Variation of Critical Heat Flux by Flow Oscillation in a Small Vertical Channel

Abstract

The effect of sinusoidal oscillation of inlet mass flux on the critical heat flux (CHF) in forced convective boiling was investigated in experiment and numerical calculation. In the experiment, the test section was a small stainless steel round tube of 5 mm in inside diameter, filtrated and deionized tap water was used as a test fluid, and the flow direction was set to vertical upward. The heated length was 1,600 mm. Electric power supplied to a circulation pump was varied periodically to oscillate the inlet mass flux sinusoidally. Direct current was passed through the test section tube to heat it ohmically. The occurrence of critical heat flux condition was detected using the signal from the thermocouples that were spot-welded on the outer wall of the test section tube. In the present experimental conditions, it was expected that the critical heat flux condition was triggered by the dryout of liquid film in annular two-phase flow regime. The main experimental parameters were the time-averaged inlet mass flux and the amplitude and period of flow oscillation. The system pressure was also used as an important experimental parameter since a boiling water reactor is operated under high pressure condition. If the oscillation period is long enough, it is expected that the critical heat flux under the flow oscillation condition is close to that for the steady state when the flow rate is equal to the minimum flow rate in the oscillatory condition. On the other hand, the decrease of the critical heat flux would be mitigated if the oscillation period is shortened, since interaction would take place between the thin and thick film regions within a boiling channel. In accordance with this expectation, the critical heat flux measured under the flow oscillation condition was reduced with an increase in the oscillation period. It was demonstrated that the reduction of critical heat flux under flow oscillation condition can be correlated fairly well using the concept of dimensionless heated length. Numerical calculations using a one-dimensional three-fluid model were also carried. The calculated critical heat fluxes for flow oscillation conditions increased with increased value of dimensionless heated length, as in the present experiment.