Flow regimes during condensation from superheated vapor

Two-phase flow during condensation in smooth horizontal round tubes of R245fa, R1233zd(E), R1234ze(E), R134a, R32 from superheated vapor is visualized and presented in this paper. Flow regimes under different mass fluxes, heat fluxes, saturation pressures, specific enthalpies and tube sizes (1, 4, 6 mm) are identified. The paper describes to the flow regime transitions according to the visualizations. The driving force behind the annular-stratified flow transition is identified to be the force balance between shear, gravity and surface tension. The mechanism that dictates the annular-intermittent flow transition is the comparison between wave-height and the tube size. The slip ratio, which generates the Kelvin-Helmholtz instability, is considered to be the reason of transition from stratified-wavy to the fully-stratified flow. The more complicated scenarios where characteristics of different flow regimes coexist are detailed and methods for simplification are provided. The results are also compared to two different flow regime maps. The flow regime map that does not consider the non-equilibrium effects does not provide information beyond bulk quality 1 and 0. Additionally, it does not capture the annular entrance during condensation either. The flow regime map with non-equilibrium taken into account addresses issues above while having its own defects. For instance, it is highly empirical and some transition lines do not properly reflect experimental observations. A more mechanistic flow regime map is recommended.