Experimental study on the CHF and the heat transfer characteristics under supercritical pressures

A series of experimental works were performed in a vertical tube of 9.4 mm inner diameter using the Freon, HFC-134a (Critical pressure: 4.059 MPa) as working fluid medium to provide a reliable heat transfer database and investigate the heat transfer characteristics during the pressure transient conditions. As a reference case, the steady-state heat transfer experiments were performed under the wide range of experimental parameters which covered the mass flux from 600 to 2000 kg/m2s and the pressures from 4.1 to 4.5 MPa. The pressure-transient heat transfer experiments were carried out for two cases of pressure increasing and decreasing transients from 3.8 to 4.5 MPa with different pressure transient rates. For the various mass flux and inlet subcooling of fluid, the CHF experiments were performed and total number of 83 CHF data was obtained. Through the steady-state heat transfer experiment, heat transfer correlation was developed considering the properties variation near the wall. The proposed correlation predicts the Nusselt number within ±20% accuracy for the 94.6% of present 7022 experimental data. As for the applicability of steady-state heat transfer correlation to the pressure transient sequences, the heat transfer correlation always overestimates the Nusselt number measured in the pressure transient heat transfer experiments by 10 to 40%. The prediction performances for the present CHF data using the Katto and Ohno correlation and the Groeneveld's CHF look-up table get remarkably worse at near critical pressure. (author)