Abstract
The present research is conducted to investigate the instability of gaseous detonations by emphasizing the role of the initiation step. To proceed with the study, two characteristic times are defined for the initiation and the branching steps (τI and τb, respectively). Depending on the values of these characteristic times, three different modes of detonation failure are observed. It is seen that the so-called cross over temperature detonability criterion is applicable only for high values of τb. The present work, in a systematic fashion, revealed the importance of two length scales that control the instability as well as the failure of gaseous detonation. These length scales are the induction length and the length of the main reaction layer. It appears that these two length scales may model a vast range of chemical kinetics.
For values of τb which are less than 0.033, the front propagates with a constant, non-oscillatory velocity. For high values of τI, an abrupt failure occurs. However, for values of τb larger than 0.033, the front propagates in an oscillatory manner. For values of τb between 0.033 and 0.053, the formation of unburned pockets of the reactant behind the shock is recognized as the reason of failure for high values of τI. Further varying of the two parameters cause the appearance of other modes of the front propagation.
Notes
Case 1: Overdriven detonation, f = 1.4, stoichiometric H2–O2 mixture
Case 2: CJ detonation, f = 1.0, stoichiometric H2–O2 mixture
Case 3: CJ detonation, equivalence ratio = 0.6, H2–O2 mixture with 75% N2 dilution Initial pressure and temperature in all cases are 1 atm and 300 ′K, respectively.
Reactions A (cm-mole-s) β Ea (kj/mole)
I1: H2 + M → 2H + M 6.99E18 −1. 436.08
I2: H2 + O2 → HO2 + H 6.84E13 0.0 243.10
b1: O2 + H → OH + O 2.00E14 0.0 70.30
b2: O + H2 → OH + H 5.06E4 2.67 26.30
C1: H + OH + M → H2O + M 2.20E22 −2.0 0.00
C2: H2O2 + OH → H2O + HO2 5.4E12 0.0 4.2