Abstract
Many of unintrusive velocimetry techniques measure the velocity of tracer particles mixed in fluid. In gas-liquid two-phase bubbly flow, the tracer particles accumulate on the gas-liquid interface and therefore the flow around a bubble might be affected by a possible change of the boundary condition on the interface. To avoid the influence of the tracer particles on the flow, molecular tagging velocimetry is one of the preferable methods because it tracks moleculartags and does not require the tracer particles to measure the fluid velocity. In the present study, we developed a molecular tagging velocimetry using photobleaching which is a photodegradation of fluorescent dye induced by intense illumination. The regions in which photobleaching took place were visualized by a laser sheet and the liquid velocity was measured by tracking the region. The method was applied to laminar pipe flows and the flows around single bubbles. As a result, we confirmed that (1) the measured liquid velocity in a laminar pipe flow agrees well with the analytical value, (2) the present method enables us to measure the velocity in the vicinity of bubbles and (3) the shear stress can be evaluated by measuring the deformation rate of the tags.
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