Date of Award


Degree Type


Degree Name

Doctor of Philosophy (PhD)


Nuclear Engineering


Dr. J.S. Chang


The ability to predict the flow patterns and flow pattern transitions in a two-phase flow process is useful for an accurate prediction of the pressure drop, heat and mass transfer rates, and also for the choice of appropriate two-phase flow design parameters for the system. During a loss of coolant accident (LOCA) in a nuclear reactor, two-phase flow may exist in the primary heat transport loop, and a knowledge of the flow patterns that are occurring at the various flow conditions is needed to accurately model the accident scenario.

Horizontal gas-liquid two-phase flow patterns and flow pattern transitions have been investigated both theoretically and experimentally for a pipe of 5.08 cm i.d., annulus geometries of outer tube diameter 5.08 cm i.d. and inner-to-outer diameter ratios from 0.375 to 0.625, and for a 37-rod nuclear fuel type bundle flow system having an outer tube diameter of 10.16 cm i.d. and rods of diameter, 1.27 cm. The 28-rod bundle flow geometry was also studied theoretically. The flow conditions were at inlet pressures of about 1 to 2 bar and at near room temperature.

In this study, the time averaged void fraction and pressure drop measurements were also successfully obtained. The instantaneous and time averaged void fraction measurements were achieved by the ring type capacitance transducers based on the differences in the dielectric constants of the liquid and gas phases. The various flow patterns occurring in the pipe, annulus and rod bundle flow systems were successfully characterized by direct visual observation through the transparent test sections and also from the signal waveforms of the instantaneous fluctuations in the void fraction, the pressure drop measurements and the ultrasonic transmission waveforms. Flow pattern transitions were determined from both the results of the measured void fraction and direct visual observation.

The experimental results show that the flow pattern structures occurring in horizontal annulus and rod bundle geometries are similar to those observed for the pipe flow case, except the annulus flow system where we characterized two additional flow patterns, namely, "Annulus-Slug" and "Annulus-Plug". These flow patterns are similar to the Slug and Plug flow structures observed for the pipe geometry, but are restricted to the lower annulus channel gap below the annulus rod. These occur at the flow conditions that would otherwise lead to Stratified flow patterns for pipe flow cases.

The results show that the flow pattern transitions for the annulus and rod bundle flow geometries are significantly different from those of the normal pipe flow. The flow pattern transitions for the annulus flow geometries were observed to be significantly influenced by different inner-to-outer diameter ratios, except the Stratified Smooth to Stratified Wavy transition. The Stratified to Intermittent and the Intermittent to Dispersed Bubble transitions occur at lower superficial liquid velocities, while the Intermittent to Annular transition occurs at higher superficial gas velocities for larger inner-to-outer diameter ratios. In the rod bundle geometries, the flow pattern transitions were observed to vary slightly with the particular angle of orientation of the bundle within the enclosing tubeshell. The various influences on the flow pattern transitions observed in the present study are mainly due to the differences in geometries and force distributions.

From direct visual observation results, we also observed that interfacial waves in the rod bundle flow geometry were generated and dissipated at the rod bundle end plates. No significant effect of the rod bundle end plates on the other flow patterns was observed, except a slight effect on the regularity of these intermittent flow patterns, usually becoming more apparent at higher flow rates.

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