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Experimental work was carried out to determine the flow pattern map in vertical heated pipes under steady-state and transient conditions, using Freon 12 in forced convective flow as working fluid and optical probes for the measurements Existing maps are based on adiabatic tests, steady-state conditions, and fluids different from Freon 12. Signals from optical probes (whose response is based on the variations in fluid refractive index) are analyzed in terms oflocal void fraction, using either the probability density function (PDF) or the ratio between the average and maximum values of the signal. From the analysis of the experimental measurements the definition of a map for annular and intermittent flow regimes was achieved. The map turned out to be in good agreement with the Weisman and Kang map developed in adiabatic, steady-state conditions Qualitative results for the transient conditions are also presented.

The results of an experimental investigation on critical heat flux in forced convective flow during transients caused by simultaneous variations of flow rate and thermal power are reported. The two parameters were varied according to an exponential law for the flow rate decrease and according to a ramp and a step law for the input power increase. Experiments were carried out with a tubular test section that was electrically and uniformly heated. Test parameters included the flow rate, half-flow decay time, several values of the initial power (before the transient) and 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. The pressure was kept constant during the transients. An analysis of the experimental data was conducted on the basis of the local conditions approach and applying the quasi-steady-state method. The effect of the simultaneous variation of mass flow rate and power on the time to crisis was also analyzed with respect to transients in which only one of the two parameters was varied.