In this paper a criterion to predict ignition delay time is proposed that the aluminum particles can be ignited behind shock waves at the melting point of aluminum 931.7K and all of the aluminum of the particle is melted. The process of rising of temperature of particle is calculated. The ignition delay times versus Mach number of incident shock waves are obtained and compared with experimental results of aluminum particles ignition behind shock waves in oxygen with initial pressure 0.0025 MPa. The results show that as particle temperature reaches the melting point of aluminum and all of the aluminum of the particle is melted behind shock waves, an aluminum particle can be ignited if gas flow behind shock waves is able to enlarge the cracks on oxide coating and cause the disruption of oxide of aluminum particle. The ignition delay time obtained by this criterion is best agreed with the experimental results as the Mach number is larger than 5.86 with the condition of experiments. If the cracks on oxide coating can not be disrupted before its temperature rises to oxide melting point, aluminum particle can only be ignited as temperature reaches oxide melting point.
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