Nonequidiffusive premixed-flame propagation in obstructed channels with open, nonreflecting ends.

Equidiffusive premixed combustion in obstructed channels with both open, nonreflecting ends exhibits various forms of flame propagation: oscillations, acceleration or a combination of both regimes. Given the limited practicality of equidiffusive premixed combustion, it is important to understand how these modes of combustion are altered at nonequidiffusive conditions, characterized by a nonunity Lewis number (thermal to mass diffusivity ratio) Le≠1. To achieve this, the impacts of Le on the flame dynamics and morphology are analyzed by means of the computational simulations of the reacting flow equations, with Arrhenius chemical kinetics, fully compressible hydrodynamics, and transport properties. In addition to varying Le, the parametric study includes various blockage ratios, channel widths, obstacle spacing and thermal expansion ratios. It is identified how these parameters influence the burning velocities as well as the scaled oscillation amplitude and frequency. Specifically, in the narrow channels with small blockage ratios, the amplitude and frequency of the oscillations vary with Le, with the frequency decreasing and the amplitude increasing as Le grows from 0.3 to 2. In other conditions, a transition from the flame oscillations to sudden flame acceleration or its propagation at a constant velocity is singularly influenced by Le, or by the interplay of Le with the geometric parameters of a channel. The delay time before the onset of flame acceleration, especially at Le<1, also varies as the channel width and the blockage ratio change. In all cases, Le has both quantitative and qualitative effects on flame propagation in obstructed channels with both open, nonreflecting ends.

Analysis of flame acceleration in open or vented obstructed pipes.

While flame propagation through obstacles is often associated with turbulence and/or shocks, Bychkov et al. [V. Bychkov et al., Phys. Rev. Lett. 101, 164501 (2008)10.1103/PhysRevLett.101.164501] have revealed a shockless, conceptually laminar mechanism of extremely fast flame acceleration in semiopen obstructed pipes (one end of a pipe is closed; a flame is ignited at the closed end and propagates towards the open one). The acceleration is devoted to a powerful jet flow produced by delayed combustion in the spaces between the obstacles, with turbulence playing only a supplementary role in this process. In the present work, this formulation is extended to pipes with both ends open in order to describe the recent experiments and modeling by Yanez et al. [J. Yanez et al., arXiv:1208.6453] as well as the simulations by Middha and Hansen [P. Middha and O. R. Hansen, Process Safety Prog. 27, 192 (2008)10.1002/prs.10242]. It is demonstrated that flames accelerate strongly in open or vented obstructed pipes and the acceleration mechanism is similar to that in semiopen ones (shockless and laminar), although acceleration is weaker in open pipes. Starting with an inviscid approximation, we subsequently incorporate hydraulic resistance (viscous forces) into the analysis for the sake of comparing its role to that of a jet flow driving acceleration. It is shown that hydraulic resistance is actually not required to drive flame acceleration. In contrast, this is a supplementary effect, which moderates acceleration. On the other hand, viscous forces are nevertheless an important effect because they are responsible for the initial delay occurring before the flame acceleration onset, which is observed in the experiments and simulations. Accounting for this effect provides good agreement between the experiments, modeling, and the present theory.

[Mandibular advancement: bilateral sagittal split versus -distraction osteogenesis]. / Verlenging van de onderkaak: bilaterale sagittale splijtingsosteotomie versus distractieosteogenese.

In the 1990s intra-oral distraction osteogenesis (DO) became available as an alternative for bilateral sagittal splitosteotomy (BSSO) for advancement of the mandible. It was thought that DO would lead to more stability in the results and fewer neurosensory disturbances of the inferior alveolar nerve. However, there was no scientific evidence for this assumption. This article describes a number of recently published, prospective studies that demonstrate that BSSO is not inferior to DO with respect to stability and neurosensory disturbances of the inferior alveolar nerve. They also demonstrate that BSSO leads to less pain in patients and to lower total costs. It can be concluded that BSSO should be considered the standard therapy for mandibular advancement up to 10 mm in non-syndromal patients.

Accelerative propagation and explosion triggering by expanding turbulent premixed flames.

The dynamics and morphology of outwardly propagating, accelerating turbulent premixed flames and the effect of flame acceleration on explosion triggering are analyzed. Guided by recent theoretical results and substantiated by experiments, we find that an expanding flame front in an externally forced, near-isotropic turbulent environment exhibits accelerative propagation given by a well-defined power law based on the average global flame radius. In this context the limits of the power-law exponent and the effective turbulence intensity experienced by the flame are derived. The power-law exponent is found to be substantially larger than that for the hydrodynamically unstable cellular laminar flames, hence facilitating the possibility of detonation triggering in turbulent environments. For large length scales, hydrodynamic instability is expected to provide additional acceleration, thus further favoring the attainment of detonation triggering.

Spectral formulation of turbulent flame speed with consideration of hydrodynamic instability.

Effects of Darrieus-Landau (DL) instability on the structure and propagation of turbulent premixed flame fronts are considered. By first hypothesizing separation of time scales of instability and turbulence, we estimate whether the instability can develop in the presence of turbulence of given flow rms-velocity and integral length scale. As a result, we modify the standard turbulent premixed combustion regime diagram by introducing new boundaries, limiting the domain where the instability influences the global flame shape and speed. Based on this analysis, a "turbulence-induced DL cutoff" as a function of turbulence and instability parameters is introduced, which when combined with a turbulent flame speed without DL instability yields the turbulent flame speed accounting for the instability. The consumption turbulent flame speed for no DL instability is formulated from the spectral closure of the G equation, thus accounting for the scale-dependent "turbulent" nature of the problem. Finally, an analytical form of the turbulent flame speed is derived, which is found to agree well with the corresponding experimentally measured turbulent flame speed from literature over wide ranges of normalized turbulence intensities and length scales.

Speedup of doping fronts in organic semiconductors through plasma instability.

The dynamics of doping transformation fronts in organic semiconductor plasma is studied for application in light-emitting electrochemical cells. We show that new fundamental effects of the plasma dynamics can significantly improve the device performance. We obtain an electrodynamic instability, which distorts the doping fronts and increases the transformation rate considerably. We explain the physical mechanism of the instability, develop theory, provide experimental evidence, perform numerical simulations, and demonstrate how the instability strength may be amplified technologically. The electrodynamic plasma instability obtained also shows interesting similarity to the hydrodynamic Darrieus-Landau instability in combustion, laser ablation, and astrophysics.

Self-similar accelerative propagation of expanding wrinkled flames and explosion triggering.

The formulation of Taylor on the self-similar propagation of an expanding spherical piston with constant velocity was extended to an instability-wrinkled deflagration front undergoing acceleration with R(F)∝t(α), where R(F) is the instantaneous flame radius, t the time, and α a constant exponent. The formulation describes radial compression waves pushed by the front, trajectories of gas particles, and the explosion condition in the gas upstream of the front. The instant and position of explosion are determined for a given reaction mechanism. For a step-function induction time, analytic formulas for the explosion time and position are derived, showing their dependence on the reaction and flow parameters including thermal expansion, specific heat ratio, and acceleration of the front.

Role of compressibility in moderating flame acceleration in tubes.

The effect of gas compression on spontaneous flame acceleration leading to deflagration-to-detonation transition is studied theoretically for small Reynolds number flame propagation from the closed end of a tube. The theory assumes weak compressibility through expansion in small Mach number. Results show that the flame front accelerates exponentially during the initial stage of propagation when the Mach number is negligible. With continuous increase in the flame velocity with respect to the tube wall, the flame-generated compression waves subsequently moderate the acceleration process by affecting the flame shape and velocity, as well as the flow driven by the flame.

Different stages of flame acceleration from slow burning to Chapman-Jouguet deflagration.

Numerical simulations of spontaneous flame acceleration are performed within the problem of flame transition to detonation in two-dimensional channels. The acceleration is studied in the extremely wide range of flame front velocity changing by 3 orders of magnitude during the process. Flame accelerates from realistically small initial velocity (with Mach number about 10(-3)) to supersonic speed in the reference frame of the tube walls. It is shown that flame acceleration undergoes three distinctive stages: (1) initial exponential acceleration in the quasi-isobaric regime, (2) almost linear increase in the flame speed to supersonic values, and (3) saturation to a stationary high-speed deflagration velocity. The saturation velocity of deflagration may be correlated with the Chapman-Jouguet deflagration speed. The acceleration develops according to the Shelkin mechanism. Results on the exponential flame acceleration agree well with previous theoretical and numerical studies. The saturation velocity is in line with previous experimental results. Transition of flame acceleration regime from the exponential to the linear one, and then to the constant velocity, happens because of gas compression both ahead and behind the flame front.

Violent folding of a flame front in a flame-acoustic resonance.

The first direct numerical simulations of violent flame folding because of the flame-acoustic resonance are performed. Flame propagates in a tube from an open end to a closed one. Acoustic amplitude becomes extremely large when the acoustic mode between the flame and the closed tube end comes in resonance with intrinsic flame oscillations. The acoustic oscillations produce an effective acceleration field at the flame front leading to a strong Rayleigh-Taylor instability during every second half period of the oscillations. The Rayleigh-Taylor instability makes the flame front strongly corrugated with elongated jets of heavy fuel mixture penetrating the burnt gas and even with pockets of unburned matter separated from the flame front.

Explosion triggering by an accelerating flame.

The analytical theory of explosion triggering by an accelerating flame is developed. The theory describes the structure of a one-dimensional isentropic compression wave pushed by the flame front. The condition of explosion in the gas mixture ahead of the flame front is derived; the instant of the explosion is determined provided that a mechanism of chemical kinetics is known. As an example, it is demonstrated how the problem is solved in the case of a single reaction of Arrhenius type, controlling combustion both inside the flame front and ahead of the flame. The model of an Arrhenius reaction with a cutoff temperature is also considered. The limitations of the theory due to the shock formation in the compression wave are found. Comparison of the theoretical results to the previous numerical simulations shows good agreement.

Theory and modeling of accelerating flames in tubes.

The analytical theory of premixed laminar flames accelerating in tubes is developed, which is an important part of the fundamental problem of flame transition to detonation. According to the theory, flames with realistically large density drop at the front accelerate exponentially from a closed end of a tube with nonslip at the walls. The acceleration is unlimited in time; it may go on until flame triggers detonation. The analytical formulas for the acceleration rate, for the flame shape and the velocity profile in the flow pushed by the flame are obtained. The theory is validated by extensive numerical simulations. The numerical simulations are performed for the complete set of hydrodynamic combustion equations including thermal conduction, viscosity, diffusion, and chemical kinetics. The theoretical predictions are in a good agreement with the numerical results. It is also shown how the developed theory can be used to understand acceleration of turbulent flames.

Coordinate-free description of corrugated flames with realistic density drop at the front.

The complete set of hydrodynamic equations for a corrugated flame front is reduced to a system of coordinate-free equations at the front using the fact that the vorticity effects remain relatively weak even for corrugated flames. It is demonstrated how small but finite flame thickness may be taken into account in the equations. Similar equations are obtained for turbulent burning in the flamelet regime. The equations for a turbulent corrugated flame are consistent with the Taylor hypothesis of "stationary" external turbulence.