Occurrence of critical heat flux during blowdown with flow reversal

Abstract

A small-scale experiment using Freon-11 at 54°C and 450 kPa in a transparent annular test section was used to study the occurrence of critical heat flux (CHF) during blowdown with flow reversal. The transients were initiated by simultaneous operation of three air-actuated values. The inner tube of the annulus was uniformly heated over its 0.61-m length while the outer transparent pyrex wall was unheated. Test section instrumentation included seven pressure taps, inlet and outlet capacitance void probes, inlet and outlet turbine flowmeters, inlet and outlet fluid thermocouples, and 22 wall thermocouples. High-speed motion pictures were taken of the lower end of the test section where the intial CHF occurred. From these high-speed pictures, the flow reversal was observed to occur between 60 and 80 ms followed by a rapid thermal excursion at about 400 ms in the lower regions of the test section. Consequently, this measured CHF occurred well after the flow had reversed and re-established itself in the downward direction. At about 300 ms an annuflar flow pattern appeared and was well-developed at 400 ms. Therefore, the early CHF measured were all associated with the transition from bubbly to annular flows. In some cases this early CHF was rewet and the heated section remained in a stable coolable state for a considerable length of time, and experienced a later CHF when the liquid was nearly depleted. This long-term dryout was a function of the liquid volume contained in the system, whereas the early CHF was independent of the system volume. In addition, the early CHF did not show any significant sign of propagation whereas the latter one was observed to propagate smoothly upward.