Mechanism of the onset of film dryout under flow oscillation condition

Accurate prediction of the critical heat flux in forced-convective boiling is one of the most crucial issues in the design of various industrial plants including nuclear power reactors. Many experimental and analytical studies were hence carried out to improve the prediction models of critical heat flux. Since significant temporal fluctuation of coolant flow rate could occur particularly when the power density is increased, several researchers investigated the effect of flow oscillation on the critical heat flux. It was shown experimentally that the flow oscillation can induce drastic reduction of critical heat flux, but the detailed mechanisms of CHF reduction remain as an area to be fully understood. The present work reports the critical heat flux in a uniformly heated vertical round tube. In the experiments, forced sinusoidal oscillation was applied to the inlet mass flux to investigate the effect of flow oscillation on the critical heat flux. To elucidate the mechanism of CHF reduction, the amplitude and the period of flow oscillation were used as the main experimental parameters. It was shown that the dimensionless heated length defined by L/ufTOSC is a promising parameter to characterize the critical heat flux under flow oscillation condition; here, L is the heated length, uf is the characteristic film velocity and TOSC is the oscillation period. (author)