Stretch Rate Research Articles

An Experimental Measurement on Laminar Burning Velocities and Markstein Length of Iso-Butane-Air Mixtures at Ambient Conditions

In the present work, experimental investigation on laminar combustion of iso-butane-air mixtures was conducted in constant volume explosion vessel. The experiments were conducted at wide range of equivalence ratios ranging between Ф = 0.6 and 1.4 and atmospheric pressure of 0.1 MPa and ambient temperature of 303K. Using spherically expanding flame method, flame parameters including stretched, unstretched flame propagation speeds, laminar burning velocities and Markstein length were calculated. For laminar burning velocities the method of error bars of 95% confidence level was applied. In addition, values of Markstein lengths were measured in wide range of equivalence ratios to study the influence of stretch rate on flame instability and burning velocity. It was found that the stretched flame speed and laminar burning velocities increased with equivalence ratios and the peak value was obtained at equivalence ratio of Ф = 1.1. The Markstein length decreased with the increases in equivalence ratios, which indicates that the diffusion thermal flame instability increased at high equivalence ratios in richer mixture side. However, the total deviations in the laminar burning velocities have discrepancies of 1.2-2.9% for all investigated mixtures.

OH-PLIF/ステレオPIVによる火炎形状と流れ場の同時計測

It is difficult to understand turbulent combustion, because the combustion process is very complex and unsteady. One of the useful parameters for discussion of the unsteady flame behaviors is the flame stretch rate, which is evaluated by local flame curvature and strain rate. In this study, we conducted simultaneous OH-PLIF/Stereo PIV measurements of turbulent premixed flames. Flame shape and flow field were obtained. Then, local flame curvature and strain rate were discussed. The turbulent flame was established by a cyclone-jet combustor for propane/air mixtures. The main jet velocity was Um = 10, 30 m/s, and the equivalence ratio was Φm = 0.75, 0.90. The equivalence ratio of the cyclone jet was the same as that of the main jet, but the velocity of the cyclone jet was the constant of 10 m/s. It was found that the variation of flame curvature was large when the main jet velocity was high. Difference between curvature convex to unburned gas and curvature convex to burned gas was relatively small. Frequency of larger strain rate was high when the main jet velocity was large and equivalence ratio was small. It is suggested that the local flame extinction could be caused by the large strain rate because the local flame extinction occurs more frequently. At all conditions, compared with the positive strain rate, the frequency of negative strain rate was high.

Extinction characteristics of ammonia/air counterflow premixed flames at various pressures

Ammonia has promising features as a carbon-free fuel without greenhouse gas emission. However, due to its low combustion intensity, the combustion characteristics of ammonia have been rarely investigated. The design of ammonia based industrial applications requires the development of effective turbulent ammonia combustion models. Thus, a study of the flame stretch effect in ammonia/air premixed combustion is necessary. The objective of this research was to study the ammonia flame extinction stretch rate both experimentally and numerically and to investigate the effects of pressure on its extinction characteristics. Experiments were conducted using a counterflow flame burner. Numerical simulations were run on CHEMKIN-PRO, using the PREMIX opposedflow model, for various detailed chemistry mechanisms. The effects of pressure on the extinction stretch rate of ammonia/air premixed flame were compared with that of methane/air flame, and the effects of pressure on the detailed reaction paths were clarified. It was found that the extinction stretch rate of ammonia/air flame is low compared with that of methane/air flame, as could be expected from its low laminar burning velocity. However, the increase of extinction stretch rate with pressure was found to be greater in the case of ammonia/air flame. From detailed chemistry analysis, it was found that the different dependence on pressure of the reaction path of the two fuels could explain this difference. Indeed, the heat release process and flame strength are affected by a greater dependence on pressure of the reactions contributing the most to heat released in the case of methane/air flame. For methane/air flame, CH3 is consumed in the main heat releasing reactions, and they experience competition by the third body recombination, 2CH3+M⇋C2H6+M at high pressure. In the case of ammonia/air flame, the heat release process is mainly related to NH3+OH ⇋ NH2+H2O, NH2+OH ⇋ NH+H2O and NH2+NO⇋N2+H2O, which remain preponderant when pressure increases. Thus, the decrease of characteristic reaction time with pressure was found to be greater in the case of ammonia/air flame, explaining a larger increase in extinction stretch rate when pressure increases.

Effect of flame thickness variation on the mass burning rate of premixed counterflow flames with an oscillating strain rate

The mass-based stretch rate is used to study the response of premixed axisymmetric counterflow flames subject to an oscillating strain rate. Integral analysis is used to estimate the mass burning rate of the oscillating counterflow flames. From this study it can be concluded that the flame responds in a nonlinear manner. With an increase of the applied strain frequencies, it is found that unsteady stretch effects arising due to flame thickness variations become significant and the mass-based stretch rate is able to capture these nonlinear effects. The inclusion of these unsteady stretch effects in the mass-based stretch helps the integral analysis to predict the mass-burning rate of oscillating flames more accurately.

Extinguishment of counter-flow diffusion flame by water mist derived from aqueous solutions containing chemical additives

A comparative study on the suppression efficiency of different water mist additives was conducted in a counter-flow flame apparatus. The introduction of 1 mL/min ultrapure water mist reduced the critical stretch rate by 14%. The chemical additives could improve the suppression efficiency of water mist except FeSO4 and H3PO4. Their suppression efficiency could be generally sorted in a descending order: organic alkali metal salts (K2C2O4, C2H3KO2), inorganic alkali metal salts (KCl, KHCO3, K3PO4), phosphorus-containing compounds (K3PO4, NH4H2PO4, H3PO4), and inorganic ferrous compounds (FeCl2, FeSO4). The morphology of the particles deviated from the previous sphere shape assumptions. It is assumed that the efficiency of chemical additives in suppressing highly stretched diffusion can depend on the following factors: (1) the chemical nature of the additives, (2) the residence time controlled by the flow conditions, and (3) the residual particles formed by the additive-containing water mist.

Fracture of dual crosslink gels with permanent and transient crosslinks

We have carried out systematic fracture experiments in a single edge notch geometry over a range of stretch rates on dual crosslink hydrogels made from polyvinyl alcohol chains chemically crosslinked with glutaraldehyde and physically crosslinked with borate ions. If the energy release rate necessary for crack propagation was calculated conventionally, by using the work done to deform the sample to the critical value of stretch λc where the crack propagates, we found that the fracture energy Γ peaks around λ̇∼0.001s−1 before decreasing sharply with increasing stretch rate, in contradiction with the measurements of crack velocity. Combining simulations and experimental observations, we propose therefore here a general method to separate the energy dissipated during loading before crack propagation, from that which is dissipated during crack propagation. For fast loading rates (with a characteristic strain rate only slightly lower than the inverse of the typical breaking time of physical bonds), this improved method to estimate a local energy release rate glocal at the onset of crack propagation, gives a value of the local fracture energy Γlocal which is constant, consistent with the constant value of the crack propagation velocity measured experimentally. Using this improved method we also obtain the very interesting result that the dual crosslink gels have a much higher value of fracture energy at low loading rates than at high loading rates, contrary to the situation in classical chemically crosslinked elastic networks.

A diffuse interface method for simulating the dynamics of premixed flames

A diffuse interface method for simulating dynamics of premixed flames is proposed. The flame is treated as a diffuse moving front with its propagation automatically captured by the convection–diffusion equation of the progress variable. The diffusion and reaction terms are constructed using the flame speed and flame thickness, and the flame speed is taken as a function of the local stretch rate to incorporate the stretch effect. This equation is coupled with the continuity equation and the momentum equation to describe flame dynamics in complex flows, and the lattice Boltzmann method is employed in the present study as the computational platform, since its feature of explicit computation is highly consistent with the advantage of flame auto-capturing of the present method. To test the performance of the method, simulations including 1-D flame propagation, 2-D Darrieus–Landau instability, 2-D cylindrical flame propagation with stretch effect, and 2D flame–vortex interaction are conducted, and the consequent results are in good agreement with analytical solutions.

A flame particle tracking analysis of turbulence–chemistry interaction in hydrogen–air premixed flames

Interactions of turbulence, molecular transport, and energy transport, coupled with chemistry play a crucial role in the evolution of flame surface geometry, propagation, annihilation, and local extinction/re-ignition characteristics of intensely turbulent premixed flames. This study seeks to understand how these interactions affect flame surface annihilation of lean hydrogen–air premixed turbulent flames. Direct numerical simulations (DNSs) are conducted at different parametric conditions with a detailed reaction mechanism and transport properties for hydrogen–air flames. Flame particle tracking (FPT) technique is used to follow specific flame surface segments. An analytical expression for the local displacement flame speed (Sd) of a temperature isosurface is considered, and the contributions of transport, chemistry, and kinematics on the displacement flame speed at different turbulence-flame interaction conditions are identified. In general, the displacement flame speed for the flame particles is found to increase with time for all conditions considered. This is because, eventually all flame surfaces and their resident flame particles approach annihilation by reactant island formation at the end of stretching and folding processes induced by turbulence. Statistics of principal curvature evolving in time, obtained using FPT, suggest that these islands are ellipsoidal on average enclosing fresh reactants. Further examinations show that the increase in Sd is caused by the increased negative curvature of the flame surface and eventual homogenization of temperature gradients as these reactant islands shrink due to flame propagation and turbulent mixing. Finally, the evolution of the normalized, averaged, displacement flame speed vs. stretch Karlovitz number are found to collapse on a narrow band, suggesting that a unified description of flame speed dependence on stretch rate may be possible in the Lagrangian description.

Compositional- and time-dependent dissipation, recovery and fracture toughness in hydrophobically reinforced hybrid hydrogels

The compositional- and time-dependent mechanical properties, recovery, and fracture toughness of hybrid gels having chemical and transient cross-links are systematically studied. The hybrid gels are prepared via micellar copolymerization of acrylamide (AAm) with n-Butyl acrylate (nBA) and the hydrophobic associations of nBA form the transient cross-links. The hybrid gels exhibited excellent mechanical properties (ultimate strength ∼ 400 kPa), fast recovery, and high tolerance to cracks (fracture toughness up to 378.0 J m−2). Swelling, tensile, and pure shear measurements all revealed that the reinforcement is more pronounced when the mole fraction of nBA is above 0.05 and/or the stretch rate is above 0.1 s−1. The compositional dependence is related to the onset of the formation of transient network while the time-dependent mechanical responses are ascribed to the dynamics of the hydrophobic associations. We also confirmed the separability of strain and time in hydrophobically reinforced hybrid gels for the first time. Our results emphasize the role of the transient network and its responses under external loading in the design of next-generation tough gels.

Ventricular fiber optimization utilizing the branching structure

In this paper, we propose an algorithm that optimizes the ventricular fiber structure of the human heart. A number of histological studies and diffusion tensor magnetic resonance imaging analyses have revealed that the myocardial fiber forms a right-handed helix at the endocardium. However, the fiber formation changes its orientation as a function of transmural depth, becoming a left-handed helix at the epicardium. To determine how nature can construct such a structure, which obtains surprising pumping performance, we introduce macroscopic modeling of the branching structure of cardiac myocytes in our finite element ventricular model and utilize this in an optimization process. We put a set of multidirectional fibers around a central fiber orientation at each point of the ventricle walls and simulate heartbeats by generating contraction forces along each of these directions. We examine two optimization processes using the workloads or impulses measured in these directions to update the central fiber orientation. Both processes improve the pumping performance towards an optimal value within several tens of heartbeats, starting from an almost-flat fiber orientation. However, compared with the workload optimization, the impulse optimization produces better agreement with experimental studies on transmural changes of fiber helix angle, streamline patterns of characteristic helical structures, and temporal changes in strain. Furthermore, the impulse optimization is robust under geometrical changes of the heart and tends to homogenize various mechanical factors such as the stretch and stretch rate along the fiber orientation, the contraction force, and energy consumption. Copyright © 2015 John Wiley & Sons, Ltd.

Stretch rate effects and flame surface densities in premixed turbulent combustion up to 1.25 MPa

Independent research at two centres using a burner and an explosion bomb has revealed important aspects of turbulent premixed flame structure. Measurements at pressures and temperatures up to 1.25 MPa and 673 K in the two rigs were aimed at quantifying the influences of flame stretch rate and strain rate Markstein number, Masr, on both turbulent burning velocity and flame surface density. That on burning velocity is expressed through the stretch rate factor, Io, or probability of burning, Pb0.5. These depend on Masr, but they grow in importance as the Karlovitz stretch factor, K, increases, and are evaluated from the associated burning velocity data. Planar laser tomography was employed to identify contours of reaction progress variable in both rigs. These enabled both an appropriate flame front for the measurement of the turbulent burning velocity to be identified, and flame surface densities, with the associated factors, to be evaluated. In the explosion measurements, these parameters were derived also from the flame surface area, the derived Pb0.5 factor and the measured turbulent burning velocities. In the burner measurement they were calculated directly from the flame surface density, which was derived from the flame contours.A new overall correlation is derived for the Pb0.5 factor, in terms of Masr at different K and this is discussed in the light of previous theoretical studies. The wrinkled flame surface area normalised by the area associated with the turbulent burning velocity measurement, and the ratio of turbulent to laminar burning velocity, ut/ul, are also evaluated. The higher the value of Pb0.5, the more effective is an increased flame wrinkling in increasing ut/ul. A correlation of the product of k and the laminar flame thickness with Karlovitz stretch factor and Markstein number is explored using the present data and those of other workers. Some generality is revealed, enabling the wave length associated with the spatial change in mean reaction progress variable to be expressed by the number of laminar flame thicknesses, and the flame volume to be found.

The effects of hydrodynamic stretch on the flame propagation enhancement of ethylene by addition of ozone.

The effect of O(3) on C(2)H(4)/synthetic-air flame propagation at sub-atmospheric pressure was investigated through detailed experiments and simulations. A Hencken burner provided an ideal platform to interrogate flame speed enhancement, producing a steady, laminar, nearly one-dimensional, minimally curved, weakly stretched, and nearly adiabatic flame that could be accurately compared with simulations. The experimental results showed enhancement of up to 7.5% in flame speed for 11 000 ppm of O(3) at stoichiometric conditions. Significantly, the axial stretch rate was also found to affect enhancement. Comparison of the flames for a given burner exit velocity resulted in the enhancement increasing almost 9% over the range of axial stretch rates that was investigated. Two-dimensional simulations agreed well with the experiments in terms of flame speed, as well as the trends of enhancement. Rate of production analysis showed that the primary pathway for O(3) consumption was through reaction with H, leading to early heat release and increased production of OH. Higher flame stretch rates resulted in increased flux through the H+O(3) reaction to provide increased enhancement, due to the thinning of the flame that accompanies higher stretch, and thus results in decreased distance for the H to diffuse before reacting with O(3).

Gas turbine combustion characteristics of H2/CO synthetic gas for coal integrated gasification combined cycle applications

This paper describes the gas turbine combustion characteristics of coal-derived synthetic gas (syngas), particularly for the syngases at the Taean IGCC plant in Korea and the Buggenum IGCC plant in the Netherlands. To evaluate the combustion performance of these syngases, we conducted combustion tests with elevated temperature and ambient pressure in a GE7EA model combustor. We observed flame stability, dynamic pressure characteristics, NOx and CO emissions, temperature in the combustion chamber, and flame structures while varying the heat input and diluent integration ratio. Without dilution, Buggenum's stable regime is larger than that of Taean, since a higher hydrogen content causes sustained flames, even at a very high flame stretch rate. However, when considering nitrogen dilution, Taean's syngas has a more stable burn than that of Buggenum, since an increase in nitrogen at Buggenum has negatively affected flame stability. From the results of NOx/CO emissions and combustion efficiency, we report that both syngases are sufficient to control NOx emissions at under 5 ppm with almost complete and stable combustion; however, quantitatively, the effect of nitrogen dilution is slightly different at Taean and Buggenum due to slight differences in the H2/CO ratio and diluent heat capacity. All the tested results and conclusions drawn are considered for optimal operation and trouble shooting at the Taean IGCC plant, which is scheduled to be complete toward the end of 2016.

Numerical simulations of premixed cool flames of dimethyl ether/oxygen mixtures

The formation and dynamics of premixed cool flames are numerically investigated by using a detailed kinetic mechanism of dimethyl ether mixtures in both freely-propagating and stretched counterflow flames with and without ozone sensitization. The present study focuses on the dynamics and transitions between cool flames and high temperature flames. The impacts of mixture temperature, inert gas temperature, and ozone concentration on low temperature ignition, cool flame formation, and flammable regions of different flame regimes are investigated. For the freely-propagating flames, three different flame structures (high temperature flames, double flames, and cool flames) are found. The present study shows that the flammability limit of dimethyl ether is significantly extended by the appearance of cool flames and that the conventional concept of the flammability limit of a high temperature flame ought to be reconsidered. Furthermore, the results demonstrate that the cool flame propagation speed can be significantly higher than that of near-limit high temperature flames and that ozone addition dramatically accelerates the formation of cool flames at low temperatures and extends the flammability limit. A schematic of a modified flammability limit diagram including both high temperature flames and cool flames is proposed. For stretched counterflow flames, the results also show that multiple flame regimes exist with and without ozone addition. It is demonstrated that at the same mixture enthalpy, ozone addition kinetically extends the cool flame extinction limit to a higher stretch rate. Moreover, with ozone addition, two different cool flame transition regimes: a low temperature ignition transition and a direct cool flame transition without an ignition limit at higher temperature, are predicted. The present results suggest that cool flames can be an important combustion process in affecting flammability limits and flame regimes as the mixture temperature, turbulent mixing, and radical production/recirculation are increased. The results also provide guidance in observing self-sustaining premixed cool flames in experiments.

Bayesian uncertainty analysis of finite deformation viscoelasticity

The viscoelasticity of the dielectric elastomer, VHB 4910, is experimentally characterized, modeled, and analyzed using Bayesian uncertainty analysis. Whereas these materials are known for their large-field induced deformation and broad applications in smart structures, the rate-dependent viscoelastic effects are not well understood. To address this issue, we quantify both the hyperelastic and viscoelastic constitutive behavior and use Bayesian uncertainty analysis to assess several key modeling attributes. Specifically, we compare an Ogden-based phenomenological model to a nonaffine hyperelastic model and couple hyperelasticity to both linear and nonlinear viscoelasticity. The utilization of Bayesian statistics is shown to provide insight into quantifying nonlinear viscoelasticity behavior as a function of internal state variables. The results are validated experimentally in the finite deformation regime over a range of stretch rates spanning four orders of magnitude (6.7×10-5–0.67Hz). A unique set of hyperelastic parameters are identified, independent of the stretch rate. In addition, comparisons of the linear and nonlinear viscoelastic models demonstrate a reduction in modeling error by approximately a factor of three. Finally, the viscoelastic time constant is shown to produce an inverse stretch rate power law dependence regardless of which hyperelastic model is used.

Injection Blow Moulding Single Stage Process – Approach of the Material Behaviour in Process Conditions and Numerical Simulation

Single stage injection blow moulding process, without preform storage and reheat, could be run on a standard injection moulding machine, with the aim of producing short series of specific hollow parts. In this process, the preform is being blow moulded after a short cooling time. Polypropylene (Random copolymer) is a suitable material for this type of process. The preform has to remain sufficiently melted to be blown. This single stage process introduces temperature gradients, molecular orientation, high stretch rates and high cooling rates. These constraints lead to a small processing window, and in practice, the process takes place between the melting temperature and the crystallization temperature. To investigates the mechanical behaviour in conditions as close to the process as possible, we ran a series of experiments: First, Dynamical Mechanical Analysis was performed starting from the solid state at room temperature and ending in the vicinity of the melting temperature. Conversely, oscillatory rheometry was also performed starting this time from the molten state at 200°C and decreasing the temperature down to the vicinity of the crystallization temperature. The influence of the shear rate and of the cooling kinetics on the enhancement of the mechanical properties when starting from the melt is discussed. This enhancement is attributed to the crystallization of the material. The question of the crystallization occurring at such high stretch rates and high cooling rates is open. A viscous Cross model has been proved to be relevant to the problem. Thermal dependence is assumed by an Arrhenius law. The process is simulated through a finite element code (POLYFLOW software) in the Ansys Workbench framework. Thickness measurements using image analysis are performed and comparison with the simulation results is satisfactory.

Direct Numerical Simulation of Complex Fuel Combustion with Detailed Chemistry: Physical Insight and Mean Reaction Rate Modeling

Direct numerical simulation (DNS) of freely-propagating premixed flames of a multi-component fuel is performed using a skeletal chemical mechanism with 49 reactions and 15 species. The fuel consists of CO, H2, H2O, CH4, and CO2 in proportions akin to blast furnace gas or a low calorific value syngas. The simulations include low and high turbulence levels to elucidate the effect of turbulence on realistic chemistry flames. The multi-component fuel flame is found to have a more complex structure than most common flames, with individual species’ reaction zones not necessarily overlapping with each other and with a wide heat releasing zone. The species mass fractions and heat release rate show significant scatter, with their conditional average however remaining close to the laminar flame result. Probability density functions of displacement speed, stretch rate, and curvature are near-Gaussian. Five different mean reaction rate closures are evaluated in the RANS context using these DNS data, presenting perhaps the most stringent test to date of the combustion models. Significant quantitative differences are observed in the performance of the models tested, especially for the higher turbulence level case.

Effect of porous media properties on the onset of polymer extensional viscosity

Abstract Based on experimental results, it has been observed that, although the applied polymer solutions in EOR generally demonstrate shear thinning properties in rheometer, in porous media above a critical flow rate, apparent viscosity increases. This behavior is interpreted as the consequence of both elastic properties of polymer solution and rock properties. In this study, elastic properties of polymer solution is imported into numerical simulation by using modified form of the Carreau model and only effect of rock properties on the onset of extensional viscosity are studied. Results proved that for a more tortuous rock sample, extensional viscosity happens at a lower Darcy velocity. To define the onset of extensional viscosity, Deborah number explanation (De= eτ r ) is applied. A linear correlation is proposed as the relationship between stretch rate and Darcy velocity, in which the linearity coefficient is related to the tortuosity of rock sample. By calculating the stretch rate using this modification, De at the onset of extensional viscosity is the same for all rock samples. Furthermore some microscopic properties of pore geometries such as aspect ratio and length of pore throat are studied by using simplified 2D contraction–expansion channels. Initially the simulation is well validated by using experimental results reported by Chauveteau (1981) . By increasing the aspect ratio, the onset of extensional viscosity occurs at lower Darcy velocity as polymer molecules are being exposed to larger stretch rate values. On the other hand by increasing the length of contracted part, the onset of extensional viscosity happens at higher Darcy velocity. This is due to the fact that polymer molecules have more time to be relaxed after being deformed due to the contraction channel.

Maximum stretched flame speeds of laminar premixed counter-flow flames at variable Lewis number

This paper examines Lewis-number effects on stretched, laminar, premixed flames near extinction. It presents the experimental measurement of maximum stretched flame speed and extinction limit for the premixed laminar combustion of selected low, unity and high-Lewis number mixtures. Stretched, fuel-lean, laminar flames of methane with Le≅1, propane with Le>1 and hydrogen with Le≪1 are studied experimentally in a counter-flow flame configuration. Flow velocity is measured in these flames by particle tracking velocimetry. Results show that a maximum reference flame speed exists for mixtures with Le≳1 at lower flame-stretch values than the extinction stretch rate. In contrast, a continually-increasing reference flame speed is measured for Le≪1 mixtures until extinction occurs when the flame is constrained by the stagnation point. Laminar flame results are also compared to numerical simulations employing a one-dimensional stagnation flame model. The chemical-kinetic models for each respective mixture capture the important trends of a maximum in su,ref ahead of the extinction stretch rate for methane and propane, and an increasing su,ref to extinction for hydrogen. These results are important to the investigation of the leading edge theory of premixed turbulent combustion, in which maximum stretched flamelet speed is a key parameter.

The motion induced by the orthogonal stretching and shearing of a membrane beneath a quiescent fluid

The flow induced above an impermeable membrane undergoing orthogonal linear stretching and orthogonal linear shearing is investigated. For an exact solution of the Navier–Stokes equations, the orthogonal shearing motions must be related through the constant σ = γ δ, where γ and δ are the dimensionless streamwise and transverse shear rates, respectively. The resulting similarity reduction leads to three nonlinearly coupled ordinary differential equations governed by σ and the ratio of membrane stretch rates β. All possible solutions of these equations are found either numerically or, in special cases, analytically. Features of the σ = 0 solutions at β = 0 and asymptotically as β → ∞ are found to be in excellent agreement with numerical calculations. An aside calculation shows that orthogonal shearing in the absence of any plate stretching cannot exist. However, shearing in one coordinate direction is possible as long as the membrane stretches in at least one direction with the caveat that there exists uniform suction through a porous membrane.