In this study, a mechanistic model is developed to predict flow patterns, pressure gradient and liquid holdup for gas-liquid flow in upward vertical annuli. The flow pattern transition model consists of modified Zhang et al. (2003a) unified model for dispersed bubble flow and annular flow pattern transitions, Caetano (1986) model for bubbly flow transition and modified Kaya et al. (2001) model for slug to churn flow transition. The hydrodynamic models are developed based on the dynamics of slug flow and the film zone is used as the control volume. The two liquid films are taken into account in the annulus slug flow and annular flow model developments. The churn flow model is developed based on the Zhang et al. unified model for pipe flow by using a much shorter slug length.

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