9.121. Our discussion of flow boiling (§9.94 et seq.) introduced the complexity of two-phase flow. As we shall see, the ability to predict flow rates is of major importance in evaluating the safety of water-cooled re­actors. Therefore, developing a good understanding of two-phase flow phenomena has been the objective of much recent work [15]. However, space limits our presentation to a few introductory ideas.

9.122. In single component two-phase flow, we have a liquid flowing concurrently with its own vapor with evaporation or condensation from one phase to the other occurring in accordance with whatever heat transfer is taking place involving the system. The vapor phase, which is a gas, follows the laws of compressible flow as described in a significant branch of fluid mechanics. To develop a model for the system, a classical approach, as described in various texts [15], is to write equations for the conservation of mass, momentum, and energy over an element of the flow channel and to evaluate the pressure gradient. The necessary, rather complicated, set of relationships can best be managed by appropriate computer codes. For a reactor core, for which the picture is further complicated by intercon­nected flow regions, the calculational development is known as subchannel analysis (§9.135).