DEVELOPMENT OF A HIGH-SPEED PARTICLE IMAGE VELOCIMETRY TECHNIQUE USING FLUORESCENT TRACERS TO STUDY STEAM BUBBLE COLLAPSE
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Use of the particle image velocimetry flow visualization techique, with digital cameras for data acquisition, to study high speed fluid flows is usually limited by the camera frame acquisition rate. The velocity of the fluid under study must be limited to insure that particles suspended in the flow field remain in the camera's focal plane in successive images. However, the use of digital cameras for data acquisition is desirable to simplify and expedite the data analysis. A method is presented which will measure changes in the flow field that occur at different framing rates. This method was tested by acquiring the velocity field during experiments involving the injection of saturated steam into subcooled water. Firstly, changes in the position of tracer particles at a framing rate of 53.8 ms per frame were measured. White polystyrene tracer particles were used to follow the flow over an area of 18.9 16.5 mm2. Since the area under study is wide, the beam had to be expanded. This step lowers the available energy and highly-scattering tracer particles were necessary to obtain a sufficient exposure of the cameras. Secondly, changes at the relatively fast framing rate of 100 to 320 s per frame were measured. In this case the view area was smaller (7.3 5.7 mm2), so the beam did not have to be expanded vertically and the energy was sufficient to allow the use of fluorescent particles. In order to achieve this fast framing rate, the laser had to be operated in a double pulse mode with one camera exposed twice. The directional ambiguity presented by the double exposure can be resolved by capturing a single exposure of the second pulse with a second digital camera. The study of collapsing steam bubbles also presents the problem of distinguishing between the light reflected by the liquid-gas interface and the light refracted by the surrounding tracer particles. This problem was resolved through the use of fluorescent seeds and appropriate filters. The filter blocked most of the green light reflected by the bubbles and passed the red light emitted by the fluorescent seeds. 1994.
