University of Toronto
Toronto, Ontario, Canada
Activation of a photochromic dye material dissolved in a transparent liquid by a UV laser beam can induce a change in liquid color and formation of dye traces whose motion can be photographed to study the instantaneous, two-dimensional flow fields in the liquid phase. The authors group has made use of this Photochromic Dye Activation (PDA) technique to study both qualitatively and quantitatively the flow structures in various gas-liquid two-phase flow regimes, including annular and stratified/wavy flows in a horizontal pipe, slug flow in horizontal and vertical pipes, and falling films and countercurrent annular flows in vertical channels.
The measurement of instantaneous velocity profiles at or near the gas-liquid interface is necessary to yield information concerning the structure of turbulence which is the key to understanding the mechanisms of mass, momentum and energy transport between the gas and liquid phases. Such information has been difficult to obtain using the conventional measurement techniques such as hot film and Laser Doppler Anemometry, when the interface position fluctuates continuously as in wavy surfaces. The application of the PDA technique to measure the two- dimensional velocity profiles and turbulence intensity profiles has been successfully accomplished to reveal the effects of interfacial waves on interfacial shear, turbulence and/or wall shear stress in wavy falling films and stratified/wavy flows.
In a study of horizontal annular flow, the mechanism of liquid transport around the tube wall in the circumferential direction has not been well understood. The PDA technique has been used to determine that the disturbance waves are mainly responsible for the liquid transport from the bottom to the top of the horizontal tube, and that the smooth film after the passage of the disturbance waves drains continuously due to gravity. These results suggest that prevention of liquid film dryout in horizontal boiler tubes may be achieved by engineered control of the disturbance wave generation and passage.
The highlights of these and other findings from the recent experimental research based on the PDA measurements will be presented along with the video footage of interesting flow structures in gas-liquid two-phase flow.