(202e) A Numerical Study On Behavior Of Single CO2 Droplet Released Into The Ocean

This study deals with dynamics of single liquefied CO2 droplet released into the seawater to carry out the fundamental design for our new concept, COSMOS, which is lead by National Marine Research Institute (NMRI), in cooperation with National Institute of Advanced Industrial Science and Technology (AIST), and IHI Marine United Inc. This concept is proposed to contribute to sequestration and storage of CO2 in the deep part of the ocean for injecting liquefied CO2 droplets onto the sea bottom from an injection nozzle installed by an offshore floating system structure, as one of the options for mitigating the global warming effect. These CO2 droplets are made by a mixture of liquefied CO2 and dry ice at the CO2 triple point (0.52 MPa and 216.6 K), whose density is typically larger than that of the seawater. An interaction of these CO2 droplets and the turbulent seawater in the deep ocean will be one of the key issues to be clarified for realization of the newly proposed concept.

We implement a numerical assessment for conducting our feasibility study by investigating fluid dynamics of liquefied CO2 in the turbulent seawater. Single CO2 droplet is considered in the present study to simplify the present flow situation. This droplet is assumed spherical initially, and very early stage of behavior is assessed by a computational fluid dynamics (CFD) technique. The seawater is considered incompressible and Newtonian, and turbulence in this is homogeneous and isotropic, which is an assumption of marine turbulence in the deep seawater without influence of the sea bottom. We carry out the present assessment based on a direct numerical simulation of turbulent seawater with single droplet by changing the Reynolds and Froude numbers. This parameterization is equivalent to the assessment for discussing the effects turbulence and gravitation on dynamics of the droplet. The effect of the surface tension between the droplet and the seawater is neglected here for simplicity of the problem.

The results of this study suggest that the effect of turbulence distorts the droplet very rapidly and violently, in particular under large gravitational effect, which corresponds to the situation of large density difference between the droplet and the seawater. On the other hand, deformation of the droplet is very calm in the case of weak turbulence, even if the gravitational effect is large. The present assessment indicates the possibility of droplet breakup by the secondary flows between the droplet and seawater. The assessment warns us that careful selection for the location of the device installation has to be made to achieve sequestration and storage of CO2 droplet on the sea bottom without dispersion of the greenhouse gas into the ocean.