Onset of significant void in internal flow, forced convection, subcooled boiling.

摘要

The observation of the void behavior in internal, forced convection, boiling water flows showed the existence of two regions. The first region is one of little or no net vapor generation, and thus of very low void fraction. The second region is one of significant net vapor generation where the void fraction increases rapidly with the heated length. The physical boundary between these two regions is called the onset of significant void (OSV) or onset of significant vapor generation, and is the subject of this report. This onset of significant vapor generation generally signals the onset of flow instability in a system with pressure-driven boundary conditions. This is because the sudden decrease in the mean flow density results in an increased pressure gradient owing to the combined effects of friction and acceleration. With pressure boundary conditions, rather than specified flowrates, this can lead rapidly to flow instabilities. In the particular case of a liquid cooled nuclear reactor the consequences can be dryout and overheating. Because this work was done as part of a research program under contract to the Savannah River Company, in support of the development of a specific nuclear reactor, the prediction of the subcooling at OSV for fully developed turbulent water flow at low pressures (p < 5 bar) was of particular interest.