Effects of Shock Waves, Boundary Layer and Turbulence on Flame Acceleration and DDT in Highly Reactive Mixtures (original) (raw)
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Combustion Science and Technology, 2017
The entire process of deflagration-to-detonation transition (DDT) is studied through direct numerical simulations in narrow channels. Calculations with adiabatic and heat-loss boundaries are conducted to investigate the effect of heat loss to walls on flame acceleration and DDT. The numerical results show that heat loss reduces the flame acceleration rate and delays the occurrence of DDT. In the adiabatic channel, flame acceleration is caused mainly by viscosity friction with walls; ultra-fast flame in boundary layers plays a key role in the occurrence of DD. However, in the channel with heat loss the growth of the pressure pulse and the interaction of the leading shock with the boundary layers are weakened. Ultra-fast flame cannot be formed at the boundary layer in front of the flame surface and the occurrence of DDT is attributed to early burning in front of the flame.
Deflagration-to-detonation transition in highly reactive combustible mixtures
Acta Astronautica, 2010
The paper presents experimental, theoretical, and numerical studies of deflagration-to-detonation transition (DDT) in highly reactive hydrogen–oxygen and ethylene–oxygen mixtures. Two-dimensional reactive Navier–Stokes equations for a hydrogen–oxygen gaseous mixture including the effects of viscosity, thermal conduction, molecular diffusion, and a detailed chemical reaction mechanism are solved numerically. It is found that mechanism of DDT is entirely determined by the features of the flame acceleration in tubes with no-slip walls. The experiments and computations show three distinct stages of the process: (1) the flame accelerates exponentially producing shock waves far ahead from the flame, (2) the flame acceleration decreases and shocks are formed directly on the flame surface, and (3) the final third stage of the actual transition to a detonation. During the second stage a compressed and heated pocket of unreacted gas adjacent ahead to the flame—the preheat zone is forming and the compressed unreacted mixture entering the flame produces large amplitude pressure pulse. The increase of pressure enhances reaction rate and due to a positive feedback between the pressure peak and the reaction the pressure peak grows exponentially, steepens into a strong shock that is coupled with the reaction zone forming the overdriven detonation wave. The proposed new physical mechanism of DDT highlights the features of flame acceleration in tubes with no-slip walls, which is the key factor of the DDT origin.
1998
The High-Temperature Combustion Facility at BNL was used to conduct deflagration-to-detonation transition (DDT) experiments. Periodic orifice plates were installed inside the entire length of the detonation tube in order to promote flame acceleration. The orifice plates are 27.3-cm-outer diameter, which is equivalent to the inner diameter of the tube, and 20.6-cm-inner diameter. The detonation tube length is 21 .&meters long, and the spacing of the orifice plates is one tube diameter. A standard automobile diesel engine glow plug was used to ignite the test mixture at one end of the tube. Hydrogen-air-steam mixtures were tested at a range of temperatures up to 650K and at an initial pressure of 0.1 MPa. In most cases, the limiting hydrogen mole fraction which resulted in DDT corresponded to the mixture whose detonation cell size, A, was equal to the inner diameter of the orifice plate, d (e.g., d/A=l). The only exception was in the dry hydrogen-air mixtures at 650K where the DDT limit was observed to be 11 percent hydrogen, corresponding to a value of d/A equal to 5.5. For a 10.5 percent hydrogen mixture at 650K, the flame accelerated to a maximum velocity of about 120 m/s and then decelerated to below 2 m/s. By maintaining the first 6.1 meters of the vessel at the ignition end at 400K, and the rest of the vessel at 650K, the DDT limit was reduced to 9.5 percent hydrogen (d/A=4.2). This observation indicates that the d/A=l DDT limit criteria provides a necessary condition but not a sufficient one for the onset of DDT in obstacle laden ducts. In this particular case, the mixture initial condition (Le., temperature) resulted in the inability of the mixture to sustain flame acceleration to the point where DDT could occur. It was also observed that the distance required for the flame to accelerate to the point of detonation initiation, referred to as the run-up distance, was found to be a function of both the hydrogen mole fraction and the mixture initial temperature. Decreasing the hydrogen mole fraction or increasing the initial mixture temperature resulted in longer run-up distances. The density ratio across the flame and the speed of sound in the unburned mixture were found to be two parameters which influence the run-up distance.
Physics Letters A 373 (2009) 501–510, 2009
The Letter presents analytical, numerical and experimental studies of the mechanism underlying the deflagration-to-detonation transition (DDT). Insight into how, when, and where DDT occurs is obtained by analyzing analytically and by means of multidimensional numerical simulations dynamics of a flame accelerating in a tube with no-slip walls. It is shown that the deflagration-to-detonation transition exhibits three separate stages of evolution corroborating majority experimental observations. During the first stage flame accelerates and generates shocks far ahead of the flame front. During the second stage the flame slows down, shocks are formed in the immediate proximity of the flame front and the preheated zone ahead of the flame front is created. The third stage is self-restructuring of the steep temperature profile within the flame, formation of a reactivity gradient and the actual formation of the detonation wave itself. The mechanism for the detonation wave formation, given an appropriate formation of the preheated zone, seems to be universal and involves a reactivity gradient formed from the initially steep flame temperature profile in the presence of the preheated zone. The developed theory and numerical simulations are found to be well consistent with extensive experiments of the DDT in hydrogen–oxygen and ethylene–oxygen mixtures in tubes with smooth and rough walls.
The Influence of Turbulent Mixing on Deflagration to Detonation Transition
2017
In the current study, the influence of turbulent mixing on Deflagration to Detonation Transition (DDT) is investigated, using a state-of-the-art Large Eddy Simulation (LES) strategy, for conditions which correspond to recent experiments [1–3]. This investigation follows the procedure of Radulescu and Maxwell [4] by considering the re-ignition of fully quenched detonations, following the detonation interaction with a porous medium, as shown in Figure 1. This type of DDT has also been examined experimentally for detonation interactions with perforated plates [5], or a series of obstacles, or blockages [6,7]. Currently, the quenching process of detonations is well understood. As a detonation wave diffracts around an object, the sudden change in area causes volumetric expansion of the gas behind the leading shock wave. Eventually, the detonation can become quenched when local cooling due to this expansion overcomes local heating due to chemical reactions [8–10]. The result is a de-coupl...
Flame acceleration in channels with obstacles in the deflagration-to-detonation transition
Combustion and Flame, 2010
It was demonstrated recently in Bychkov et al., Phys. Rev. Lett. 101 (2008) 164501, that the physical mechanism of flame acceleration in channels with obstacles is qualitatively different from the classical Shelkin mechanism. The new mechanism is much stronger, and is independent of the Reynolds number. The present study provides details of the theory and numerical modeling of the flame acceleration. It is shown theoretically and computationally that flame acceleration progresses noticeably faster in the axisymmetric cylindrical geometry as compared to the planar one, and that the acceleration rate reduces with increasing initial Mach number and thereby the gas compressibility. Furthermore, the velocity of the accelerating flame saturates to a constant value that is supersonic with respect to the wall. The saturation state can be correlated to the Chapman-Jouguet deflagration as well as the fast flames observed in experiments. The possibility of transition from deflagration to detonation in the obstructed channels is demonstrated. arXiv:1211.0655v1 [physics.flu-dyn]
Quasi-steady stages in the process of premixed flame acceleration in narrow channels
The present paper addresses the phenomenon of spontaneous acceleration of a premixed flame front propagating in micro-channels, with subsequent deflagration-todetonation transition. It has recently been shown experimentally R. Yetter, Proc. Combust. Inst. 31, 2429 (2007)], computationally [D. Valiev, V. Bychkov, V. Akkerman, and L.-E. Eriksson, Phys. Rev. E 80, 036317 (2009)], and analytically [V. Bychkov, V. Akkerman, D. Valiev, and C. K. Law, Phys. Rev. E 81, 026309 (2010)] that the flame acceleration undergoes different stages, from an initial exponential regime to quasi-steady fast deflagration with saturated velocity. The present work focuses on the final saturation stages in the process of flame acceleration, when the flame propagates with supersonic velocity with respect to the channel walls. It is shown that an intermediate stage may occur during acceleration with quasi-steady velocity, noticeably below the Chapman-Jouguet deflagration speed. The intermediate stage is followed by additional flame acceleration and subsequent saturation to the Chapman-Jouguet deflagration regime. We elucidate the intermediate stage by the joint effect of gas pre-compression ahead of the flame front and the hydraulic resistance. The additional acceleration is related to viscous heating at the channel walls, being of key importance at the final stages. The possibility of explosion triggering is also demonstrated. C 2013 AIP Publishing LLC. 096101-2 Valiev et al. Phys. Fluids 25, 096101 (2013) at the rough walls, and viscous drag in relation to DDT have been also discussed. 20-23 An important theoretical prediction of Refs. 18 and 19 was the fast flame front acceleration in narrow channels. Following this prediction, experiments on DDT in micro-channels with diameters about 1 mm have been performed 24-27 using ethylene-oxygen mixtures. In addition to supporting the main theoretical predictions, the experiments also demonstrated several additional features beyond the scope of the theory, such as the possibility of fast steady or quasi-steady deflagration propagating with supersonic speed with respect to the channel wall. Since the theory 18, 19 was restricted to the initial incompressible stage of flame acceleration, it did not account for compressibility effects.
Flame acceleration and DDT of hydrogen–oxygen gaseous mixtures in channels with no-slip walls
International Journal of Hydrogen Energy, 2011
Hydrogeneoxygen flame acceleration and transition from deflagration to detonation (DDT) in channels with no-slip walls were studied theoretically and using high resolution simulations of 2D reactive NaviereStokes equations, including the effects of viscosity, thermal conduction, molecular diffusion, real equation of state and a detailed chemical reaction mechanism. It is shown that in "wide" channels (D > 1 mm) there are three distinctive stages of the combustion wave propagation: the initial short stage of exponential acceleration; the second stage of slower flame acceleration; the third stage of the actual transition to detonation. In a thin channel (D < 1 mm) the flame exponential acceleration is not bounded till the transition to detonation. While velocity of the steady detonation waves formed in wider channels (10, 5, 3, 2 mm) is close to the Chap-maneJouguet velocity, the oscillating detonation waves with velocities slightly below the CJ velocity are formed in thinner channels (D < 1.0 mm). We analyse applicability of the gradient mechanism of detonation ignition for a detailed chemical reaction model to be a mechanism of the deflagration-to-detonation transition. The results of high resolution simulations are fully consistent with experimental observations of flame acceleration and DDT in hydrogeneoxygen gaseous mixtures.
The influence of initial temperature on flame acceleration and deflagration-to-detonation transition
Symposium (International) on Combustion, 1996
The influence of initial mixture temperature on deflagration-to-detonation transition (DDT) has been investigated experimentally. The experiments were carried out in a 27-cm-inner diameter, 2 1.3-meter-long heated detonation tube, which was equipped with periodic orifice plates t o promote flame acceleration. Hydrogen-air-steam mixtures were tested at a range of temperatures up t o 650K and at an initial pressure of 0.1 MPa. In most cases, the limiting hydrogen mole fraction which resulted in transition to detonation corresponded t o the mixture whose detonation cell size, A, was approximately equal to the inner diameter of the orifice plate, d (e.g., d/A=1). The only exception was in dry hydrogen-air mixtures at 650K where the DDT limit was observed t o be 11 percent hydrogen, corresponding t o a value of d/A equal to 5.5. For a 10.5 percent hydrogen mixture at 650K, the flame accelerated to a maximum velocity of about 1 2 0 m/s and then decelerated to below 2 m/s. This observation indicates that the d/A= 1 DDT limit criterion provides a necessary condition but not a sufficient one for the onset of DDT in obstacle-laden ducts. In this particular case, the mixture initial condition (i.e., temperature) resulted in the inability of the mixture t o sustain flame acceleration t o the point where DDT could occur. It was also observed that the distance required for the flame to accelerate to the onset of detonation was a function of both the hydrogen mole fraction and the mixture initial temperature. For example, decreasing the hydrogen mole fraction or increasing the initial mixture temperature resulted in longer transition distances.