Characterization of swirling-flow behavior in complex pipeline using bubble trajectory method with stereo particle tracking/image velocimetry

To understand the swirling-flow behavior in a complex pipeline geometry comprising an elbow and orifice, a quantitative flow visualization technique is introduced into the flow-field observation using a bubble trajectory method combined with stereo particle tracking/image velocimetry. For the chosen bubble sizes of approximately 2 mm un a pipe of 56 mm in diameter, their motion is correlated with the mean swirling flow, and when analyzed in a straight swirling flow, the bubbles cluster close to the point of maximum axial velocity for swirl intensities larger than 0.3. Subsequently, this experimental technique is applied to the characterization of a swirling flow in the Mihama pipeline model, for which the streamwise variation of the location of the maximum axial velocity and its magnitude are measured over the full range of the pipeline model. A spiral swirling flow is observed downstream of the elbow owing to the combined effect of the swirling and secondary flows in the elbow. The spiral swirling flow persists in the following straight pipe down to the orifice, where a highly accelerated flow with a high turbulence is generated towards the top wall of the pipe. This result agrees closely with the observation of pipe-wall thinning in the prototype Mihama pipeline.