The general understanding of transient critical heat fluxes in a pool of water at various pressures due to exponentially increasing heat inputs is necessary as the database for the correct prediction of the severe nuclear reactor accident due to a power burst. The transitions from non-boiling regime to film boiling occur instantaneously at the critical heat fluxes and then the film boiling leads to the actual burnout of the fuel rod. The purpose of the present work is to investigate the effect of surface conditions of platinum ribbon heaters with different surface conditions such as commercial and rough surfaces finished by Emery paper on the steady and transient critical heat flux (CHF) in saturated condition under atmospheric pressure due to the exponentially increasing heat inputs with the wide range of exponential periods. Namely the heat inputs correspond to those with the increasing rates from quasi-steadily to rapid ones. The dynamic heat transfer processes including boiling incipience and transition to film boiling due to exponential heat input were measured for a horizontal vertically oriented ribbon with 4 mm in width in a pool of water for the pressures from 101.3 kPa to 984 kPa, the periods from 10 ms to 20 s and for the subcoolings from 0 to 50 K. The steady-state critical heat fluxes for the subcoolings ranging from 0 to 50 K at pressures were measured with different surface conditions. The data obtained for lower and higher subcoolings were compared with ones given by Sakurai and Fukuda. The typical trend of critical heat fluxes for the exponential periods is that the critical heat flux gradually increases from the steady-state value, then decreases and again increases with the decrease in period from the longest one tested here, though the trend was not observed completely for the short period range at high pressures. The critical heat fluxes for both cases which increase with the decrease in period from the minimum critical heat flux become almost in agreement with each other for short periods. The mechanism of the transition at the critical heat flux from non-boiling regime to film boiling regime was assumed to be a consequence of the heterogeneous spontaneous nucleation in originally flooded cavities on the cylinder surface. The effects of subcooling and pressures on the transition to film boiling due to the heat inputs with a wide range of increasing rate were investigated for the both surfaces.
