Based on high speed camera

nitrogen-water two-phase vertical upward flow characteristics was studied in a small rectangular channel with hydraulic diameter of 1.15 mm.Four typical flow pattern images（bubbly flow

slug flow

churn flow and annular flow） were observed.According to non-overlapping characteristics among bubbles in the small rectangular channel

four typical flow patterns images were enhanced by using digital image processing techniques.After detecting bubble’s edge and filling fully

gas void fraction was obtained by the proposed models.Two-phase flow was distinguished by the liquid Reynolds number as laminar region

transition region and turbulent region

and a modified Chisholm equation was proposed

The results showed that the modified Chisholm correlation equation could better predict pressure drop in this study.