High Stretch Rate Research Articles

Stretch triggers microtubule stabilization and MARCKS-dependent membrane incorporation in the shaft of embryonic axons

Neurons have a unique polarized nature that must adapt to environmental changes throughout their lifespan. During embryonic development, axon elongation is led by the growth cone,1 culminating in the formation of a presynaptic terminal. After synapses are formed, axons elongate in a growth cone-independent manner to accompany body growth while maintaining their ultrastructure and function.2,3,4,5,6 To further understand mechanical strains on the axon shaft, we developed a computer-controlled stretchable microfluidic platform compatible with multi-omics and live imaging. Our data show that sensory embryonic dorsal root ganglia (DRGs) neurons have high plasticity, with axon shaft microtubules decreasing polymerization rates, aligning with the direction of tension, and undergoing stabilization. Moreover, in embryonic DRGs, stretch triggers yes-associated protein (YAP) nuclear translocation, supporting its participation in the regulatory network that enables tension-driven axon growth. Other than cytoskeleton remodeling, stretch prompted MARCKS-dependent formation of plasmalemmal precursor vesicles (PPVs), resulting in new membrane incorporation throughout the axon shaft. In contrast, adolescent DRGs showed a less robust adaptation, with axonal microtubules being less responsive to stretch. Also, while adolescent DRGs were still amenable to strain-induced PPV formation at higher stretch rates, new membrane incorporation in the axon shaft failed to occur. In summary, we developed a new resource to study the biology of axon stretch growth. By unraveling cytoskeleton adaptation and membrane remodeling in the axon shaft of stretched neurons, we are moving forward in understanding axon growth.

Spherically expanding flame simulations using Cantera coupled to an unsteady Lagrangian formulation

A Lagrangian-based one-dimensional approach has been applied, using Cantera libraries, to study the dynamics of spherically expanding flames. Detailed reaction models have been employed to examine spherical flame propagation in hydrogen-air mixtures. In the first part, our approach was validated against laminar flame speed and Markstein number data from the literature. It was shown that the laminar flame speed was predicted within 5% on average but that significant discrepancies were observed for the Markstein number, especially for rich mixtures. This suggests that even application of detailed reaction models can lead to inaccurate prediction of the stability of rich flames, which can be detrimental to the prediction of flame acceleration and the deflagration to detonation transition process. In the second part, a detailed analysis of the thermo-chemical dynamics along the path of Lagrangian particles propagating in stretched flames was performed. For mixtures with negative Markstein lengths, it was found that at high stretch rates, the mixture entering the reaction-dominated period is less lean with respect to the initial mixture than at low stretch rates. This induces faster rates of chemical heat release and active radical production which results in higher flame propagation speed. Opposite effects were observed for mixtures with positive Markstein lengths, for which slower flame propagation was observed at high stretch rates compared to low stretch rates.

On the ignition kernel formation and propagation: an experimental and modeling approach

The next generation of advanced combustion devices is being developed to operate under ultra-high-pressure conditions. However, under such extreme conditions, flame tends to become unstable and measurement of fundamental properties such as the laminar flame speed becomes challenging. One potential method to resolve this issue is measuring the ignition-affected region during spherically expanding flame experiments. The flame in this region is more resistant to perturbations and remains smooth due to the high stretch rates (i.e. small radii). Stable flame propagation allows for improved flame measurement, however, the experimentally observed kernel propagation is a function of both inflammation and ignition plasma. Therefore, the goal of the present study is to better understand the plasma formation and propagation during the ignition process, which would allow for reliable laminar flame speed measurements. To accomplish this goal, thermal plasma operating at high pressures is studied with emphasis on the spark energy effects on the formation of the ignition kernel. The thermal effect of the plasma is experimentally observed using a high-speed Schlieren imaging system. The energy dissipated within the plasma is measured with the use of voltage and current probes with a measurement of plasma sheath voltage drop as an input to numerical modeling. The measured kernel propagation rate is used to assess the accuracy of the model. The experiments and modeling are conducted in dry air at 1, 3, and 5 atm as well as in CH4-N2 mixtures at 1 atm, and kernel radius, temperature, and mass are reported. The voltage-drop (as a non-thermal loss) is measured to be approximately 330 5 V (dry air at 1 atm) for glow plasma with a large dependency on pressure, gas composition, electrode surface quality, electrode geometry, electrode shape, and current density. The same loss within the arc plasma is measured to be 15 5 V, however the arc phase loss which agrees with arc propagation is significantly higher (∼45 V) which suggest additional unaccounted for phenomena occurring during the arc phase. With these losses, the modeling results are shown to predict the final kernel radius within 10%–20% of the observed kernel size. The difference found between the modeling and experimental results is determined to be a result of assuming that the primary loss mechanism (voltage drop across sheath formation) remains constant for the duration of glow discharge. The discrepancy for arc discharge is discussed with several potential sources, however, additional studies are required to better understand how the arc formation affects the kernel propagation.

Fracture of dual crosslink gels with permanent and transient crosslinks: Effect of the relaxation time of the transient crosslinks

We investigate the fracture properties of poly(acrylamide-co-1-vinylimidazole) dual crosslink hydrogels [P(AAm-co-VIm)-M2+ gels] containing a small fraction of covalent bonds and a majority of dynamic bonds based on metal coordination bonds (Ni2+ or Zn2+). Unlike a previous study on a different dual crosslink hydrogel system having slower dynamic bonds based on poly(vinylalcohol) and borate ions (PVA-Borax gels), the presence of these faster dynamic coordination bonds has two main effects: They significantly toughen the P(AAm-co-VIm)-M2+ gels even at high stretch rates, where the dynamic bonds should in principle behave as covalent bonds at the crack tip, and they toughen the gels at very low stretch rates, where the dynamic bonds are invisible during the loading stage. We propose two additional molecular mechanisms to rationalize this behavior of P(AAm-co-VIm)-M2+ gels: we hypothesize that fast exchanging dynamic bonds remain slow compared to the characteristic time of bond scission and are, therefore, able to share the load upon covalent bond scission even at low loading rates. We also argue of the existence of longer-lived clusters of dynamic bonds that introduce a stretch rate-dependent strain hardening in uniaxial tension and stabilize and increase the size of the dissipative zone at the crack tip, thereby introducing a strain-dependent dissipative mechanism.

Finite deformation and fractional order viscoelasticity of an auxetic foam

Auxetic foams exhibit novel mechanical properties due to their unique microstructure for improved energy-absorption and cavity expansion applications that have fascinated the scientific community since their inception. Given the advancements in material processing and performance of polymer open cell auxetic foams, there is a strong desire to fully understand the nonlinear rate-dependent deformation of these materials. The influence of nonlinear compressibility is introduced here along with relaxation effects to improve model predictions for different stretch rates and finite deformation regimes. The viscoelastic behavior of the material is analyzed by comparing fractional order and integer order calculus models. All results are statistically validated using maximum entropy methods to obtain Bayesian posterior densities for the hyperelastic, auxetic, and viscoelastic parameters. It is shown that fractional order viscoelasticity provides [Formula: see text]–[Formula: see text] improvement in prediction over integer order viscoelastic models when the model is calibrated at higher stretch rates where viscoelasticity is more significant.

Probing local nonlinear viscoelastic properties in soft materials

Minimally invasive experimental methods that can measure local rate dependent mechanical properties are essential in understanding the behaviour of soft and biological materials in a wide range of applications. Needle based measurement techniques such as Cavitation Rheology (Zimberlin et al., 2007) and Volume Controlled Cavity Expansion (VCCE, Raayai-Ardakani et al. (2019a)), allow for minimally invasive local mechanical testing, but have been limited to measuring the elastic material properties. Here, we propose several enhancements to the VCCE technique to adapt it for characterisation of viscoelastic response at low to medium stretch rates (10−2 - 1 s−1). Through a carefully designed loading protocol, the proposed technique performs several cycles of expansion–relaxation at controlled stretch rates in a cavity expansion setting and then employs a large deformation viscoelastic model to capture the measured material response. Application of the technique to soft PDMS rubber reveals significant rate dependent material response with high precision and repeatability, while isolating equilibrated states that are used to directly infer the quasistatic elastic modulus. The technique is further established by demonstrating its ability to capture changes in the rate dependent material response of a tuneable PDMS system. The measured viscoelastic properties of soft PDMS samples are used to explain earlier reports of rate insensitive material response by needle based methods: it is demonstrated that the conventional use of constant volumetric rate cavity expansion can induce high stretch rates that lead to viscoelastic stiffening and an illusion of rate insensitive material response. We thus conclude with a cautionary note on possible overestimation of the quasistatic elastic modulus in previous studies and suggest that the stretch rate controlled expansion protocol, proposed in this work, is essential for accurate estimation of both quasistatic and dynamic material parameters.

Burning characteristics of PMMA with varied stretch rates under stagnation-point diffusion flames

This study investigated the burning characteristics of PMMA with varied stretch rates under stagnation-point diffusion flames both theoretically and experimentally, focusing on the manner in which the surface energy balance and heat losses affected burning rate with varied stretch rates. The burning rate versus stretch rate was firstly investigated by introducing a modified Spalding B number, where the burning rate was proved to equal the buoyancy or forced flow induced stretch rate to the power of 0.5. At lower stretch rate, burning was largely affected by heat losses, associated with a lower B number, and decreased up to the quenching limit, similar to the microgravity results. At higher stretch rate, burning was controlled by flame radiation with a larger B number and increased burning rate. Additionally, ice bath increased the solid in-depth conduction heat loss due to the lower temperature maintained at the upper surface of the sample, and further influenced the energy balance with a larger heat losses fraction, leading to reduced B number and burning rate. This study provided an insightful investigation of the burning behaviors of stagnation-point diffusion flames with low stretch rates, which was used in a terrestrial environment to simulate microgravity flames.

Study on counterflow premixed flames using high concentration ammonia mixed with methane

In the present study, combustion characteristics of ammonia/methane were evaluated computationally utilizing counterflow premixed ammonia/methane/air flames at normal temperature and pressure conditions. Investigations were carried out over a wide range of flame strain rates, fuel blend compositions and equivalence ratios. Extinction characteristic results show that methane addition and equivalence ratio have significant impact on the extinction limits of the stretched flames. With more methane in the ammonia/methane fuel blends, the flame sustainability to high stretch rate substantially increases. Flame structures of the stretched flames were then investigated to have a deep insight into the combustion process of the ammonia/methane premixed combustion. Results show that temperature, heat release rate and important intermediate radicals of the flames are enhanced. However, the considerably increased carbon oxides and nitrogen oxides emissions suggest emission reduction is a key issue in the ammonia/methane combustion process. Pathway and sensitivity analyses reveal the chemistry of NOx emission characteristics of different ammonia/methane flames.

Effect of temperature on the rupture behavior of highly stretchable acrylic elastomer

Dielectric elastomer has been recently explored extensively to make diverse soft actuators and energy harvesting devices. The lack of study on the rupture behavior under the influence of temperature hinders further applications where heat generation and accumulation are unavoidable. In this paper, an experimental study has been carried out to investigate the effect of temperature on the rupture behavior of acrylic dielectric elastomer. By using VHB 4910 films with and without an initial crack, the fracture energy at different temperature and stretch rate is measured by pure shear test. The storage modulus and phase angle have been investigated by dynamic mechanical analysis (DMA). The images of defects and rupture surface are provided by scanning electron microscope (SEM). It is found that the stretch at rupture is insensitive to the temperature for both pristine and precut samples. In addition, the maximum nominal stress and fracture energy linearly decrease with environmental temperature, especially at high stretch rate. Furthermore, we measure the stretch at rupture for rectangular strips with a single edge-notch under uniaxial tension and compare them with the theoretical prediction using nonlinear fracture mechanics based on the measured fracture energy. The results obtained in this paper will give a reference to the engineering design and applications of dielectric elastomer, especially for those working at different temperatures.

Rate-dependency of the mechanical behaviour of semilunar heart valves under biaxial deformation

This paper presents an experimental investigation and evidence of rate-dependency in the planar mechanical behaviour of semilunar heart valves. Samples of porcine aortic and pulmonary valves were subjected to biaxial deformations across 1000-fold stretch rate, ranging from λ̇=0.001 to 1 s−1. The experimental campaign encompassed protocols covering (i) tests on samples without preconditioning, (ii) preconditioning immediately followed by tensile tests; and (iii) tensile tests at different rates performed on the same preconditioned specimen. Our results indicate that under all employed loading protocols, heart valve samples exhibit a marked rate-dependency in their deformation behaviour. This rate-dependency is reflected in stress-stretch curves and the calculated ensuing gradients, where samples typically show stiffening with increased rate. These results underpin one conclusive outcome: the in-plane mechanical behaviour of semilunar valves is rate-dependent (p<0.05 for Cauchy stress levels ≥50 kPa). This outcome implies that the rate of deformation for characterising the mechanical behaviour of semilunar heart valves may not be chosen arbitrarily low, and models that incorporate rate-effects may be more appropriate for better capturing the mechanical behaviour of heart valves. Statement of SignificanceThis study presents for the first time a comprehensive set of results and evidence of rate-dependency in the mechanical behaviour of semilunar heart valves under biaxial deformation. Our results challenge the widely-applied assumption in the bulk of the existing literature, where an implicit rate-independency is assumed in both experimental and modelling propositions related to the biomechanics of the aortic and pulmonary valves. This study therefore creates a solid platform for future research in heart valve biomechanics with two important implications. First, experimental campaigns have to be carried out at high stretch rates; ideally as close to the physiological rate as possible. Second, new continuum/computational models are required to address the rate-dependent mechanical behaviour of the semilunar valves.

On the effect of labour durations using an anisotropic visco-hyperelastic-damage approach to simulate vaginal deliveries

Injuries sustained by the pelvic floor muscles during childbirth are one of the major risk factors for the development of pelvic floor dysfunctions. The ability to predict the loss of the tissue integrity and the most affected regions prior to the childbirth would represent a compelling difference in choosing the appropriate management of labour.Previous biomechanical studies, using the finite element method, were able to simulate a vaginal delivery and analyse the mechanical effects on the pelvic floor muscles during the passage of the foetus. Complementing these studies, the aim of this work is to improve the characterization of the pelvic floor muscles, by using an anisotropic visco-hyperelastic constitutive model, including a continuum mechanics damage model. Viscoelasticity is a key feature to obtain more realistic results since biological tissues present relaxation effects that allow larger deformations without damage. This work analyses the reaction forces and the loss of tissue integrity sustained by the pelvic floor and evaluates the effects of different durations of labour. A delaying pushing technique of rest and descend is also studied in this work.The results obtained showed that the reaction forces vary with the duration of labour, with higher force levels associated with higher stretch rates. The pubovisceral muscle is the most affected of the levator ani, presenting an affected region of approximately 30%. The relaxation properties of the tissue contribute to diminish the damage levels, supporting the theory of delayed pushing applied in the second stage of labour.

Impact of the bluff-body material on the flame leading edge structure and flame–flow interaction of premixed CH4/air flames

In this paper we investigate the interaction between the flame structure, the flow field and the coupled heat transfer with the flame holder of a laminar lean premixed CH4/air flame stabilized on a heat conducting bluff body in a channel. The study is conducted with a 2-D direct numerical simulation with detailed chemistry and species transport and with no artificial flame anchoring boundary conditions. Capturing the multiple time scales, length scales and flame-wall thermal interaction was done using a low Mach number operator-split projection algorithm, coupled with a block-structured adaptive mesh refinement and an immersed boundary method for the solid body. The flame structure displays profiles of the main species and atomic ratios similar to previously published experimental measurements on an annular bluff body configuration for both laminar and turbulent flow, demonstrating generality of the resolved flame leading edge structure for flames that stabilize on a sudden expansion. The flame structure near the bluff body and further downstream shows dependence on the thermal properties of the bluff body. We analyze the influence of flow strain and heat losses on the flame, and show that the flame stretch increases sharply at the flame leading edge, and this high stretch rate, together with heat losses, dictate the flame anchoring location. By analyzing the impact of the flame on the flow field we reveal that the strong dependence of vorticity dilatation on the flame location leads to high impact of the flame anchoring location on the flow and flame stretch downstream. This study sheds light on the impact of heat losses to the flame holder on the flame–flow feedback mechanism in lean premixed flames.

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.

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.

Numerical simulation and optimization of the injection blow molding of polypropylene bottles - a single stage process

Single stage injection blow molding process, without preform storage and reheat, could be run on a standard injection molding machine, with the aim of producing short series of specific hollow parts. The polypropylene bottles are blow molded right after being injected. This implies that the preform has to remain sufficiently malleable to be blown while being viscous enough to avoid being pierced during the blow molding stage. These constraints lead to a small processing window, and so the process takes place between the melting temperature and the crystallization temperature, where the polypropylene is in its molten state but cool enough to enhance its viscosity without crystallizing. This single stage process introduces temperature gradients, high stretch rate and high cooling rate. Melt rheometry tests were performed to characterize the polymer behavior in the temperature range of the process, as well as Differential Scanning Calorimetry. A viscous Cross model is used with the thermal dependence assumed by an Arrhenius law. The process is simulated through a finite element code (POLYFLOW) in the ANSYS Workbench framework. The geometry allows an axisymmetric approach. The transient simulation is run under anisothermal conditions and viscous heating is taken into account. Sensitivity studies are carried out and reveal the influence of process parameters such as the material behavior, the blowing pressure and the initial temperature field. Thickness measurements using image analysis are performed and the simulation results are compared to the experimental ones. The simulation shows broad agreements with the experimental results. An optimization loop is run to determine the optimal initial thickness repartition. Design points are defined along the preform and the optimization modifies the thickness at these locations.

Inhibition of counterflow methane/air diffusion flame by water mist with varying mist diameter

In the present study, the effect of fine water mist on extinguishment of a methane–air counterflow diffusion flame was investigated to understand the underlying physics of fire extinguishment of highly stretched diffusion flame by water mist. Twin-fluid atomizers were used to generate polydisperse water mist of which Sauter mean diameters were 10, 20, 40, and 60μm. When water mist is not added, the critical stretch rate at extinguishment is 439s−1 as compared to the theoretical value of 460s−1. For the case with water mist addition, when the stretch rate is small enough, almost all the water mist evaporates within the flame zone. On the other hand, for high stretch rate case, large mist droplets pass through the flame zone and can reach the stagnation plane. However, no oscillatory motion was found around the stagnation plane. Critical stretch rate at extinguishment decreases monotonously with the mass fraction of water mist independently of the mist diameter within the range of D32 from 10μm to 60μm. On the other hand, with increase in the surface area parameter, the critical stretch rate at extinguishment decreases rapidly and becomes less sensitive at large surface area parameter, of which tendency is qualitatively in good agreement with theoretical predictions. For a constant surface area parameter, the critical stretch rate decreases with mist diameter because the mass fraction of water mist should increase in proportion to the mist diameter to keep the surface area parameter constant. When the water mist evaporates completely in the flame zone as in the present study, the mass fraction of the water mist is the dominant factor for fire extinguishment, rather than the surface area parameter. Therefore, an appropriate combination of stretch rate and water mist mass fraction should be provided to suppress effectively a given fire with a small amount of water mist.

S0520101 粘弾性濃厚サスペンションの伸長変形挙動([S052-01]複雑流体の流動現象(1))

We examined the elongational deformation of concentrated suspensions using a viscoelastic fluid as a solvent. The stretch deformation subjected in fluid was given by a filament stretch method. Test fluid was mixture of potato starch and PAA1.0wt% with a ratio of concentration of 1:1. In the results in elongational deformation, it was found that the existence of solid particles in fluid affected the transient response of filament shape and force acting on fluid. Especially, at high stretch rates, the aggregation of particles occurred in a liquid filament locally, and the profile of the filament achieved to an asymmetric shape. The aggregation of particles seems to be caused by a compressive deformation in the radial direction of the filament. This result is relevant to jamming of particles reported in shear flow. Furthermore, in the evaluation of elongnational viscosity, although the results of concentrated suspension were larger than those of solvent, the difference in the stretch-thickening of elongational viscosity between both fluids was not remarkable.

Evaluation of chemical-kinetics models for n-heptane combustion using a multidimensional CFD code

Computational fluid dynamics (CFDs)-based predictions are presented for nonpremixed and partially premixed flames burning vaporized n-heptane fuel. Three state-of-the-art chemical kinetics models are incorporated into a time-dependent, two-dimensional, CFD model known as UNICORN. The first mechanism is the San Diego (SD) mechanism (52 species and 544 reactions), the second one is the Lawrence Livermore National Laboratory (LLNL) mechanism (160 species and 1540 reactions), and the third one is the National Institute of Standards and Technology (NIST) mechanism (197 species and 2926 reactions). Soot model based on acetylene, and radiation model based on optically thin media assumption are included. Two-dimensional calculations are made for the detailed structures of nonpremixed and partially premixed flames, strain-induced extinction and diffusion-controlled autoignition and the results are compared with the available experimental data. Diffusion-controlled autoignition characteristics are also compared with the ignition delay times calculated in homogeneous stoichiometric mixture of n-heptane and air. Through the simulation of complete flowfields between the opposing fuel and air ducts reasons for the flame curvature seen in some experiments are explained. Compared to the traditional one-dimensional models for opposing-jet flames, two-dimensional simulations are found to give results closer to the experimental values when the flames are highly stretched. While LLNL mechanism predicted extinction of a nonpremixed flame better, NIST mechanism predicted the autoignition behavior in the flowfield established by the opposing jets of fuel and heated air better. However, all three mechanisms predicted both the nonpremixed and partially premixed n-heptane flames very well. Surprisingly, SD mechanism with less than one-third of the species used in the other two mechanisms predicted flame structures with nearly the same accuracy. Comparisons made with the available experimental data could not suggest which mechanism is better in predicting the minor species concentrations. Computations could not predict the temperature rise detected in the experiments in the premixed-combustion zone of a partially premixed flame when it was subjected to a moderately high stretch rate.

Blowoff mechanism of two dimensional bluff-body stabilized turbulent premixed flames in a prototypical combustor

Near blowoff dynamics and characteristics of turbulent premixed flames stabilized by a triangular flame holder in the midspan of a rectangular duct were studied using high speed imaging and simultaneous particle imaging velocimetry and OH planar laser-induced fluorescence. Near blowoff dynamics manifested by the onset of asymmetric vortex shedding and local extinction were observed. It has been proposed that a partial or total extinction of the flame sheet along the shear layers is the major factor that determines the final blowoff event. Observation of flame kernels within the recirculation zone, which under stable conditions contain only combustion products, is further evidence of the shear layer extinction. For a stably burning planar V flame away from blowoff, the flame front envelopes the Kelvin Helmholtz vortices. Near blowoff, the two flame fronts become more aligned with the flow direction due to reduction in flame speed and interact with the vortices emanating from the shear layer. This overlap induces high local stretch rates that exceed the extinction stretch rates, resulting in local flame extinction along the shear layers. Following extinction, fresh reactants are entrained into and react within the recirculation zone, with all other parts of the flame extinguished. This flame kernel within the recirculation zone may survive for time scales of about one hundred milliseconds, potentially reigniting the shear layers such that the entire flame is re-established for a short period. This shear layer extinction and re-ignition event can happen several times before final blowoff, which occurs when the flame kernel fails to reignite the shear layers before being extinguished itself, thus leading to global flame extinction.

Blowoff dynamics of bluff body stabilized turbulent premixed flames

This article concerns the flame dynamics of a bluff body stabilized turbulent premixed flame as it approaches lean blowoff. Time resolved chemiluminescence imaging along with simultaneous particle image velocimetry and OH planar laser-induced fluorescence were utilized in an axisymmetric bluff body stabilized, propane-air flame to determine the sequence of events leading to blowoff and provide a quantitative analysis of the experimental results. It was found that as lean blowoff is approached by reduction of equivalence ratio, flame speed decreases and the flame shape progressively changes from a conical to a columnar shape. For a stably burning conical flame away from blowoff, the flame front envelopes the shear layer vortices. Near blowoff, the columnar flame front and shear layer vortices overlap to induce high local stretch rates that exceed the extinction stretch rates instantaneously and in the mean, resulting in local flame extinction along the shear layers. Following shear layer extinction, fresh reactants can pass through the shear layers to react within the recirculation zone with all other parts of the flame extinguished. This flame kernel within the recirculation zone may survive for a few milliseconds and can reignite the shear layers such that the entire flame is reestablished for a short period. This extinction and reignition event can happen several times before final blowoff which occurs when the flame kernel fails to reignite the shear layers and ultimately leads to total flame extinguishment.