Measurements of Near Wall Turbulent Structure in a Microbubble Flow Using a Highly Magnifying Telecentric PIV/PTV System

An experimental investigation has been made regarding a turbulent structure of a horizontal channel flow with microbubbles. In the present study, we approached by two techniques, one is a Highly Magnifying Telecentric PIV/PTV system, and another is a measurement using a shear stress sensor. In bubbly flow, the PTV measurement is hampered by existence of a large number of bubbles in optical path and scatter light on the bubble surface. The system enables clear measurements of bubbles and liquid phase from closer distance by using rectangular prisms and telecentric lens, and bubbles' form measurements by using laser induced fluorescent and shadow imaging technique. The experiments have been carried out for flow conditions with Reynolds number ranging from 10,000 to 50,000 and void fraction up to 0.04. In the case of Reynolds number is 10,000, the large bubbles of approximately 5mm in diameter, which creep on the wall, increased a velocity gradient near the wall. The velocity profile was reduced with the increasing of the air injection by the bubble's diameter and form for Reynolds number of 30,000. In contrast, there was no noticeable change of the flow structure by bubbles for Reynolds number of 50,000, since bubbles diffused to the distant region from the wall. The shear stress measurements indicated that an increase by up to 40% of the drag force for Reynolds number is 10,000. On the other hand, the drag force was decreasing with an increasing of void fraction for Reynolds number is 30,000 to 50,000. The results of the shear stress sensor confirmed the Highly Magnifying Telecentric PIV/PTV measurements.