2-3

Achieving a High-Reliability Numerical Simulation for Gas-Liquid Two-Phase Flow and Expanding Applications for Design and Safety Evaluation



Fig. 2-6 Grasping a key parameter

We recently developed numeric models for bubble diameter and bubble turbulent diffusion irrespective of the flow path scale. These are based on our findings that these characteristics are affected by the relative relationship between Db and Lv.



Fig. 2-7 Prediction of bubbly flow along a large vertical pipe (48 cm inner diameter)

The bubble distribution under two-phase flow circulation can be determined precisely by this study.



Fig. 2-8 Pressure response analyses in supercritical experiments at TRACY

We analyzed pressure responses in uranyl nitrate solution under different reactivity insertion rates. Based on these analyses, we developed a prediction model that can trace pressure transients well. The single large pressure pulse is caused by rapidly produced radiolytic gas that is expelled from the solution. The oscillatory response results from the solution inertia and the restoring force due to the gas expansion and contraction.



For understanding gas-liquid two-phase flow in a large flow path, a large volume like an upper plenum above the reactor core or a water pool is indispensable for the design and safety evaluation of passive safety systems. Previous evaluation methods were based on experimental data from small-scale flow paths, up to 5 cm inner diameter. These methods were unable to predict the behavior well in such a flow path or volume. In this study, experiments were conducted with a large vertical pipe, 20 cm inner diameter. We discovered that the effect of flow path scale on void fraction distribution can be explained by the relative relationship between the bubble diameter (Db) and the vortex scale in the liquid phase (Lv) (Fig. 2-6). The model developed based on this finding was verified using various databases (Inner diameter: 1-48 cm, Pressure:0.1-5 MPa) and was confirmed to predict the behavior of two phase flow well by the concept using the relationship between Db and Lv (Fig. 2-7).
The developed model was incorporated into the ACE-3D code (JAERI public code) that can simulate complex geometries by either a porous-approximation function or a boundary-fitting coordinate function. The ACE-3D code is now widely used for various applications, such as the design analysis of a passive containment cooling system (PCCS) with a horizontal U-tube type heat exchanger and the analytical investigation of pressure response due to radiolytic gas in supercritical experiments at TRACY (Fig. 2-8). Using the ACE-3D code, we are reducing the cost of experiments by reducing the number of experiments. We are also obtaining more detailed physical knowledge to elucidate experimental results.


Reference
A. Ohnuki et al., Model Development for Bubble Turbulent Diffusion and Bubble Diameter in Large Vertical Pipes, J. Nucl. Sci. Technol., 38(12), 1074 (2001).

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Persistent Quest Research Activities 2002
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