Two-phase air-water stratified flow was studied in a horizontal duct. The two-phase air-water flow was considered under atmospheric conditions. The non-linear field equations for air and water were coupled and solved using a finite differencemethod. For this purpose, the two-phase flow analyzer 'MINCS' which was initially developed by Japan Atomic Energy Research Institute was used. The initial and boundary conditions were established in order to create a slug. The criterion for two-phasestratified flow instability was compared with the instability criteria based on linear analysis and the empirical correlations of other researchers. It was found that the gas velocity required for initiation of slugging is much lower than defined by theKelvin-Helmholtz instability criteria. But the instability criteria for the on set of slugging agreed well with Mishima and Ishii prediction. The reason for the necessary lower air velocity in comparison with Kelvin-Helmholtz relation is the growth ofshort waves which create slugs which is discussed here. The dynamical behavior of slug initiation which was generated by short waves was presented. The results showed that as void fraction was decreased, slug generated with lower air velocities in ashorter duct length and in a shorter period of time with a bigger wetted area. Also, the higher air velocities create slug with higher growth rate in a shorter duct length.
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