Shadowgraph visualisations of salt-stratified turbulence obtained in a stratified inclined duct (SID) laboratory experiment

A collection of 113 experimental datasets corresponding to flow visualisations obtained in the G. K. Batchelor Laboratory (Department of Applied Mathematics and Theoretical Physics, University of Cambridge). The experiments were setup to create stratified turbulence generated via an exchange flow through a long tilted duct (L=2000, H=50, W=100 mm) connecting two large reservoirs (400 litres each) containing saltwater (NaCl) at different densities. The movies consist of shadowgraph visualisations, where parallel light was shone through the flow and projected onto a semi-transparent screen, where it was recorded by a video camera. The movies show the evolution of density (salinity) contrasts in the flow, including the formation and break up of density interfaces, as well as scouring and overturning behaviours. Each of the 113 movies corresponds to a distinct experiment, collectively covering a two-dimensional parameter space (Reynolds number and tilt angle) where various turbulent behaviours take place. This allows the study of a range of turbulent mixing processes responsible for transporting mass and momentum in stratified fluid systems like the oceans. Please refer to the READ_ME file for detailed information on the folder contents and organisation.European Union’s Horizon 2020 research and innovation Grant No 742480 ‘Stratified Turbulence And Mixing Processes’ (STAMP) for the experimental facility, X. Jiang, and G. Kong. Leverhulme Trust Early Career Fellowship for A. Lefauve

Experimental study of gas-Liquid Mass transfer in a rectangular microchannel by digital image analysis method

Study of mass transfer in microchannels is indispensable for the design of microreactors. Gas-phase volume monitoring method has been widely used to study the mass transfer process.When using this method, bubble length was measured in most studies to calculate the bubble volume by assuming a symmetrical bubble shape. Therefore, this method is not suitable for asymmetric bubbles. The present study focuses on the mass transfer of CO2 bubbles in a flat rectangular microchannel by using the method of digital image analysis (DIA), especially for deformed bubbles. The dynamics of gas-liquid flow at different volumetric flow rates were observed by a high-speed recording system. Flow patterns were mapped and scaling laws were given for bubble size and bubble velocity. The results showed that the bubble volume increases as gas flow rate increases, while decreases as liquid flow rate increases. It can be explained by the bubble breakup mechanism. Besides, the bubble velocity increases as gas and liquid flow rates increase. The mass transfer of CO2 from bubbles to liquid slugs was quantitatively characterized by volumetric mass transfer coefficient kLa. The results showed that kLa and kL increase with increasing of superficial gas and liquid velocities. The same tendencies can be found in the literature. Finally, new mass transfer correlations were proposed. Predictions from the correlations showed a good agreement with the experimental data

Mass transfer and modeling of deformed bubbles in square microchannel

Understanding of mass transfer in gas-liquid slug flow is imperative to design and optimize micro-reactors. There exist extensive studies on symmetric bubbles by the phase volume monitor technique, whereas deformed bubbles are rarely studied due to the limitation of volume calculation methods. In this work, CO2-water and N2-water two-phase flows were investigated in a square microchannel, obtaining annular flow, slug flow, and bubbly flow. A flow pattern map was then proposed and compared with the literature. A 3D slicing technique was developed to measure the volume and interfacial area of bubble, including symmetric bubbles and deformed bubbles, by slicing the bubble along the streamwise direction. Scaling laws of the important parameters that characterize the micro-reactors were proposed. Mass transfer coefficients kLa were quantified from the time-changing volume. The empirical correlation involving dimensionless numbers were fitted, which shows accurate predictive performance for mass transfer coefficients in this study and literatures. The bigger index of Reynolds number ReG indicated that gas flow condition is the main influencing factor during mass transfer process. To have a better universality, a new semi-theoretical model involving the ratio of the size of the liquid and gas phases LL/LG was developed based on the Pigford and Higbie penetration theory because experimental data confirms that the degree of bubble deformation is related to LL/LG. The semi-theoretical model shows a satisfactory agreement over the whole range of slug flow in this study

Hydrodynamics of gas-liquid displacement in porous media: fingering pattern evolution at the breakthrough moment and the steady state

Gas-liquid displacement in porous media widely exists in many terrestrial/extraterrestrial subsurface resource extraction and utilization applications. The typical fingering displacement during gas invading has been well identified through extensive research efforts. Yet, the evolution of fingering structures after invading breakthrough is rarely reported. Herein, through a joint approach of experimental flow imaging and digital image processing, we investigated the gas-liquid fingering displacement in a porous-patterned microfluidic chip from the breakthrough moment until reaching the steady state. With a wide range of capillary number Ca and viscosity ratio M, we visualized the evolution of finger morphologies in different flow regimes including capillary fingering (CF), viscous fingering (VF), and crossover zone (CZ). Interestingly, we found that finger structures of CF regime remain the same after the breakthrough, whereas fingers of VF regime keep expanding until almost all the pore space is invaded and eventually reaches to steady state. Followed with experimental observations, a comparative quantification of fingering patterns was also conducted in terms of invasion velocity, phase saturation and fractal dimension. A dramatic increase of gas saturation, from 0.15 to 0.60 at the case of Log10Ca=- 5.17 and Log10M=-2.78, is obtained in the VF regime when the steady state is reached, so is the fractal dimension (from 0.14 to 0.16, even higher than one of CF). The underlying mechanism of such fingering expansion in VF is further revealed from the time evolution of fingering after breakthrough. A previously unobserved fingering cycle, consisting of new finger forming, cap invading, breakthrough and finger vanishing, keeps repeating until the saturation reaches the maximum. We believe that these findings are of significance in evaluating extraction effectiveness, economic benefits and storage safety for subsurface applications

Hydrodynamic interaction of bubbles rising side‑by‑side in viscous liquids

Abstract: Detailed experiments are conducted to study hydrodynamic effects of two simultaneously released bubbles rising in viscous liquids. Different types of interactions are observed as a function of the liquid viscosities, leading to different bubble shapes, ranging from rigid spheres and spheroids to deformable spheroids. Bubble velocities are obtained by an automated smooth spline technique, which allows for an accurate calculation of the lift and drag forces. The results obtained for spherical bubbles are in agreement with predictions of Legendre et al. (J Fluid Mech 497:133–166, 2003). The observations of deformed bubbles show that a very small equilibrium distance can be established due to the induced torque arising from the deformation. In terms of the lateral interaction, different separation distances can be observed depending on the initial distance. For deformable bubbles, the results are limited to a qualitative analysis due to limitations of the processing technique to handle strong shape irregularities. Nevertheless, the observations reveal that the deformation plays an important role with respect to bubble interactions and path instability of which the latter can be triggered by the presence of other bubbles. Graphic abstract: [Figure not available: see fulltext.]

Influence of wetting conditions on bubble formation from a submerged orifice

The formation of gas bubbles by submerged orifices is a fundamental process encountered in various industrial applications. The dynamics of the contact line and the contact angle may have a significant influence on the detached bubble size depending on the wettability of the system. In this study, the influence of wetting conditions on the dynamics of bubble formation from a submerged orifice is investigated experimentally and numerically. The experiments are performed using a hydrophobic orifice plate and a series of ethanol–water solutions to vary the wettability where the key characteristics of the bubbles are measured using a high-speed, high-resolution camera. An extensive analysis on the influence of wetting conditions on the bubble size, bubble growth mechanism and the behavior of the contact line is given. Bubble growth stages, termed (1) hemispherical spreading, (2) cylindrical spreading, (3) critical growth and (4) necking, are identified based on key geometrical parameters of the bubble and relevant forces acting on the bubble during the growth. The experimental results show that the apparent contact angle varies in a complicated manner as the bubble grows due to the surface roughness and heterogeneity. The experimental findings are finally used to validate the local front reconstruction method with a contact angle model to account for the contact angle hysteresis observed in the experiments. Graphic abstract: [Figure not available: see fulltext.