The Experts below are selected from a list of 150 Experts worldwide ranked by ideXlab platform
Elaine S Oran - One of the best experts on this subject based on the ideXlab platform.
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chemical diffusive models for flame acceleration and Transition to detonation genetic algorithm and optimisation procedure
Combustion Theory and Modelling, 2019Co-Authors: Carolyn R Kaplan, Alp Ozgen, Elaine S OranAbstract:This paper presents a general approach for developing an automated, fast and flexible procedure to determine the reaction parameters for a simplified chemical-diffusive model to simulate flame acce...
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flame acceleration and Transition to detonation effects of a composition gradient in a mixture of methane and air
Proceedings of the Combustion Institute, 2019Co-Authors: W Zheng, Carolyn R Kaplan, R W Houim, Elaine S OranAbstract:Abstract 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.
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premixed flame stability and Transition to detonation in a supersonic combustor
Combustion and Flame, 2018Co-Authors: Gabriel B Goodwin, Elaine S OranAbstract:Abstract Simulations of a supersonic, reacting, premixed flow in a channel were performed to investigate the effect of flow speed on ignition, flame stability, and Transition to detonation. The configuration studied was a rectangular channel with a supersonic inflow of stoichiometric ethylene–oxygen, a transmissive outflow boundary, and no-slip adiabatic walls. The compressible reactive Navier–Stokes equations were solved by a high-order numerical algorithm on an adapting mesh for inflow Mach numbers, M∞, of 3 to 10. For M ∞ = 3, the fuel-oxidizer mixture does not reach a sufficient temperature for autoignition. Boundary layers that form on the top and bottom walls deflect the incoming flow, resulting in the formation of an oblique shock train. For M∞ ≥ 5, the fuel-oxidizer mixture ignites in the boundary layers and the flame front expands into the channel. The flame front becomes unstable and turbulent with time due to a Rayleigh–Taylor (RT) instability at the interface between the low-density burned gas and high-density unburned gas. Detonation is initiated in several locations at the flame front and in the unburned gas through an energy-focusing mechanism. As M∞ increases, the time scales for growth of the RT instability at the flame front and eventual detonation increase significantly. Despite the difference in time scales, the flame evolution process is qualitatively independent of M∞: ignition in the boundary layer, laminar flame expansion, growth of an RT instability at the flame front, turbulent flame expansion, and deflagration-to-detonation Transition.
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flame instability and Transition to detonation in supersonic reactive flows
arXiv: Fluid Dynamics, 2017Co-Authors: Gabriel B Goodwin, Elaine S OranAbstract:Multidimensional numerical simulations of a homogeneous, chemically reactive gas were used to study ignition, flame stability, and deflagration-to-detonation Transition (DDT) in a supersonic combustor. The configuration studied was a rectangular channel with a supersonic inflow of stoichiometric ethylene-oxygen and a transimissive outflow boundary. The calculation is initialized with a velocity in the computational domain equal to that of the inflow, which is held constant for the duration of the calculation. The compressible reactive Navier-Stokes equations were solved by a high-order numerical algorithm on an adapting mesh. This paper describes two calculations, one with a Mach 3 inflow and one with Mach 5.25. In the Mach 3 case, the fuel-oxidizer mixture does not ignite and the flow reaches a steady-state oblique shock train structure. In the Mach 5.25 case, ignition occurs in the boundary layers and the flame front becomes unstable due to a Rayleigh-Taylor instability at the interface between the burned and unburned gas. Growth of the reaction front and expansion of the burned gas compress and preheat the unburned gas. DDT occurs in several locations, initiating both at the flame front and in the unburned gas, due to an energy-focusing mechanism. The growth of the flame instability that leads to DDT is analyzed using the Atwood number parameter.
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chemical diffusive models for flame acceleration and Transition to detonation genetic algorithm and optimization procedure
arXiv: Fluid Dynamics, 2017Co-Authors: Carolyn R Kaplan, Alp Ozgen, Elaine S OranAbstract:One of the most important and difficult parts of constructing a multidimensional numerical simulation of flame acceleration and deflagration-to-detonation Transition (DDT) in a reacting flow is finding a reliable and affordable model of the chemical and diffusive properties. For simulations of realistic scenarios, full detailed chemical models are computationally prohibitive. In addition, they are usually inaccurate for high-temperature and high-pressure shock-laden flows. This paper presents a general approach for developing an automated procedure to determine the reaction parameters for a simplified chemical-diffusive model to simulate flame acceleration and DDT in stoichiometric methane-air and ethylene-oxygen mixtures. The procedure uses a combination of a genetic algorithm and Nelder-Mead optimization scheme to find the optimal reaction parameters for a reaction rate based on an Arrhenius form for conversion of reactants to products. The model finds six optimal reaction parameters that reproduce six target flame and detonation properties. Results from the optimization procedure show that the optimal reaction parameters, when used in 1-D reactive Navier-Stokes simulations, closely reproduce the target flame and detonation properties for the stoichiometric methane-air and ethylene-oxygen mixtures. When the reaction parameters are used as input in a 2-D simulation of flame acceleration and DDT in an obstacle-laden channel containing stoichiometric methane-air, the simulation results closely follow the Transition to detonation observed in experiments. This automated procedure for finding parameters for a proposed reaction model makes it possible to simulate the behavior of flames and detonations in large, complex scenarios, which would otherwise be an incalculable problem.
Matei I Radulescu - One of the best experts on this subject based on the ideXlab platform.
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modelling of the Transition of a turbulent shock flame complex to detonation using the linear eddy model
Combustion and Flame, 2018Co-Authors: Andrzej Pekalski, Brian Maxwell, Matei I RadulescuAbstract:Abstract In the current study, the influence of turbulent mixing and local reaction rates on the Transition to detonation of a turbulent shock-flame complex was investigated using a state-of-the-art large eddy simulation (LES) strategy. Specifically, detonation attenuation by a porous medium, and the subsequent re-initiation for methane–oxygen, a moderately unstable mixture, was considered. The purpose of the investigation was to validate the numerical strategy with previous experimental observations, and to determine what specific roles turbulent mixing and shock compression have on flame acceleration during the final stages of deflagration to detonation Transition (DDT). The modelling procedure adopted was a grid-within-a-grid approach: The compressible linear eddy model for large eddy simulation (CLEM-LES). It was found that average turbulent velocity fluctuations greater than the laminar flame speed by an order of magnitude were required in order to maintain wave velocities above the Chapman–Jouguet (CJ)-deflagration velocity threshold, a precursor requirement for detonation re-initiation to occur. It was also found that sufficient turbulent burning on the flame surface was required in order to drive pressure waves to sufficiently strengthen the leading shock wave, locally, in order to trigger auto-ignition hot spots in the wave front. These local explosion events, which were found to burn out through turbulent surface reactions, drive transverse pressure waves outward. Upon subsequent shock reflections or interactions of the transverse waves, new local explosion events occurred, which further strengthened the adjacent leading shock wave above the CJ-detonation speed. Eventually, through this process, the wave sustained the CJ-detonation speed, on average, through the cyclic mechanism of local explosion events followed by turbulent surface reactions. Finally, combustion of the flame acceleration process was found to lie within the thin-reaction zones regime.
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chapman jouguet deflagrations and their Transition to detonation
Proceedings of the Combustion Institute, 2017Co-Authors: Mohamed Saif, Wentian Wang, Andrzej Pekalski, M E Levin, Matei I RadulescuAbstract:Abstract We study experimentally fast flames and their Transition to detonation for five different hydrocarbons, namely methane, ethane, ethylene, acetylene, and propane with oxygen as the oxidizer. Following the interaction of a detonation wave with a column of cylinders of varying blockage ratio, the experiments demonstrate that the fast flames established are Chapman–Jouguet deflagrations, in excellent agreement with the self-similar model of Radulescu et al. (2005). The experiments indicate that these Chapman–Jouguet deflagrations dynamically restructure and amplify into fewer stronger modes until the eventual Transition to detonation. The Transition length to a self-sustained detonation was found to correlate very well with the mixtures’ sensitivity to temperature fluctuations, reflected by the χ parameter introduced by Radulescu, which is the product of the non-dimensional activation energy Ea/RT and the ratio of chemical induction to reaction time ti/tr. Correlation of the measured deflagration to detonation Transition (DDT) lengths determined that the relevant characteristic time scale from chemical kinetics controlling DDT is the energy release or excitation time tr. Correlations with the cell size also capture the dependence of the DDT length on χ for fixed blockage ratios.
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chapman jouguet deflagrations and their Transition to detonation
arXiv: Fluid Dynamics, 2015Co-Authors: Mohamed Saif, Wentian Wang, Andrzej Pekalski, M E Levin, Matei I RadulescuAbstract:We study experimentally fast flames and their Transition to detonation in mixtures of methane, ethane, ethylene, acetylene, and propane mixtures with oxygen. Following the interaction of a detonation wave with a column of cylinders of varying blockage ratio, the experiments demonstrate that the fast flames established are Chapman-Jouguet deflagrations, in excellent agreement with the self-similar model of Radulescu et al. (2015). The experiments indicate that these Chapman-Jouguet deflagrations dynamically restructure and amplify into fewer stronger modes until the eventual Transition to detonation. The Transition length to a self-sustained detonation was found to correlate very well with the mixtures' sensitivity to temperature fluctuations, reflected by the $\chi$ parameter introduced by Radulescu, which is the product of the non-dimensional activation energy $E_a/RT$ and the ratio of chemical induction to reaction time $t_i/t_r$. Correlation of the measured DDT lengths determined that the relevant characteristic time scale from chemical kinetics controlling DDT is the energy release or excitation time $t_r$. Correlations with the cell size also capture the dependence of the DDT length on $\chi$ for fixed blockage ratios.
Jonathan B Freund - One of the best experts on this subject based on the ideXlab platform.
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numerical modeling of shock to detonation Transition in energetic materials
Combustion and Flame, 2012Co-Authors: Ju Zhang, T L Jackson, J Buckmaster, Jonathan B FreundAbstract:Abstract Determining the hazard classification of energetic materials is important for transportation safety and storage concerns. to avoid costly grain redesign and additional testing, a model that adequately predicts the shock sensitivity of energetic materials is required, particularly the outcome of the Naval Ordnance Laboratory Large Scale Gap Test. The goals of this effort are to develop and validate computational tools that predict the shock sensitivity of energetic materials. Specifically, to use our packing code, Rocpack, to generate morphologies of interest for shock sensitivity assessments, and to use our CFD code, RocSDT, to propagate shocks of various strengths through the pack to predict the onset of detonation. Dealing accurately with the material interfaces in this problem is a long-standing challenge, as familiar strategies lead to spurious temperature spikes, and therefore spurious reaction rate spikes. We describe a new strategy, which does not generate spurious spikes, and demonstrate via a number of test problems that numerical convergence can be achieved. We also examine two problems that are stepping stones to a complete simulation; both are planar. In the first, we consider the passage of a shock wave through pure HMX in which a line of hot spots of the kind generated by void collapse are located a short distance behind the shock. When the hot spot spacing is large, the shock remains a shock; when small, Transition to detonation occurs. In the second problem we also insert hot spots, but into a matrix of HMX particles and binder.
W M Breitung - One of the best experts on this subject based on the ideXlab platform.
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deflagration to detonation Transition in large confined volume of lean hydrogen air mixtures
Combustion and Flame, 1996Co-Authors: S B Dorofeev, V P Sidorov, A E Dvoinishnikov, W M BreitungAbstract:The results of large-scale experiments on turbulent flame propagation and Transition to detonation in a confined volume of lean hydrogen-air mixtures are presented. The experiments were in a strong concrete enclosure of 480 m3, and 69.9 m length. The experimental volume consists first of a channel (34.6 m length, 2.3 m height, 2.5 m width) with or without obstacles, a canyon (10.55∗6.3∗2.5 m), and a final channel. Ignition was with a weak electric spark at the beginning of the first channel. The effect of hydrogen concentration (9.8%–14% vol.) on turbulent flame propagation and Transition to detonation was studied. The obstacle configuration in the first channel (blockage ratio 0.3, 0.6, and no obstacles), exit cross section to the canyon (1.4, 2, and 5.6 m2), and vent area at the end (0, 2.5, and 4 m2) were varied in the tests. Details of turbulent flame propagation, of pressure field, and of detonation onset are presented. A minimum of 12.5% of hydrogen was found to be necessary for Transition to detonation. This is a much less sensitive mixture than those in which the onset of spontaneous detonation has previously been observed (minimum of 15% of hydrogen in air). The effect of scale on the onset conditions for spontaneous detonation is discussed. The characteristic geometrical size of the mixture for Transition to detonation is shown to be strongly related to the mixture sensitivity.
G O Thomas - One of the best experts on this subject based on the ideXlab platform.
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experimental studies of ignition and Transition to detonation induced by the reflection and diffraction of shock waves
Shock Waves, 2000Co-Authors: C J Brown, G O ThomasAbstract:This paper presents results from a program of experimental studies of ignition induced by the interaction of an initially planar shock wave with an obstacle in its path. With the aid of pressure measurements, spark schlieren photography and smoked foil techniques it is shown how, given favourable initial conditions, the two-dimensional multiple shock reflection and diffraction can promote ignition and Transition to detonation in reactive gaseous mixtures. Comparison of the results with those of a non-reactive gas distinguishes the gas dynamic and chemical processes involved, and experimentally determined detonation cell sizes are compared with values predicted using chemical kinetic rate data. The systems investigated were argon, air, propane-air, propane-oxygen-argon and ethylene-oxygen-argon.
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experimental studies of shock induced ignition and Transition to detonation in ethylene and propane mixtures
Combustion and Flame, 1999Co-Authors: C J Brown, G O ThomasAbstract:Abstract Results are presented of ignition delay measurements, over the temperature range 1073–2211 K, obtained by monitoring CH emission from ethylene and propane mixtures. The relative influence of argon and nitrogen as diluents is also investigated. Comparisons with other experimental data are made and the validity of chemical kinetic reaction mechanisms discussed. Spark schlieren photographs of the reflection and subsequent ignition processes are also presented, and the effects of varying the diluent and degree of dilution away from ideal conditions investigated. In some cases, acceleration of the reaction wave has been observed, with Transition to detonation occurring a short time later.