CHF BEHAVIOR DURING PRESSURE, POWER AND OR FLOW-RATE SIMULTANEOUS VARIATIONS

The results of an experimental investigation are presented on critical heat flux in forced convective flow boiling during transients caused by simultaneous variations of either two or three parameters among pressure, flow rate and thermal power. The three parameters are varied according to an exponential law for the flow rate and the pressure decrease, and to a ramp and a step law for the input power increase. Experiments are carried out employing a tubular test section which is electrically and uniformly heated. Test parameters include the flow rate half-flow decay time, several values of the initial power (before the transient) and the final power (at the end of the transient) in the case of step transients, and the slope of the ramp in the case of ramp transients, and the depressurization rate. An analysis of the experimental data is performed using the local conditions approach, and applying the quasi-steady-state method. The effect of the simultaneous variation of either two or three main parameters on the time-to-crisis is also analysed for transients in which only one of the parameters is varied.