Transition from plug/slug flow to annular flow in microchannel tube: A database and a model

A flow pattern map is a tool to predict flow patterns at a specific condition for a fluid. The map can also be used in heat transfer or pressure drop models when the detail of two-phase flow is important. In this work, a model for predicting flow patterns in microchannel tubes is proposed. Six fluids (R32, R1234yf, R134a, R1234ze(E), R1233zd(E), and R1336mzz(Z)) were measured in a 0.643 mm microchannel tube. The flow patterns are presented and reviewed. Comparison is made among the six fluids at 150 kg-m⁻²s⁻¹. As saturation pressure is higher, the occurrence of transitional flow and annular flow delay to higher quality. In the model, the boundary from plug/slug flow to transitional flow is calculated based on the force balance of liquid shear, gravitation, and surface tension. It shows that when liquid velocity is large enough to counter the surface tension and gravitation, the liquid slug breaks down, and transitional flow occurs. The boundary from transitional flow to annular flow is calculated based on the kinetic energy ratio. It was suggested by Field and Hrnjak (2007), that when the ratio of vapor to liquid kinetic energy is between one to three, the annular flow starts. Five of the fluids except for R1336mzz(Z) in the database are used to build the flow pattern map. R1336mzz(Z) validates the flow pattern map.