Analysis of the characteristic lengths in the bubble and slug flow regimes generated in a capillary T-junction

We present an analysis of the geometry of the continuous and disperse phases in the bubble and slug flow regimes in air–water mixtures generated in a capillary T-junction of 1 mm internal diameter. Bubble size dispersion is very low in the considered flow patterns. The concept of unit cell is used to identify two characteristic lengths of the two-phase flow, namely, the unit cell length and the bubble length. The relationship between these lengths and the gas and liquid superficial velocities, gas mean velocity, bubble generation frequency and volume average void fraction is analysed. We conclude that in the considered configuration the unit cell and bubble lengths can be predicted either by the ratio of the gas–liquid superficial velocities or the volume average void fraction.Peer ReviewedPreprin

A 3D CFD numerical study of the bubble generation process into a bubble Tjunction generator and its comparison with experimental data: Part I

This work presents a 3D numerical study of the bubble generation process into a bubble generator obtained with the commercial Computational Fluid Dynamics solver ANSYS Fluent v15.0.7, and its comparison with experimental data reproducing the same conditions. The bubble generator is formed by two perpendicular capillaries in which liquid and gas are injected at perpendicular directions into a 1 mm internal diameter capillary T-junction with a total length of 10 mm. The fluids used in experiments and CFD simulations are air and water, both of them considered incompressible and isothermal, at a room temperature of 25º. A total of 23 different cases are studied for different injection conditions, and results between numerical simulations and experiments are compared. In this first part of the analysis, we focus on the flow pattern regimes and the dynamics of the bubble generation process. In addition to the new numerical simulations presented here, a new model has been used to predict the bubble generation frequency and tested with both experimental and numerical data. Results on bubble generation frequency are also presented by means of the non-dimensional Strouhal number. Same types of patterns, bubble and slug flow regimes, are obtained in simulations and experiments. In order to perform an exhaustive validation and comparison of numerical simulations with experimental data, several parameters have been selected: bubble velocity, volumetric void fraction, bubble generation frequency, Strouhal number and bubble equivalent diameter. Numerical simulations agree qualitatively, but not always quantitatively, with experimental results.Peer ReviewedPostprint (published version

Influence of contact angle boundary condition on CFD simulation of T-Junction

“This is a post-peer-review, pre-copyedit version of an article published in Microgravity Science and Technology. The final authenticated version is available online at: https://doi.org/10.1007/s12217-018-9605-x ”.In this work, we study the influence of the contact angle boundary condition on 3D CFD simulations of the bubble generation process occurring in a capillary T-junction. Numerical simulations have been performed with the commercial Computational Fluid Dynamics solver ANSYS Fluent v15.0.7. Experimental results serve as a reference to validate numerical results for four independent parameters: the bubble generation frequency, volume, velocity and length. CFD simulations accurately reproduce experimental results both from qualitative and quantitative points of view. Numerical results are very sensitive to the gas-liquid-wall contact angle boundary conditions, confirming that this is a fundamental parameter to obtain accurate CFD results for simulations of this kind of problems.Peer ReviewedPostprint (published version

Experimental analysis of the bubble-slug transition in a minichannel in microgravity conditions

Best student presentation in Physical SciencePeer ReviewedAward-winnin

Characterization of bubbly and slug flow regimes generated in a minichannel in microgravity conditions

We performed a characterization of the bubbly and slug flow regimes in minichannels in conditions relevant to microgravity. Two-phase flows were generated on ground by means of a T-junction device (1 mm i.d.), whose operation is independent to changes in gravity level [1]. Air and water were injected at several superficial velocities ranging from 0 up to 2 m/s. The generation and detachment of the minibubbles is provided by the liquid cross-flow (see figure). In nominal conditions small Bond and Weber numbers are achieved for the air/water mixture flow. Therefore, capillary forces dominate over buoyancy and inertial forces [2]. Laminar regime can also be assumed. Bubbly, slug, churn and annular flows were observed (see figure). The characterization consists in the analysis of the bubble and the unit cell lengths, the bubble velocity and the void fraction in both bubbly and slug flow regimes.Peer Reviewe

Bubble and slug flows characteristic lengths in a microchannel

Peer ReviewedPostprint (published version

Comparison of two methods for generating small bubbles in microgravity conditions

Peer ReviewedPostprint (published version

Impact study of contact angle boundary conditions for CFD simulation of two-phase flow generation in a T-junction

Gas – liquid two - phase flows dominated by capillary forces occur in countless modern industrial applications. Some examples can be found in normal gravity (mixing process, chemical reactions, emulsion technology, materials synthesis, medical science, persona l care products, etc), as well as in space - based systems (life - support systems for human exploration, thermal management systems, propulsion systems, chemical contactors, space bioreactors, etc). A better understanding and an improvement of two - phase flows generation techniques are thus mandatory for the optimal development of these technologies, both in normal and reduced - gravity environments. In the past, great efforts have been conducted to fulfil this goal, and several methods have been proposed, aiming to generate and control two - phase flows in a very accurate way. T-junction bubble generators have arisen as an efficient method providing trains of bubbles, immersed into a continuous liquid, with small dispersion in bubble size.Peer ReviewedPostprint (published version

Comparison of two methods for generating small bubbles with a T-junction in microgravity conditions

Peer ReviewedPostprint (published version

Perspectives on OpenFOAM numerical simulations of slope winds in Mars

Planetary boundary layer phenomena, such as slope winds and nocturnal low-level jets, are frequent on Mars, e.g., whenever diurnal surface temperature variations are significant, and large-scale winds are weak. In particular, thermally-driven winds are very common over sloping regions; they can reach significantly high speeds for steep slopes, and govern near-surface winds in many sites. Simulations of this type of flows on Earth have usually been conducted using commercial computational fluid dynamics software (CFD). In this work, a set-up is described for numerically simulating slope flows on Mars within the framework of the open-source code OpenFOAM CFD. For this purpose, thermally-driven winds are analysed in an idealised 2D Martian mountain-valley system with realistic values of parameters such as the slope angle, temperature difference between the atmosphere and slope, etc. The fluid domain dimensions, and the temperature and pressure initial and boundary conditions used in this work were tested and proven suitable in previous similar investigations for Earth applications. Large-eddy simulations (LES) turbulent model is used here. The results obtained for velocity and temperature profiles are shown both for anabatic (up-slope) and katabatic (down-slope) winds. This study shows the potential of the proposed methodology and the performance of OpenFOAM. Future work aims at facilitating the screening of large numbers of candidate landing sites for future Mars missions, and at estimating the thermally-driven wind speeds for wind resource assessment.This work has been supported by the project PID2019-105162RB-I00 funded by MCIN/AEI/10.13039/501100011033 and by the project 2017 SGR 1278 from the AGAUR from Generalitat de CatalunyaPeer ReviewedObjectius de Desenvolupament Sostenible::7 - Energia Assequible i No Contaminant::7.2 - Per a 2030, augmentar substancialment el percentatge d’energia renovable en el con­junt de fonts d’energiaObjectius de Desenvolupament Sostenible::7 - Energia Assequible i No ContaminantObjectius de Desenvolupament Sostenible::7 - Energia Assequible i No Contaminant::7.1 - Per a 2030, garantir l’accés universal a serveis d’energia assequibles, confiables i modernsPostprint (published version