The Speed and Temperature of an Edge Flame Stabilized in a Mixing Layer: Dependence on Fuel Properties and Local Mixture Fraction Gradient

Experiments were conducted in a two-stream planar mixing layer at convective Mach numbers,Mc, of 0.28, 0.42, 0.50, 0.62 and 0.79. Planar laser Mie scattering (PLMS) from a condensed alcohol fog and planar laser-induced fluorescence (PLIF) of nitric oxide were used for flow visualization in the side, plan and end views. The PLIF signals were also used to characterize the turbulent mixture fraction fluctuations.Visualizations using PLMS indicate a transition in the turbulent structure from quasi-two-dimensionality at low convective Mach number, to more random three-dimensionality for$M_c\geqslant 0.62$. A transition is also observed in the core and braid regions of the spanwise rollers as the convective Mach number increases from 0.28 to 0.62. A change in the entrainment mechanism with increasing compressibility is also indicated by signal intensity profiles and perspective views of the PLMS and PLIF images. These show that atMc= 0.28 the instantaneous mixture fraction field typically exhibits a gradient in the streamwise direction, but is more uniform in the cross-stream direction. AtMc= 0.62 and 0.79, however, the mixture fraction field is more streamwise uniform and with a gradient in the cross-stream direction. This change in the composition of the structures is indicative of different entrainment motions at the different compressibility conditions. The statistical results are consistent with the qualitative observations and suggest that compressibility acts to reduce the magnitude of the mixture fraction fluctuations, particularly on the high-speed edge of the layer.

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Frequency spectra of reactant fluctuations in turbulent flows

Journal of Fluid Mechanics ◽

10.1017/s0022112093000230 ◽

1993 ◽

Vol 246 ◽

pp. 489-502 ◽

Cited By ~ 9

Author(s):

George Kosály

Keyword(s):

Turbulent Flows ◽

Cross Correlation ◽

Mixture Fraction ◽

Spectral Characteristics ◽

Mixing Layer ◽

Correlation Coefficients ◽

Frequency Spectra ◽

Fast Chemistry ◽

Limiting Values ◽

Intermediate Rate

Bilger, Saetran & Krishnamoorthy (1991) give measured values of the variance, cross-correlation coefficient, autospectra, coherence and phase shift of the reactant concentration fluctuations for an irreversible second-order reaction in an incompressible turbulent scalar mixing layer. The present paper approaches the interpretation of the measured data by evaluating the above quantities in the frozen (slow) and equilibrium (fast) chemistry limits. We assume that the limiting values bracket the corresponding intermediate rate data.The analysis leads to values that correspond with the measured variances and correlation coefficients. The paper offers simple procedures for experimenters to evaluate the fast chemistry limit of the spectral characteristics from the measured mixture fraction fluctuations. The investigation of the limiting spectra suggests that, in the frequency region considered in the Bilger et al. measurements, the shape of the autospectrum is quite insensitive to the chemistry rate. The cross-spectrum is much more sensitive to the chemistry than the autospectrum. The analysis predicts correctly that the coherence decreases with increasing frequency while the phase stays equal to π until the decrease of the coherence leads to indeterminate phase results.

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Compressibility effects in a turbulent annular mixing layer. Part 2. Mixing of a passive scalar

Journal of Fluid Mechanics ◽

10.1017/s0022112000001634 ◽

2000 ◽

Vol 421 ◽

pp. 269-292 ◽

Cited By ~ 33

Author(s):

JONATHAN B. FREUND ◽

PARVIZ MOIN ◽

SANJIVA K. LELE

Keyword(s):

Mach Number ◽

High Speed ◽

Mixture Fraction ◽

Mixing Layer ◽

Passive Scalar ◽

Momentum Equation ◽

Mixing Efficiency ◽

Scalar Flux ◽

Mach Numbers ◽

Highly Correlated

The mixing of fuel and oxidizer in a mixing layer between high-speed streams is important in many applications, especially air-breathing propulsion systems. The details of this process in a turbulent annular mixing layer are studied with direct numerical simulation. Convective Mach numbers of the simulations range from Mc = 0.1 to Mc = 1.8. Visualizations of the scalar field show that at low Mach numbers large intrusions of nearly pure ambient or core fluid span the mixing region, whereas at higher Mach numbers these intrusions are suppressed. Increasing the Mach number is found to change the mixture fraction probability density function from non-marching to marching and the mixing efficiency from 0.5 at Mc = 0.1 to 0.67 at Mc = 1.5. Scalar concentration fluctuations and the axial velocity fluctuations become highly correlated as the Mach number increases and a suppressed role of pressure in the axial momentum equation is found to be responsible for this. Anisotropy of scalar flux increases with Mc, and is explained via the suppression of transverse turbulence lengthscale.

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The evaporatively driven cloud-top mixing layer

Journal of Fluid Mechanics ◽

10.1017/s0022112010002831 ◽

2010 ◽

Vol 660 ◽

pp. 5-36 ◽

Cited By ~ 58

Author(s):

JUAN PEDRO MELLADO

Keyword(s):

Mixture Fraction ◽

Evaporative Cooling ◽

Mixing Layer ◽

Molecular Transport ◽

Buffer Zone ◽

Turbulent Convection ◽

Constant Thickness ◽

Mean Velocity ◽

Turbulent Layer ◽

Entrainment Velocity

Direct numerical simulations of the turbulent temporally evolving cloud-top mixing layer are used to investigate the role of evaporative cooling by isobaric mixing locally at the stratocumulus top. It is shown that the system develops a horizontal layered structure whose evolution is determined by molecular transport. A relatively thin inversion with a constant thickness h = κ/we is formed on top and travels upwards at a mean velocity we ≃ 0.1(κ |bs|χc2)1/3, where κ is the mixture-fraction diffusivity, bs < 0 is the buoyancy anomaly at saturation conditions χs and χc is the cross-over mixture fraction defining the interval of buoyancy reversing mixtures. A turbulent convection layer develops below and continuously broadens into the cloud (the lower saturated fluid). This turbulent layer approaches a self-preserving state that is characterized by the convection scales constructed from a constant reference buoyancy flux Bs = |bs|we/χs. Right underneath the inversion base, a transition or buffer zone is defined based on a strong local conversion of vertical to horizontal motion that leads to a cellular pattern and sheet-like plumes, as observed in cloud measurements and reported in other free-convection problems. The fluctuating saturation surface (instantaneous cloud top) is contained inside this intermediate region. Results show that the inversion is not broken due to the turbulent convection generated by the evaporative cooling, and the upward mean entrainment velocity we is negligibly small compared to the convection velocity scale w* of the turbulent layer and the corresponding growth rate into the cloud.

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LB SIMULATION ON COMBUSTION WITH TURBULENCE

International Journal of Modern Physics B ◽

10.1142/s0217979203017321 ◽

2003 ◽

Vol 17 (01n02) ◽

pp. 197-200 ◽

Cited By ~ 13

Author(s):

KAZUHIRO YAMAMOTO

Keyword(s):

Mixture Fraction ◽

Mixing Layer ◽

Vortex Pair ◽

Lewis Number ◽

Spatial Location ◽

Flamelet Model ◽

Two Fluids ◽

Fast Chemistry ◽

Boltzmann Method ◽

Instantaneous Chemical

In this study, we simulate combustion field by the lattice Boltzmann method. We use a compressible model to describe the behavior of a turbulent non-premixed flame in the mixing layer of two co-flowing streams, one of propane and the other of air. By the assumption of fast-chemistry and unity Lewis number, we adopt the conserved scalar approach to reduce computational costs. In this case, the instantaneous chemical composition of the mixture at a given spatial location is at chemical equilibrium. This is so-called laminar flamelet model where the temperature and concentration are obtained by mixture fraction, which is determined by the degree of mixing of fuel and oxidizer. Results show that a thin flame zone separates two fluids of fuel and oxidizer, and the interaction between flame and a vortex pair is observed.

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Effects of initial mixture fraction on the development of a turbulent reactive mixing layer

Journal of Turbulence ◽

10.1080/14685248.2011.607167 ◽

2011 ◽

Vol 12 ◽

pp. N33

Author(s):

Mani Fathali ◽

Bamdad Lessani

Keyword(s):

Mixture Fraction ◽

Mixing Layer ◽

Initial Mixture ◽

Reactive Mixing

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Conditional scalar dissipation statistics in a turbulent counterflow

Journal of Fluid Mechanics ◽

10.1017/s0022112098008635 ◽

1998 ◽

Vol 361 ◽

pp. 1-24 ◽

Cited By ~ 33

Author(s):

KATERINA SARDI ◽

A. M. K. P. TAYLOR ◽

J. H. WHITELAW

Keyword(s):

Diffusion Flames ◽

Probability Distributions ◽

Mixture Fraction ◽

Mixing Layer ◽

Scalar Dissipation ◽

Jet Flows ◽

Residence Times ◽

Scalar Turbulence ◽

The Mean ◽

Log Normal

Cold-wire measurements of a scalar, temperature, its fluctuations and the axial and radial components of the scalar dissipation between two opposed turbulent jet flows, where one jet was slightly heated, show that the residence times of the scalar in the mixing layer were short, that the scalar fluctuations and their dissipation were strongly correlated and that the probability distributions of the conditional scalar dissipation components were log-normal at values of the dissipation larger than the mean. The first finding is consistent with the fact that the scalar turbulence was ‘young’, in the sense that residence times were shorter than the large-eddy turn-over time, so that the results are likely to be representative of scalar turbulence when scalar mixing first takes place between two streams, for example close to the stabilization region of turbulent diffusion flames. The second implies that the mean scalar dissipation, conditional on the stoichiometric mixture fraction, is larger than the unconditional mean by up to an order of magnitude. Dependence of the distributions of the mean and r.m.s. conditional scalar dissipation on the shape of the scalar p.d.f. was demonstrated by relating the largest conditional dissipation values to the rarest scalar fluctuations and it was found that this dependence was also valid in other flows where scalar dissipation has been measured. The third finding implies that the use of a log-normal distribution to describe the p.d.f. of the conditional scalar dissipation, in the context of flame extinction modelling, will be in error by only 15% provided that the mean and the r.m.s. conditional scalar dissipation are accurately known.

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