One of the most crucial parameters for detonation transition of two-phase fuel/air mixtures is the amount of pre-vaporized fuel prior to ignition. Phase equilibrium states and droplet equilibration lifetime to achieve the phase equilibrium are calculated in an attempt to assess whether the fraction of vaporized fuel is sufficient and to establish a criterion for detonation transition of two-phase mixtures on the basis of the ratio between PDE filling time and the droplet equilibration lifetime t_f/t_e. The results show that the most important parameters which determine the critical conditions for liquid-fuel detonation are the mixture temperature and the spray droplet size. Based on the critical conditions found from experiments, a simple criterion for detonation transition of Jet A-l/air mixtures has been established. Because the DDT run-up distance of Jet A-l/ air mixture is around 40 times of the cellular width, a semiempirical equation for the criterion function of cellular width, initial droplet diameter, evaporation coefficient, and fraction of vaporized fuel at phase equilibrium state is found (see Eq. (2)).
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