This study presents results of multi-phase flows in small scale pipes. Experiments are carried out for water-air, paraffin-air, water-paraffin and water-paraffin-air flows in pipes with inner diameters of 5.6 mm and 7.0 mm, respectively. Deionized water, compressed dried air and a very low viscosity paraffin oil with a density of and a viscosity  at 20º C are used as working fluids. The flow test facility was especially designed to provide steady volume flows without any pulsation even for very small volume flow rates. This is achieved through the use of pressurized storage vessels for the three fluids instead of rotational pumps. The flow rates of the two liquids are controlled by state-of-the-art mass flow controllers, whose measuring principle is based on the coriolis effect. The flow rate of the gas is controlled by three thermal mass flow controllers corresponding to different flow rate capacities. The experiments are conducted with respect to the developing flow patterns and the pressure drops caused by the flow. High accuracy glass pipes are used, concerning outer diameter and wall thickness. The flow is illuminated on the one hand by a stroboscope, brightening the whole pipe volume and on the other hand by a laser sheet, brightening a vertical plane that cuts through the axis of the pipe. A comprehensive simulation of the light distortion caused by the different refraction indices of the fluids and the curved pipe surfaces shows that pictures taken directly from the pipe exhibit tremendous distortions. These are reduced by the use of compensation boxes containing water or paraffin, corresponding to the continuous phase inside the pipe. This reduces the distortion to a marginal area.
The resulting flow pattern data are presented analogous to presentations of corresponding data in literature. The comparison of the flow pattern maps with literature data shows that a variation in pipe diameters in the range of several centimeters to several millimeters causes an essential change in the flow pattern transitions. Especially the Bond number which represents the ratio of gravitational forces to surface tension forces, reaches the order of O(1) if gas is present in the flow. For Bond numbers , surface tension forces are dominant. This is proven by the fact that almost no stratified flow was observed in the 7.0 mm pipe. In the 5.6 mm pipe, absolutely no stratification was observed.
The dominance of surface tension results in intermittent flows being the flow pattern most frequently observed in liquid-gas and liquidliquid-gas flows. In liquid-liquid flows, the same effect of the reduction in pipe diameter is discovered. However, intermittent flow is no longer the dominant flow pattern as annular flow patterns occur for wide ranges of flow conditions. Flow pattern prediction methods are tested for their ability to predict the experimental results. No precise agreement was found, but some models show trends corresponding to the experiments.
The experimental pressure drop shows comparable behavior to results from other authors published in literature. The ability of theoretical pressure drop calculation methods to predict the experimental values has been tested. For liquid-gas and liquid-liquid flows, several applicable models are found. By contrast, the only model explicitly developed for the use with liquid-liquid-gas flows, predicts values that are far too low.