Stretch Rate Research Articles

Laminar burning velocities and Markstein numbers of syngas–air mixtures

In the currently reported work, three typical mixtures of H 2, CO, CH 4, CO 2 and N 2 have been considered as representative of the producer gas coming from wood gasification. Laminar burning velocities have been determined from schlieren flame images at normal temperature and pressure, over a range of equivalence ratios within the flammability limits. The study of the effects of flame stretch rate was also performed. Combustion demonstrates a linear relationship between flame radius and time for syngas–air flames. The maximum value of syngas–air flame speeds is observed at the stoichiometric equivalence ratio, while lean or rich mixtures have lower flame speeds. The higher is the syngas heat value the higher is the laminar burning velocity of the syngas mixture. Markstein numbers show that typical syngas–air flames are generally unstable. Karlovitz numbers indicates that typical syngas–air flames are little influenced by stretch rate. Based on the experimental data, a formula for calculating the laminar burning velocities of syngas–air flames is proposed. The magnitude of laminar burning velocity for typical syngas compositions is comparable to that of a simulated mixture comprising 5% H 2/95% CO and proved to be similar to methane, although somewhat slower than propane.

Observing the chain stretch transition in a highly entangled polyisoprene melt using transient extensional rheometry

We measure the viscoelastic properties of a highly entangled narrow molecular weight polyisoprene melt with approximately 280 entanglements per chain in steady and transient shear and in elongational flows. The storage and loss moduli of the melt are found to be well described by the Milner and McLeish model. The relaxation modulus G(t,γ) is measured using stress relaxation after a sudden shearing displacement and we experimentally determine the Rouse time τR by observing strain-time separability G(t,γ)=G(t)h(γ) for t>τR. The transient elongational properties are measured using three distinct instruments: the Sentmanat extensional rheometer (SER) universal testing platform from Xpansion Instruments, its counterpart, the extensional viscosity fixture (EVF) from TA Instruments, and a filament stretching rheometer (FSR). The kinematics obtained in each device is sensitive to the aspect ratio of the sample and care must be taken to achieve homogeneous deformation conditions. Especially for the SER and EVF instruments, a second aspect ratio associated with the rectangular cross-section of the sample is also important. We find that the initial growth in the tensile stress follows the prediction given by the Doi–Edwards reptation model for Deborah numbers based on the Rouse time less than about DeR=0.04. For DeR=0.04, the stress difference follows more or less the Doi–Edwards prediction in the limit of infinite stretch rates and, for DeR>0.04, the measured stresses are well above those that can be predicted by the basic Doi–Edwards model. When DeR>1, the stress difference also exceeds the linear viscoelastic prediction. In conjunction with this strain-hardening response, a stabilization is obtained whereby the limiting Hencky strain before sample rupture is markedly increased. We compare our observations in the regime 0.04<DeR<1 with available experiments and theories. The stabilization for DeR>1 is interpreted as a signature of chain stretching for elongational deformation rates faster than the inverse Rouse time.

In Vitro Stretches of the Mitral Valve Anterior Leaflet Under Edge-to-Edge Repair Condition

Mitral valve edge-to-edge repair (ETER) alters valve mechanics, which may impact efficacy and durability of the repair. The objective of this paper was to quantify stretches in the central region of the anterior leaflet of the mitral valve after ETER with a single suture and 6 mm suture. Sixteen markers, forming a 4x4 array, were attached onto the central region of the mitral valve anterior leaflet. The mitral valve was subjected to ETER with a single suture and 6 mm suture, and mounted in an in vitro flow loop simulating physiological conditions. Images of the marker array were used to calculate marker displacement and stretch. A total of 9 mitral valves were tested. Two peak stretches were observed during a cardiac cycle, one in systole and the other in diastole under mitral valve edge-to-edge repair condition. The major principal (radial) stretch during systole was significantly greater than that during diastole. However, there was no significant difference between the minor principal (circumferential) stretch during diastole and that during systole. In addition, there were no significant differences in the radial and circumferential, or areal stretches and stretch rates during diastole between the single suture and 6 mm suture. The ETER subjects the mitral valve leaflets to double frequency of loading and unloading. Minor change in suture length may not result in a significant load difference in the central region of the anterior leaflet during diastole.

Rarefied flow effects on stabilization and extinction of rotating-disk flame at low pressures

An activation energy asymptotic analysis with one-step overall reaction was performed for the stabilization and extinction of a premixed flame over a rotating disk at sufficiently low pressures, for its relevance in low-pressure CVD (chemical vapor deposition) operations in which the flow is weakly rarefied. Extinction criteria based on the critical Damköhler number were obtained through the S-curve concept, parametrically demonstrating the influence of the CVD operating conditions, such as the spin rate and temperature of the disk, on flame extinction. It is further shown that, while decreasing pressure and hence the reactivity of the mixture tends to extinguish the flame, the trend can be substantially weakened by taking into account of the influence of the Knudsen layer, which reduces the heat loss to the disk as well as the flow stretch rate at the flame.

Experimental study of limit lean methane/air flame in a standard flammability tube using particle image velocimetry method

Lean limit methane/air flame propagating upward in a standard 50 mm diameter and 1.8 m length tube was studied experimentally using particle image velocimetry method. Local stretch rate along the flame front was determined by measured gas velocity distributions. It was found that local stretch rate is maximum at the flame leading point, which is in agreement with earlier theoretical results. Similar to earlier observations, extinction of upward propagating limit flame was observed to start from the flame top. It is stated that the observed behavior of the extinction of the lean limit methane/air flame can not be explained in terms of the coupled effect of flame stretch and preferential diffusion. To qualitatively explain the observed extinction behavior, it is suggested that the positive strain-induced flame stretch increases local radiation heat losses from the flame front. An experimental methodology for PIV measurements in a round tube is described.

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.

Behavior of Extinction Limits of Lean Premixed Counterflow Flames at Elevated Temperatures

The effects of unburned gas temperature (Tu), mole fraction of oxygen (C(O2)) in a mixture and pressure on the extinction limit of lean CH4/O2/N2 premixed counterflow flames were experimentally and numerically investigated. Increments of extinction stretch rates with an increase in Tu were measured at a range of 300 K < Tu < 700 K, and those with an increase in C(O2) were measured at a range of 17% < C(O2) < 22%. The extinction stretch rate of mixtures with low C(O2) decreased with pressure. This decreasing trend changed into an increasing trend with increasing Tu or C(O2). When the density-weighted stretch rate was plotted in place of the normal stretch rate, the extinction limit monotonically expanded with pressure even for weakly burning flames. Moreover, the study investigated the applicability of the local Karlovitz number (KaL) and the density-weighted stretch rate to the global description of behavior of stretched flames in various conditions. The KaL remained at almost unity in wide ranges of Tu and C(O2).

Effects of Mechanical Stretch on Collagen and Cross-Linking in Engineered Blood Vessels

It has been shown that mechanical stimulation affects the physical properties of multiple types of engineered tissues. However, the optimum regimen for applying cyclic radial stretch to engineered arteries is not well understood. To this end, the effect of mechanical stretch on the development of engineered blood vessels was analyzed in constructs grown from porcine vascular smooth muscle cells. Cyclic radial distension was applied during vessel culture at three rates: 0 beats per minute (bpm), 90 bpm, and 165 bpm. At the end of the 7-week culture period, harvested vessels were analyzed with respect to physical characteristics. Importantly, mechanical stretch at 165 bpm resulted in a significant increase in rupture strength in engineered constructs over nonstretched controls. Stress-strain data and maximal elastic moduli from vessels grown at the three stretch rates indicate enhanced physical properties with increasing pulse rate. In order to investigate the role of collagen cross-linking in the improved mechanical characteristics, collagen cross-link density was quantified by HPLC. Vessels grown with mechanical stretch had somewhat more collagen and higher burst pressures than nonpulsed control vessels. Pulsation did not increase collagen cross-link density. Thus, increased wall thickness and somewhat elevated collagen concentrations, but not collagen cross-link density, appeared to be responsible for increased burst strength.

Theoretical and numerical analysis of a spreading opposed-flow diffusion flame

This article describes the macroscopic and microscopic features of flames spreading over solid-fuel surfaces by examining and comparing three models. The first model examines ignition and flame spread over a solid-fuel surface using a two-dimensional numerical simulation code. This model employs variable density, variable thermophysical properties and one-step global finite-rate chemistry. The second model, a macroscopic ‘field’ model, is solved in terms of the mixture fraction ( Z ) and total enthalpy ( H ) functions. Comparisons are made with numerical predictions for primitive quantities: temperature, species distributions and velocity fields; and derived quantities: heat flux, mass flux, mixture fraction, enthalpy function and flame stretch rate. The third model yields a ‘localized’ flame structure description near the flame attachment point. Theoretical formulas are produced for the quenching distance, the leading edge heat flux, and the flame structure, as characterized by reactivity, temperature field and species distributions. The analytical predictions are compared with numerical simulations to derive flame microstructure scaling parameters.

Flame-synthesis limits and self-catalytic behavior of carbon nanotubes using a double-faced wall stagnation flow burner

Flame-synthesis limits of carbon nanotubes (CNTs) are measured using a double-faced wall stagnation flow (DWSF) burner that shows potential in mass production of CNTs. With nitrogen-diluted premixed ethylene–air flames established on the nickel-coated stainless steel double-faced plate wall, the limits of CNT formation are determined using field-emission scanning and transmission electron microscopies and Raman spectroscopy. Also, self-catalytic behavior of the synthesized CNTs is evaluated using the DWSF burner with a CNT-deposited stainless steel double-faced plate wall. Results show narrow fuel–equivalence ratio limits of multi-walled CNT (MWCNT)-synthesis at high flame stretch rates and substantially extended limits at low flame stretch rates. This implies that the synthesis limits are very sensitive to the fuel–equivalence ratio variation for the high stretch rate conditions, yielding a lot of impurities and soot rather than MWCNTs. The enhanced ratio of tube inner diameter to wall thickness of the MWCNTs synthesized using a CNT self-catalytic flame-synthesis process is observed, indicating that the quality of metal-catalytic, flame-synthesized MWCNTs can be much improved via the process. Thus, using a DWSF burner with the CNT self-catalytic process has potential in mass production of MWCNTs with improved quality.

Combustion and the design of future engine fuels

The fast diminution of readily extractable sources of fossil fuels, particularly oil, and concerns about global warming are leading to the creation of many new potential fuels. Their practicability must be assessed in terms of a wide range of physico-chemical properties, in relation to the operational aerodynamics in different engines. This article concentrates on those properties related to combustion and these are discussed in detail for some fuels with contrasting properties. Intrinsic fuel properties include volumetric energy, vapour pressure, heat of reaction, latent enthalpy of vaporization, and the relative volumes of energy that engines can breathe. Important combustion properties include the minimum ignition energy, laminar burning velocity, Markstein numbers for strain and curvature, flame extinction stretch rates for positive and negative stretch, stretch factor for flame instability, turbulent flame burning and quenching, autoignition delay time, excitation time for autoignition heat release, research and motor octane numbers and cetane number (CN). Such properties are manifest in a variety of aerodynamic contexts: for example, the nature of autoignition depends on spatial reactivity gradients and the acoustic speed. Particular problems can arise in characterizing engine knock when operational regimes are outside those in which the octane and CNs are determined. The approach adopted in this article does not assume any unique alignment between fuels, mode of combustion, and power unit, and different possibilities are discussed.

Numerical Study of Thickness Distribution of Stretch-Blow Bottles for Defects Prediction

Nowadays, polyethylene terephthalate (PET) bottles have been increasingly used as drink containers. They are usually manufactured by a stretch-blow molding process. The improper parameters set in the stretch blow molding process may lead to many defects in the stretch-blow bottle. Finite Element (FE) simulations of the forming process were performed in this paper. The influences of the technological parameters, such as the balance between stretching and blowing rate, the movement of the stretch rod and the inflation pressure, were studied. As a result, the defects, such as over-thin area, cracking and deformation, can be predicted by this method. Especially, it is shown that the cracking in the bottom of products may result from the improper values of the dwell time and the stretch rate. The trends shown by the simulation results are in good agreement with the experimental results. The method can be applied to predict the probable defects, assess the structural properties, and optimize the processing parameters of the stretch blow molding process.

Nonlinear effects in the extraction of laminar flame speeds from expanding spherical flames

Various factors affecting the determination of laminar flames speeds from outwardly propagating spherical flames in a constant-pressure combustion chamber were considered, with emphasis on the nonlinear variation of the stretched flame speed to the flame stretch rate, and the associated need to nonlinearly extrapolate the stretched flame speed to yield an accurate determination of the laminar flame speed and Markstein length. Experiments were conducted for lean and rich n-butane/air flames at 1 atm initial pressure, demonstrating the complex and nonlinear nature of the dynamics of flame evolution, and the strong influences of the ignition transient and chamber confinement during the initial and final periods of the flame propagation, respectively. These experimental data were analyzed using the nonlinear relation between the stretched flame speed and stretch rate, yielding laminar flame speeds that agree well with data determined from alternate flame configurations. It is further suggested that the fidelity in the extraction of the laminar flame speed from expanding spherical flames can be facilitated by using small ignition energy and a large combustion chamber.

Explosion bomb measurements of ethanol–air laminar gaseous flame characteristics at pressures up to 1.4 MPa

The principal burning characteristics of a laminar flame comprise the fuel vapour pressure, the laminar burning velocity, ignition delay times, Markstein numbers for strain rate and curvature, the stretch rates for the onset of flame instabilities and of flame extinction for different mixtures. With the exception of ignition delay times, measurements of these are reported and discussed for ethanol–air mixtures. The measurements were in a spherical explosion bomb, with central ignition, in the regime of a developed stable, flame between that of an under or over-driven ignition and that of an unstable flame. Pressures ranged from 0.1 to 1.4 MPa, temperatures from 300 to 393 K, and equivalence ratios were between 0.7 and 1.5. It was important to ensure the relatively large volume of ethanol in rich mixtures at high pressures was fully evaporated. The maximum pressure for the measurements was the highest compatible with the maximum safe working pressure of the bomb. Many of the flames soon became unstable, due to Darrieus–Landau and thermo-diffusive instabilities. This effect increased with pressure and the flame wrinkling arising from the instabilities enhanced the flame speed. Both the critical Peclet number and the, more rational, associated critical Karlovitz stretch factor were evaluated at the onset of the instability. With increasing pressure, the onset of flame instability occurred earlier. The measured values of burning velocity are expressed in terms of their variations with temperature and pressure, and these are compared with those obtained by other researchers. Some comparisons are made with the corresponding properties for iso-octane–air mixtures.

Measurements of laminar burning velocities and flame stability analysis for dissociated methanol–air–diluent mixtures at elevated temperatures and pressures

The laminar burning velocities and Markstein lengths for the dissociated methanol–air–diluent mixtures were measured at different equivalence ratios, initial temperatures and pressures, diluents (N 2 and CO 2 ) and dilution ratios by using the spherically outward expanding flame. The influences of these parameters on the laminar burning velocity and Markstein length were analyzed. The results show that the laminar burning velocity of dissociated methanol–air mixture increases with an increase in initial temperature and decreases with an increase in initial pressure. The peak laminar burning velocity occurs at equivalence ratio of 1.8. The Markstein length decreases with an increase in initial temperature and initial pressure. Cellular flame structures are presented at early flame propagation stage with the decrease of equivalence ratio or dilution ratio. The transition positions can be observed in the curve of flame propagation speed to stretch rate, indicating the occurrence of cellular structure at flame fronts. Mixture diluents (N 2 and CO 2 ) will decrease the laminar burning velocities of mixtures and increase the sensitivity of flame front to flame stretch rate. Markstein length increases with an increase in dilution ratio except for very lean mixture (equivalence ratio less than 0.8). CO 2 dilution has a greater impact on laminar flame speed and flame front stability compared to N 2 . It is also demonstrated that the normalized unstretched laminar burning velocity is only related to dilution ratio and is not influenced by equivalence ratio.

Numerical evaluation of NOx mechanisms in methane-air counterflow premixed flames

The control of nitrogen oxides (NOx) has been a major issue in designing combustion systems, since NOx play a key role in ozone depletion and the generation of photochemical smog. The characteristics of NOx emission can be essential information for the development of a clean combustor having suitable reduction methodologies. In the present study, NOx emission characteristics were evaluated numerically, accounting for the effect of equivalence ratio, stretch rate, pressure, and initial temperature. In general, peak NOx emission appeared near the equivalence ratio of unity case, and NOx emission increased with pressure and initial temperature due to the temperature sensitivity in NOx mechanism. NOx decreased with stretch rate due to the decrease in residence time in high temperature region. Furthermore, the thermal and prompt mechanisms were evaluated with equivalence ratio for two calculation methods. The conventional methods ignore the interaction of coupled mechanism of thermal and prompt NOx. The reaction path diagram was introduced to understand effective reaction pathways in various conditions.

Improvement of flame stability and NOx reduction in hydrogen-added ultra lean premixed combustion

Lean premixed combustion is a well known method in gas turbine combustors that can reduce fuel consumption and decrease flame temperature. In lean premixed flames, flame instabilities can occur because the combustion takes place near the lean flammable limit. For the purpose of increasing flame stability, a small amount of hydrogen was added into a fuel, which has ultra low lean flammable limit. The extinction stretch rate increased and total equivalence ratio at extinction decreased with hydrogen addition; consequently, ultra lean premixed combustion was possible and flame stability could be achieved at low temperature conditions. The NOx emission increased with hydrogen addition for the same stretch rate and equivalence ratio, but the extinction stretch rate and lean flammability limit was enlarged. Consequently, NOx emission decreased with hydrogen addition in the near extinction conditions. Hydrogen addition could improve flame stability and reduce NOx emission in ultra lean premixed combustion.

Measurement of laminar burning velocities and analysis of flame stabilities for hydrogen-air-diluent premixed mixtures

The laminar burning velocities and Markstein lengths of the hydrogen-air-diluent mixtures were measured at different equivalence ratios (0.4–1.5), different diluents (N2, CO2 and 15%CO2+85%N2) and dilution ratios (0, 0.05, 0.10 and 0.15) by using the outwardly expanding flame. The influences of flame stretch rate on the flame propagation characteristics were analyzed. The results show that both the laminar burning velocities and the Markstein lengths of the hydrogen-air-diluent mixtures decrease with the increase of dilution ratio. The decrease in Markstein lengths means that adding diluents into the hydrogen-air mixtures will decrease the diffusional-thermal instability of the flame front. For a specified dilution ratio, the laminar burning velocities give their maximum values at an equivalence ratio of 1.8. The Markstein lengths increase with the increase of the equivalence ratio monotonously regardless of the diluents. The study shows that CO2 as the diluent has a greater impact on the laminar flame speed and the flame front stability than N2 as the diluent.

Characteristics of Pyrolysis and Combustion of Polymers in Stagnation-Point Flow for Preheated and Diluted Air with H2O and CO2

Combustion experiments on polypropylene (PP) and polyethylene terephethalate (PET) in stagnation-point flow were performed to investigate the fundamental characteristics of these polymers in high-temperature air combustion (HiTAC). Numerical study was also performed to estimate the regression rate and to determine the kinetic parameter of pyrolysis. In the experiments on PP combustion, extinction limits and sooting limits were found to be extended when highly preheated air was used. In the case of H2O and CO2 dilutions, the dilution enhanced regression rates at low stretch rates. In the case of PET combustion, results indicated that regression rates depended on the production rate of char. The kinetic parameters of PP pyrolysis under the combustion conditions were estimated using a new method in which experiments and numerical simulations were combined. The regression rates calculated using the kinetic parameters obtained in the present study were in good agreement with those of the experiments in contrast with the numerical results using the kinetic parameters obtained using the previously reported thermal gravimetric analysis (TGA).

Laminar burning velocities at high pressure for primary reference fuels and gasoline: Experimental and numerical investigation

Spherical flames of n-heptane, iso-octane, PRF 87 and gasoline/air mixtures are experimentally investigated to determine laminar burning velocities and Markstein lengths under engine-relevant conditions by using the constant volume bomb method. Data are obtained for an initial temperature of 373 K, equivalence ratios varying from ϕ = 0.7 to ϕ = 1.2 , and initial pressures from 10 to 25 bar. To track the flame front in the vessel a dark field He–Ne laser Schlieren measurement technique and digital image processing were used. The propagating speed with respect to the burned gases and the stretch rate are determined from the rate of change of the flame radius. The laminar burning velocities are obtained through a linear extrapolation to zero stretch. The experimentally determined Markstein numbers are compared to theoretical predictions. A reduced chemical kinetic mechanism for n-heptane and iso-octane was derived from the Lawrence Livermore comprehensive mechanisms. This mechanism was validated for ignition delay times and flame propagation at low and high pressures. In summary an overall good agreement with the various experimental data sets used in the validation was obtained.