Baixin Chen

Experimental and numerical investigation of acoustic streaming excited by using a surface acoustic wave device on a 128° YX-LiNbO3 substrate

This work uses a finite volume method to investigate three-dimensional acoustic streaming patterns produced by surface acoustic wave (SAW) propagation within microdroplets. A SAW microfluidic interaction has been modelled using a body force acting on elements of the fluid volume within the interaction area between the SAW and fluid. This enables the flow motion to be obtained by solving the laminar incompressible Navier–Stokes equations driven by an effective body force. The velocity of polystyrene particles within droplets during acoustic streaming has been measured and then used to calibrate the amplitudes of the SAW at different RF powers. The numerical prediction of streaming velocities was compared with the experimental results as a function of RF power and a good agreement was observed. This confirmed that the numerical model provides a basic understanding of the nature of 3D SAW/liquid droplet interaction, including SAW mixing and the concentration of particles suspended in water droplets.

Small-scale modelling of the physiochemical impacts of CO2 leaked from sub-seabed reservoirs or pipelines within the North Sea and surrounding waters

A two-fluid, small scale numerical ocean model was developed to simulate plume dynamics and increases in water acidity due to leakages of CO2 from potential sub-seabed reservoirs erupting, or pipeline breaching into the North Sea. The location of a leak of such magnitude is unpredictable; therefore, multiple scenarios are modelled with the physiochemical impact measured in terms of the movement and dissolution of the leaked CO2. A correlation for the drag coefficient of bubbles/droplets free rising in seawater is presented and a sub-model to predict the initial bubble/droplet size forming on the seafloor is proposed. With the case studies investigated, the leaked bubbles/droplets fully dissolve before reaching the water surface, where the solution will be dispersed into the larger scale ocean waters. The tools developed can be extended to various locations to model the sudden eruption, which is vital in determining the fate of the CO2 within the local waters.

Streaming phenomena in microdroplets induced by Rayleigh surface acoustic wave

This paper reports the numerical simulation and experimental characterization of three-dimensional acoustic streaming behavior of a liquiddroplet subjected to a Rayleigh surface acoustic wave. The streaming behavior of the droplet was studied as a function of radio-frequency (RF) power, aperture of the interdigitated transducer, and size of the liquiddroplet. The hydrodynamic flow field within the droplet was determined by solving the laminar incompressible Navier–Stoke’s equations. The numerical and experimental results are shown to be in good agreement over the range of parameters examined. The ratios of the position of butterfly central line (axis of rotation) to radius of the droplet are demonstrated to be fairly constant for moderate droplet volumes and to vary by less than 12% at large droplet volumes. Besides that, an increase in the RF power and a decrease in the droplet size result in an increased surface acoustic wave(SAW) streaming velocity. The numerical results also suggest that a maximum streaming velocity is achieved when the SAW width is approximately half of the droplet radius.

Frequency effect on streaming phenomenon induced by Rayleigh surface acoustic wave in microdroplets

Acoustic streaming of ink particles inside a water microdroplet generated by a surface acoustic wave(SAW) has been studied numerically using a finite volume numerical method and these results have been verified using experimental measurements. Effects of SAW excitation frequency, droplet volume, and radio-frequency (RF) power are investigated, and it has been shown that SAW excitation frequency influences the SAWattenuation length, lSAW , and hence the acoustic energy absorbed by liquid. It has also been observed that an increase of excitation frequency generally enhances the SAW streaming behavior. However, when the frequency exceeds a critical value that depends on the RF power applied to the SAW device, weaker acoustic streaming is observed resulting in less effective acoustic mixing inside the droplet. This critical value is characterised by a dimensionless ratio of droplet radius to SAWattenuation length, i.e., Rd/lSAW . With a mean value of Rd/lSAW  ≈ 1, a fast and efficient mixing can be induced, even at the lowest RF power of 0.05 mW studied in this paper. On the other hand, for the Rd/lSAW ratios much larger than ∼1, significant decreases in streaming velocities were observed, resulting in a transition from regular (strong) to irregular (weak) mixing/flow. This is attributed to an increased absorption rate of acoustic wave energy that leaks into the liquid, resulting in a reduction of the acoustic energy radiated away from the SAW interaction region towards the droplet free surface. It has been demonstrated in this study that a fast and efficient mixing process with a smaller RF power could be achieved if the ratio of Rd/lSAW  ≤ 1 in the SAW-droplet based microfluidics.

Measurement on CO_2 Solution Density by Optical Technology

The optical technology based on Mach-Zehnder interferometry was successfully applied to a high-pressure liquid CO2 and water system to measure CO2 solution density. Experiments were carried out at a pressure range of from 5.0 to 12.5 MPa, temperatures from 273.25 to 284.15 K, and CO2 mass fraction in solution up to 0.061. CO2 solution density data were obtained from two sets of experiments. These data were calculated through the fringe shifts induced by density changes inside of the high-pressure vessel, which were directly recorded during the experiments, and a modified version of Lorentz-Lorenz formulation. The experimental results indicated that the density ratio of CO2 solution to that of pure water at the same pressure and temperature is monotonically linear with the CO2 concentration in the solution. The slope of this linear function, calculated by the experimental data fitting, is 0.275.

Measurement of the Density of CO2 Solution by Mach-Zehnder Interferometry

Abstract: The density of CO2 solution was measured by using Mach‐Zehnder interferometry in the pressure range from 5.0 to 12.5 MPa, at temperatures from 273.25 to 284.15 K, and CO2 mass fraction in solution up to 0.061. It was found that the density difference between the CO2 solution and pure water at the same pressure and temperature is monotonically linear with the CO2 mass fraction. The slope of this linear function, calculated by experimental data fitting, is 0.275.

13 – Environmental risks and performance assessment of carbon dioxide (CO2) leakage in marine ecosystems

Abstract: This chapter describes the state of the current understanding of the potential for CO 2 leaked from carbon dioxide (CO 2 ) capture and storage (CCS) to impact the marine ecosystem. This is a complex problem as it requires an understanding of physical dispersion, the behaviour of plumes, marine chemistry, organism physiology and ecological relationships. Aside from predicting the likelihood of a leak event, the key issue is to understand the spread, persistence and impact of a hypothetical CCS derived leak and contrast this with, for example, trawling impacts and the global long-term consequences of climate change and the uptake of anthropogenically created atmospheric CO 2 (ocean acidification), which CCS seeks to mitigate. Excess CO 2 in the marine system is undoubtedly harmful to many organisms. In the vicinity of a leak event, it is likely that significant ecological alteration would occur. Initial research indicates that only persistent leaks of a significant proportion of reservoir capacities would cause widespread and unacceptable impacts. However, much more research is required to determine critical leak magnitudes, within sediment interactions and ecosystem recovery before any comprehensive risk assessment of CCS can be delivered.

CO2 release in deep ocean by moving ship

Publisher Summary This chapter illustrates that the moving-ship type of CO2 ocean storage is a concept whereby captured and liquefied CO2 is delivered by ship to a site and injected into the ocean depths by means of a pipe suspended beneath it as it slowly moves through the water. In addition to the horizontal movement of the release point, the rising and gradual dissolving behavior of CO2 droplets contributes to the dilution of CO2 in seawater. From the point of view of dilution, deeper injection and faster movement of the release point are preferable, and a technology to make droplets of an appropriate initial size is needed to control the distance of the vertical journey of CO2 droplets. It discusses that the engineering feasibility of a long pipe suspended and towed by a moving ship is first investigated. A streamlined nozzle for making droplets of an appropriate initial size is designed and examined in a towing tank and also in a high-pressure tank. The terminal velocity of a rising CO2 droplet under deep ocean circumstances is measured in a high-pressure tank taking into direct account the influence of a covering hydrate film. Finally, a numerical case study is performed to assess the effects of initial CO2 droplets on the plume dynamics, implementing the observation data taken from the experiments to a two-fluid CO2 plume evolution model.

Numerical Simulation of the Inner Structure of a Two-phase Plume Formed in a Stratification Environment

The inner structure of a two-phase plume, driven by air bubble buoyancy and formed in a stratification ambient fluid in a rectangular tank, is numerically simulated by means of two-phase flow theory and Large-eddy simulation technology. Focusing on the discrete nature of the buoyant dispersed phase and on the role of momentum exchange between two phases during plume formation, we investigated the phenomena of mass “entraining-in” and “peeling-out” that occurs inside the stratified ambient plume. These phenomena are thought to result from an intricate interplay among phase interaction, static stability of the stratification ambient fluid itself, and dynamic stability due to turbulence. Numerical simulations show that there exists an inner-out structure of the stratified ambient plume, while at the same time predicting that the re-entraining-in mass flux is on the same order of magnitude as that of the inner peeling-out mass flux within the annular region centered around the plume. This further explains the mechanism underlying the formation of multi-scale eddies at the edge of the air bubble plume, which also constitutes the boundary between the inner and outer zones of this inner-out stratified fluid plume. Within the inner part of the plume, the mass entraining-in and peeling-out appeared as a spatial discontinuity. The numerically visualized three-dimensional density fields are consistent with the two-phase plume characteristics.

Multipseudopotential interaction: A consistent study of cubic equations of state in lattice Boltzmann models

A method is developed to analytically and consistently implement cubic equations of state into the recently proposed multipseudopotential interaction (MPI) scheme in the class of two-phase lattice Boltzmann (LB) models [S. Khajepor, J. Wen, and B. Chen, Phys. Rev. E 91, 023301 (2015)]10.1103/PhysRevE.91.023301. An MPI forcing term is applied to reduce the constraints on the mathematical shape of the thermodynamically consistent pseudopotentials; this allows the parameters of the MPI forces to be determined analytically without the need of curve fitting or trial and error methods. Attraction and repulsion parts of equations of state (EOSs), representing underlying molecular interactions, are modeled by individual pseudopotentials. Four EOSs, van der Waals, Carnahan-Starling, Peng-Robinson, and Soave-Redlich-Kwong, are investigated and the results show that the developed MPI-LB system can satisfactorily recover the thermodynamic states of interest. The phase interface is predicted analytically and controlled via EOS parameters independently and its effect on the vapor-liquid equilibrium system is studied. The scheme is highly stable to very high density ratios and the accuracy of the results can be enhanced by increasing the interface resolution. The MPI drop is evaluated with regard to surface tension, spurious velocities, isotropy, dynamic behavior, and the stability dependence on the relaxation time.