Based on the observed flow pattern by high speed camera

experimental study was conducted to investigate flow characteristics for nitrogen and water flows in a vertical square mini-channel with hydraulic diameter of 1 mm.The perimeter and area of gas phase was calculated with a method of image processing technique.The void fractions were obtained by computing the parameters with the proposed hypothetical cylinder model.Comparisons with the literature available showed that the Chisholm model provided the best agreement with experimental data.For low superficial velocities

the Lee & Lee model and Dukler model showed a better predictive ability than the other experimental correlations.