Mechanism of non-axisymmetric pipe-wall thinning in pipeline with elbow and orifice under influence of swirling flow

In this study, the mechanism of non-axisymmetric pipe-wall thinning that led to a pipeline break in the Mihama nuclear power plant in 2004 is evaluated in a scale-model experiment in a water tunnel having an elbow and orifice under the influence of swirling flow. The velocity fields are measured by stereo particle image velocimetry, and the mass transfer rate is measured by a benzoic acid dissolution method at Reynolds number Re = 3 × 104 with and without swirling flow. The non-axisymmetric swirling flow is found to be generated behind the elbow, even when the axisymmetric swirling flow is supplied in the upstream of the elbow. The secondary flow generated in the elbow is not suppressed in the pipeline 10 diameters downstream of elbow in the swirling flow, and in this flow geometry, the non-axisymmetry of the flow is greatly magnified downstream of the orifice. The measured mass transfer rates downstream of the orifice under the influence of swirling flow indicate that the Sherwood number distribution on one side of the pipe is enhanced and that on the other side is reduced owing to the appearance of the non-axisymmetric swirling flow, which results in the occurrence of non-axisymmetric pipe-wall thinning downstream of the orifice.