Transition in the propagation mechanism during flame acceleration in porous media

The combustion of stoichiometric hydrogen-air at various initial pressures was investigated in a 7.62 cm square cross-section channel filled with 1.27 cm diameter beads. The flame time-of-arrival and pressure time history along the channel were obtained by ionization probes and piezoelectric pressure transducers. Flame acceleration was found to be very rapid, e.g. at an initial pressure of 45 kPa the flame achieves a velocity of over 600 m/s in roughly 0.3 m. It was determined that at this high speed a well defined planar shock wave precedes a thick reaction zone. It was also shown that there is a transition in the flame propagation mechanism, similar to that observed in an obstacle laden channel [G. Ciccarelli and C. Johansen, The role of shock-flame interactions on flame acceleration in an obstacle laden channel, Proc. 22nd International Colloquium on the Dynamics of Explosions and Reactive Systems, Minsk, 2009]. By varying the initial pressure of the mixture, changes in the axial location of the transition between combustion propagation regimes was also observed. A soot foil technique was used to identify the transition in the propagation mechanism, as well as to provide information concerning the local flow field around the beads and the overall average flow direction.     

The study of geometric effects on the explosion front propagation in a horizontal channel with a layer of spherical beads

Experiments were performed in a horizontal channel partially filled with a layer of 12.7 mm ceramic-oxide beads filled with a nitrogen-diluted stoichiometric methane–oxygen mixture, i.e., CH₄ + 2(O₂ + 2/3N₂). Ionization probes and pressure transducers were used to track the explosion front velocity in the 1.22 m long, 76 mm wide and 152 mm high horizontal channel. Schlieren photography and smoked foil techniques are used to gain insight into the explosion front structure. The explosion propagation phenomenon was characterized by the combustion in the bead layer and the unobstructed gap above. It was determined that for a fixed gap height the bead layer thickness had very little effect on the explosion propagation phenomenon. However, for a fixed bead layer height the explosion propagation was strongly influenced by the gap height. The combustion products vented from the bead layer behind the flame propagating in the gap affects the structure of the shock-flame front in the gap and the maximum flame velocity achieved. The coupling between the vented products and the flame velocity in the gap was strongly influenced by the gap height. The gap height also affects the structure of the detonation wave propagating in the gap following DDT that always occurred in the gap. The DDT run-up distance was found to increase with increasing gap height and inversely with initial pressure.     

Flame acceleration and transition to detonation: Effects of a composition gradient in a mixture of methane and air

The effects of a composition gradient on flame acceleration and transition to detonation in a mixture of methane and air were studied by numerically solving the unsteady, fully compressible, reactive Navier–Stokes equations. The specific problem addressed here is for ignition in a two-dimensional, obstructed channel where there is a spatial gradient of equivalence ratios perpendicular to the propagation direction of the reaction wave. The solution method uses a calibrated, optimized chemical-diffusive model that reproduces correct flame and detonation properties for methane–air mixtures over a range of equivalence ratios. Comparisons were made to a stoichiometric, homogeneous mixture in order to focus on the worst-case scenario for safety concerns. The results showed that the flame speed is smaller and the average total heat release are lower, but the maximum flame surface area is larger in the inhomogeneous mixture. This is because there is more unburned material between obstacles but less energy released from this increased flame surface area in the fuel-lean region, leading to the reduction of the total heat release. The transition to detonation is delayed in the inhomogeneous mixture, because the hot spot forms in the fuel-lean region and the strength of the Mach stem that hits the obstacle is weaker. The detonation front tends to decouple into a shock and a flame earlier in the inhomogeneous mixture, due to the incomplete mixing throughout the entire domain during the detonation propagation process.     

Flame acceleration and the transition to detonation of stoichiometric ethylene/oxygen in microscale tubes

Flame propagation in capillary tubes with smooth circular cross-sections and diameters of 0.5, 1.0, and 2.0 mm are investigated using high-speed photography. Flames were found to propagate and accelerate to detonation speed in stoichiometric ethylene and oxygen mixtures initially at room temperature in all three tube diameters. Ignition occurs at the midpoint along the length of the tube. We observe for the first time transition to detonation in micro-tubes. Detonation was observed with both spark and hot-wire ignition. Tubes with larger diameters take longer to transition to detonation. In fact, transition distance scales with the diameter in our 1.0 and 2.0 mm cases with spark ignition. Flame structures are observed for various stages of the process. Three types of flame propagation modes were observed in the 0.5 mm tube with spark ignition: (a) acceleration to Chapman–Jouguet (CJ) detonation speed followed by constant CJ wave propagation, (b) acceleration to CJ speed, followed by the detonation wave failure, and (c) flame acceleration to a constant speed below the CJ speed of approximately 1600 m/s. The current detonation mechanism observed in capillary tubes is applicable to predetonators for pulsed detonation, micro propulsion devices, safety issues, and addresses fundamental issues raised by recent theoretical and numerical analyses.     

激光加速高能质子实验研究进展及新加速方案

利用超强激光与等离子体相互作用来加速高能离子是激光等离子体物理及加速器物理领域的研究热点.经过了近20年的发展,激光离子加速已取得丰硕成果,催生了一批新的应用.本文概述了国内外激光离子加速所取得的标志性实验研究进展,围绕高能质子的产生这一关键问题进行了深入的探讨,介绍了近几年来发展的有潜力的新加速方案.     

Flame dynamics

This lecture describes recent theoretical developments associated with the dynamics of flames, obtained primarily by exploiting the various temporal and length scales involved in the combustion process. In premixed flames the focus is on flame-flow interactions that occur during the nonlinear development of hydrodynamically unstable large-scale flames, or during the propagation of curved flames in two-dimensional channels. The second part of the paper deals with non-premixed and partially premixed flames, where the focus is on understanding the nature of diffusive-thermal instabilities including the effect of thermal expansion, and on stabilization mechanisms of edge flames, which possess characteristics of both premixed and diffusion flames. The results presented in this talk illustrate how simplified models, when analyzed to their extreme, yield predictions of qualitative nature with physical insight that have advanced our understanding of combustion. This insight can be used to guide the experimental efforts, explain observations and validate large-scale numerical simulations.     

Unsteady deflagration speed of an auto-ignitive dimethyl-ether (DME)/air mixture at stratified conditions

The propagation speed of an auto-ignitive dimethyl-ether (DME)/air mixture at elevated pressures and subjected to monochromatic temperature oscillations is numerically evaluated in a one-dimensional statistically stationary configuration using fully resolved numerical simulations with reduced kinetics and transport. Two sets of conditions with temperatures within and slightly above the negative temperature coefficient (NTC) regime are simulated to investigate the fundamental aspects of auto-ignition and flame propagation along with the transition from auto-ignitive deflagration to spontaneous propagation regimes under thermal stratification. Contrary to the standard laminar flame speed, the steady propagation speed of an auto-ignitive front is observed to scale proportionally to its level of upstream reactivity. It is shown that this interdependence is primarily influenced by the characteristic residence time and the homogeneous auto-ignition delay. Furthermore, the unsteady reaction front in either of the two cases responds distinctly to the imposed stratification. Specifically, the results in both cases show that the dynamic flame response depends on the mean temperature at the flame base T_b and the time-scale of thermal stratification. It is also found that, based on T_b and the propensity of the mixture to two-stage chemistry, the instantaneous peak propagation speed and the overall time taken to achieve that speed differs considerably. A displacement speed analysis is carried out to elucidate the underlying combustion modes that are responsible for such a variation in flame response.     

Studies on Temporal Resolution Characteristics of Flame Temperature for Flame Atomic Absorption Spectroscopy

In this paper we report a method and experiment system developed by us to useful for temporal and spatial temperature distribution of the flame for atomic obsorption spectroscopy in detail. The method and system principle is based on the modified sodium line reversal method. The studies on temporal (20–50 μsec range) and spatial (π 1mm) resolution of the flame temperature aim at establishing optimum analysis conditions and improving analysis characteristics of flame atomic absorption/emission spectroscopy.     

Influence of the ignition delay time on the explosion parameters of hydrocarbon–air–oxygen mixtures at elevated pressure and temperature

Combustion phenomena change as the conditions in which they are occurring change. Proper understanding and reliable prediction of these phenomena, including important explosion indexes (e.g., flammability limits, explosion pressures), are required for achieving safe and optimal performance of industrial processes and creating new applications. To this end, we investigated the influence of the residence time on aforementioned parameters of n-butane–oxygen mixture and a typical mixture for ethylene oxide production: methane–ethylene–oxygen, focusing on how elevated conditions affect the upper explosion limit and the explosion pressure. Elevated initial conditions (T = 230 °C, P = 4–16 bar) cause pre-ignition reactions to occur in the regime of the low temperature oxidation mechanism (LTOM). These reactions change the mixture composition prior to ignition. For both mixtures investigated, these changes in the initial mixture composition, due to pre-ignition reactions, result in a different explosion pressure. This is significant, because pressure rise is used as the ignition criterion. Consequently, a different classification of the investigated mixtures, as flammable or non-flammable, is possible, depending on the residence time prior to ignition. The experimental results are compared with theoretical calculations performed using detailed reaction kinetic models.     

Role of stochastic heating in wakefield acceleration monitored by optical injection

It is shown that stochastic heating can play an important role in Laser Wake Field Acceleration. When considering low density plasma interacting with a high intensity wave perturbed by a low intensity counterpropagating wave, stochastic heating can provide electrons with the right momentum for trapping in the wake field. The influence of stochastic acceleration on the trapping of electrons is compared to the one of cold injection by considering several polarizations of the colliding pulses. For some value of the plasma density and pulse duration, a transition from an injection due to stochastic acceleration to a cold injection dominated regime – regarding the trapped charge – has been observed from PIC code simulations. When the plasma density exceeds some value, stochastic heating becomes important and is necessary in some circumstances to get electrons trapped into the wakefield.