Modelling of flow pattern transitions in small diameter horizontal and inclined tubes

Flow pattern maps in adiabatic mini and micro tubes in gas–liquid and liquid–liquid systems are discussed, emphasizing the phenomena that characterize two-phase flows in small-scale systems. Mechanistic models developed for gas–liquid and liquid–liquid systems in conventional-size tubes, but account for the effects of the Eotvos number Eo_D=ΔρgD²/σ, are examined. It is shown that mechanistic models, which are applicable for a large range of Eo_D in conventional-size tubes, are also applicable to mini/micro tubes and yield reasonable predictions of their flow pattern maps. These mechanistic models are also used to identify the other controlling dimensionless groups (and their critical values) associated with the various flow pattern transitions. Additionally, the effect of gravity on flow pattern transitions in mini-tubes is addressed in an attempt to identify the conditions where the pipe inclination should still be considered. Indeed, it is found that the tube inclination has a significant effect on the flow pattern transitions for Eo_D > 1, even in small diameter and capillary tubes. It is shown that Eo_D ∼ 1 can be considered as a characteristic threshold value, below which bubble/drop length scales become constrained by the tube diameter (<2.5 mm for air–water, <1mm for refrigerants), hence the features of low Eo_D systems (i.e., mini/micro tubes) are manifested in the flow pattern map. Finally, the conditions under which counter-current flow can still be considered feasible are also addressed.