9.94. In practical reactor systems the coolant is not stationary, and the boiling which takes place, called flow boiling, is hydrodynamically quite different from pool boiling. Flow boiling commonly occurs under forced- convection conditions, as in boiling-water reactors (BWRs) and to some extent in pressurized-water reactors (PWRs). It can also be experienced, however, when there is natural circulation in a loop configuration, such as may be present during the transient conditions that arise when a coolant circulation pump fails.

9.95. Suppose that water, below the saturation temperature, is forced through a channel between or around the solid fuel elements of a reactor; heat is then transferred from the solid surface (or wall) to the water. As long as the fuel-wall temperature, which increases along its length (§9.143 eq seq.), remains below the steam saturation temperature, single-phase heat transfer only will occur. In PWRs, the pressure on the cooling system is increased in order to raise the saturation temperature and thus prevent bulk boiling, but some local boiling is tolerated; PWRs and BWRs may therefore be regarded as having certain features in common.

9.96. The flow patterns in boiling two-phase flow are complex. Primarily for conceptual purposes, we show the classical Fig. 9.14 representation for flow boiling in a vertical hollow tube [10]. At first, the rising flowing water is merely heated by convective transfer with no boiling occurring. As the temperature of the flowing water increases, a point is reached where the temperature of the water in the slowly moving laminar layer next to the wall is above the saturation temperature although the temperature at the same level in the more rapidly moving core is still below saturation. Initial vaporization therefore occurs along the wall under subcooled (local boiling) conditions. Bubbles grow and are carried along in the superheated layer close to the wall, but they condense on being mixed with the subcooled liquid core.

9.97. After the central core reaches saturation conditions, the vapor bubbles being fed from the superheated layer near the surface no longer collapse but are carried along in the stream. As boiling progresses, the

TEMP. QUALITY

Fig. 9.14. Regimes of two-phase flow.

flow mechanism becomes quite complex and depends on how the vapor and liquid phases flowing in the same direction distribute themselves. As the vapor volume increases, the flow must accelerate. Bubble flow is char­acterized by a regular distribution of vapor bubbles in the continuous liquid
phase. With increased vaporization, and a resulting higher void fraction or quality (§9.99), a transition to annular flow occurs. This is characterized by a continuous vapor phase in the central core in which some liquid drops may be dispersed.