Pressure Drop of Two-Phase Dry-Plug Flow in Round Mini-Channels

As the fluid devices are being miniaturized, the flow channels inside the devices also become smaller, often down to about the size of 1 mm in hydraulic diameter. In such mini-channels, the interfacial tensions between solid and liquid (σ_SL) and solid and gas (σ_SG), as well as the surface tension between liquid and gas (σ_LG), play an important role in two-phase flows, which are determined by choice of tube materials. Among the various two-phase flow regimes, the plug flow appears more frequently in mini-channels than in macro-channels. The surface wettability affected the morphologies of two-phase plug flows, and eventually resulted in large difference in pressure drop. In the present experimental study, contribution of the moving contact lines to the pressure drop of air-water dry-plug flow (dry wall condition at the gas portions) in round mini-channels was investigated. The polyurethane and Teflon tubes were tested; the tube diameters were 2.16 and 1.62 mm, and the contact angles were about 75 and 110°, respectively. All the experiments were performed only in the dry-plug flow regime. The pressure drop became larger either by increasing the liquid superficial velocity or by decreasing the gas superficial velocity due to the increase of the number of the moving contact lines. It implies that the pressure drop by the energy dissipation of moving contact line is an important parameter to be considered in predicting the pressure drop in the dry-plug flow regime. Therefore, the total pressure drop of two-phase dry-plug flows consists of three parts: The frictional pressure drops at the gas- and liquid-plug portions and the pressure drop by energy dissipation at the moving contact lines. The void fraction and the number of liquid plugs could be determined from the superficial velocities of gas and liquid once the two-phase mixer is fixed. Also, the pressure drop by the energy dissipation at the moving contact lines was predicted through modification of the dynamic contact angle analysis. Finally, the simple model was proposed to predict the pressure drop of two-phase dry-plug flow in round mini-channels, which represents the experimental data within the mean deviation of 5.3%.