limited for propane when the hydrogen flow rate is small. The radiation fraction changes slightly with the increase in the flow rate of hydrogen under the same heat release rate of propane. And then, it decreases sharply when the ratio of hydrogen volume flow rate to total flow rate is larger than 0.6. Based on the theoretical analysis of laminar combustion velocity, activationenergy, and the soot generation theory, a dimensionless model has been developed to describe the evolution of radiation fraction.

Experimental study on trajectory flame length and axial temperature distribution of inclined turbulent jet flames

Uncontrollable jet fires are a typical disaster caused in process industries involving hazardous materials. Jet fires are also great safety hazards in the process of pipeline transportation. With the development of pipeline transportation, pipe network leakage events often do not occur in simple scenarios, but in more complex situations, such as in inclined jet fires. There are few studies on the physical characteristics of inclined jet flames in specific environments, which is of great significance for safety protection. In this work, experiments on turbulent diffusion jet flames in different inclined states (−90°to 90°) with nozzle diameters ranging between 2 and 6 mm were carried out. The jet flame length and temperature distribution of the central axis of the flame were analysed. In the present study, a theoretical model is derived to incorporate the effect of the jet inclined angle on the flame length. The experimental results show that both the flame length and the temperature of the central axis of the flame decrease with a decrease of the angle. Finally, the correction angle is introduced into the classical model, and a new dimensionless correlation is established, which is in good agreement with the current and other authors’ experimental results.

Ensemble Combustion of Liquid Oxygen / Gaseous Hydrogen Coaxial Flames with Transverse Acoustics

ABSTRACT Experiments were performed to investigate the ensemble combustion of a linear array of three coaxial liquid oxygen/gaseous hydrogen turbulent non-premixed jet flames, under the influence of transverse acoustic forcing. The results are systematically compared to the results from a predecessor study of the same ensemble of flames under the same conditions, but without acoustic forcing. Both pressure nodes (PN) and pressure antinodes (PAN) were investigated as a function of the outer-to-inner momentum flux ratio. Shadowgraphs and OH* emission imaging showed that PNs distort the flames more than PANs, similar to previous observations in single flames. A wave amplification mechanism (WAM) identified previously to be present both in single flames and in the unforced ensemble of flames was found to also be present under acoustic forcing. In the predecessor study without acoustics, the WAM tended to self-organize within the ensemble into an out-of-phase varicose nesting that minimized large-scale collisions. In the present study, PN forcing tended to organize the WAM structures into a sinuous nesting, which also minimized collisions. PAN forcing, on the other hand, promoted destructive in-phase varicose nesting that promoted large-scale collisions, causing subsequent breakdown into smaller scales. Dynamic Mode Decomposition revealed natural frequencies and modes in addition to the acoustic quantities. Discrete combustion waves related to the WAM propagated at the same normalized average velocities regardless of whether acoustics were present in the ensemble or not. However, the trajectories are grouped into different curves than for single-element flames.

Three-Dimensional Direct Numerical Simulation of Near-Field Ozone-Enhanced Lean Premixed Syngas Turbulent Jet Flame

Due to its enhancement in the flame speed, ozone added in lean premixed syngas turbulent jet flame was investigated by the three-dimensional direct numerical simulation method in the near field of the flame. In the present study, numerical simulations were conducted in the lean premixed syngas turbulent jet flame configuration to explore the effects of ozone addition on freely-propagating turbulent flames. It was seen that turbulence began to significantly affect the flame surface to produce wrinkles in lean premixed gas flame with ozone added after 4D; ozone started to affect the composition field and temperature field after 8D; it accelerated the generation of intermediate products, OH and O radicals; and it will promote the production of CO2 in the near field range. Ozone will increase the flame surface area of the lean premixed syngas flame during the ignition period and can promote the ignition process and make the combustion occur earlier. The flame surface of the case with ozone added is more easily stretched by turbulence, and ozone can improve the stability of combustion. Ozone does not affect the effective radius of the flame curvature but will broaden the distribution of the curvature term because of the enhancement effect on the displacement speed of the flame surface.

Combustion characteristics of a confined turbulent jet flame

This work studied the combustion characteristics of a highly-turbulent premixed jet flame influenced by a confined space. A piloted premixed jet burner is used to generate a supersonic methane-air fuel-rich flow and a heat-resisting quartz tube is located coaxially above the jet flame which acts as the confined space. Flame luminosity, OH* and CH* chemiluminescence were captured at different distances between the tube and the burner (H). Jet flame characteristics have been comprehensively investigated from flame extinction to combustion enhancement with the increasing of H. A critical value of H has been identified, below which the flames will be extinguished by the ejection effect of the confined space and the central jet. For the self-sustained confined jet flames, it can be concluded that there are three representative flame structures with the increasing of H, namely the V-shaped flame, converging flame and fully developed flame. The ignition failure of the central region in the fuel jet is found to be the immediate cause for the presence of the V-shaped flame. In the converging flame, a region where strong combustion occurs (the first intense combustion region, FICR) can be observed around the entrance of the confined space, which is mainly formed by the chemical reactions of CH* and OH*. For the fully developed flame, besides FICR, a second intense combustion region (SICR) that mainly related to OH* reaction pathways will emerge in the confined space.

Ensemble Combustion of Liquid Oxygen/ Gaseous Hydrogen Coaxial Flames

ABSTRACT Experiments were performed to investigate the ensemble combustion of a linear array of three coaxial liquid oxygen/gaseous hydrogen turbulent non-premixed jet flames. The experimental conditions were carefully controlled to duplicate those of a previous single element experiment as closely as possible, so that the single element flame could be closely compared with the present three element flame. All ensemble and inter-element effects were due to the turbulent combustion only; acoustic effects were intentionally suppressed. Abundant interactions were found both in the time averaged sense as well as in the dynamics of the larger scale motions. The multielement flames were found to be axially longer, and dominant frequencies were found to emerge for the multielement flames that were not present for the single element flames. A dynamic wave amplification mechanism previously identified for single element flames was found to remain present for multielement flames. However, the mechanism did not operate independently for each element in the ensemble, but instead entered into phase relationships with the neighboring elements. The phase relationship possibly encouraged the dynamics between jet flames that allowed the dominant frequencies to emerge. Several indications of such a phase relationship were found, but not all indications were observed to occur all of the time. It is possible that the coordination of the wave amplification mechanism between the flames was weak or intermittent. Other simultaneous processes probably also contributed to the overall flame behavior.

The partially stirred reactor model for combustion closure in large eddy simulations: Physical principles, sub-models for the cell reacting fraction, and open challenges

For their ability to account for finite-rate chemistry, reactor-based models are well suited Turbulence–Chemistry Interactions (TCI) Sub-Grid Scale (SGS) closures for Large Eddy Simulations (LES). The SGS closure in the Partially Stirred Reactor (PaSR) model relies on the determination of the reacting fraction of each computational cell, whose definition is based on estimates of the characteristic mixing and chemical time scales. Direct Numerical Simulations (DNS) of turbulent combustion can supply key information on TCI for the development, validation, and comparison of combustion models. In particular, a priori testing allows the direct validation of model assumptions. In the present work, an a priori assessment of the PaSR model is conducted. Its ability to reconstruct thermo-chemical quantities of interest is investigated along with model assumptions. Sub-grid quantities are extracted from the DNS to investigate the role of the cell reacting fraction. Various definitions are then proposed to estimate the characteristic chemical timescale in the PaSR model. Modeled chemical source terms and heat release rates are compared against the filtered quantities from DNS data of a two-dimensional, spatially developing, turbulent nonpremixed jet flame with detailed kinetics. The results demonstrate the importance of accounting for the fine structures quantities in the context of reactor-based models. A new formulation of the chemical timescale is proposed and provides improved overall predictions. Several issues are raised in the discussion, representing realistic prospects for further developments of the PaSR model as a SGS combustion closure for LES.

Self-excited instability regimes of a confined turbulent jet flame at elevated pressure

The dynamic response of a confined, premixed turbulent jet flame is investigated at high thermal power densities (∼25 MW/m2/bar) and turbulence levels (Rejet∼ 5 ×105). As equivalence ratio and inlet jet velocity are varied at these conditions, multiple instability modes coexist: a low-amplitude instability (p′/pC≲ 9%) with longitudinal-mode fluctuations (1L and 2L) and two high-amplitude (p′/pC≲ 20%), high-frequency, transverse instability modes. While the axial modes are ubiquitous across the operational envelope, the transverse mode selection is sensitive to the equivalence ratio and reactant jet velocity. A linear stability analysis (LSA) of the confined base flow is performed to explore the flame perturbation in response to the density and temperature gradients, and the shear-layer instabilities in the flow. The high-frequency combustion instabilities are associated with a combined azimuthal hydrodynamic mode of the reactant jet, (1) at the combustion chamber near field and, (2) downstream in the fully developed region of the combustor. An excellent agreement was observed between the convectively unstable modes identified by the temporal LSA and the self-excited combustion instabilities in the experiment.

Combustion characteristics of aluminum particle jet flames in a hot co-flow

Aluminum(Al) powder has been proposed as a promising dense energy carrier to store and transport abundant renewable power for a future carbon-neutral society. In this work, a detailed model for simulating Al particle cloud combustion with multiple oxidizers in the context of large eddy simulation is developed. The Al particles are tracked in the lagrangian framework with various sub-models including phase change, heterogeneous surface reaction, evaporation and radiation. The model is then employed to simulate turbulent jet flames with Al particles(d32=25μm) in a hot co-flow. Tunable diode laser absorption spectroscopy(TDLAS) is used to measure the temperature and H2O distribution. The present model is first validated by comparing the predicted condensed Al2O3 distribution with the measured Mie scattering signals. The results indicate that the simulation results are in good agreement with the experimental measurements. Further comparisons of the temperature and H2O profile are then conducted. The numerical simulation predicts weaker mass and heat transport in the upstream and middle of the flame along the centerline of the burner, compared with the reconstructed TDLAS results, but the downstream scalar profiles of the jet flame agree well with the measurements. Discrete characteristics are observed in the simulated instantaneous temperature and condensed Al2O3 snapshots, which are due to the nature of heterogeneous combustion of metal fuel particles under fuel-lean conditions. Finally, the influence of the oxidizer mole fraction and the initial temperature of the primary jet on the ignition distance is analyzed. The results demonstrate that the ignition distance can be decreased effectively by increasing the oxidizer concentration of the hot co-flow or the initial temperature of the Al particles and carrier gas.

Hybrid constraint multi-line absorption spectroscopy for non-uniform thermochemical measurements in axisymmetric laminar and jet flames

We propose a novel strategy for the accurate determination of non-uniform thermochemical distributions in axisymmetric flames using line-of-sight laser absorption spectroscopy with the constraints of hybrid information. In this method, the computational fluid dynamics (CFD) simulation or other diagnostic method (e.g., thermocouple measurement) is used to qualitatively characterize the thermochemical properties, whereas the kinetic modelling, chemical equilibrium calculation and thermocouple measurement are used to constrain the range of unknown parameters. Experimental demonstrations were firstly performed in a standard laminar CH4/air premixed flame stabilized on a McKenna burner at different equivalence ratios (Φ = 0.8-1.2). With proper hybrid constraints, multiple solutions are avoided, and the deviation and convergence time can be simultaneously reduced comparing to the conventional method. For further demonstration, this strategy was then applied to a turbulent jet flame to recover the significant thermochemical gradient without the need for tomographic reconstruction techniques.

Laser diagnostic investigation of a confined premixed turbulent jet flame stabilized by recirculation

A preheated premixed CH4/air jet flame was operated in an optically accessible rectangular combustion chamber in order to study the flame stabilization and flame expansion mechanisms. Due to an off-center position of the jet, a pronounced lateral recirculation zone had formed in the combustion chamber which stabilized the lifted flame by mixing burned and unburned gases in the shear layer. Simultaneous single shot measurements of the flow field by particle image velocimetry (PIV), of OH by planar laser induced fluorescence (PLIF) and of heat release by OH chemiluminescence imaging from two directions were performed at a frame rate of 5 kHz. The image sequences provided a detailed insight into the interaction between the flow field and the flame, particularly into the effects of the vortices that developed in the shear layer between the jet and the recirculation zone. Special interest was devoted to the role of autoignition and turbulent flame propagation near the flame base.