STUDIES OF TWO-PHASE PLUMES IN STRATIFIED ENVIRONMENTS

Abstract

Two-phase plumes play an important role in the more practical scenarios for ocean sequestration of CO{sub 2}--i.e. dispersing CO{sub 2} as a buoyant liquid from either a bottom-mounted or ship-towed pipeline. Despite much research on related applications, such as for reservoir destratification using bubble plumes, our understanding of these flows is incomplete, especially concerning the phenomenon of plume peeling in a stratified ambient. To address this deficiency, we have built a laboratory facility in which we can make fundamental measurements of plume behavior. Although we are using air, oil and sediments as our sources of buoyancy (rather than CO{sub 2}), by using models, our results can be directly applied to field scale CO{sub 2} releases to help us design better CO{sub 2} injection systems, as well as plan and interpret the results of our up-coming international field experiment. The experimental facility designed to study two-phase plume behavior similar to that of an ocean CO{sub 2} release includes the following components: 1.22 x 1.22 x 2.44 m tall glass walled tank; Tanks and piping for the two-tank stratification method for producing step- and linearly-stratified ambient conditions; Density profiling system using a conductivity and temperature probe mounted to an automated depth profiler; Lighting systems, including a virtual point source light for shadowgraphs and a 6 W argon-ion laser for laser induced fluorescence (LIF) imaging; Imaging system, including a digital, progressive scanning CCD camera, computerized framegrabber, and image acquisition and analysis software; Buoyancy source diffusers having four different air diffusers, two oil diffusers, and a planned sediment diffuser; Dye injection method using a Mariotte bottle and a collar diffuser; and Systems integration software using the Labview graphical programming language and Windows NT. In comparison with previously reported experiments, this system allows us to extend the parameter range of our experiments to better match CO{sub 2} injection scenario field conditions and to make detailed measurements of the experimental plumes. To date we have run a basic suite of experiments covering a range of injected fluids (air and oil), flow rates (0.004 to 0.083 1/s), bubble/droplet sizes (0.05 to 1.5 cm), bubble slip velocities (5 to 35 cm/s), and ambient stratification types (linear and step). Qualitative observations of plume type (frequency and nature of peeling) from these experiments, as well as other experiments from the literature, have been correlated with dimensionless parameters to help us better characterize plume behavior. At the same time, quantitative data are being compared with a new analytical model which treats the flow as an upward-moving inner plume, coupled with an annular, downward-flowing outer plume which entrains from and intrudes into the ambient seawater. The model also includes CO{sub 2} specific features such as bubble/droplet mass transfer, solute dissolution effects on plume buoyancy, and change in total CO{sub 2} concentration and pH. We are continuing to conduct experiments which will provide additional data for model calibration. In turn, the model will be applied to CO{sub 2} injection to help us better understand field-scale CO{sub 2} releases, including those associated with the up-coming international field experiment.