Opposed Jet Flow Research Articles

Numerical study on transport and deposition characteristics of particles associated with heat setting process in opposed jet flow field

The transport and deposition of particles accompanying the heat setting process in the opposed jet air supply system have a significant impact on the equipment safety. This paper used the Realizable k–ε model to simulate the flow field distribution of the impinging jet and the Lagrange method to track the trajectory of particles, by considering factors, such as jet velocity (7–13 m/s), oven temperature (413–473 K), and particle size (1–30 μ m ) changes, the instantaneous force process of particles was analyzed and their deposition mechanism was revealed. The numerical results have been well validated, with the root mean square error of within 10%. The results indicate that the flow field form of the opposed jet significantly affects particle placement on the surface of the air duct, the maximum turbulent vortex structure appears at the inlet of the air duct on both sides of the protruding nozzle, enhancing the capture of particles. Under the influence of gravity, the lower surface of the air duct becomes the main deposition surface, this phenomenon becomes more pronounced as the particle size increases and the maximum deposition rate difference between the upper and lower layers is 37%. The number of particles deposited on the rear end of the air duct is greater than that on the front end, and the deposition rate decreases with the increase in jet velocity. In the initial stage, a temperature gradient appeared in the machine room, which provided upward lift for the particles, resulting in more small diameter particles ( d p < 10 μ m ) depositing on the upper surface of the air duct. The results can provide theoretical support for the fire prevention and pollutant purification of equipment, such as heat setting machines.

Studies on flow field, and instabilities in a large diameter opposed jet burner

This work reports studies on flow fields, and instabilities exhibited by opposed jets at equal momenta, for Reynolds numbers (Re) ranging from 178 to 5000 using a large diameter counterflow jet setup. Flow instability investigations were conducted over a range of aspect ratios (α) too. This study identifies the regions of bi-stability and those of oscillatory stagnation plane offsets identified by Re. Experiments on flow field were carried out using particle image velocimetry technique, and the paper presents the axial and radial velocity profiles at various locations as well as their gradients. A decreasing trend in stagnation plane displacements with the Reynolds number was observed. The experiments in comparison with the past literature suggest a possible dependence of the stagnation plane displacements on a nozzle-exit diameter. The trends in maximum stagnation plane displacements (δmax), as well as the critical Reynolds numbers (Recr) with aspect ratios (α), are analyzed and compared. The flow field studies reveal the need for two dimensional axisymmetric simulations with realistic velocity boundary conditions to predict opposed jet flow phenomena accurately. Reacting flow instability studies were also carried out for equal momenta using methane-air and ethylene–air flames at various aspect ratios. The results show an enhanced bistability for ethylene–air flames over methane–air flames.

Laser-induced breakdown spectroscopy for local equivalence ratio measurement in opposed jet methane-air flames

Laser-induced breakdown spectroscopy (LIBS) has been applied to both non-reacting and reacting opposed jet flows to evaluate the measurement accuracy of instantaneous, local air-fuel ratio. However, the LIBS signal is much weaker in flames than in the corresponding non-reacting gases and more work is required to investigate the potential of this technique for instantaneous measurements in reacting flows and quantify measurement uncertainties. The present study reported local equivalence ratio measurements of methane/air flames in an opposed-jet burner, which allowed independent control of flow rate and air-fuel ratio. Images and emission spectra of laser-induced plasma were investigated simultaneously using both an Intensified CCD (ICCD) and a spectrometer with a high degree of spatial and temporal resolution. The influences of the camera delay time, exposure time and laser pulse energy on the LIBS measurements were investigated as well. The dependence of spectral intensity ratios of H/O and C2/CN on air-fuel ratio was quantified over a wide range of conditions extending from pure air to pure fuel. It was found that H/O and C2/CN intensity ratios depend monotonically on the mole fraction of methane in the ranges of 0.0–0.8 and 0.3–1.0 respectively. The presence of a flame within the laser beam led to significant measurement deterioration relative to the corresponding non-reacting flows. This could lead to increased measurement uncertainty, and was therefore corrected by increasing the laser pulse energy and applying a proposed data processing method; the proposed correction method was able to reduce the equivalence ratio measurement uncertainty to be within 10% for mixtures with methane mole fractions lower than 50% and within 15% for mixtures with higher methane mole fraction. The established correlations between the intensity ratios of H/O and C2/CN with local air-fuel ratio allowed measurement of the spatial gradient of air-fuel ratio across opposed jet diffusion flames. The LIBS measurements of air-fuel ratio in non-premixed flames were finally compared successfully with CHEMKIN simulations, which demonstrates the ability of LIBS to accurately measure the spatial gradient of the air-fuel ratio.

Lattice Boltzmann method for miscible gases: A forcing-term approach.

A lattice Boltzmann method for miscible gases is presented. In this model, the standard lattice Boltzmann method is employed for each species composing the mixture. Diffusion interaction among species is taken into account by means of a force derived from kinetic theory of gases. Transport coefficients expressions are recovered from the kinetic theory. Species with dissimilar molar masses are simulated by also introducing a force. Finally, mixing dynamics is recovered as shown in different applications: an equimolar counterdiffusion case, Loschmidt's tube experiment, and an opposed jets flow simulation. Since collision is not altered, the present method can easily be introduced in any other lattice Boltzmann algorithms.

Experimental and numerical study of laminar flame extinction for syngas and syngas-methane blends

ABSTRACTExtinction strain rates of syngas and syngas–methane blends at atmospheric pressure were investigated in laminar counter-flow diffusion flames as a function of nitrogen dilution through both experiment and two-dimensional axisymmetric full-domain numerical simulations. Three representative compositions of syngas were examined along with two syngas–methane blends for a range of fuel mole fractions. An opposed-jet burner configuration with straight-tube fuel and air nozzles was used in the experiments and an advanced, computational, solution algorithm was used to obtain the corresponding simulation results. Very good agreement was found between predicted and experimentally observed flame structure as well as global strain rate extinction limits for all syngas fuel and syngas–methane blends over the full range of flow rates and fuel mole fractions considered. Additionally, the local strain rate near extinction was shown to correlate with the global strain rate and an explanation of the differences between these two values was provided by the predicted changes in the nozzle exit plane velocity profiles as a function of flow rate. In particular, non-zero gradients in the axial center-line velocity were predicted at the nozzle exit plane for higher flow rates due to the pressure field created by the opposed-jet flows. The syngas extinction limits as a function of fuel composition were examined. While the amount of hydrogen was found to have a dominant effect on the extinction strain limits, methane and carbon monoxide were also found to induce early extinction and increase reactivity, respectively, if present in sufficiently large quantities.

Drag and heat reduction efficiency research on opposing jet in supersonic flows

Opposing jet has been proven as a feasible strategy to achieve drag and heat reduction in hypersonic vehicles. In actual flight, the reduction efficiency of the drag and heat flux of opposing jet should be maximally enhanced to reduce the size of the coolant supply system, which occupies considerable space and weight. The current study investigates the drag reduction efficiency (Ef) and heat flux reduction efficiency (Eh) of unit mass opposing jet. An opposing jet flow around a blunt body is solved by using a Navier–Stokes equation with the SST k–ω turbulence model. Flow fields, drag force, and heat flux have been obtained, and typical results have been validated with experiments performed in the literature. Detailed results show that a high jet temperature is beneficial for strengthening Ef but exerts minimal influence on drag reduction. Eh is influenced by jet temperature and pressure. A high jet temperature weakens heat flux reduction in opposing jet but a reasonable jet temperature can promote the Eh though it is higher than normal temperature. The reduction efficiency of drag and heat flux can be improved with optimized jet temperature and pressure in the preconditioning requirement of drag and heat flux reduction, which benefits from saving the occupied space, reducing system weight, and promoting vehicle loading.

Effects of confinement, geometry, inlet velocity profile, and Reynolds number on the asymmetry of opposed-jet flows

The opposed-jet counterflow configuration is widely used to measure fundamental flame properties that are essential targets for validating chemical kinetic models. The main and key assumption of the counterflow configuration in laminar flame experiments is that the flow field is steady and quasi-one-dimensional. In this study, experiments and numerical simulations were carried out to investigate the behavior and controlling parameters of counterflowing isothermal air jets for various nozzle designs, Reynolds numbers, and surrounding geometries. The flow field in the jets’ impingement region was analyzed in search of instabilities, asymmetries, and two-dimensional effects that can introduce errors when the data are compared with results of quasi-one-dimensional simulations. The modeling involved transient axisymmetric numerical simulations along with bifurcation analysis, which revealed that when the flow field is confined between walls, local bifurcation occurs, which in turn results in asymmetry, deviation from the one-dimensional assumption, and sensitivity of the flow field structure to boundary conditions and surrounding geometry. Particle image velocimetry was utilized and results revealed that for jets of equal momenta at low Reynolds numbers of the order of 300, the flow field is asymmetric with respect to the middle plane between the nozzles even in the absence of confining walls. The asymmetry was traced to the asymmetric nozzle exit velocity profiles caused by unavoidable imperfections in the nozzle assembly. The asymmetry was not detectable at high Reynolds numbers of the order of 1000 due to the reduced sensitivity of the flow field to boundary conditions. The cases investigated computationally covered a wide range of Reynolds numbers to identify designs that are minimally affected by errors in the experimental procedures or manufacturing imperfections, and the simulations results were used to identify conditions that best conform to the assumptions of quasi-one-dimensional modeling.

A particle manipulation method and its experimental study based on opposed jets.

A particle manipulation method was presented in this paper based on opposed jets. In such a method, particles were trapped near the stagnation point of the flow field and moved by controlling the position of the stagnation point. The hold direction of the flow to the particle was changed by changing the orientation of the opposed-jet flow field where a particle is trapped. Subsequently, the directional and quantitative movement of the particle in any direction was achieved. Taking micron particles as examples, we analyzed the control mechanism of particles based on opposed jets and evaluated the influence of jet velocity, inner diameter, distance of end face, radial error, and position of capillaries on the particle control performance by simulations. The feasibility of the proposed method was proved by a great number of experiments, and the results demonstrated that particles with the arbitrary size and shape can be trapped and moved directionally and quantitatively by constructing an opposed-jet flow field. The trapping and position control of particles can be manipulated without any contact with proper flow field parameters.

Penetration, accumulation, and swing characteristics of particle cloud in a turbulent axisymmetric opposed-jet flow

The behavior of particle cloud in a high Reynolds number (13500) turbulent opposed-jet flow with a moderate nozzle separation (12 times the nozzle diameter) is investigated by a two-phase large eddy simulation. Euler/Lagrangian approaches are applied to simulate gas and particle phases, respectively. Two-way coupling is considered, and a deterministic hard-sphere collision model is used to deal with the interparticle collision. Three particle Stokes numbers (8, 37, and 180) and three particle volume fractions (2 × 10−5, 1.5 × 10−4, and 4.8 × 10−3) are tested. Particle inertia and interparticle collisions are found to exert a significant effect on particle distribution and velocity characteristics, a strong interaction is observed between particle cloud and gas impingement plane. Particle inertia strengthens the penetration of particles in the opposite stream, thus widening the particle aggregation region and decreasing the peak value of particle concentration. The mixing of rightward- and leftward-moving particles in the particle penetration distance noticeably decreases the particle axial mean velocity and increases the particle axial fluctuation velocity. Furthermore, interparticle collisions suppress the reciprocating penetration of particles in the opposed jets and force the particles to accumulate near the impingement plane. Meanwhile, interparticle collisions increase the particle radial mean and radial fluctuation velocities by energy transfer from the axial direction to the radial direction. The unstable gas impingement plane can drive the swing of particle aggregation region, especially for the small inertia particles and massive interparticle collisions. By contrast, the particle cloud can enhance the stability of gas impingement plane, which significantly reduces the gas velocity fluctuation in the impinging region.

Bifurcation analysis of laminar isothermal planar opposed-jet flow

For a fixed geometric configuration, hydrodynamic instabilities and bifurcation processes of laminar isothermal planar opposed-jet flows with symmetric and slightly asymmetric inlet boundary conditions are investigated numerically using a high-resolution approach based on spectral element method. In current configuration, when inlet boundary conditions are symmetric, in the range of the Reynolds number considered (Re ≤ 200), multiple new symmetry-breaking bifurcations are observed and four new flow patterns are identified. Their hydrodynamic characteristics are analyzed, in particular their symmetries. In addition, the case that inlet boundary conditions are slightly asymmetric is investigated. It is found that bifurcation processes are extremely sensitive to this small symmetry-breaking imperfection and much different from those in the symmetric case. Furthermore, model equations are constructed by symmetry consideration to explain the numerical results based on hydrodynamic equations.

A Simple Approach for Specifying Velocity Inflow Boundary Conditions in Simulations of Turbulent Opposed-Jet Flows

A new methodology is developed to specify inflow boundary conditions for the velocity field at the nozzle exit planes in turbulent counterflow simulations. The turbulent counterflow configuration consists of two coaxial opposed nozzles which emit highly-turbulent streams of varying species compositions depending on the mode considered. The specification of velocity inflow boundary conditions at the nozzle exits in the counterflow configuration is non-trivial because of the unique turbulence field generated by the turbulence generating plates (TGPs) upstream of the nozzle exits. In the method presented here, a single large-eddy simulation (LES) is performed in a large domain that spans the region between the TGPs of the nozzles, and the time series of the velocity fields at the nozzle exit planes are recorded. To provide inflow boundary conditions at the nozzle exit planes for simulations under other conditions (e.g., different stream compositions, bulk velocity, TGP location), transformations are performed on the recorded time series: the mean and r.m.s. (root-mean-square) quantities of velocity, as well as the longitudinal integral length scale on the centerline, at the nozzle exits in simulations are matched to those observed in experiments, thereby matching the turbulent Reynolds number R e t . The method is assessed by implementing it in coupled large-eddy simulation/probability density function (LES/PDF) simulations on a small cylindrical domain between the nozzle exit planes for three different modes of the counterflow configuration: N 2 vs. N 2; N 2 vs. hot combustion products; and C H 4/N 2 vs. O 2. The inflow method is found to be successful as the first and second moments of velocity from the LES/PDF simulations agree well with the experimental data on the centerline for all three modes. This simple yet effective inflow strategy can be applied to eliminate the computational cost required to simulate the flow field upstream of the nozzle exits. It is also emphasized that, in addition to the predicted time series data, the availability of experimental data close to the nozzle exit planes plays a key role in the success of this method.

Eulerian and Lagrangian stagnation plane behavior of moderate Reynolds number round opposed-jets flow

Turbulent opposed jet (TOJ) flow is characterized by an unsteady stagnation plane formed by two counter-flowing jets. The complicated behavior of the stagnation plane significantly affects the turbulence characteristics in the impinging zone. In this study, direct numerical simulations (DNS) were performed to study the dynamic behavior of the stagnation plane in a three-dimensional round opposed-jet flow with a moderate Reynolds number (Re=4050). Proper orthogonal decomposition (POD) analysis was used to study the behavior of the Eulerian stagnation plane (ESP) and vortex structures. The results show that the first POD mode dominates the axial movement of the stagnation plane, and it contributes most of the axial velocity fluctuation. The stagnation plane is found to tilt relative to the normal direction of the jet flow by the second to fourth POD modes. The shape of the stagnation plane can be reconstructed to a large extent through the dominant POD modes (namely the first four modes). The Lagrangian property of the stagnation plane was investigated through calculation of the field of finite-time Lyapunov exponents (FTLE), in which the ridges can identify the Lagrangian coherent structures (LCSs) in the turbulent opposed jet flow. Throughout the domain three different types of LCSs were found, corresponding to the axial jet region, impinging zone and radial jet area. A Lagrangian stagnation plane (LSP) is defined as the LCS in the impinging zone, which indicates the boundary of the fluid parcels from the two nozzles. A discrepancy between the ESP and LSP was displayed, and this finding implies a backflow phenomenon, which means the moving impermeable LSP always forces the fluid parcels ahead of the LSP to move toward the ESP. As a result of this analysis, the unstable behavior of the stagnation plane is demonstrated.

A dog is not a mink is not a goat but they all can benefit from behavioral measures that tell us who they are

Turbulence modifications in an axisymmetric opposed jets flow are experimentally investigated by means of a simultaneous two-phase PIV measurement technique. The measurements are conducted at a Reynolds number of 14,500. Glass beads with an averaged diameter of 100 μm (corresponding to a Stokes number of 59.5) are used as dispersed phase. Relatively low mass loading ratios (0.04) and large nozzle separation (12 times nozzle diameter) are tested. It is found that the presence of the particles can distinctly modify the gas-phase characteristics, including both macroscopic turbulence statistics and mesoscopic turbulence structures. In the present dilute gas-particle opposed jets flow, inter-particle collision can be neglected, and particles can penetrate into the opposite stream until to the vicinity of the opposite nozzle. The penetrating and counter-moving particles bring about a lower particle velocity and a larger relative velocity between gas- and particle-phase, which result in a more noticeable modulation of opposed jets. The presence of particles decreases the time-averaged velocity of the opposed jets, increases the turbulence in the axial jet and suppresses the transfer between gas reactants in the impingement region. Moreover, the presence of particles also significantly modifies the axial jet structures, representing the coherent vortex elongating in streamwise direction and shortening in transverse direction, meanwhile noticeably suppresses the turbulence kinetic energy transport from the axial jet to the radial jet.

Numerical study of active mixing over a dynamic flow field in a T-jets mixer—Induction of resonance

The flow field in a 2D T-jets mixer was simulated to study the effect of the pulsation/modulation of the jets flow rate on the dynamics of mixing. Different strategies, frequencies and amplitudes of the opposed jets flow rate modulation were tested. The modulation frequencies were set as multiples of the natural oscillation frequencies of the dynamics flow field. The natural flow frequencies are determined from the unforced flow, i.e., when the jets are not modulated. It is found that out phase modulation of the opposed jets, with frequencies close to the natural frequencies, cause resonance of the flow enhancing the order of the system, which results in a flow field with a well-defined repetitive generation of vortices. Conversely, when the pulsation frequencies were different from the natural frequencies the flow disorder was enhanced, i.e., the vortices evolution throughout the T-jets mixers is less repetitive. The impact of the jets flow rate modulation on the flow field dynamics increases with the modulation amplitude up to the extreme case where it completely drives the dynamics of the system. A design equation for the most energy efficient pulsation of the jets feed streams in opposed jets mixers is proposed.

Experimental investigation on turbulence modification in a dilute gas-particle axisymmetric opposed jets flow

Turbulence modifications in an axisymmetric opposed jets flow are experimentally investigated by means of a simultaneous two-phase PIV measurement technique. The measurements are conducted at a Reynolds number of 14,500. Glass beads with an averaged diameter of 100μm (corresponding to a Stokes number of 59.5) are used as dispersed phase. Relatively low mass loading ratios (0.04) and large nozzle separation (12 times nozzle diameter) are tested. It is found that the presence of the particles can distinctly modify the gas-phase characteristics, including both macroscopic turbulence statistics and mesoscopic turbulence structures. In the present dilute gas-particle opposed jets flow, inter-particle collision can be neglected, and particles can penetrate into the opposite stream until to the vicinity of the opposite nozzle. The penetrating and counter-moving particles bring about a lower particle velocity and a larger relative velocity between gas- and particle-phase, which result in a more noticeable modulation of opposed jets. The presence of particles decreases the time-averaged velocity of the opposed jets, increases the turbulence in the axial jet and suppresses the transfer between gas reactants in the impingement region. Moreover, the presence of particles also significantly modifies the axial jet structures, representing the coherent vortex elongating in streamwise direction and shortening in transverse direction, meanwhile noticeably suppresses the turbulence kinetic energy transport from the axial jet to the radial jet.

Numerical study of particle behavior in laminar axisymmetric opposed-jet flows

The particle behavior in laminar axisymmetric opposed-jet flows is investigated numerically using a high order finite difference method and Lagrangian particle tracking. The deterministic hard-sphere collision model is used to deal with the inter-particle collisions, and the feedback of the particles on the carrier phase is neglected in the present work. In the scope of particle volume fractions and particle Stokes numbers studied, the particle collision rates in the impinging zone can agree with the prediction of cylinder formula (Saffman & Turner, 1956). The results found that inter-particle collisions have significant influence on particle distribution, velocities and residence time. When particle volume fraction at nozzle outlet is smaller than O(10−4), inter-particle collisions make little difference on the particle concentration profile, which is primarily decided by the maximum penetration distance (MPD) of a particular particle. Particle will prefer to concentrate at the location of MPD when the MPD is within the flow domain, otherwise particles will evenly distribute along the axis of two nozzles. When particle volume fraction is higher than O(10−3), the effect of inter-particle collisions become significant, which can impede particles reaching the MPD so that particles will prefer to concentrate at the impinging plane. Inter-particle collisions can restrain the penetration process of particles, and shorten the particle residence time at the same time. Due to massive collisions, particle mean velocity profile has the similar shape with the profile of gas velocity in the impinging zone, and slip velocity between particles and gas will be decreased.

A multi-component lattice Boltzmann method in consistent with Stefan–Maxwell equations: Derivation, validation and application in porous medium

In this paper, a multi-component lattice Boltzmann method (LBM) with different lattice speeds and multiple linearized collision terms is improved. The molecular weights, viscosities of each component and the diffusivities can be tuned separately. An exact calculation for relaxation times of the cross-collision terms is derived, which makes the model to be consistent with the Stefan–Maxwell equations. The derivation also demonstrates that the tuning-molecular-weight strategy is only available for the ternary diffusion. The expressions of the fluxes and the boundary conditions are proposed. A simulation of the one-dimensional diffusion demonstrates that the second-order interpolation is better than the first-order one to deal with the different lattice speed. The simulation of Couette flow shows that the model behaves like a single fluid with high Schmidt number, and the components tend to flow independently with small Schmidt number. The model is validated by the simulation of one-dimensional diffusion and two-dimensional opposed jet flow, where the LBM results coincide well with the results of the Stefan–Maxwell equations. Finally, the diffusion in a porous media is simulated as an example of the application of the model. This lattice Boltzmann model is suitable for simulating multi-component convection–diffusion problems with complex boundary conditions.

Flame Synthesis of Carbon Nanotubes and Nano-Onions in a Rotating Opposed-Jets Flow

The influence of flow rotation on the synthesis of carbon nano-structures using rotating opposed flow ethylene diffusion flames and a catalytic Ni substrate was investigated. In the experiments, the flame parameter was kept constant with fuel and oxidizer compositions of 20%C2H2+80%N2 and 40%O2+60%N2 in the upper and lower burners, respectively, whereas the strain rate was varied by adjusting the rotation speed. Stain rate affects carbon nano-structures synthesis either through the residence time of the flow or carbon sources available for the growth of carbon nanotubes (CNTs) and onions. A diffusion flame at low strain rate is stronger than a weak flame at high strain rate and produces more carbon sources because of the longer residence time of the flow. At a higher strain rate, curved and entangled tubular multi-walled CNTs were harvested, however, at a lower strain rate carbon nano-onions (CNOs) were synthesized. It is verified that flow rotation associated with residence time plays an important role in the synthesis of carbon nanostructures.

Flow and mixing characteristics of pulsed confined opposed jets in turbulent flow regime

A numerical study is performed on a two-dimensional confined opposed-jet configuration to gain basic understanding of the flow and mixing characteristics of pulsed turbulent opposed-jet streams. The sinusoidal pulsating flows with different temperature are imposed at opposed-jet inlets, which are mixed with each other in a confined flow channel. The current mathematical model taking the effect of temperature-dependent thermo-physical properties of fluid into account can present a good prediction for opposed-jet streams compared with experimental data. The numerical results indicate that introduction of temperature difference between opposed jet flows can lead to an asymmetric flow field immediately after jet impact, and the sinusoidal flow pulsations can effectively enhance mixing rate of opposed jets. Parameter studies are conducted for optimization of pulsed opposed jets. The effect of Reynolds number and flow pulsation as well as the configuration geometry on the mixing performance are discussed in detail. Examination of the flow and thermal field shows that the mixing rate is highly dependent on the vortex-induced mixing and residence time of jet fluid in the exit channel.

Scalar dissipation rate statistics in turbulent flows using Planar Laser Induced Fluorescence measurements

A shearless mixing layer with isotropic decaying turbulence has been examined with a turbulent axisymmetric opposed-jet flow and a plume in pipe flow. The study has been carried out experimentally with Planar Laser Induced Fluorescence of acetone (used as a tracer to measure the passive scalar ξ) to study the conditional statistics of the scalar dissipation χ. For all flows examined, it has been found that: (i) the PDFs of the conditional χ show a consistent small deviation from log-normality at high values of ξ; (ii) the ratio F=〈χ′2|η〉1/2/〈χ|η〉 is independent of η the sample space variable for ξ), within statistical error; (iii) F increases from zero (at the injector in the plume experiment) to a value between 1 and 1.4 downstream and at later times; (iv) the rise of F to this value occurs over a few Kolmogorov timescales; and (v) the long-time value of F is close between the two experiments and seems to increase with the turbulent Reynolds number. The development of conditional scalar dissipation fluctuations is being discussed by comparing the “early” and “late” mixing regions in the plume and the counterflow. The agreement between the PLIF measurements, which can only resolve two-dimensional fields, suggests that the present conclusions are not limited by resolution or the lack of measuring the third dimension of the scalar gradient. The results are useful for turbulent reacting flow models such as advanced flamelet, CMC, and transported PDF closures.