On void fraction distribution during two-phase boiling flow instability

[article extract] Flow instability is an important consideration in the design of nuclear reactors because of the possible occurrence of flow excursion during a postulated accident. The present investigation addresses the static flow instability in a heated channel. Static flow instability may arise in narrow cooling channels due to steam formation in the case of loss of coolant accidents. The formation of steam in a heated channel can have a significant effect on the overall pressure drop along the channel. For fully developed single-phase flow, at high coolant velocities, the pressure drop is comprised primarily of effects due to wall friction, associated with surface roughness condition, which decreases the velocity. With a reduction of the flow velocity, two-phase effects initially appear in a single-phase liquid at the point of incipient boiling (IB) and become more pronounced with the onset of significant voiding (OSV) as demonstrated in Fig. 1. With the presence of vapour in the bulk liquid during boiling, it causes an increase of frictional drag, and also acts to affect the pressure drop through acceleration and buoyancy effects. At some intermediate velocity, the increase in pressure drop due to boiling completely offsets the decrease in pressure drop due to channel frictional components. Further velocity reductions cause the pressure drop to rise which results in a minimum point, the onset of flow instability (OFI) point, in the pressure drop versus velocity curve (see Fig. 1). If parallel flow paths exist, such as in fuel element designs, this increase in pressure drop in one channel may cause flow to be diverted to alternate channels, destabilising the system, and resulting in excursive or Ledinegg stability.