A two-step approach has been used to obtain a new onset of slug formation criterion for the stratified and nonstratified flows in horizontal pipes: (1) In the first step, a more general expression than the existing models for the onset of slug flow criterion has been derived from the anlaysis of singular points and neutral stability conditions of the transient one-dimensional two-phase flow equations of two-fluid model. (2) In the second step, introducing simplifications and incorporating a parameter into the general expression obtained in the first step to satisfy a number of physical conditions a priori specified, a new onset of slug flow criterion for horizontal pipes has been derived. Comparisons of the present model with the experimental data and existing theories show that the present model agrees very cloely with the best existing model.
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