An energy model for the transient flow boiling crisis under highly subcooled conditions at atmospheric pressure

We present an original model describing the transient flow boiling crisis of water at high subcooling and atmospheric pressure. We hypothesize that in such conditions, the mechanism of the boiling crisis is the prevention of the bubbles recondensation when a thin fluid layer near the heated wall reaches temperature saturation conditions. To capture this phenomenon, we propose an energy model describing the heat exchanges in the thin fluid layer throughout the entire transient from the initiation to the boiling crisis. We bring to light a non-dimensional mathematical relation capturing 168 working points in the investigated range of power escalation period (from 5 to 500 ms), subcooling (from 25 to 75 K) and Reynolds number (8,500 to 35,000) at atmospheric pressure. Its fitting accuracy is excellent for the high subcooling (50 K and above): more than 75 % of these points are predicted with +/- 5 % error. This relationship enables the prediction of the transient critical heat flux based on the steady-state value or a single tuning constant. The non-dimensional groups deduced from the study are relevant tools to identify the major physical phenomena involved in transient boiling crisis and to quantify the impact of the different operating parameters.