The gas–liquid separator is a key component in the gas removal system in thorium molten salt reactor (TMSR). In this paper, an experimental study focusing on the gas core formation in the gas–liquid separator was carried out. We observed that formation of the air core depends primarily on the back pressure in the separator. Gas core formation was visualized for a range of back pressures, swirl numbers, and Reynolds numbers. Analysis of flow patterns indicated that gas core formation may be defined as four stages: “air core with suction,” “tadpole-shaped core,” “cloudy core,” and “rod core.” When rod core is achieved, gas bubbles will be separated completely and that particular back pressure is defined as critical back pressure. The critical back pressure depends on swirl number and Reynolds number. The trends how the critical back pressures vary with the Reynolds number and the swirl number were analyzed.

For pump turbines, an S-shaped curve can lead to failures in synchronization. To improve the hydraulic design, the component that is responsible for the formation of the S-shaped curve was identified by a hydraulic loss analysis using previous computational fluid dynamics (CFD) results, which indicates that the formation of the S-shaped curve can be ascribed to the runner. To improve the hydraulic design of the runner, a simple numerical approach for direct problem analysis was proposed, based on the blade-loading distributions of runners with and without an S-shaped curve, and directly analyzed. It was implied from the differences in the blade-loading distributions that, when the meridional passage was broadened, the formation of the S-shaped curve was suppressed. To validate this, two runners with different meridional sections were designed by means of the inverse design method. Through model tests, it was verified that the S-shaped curve was eliminated completely and the performance curve of the modified hydraulic model satisfied the requirements for safe operation in a pumped storage plant.

An improved shadow-based imaging method combined with a ray-tracing algorithm is proposed for three-dimensional (3D) reconstruction of bubble mesostructures. The 3D micro-scale interface measurement of bubbles categorized as transparent objects is challenging at the forefront of research in the field of multiphase flows. In the study, a reconstruction algorithm is developed using a single high-speed camera under a small-angle diffused backlight source based on Helmholtz reciprocity, Gaussian optics, and Snell’s law. The accuracy of the algorithm is assessed by comparing the reconstruction results of the rendered images generated from the 3D model using Blender. Standard models include a transparent hollow sphere model and several irregular bubble models. The accuracy of the algorithm is determined by analyzing and comparing the contour lines along the edges of the bubble models. The error of the method in the paper can be improved below 5% with the ray tracing optimization. The average uncertainty U _c is 2.8%, according to the evaluation of the models in Blender. The extreme deformation of the bubbles in a turbulent flow field enhanced by ultrasonic is performed to evaluate the 3D reconstruction algorithm. Prior to the fragmentation of bubbles into micro/nanobubbles by ultrasonication, the bubble surface undergoes severe deformation under continuous bombardment by small-scale turbulence eddies. The proposed algorithm exhibited high accuracy and easily performing in experiments.

Non-uniform inflow at the inlet of a reactor coolant pump generates performance variations, such as in the head, efficiency, cavitation, and vibrations. The three-dimensional velocity and pressure for non-uniform inflow are reconstructed via time-resolved tomographic particle image velocimetry to evaluate its effect on the reactor coolant pump. Five volume velocity fields were reconstructed through the multiplicative algebraic reconstruction technique, which constitutes the entire non-uniform flow field. The statistics of the velocity fields were used to study the non-uniform inflow characteristics. The non-uniform inflow contains two large-scale vortices in the form of counter-rotating vortex pairs. The influence of non-uniform inflow on the performance of the pump may be caused by the non-uniform pressure field. For power fluctuations during reactor operations, this non-uniform pressure distribution may generate a low-pressure region on the impeller, which induces cavitation. The non-uniform inflow shown by inhomogeneous vectors was evaluated statistically through the turbulent kinetic energy, which represents the velocity variance in each direction. For a constant head, the non-uniformity of the flow field increased with the flow rate, and the scale of small-scale turbulent vortices decreased. With proper orthogonal decomposition analysis, 90% of the energy region and flow structures were dominated by the previous 412 modes. Furthermore, the temporal modes 1, 2, 3, 100, 200, and 400 show that the frequency of large-scale eddy turnover was about 6.6–13.2 Hz. However, the large-scale eddy could be characterized within the first mode of the spatial distribution.