Application of the laser-induced fluorescence to study local characteristics of a gas-liquid flow in rectangular microchannel

The Laser-Induced Fluorescence (LIF) method was used to characterize liquid phase distribution in rectangular slit microchannel with cross-section 200×1205 μm for horizontal gas-liquid flow. Ethanol and nitrogen were used as working liquid and gas accordingly. The feature of this study is an application of hydraulic focusing cross-junction mixer for obtaining elongated bubble and transition flows in the microchannel with a high aspect ratio. Using LIF measurements for elongated bubble and transition flows the liquid film distributions were obtained for different distances from the bubble top and average liquid film thickness was compared with the prediction according to Taylor’s law.

Computational Study of the Dynamics of the Taylor Bubble

We perform high-resolution numerical simulations of three-dimensional dynamics of an elongated bubble in a microchannel at moderate Reynolds numbers up to 1800. For this purpose, we use the coupled Brinkman penalization and volume of fluid methods implemented in the open-source framework Basilisk. The new results are validated with available experimental data and compared with previous numerical and theoretical predictions. We extend existing results to regimes with significant inertia, which are characterized by intense deformations of the bubble, including cases with azimuthal symmetry breaking. Various dynamical features are analyzed in terms of their spatiotemporal characteristics, such as frequencies and wavelengths of the bubble surface undulations and vortical structures in the flow.

Subcooled Flow Boiling on Micro-Porous Structured Copper Surface in a Vertical Mini-Gap Channel

An experimental investigation of subcooled flow boiling in a rectangular mini-gap channel with the dimension of 0.5 mm × 5 mm was conducted with deionized water as the working fluid. Fabricated by electroless plating method and high-temperature treatment, the copper-based hydrophobic micro-porous surface was utilized in the experiments. High-speed flow visualization was conducted to picture the flow patterns during the experiment. The mass fluxes were in the range of 200–400 kg/m2s, and the wall heat fluxes were spanned from 35 to 350 kW/m2. The onset of flow boiling, heat transfer coefficient, and pressure drop were discussed with the variation of heat fluxes and mass fluxes, the trends of which were analyzed along with the flow patterns. Because of the numerous nucleation sites on micro-porous surface, the superheat required for the onset of boiling are of small amounts of about 2 K. Due to the intense nucleation process, the boiling curves appeared to be a negative slope after the onset of boiling, which was more obvious in the lower mass flux conditions. In the high heat flux conditions, heat transfer coefficients under lower mass flux condition were higher because the intense nucleation process occurred and the elongated bubble flow along with the film evaporation heat transfer was formed. The film evaporation heat transfer inside the elongated bubble is more efficient to release the latent heat than the nucleate boiling. However, the appearance of the elongated bubble flow would attribute to higher pressure drop and severer pressure drop fluctuation due to its expansion toward upstream.

Simulation of Intermittent Flow Development in a Horizontal Pipe

In this study, detailed three-dimensional (3D) numerical simulations of intermittent multiphase flows were carried out to investigate the slug initiation process and various features of intermittent flows inside a horizontal pipe. Air and water are used as working fluids. The domain used for simulations is a 14.4 m long pipe with 54 mm inner diameter. The volume of fluid (VOF) model was used to capture the air/water interface and its temporal evolution. Using the developed computational fluid dynamics (CFD) model, the slug formation and propagation along horizontal circular pipe were successfully predicted and studied comprehensively. Slug length and the frequency of slug formation, as two main features of intermittent flow, were used to validate the model against experimental results and available correlations in the literature. Three-dimensional numerical simulation of intermittent flow proved to be a powerful tool in tackling limitations of experiments and providing detailed data about various features of the intermittent flow. The effect of gas and liquid superficial velocities on the liquid slug and elongated bubble length was explored. Moreover, the study revealed new findings related to the elongated bubble shape and velocity field in the slug unit.