(An) experimental study on the critical heat flux and the pressure transient heat transfer up to supercritical pressures

Abstract

A Super-Critical pressure Water cooled Reactor (SCWR) is considered as one of the GEN-IV (Generation IV) innovation nuclear reactors for its advantage in terms of thermal efficiency as well as compactness and simplicity. Although supercritical pressure fluids do not undergo a change of phase, when the SCWRs are operated with a sliding pressure start-up mode, a critical heat flux (CHF) should be avoided during the power-increasing phases under sub-critical pressure conditions. Moreover, in order to ensure the reliability of safety analyses for abnormal pressure decreasing transients, it is highly necessary to understand the CHF performance near the critical pressure and heat transfer characteristics during the pressure transients. The supercritical pressure fluid has already been practically utilized in the field of fossil-fired power plants and a lot of data have been accumulated since 1950s. The most previous studies, however, have been focused on the steady-state heat transfer regime to investigate the peculiar heat transfer characteristics and to develop the heat transfer coefficient. From a quantitative point of view, experimental study for the pressure transient conditions has not been performed yet. In this study, a series of experimental works were performed in a vertical tube of 9.4 mm inner diameter using the Freon, HFC-134a 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 $2000 kg/m^2 s$ 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...