Review of flow regime transition criteria for adiabatic co-current upward gas-liquid flow in vertical multi-scale channels

Predicting the vertical gas-liquid flow regime in multi-scale channels is essential for optimizing system performance and design in engineering fields such as heat dissipation, petrochemical processing, and nuclear energy. This paper provides a comprehensive review of the flow regime transition mechanisms and criteria for adiabatic co-current upward gas-liquid flow in multi-scale vertical channels. Firstly, the widely accepted definitions of scale classifications and the characteristics of flow regimes at different scales are summarized. On that basis, the transition mechanisms and criteria for multi-scale channels across different flow regimes are reviewed, including bubbly flow, slug flow, churn flow, and annular flow. Finally, the existing criteria are assessed with experimental data banks, and refined methods for predicting flow regimes in multi-scale channels are proposed, incorporating new boundaries for medium-to-large scales and accounting for flow regime transition types.