Development of a submersible small fiber LDV probe and its application to flows in a 4 × 4 rod bundle

Optical measurement is an effective and accurate method to understand flow structures of multiphase flows. However, we frequently encounter difficulty in optical access due to the presence of interface and the complex geometry of a flow field. In the present study, we have developed small LDV optics to avoid the difficulty in optical access by installing the LDV probe into a channel component or by submerging the LDV probe into a flow field. The developed LDV optics consists of a manipulator and the probe. The manipulator splits a beam emitted from an Ar-ion laser into four beams. Each beam is introduced into an optical fiber through beam adjustment optics, and is transmitted to the probe through the fiber. The diameter of the probe is 7 mm. The laser beams emitted from the fibers in the probe are focused by a front lens of the probe and form a measurement volume. Scattered light from tracer particles are received by the probe and collected into a receiving fiber installed in the probe. The developed system is applied to laminar and turbulent flows in a rectangular duct for validation. Comparisons with a conventional LDV system show that the accuracy and reliability of the developed system are the same level as the conventional system. The application of the developed system to single-phase and bubbly flows in a 4 × 4 rod bundle demonstrates that the system has a large potential of measuring single-phase and multiphase flows in a channel with a complex geometry.