(An) experimental study on effect of surface wettability on two-phase flow pattern and pressure drop in round mini-channels

Abstract

In the present experimental study, the effect of the surface wettability on the two-phase flow pattern and pressure drop in round mini-channels was investigated for three different tube materials, i.e., glass, polyurethane and Teflon, respectively, with their inner diameters ranging from 1.46 to 2.16 mm. Water and methanol were used as the liquid phase whereas air as the gas phase. The surface wettability was divided into three ranges: Highly wetting (θ < 50°), marginally wetting (50° < θ < 90°) and poorly wetting (θ > 90°) cases. Through a series of experiments, all the flow patterns could be classified into two groups, i.e., the wet- and the dry-flows. The wet-flow includes the wet-plug, wet-slug and annular flows as well as the bubbly flow. The dry-flow includes the dry-plug, dry-slug and the rivulet flows as well as the wavy flow. In general, the dry-flows appeared at the lower liquid superficial velocity region. Corresponding to each wettability range, the normalized liquid superficial velocities ($j_L^+$) were proposed and the condition of $j_L^+$ = 1 well represents both the lower limit of the wet-flow region and the upper limit of the dry-flow region. Finally, the generalized flow pattern maps pertinent to each wettability range were constructed and in good agreement with the previous report. Among the various two-phase flow regimes in mini-channels, the plug flow appears very frequently. Here, the surface wettability determined the morphologies of two-phase plug flows, and eventually resulted in large difference in pressure drop. In the wet-plug flow regime (wet wall condition at the gas portions), the pressure drop was predicted by the homogeneous flow model or by the correlations of Mishima and Hibiki (1996) and Chisholm (1967) with reasonable accuracies. On the other hand, in the dry-plug flow regime (dry wall condition at the gas portions), the role of the moving contact lines turned out to be significant. That is, the pressure drop by the mov...