Two-phase liquid-gas flow occurs in many safety systems of nuclear reactors as well as in reactor cores. To further improve both safety and commercial performance of nuclear reactors, it is important to improve numerical codes and deepen the understanding of two-phase flow with experiments on gas behaviour in liquids. Among several available measurement methods, ultrasound based methods are affordable and easy to use even for high pressure/temperature flows in non-transparent pipes. Ultrasound Reflector Recognition and Tracking Technique (URRTT) has been developed as a new technique. It uses an ultrasound transducer, which emits ultrasound beam into the liquid with gas bubbles. The phase interface reflects the beam and because of that, the phase interface can be recognised in the reflected signal and the distance (from the transducer) can be calculated. The core of this technique is the tracking algorithm that can separate data of different bubbles from each other and obtain their one dimensional trajectories along the measurement line. Trajectories measured simultaneously by more transducers (at different positions or from different directions) can be combined. That means trajectory of the bubble interface from one transducer can be connected to trajectory from a different transducer and by doing so, a secondary data can be obtained using the information that those trajectories belong to the same bubble. As an example, the average two dimensional velocity between two parallel measurement lines can be obtained. Another example is the measurement of the bubble size using one measurement line with two oppositely oriented transducers. Experiments have been conducted to prove the concept of URRTT. Results have been validated to data obtained by the image processing of footage taken by a high speed camera. The results obtained by URRTT can be of high value since each detected bubble is measured individually and thus, difference in the bubble behaviour based on the size, velocity or history of the bubble can be described.

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