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Phenomenology of flame acceleration and deflagration to detonation transition

The sequence of events leading to detonation in a tube containing explosive gases can be summarized as follows:

  • Generation of compression waves ahead of an accelerating laminar flame (see Fig. 1a). The laminar flame front is wrinkled at this stage.
  • Formation of a shock front due to coalescence of compression waves (Fig.1a).
  • Movement of gases induced by shock, causing the flame to break into a turbulent brush (Fig. 1a).
  • Onset of "an explosion in an explosion" at a point within the turbulent reaction zone, producing two strong shock waves in opposite directions and transverse oscillations in between. These oscillations are called transverse waves (see Fig. 1b). The forward shock is referred to as superdetonation and moves into unburned gases. In the opposite direction, a shock moves . into the burned gases and is known as retonation.
  • Developments of spherical shock wave at the onset of the "explosion in an explosion," with a center located in the vicinity of the boundary layer (Fig.2).
  • Interaction of transverse waves with shock front, retonation wave, and reaction zone.
  • Establishment of a final "steady wave" as a result of a long sequence of wave interaction processes that lead finally to the shock-deflagration ensemble: the self-sustained C-J detonation wave.

Figure 1a. The development of detonation in stoichiometric H2-O2 mixture
initially at normal temperature and pressure, showing the generation of
pressure waves ahead of the accelerating flame.

Figure 1b. The onset of retonation. For both photographs spark ignition
by discharging 1.0 mJ. Electrodes located at closed end of 25 x 37 mm
cross-section tube (Oppenheim et al., 1963). Streak schlieren photographs.

The forward-mowing wave was studied using wall imprints of the detonation process, an example of which is shown in Fig. 3. The characteristic fish-scale pattern, which corresponds to inception of the forward shock, is a distinguished feature of a self-sustained detonation front.

There are generally four different modes of transition process observed and classified, based on the location of the onset of an “explosion in an explosion”: between flame and shock front (Fig.4a), at the flame front (Fig.4b), at shock front (Fig.4c) and at the contact discontinuity (Fig.4d). The oset of detonation depends on the particular pattern of shock fronts created by the accelerating flame. The process of DDT is unreproducible in its detailed sequence of events.

DDT refers to the phenomena where the critical conditions for the onset of detonation are established by the combustion process itself without external energy source. There are several ways by which the conditions necessary for transition can be achieved. These include:

  1. flame acceleration to some critical speed,
  2. ignition of a turbulent pocket, and
  3. jet ignition.

Figure 2. Flash schlieren photograph of the onset of retonation in
a stoichiometric H2-O2 mixture initially at normal temperature and
pressure at an instant marked by A on streak schlieren photo
(Fig.1)(Oppenheim et al., 1963).

Figure 3. Wall imprints of the transition process.

Figure 4. Various modes of DDT observed in 2H2+O2 mixture;
a) onset occurring between flame and shock, b) onset occurring at flame front,
c) onset occurring at shock front, d) onset occurring at contact discontinuity.
(Urtiew and Oppenheim, 1966).

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Page last modified on December 10, 2008, at 02:30 PM