CMC simulations of lifted turbulent jet flame in a vitiated coflow

Lifted turbulent jet diffusion flame is simulated using Conditional Moment Closure (CMC). Specifically, the burner configuration of Cabra et al. [R. Cabra, T. Myhrvold, J.Y. Chen, R.W. Dibble, A.N. Karpetis, R.S. Barlow, Proc. Combust. Inst. 29 (2002) 1881–1887] is chosen to investigate H₂/N₂ jet flame supported by a vitiated coflow of products of lean H₂/air combustion. A 2D, axisymmetric flow-model fully coupled with the scalar fields, is employed. A detailed chemical kinetic scheme is included, and first order CMC is applied. Simulations are carried out for different jet velocities and coflow temperatures (T_c). The predicted liftoff generally agrees with experimental data, as well as joint-PDF results. Profiles of mean scalar fluxes in the mixture fraction space, for T_c=1025 and 1080 K reveal that (1) Inside the flame zone, the chemical term balances the molecular diffusion term, and hence the structure is of a diffusion flamelet for both cases. (2) In the pre-flame zone, the structure depends on the coflow temperature: for the 1025 K case, the chemical term being small, the advective term balances the axial turbulent diffusion term. However, for the 1080 K case, the chemical term is large and balances the advective term, the axial turbulent diffusion term being small. It is concluded that, lift-off is controlled (a) by turbulent premixed flame propagation for low coflow temperature while (b) by autoignition for high coflow temperature.     

Visualization of dilute hydrogen jet flame in air flow

The structure of hydrogen jet flame diluted by CO₂ in air flow is studied by various visualization techniques, such as schlieren, direct photograph, tracer injection and reactive Mie scattering method, which allow understanding of the influence of CO₂ on the characteristics of the hydrogen jet flame. The experimental result indicates that the flame structure consists of laminar fuel jet and surrounding reaction zone near the nozzle exit. When the CO2 fraction is increased, the width of the fuel jet grows and the reaction zone is reduced in size. These observations are further confirmed by quantitative measurements of temperature and velocity fields in the flame, which are evaluated by thermocouple and particle image velocimetry (PIV), respectively. These results indicate that the flame temperature is decreased and the flow rate of the fuel jet is increased by the influence of diluents, which are due to the reduced calorific value and larger density of fuel, respectively.     

Transported PDF simulation of auto-ignition of a turbulent methane jet in a hot,vitiated coflow

In the current work, the auto-ignition of a turbulent round methane jet is studied numerically by means of a transported probability density function (PDF) method. The methane jet is issued into a hot, vitiated coflow, where it ignites to form a steady lifted flame. For this flame, experimental data of hydroxyl, temperature and mixture fraction are provided in the area where the fuel auto-ignites. To model this experiment, the transport equation for the thermochemical PDF is solved using a hybrid finite volume / Lagrangian Monte-Carlo method. Turbulence is modelled using the k-? turbulence model including a jet-correction. Computational results are compared to experimental data in terms of mean quantities, variances and lift-off height. Moreover, the structure of the one-point, one-time marginal PDF of temperature is analysed and compared to experimental data which are provided in this work. It is found that the transported PDF method in conjunction with the k-? model is capable of reproducing these statistical data very well. In particular the effect of ignition on the marginal PDF of temperature can be well reproduced with this approach. To further analyse the relevant processes in the evolution of the temperature PDF, a statistically homogeneous system is studied both numerically and analytically.     

Auto-ignition and flame stabilization of pulsed methane jets in a hot vitiated coflow studied with high-speed laser and imaging techniques

The auto-ignition of a pulsed methane jet issuing into a laminar coflow of hot exhaust products of a lean premixed hydrogen/air flat flame was examined using high-speed laser and optical measurement techniques with frame rates of 5 kHz or more. OH* chemiluminescence was used to determine the downstream location of the first auto-ignition kernel as well as the stabilization height of the steady-state lifted jet flame. OH planar laser-induced fluorescence (PLIF) was used to determine further details of the auto-ignition with a higher spatial resolution. Simultaneous imaging of broadband luminosity from a viewing angle perpendicular to the OH* chemiluminescence was applied, to three-dimensionally reconstruct the ignition kernel location in space and to determine whether the first occurrence of the kernel was within or beyond the PLIF laser sheet. The development and expansion of the jet was characterized by high-speed Schlieren imaging. Statistics have been compiled for both the ignition time as well as the downstream location of the first auto-ignition kernel and the stabilization height of the steady-state lifted jet flame. From the PLIF images it was found that auto-ignition tended to occur at the interface between bulges of the inflowing jet and the coflow. For steady-state conditions, auto-ignition kernels were observed frequently below the flame base, emphasizing that the lifted jet flame is stabilized by auto-ignition.     

A DNS study on the flame structures and flame stabilization mechanism of a laboratory-scale lean premixed jet flame in crossflow

In the present work, direct numerical simulation (DNS) of a laboratory-scale lean premixed reacting jet flame in crossflow was performed to understand the flame structures and the flame stabilization mechanism. In the DNS, an ethylene-air jet with an equivalence ratio of 0.6 was injected into a hot vitiated crossflow. The jet Reynolds number reaches 6161. The DNS results were compared with those of the experiment with a good agreement. It was found that the windward and leeward branches of the flame show significantly different behaviors. The windward flame branch, appearing lifted and discontinuous, is located in the shear layer regions with high temperature, low vorticity and low scalar dissipation rate. The location of the peak heat release rate shifts to a higher mixture fraction with increasing distance from the jet exit. The leeward branch of the flame anchors in the shear layer near the jet exit. The recirculation zone in the wake of the jet facilitates the stabilization of the leeward flame. The chemical explosive mode analysis (CEMA) and species budget analysis were employed to characterize the local combustion mode. Auto-ignition plays a key role in the stabilization of the windward flame where a large range of extinction is also found due to the high strain rate. In contrast, premixed flame propagation is dominant on the leeward side.     

Visualization of multiple scalar and velocity fields in a lifted jet flame

The stabilization of lifted jet diffusion flames has long been a topic of interest to combustion researchers. The flame and flow morphology, the role of partial premixing, and the effects of large scale structures on the flame can be visualized through advanced optical imaging techniques. Many of the current explanations for flame stabilization can benefit from the flow and flame information provided by laser diagnostics. Additionally, the images acquired from laser diagnostic experiments reveal features invisible to the eye and line-of-sight techniques, thereby allowing a deeper insight into flame stabilization. This paper reports visualizations of flame and flow structures from Particle Image Velocimetry (PIV), Planar Laser-Induced Fluorescence (PLIF) and Rayleigh scattering. The techniques are surveyed and the success of visualization techniques in clarifying and furthering the understanding of lifted-jet flame stabilization is discussed.     

Mechanism on oscillating lifted flames in nonpremixed laminar coflow jets

The oscillating lifted flame in a laminar nonpremixed nitrogen-diluted fuel jet is known to be a result of buoyancy, though the detailed physical mechanism of the initiation has not yet been properly addressed. We designed a systematic experiment to test the hypothesis that the oscillation is driven by competition between the positive buoyancy of flame and the negative buoyancy of a fuel stream heavier than the ambient air. The positive buoyancy was examined with various flame temperatures by changing fuel mole fraction, and the negative buoyancy was investigated with various fuel densities. The density of the coflow was also varied within a certain range by adding either helium or carbon dioxide to air, to study how it affected the positive and negative buoyancies at the same time. As a result, we found that the range of oscillation was well-correlated with the positive and the negative buoyancies; the former stabilized the oscillation while the latter triggered instability and became a source of the oscillation. Further measurements of the flow fields and OH radicals evidenced the important role of the negative buoyancy on the oscillation, detailing a periodic variation in the unburned flow velocity that affected the displacement of the flame.     

Large eddy simulation of a turbulent nonpremixed piloted methane jet flame (Sandia Flame D)

Large eddy simulation (LES) is conducted of the Sandia Flame D [Proc. Combust. Inst. 27 (1998) 1087, Sandia National Laboratories (2004)], which is a turbulent piloted nonpremixed methane jet flame. The subgrid scale (SGS) closure is based on the scalar filtered mass density function (SFMDF) methodology [J. Fluid Mech. 401 (1999) 85]. The SFMDF is basically the mass weighted probability density function (PDF) of the SGS scalar quantities [Turbulent Flows (2000)]. For this flame (which exhibits little local extinction), a simple flamelet model is used to relate the instantaneous composition to the mixture fraction. The modelled SFMDF transport equation is solved by a hybrid finite-difference/Monte Carlo scheme. This is the first LES of a realistic turbulent flame using the transported PDF method as the SGS closure. The results via this method capture important features of the flame as observed experimentally.     

A numerical study on the formation of diffusion flame islands in a turbulent hydrogen jet lifted flame

This paper presents a numerical study on the formation of diffusion flame islands in a hydrogen jet lifted flame. A real size hydrogen jet lifted flame is numerically simulated by the DNS approach over a period of about 0.5 ms. The diameter of hydrogen injector is 2 mm, and the injection velocity is 680 m/s. The lifted flame is composed of a stable leading edge flame, a vigorously turbulent inner rich premixed flame, and a number of outer diffusion flame islands. The relatively long-term observation makes it possible to understand in detail the time-dependent flame behavior in rather large time scales, which are as large as the time scale of the leading edge flame unsteadiness. From the observation, the following three findings are obtained concerning the formation of diffusion flame islands. (1) A thin oxygen diffusion layer is developed along the outer boundary of the lifted flame, where the diffusion flame islands burn in a rather flat shape. (2) When a diffusion flame island comes into contact with the fluctuating inner rich premixed flame, combustion is intensified due to an increase in the hydrogen supply by molecular diffusion. This process also works for the production of the diffusion flame islands in the oxygen diffusion layer. (3) When a large unburned gas volume penetrates into the leading edge flame, the structure of the leading edge flame changes. In this transformation process, a diffusion flame island comes near the leading edge flame. The local deficiency of oxygen plays an important role in this production process.     

Two-point time-series measurements of minor-species concentrations in a turbulent nonpremixed flame

We report a technique that is capable of making simultaneous two-point time-series measurements of minor-species concentrations in turbulent flames. The experimental setup, which incorporates picosecond time-resolved laser-induced fluorescence, has a spatial resolution of less than 250 microm and a temporal resolution of less than 100 micros, which spatially and temporally resolve microscales in many turbulent flows. Two-point time-series data are given for a standard turbulent nonpremixed flame at Re= 10,000, including a discussion of potential implications.     

Flow-field dynamics of the non-reacting and reacting jet in a vitiated cross-flow

The dynamics of a premixed ethylene-air jet injected transverse to a vitiated cross-flow were investigated using high-repetition rate particle image velocimetry (PIV). Both non-reacting and reacting jets were found to be characterized by a dominant frequency associated with the jet wake vortex system. For the isothermal jet, increasing the momentum flux ratio (J) has only a slight effect on the frequency of the oscillation but significantly increases its magnitude. The reacting jet was found to exhibit different behavior, with a monotonic increase in the dominant frequency with J. The jet equivalence ratio (?_j) was found to have little effect on the rate of wake vortex shedding but affects the overall magnitude of the oscillation. Comparison with data reported in the literature suggests the relationship between the wake Strouhal number (St_w) and J is fuel dependent. Application of a vortex detection algorithm shows a stark difference in the location of the wake vortices under non-reacting and reacting conditions. Under isothermal conditions, the vortices are found close to the jet centerline and dissipate relatively quickly. Reaction confines the vortices to a narrow shear layer until a farther distance downstream and the vortices convect through the entire area of interest. Additionally, the vortex circulation strength was found to increase with J. Proper orthogonal decomposition (POD) analysis of the non-reacting and reacting jets demonstrates the dominance of the wake vortex structures in the oscillating flow fields. In both cases, the temporal information extracted from the most energetic modes is identical to the dominant frequencies measured in the flow fields. The primary effect of heat release is to reduce the overall amount of coherence and to delay the appearance of elevated coherence levels until a larger streamwise distance from the jet exit.     

Blowout limits of inclined nonpremixed turbulent jet flames

The blowout behavior of inclined nonpremixed turbulent jet flames is investigated by varying the jet inclined angle in the range of -90° to 90° The critical jet velocity at blow-out limit is quantified experimentally for various nozzle diameters, different fuels and inclined angles. Numerical simulations are performed to emphasize the flow field difference for the positive and negative inclined angles. Physical modeling is conducted to incorporate the effect of the inclined angle on blow-out behavior. Major findings include: (1) The negatively inclined jet flames show more intense yellow luminosity with larger sooting zones than the positively inclined jet flames; (2) The blowout limit decreases appreciably with the jet inclined angle for the negatively inclined flames, while for the positively inclined jet flames, this decrease is relatively small; (3) Physical analysis of the flow development of inclined jets is conducted, indicating the centerline velocity along the jet trajectory decreases faster for the flame with smaller inclined angle. And the decrease rate is relatively larger for the negatively inclined jet flames; (4) Based on the analysis of the flow development as well as the characteristic velocity with the inclined angle variation, a model based on the Damköhler number (Da) accounting for the effect of jet inclined angle is developed to characterize the blowout limits of inclined jet flames. The proposed model successfully correlates the experimental data. The present findings provide new data and a basic scaling law for the blowout limit of nonpremixed inclined turbulent jet flames, revealing the effect of the relative angle between the jet momentum and buoyancy.     

Ignition of hydrogen in unsteady nonpremixed flows

The effects of unsteady strain on hydrogen (H₂) ignition in nonpremixed flows are investigated with both experimental measurements and numerical computations. A mixing layer is established in a counterflow configuration with a fuel stream containing N₂–diluted H₂ (X_H2=0.08) flowing against heated air. A reproducible ignition process is initiated by introducing atomic oxygen into the mixing layer with a pulsed ArF excimer laser, which photodissociates heated O₂ from the oxidizer stream. The temporal evolution of OH during ignition is measured by planar laser-induced fluorescence. Following the induction phase, the measured OH mole fraction increases rapidly to a super-equilibrium value that is 60% greater than the OH mole fraction in a steady diffusion flame. The peak OH mole fraction occurs at approximately 6 ms after the excimer laser pulse. To study the OH time history under transient strain, the fuel stream is pulsed at a fixed time after the initiation of ignition. The response of the ignition kernel is extremely sensitive to the time delay of the flow transient. The unsteady strain can delay the ignition time or extinguish the kernel. Comparisons between computations and experiments are made for the evolution of OH during autoignition both for steady and unsteady strain. For both steady and unsteady strain, the transient one-dimensional counterflow computations show excellent agreement with the experiment in terms of predicting ignition delays and the rate of OH accumulation during the induction period. The computations also capture the super-equilibrium OH during the transition to the formation of a steady flame, although not to the degree observed experimentally. The computations are further used to understand the influence of unsteady strain on the kernel evolution. It is found that the degree of super-equilibrium OH is sensitive to strain transients applied close to the time of thermal runaway.     

Fluctuating characteristics of radiative source term in hydrogen turbulent jet diffusion flame

The laminar flamelet model in combination with joint probability density function transport equation of mixture fraction and turbulence frequency is used to simulate turbulent jet diffusion flames of hydrogen. The frequency distributions of radiative source terms are calculated for four important infrared bands of water vapor. The results show that, for the given ensemble, about 95% samples of radiative source term for each band locate within the region of ±3.0 standard deviation of the mean radiative source term. Due to the different relation between band intensity parameters and temperature for every band, the symmetrization of frequency distributions for each band is different.     

Investigation of a turbulent wall jet in forced convection issuing into a directed coflow stream

In the present paper, we have studied numerically the directed coflow stream effects on mean and turbulent flow properties of a turbulent plane wall jet in forced convection emerging into a directed coflow stream. The system of equations governing the studied configuration is solved with a finite difference scheme using a staggered grid for numerical stability, not uniform in the two directions of the flow. The modified version of the first-order low Reynolds number k–? turbulence model is used and compared to existing experimental findings. It is found that predicted results are in satisfactory agreement with the experimental data and that the wall jet fluid decays faster in presence of a directed coflow stream. Results show also that the increase of coflow deviation angles causes an increase of the growth rates of the dynamic and thermal half-width of the jet and enhances the turbulent mixing. It is found that the longitudinal development of normalised forms of the jet characteristics parameters at different directed coflow velocity ratios can be reasonably well collapsed onto universal trends through the use of momentum length scale.