Global Stretch Rate Research Articles

Uniaxial and biaxial experimental investigation of glassy polymers

The alignment of polymer chains within polycarbonate (PC) significantly influences the mechanical properties of the deformed specimen. This influence can be affected by both the global stretch rate and the type of loading. The objectives of the current work are to investigate uniaxial film, bar specimens, and biaxial thick-walled specimens using 3D-optical measurements.Our approach involves developing different methods to determine the local inhomogeneous strain fields for both thin and thick-walled uniaxial specimens. In the pure tension case and under the assumption of uniaxial plane stress, we utilize the strains to calculate the local volume ratio and, consequently, approximate the local true stress at different axial positions of the specimen. Additionally, we investigate the effect of variations in global stretch rates on local stresses. Furthermore, we explore induced anisotropy for different global stretching ratios.Another objective is to investigate biaxial specimens under sequential and simultaneous biaxial stretching conditions, examining the impact of both stretching types on the mechanical and structural properties of the material.

Improvement of Turbulent Burning Velocity Measurements by Schlieren Technique, for High Pressure Isooctane-Air Premixed Flames

ABSTRACTAs in most industrial cases, the expansion of premixed flames in spark-ignition (SI) engines is strongly affected by turbulence flow fields. However, as the flame radius and curvature are non-negligible during combustion development, the global stretch effect can drastically change the turbulent flame propagation speed. In this paper, spherical turbulent premixed flames were studied inside a constant-volume vessel for a wide range of air-isooctane mixtures, initial turbulent intensities and pressures by using simultaneously Mie scattering tomography and 2-View Schlieren imaging. The comparison between different radii enabled non-convected flames with a global spherical shape to be selected and the validity of a correction factor, first introduced by Bradley et al., was assessed. A set of turbulent flame speeds was obtained and the evolution as a function of the global stretch rate was analyzed. Various turbulent flame speed definitions were also evaluated on two different datasets.

Non-premixed flame dynamics excited by flow fluctuations generated from Dielectric-Barrier-Discharge plasma

Plasma-induced flow perturbation and its subsequent dynamic effects on a flame are decoupled, both experimentally and theoretically, from the plasma assisted combustion system. A coaxial Dielectric-Barrier-Discharge (DBD) plasma generator is designed, and the discharge is characterized using probes and cameras. Particle tracing method is elaborately demonstrated to map the velocity of flow fluctuation induced by the plasma, with uncertainty estimated to be less than 5%. Then the plasma generator is coupled with the fuel nozzle of the non-premixed counterflow burner. By using multiple diagnostics, several turbulent-like features are observed from the upstream laminar flow (Re ≈ 300) and the downstream non-premixed flat flame, including the distorted velocity profile, fluctuation intensity above 50%, wrinkled flame sheet, and near −5/3 slope of frequency spectra for both fluctuation velocity and CH* chemiluminescence intensity. The aerodynamic effect on the flame is resolved by more than 90% over frequency spectra and then, characterized using flame transfer functions (FTFs). The experimental results show a negative linear correlation between the FTFs’ gain and perturbation frequency on the logarithmic plot, which is then verified by a theoretical model derived from the Z-equation of non-premixed counterflow flame. Further, the model indicates that for small perturbations, the influence of the global stretch rate on the FTFs is linear, while the effect of the imposed amplitude is negligible, and the dimensionless perturbation frequency (St) scaled by the global stretch rate becomes the only variable.

Propagation speed of wrinkled premixed flames within stoichiometric hydrogen-air mixtures under standard temperature and pressure

To explore the influence mechanism of initial turbulence on propagation speed of wrinkled flames, the turbulent combustion behavior of wrinkled stoichiometric hydrogen premixed flames was studied in a spherical fanstirred closed vessel under standard temperature and pressure. The variations on flame structure were first observed; turbulent flames first were distorted and then became cellular, and both first and second critical flame radii of cellularity declined with a increased rate as turbulent intensity rose. Then, the variations of stretch effects were compared to laminar flame; the global stretch rate on turbulent flame at a same flame size was raised while the enhancement extent was obviously enlarged with the increase of initial turbulent intensity and/or the growth of flame size. Finally, the variation regulations of propagation speed induced by varying turbulent intensity were analyzed; the nexus between propagation speed and initial turbulence was discussed with the considerations of cellularity phenomenon and stretch effects.

Propagation characteristics of laminar spherical flames within homogeneous hydrogen-air mixtures

Taking the laminar spherical flames propagate within homogenous hydrogen-air mixture as the studied object, the effects of initial conditions (including equivalence ratio, initial pressure, and initial temperature) on propagation characteristics are systematically investigated. During propagation, global stretch rate monotonously declines towards convergence, it first rises then declines with the increase of equivalence ratio (φ) from 0.5 to 4.0 and the maximal value is attained at φ = 1.8. With the declines of global stretch rate, the propagation speed within lean mixtures first declines and then rises, but it monotonously rises within stoichiometric and rich mixtures. Markstein length is sensitive to equivalence ratio and initial pressure rather than initial temperature. Unstretched laminar burning velocity isn't monotonously changed with the variation of equivalence ratio but it monotonously verifies with the variation of initial thermodynamic condition. Owing to the wane of stretch effects, flame develops towards unstable, the nexus between critical flame radius of cellularity behaviours and initial conditions are analysed based upon hydrodynamic and thermal-diffusive effects. In addition, the critical Peclet number is observed linear to equivalence ratio but less sensitive to initial ambient conditions.

Extinction studies of near-limit lean premixed syngas/air flames

Extinction studies of weakly-stretched near-limit lean premixed syngas/air flames were conducted in a twin-flame counterflow configuration. Experiments showed that buoyancy-induced natural convection at normal gravity strongly disturbed these flames. In order to validate the simulation, accurate extinction data was obtained at micro-gravity. Experimental data obtained from the 3.6 s micro-gravity drop tower showed that the extinction equivalence ratio increased with the increasing global stretch rate and decreased with the increasing H2 mole fraction in the fuel. Numerical simulation was conducted with CHEMKIN software using GRI 3.0 and USC-Mech II mechanisms. The predicted extinction limit trend was in agreement with the micro-gravity experimental data. Sensitivity analyses showed that the competition between the main branching reaction H + O2 ⇔ O + OH and the main termination reaction H + O2 + M ⇔ HO2 + M in the H2/O2 chemistry determined the extinction limits of the flames. The dominant species for syngas/air flame extinction was the H radical. The key exothermal reaction changed from OH + CO ⇔ H + CO2 to OH + H2 ⇔ H + H2O with the increasing H2 mole fraction in the fuel. Also, the mass diffusion played a more important role than chemical kinetics in the flame extinction. When the H2 mass diffusion was suppressed, the reaction zone was pushed to the stagnation plane and the flame became weaker; while H mass diffusion is suppressed, the reaction zone slightly shifted towards the upstream and the flame was slightly strengthened.

Flame Interactions in Turbulent Premixed Twin V-Flames

Multiple flame–flame interactions in premixed combustion are investigated using direct numerical simulations of twin turbulent V-flames for a range of turbulence intensities and length scales. Interactions are identified using a novel automatic feature extraction (AFE) technique, based on data registration using the dual-tree complex wavelet transform. Information on the time, position, and type of interactions, and their influence on the flame area is extracted using AFE. Characteristic length and time scales for the interactions are identified. The effect of interactions on the flame brush is quantified through a global stretch rate, defined as the sum of flamelet stretch and interaction stretch contributions. The effects of each interaction type are discussed. It is found that the magnitude of the fluctuations in flamelet and interaction stretch are comparable, and a qualitative sensitivity to turbulence length scale is found for one interaction type. Implications for modeling are discussed.

A structural study of premixed hydrogen-air cellular tubular flames

Two dimensionally spatially resolved structural measurements are reported for cellular phenomena in lean laminar premixed hydrogen-air tubular flames. Laser-induced Raman scattering and chemiluminescence imaging are combined to investigate low Lewis number lean hydrogen-air flames. The strong effect of thermal-diffusive imbalance is observed in radial profiles interpolated through the centers of reaction and extinction zones. In the flame cell, the equivalence ratio is ∼80% higher than the inlet mixture, resulting in a peak flame temperature of 1600K that is 550K above the adiabatic flame temperature of the inlet mixture (1055K). In the adjacent extinction zone, the temperatures are ∼900K lower than the peak flame temperature and the equivalence ratio is similar to the inlet mixture. Despite doubling the global stretch rate from 200s−1 to 400s−1, the enhancement of local equivalence ratio and peak temperature in the flame cell remain similar. This enhancement seems dependent on the local cellular flame curvature, that is similar between both cases. With strong preferential diffusion effects, cellular flames offer unique validation data to improve the accuracy of current molecular transport modeling techniques.

The effect of stretch on cellular formation in non-premixed opposed-flow tubular flames

Cellular formation in non-premixed flames is experimentally studied in an opposed-flow tubular burner. This burner allows independent variation of the global stretch rate and overall flame curvature. In opposed-flow flames formed by 21.7% hydrogen diluted in carbon dioxide versus air, cells are formed near extinction with a low fuel Lewis number and a low initial mixture strength. Using an intensified CCD camera, the flame chemiluminescence is imaged to study cellular formation from the onset of cells to near extinction conditions. The experimental onset of cellular instability is found to be at or at a slightly lower Damköhler number than the numerically determined extinction limit based on a two-point boundary value solution of the tubular flame. For fuel Lewis numbers less than unity, concave curvature towards the fuel retards combustion and weakens the flame and convex curvature towards the fuel promotes combustion and strengthens the flame. In the cell formation process, the locally concave flame cell midsection is weakened and the locally convex flame cell ends are strengthened. With increasing stretch rate, the flame breaks into cells and the cell formation process continues until near-circular cells are formed with no concave midsection. Further increase in the stretch rate leads to cell extinction. With increasing stretch rate, the flame thickness at the cell midsection decreases similar to a planar opposed-flow flame while the flame thickness at the cell edges is unchanged and can even increase due to the strengthening effect of convex curvature at the flame edges toward the low Lewis number fuel. The results show the existence of cellular flames well beyond the two-point boundary value extinction limit and the importance of local flame curvature in the formation of flame cells.

Analysis of entropy generation in non-premixed hydrogen versus heated air counter-flow combustion

In this paper, entropy generation in non-premixed hydrogen versus heated air counter-flow combustion confined by planar opposing jets is investigated for the first time. The effects of the volume percentage of hydrogen in fuel mixture and the inlet Reynolds number (corresponding to the global stretch rate) on entropy generation are studied by numerical evaluating the entropy generation equation. The lattice Boltzmann model proposed in our previous work, instead of traditional numerical methods, is used to solve the governing equations for combustion process. Through the present study, three interesting features of this kind of combustion, which are quite different from that reported in previous literature on entropy generation analysis for diffusion hydrogen-air flames, are revealed. Moreover, it is observed that the whole investigated domain can be divided into two parts according to the predominant irreversibilities. The total entropy generation number can be approximated as a linear increasing function of the volume percentage of hydrogen in fuel mixture and the inlet Reynolds number for all the cases under the present study. It is very interesting that most characteristics in this kind of diffusion combustion also can be found in its premixed counterpart investigated in our previous work. Especially, the critical Reynolds numbers determining the order of the predominant irreversibilities in this type of diffusion flame are same as its premixed counterpart, although the flow and scalar fields between them are quite different.

Analysis of entropy generation in counter-flow premixed hydrogen–air combustion

In this paper, entropy generation in counter-flow premixed hydrogen–air combustion confined by planar opposing jets is investigated for the first time. The effects of the equivalence ratio and the inlet Reynolds number (corresponding to the global stretch rate) on entropy generation are studied by numerical evaluating the entropy generation equation. The lattice Boltzmann model proposed in our previous work, instead of traditional numerical methods, is used to solve the governing equations for combustion process. Through the present study, three interesting features of this kind of combustion, which are quite different from that reported in previous literature on entropy generation analysis for reactive flows, are revealed. Moreover, it is observed that the whole investigated domain can be divided into two parts according to the predominant irreversibilities. The total entropy generation number can be approximated as a linear increasing function of the equivalence ratio and the inlet Reynolds number for all the cases under the present study.

The effects of hydrogen substitution on turbulent premixed methane–air kernels using direct numerical simulation

Abstract Direct numerical simulations (DNS) are used to assess the effects of hydrogen substitution on lean premixed methane–air kernels during the early stages of growth in freely decaying turbulence. Two-dimensional simulations with a detailed 68-step reaction mechanism are carried out at equivalence ratios of ϕ = 0.53 and ϕ = 0.625, both with and without the substitution of a 30% fuel mole fraction of hydrogen. A comparative analysis is made into the changes in turbulent flame speeds, global stretch rates, and flame wrinkling at different turbulence intensities. The underlying causes of these changes are investigated through the distributions of the surface-conditioned displacement speed, strain rate and curvature. Direct comparison is made with the planar flame results of Hawkes and Chen [1] to assess the qualitative effects of kernel geometry in combination with a hydrogen-enriched fuel. It was found that the reduced effective fuel Lewis number and preferential diffusion of hydrogen, combined with the higher stretch rates and mean positive curvature of the kernel make the effects of hydrogen enrichment much more pronounced in kernels compared to planar flames.

Dynamic behavior of a freely-propagating turbulent premixed flame under global stretch rate oscillations

Dynamic features of a freely propagating turbulent premixed flame under global stretch rate oscillations were investigated by utilizing a jet-type low-swirl burner equipped with a high-speed valve on the swirl jet line. The bulk flow velocity, equivalence ratio and the nominal mean swirl number were 5 m/s, 0.80 and 1.23, respectively. Seven velocity forcing amplitudes, from 0.09 to 0.55, were examined with a single forcing frequency of 50 Hz. Three kinds of optical measurements, OH-PLIF, OH ∗ chemiluminescence and PIV, were conducted. All the data were measured or post-processed in a phase-locked manner to obtain phase-resolved information. The global transverse stretch rate showed in-phase oscillations centering around 60 (1/s). The oscillation amplitude of the stretch rate grew with the increment of the forcing amplitude. The turbulent flame structure in the core flow region varied largely in axial direction in response to the flowfield oscillations. The flame brush thickness and the flame surface area oscillated with a phase shift to the stretch rate oscillations. These two properties showed a maximum and minimum values in the increasing and decreasing stretch periods, respectively, for all the forcing amplitudes. Despite large variations in flame brush thickness at different phase angles, the normalized profiles collapse onto a consistent curve. This suggests that the self-similarity sustains in this dynamic flame. The global OH ∗ fluctuation response (i.e. response of global heat-release rate fluctuation) showed a linear dependency to the forcing velocity oscillation amplitudes. The flame surface area fluctuation response showed a linear tendency as well with a slope similar to that of the global OH ∗ fluctuation. This indicated that the flame surface area variations play a critical role in the global flame response.

Flame Extinction Limits of H2‐CO Fuel Blends

The flame extinction limits of syngas (H2‐CO) flames were measured using a twin-flame counterflow burner. Plots of extinction limits (%f: volumetric percent of fuel in air) versus global stretch rates were generated at different fuel blend compositions and were extrapolated to determine the flame extinction limit corresponding to an experimentally unattainable zero-stretch condition. The zero-stretch extinction limit of H2‐CO mixtures decreases with the increase in H2 concentration in the mixture. The average difference between the measured flame extinction limit and the Le Chatelier’s calculation is around 7% of the mean value. The measured OH chemiluminescence data indicates that regardless of blend composition the OH radical concentration reduces to a critical value prior to the flame extinction. The measured laminar flame velocity close to the extinction indicates that regardless of fuel composition, the premixed flame of hydrogen fuel blends extinguishes when the mixture laminar flame velocity falls below a critical value.

Cellular instabilities and self-acceleration of outwardly propagating spherical flames

Using a recently developed constant and high-pressure combustion chamber, an experimental study wasconducted on several aspects of cellular instabilities of outwardly propagating spherical premixed flames. Propane/air and hydrogen/oxygen/nitrogen flames of different concentrations and under elevated pressures were used to systematically identify the influences of thermal expansion ratio, flame thickness, global activation energy, mixture Lewis number, and global stretch rate on the generation of hydrodynamic and diffusional-thermal cells over the flame surface. In particular, it was demonstrated that hydrodynamic instability is greatly enhanced with increasing pressure and hence decreasing flame thickness, although the influence can also be moderated by the progressively important three-body termination reactions as the pressure increases. The onset of cellular instability was examined in light of the theory of Bechtold and Matalon, and satisfactory qualitative and acceptable quantitative comparisons were observed. The cellular flames were found to be self-accelerating, including those that are diffusionally unstable, with fractal dimensions between 2.20 and 2.25.

Dynamics of flow stretch on coal ignition mechanism

The dynamics of flow stretch on the mechanism of coal ignition has been investigated experimentally. The hydrodynamic and global stretch rates in this mechanism are proposed whereby the ignition position and flame configuration are correlated. There is a limit of stagnation point ignition, when the specific global stretch rate is 50 1/s for a constant flow temperature of 1000 K and oxygen concentration of 21%. When the global stretch rate is less than the specific global stretch rate, the coal ignites near the stagnation point of the particle and the shape of the developing flame is spherical. When the global stretch rate exceeds the specific global stretch rate, the coal ignites at the rear of the particle and the configuration resembles a Bunsen flame. The coal ignited in the middle zone of the particle at the transition state is interpreted by the definition of the hydrodynamic stretch rate. The effect of the ignition stretch factor on the ignition mechanism compared with the effect of the flame stretch factor on the mechanism of flame extinction is discussed in detail.

Velocity and scalar fields of turbulent premixed flames in stagnation flow

Detailed experimental measurements of the scalar and velocity statistics of premixed methane/air flames stabilized by a stagnation plant are reported. Conditioned and unconditioned velocity of two components and the reaction progress variables are measured by using a two-component laser Doppler velocimetry techniques and Mie scattering techniques, respectively. Experimental conditions cover equivalence ratios of 0.9 and 1.0, incident turbulence intensities of 0.3 to 0.45 m/s, and global stretch rates of 100 to 150 sec/sup /minus/1/. The experimental results are analyzed in the context of the Bray-Moss-Libby flamelet model of these flames. The results indicate that there is no turbulence production within the turbulent flame brush and the second and third order turbulent transport terms are reduced to functions of the difference between the conditioned mean velocity. The result of normalization of these relative velocities by the respective velocity increase across laminar flames suggest that the mean unconditioned velocity profiles are self-similar. 13 refs., 5 figs., 1 tab.

Structure and propagation of turbulent premixed flames astabilized in a stagnation flow

Turbulence statistics in premixed methane/air flames stabilized by a stagnation plate have been measured using the two-component laser Doppler velocimetry technique. Experimental conditions cover equivalence ratios of 0.67 to 1.16, incident turbulence intensities of 0.1 to 0.7 m/s, and global stretch rates of 100 to 230 sec−1. Results on turbulence statistics show that in crossing the flame the increase in fluctuation is more pronounced in the normal component while the Reynolds stress remains almost zero, and that the velocity joint probability density function is bimodal for rich flames and mono-modal for lean flames. It is also demonstrated that the present stagnation flame configuration can be usefully applied for the determination, of the turbulent flame speed ST. Values of ST determined as functions of the equivalence ratio and turbulence intensity agree well with those in the literture. It is suggested that the stagnation flame offers potential as a viable configuration for the study of planar, turbulent premixed flames uncomplicated by fluctuations generated by the stabilization regions associated with the conventional conical Bunsen flames and V-flames