| 1. |
- Liberman, Michael, et al.
(författare)
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Dynamics of flames in tubes with no-slip walls and the mechanism of tulip flame formation1
- 2023
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Ingår i: Combustion Science and Technology. - : Informa UK Limited. - 0010-2202 .- 1563-521X. ; 195:7, s. 1637-1665
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Tidskriftsartikel (refereegranskat)abstract
- A hydrogen/air flame propagation and the development of tulip-shaped flame in 2D tubes of different aspect ratios with both closed ends and in a half-open rectangular channel were studied using high resolution direct numerical simulations of the fully compressible Navier–Stokes equations coupled with a detailed chemistry. Flame propagation in a 3D rectangular channel was studied using large eddy simulations and compared with the results of direct numerical simulations of flame propagation in a 2D rectangular channel with the same aspect ratio. It is shown that the interaction of the rarefaction wave generated by the flame at the deceleration stage with the “positive” flow of unburned gas generated by the flame at the previous accelerating stage leads to a significant decrease of the velocity of the unburned gas flow in the near field zone ahead of the flame front. As a result, the thickness of the boundary layer in the near-field zone ahead of the flame increases significantly, and the profile of the axial velocity of the unburned gas in the near-field zone ahead of the flame front takes the form of a tulip or an inverted tulip, which leads to corresponding changes in the velocities of different parts of the flame front, the flame front inversion, and the formation of a tulip-shaped flame.
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| 2. |
- Liberman, Michael, et al.
(författare)
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Influence of chemical kinetics on spontaneous waves and detonation initiation in highly reactive and low reactive mixtures
- 2019
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Ingår i: Combustion theory and modelling. - : Informa UK Limited. - 1364-7830 .- 1741-3559. ; 23:3, s. 467-495
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Tidskriftsartikel (refereegranskat)abstract
- Understanding the mechanisms of explosions is important for minimising devastating hazards. Due to the complexity of real chemistry, a single-step reaction mechanism is usually used for theoretical and numerical studies. The purpose of this study is to look more deeply into the influence of chemistry on detonation initiated by a spontaneous wave. The results of high-resolution simulations performed for one-step models are compared with simulations for detailed chemical models for highly reactive and low reactive mixtures. The calculated induction times for H-2/air and for CH4/air are validated against experimental measurements for a wide range of temperatures and pressures. It is found that the requirements in terms of temperature and size of the hot spots, which can produce a spontaneous wave capable to initiate detonation, are quantitatively and qualitatively different for one-step models compared to detailed chemical models. The time and locations when the exothermic reaction affects the coupling between the pressure wave and spontaneous wave are considerably different for a one-step and detailed models. The temperature gradients capable to produce detonation and the corresponding size of hot spots are much shallower and, correspondingly, larger than those predicted using one-step models. The impact of the detailed chemical model is particularly pronounced for the methane-air mixture. In this case, not only the hot spot size is much greater than that predicted by a one-step model, but even at the elevated pressure, the initiation of detonation by a temperature gradient is possible only if the temperature outside the gradient is rather high, so that can ignite a thermal explosion. The obtained results suggest that the one-step models do not reproduce correctly the transient and ignition processes, so that interpretation of the simulations performed using a one-step model for understanding mechanisms of flame acceleration, DDT and the origin of explosions must be considered with great caution.
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| 3. |
- Qian, Chengeng, et al.
(författare)
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Convergence properties of detonation simulations
- 2020
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Ingår i: Geophysical and Astrophysical Fluid Dynamics. - : Taylor & Francis. - 0309-1929 .- 1029-0419. ; 114:1-2, s. 58-76
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Tidskriftsartikel (refereegranskat)abstract
- We present a high-resolution convergence study of detonation initiated by a temperature gradient in a stoichiometric hydrogen-oxygen mixture using the PENCIL CODE and compare with a code that employs a fifth order weighted essentially non-oscillating (WENO) scheme. With Mach numbers reaching 10-30, a certain amount of shock viscosity is needed in the PENCIL CODE to remove or reduce numerical pressure oscillations on the grid scale at the position of the shock. Detonation is found to occur for intermediate values of the shock viscosity parameter. At fixed values of this parameter, the numerical error associated with those small wiggles in the pressure profile is found to decrease with decreasing mesh width like down to . With the WENO scheme, solutions are smooth at , but no detonation is obtained for . This is argued to be an artifact of a decoupling between pressure and reaction fronts.
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| 4. |
- Qian, Chengeng, et al.
(författare)
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On the mechanism of “tulip flame” formation : The effect of ignition sources
- 2023
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Ingår i: Physics of fluids. - : AIP Publishing. - 1070-6631 .- 1089-7666. ; 35:11
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Tidskriftsartikel (refereegranskat)abstract
- The initial stages of hydrogen-air flame propagation in tubes and the mechanism of tulip flame formation are investigated using a high-order numerical code to solve the fully compressible reactive Navier-Stokes equations for a spark or planar igniting flame at the closed end of a tube and propagating to the opposite closed or open end. It is shown that the mechanism of tulip flame formation is universal for both sparked and planar ignited flames in tubes with both ends closed. Flame front inversion results from the tulip-shaped profile of the unburned gas axial velocity near the flame front, which is the result of the superposition of the unburned gas flow generated by the accelerating flame and the reverse flow generated by the rarefaction wave during flame deceleration. In a half-open tube, this mechanism is valid for spark ignited flames. In the case of planar ignition, there is no rarefaction wave, but the growth of bulges on the sidewalls leads to the formation of a tulip flame.
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| 5. |
- Wang, Cheng, et al.
(författare)
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Influence of chemical kinetics on detonation initiating by temperature gradients in methane/air
- 2018
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Ingår i: Combustion and Flame. - : Elsevier BV. - 0010-2180 .- 1556-2921. ; 197, s. 400-415
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Tidskriftsartikel (refereegranskat)abstract
- Different simplified and detailed chemical models and their impact on simulations of combustion regimes initiating by the initial temperature gradient in methane/air mixtures are studied. The limits of the regimes of reaction wave propagation depend upon the spontaneous wave speed and the characteristic velocities of the problem. The present study mainly focus to identify conditions required for the development a detonation and to compare the difference between simplified chemical models and detailed chemistry. It is shown that a widely used simplified chemical schemes, such as one-step, two-step and other simplified models, do not reproduce correctly the ignition process in methane/air mixtures. The ignition delay times calculated using simplified models are in orders of magnitude shorter than the ignition delay times calculated using detailed chemical models and measured experimentally. This results in considerably different times when the exothermic reaction affects significantly the ignition, evolution, and coupling of the spontaneous reaction wave and pressure waves. We show that the temperature gradient capable to trigger detonation calculated using detailed chemical models is much shallower (the size of the hot spot is much larger) than that, predicted by simulations with simplified chemical models. These findings suggest that the scenario leading to the deflagration to detonation transition (DDT) may depend greatly on the chemical model used in simulations and that the Zel'dovich gradient mechanism is not necessary a universal mechanism triggering DDT. The obtained results indicate that the conclusions derived from the simulations of DDT with simplified chemical models should be viewed with great caution.
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