Occurrence of critical heat flux during blowdown with flow reversal. [LWR]

Abstract

A small-scale experiment using Freon-11 at 130°F and 65 ps.ta in a well-instrurneiitrj transparent annular test section was used to study the occurrence of critical heat flux (CHF) during blowdown with flow reversal. The inner stainless steel tube of the annulus was uniformly heated over its 2 ft length. Inlet and exit void fractions were measured by a capacitance technique. Flow regime transition was observed with high speed photography. A J-hr contact time between Freon-11 and nitrogen at 13O°F and 60 psig wr.is found to greatly affect the steady-state subcooled boiling initial conditions. Delay in bubble growth was observed in adiabatic blowdown runs. This was caused by the thermodynamic nonequilibrium conditions required for the unstable bubble growth. For the diabatic runs, equilibrium was more closely approached in the test section during the early phase of blowdown. Critical heal flu>: did not occur immediately during the flow decay D in an .ippi oxim.tt eJ y h() msec reversal period. The first or early CHF which occurred at: about 400 nsoc was independent of the blowdown volume and did not propagate upward. An annular flow pattern appeared at the onset of Chis C'HF which occur red onlv at the lower 8 in. of the heated x v 11 For some cases the early (IMF was rewet and a second CHK was observed at near depletion of liquid in the heated channel. This CHF however propagated smoothly upward and the time scale for this event was dependent on the system blowdown volume. xviii CHAH'KK 1