Effects of void fraction correlations on pressure gradient separation of air–water two-phase flow in vertical mini rectangular ducts

•Effects of void fraction on pressure gradient separation were investigated.•Void fraction correlations were evaluated indirectly for different flow patterns.•Void fraction doesn't affect frictional pressure gradient of bubble and annular flow.•kβ and drift-flux type are more compatible than slip ratio and general type.•A correlation is recommended for pressure gradient separation in rectangular duct.

Effects of void fraction correlations on pressure gradient separation of air–water two-phase flow in vertical rectangular ducts were investigated experimentally under ambient temperature and pressure. The superficial gas and liquid velocities ranged from 0.58 to 32 m/s and from 0.16 to 3.8 m/s respectively. The results indicate that although the void fraction correlation has great influence on gravitational pressure gradient of slug flow, churn flow and annular flow, it nearly has no influence on that of bubbly flow. For the separating frictional pressure gradient, the void fraction correlation nearly has no effect on bubbly flow and annular flow, while it has significant influence on that of slug flow and churn flow. The correlations of kβ type and drift-flux type are more compatible with each other than the slip ratio and general type, when they are applied to separate the frictional component from the total pressure gradient. The drift velocity correlation of Jones and Zuber (1979) agrees well with Ishii (1977) for bubbly flow, slug flow and churn flow, so it is recommended for pressure gradient separation in vertical mini rectangular duct. Furthermore, the void fraction correlation has less influence on the pressure gradient separation as the duct size decreases.