Thermofluid analysis of two-phase slug flow in microchannels with applications to leakage modeling in rotary-vane turbomachinery

Understanding the characteristics of two-phase slug flow inside microchannels is a challenging problem that has many applications in chemical, mechanical and biological processes. Even though many experimental studies focused on two-phase flows in microchannels, considerable discrepancies among the data still exist, mainly due to the difficulties in setting up the experiments and/or in conducting the measurements. Similarly, numerical modeling of these flows has failed to accurately capture the flow physics, especially those pertaining to the characteristics of the slug flow regime. The presence of a thin liquid film on the order of 10 mum around the bubble may be a contributing factor to the above difficulties.;This study describes slug flow hydrodynamics and heat transfer in microchannels and tries to overcome the aforementioned modeling difficulties.;The effects of the surface tension on hydrodynamics and dimensionless wall shear stress are investigated and a general Moody friction factor for a typical two-phase slug flow in microchannels is defined.;Formation of the slugs for different superficial velocities, capillary numbers, and gas volume fractions are investigated with the help of Volume-of-Fluid (VOF) method.;Innovative analytical models for the calculation of two-phase slug flow pressure drop in microchannels encountering a sudden contraction are introduced. In contrast to the existing empirical correlations which were tuned for a specific geometry, the new analytical models are capable of taking geometrical parameters as well as flow conditions into account.;A fundamental study of heat transfer characteristics of two-phase slug flow in microchannels showed that the convective heat transfer coefficient in the film region is significant. A strong coupling between the conductive heat transfer in the solid wall and the convective heat transfer in the flow field is observed and characterized. Local and time-averaged Nusselt numbers for slug flows are reported for the first time. Significant improvements in the heat transfer coefficient could be achieved by short slugs where the Nu number was found to be 610% higher than in single-phase flows.;Understanding the physics of two-phase flows in micro-scale leakage paths in any devices is one of the applications of this study. This study demonstrates that large internal leakage that inherently exists in rotary-vane expanders undermines its functionality. The developed models in this study employed to estimate the internal leakages in a rotary-vane expander.