MEASUREMENT AND CHARACTERIZATION OF DISTURBANCE WAVES IN PRESSURIZED TWO-PHASE FLOW WITH X-RAY DENSITOMETRY AND NOVEL PROCESSING METHODS

Abstract

This thesis investigates the formation and dynamics of disturbance waves in pressurized two-phase flow using a novel X-ray densitometry technique. Disturbance waves—large amplitude interfacial structures that develop along the liquid film in annular flow—play a critical role in momentum and mass transfer, droplet entrainment, and the onset of dryout in nuclear thermal-hydraulic systems. Despite their importance, accurate quantitative measurement of disturbance wave properties under prototypical reactor conditions remains a significant challenge due to limitations of conventional optical and electrical techniques. To address this gap, a high-speed X-ray imaging system was developed and integrated into a thermal-hydraulic loop capable of operating at pressures up to 10 MPa. A custom-designed, ultra-thin wall test section and a single-photon counting detector enabled high-resolution, non-intrusive measurements of flow structure. A novel image processing framework was introduced to correct for noise sources such as non-uniform illumination, frame instability, and count-rate fluctuations. This approach enabled accurate extraction of wave velocity, frequency, amplitude, and width in high-pressure, high-temperature water-steam two-phase flow. Experimental results demonstrated the capability of the X-ray system to resolve dynamic features of disturbance waves, revealing trends in wave properties as a function of inlet subcooling, pressure, and flow rates. These findings were compared with existing literature and empirical correlations, highlighting the effect of working fluid properties and test section length on wave development. This research advances the state-of-the-art in two-phase flow diagnostics by demonstrating the feasibility and advantages of X-ray densitometry for wave characterization. The findings provide essential data for improving closure models in system codes and subchannel analyses, contributing to more accurate prediction of flow behavior in nuclear reactor systems.