Turbulent Counterflow Research Articles

NO and CO Emission Characteristics of Laminar and Turbulent Counterflow Premixed Hydrogen-Rich Syngas/Air Flames

Burning hydrogen-rich syngas fuels derived from various sources in combustion equipment is an effective pathway to enhance energy security and of significant practical implications. Emissions from the combustion of hydrogen-rich fuels have been a main concern in both academia and industry. In this study, the NO and CO emission characteristics of both laminar and turbulent counterflow premixed hydrogen-rich syngas/air flames were experimentally and numerically studied. The results showed that for both laminar and turbulent counterflow premixed flames, the peak NO mole fraction increased as the equivalence ratio increased from 0.6 to 1.0 and decreased as the strain rate increased. Compared with the laminar flames at the same bulk flow velocity, turbulent flames demonstrated a lower peak NO mole fraction but broader NO formation region. Using the analogy theorem, a one-dimensional turbulent counterflow flame model was established, and the numerical results indicated that the small-scale turbulence-induced heat and mass transport enhancements significantly affected NO emission. Considering NO formation at the same level of fuel consumption, the NO formation of the turbulent flame was significantly lower than that of the laminar flame at the same level of fuel consumption, implying that the turbulence-induced heat and mass transfer enhancement favored NOx suppression.

Theory of anisotropic superfluid 4He counterflow turbulence.

We develop a theory of strong anisotropy of the energy spectra in the thermally driven turbulent counterflow of superfluid 4He. The key ingredients of the theory are the three-dimensional differential closure for the vector of the energy flux and the anisotropy of the mutual friction force. We suggest an approximate analytic solution of the resulting energy-rate equation, which is fully supported by our numerical solution. The two-dimensional energy spectrum is strongly confined in the direction of the counterflow velocity. In agreement with the experiments, the energy spectra in the direction orthogonal to the counterflow exhibit two scaling ranges: a near-classical non-universal cascade dominated range and a universal critical regime at large wavenumbers. The theory predicts the dependence of various details of the spectra and the transition to the universal critical regime on the flow parameters.This article is part of the theme issue ‘Scaling the turbulence edifice (part 2)’.

Effects of evaporation on chemical reactions in counterflow spray flames

The role of evaporation on chemical reactions in counterflow spray flames remains a key issue due to the non-linear inter-phase interactions through mass, momentum, and energy conservation. An extended chemical explosive mode analysis to illustrate the role of evaporation (ECEMA) was previously developed by separating the evaporation source term from non-chemical terms and projecting them onto the chemical explosive mode (CEM). Evaporation was found to promote chemical reactions when the fuel supply effect dominated the evaporative cooling and to inhibit reactions otherwise. In this work, ECEMA is applied to one-dimensional laminar and three-dimensional turbulent counterflow spray flames. For laminar cases, ECEMA is applied to multi-modal spray flame solutions including the distributed, collocated, and cool flame. The analysis for the collocated and cool flame shows similar behavior, namely, that evaporation inhibits chemical reactions due to heat absorption near the fuel injection region and enhances chemistry around the zero-crossing CEM. For the distributed flame, the promotion effect is observed for most of the domain. For the turbulent counterflow flame, three CEM regions are identified, namely, a hybrid region, an inhibition region, and a promotion region. In particular, for the hybrid region near the spray injection side, the contribution of evaporation to chemical reactions changes from inhibition to promotion. For the inhibition region in the middle, significant suppressing of chemical reactions by evaporation is observed. In the promotion region near the oxidizer side, evaporation remains promoting chemical reactions. For the case investigated, the dominant combustion modes are the assisted ignition and local extinction.

Ubiquity of particle–vortex interactions in turbulent counterflow of superfluid helium

Abstract

A regularized deconvolution model for sub-grid dispersion in large eddy simulation of turbulent spray flames

In this study, a sub-grid scale (SGS) dispersion model is developed for simulating turbulent spray flames. This model employs a regularized deconvolution method (RDM) to determine the sub-grid fluctuations from the filtered velocity field. The model is examined in simulations of a turbulent counterflow n-dodecane spray flame, and benchmarked against a simplified Langevin model (SLM) and simulations without dispersion closure (NOM). To identify conditions that characterize the importance of the spray-flame coupling, a regime diagram is constructed from physical arguments and theoretical analysis by considering idealized conditions. Based on the proposed criteria, a range of operating conditions is considered, involving different Stokes- and Reynolds numbers as well as inert and reacting conditions. A priori DNS analyses are performed to compare the probability density functions (PDFs) of slip velocities. Significant discrepancies between models are observed for smaller Stokes numbers. Subsequently, large eddy simulations (LES) are performed and effects of mesh resolution on statistical flow-field quantities are investigated. Qualitative and quantitative differences are observed for coarse-grid simulations with larger droplets, where a double-flame structure is predicted by DNS and LES-RDM, whereas a single flame is predicted by LES-NOM and LES-SLM. For the finer grid, all three LES models predict the double-flame structure. Turbulence modulation by the spray is expected to play a role in creating different flame topologies, and the RDM-based SGS-closure model is shown to provide improved predictions for the dispersed and carrier phases for a wider range of grid resolution.

An a priori study of different tabulation methods for turbulent pulverised coal combustion

In many practical pulverised coal combustion systems, different oxidiser streams exist, e.g. the primary- and secondary-air streams in the power plant boilers, which makes the modelling of these systems challenging. In this work, three tabulation methods for modelling pulverised coal combustion are evaluated through an a priori study. Pulverised coal flames stabilised in a three-dimensional turbulent counterflow, consisting of different oxidiser streams, are simulated with detailed chemistry first. Then, the thermo-chemical quantities calculated with different tabulation methods are compared to those from detailed chemistry solutions. The comparison shows that the conventional two-stream flamelet model with a fixed oxidiser temperature cannot predict the flame temperature correctly. The conventional two-stream flamelet model is then modified to set the oxidiser temperature equal to the fuel temperature, both of which are varied in the flamelets. By this means, the variations of oxidiser temperature can be considered. It is found that this modified tabulation method performs very well on prediction of the flame temperature. The third tabulation method is an extended three-stream flamelet model that was initially proposed for gaseous combustion. The results show that the reference gaseous temperature profile can be overall reproduced by the extended three-stream flamelet model. Interestingly, it is found that the predictions of major species mass fractions are not sensitive to the oxidiser temperature boundary conditions for the flamelet equations in the a priori analyses.

Effects of turbulence and strain rate on hydrogen-enriched high Karlovitz number lean premixed methane flames

Effects of hydrogen addition at constant heat release, with the influence of turbulence and strain for lean methane-air turbulent counterflow premixed flames in the distributed regime (high Karlovitz number), are investigated using large eddy simulation (LES). Fuel mixtures of up to 60% of hydrogen in volume are considered, and sub-filter turbulence/chemistry interaction is modelled using the finite rate chemistry partially stirred reactor (PaSR) approach combined with detailed kinetics of a skeletal mechanism based on GRI-MECH3.0. Local flame structure and species formation are examined at different turbulence conditions. With constant heat release, hydrogen addition shows its benefits as species emission decreases with a more stable flame. By increasing integral length scale of turbulent structures, global strain rate of the flame is more significant, leading to an increase in flame thickness and reduction in the formation of intermediate species, pollutant NO and heat release. At higher strain rate, ratio of turbulent to laminar flame speed increases faster with increased hydrogen content of fuel, and the flame structure displays more wrinkling and is shorter as a consequence of faster mixing. It is also found that flame and heat release are less influenced by stretch when hydrogen is present within the fuel mixture, indicating the potential of hydrogen substitution for reaching a defined power output while reducing pollutant emissions.

Mathematical model and calculation of water-cooling efficiency in a film-filled cooling tower

Different approaches to simulation of momentum, mass, and energy transfer in packed beds are considered. The mathematical model of heat and mass transfer in a wetted packed bed for turbulent gas flow and laminar wave counter flow of the fluid film in sprinkler units of a water-cooling tower is presented. The packed bed is represented as the set of equivalent channels with correction to twisting. The idea put forward by P. Kapitsa on representation of waves on the interphase film surface as elements of the surface roughness in interaction with the gas flow is used. The temperature and moisture content profiles are found from the solution of differential equations of heat and mass transfer written for the equivalent channel with the volume heat and mass source. The equations for calculation of the average coefficients of heat emission and mass exchange in regular and irregular beds with different contact elements, as well as the expression for calculation of the average turbulent exchange coefficient are presented. The given formulas determine these coefficients for the known hydraulic resistance of the packed bed element. The results of solution of the system of equations are presented, and the water temperature profiles are shown for different sprinkler units in industrial water-cooling towers. The comparison with experimental data on thermal efficiency of the cooling tower is made; this allows one to determine the temperature of the cooled water at the output. The technical solutions on increasing the cooling tower performance by equalization of the air velocity profile at the input and creation of an additional phase contact region using irregular elements “Inzhekhim” are considered.

Characterization of extinction/reignition events in turbulent premixed counterflow flames using strain-rate analysis

We investigate extinction/reignition events in two contrasting turbulent premixed flames of the Yale turbulent counterflow flame (TCF) burner, that are both qualitatively and quantitatively different. One of the two chosen flames is a high-burning (HB) flame with a low probability of local extinction while the other flame is a low-burning (LB) flame with a high probability of local extinction. In recent work, we successfully studied the turbulent premixed flames of the Yale TCF burner using the large-eddy simulation/probability density function (LES/PDF) methods. In this present study, the main motivation is to investigate how the compositional structure of the two turbulent flames are related to that of laminar flames. To this end, steady, one-dimensional, strained, opposed-jet laminar flame calculations are performed to investigate the effect of strain rates K on the laminar counterparts of HB and LB, and to evaluate the extinction strain rates Sext for the two flames. Subsequently, a normalized distance Z is defined in terms of the mixture fraction ξ, which is calculated based on the mass fraction of N2. The scatter plots from the particle data of (a) CH2O mass fraction YCH2O*vs. progress variable p* and (b) temperature T*vs. the normalized distance Z* are quite different for the two flames with more samples close to the extinguished laminar profile for the LB flame than for the HB flame. The cell-mean profiles of T vs. Z resemble the laminar profiles at different strain rates even though the LES/PDF are non-trivially 3D and unsteady. These cell-mean profiles are used to evaluate the instantaneous equivalent steady strain rate (ESSR) S for the two flames. The cumulative distribution function (CDF) of S, conditional on S < Sext (i.e., burning samples), is somewhat similar, with the distributions being broad without a peak close to the bulk strain rate. However, comparatively, more samples of the LB flame have the ESSR values above the extinction strain rate, i.e., S > Sext. The scatter plots of the ESSR S vs. the fresh product layer thickness Δf quantify the thinning of the product layer as the strain rate S increases. The scatter for the HB flame follow the corresponding laminar profile quite closely, whereas, the thicknesses observed for the LB flame are higher compared to its laminar prediction.

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.

An investigation of turbulent premixed counterflow flames using large-eddy simulations and probability density function methods

We report results from a coupled large-eddy simulation (LES)/probability density function (PDF) computational study of turbulent premixed flames in the Yale turbulent counterflow flame (TCF) burner. The Yale TCF burner in the premixed mode consists of two coaxial opposed nozzles: one emitting cold, fresh premixed reactants, CH4/O2/N2, and the other hot stoichiometric combustion products. This results in a turbulent premixed flame close to the mean stagnation plane. Four critical parameters are identified in the experiments, namely, the bulk strain rate, the turbulent Reynolds number, the equivalence ratio of the reactants mixture and the temperature of the hot combustion products. These are varied independently. In the conditional statistics approach, the instantaneous centerline profiles of OH mass fraction and its gradient are used to identify (i) the interface between the two counterflowing streams referred to as the gas mixing layer interface (GMLI), and (ii) the turbulent flame front using a binary reaction progress variable, c. The conditional mean of the progress variable conditioned on distance Δ from the GMLI, < c | Δ > , and the PDF of the GMLI-to-flame-front distance, Δf, are used to quantify the effects of the critical parameters on the interactions of the turbulent premixed flame with the counterflowing hot combustion products, both in the experiments and in the simulations. The LES/PDF simulations are performed in a cylindrical domain between the two nozzle exit planes. A base case simulation involving reference values of the critical parameters is simulated, and the centerline profiles (both unconditional and conditional) of the velocity statistics and the mean progress variable are found to match well with the experimental data. Additionally, the LES/PDF simulations predict the experimentally-observed trends of the effects of the critical parameters on the turbulent premixed flame very well. More importantly, the probability of localized extinction at the GMLI (i.e., 1−〈c|Δ=0〉) and the PDF of the separation distance between the GMLI and flame front, Δf, compare well with the experiments for all the flow conditions explored in the parametric study. Three independent key quantities are computed from the LES/PDF simulations of the base case to examine if the simulations can be considered to be in the direct numerical simulation (DNS) limit. They are (i) the ratio of the resolved turbulent diffusivity to the resolved molecular diffusivity, D˜T/D˜, (ii) the normalized mixing rate, ΩRτL, and (iii) the normalized grid spacing, h/δL. The ratio of D˜T/D˜ is sufficiently small ( ≤ 0.02) and the value of ΩRτL is sufficiently large ( ≈ 22) to be considered to be in the DNS limit. However, the ratio of h/δL is too large ( ≈ 0.6) and hence the LES/PDF cannot be considered to be in the DNS limit by this criterion. In spite of the poor spatial resolution, the particle-mesh method yields a flame speed close to the laminar flame speed and this likely explains the success of the present LES/PDF calculations of the TCF premixed flame over the full range of critical parameters.

One-dimensional turbulence modeling of a turbulent counterflow flame with comparison to DNS

The one-dimensional turbulence (ODT) model is applied to a reactant-to-product counterflow configuration and results are compared with DNS data. The model employed herein solves conservation equations for momentum, energy, and species on a one dimensional (1D) domain corresponding to the line spanning the domain between nozzle orifice centers. The effects of turbulent mixing are modeled via a stochastic process, while the Kolmogorov and reactive length and time scales are explicitly resolved and a detailed chemical kinetic mechanism is used. Comparisons between model and DNS results for spatial mean and root-mean-square (RMS) velocity, temperature, and major and minor species profiles are shown. The ODT approach shows qualitatively and quantitatively reasonable agreement with the DNS data. Scatter plots and statistics conditioned on temperature are also compared for heat release rate and all species. ODT is able to capture the range of results depicted by DNS. However, conditional statistics show signs of underignition.

Buoyancy-induced turbulent mixing in a narrow tilted tank

We describe a series of experiments in which a constant buoyancy flux $B_{s}$ of salty dyed water of density ${\it\rho}_{s}$ is introduced at the top of a long narrow tank of square cross-section tilted at an angle ${\it\theta}$ to the vertical. The tank is initially filled with fresh clear water of density ${\it\rho}_{0}&lt;{\it\rho}_{s}$, and we investigate the resulting buoyancy-driven turbulent mixing at various tilt angles ${\it\theta}$. Using a light-attenuation image analysis method, we determine the evolution of the reduced gravity $g^{\prime }=g({\it\rho}-{\it\rho}_{0})/{\it\rho}_{0}$ of the mixed fluid in time and space as it propagates towards the bottom of the tank. For all tilt angles tested (${\it\theta}=0^{\circ }$ to ${\it\theta}=45^{\circ }$), we focus exclusively on high-Reynolds-number experiments, where the flow remains turbulent both along the length and across the width of the tank. We find that when ${\it\theta}&gt;0^{\circ }$, the cross-tank component of gravity acts to segregate the dense fluid from the relatively lighter fluid, and a statically stable gradient of $g^{\prime }$ across the width of the tank occurs more frequently than a statically unstable gradient, i.e. $(\partial g^{\prime }/\partial x)&lt;0$ occurs more frequently than $(\partial g^{\prime }/\partial x)&gt;0$. This is in contrast to the case when ${\it\theta}=0^{\circ }$, where instantaneous cross-tank gradients of reduced gravity may be positive or negative, but are equal to zero in an ensemble average. We observe that when ${\it\theta}&gt;0^{\circ }$, the cross-tank gradient of reduced gravity induces a turbulent counterflow where dense fluid flows down the upward-facing surface of the tank and lighter fluid flows in the opposing direction above. We model the evolution of the cross-tank averaged, ensemble averaged reduced gravity $\langle \overline{g^{\prime }}\rangle _{e}$ as a diffusive process using Prandtl’s mixing length theory, building on the model of van Sommeren et al. (J. Fluid Mech., vol. 701, 2012, pp. 278–303) who considered purely vertical tanks. We model the fluctuations (from the cross-tank averaged quantity) of reduced gravity $\langle {\hat{g}}^{\prime }\rangle _{e}$ and counterflow velocity $\langle {\hat{w}}\rangle _{e}$ by characterising the mixing across the width of the tank with a cross-tank turbulent diffusivity ${\it\kappa}_{T,x}$, which we assume is constant in the cross-tank coordinate $x$. We show that the counterflow that exists when ${\it\theta}&gt;0^{\circ }$ acts directly to enhance the effective along-tank turbulent diffusivity ${\it\kappa}_{T,z}$, and from experiments, we find that the mixing length increases approximately linearly with ${\it\theta}$, and that both ${\it\kappa}_{T,x}$ and ${\it\kappa}_{T,z}$ are proportional to $(\partial \langle \overline{g^{\prime }}\rangle _{e}/\partial z)^{1/2}$.

Modeling of the Evaporative Cooling of Running-Down Liquid Films in the Slit Channel of the Spraying Device of a Cooling Tower

This paper presents the results of computational modeling of the nonstationary evaporative cooling of a liquid film running down a vertical surface cooled by a turbulent vapor-air counterflow. The heat and mass transfer problem has been formulated in conjugate form. The calculation data on the total heat flow density at the interface for various instants of time are given.

Analysis of segregation and bifurcation in turbulent spray flames: A 3D counterflow configuration

The understanding of spray combustion processes is of primary importance, as it is encountered in a wide range of industrial applications. In the present work, mesoscale-resolved simulations of a 3D turbulent counterflow spray configuration are conducted. Primary focus is on examining the effect of the coupling between turbulence, evaporation, mixing, and combustion. By considering different initial droplet diameters and through comparisons with turbulent and laminar configurations at the same operating condition, it is shown that preferential concentration can lead to conditions of locally high mixture-fraction composition. In addition, local variability in strain rate and droplet diameter introduces a bifurcation of the spray flame. This bifurcation consists of spray flame structures exhibiting single-reaction or double-reaction structures. It is shown that this bimodal behavior is linked to the existence of a hysteresis in the laminar spray flame structure for droplet diameter variations, as well as the occurrence of a bifurcation for strain rate variations. These results have direct implications for flamelet-based tabulation methods, since identifying the appropriate flamelet structure in turbulent spray flames would require informations about boundary conditions and the flamelet history.

Turbulent heat transfer in a counterflow moving porous bed using a two-energy equation model

This work investigates the influence of physical properties on heat transfer in a turbulent counterflow in a moving bed using the High and Low Reynolds number turbulence models, in which the working fluid flows in opposite direction to that of the steady movement of the permeable rigid medium. Transport equations for flow and heat transfer in a moving bed equipment are applied and discretized using the control-volume method. The system of algebraic equations obtained is relaxed via the SIMPLE algorithm. The effects of Reynolds number, solid-to-fluid velocity ratio, permeability, porosity, ratio of solid-to-fluid thermal capacity and ratio of solid-to-fluid thermal conductivity on heat transport are investigated. Results indicate that motion of solid material, contrary to the direction of the fluid, enhances heat transfer between phases. The same effect was observed for smaller Darcy number and porosity, as well as for higher solid-to-fluid thermal capacity and thermal conductivity ratios. When the intrinsic fluid velocity increases there is a greater conversion of mechanical kinetic energy into turbulence, increasing the final levels of the turbulent kinetic energy for both High and Low Reynolds number models.

Structure of a quantum vortex tangle in4He counterflow turbulence

The paper presents a comprehensive characterization of well-developed vortex tangles in a turbulent counterflow in quantum fluids (with a laminar normal fluid component). We perform and analyze extensive numerical simulations using the vortex filament method, solving the full Biot-Savart equations for the vortex dynamics in a wide range of temperatures and counterflow velocities. We start with the analysis of the macroscopic characteristics of the quantum vortex tangle such as vortex line density, its mean anisotropic and curvature parameters, the mean friction force between normal and superfluid components, the drift velocity of the vortex tangle, etc. Next we proceed to the main goal of the paper and move from the traditional macroscopic approach in terms of mean characteristics of the vortex tangle to the microscopic statistical and kinetic levels of description of quantum turbulence. These include objects that are much less studied or even totally neglected such as the vortex reconnection rates, the correlations and probability distribution functions (PDFs) of the vortex loop lengths, of the line curvature, of the mean curvatures of individual loops, the cross-correlation function between the loop length and its mean curvature, and the autocorrelation function of the vortex-line orientations. This detailed statistical information is required for a deeper understanding of quantum turbulence and for the development of its advanced theoretical description. In addition, we identify which of the studied properties are strongly affected by the choice of the reconnection criteria that are traditionally used in the vortex filament method and which of them are practically insensitive to the reconnection procedure. We conclude that the vortex filament method is sufficiently robust and well-suited for the description of the steady-state vortex tangle in the quantum counterflow.

Effect of quenching of the oxidation layer in highly turbulent counterflow premixed flames

Three lean-to-stoichiometric premixed flames were studied in the opposed-jet configuration by counterflowing fresh reactants and fully burnt products of combustion at 1850K. The study was performed experimentally under turbulent conditions and computationally under laminar ones. The flame mixtures were selected to have the same unstrained laminar flame speed. The turbulent flames were studied experimentally using simultaneous planar imaging of CO and OH laser-induced fluorescence, which, in combination, yielded a quantity proportional to the forward reaction rate for CO+OH→CO2+H. Since the turbulence characteristics of the feed streams were identical for all flames, the turbulent burning regime was also the same, with a turbulent Reynolds number of 1050 and a Karlovitz number of approximately 5. The oxidation layer of the stoichiometric flame was extinguished, whereas the lean flames exhibited substantial evidence of CO conversion as indicated by the CO+OH reaction rate imaging. To aid the interpretation of the experiments, we numerically investigated the extinction of strained laminar premixed flames with compositions identical to those of the experiments. The calculations corroborated the experimental results, indicating that the stoichiometric flame was the least robust and extinguished at the lowest strain rate. Furthermore, extinction occurred when the flames were very close to the gas stagnation plane and the oxidation layer extended beyond it, towards the burnt product side. The quenching of the oxidation layer is suggested as a possible reason for either local or overall extinction of highly strained premixed flames.

Highly turbulent counterflow flames: A laboratory scale benchmark for practical systems

We propose a highly turbulent counterflow flame as a very useful benchmark of complexity intermediate between laminar flames and practical systems. By operating in a turbulent Reynolds number regime of relevance to practical systems such as gas turbines and internal combustion engines, it retains the interaction of turbulence and chemistry of such environments, but offers several advantages including: (a) the achievement of high Reynolds numbers without pilot flames, which is particularly advantageous from a modeling standpoint; (b) control of the transition from stable flames to local extinction/reignition conditions; (c) compactness of the domain by comparison with jet flames, with obvious advantages from both a diagnostic and, especially, a computational viewpoint; and (d) the reduction or, altogether, elimination of soot formation, thanks to the high strain rates and low residence times of such a system, and the establishment of conditions of large stoichiometric mixture fraction, as required for robust flame stabilization. We demonstrate the phenomenology of such highly strained turbulent flames under conditions spanning unpremixed, partially premixed and premixed regimes. The system lends itself to the validation of DNS and other computational models. It is also well-suited for the examination of practical fuel blends – a need that is becoming more and more pressing in view of the anticipated diversification of the future fossil fuel supply.

HYDROXYL TIME-SCALE CORRELATIONS IN TURBULENT COUNTERFLOW NONPREMIXED FLAMES

Time-series measurements of OH concentration have been obtained in turbulent counterflow nonpremixed H2/CH4/air flames to study the influence of turbulence parameters on the dynamics of such flames by independently varying the Reynolds number (Re) and global strain rate (SR). The autocorrelation functions from the measured time series are found to be self-similar and can be fully characterized by their integral time scales. The integral time scale displays a power law dependence on both Re and SR. At lower strain rates, fluctuations in the OH layer are nearly independent of Reynolds number. An increase in SR changes the spatial displacement of OH, paving the way for faster fluctuations of the OH layer at higher Reynolds numbers. Both the bulk strain rate and Reynolds number scale linearly with the jet exit velocity (U). Furthermore, the combined effect of SR and Re on the integral time scale behaves approximately as U−1.35 in these counterflow nonpremixed flames. Hence, the Re−1.4 dependency previously observed for the OH integral time scale in jet diffusion flames is likely due to a combined effect of jet exit velocity on both strain rate and Reynolds number. Finally, stochastic time-series simulations are used to examine the relationship between time scales for OH and mixture fraction in turbulent nonpremixed flames. We thus demonstrate that integral time scale for mixture fraction is considerably greater for opposed-jet as compared to free-jet flames.