Transient CHF in flow boiling during flow rate and thermal simultaneous variations

The present paper deals with the results of an experimental investigation on forced flow critical heat flux during transients caused by simultaneous variations of the flow rate and of the thermal power. The two parameters were varied according to an exponential law for what concerns the flow rate decrease, and to a ramp-wise and a step-wise law for the input power increase. Experiments were carried out employing a tubular test section which was electrically and uniformly heated. The test parameters included the flow rate half-flow decay time as well as several values of the initial power (before the transient) and the final power (at the end of the transient), in the case of step-wise transients, and the slope of the ramp in the case of ramp-wise transients. The pressure was kept constant during the transients. An analysis of the experimental data was conducted on the base of the local conditions approach. The effect of the simultaneous variation of the mass flow rate and of the power on the time to crisis has also been analysed with respect to transients in which only one of the two parameters was varied