| 1. |
- Bychkov, Vitaly, et al.
(författare)
-
Physical Mechanism of Ultrafast Flame Acceleration
- 2008
-
Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 101:16, s. 164501-1-164501-4
-
Tidskriftsartikel (refereegranskat)abstract
- We explain the physical mechanism of ultrafast flame acceleration in obstructed channels used in modern experiments on detonation triggering. It is demonstrated that delayed burning between the obstacles creates a powerful jetflow, driving the acceleration. This mechanism is much stronger than the classical Shelkin scenario of flame acceleration due to nonslip at the channel walls. The mechanism under study is independent of the Reynolds number, with turbulence playing only a supplementary role. The flame front accelerates exponentially; the analytical formula for the growth rate is obtained. The theory is validated by extensive direct numerical simulations and comparison to previous experiments.
|
|
| 2. |
- Liberman, M. A., et al.
(författare)
-
Numerical Simulation of Deflagration-to-Detonation Transition : the Role of Hydrodynamic Instability
- 2006
-
Ingår i: International Journal of Transport Phenomena. - 1028-6578. ; 8, s. 253-277
-
Tidskriftsartikel (refereegranskat)abstract
- The role of the flame folding, induced by the classical Darrieus-Landau instability, on the transition from deflagration to detonation is studied by numerical simulations of premixed gas combustion spreading from the closed end of a semi-infinite, smooth-walled channel. It is found that in sufficiently wide channels the Darrieus-Landau instability may invoke nucleation of hot spots within the folds of the developing wrinkled flame, triggering an abrupt transition from deflagrative to detonative combustion. The mechanism of the transition is the temperature increase due to the influx of heat from the folded reaction zone, followed by autoignition. The transition occurs when the pressure elevation at the accelerating reaction front becomes high enough to produce a shock capable of supporting detonation. This requires the fold to be sufficiently narrow and deep. The influence of adhesive and rough walls on the transition is discussed.
|
|
| 3. |
|
|
| 4. |
- Lindblad, Erik, et al.
(författare)
-
Implicit-explicit Runge-Kutta methods for stiff combustion problems
- 2009
-
Ingår i: SHOCK WAVES, VOL 1, PROCEEDINGS. - Berlin, Heidelberg : SPRINGER-VERLAG BERLIN. ; , s. 299-+, s. 299-304
-
Konferensbidrag (refereegranskat)abstract
- New high order implicit-explicit Runge-Kutta methods have been developed and implemented into a finite volume code to solve the Navier-Stokes equations for reacting gas mixtures. If only the stiff chemistry is treated implicitly, the linear systems in each Newton iteration are simple and solved directly. Numerical simulations of deflagration-to-detonation transition (DDT) show the potential of the new time integration for computational combustion.
|
|
| 5. |
- Modestov, Mikhail, et al.
(författare)
-
Growth rate and the cutoff wavelength of the Darrieus-Landau instability in laser ablation
- 2009
-
Ingår i: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics. - 1539-3755 .- 1550-2376. ; 80:4, s. 046403-046412
-
Tidskriftsartikel (refereegranskat)abstract
- The main characteristics of the linear Darrieus-Landau instability in the laser ablation flow are investigated. The dispersion relation of the instability is found numerically as a solution to an eigenvalue stability problem, taking into account the continuous structure of the flow. The results are compared to the classical Darrieus- Landau instability of a usual slow flame. The difference between the two cases is due to the specific features of laser ablation: sonic velocities of hot plasma and strong temperature dependence of thermal conduction. It is demonstrated that the Darrieus-Landau instability in laser ablation is much stronger than in the classical case. In particular, the maximum growth rate in the case of laser ablation is about three times larger than that for slow flames. The characteristic length scale of the Darrieus-Landau instability in the ablation flow is comparable to the total distance from the ablation zone to the critical zone of laser light absorption. The possibility of experimental observations of the Darrieus-Landau instability in laser ablation is discussed.
|
|
| 6. |
- Valiev, Damir, et al.
(författare)
-
Different stages of flame acceleration from slow burning to Chapman-Jouguet deflagration
- 2009
-
Ingår i: Physical Review E. - 2470-0045 .- 2470-0053. ; 80:3, s. 036317-
-
Tidskriftsartikel (refereegranskat)abstract
- Numerical simulations of spontaneous flame acceleration are performed within the problem of flame transition to detonation in two-dimensional channels. The acceleration is studied in the extremely wide range of flame front velocity changing by 3 orders of magnitude during the process. Flame accelerates from realistically small initial velocity (with Mach number about 10(-3)) to supersonic speed in the reference frame of the tube walls. It is shown that flame acceleration undergoes three distinctive stages: (1) initial exponential acceleration in the quasi-isobaric regime, (2) almost linear increase in the flame speed to supersonic values, and (3) saturation to a stationary high-speed deflagration velocity. The saturation velocity of deflagration may be correlated with the Chapman-Jouguet deflagration speed. The acceleration develops according to the Shelkin mechanism. Results on the exponential flame acceleration agree well with previous theoretical and numerical studies. The saturation velocity is in line with previous experimental results. Transition of flame acceleration regime from the exponential to the linear one, and then to the constant velocity, happens because of gas compression both ahead and behind the flame front.
|
|
| 7. |
- Valiev, Damir, 1981-
(författare)
-
Flame Dynamics and Deflagration-to-Detonation Transition
- 2008
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Various premixed flame phenomena are studied by means of direct numerical simulations of the complete system of hydrodynamic equations. Rigorous study of flame dynamics is essential for all premixed combustion problems where multidimensional effects cannot be disregarded.The present thesis consists of six parts. The first part deals with the fundamental problem of curved stationary flames propagation in free-slip tubes of different widths. It is shown that only simple "single-hump" slanted stationary flames are possible in tubes wider than some stability limit. The flame dynamics is shown to be governed by a large-scale stability mechanism resulting in a highly slanted flame front.The second part of the thesis is dedicated to studies of acceleration and fractal structure of outward freely propagating flames. It is shown that the development of Landau-Darrieus instability results in the formation of fractal-like flame front structure. Two-dimensional simulation of radially expanding flames displays a radial growth with 1.25 power law temporal behavior. It is shown that the fractal excess for 2D geometry obtained in thenumerical simulation is in good agreement with theoretical predictions.In third part the flame acceleration in tubes with non-slip at the walls is studied in the extremely wide range of flame front velocity. Flame accelerates from small initial velocity to supersonic speed in the laboratory reference frame. Flame acceleration undergoes three stages: 1) initial exponential acceleration in the quasi-isobaric regime, 2) almost linear increase of the flame speed to supersonic values, 3) saturation to a stationary high-speed deflagration velocity, which is correlated with the Chapman-Jouguet deflagration speed. The saturation velocity is in line with previous experimental results.In fourth part 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 developed analytical theory determines temperature distribution ahead of an accelerating flame. The heating effects of viscous stress and the compression wave become comparable at sufficiently high values of the Mach number. Viscous stress makes heating and explosion of the fuel mixture preferential at the walls.In fifth part we reveal the physical mechanism of ultra-fast flame acceleration in obstructed channels used in modern experiments on detonation triggering. It is demonstrated that delayed burning between the obstacles creates a powerful jet-flow, driving the acceleration. The flame front accelerates exponentially; theanalytical formula for the growth rate is obtained. The theory is validated by extensive direct numerical simulations and comparison to previous experiments.The last part of the thesis concerns the transition from deflagration to detonation. It is found that in sufficiently wide free-slip channels and for sufficiently fast flames Landau-Darrieus instability may invoke nucleation of hot spots within the wrinkled flame folds, triggering an abrupt transition from deflagrative to detonative combustion. Results on DDT in channels with non-slip at the walls are also presented.
|
|
| 8. |
- Valiev, Damir, et al.
(författare)
-
Heating of the fuel mixture due to viscous stress ahead of accelerating flames in deflagration-to-detonation transition
- 2008
-
Ingår i: Physics Letters A. - Amsterdam : Elsevier BV. - 0375-9601 .- 1873-2429. ; 372:27-28, s. 4850-4857
-
Tidskriftsartikel (refereegranskat)abstract
- 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. The simulations were performed on the basis of compressible Navier–Stokes gas-dynamic equations taking into account chemical kinetics. In agreement with the theory, viscous stress makes heating and explosion of the fuel mixture preferential at the walls. The explosion develops in an essentially multi-dimensional way, with fast spontaneous reaction spreading along the walls and pushing inclined shocks. Eventually, the combination of explosive reaction and shocks evolves into detonation.
|
|
| 9. |
- Valiev, Damir
(författare)
-
The role of Landau-Darrieus instability in flame dynamics and deflagration-to-detonation transition
- 2007
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The role of intrinsic hydrodynamic instability of the premixed flame (known as Landau-Darrieus instability) in various flame phenomena is studied by means of direct numerical simulations of the complete system of hydrodynamic equations. Rigorous study of flame dynamics and effect of Landau-Darrieus instability is essential for all premixed combustion problems where multidimensional effects cannot be disregarded. The present thesis consists of three parts. The first part deals with the fundamental problem of curved stationary flames propagation in tubes of different widths. It is shown that only simple "single-hump" slanted stationary flames are possible in wide tubes, and "multi-hump" flames in a laminar flow are possible in wide tubes only as a non-stationary mode of flame propagation. The stability limits of curved stationary flames in wider tubes are obtained, together with the dependence of the velocity of the stationary flame on the tube width. The flame dynamics in wider tubes is shown to be governed by a large-scale stability mechanism resulting in a highly slanted flame front. The second part of the thesis is dedicated to studies of acceleration and fractal structure of outward freely propagating flames. It is shown that in direct numerical simulation the development of Landau-Darrieus instability results in the formation of fractal-like flame front structure. The fractal excess for radially expanding flames in cylindrical geometry is evaluated. Two-dimensional simulation of radially expanding flames in cylindrical geometry displays a radial growth with 1.25 power law temporal behavior after some transient time. It is shown that the fractal excess for 2D geometry obtained in the numerical simulation is in good agreement with theoretical predictions. The difference in fractal dimension between 2D cylidrical and three-dimensional spherical radially expanding flames is outlined. Extrapolation of the obtained results for the case of spherical expanding flames gives a radial growth power law that is consistent with temporal behavior obtained in the survey of experimental data. The last part of the thesis concerns the role of Landau-Darrieus instability in the transition from deflagration to detonation. It is found that in sufficiently wide channels Landau-Darrieus instability may invoke nucleation of hot spots within the folds of the developing wrinkled flame, triggering an abrupt transition from deflagrative to detonative combustion. It is found that the mechanism of the transition is the temperature increase due to the influx of heat from the folded reaction zone, followed by autoignition. The transition occurs when the pressure elevation at the accelerating reaction front becomes high enough to produce a shock capable of supporting detonation.
|
|
