Transient flow boiling crisis at high subcooling: An energy mechanistic model based on experimental observations
Résumé
In case of a reactivity insertion accident in an experimental nuclear reactor, heat generation in the core can grow exponentially in time, with a power escalation period ranging from a few to a few hundreds of milliseconds. Due to neutronic and thermohydraulic effects, boiling crisis may arise, possibly leading to an explosive reaction. If the boiling Crisis has been widely investigated in steady-state conditions, this has not been the case for fast transient heat inputs. The aim of the present work is to understand and to predict the transient flow boiling crisis in the conditions of moderate pressure and high subcooling.
To this end, we have analyzed experimental measurements making use of space and time highly resolved videos and IR thermography covering a wide range of experimental parameters. The analysis gives a better insight on the dependency of the transient Critical Heat Flux to the different parameters of interest and to highlight the underlying mechanisms. For conditions of forced flow and high subcooling, the bubbles generated at the wall present a pulsating behavior. This specific process leads to an efficient heat transfer from the wall to the neighboring fluid. Boiling crisis is stated to occur when a thin layer of liquid contacting the wall reaches the saturation temperature.
From these observations, we developed an energy model that brings to light two non-dimensional parameters useful to describe the transient nature of the process and the dominant cooling processes. This permits us to propose a method of predict the transient boiling crisis. Finally, some possible mechanistic improvement of the model is discussed.