Modelling of critical heat flux in subcooled flow boiling

The present paper reports the state-of-the-art review of the existing models for the prediction of the critical heat flux (CHF) under subcooled flow boiling conditions, with special regard to high subcooling and velocity of the coolant. These latters conditions are receiving special attention for the thermal hydraulic design of high heat flux components for fusion thermonuclear reactors, where the option of water as a coolant under high subcooled boiling conditions at high velocity is currently pursued. Apart from the specific difficulty of properly modelling the CHF phenomenon for a large range of conditions, a major drawback of many of existing theories is the use of empirical constants to adjust the model with the data, making them complex correlations which spoil the original idea of the model. Among the many existing theories available today, only two of them are receiving considerable attention: the near-wall bubble crowding theory and the liquid sublayer dryout theory. This latter is capable of predicting the CHF under a wide range of subcooled conditions, also extending its good performance to uniform and non uniform heating of the channel and swirl flow as well.