9.125. In the design of the first PWRs, local (nucleate) boiling was considered to be undesirable, and conditions were chosen to prevent its occurrence. In later designs, however, local boiling during steady-state operation became a design requirement. Local boiling will affect the pres­sure drop and, in an open lattice, will result in flow redistribution with the possibility of burnout. An underestimating of the factors affecting the pressure drop is therefore essential.

9.126. Where the parallel flow channels have the same inlet and outlet headers, the pressure drop must be equal in all channels. If there is a tendency in any channel for the resistance per unit flow rate to be higher, because of local boiling or other factors, the flow rate in that channel will automatically decrease until the pressure drop is the same as that in the other channels. Such flow distribution may result in instabilities because the decreased flow will mean a lower thermal transport and, consequently, higher surface and fluid temperatures. This could lead to ultimate failure

if the higher temperatures resulted in bulk boiling, accompanied by in­creased flow resistance, and so on (cf. §9.98).

9.127. It has been found [17] that the pressure drop for a nonboiling, open, parallel-tube system may be predicted by means of the standard Fanning friction-factor correlation based on an equivalent diameter. Under nonboiling conditions, there are only minor variations caused by differences in bulk-water temperatures. When local boiling occurs, however, there is an increase in momentum exchange and in heat transfer which may be due to the increase in hydrodynamic turbulence caused by repeated growth and collapse of steam bubbles. The bubbles may also be considered, in a broad sense, to contribute an increased surface roughness. Whatever the mechanism, there is an increase in pressure drop, and this may be repre­sented by a friction factor applicable to the local-boiling conditions.

9.128. The friction factor/for local boiling at 13.8 MPa (2000 psi) may

be expressed empirically in terms of the isothermal (nonboiling) factor /iso as a function of the bulk temperature tm alone; thus,

where the bulk temperature is between 293 and 335°C (saturation tem­perature of water) and above the local-boiling temperature. The pressure drop is equal to that for nonboiling until the water temperature reaches 335°C. Alternative procedures for local-boiling, pressure-drop calculations have been based on two-phase flow predictions.