Evaluation of Split Ratio for Plug Flow at a Meso-Scale T-Junction
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Numerous applications, such as meso-scale heat exchangers, Lab-on-Chip devices (LOC), different systems within pharmaceutical and food industry, monodispersed emulsion and several other microfluidic systems, include two-phase flow through a meso-scale T-junction. When two-phase gas-liquid flow passes through an asymmetric meso-scale T-junction, a mal-distribution occurs. The phenomenon has proven itself to be unavoidable in most cases. In some applications this phenomenon can put the operational system at risk, while in other applications it is actually preferred. The phenomenon is still far from thoroughly understood. Thus the objective of this thesis is to further investigate this mal-distribution phenomenon. Split ratio for plug flow at a meso-scale T-junction has been investigated. A model for prediction of the split ratio has been proposed. Physical ingredients for determination of the split ratio have been focused upon. Much of the conducted work is based on findings in the MSc thesis by Hong et al. (2011) who proved the importance of the bubble length when predicting the split ratio. Split ratio, bubble length and pressure has been measured through experimentation. The T-junction used in the conducted experiments has a main channel, referred to simply as the main . It is connected in a straight line with one outlet referred to as the run . The second outlet is connected perpendicularly to the main and the run, and is referred to as the branch . All channels have a square shaped cross section with a hydraulic diameter of . Water and air was used as working fluids. For all conducted experiments the flow field took on a plug flow pattern. The branch channel has been observed to be rich in gas for all cases, except when the flow rate in the run is high. The flux in the main also has to be low to reduce the viscous drag forces between the two phases and the inertial forces of the plug. For increasingly high total flow rate in the run, a turning point has been located. When the flow rate exceeds this point the run becomes rich in gas. In both extreme cases (high flow rate in the run and in the branch) separation occurs for sufficiently short bubbles. The occurrence of separation is also highly dependent on the total flux in the main. To retain separation the surface tension has to overcome the viscous drag forces acting on the interface between the two phases. In the centre regime, where bubbles always break up and a plug flow pattern occurs in both outlets, the split ratio shows a strict relation to the bubble length. This strict relation between the split ratio and the bubble length were also concluded upon in the MSc thesis by Hong et al. (2011). In the defined centre regime changes in superficial velocities showed to have a negligible effect on the split ratio in comparison to variation in the bubble length. Long bubbles yields a split ratio located closest to perfect distribution. Decreasing the bubble length yields an increase in the void fraction (gas) in the branch. A model for prediction of the split ratio has been proposed. It is primarily valid within the centre regime, and is based on the time and area averaged Bernoulli equation. The model takes the bubble length into account, and predicts the split ratio on the main assumption that an increased amount of energy is lost to friction and separation as the fraction of water in the branch is increased. This while keeping the total fluxes in each of the outlets constant. An anticipated trend has been located through evaluating the model against experimental data. Therefore the model has been concluded upon to be physically sound.