Stretch Rate Research Articles

A dynamic visco-hyperelastic model of dielectric elastomers and their energy dissipation characteristics

In this paper, we present a model describing the nonlinear dynamic visco-hyperelastic behaviors of dielectric elastomers (DE), with the purpose to explain the material’s dynamic energy dissipation mechanism, and provide convenience for actual design of DE devices. On the basic mechanical properties of the material, a visco-hyperelastic constitutive relationship, derived from Kelvin–Voigt rheological model and expressed as complex modulus, is created at first. Then, from the approximate relationship between harmonic motion frequency and the stretch rate (as well as the amplitude of stretch ratio) of the film, a new model-fitting approach is put forward to obtain the three intrinsic parameters, based on the uniaxial tensile tests for VHB 4910 DE film at different stretch rates (from 0.029 to 0.71 s−1). Applying the proposed parameters, the hysteresis and energy dissipation behaviors of the DE film are subsequently predicted, showing good agreement with the experimental results. Finally, the influences of the kinematic variable pair on energy dissipation properties are quantitatively investigated.

Examining flow-flame interaction and the characteristic stretch rate in vortex-driven combustion dynamics using PIV and numerical simulation

In this paper, we experimentally investigate the combustion dynamics in lean premixed flames in a laboratory scale backward-facing step combustor in which flame-vortex driven dynamics are observed. A series of tests was conducted using propane/hydrogen/air mixtures for various mixture compositions at the inlet temperature ranging from 300K to 500K and at atmospheric pressure. Pressure measurements and high speed particle image velocimetry (PIV) are used to generate pressure response curves and phase-averaged vorticity and streamlines as well as the instantaneous flame front, respectively, which describe unsteady flame and flow dynamics in each operating regime. This work was motivated in part by our earlier study where we showed that the strained flame consumption speed Sc can be used to collapse the pressure response curves over a wide range of operating conditions. In previous studies, the stretch rate at which Sc was computed was determined by trial and error. In this study, flame stretch is estimated using the instantaneous flame front and velocity field from the PIV measurement. Independently, we also use computed strained flame speed and the experimental data to determine the characteristic values of stretch rate near the mode transition points at which the flame configuration changes. We show that a common value of the characteristic stretch rate exists across all the flame configurations. The consumption speed computed at the characteristic stretch rate captures the impact of different operating parameters on the combustor dynamics. These results suggest that the unsteady interactions between the turbulent flow and the flame dynamics can be encapsulated in the characteristic stretch rate, which governs the critical flame speed at the mode transitions and thereby plays an important role in determining the stability characteristics of the combustor.

Extinction characteristics of CH4/O2/Xe radiative counterflow planar premixed flames and their transition to ball-like flames

Extinction characteristics of CH4/O2/Xe radiative counterflow premixed flames and their transition to ball-like flames were examined by computations and microgravity experiments. First, the flammability limit of flame ball for the mixture was estimated to be leaner than that of counterflow premixed flame by one-dimensional computations with detailed chemistry. Extinction experiments under microgravity showed that there was a ball-like flame prior to total extinction in the vicinity of the stagnation-plane in the low-speed counterflow field at stretch rate of 1.6–3.2s−1. Formation of such a ball-like flame occurred subsequent to the extinction of counterflow flames and the ball-like flame was finally extinguished when the equivalence ratio was further decreased. Two-dimensional computations indicated that the temperature of the ball-like flame increased with the decrease of equivalence ratio in the near-limit condition when it approached extinction. The temperature distribution of the computational ball-like flame was in qualitative agreement with that of the flame ball. The ball-like flame in the counterflow field was considered to be stable based on steady-state two-dimensional computation at an equivalence ratio slightly richer than the limit of a transient ball-like flame. Such a stable computational ball-like flame is not perfectly spherical. The ball-like flame observed in this study is thought to have a close correlation with the ideal flame ball which is generally established in a quiescent mixture.

Experimental study of suppressing effect of fine water droplets on propane/air premixed flames stabilized in the stagnation flowfield

Effect of fine water droplets on the laminar burning velocity of propane/air premixed flame was investigated by using a single jet-plate configuration. For the case without water droplets, the measured laminar burning velocities are in reasonably good agreement with previously reported data and the numerical simulation. The dependence of the burning velocity on the stretch rate for the case without water droplets is positive for all mixtures tested, resulting in the negative Markstein length, which coincides with previous experimental and theoretical studies. Water droplets lower significantly the laminar burning velocity and change its dependence on the stretch rate to negative. This leads to apparent positive Markstein lengths. The positive Markstein length was discussed on the basis of the droplets behavior in the stagnation flow field adopted in the present study. Even if the droplet mass loading was kept constant, the water droplets do not follow the diverting flow field when the stretch rate is high and the droplets accumulation occurs in the stagnation region where the burning velocities were measured. This fact results in the lower burning velocity as compared to that measured for uniformly dispersed water droplets.

A study of liquid fuel injection and combustion in a constant volume vessel at diesel engine conditions

Experiments were performed in a constant volume vessel, with fuel sprays injected into the vessel at selected pressure and temperature conditions that are typical of a diesel engine at cold start. N-heptane and i-octane were used as surrogate fuels for diesel and gasoline, following the establishment of a semidetailed mechanism containing skeletal mechanisms for n-heptane and i-octane, that has proved to predict autoignition delay times that agree well with experiments. Computational Fluid Dynamics (CFD) modeling of combustion for these two fuels at the selected experimental conditions were thus conducted adopting this mechanism, with the objective of studying fuel composition, temperature and pressure effects on mixing and combustion numerically. A tabulated chemistry approach called Flamelet Generated Manifolds (FGMs) has been applied for combustion modeling in the CFD solver STAR-CD. Since diesel combustion features mainly diffusion combustion, the manifold is created with 1D diffusion flame solutions from ignition and extended to steady state flamelets from low-to-high stretch rates. Effects from different fuels are examined experimentally, while effects from different conditions for n-heptane injection are examined from both experimental and simulation results.

Technical applicability of low-swirl fuel nozzle for liquid-fueled industrial gas turbine combustor

In this study, potentials of the liquid-fueled low-swirl burner technique for industrial gas-turbine combustor application are reported for the first time. A low-swirl fuel nozzle, which is a new implementation of the basic low-swirl burner design, is configured by the velocity measurement of methane–air open flames under atmospheric pressure and a low velocity (∼3m/s) condition. Flow properties, such as the axial stretch rate and virtual origins, are compared with the previously reported values with the axial vane type low-swirl injector, and it is confirmed that the flow field generated from the current implementation is of the typical low-swirl flow. Then, it is shown that the configuration successfully stabilize the lifted flame under much higher velocity (∼50m/s) condition with kerosene fuel injected by a typical pressure atomizer. Finally, the fuel nozzle is installed in a 290kW simple-cycle liquid-fueled gas turbine engine and is found to be operable over the entire operating range. The combustor inlet wall temperatures are shown to be within an acceptable range, even without the cooling air that was required for conventional combustors. This is an advantage of the lifted flame stabilized by the low-swirl technique. Although our focus is not on low emissions characteristics, NOx emissions is also found to be below maximum levels of current Japanese regulations (<84ppm@15% O2). In sum, the proposed fuel nozzle design shows promise for the application of liquid-fueled industrial gas turbine engines.

Simulation of NO Formation in Hydrogen-Air Counterflow Diffusion Flame

The combustion of hydrocarbon fuels produces large amounts of carbon dioxide. In order to cope with the challenge of greenhouse effect and global environmental protection. H2, as a cleaner and more energy-burning fuel, is being considered in many of the practical applications of combustion equipments. However, H2 fuel combustion will still produce pollutant NOx. Thus, the study of NOx emission is one of the most important topics in H2 combustion. With the classical counterflow burner, a numerical simulation of NO emission was carried out on the H2 diffusion flames. The stretch effects on flame temperature, NO concentration, and EINO are analyzed; the contribution of different NO generation routs are also quantified and analyzed. The major parameters influencing NO emission are flame temperature, radical concentration, and residence time, the observed relationship between stretch rate and NO emission is explained with the three major parameters.

Analysis of NO Formation in Counterflow Premixed Hydrogen-Air Flame

Though hydrogen fuel reduces the carbon dioxide emission, it still produces NOx. However, gaps exist in the fundamental understanding of hydrogen-air combustion and the NO emission; most previous research has focused on the flames burning with mixture such as H2 mixed with CH4, rather than pure H2 flame. Here, a computational study is presented to investigate the effects of stretch rate on NO formation in counter-flow premixed hydrogen-air flame. The simulations of premixed hydrogen flames were performed with OPPDIF code with UCSD chemical mechanism. Results indicate that the NO formation is affected by three factors: radical concentration, flame temperature, and residence time of reactants. The flame temperature, the reaction rate of NO, and the NO emission index all decrease when the stretch rate increases. Moreover, the formation of NO through thermal mechanism, NNH mechanism, and N2O mechanism are discussed, as well as the percentages of their contribution.

Measurements of the Combustion Characteristics of Early Premixed Flames Inside Closed-Vessel Combustion

The behavior of the early phase of combustion in spark-ignition (SI) engines is very crucial, since cycle-by-cycle dispersion in combustion, and hence pressure development, originates during this phase. A fundamental experimental study has been undertaken to understand and establish the behavior of early quasisteady premixed laminar flames, or generally called, early flames that exist in practical SI engines. A new, specially designed, closed-volume, spherical vessel test cell with central ignition and optical access was designed, built, and used to undertake experimental measurements for laminar premixed flame growth. A novel multiple-zones mathematical model was implemented to develop a new method for calculating the key properties of early flame behavior inside a closed vessel. Laminar premixed combustion experiments were undertaken to measure, quantify, and decouple the effects of initial fuel mixture equivalence ratio (φ = 0.8 to φ = 1.4), pressure (0.5 bar, 1 bar, 2 bars, and 3.5 bars), and temperature (298 K and 425 K) on the key characteristics of early laminar flame kernel such as timing (t) and rate of change of first 5% mass fraction burned (MFB – X 5%), flame radius (R f ), flame growth rate (dR f /dt), laminar burning velocity (S u ), flame stretch (α), flame cellularity, and unburned gas temperature and pressure. The study showed that the timings of X 5% and the rate of mass fraction burned (dX/dt) had been significantly affected by the variation in equivalence ratio, initial temperature, and pressure. Flame stretch rates during early flame growth were very high but were found to decline nonlinearly to lower, steadier values after R f > 2.5 cm. The onset of cellularity or autoturbulence was found to trigger at a critical radius (R fcr ) in the laminar premixed flames causing rapid increase in its S u and dR f /dt.

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.

Thermal and Scalar Dissipation Rates of Stretched Cylindrical Diffusion Flame

Structures of stretched cylindrical diffusion flame with large curvature were investigated experimentally. Temperature distributions of fuel-diluent mixture/air flames were measured. Fuel was diluted with diluent gases (N2, Ar and He) in order to set the Lewis number Le～1. Fuel (Air) was supplied from inside (outside) of the cylindrical flame. Thermal dissipation rates were calculated from measured temperature distribution. In the case of Le=1, the scalar dissipation rate is proportional to the thermal dissipation rate. Therefore, the stoichiometric scalar dissipation rate was evaluated using the maximum value of the thermal dissipation rates obtained from each temperature distribution. Generally, it is known that the scalar dissipation rate of the counterflow flat diffusion flame increases as the flame stretch rate increases. However, in the case of the cylindrical diffusion flames, the scalar dissipation rate has a maximum value or decreases with the stretch rate.

G061066 伸長円筒状拡散火炎の温度消散率とスカラー散逸率([G061-06] 熱工学部門,一般セッション(6) : ふく射,燃焼(2))

Structures of stretched cylindrical diffusion flame with large curvature were investigated experimentally. Temperature distributions of fuel-diluent mixture/air flames were measured. Fuel was diluted with diluent gases (N2, Ar and He) in order to set the Lewis number Le〜 1. Fuel (Air) was supplied from inside (outside) of the cylindrical flame. Thermal dissipation rates were calculated from measured temperature distribution. In the case of Le= 1, the scalar dissipation rate is proportional to the thermal dissipation rate. Therefore, the stoichiometric scalar dissipation rate was evaluated using the maximum value of the thermal dissipation rates obtained from each temperature distribution. Generally, it is known that the scalar dissipation rate of the counterflow flat diffusion flame increases as the flame stretch rate increases. However, in the case of the cylindrical diffusion flames, the scalar dissipation rate has a maximum value or decreases with the stretch rate.

Discrete Reversible Transitions in Titin at Low Forces

Titin is a giant filamentous protein which, due to its sarcomeric arrangement, molecular structure and elasticity, determines the passive mechanical properties of muscle. At low, physiologically relevant passive forces (<30 pN/molecule) titin is thought to extend continuously at the expense of reducing the configurational entropy of its tandem-Ig regions and PEVK domain. Structural transitions, such as Ig-domain unfolding, are thought to begin only at forces well exceeding the physiological regime, typically above 100 pN. To investigate the detail in the mechanical behavior of titin in the low-force regime, we manipulated individual native skeletal-muscle titin molecules by using custom-built optical tweezers operating at high spatial and temporal resolution in the constant-velocity or constant-force (force-clamp) mode.Titin molecules, purified from rabbit longissimus dorsi, were captured with two beads, one coated with the T12 sequence-specific antibody, and the other with the photoreactive cross-linker sulfo-SANPAH. One of the beads was captured in the optical trap, while the other with a micropipette, the position of which was either moved with a constant rate or adjusted rapidly so as to maintain constant force. At constant, physiologically relevant stretch rates (200-500 nm/s) we observed discrete, step-like (∼20-60 nm) structural changes at forces well below 30 pN, prior to the onset of globular-domain unfolding. In force-clamp stretch experiments these transitions decayed with a high-rate kinetic component. The low-force discrete transitions disappeared in the immediately subsequent mechanical cycle but reappeared after a rest period, indicating that they contribute to titin's mechanical fatigue. This fatigue, previously attributed to the PEVK domain, thus occurs by mechanically wearing out distinct structural elements, suggesting that long-range interactions stabilize this putative random domain. The time- and force-scales suggest that the observed discrete structural transitions may play an important role in sarcomeric stress adaptation.

A Study of the Relationship between Fabric Property and Clothing Pressure of Knitted Underwear

The objective of this study was to investigate the relationship between fabric property and Clothing Pressure of knitted underwear, using objective pressure evaluates method. Eight kinds of fabric for knitted underwear were tested on the stretching property and elasticity property, and then eight same style of underwear which were made by Eight kinds of fabric were tried on by pressure dummy. Clothing pressure of seven different points was measured and recorded. The linear relationship between fabric property and Clothing Pressure of knitted underwear was obtained. Results showed that Clothing Pressure of knitted underwear was negative correlation with elastic recovery rate of knitted fabric, and was positive correlation with tensile stretch rate.

Nanomechanics of the MPMV Capsid-Protein-Binding RNA Fragment

The interaction between RNA and binding proteins plays an important role in the structure, stability, function and dynamics of either molecular species. The Mason-Pfizer monkey retrovirus (MPMV) is an ideal model for investigating RNA-protein interactions, because part of the genomic RNA, called the packaging signal, is influenced by the binding of its capsid proteins. To explore the properties of MPMV RNA, we manipulated individual molecules of the packaging-signal sequence by using force-measuring optical tweezers.The 207-base-long segment of MPMV RNA corresponding to the packaging signal, extended on each side with 1200-base-long indifferent gene segments for mechanical handling, was cloned into a pET28a vector, then expressed in an in vitro transcription system. Complementary DNA strands were hybridized to this RNA in a thermal-ramp protocol so as to obtain RNA/DNA handle ends. The complex was then manipulated in repetitive stretch and relaxation cycles across a force range of 0-80 pN under Mg-free conditions. At a stretch rate of 250 nm/s force increased monotonically and non-linearly. Above 15 pN a two-step transition was typically observed, corresponding to the unfolding of the RNA molecule. The total length gain (∼100 nm) associated with the process is in good agreement with the theoretical contour length. The transitions were not reversible during relaxation with a rate of 250 nm/s, suggesting that the force-dependent refolding kinetics are slow in comparison with that of the experiment. However, upon introducing a ∼30-second pause before the second mechanical cycle, the unfolding transitions reappeared. Our findings suggest that the packaging signal forms a complex, stable secondary structure that unfolds in at least two major steps. Whether the kinetics of the transitions are influenced by the presence of capsid proteins is a subject of further detailed investigations.

Anomalous blow-off behavior of laminar inverted flames of ultra-lean hydrogen–methane–air mixtures

An experimental study of rod-stabilized laminar inverted flames of ultra-lean hydrogen–methane–air mixtures has been performed. For mixtures with high hydrogen content, anomalous stabilization and blow-off behavior has been observed. Flames in those mixtures could be stabilized at equivalence ratios below the lean flammability limit for a zero-stretch planar flame. Stabilization of such flames was possible only when the mixture velocity exceeded some critical value. Flames were blown off when the mixture velocity was reduced below this value. The stand-off distance above the flame holder for those flames decreased and heat transfer from the flame base to the flame holder became more intense when the mixture velocity was increased. This is opposite to the regular behavior of inverted flames. These observed unusual phenomena were attributed to the combination of a strong flame stretch and preferential diffusion effects and to the negative value of the Markstein length in mixtures with high hydrogen content. According to the suggested explanation, increasing the velocity results in an increase of the flame stretch rate at the flame base. This, in turn leads to higher flame temperatures and higher burning velocities, making the survival of the flame below the flammability limits possible. Along with the anomalous blow-off behavior, normal blow-off occurring at increased velocity was observed for mixtures with high hydrogen content at the lowest tested equivalence ratios. The observation of the flame shape evolution showed that, when the normal blow-off limit is approached, a flame narrows slightly above the flame base, forming a “neck”. The flame fronts merge at the “neck” location and flame breaks there, leading to complete flame extinction or leaving a very small flamelet near the flame holder. It is suggested that the flame local extinction in that case occurs as the result of the excessive flame stretch at the flame “neck” which leads to the flame fronts merging and to the incomplete reaction.

Experimental determination of laminar burning velocity for butanol/iso-octane and ethanol/iso-octane blends for different initial pressures

The comparison of butanol/isooctane blends with an ethanol/isooctane blend was characterized with respect to laminar combustion, by using the spherical expanding flame methodology, in a constant volume vessel. This paper presents the results obtained for blends with alcohol concentrations of 0%, 25%, 50%, 75% and 100% (in volume) and an equivalence ratio ranging from 0.7 to 1.4, at different initial pressures (0.1, 0.3, 0.5 and 1.0MPa) and an initial temperature of 423K. The addition of alcohol to isooctane increases the laminar burning velocity, but increasing the initial pressure limits this increase. From burned gas Markstein length estimates, the flame of ethanol/isooctane and air mixture seems to be less sensitive to the total stretch rate and thermo-diffusive instabilities. From experimental data, a correlation with laminar burning velocity is suggested as a function of initial pressure, equivalence ratio and alcohol concentration.

Stretch and Wrap Style Tourniquet Effectiveness With Minimal Training

The objective was to determine if proper application of the Stretch, Wrap, and Tuck Tourniquet (SWAT-T) would stop arterial flow and would occur with minimal training. Fifteen undergraduates watched a 19 second video three times, practiced twice, and applied the tourniquet to volunteers at 10 locations: 3 above the elbow or knee and 2 below. Successful occlusion (60 second Doppler signal elimination) was more frequent than proper stretch (96 versus 75), more frequent on arms than legs (59 versus 37), and achieved before completed application (16 +/- 8 versus 33 +/- 8 seconds; each p < 0.05). Proper stretch (correct alteration of shapes printed on the tourniquet) was more frequent on legs than arms (30 versus 45; p <0.05). Applications were rated Easy (101), Challenging (37), Difficult (12) with discomfort None (53), Little (62), Moderate (34), Severe (1). The 8 appliers with <70% proper stretch rates received 10 minutes additional training and then retested at mid upper arm, mid-thigh, and below knee (24 applications) for improved proper stretch and occlusion (5 versus 18 and 10 versus 20; p < 0.01). Proper application of the SWAT-T is easy and can stop extremity arterial flow but requires some training for many appliers.

Unified constitutive modeling of rubber-like materials under diverse loading conditions

This paper presents a new constitutive model that unifies the behavioral characterizations of rubber-like materials in a broad range of loading regimes. The proposed model combines a selection of existing components that are known to reflect, with suitable accuracy, two fundamental aspects of rubber behavior in finite strain: (i) rate-independent softening under deformation, also known as the Mullins effect, and (ii) hyper-viscoelasticity, including at high strain rates. The evolution model is further generalized to account for multiple rates of internal dissipation (or material time-scales). Suitable means of identifying the system’s parameters from simple uniaxial extension tests are explored. Several aspects of the model’s behavior are shown in virtual experiments of uniaxial extension, at different stretch rates. A possible directional approach extending the model to handle softening induced anisotropy is briefly 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.