Deflagration-to-detonation transition in highly reactive combustible mixtures (original) (raw)
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On the Mechanism of the Deflagration-to-Detonation Transition in a Hydrogen–Oxygen Mixture
The flame acceleration and the physical mechanism underlying the deflagration-to-detonation transition (DDT) have been studied experimentally, theoretically, and using a two-dimensional gasdynamic model for a hydrogen-oxygen gas mixture by taking into account the chain chemical reaction kinetics for eight components. A flame accelerating in a tube is shown to generate shock waves that are formed directly at the flame front just before DDT occurred, producing a layer of compressed gas adjacent to the flame front. A mixture with a density higher than that of the initial gas enters the flame front, is heated, and enters into reaction. As a result, a high-amplitude pressure peak is formed at the flame front. An increase in pressure and density at the leading edge of the flame front accelerates the chemical reaction, causing amplification of the compression wave and an exponentially rapid growth of the pressure peak, which “drags” the flame behind. A high-amplitude compression wave produces a strong shock immediately ahead of the reaction zone, generating a detonation wave. The theory and numerical simulations of the flame acceleration and the new physical mechanism of DDT are in complete agreement with the experimentally observed flame acceleration, shock formation, and DDT in a hydrogen-oxygen gas mixture.
International Journal of Hydrogen Energy, 2014
A numerical approach has been developed to simulate flame acceleration and deflagration to detonation transition in hydrogen-air mixture. Fully compressible, multidimensional, transient, reactive NaviereStokes equations are solved with a chemical reaction mechanism which is tuned to simulate different stages of flame propagation and acceleration from a laminar flame to a turbulent flame and subsequent transition from deflagration to detonation. Since the numerical approach must simulate both deflagrations and detonations correctly, it is initially tested to verify the accuracy of the predicted flame temperature and velocity as well as detonation pressure, velocity and cell size. The model is then used to simulate flame acceleration (FA) and transition from deflagration to detonation (DDT) in a 2-D rectangular channel with 0.08 m height and 2 m length which is filled with obstacles to reproduce the experimental results of Teodorczyk et al. The simulations are carried out using two different initial ignition strengths to investigate the effects and the results are evaluated against the observations and measurements of Teodorczyk et al.
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.
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.
Investigation of Transition of Deflagration to Detonation in Moving Mixtures of Combustible Gases
In the report the new experimental data about essential reduction of predetonation distance are presented in the case of detonation initiation with an weak electric discharge in a detonable gas flow. Experiments were done at the device, which allows modeling one cycle of pulse detonation engine operation. The experimental data are compared to the results of numerical calculations. The good consent with experiments demonstrates feasibility of the offered methods of calculations and allows to give explanation to the observed experimental effects. The results seem to be of practical application to the control of detonation process in PDE.
Physics Letters A, 2008
The role of viscous stress in heating of the fuel mixture in deflagration-to-detonation transition in tubes is studied both analytically and numerically. The analytical theory is developed in the limit of low Mach number; it determines temperature distribution ahead of an accelerating flame with maximum achieved at the walls. The heating effects of viscous stress and the compression wave become comparable at sufficiently high values of the Mach number. In the case of relatively large Mach number, viscous heating is investigated by direct numerical simulations.
Investigation into mechanisms of deflagration-to-detonation using Direct Numerical Simulations
E3S Web of Conferences
Detonation, a combustion phenomenon is a supersonic combustion wave which plays critical role in the theory and application of combustion. This work presents numerical investigation into indirect initiation of detonation using direct numerical simulations (DNS). The Adaptive Mesh Refinement in object–oriented C++ (AMROC) tool for parallel computations is applied in DNS. The combustion reactions take place in a shock tube and an enclosure with a tube respectively and are controlled by detailed chemical kinetics. The database produced by DNS accurately simulates the process of transition of deflagration to detonation (DDT), and investigates the influence of overpressure and kinetics on flame propagations during combustion processes. The numerical simulations showed the influence of pressure and kinetics to the transition of slow and fast flames and DDT during flame propagations. When the reaction rate is fast, DDT is achieved, but when slow, DDT will not occur and therefore, there wil...
2011
The problem of the deflagration-to-detonation transition (DDT) and a key role of shock waves, boundary layer and turbulence in the detonation preconditioning process is well known but still not resolved in the combustion theory. The ignition, flame propagation with a flow ahead of the flame, and shock waves generation with turbulent boundary layer behind the shock is the sequence of principal events leading to the deflagration-to-detonation transition in smooth channels [1-3]. As Ya. Zeldovich wrote [4], the turbulence is not only one and even not the major reason of the flame acceleration in smooth channels. Wrinkled flame stretch and non-uniformity of the flow across the channel can be the main reason of flame acceleration leading to DDT. Experimental schlieren photos indicated that location of the transition to detonation always originates somewhere within the shockwave complex, sometimes near the wall in the boundary layer, sometimes in the center of a channel. One of the main p...
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.