Void fraction thermo-kinematics for subcooled flow boiling

Abstract

This work presents some preliminary results about a new void fraction model in subcooled flow boiling based on brand new mass and energy balances developed by the author over the past years. As main novelties, the new mass balance is based on the liquid velocity and the mixture specific volume, whereas the new heat balance is a function of the mixture enthalpy, based on the thermodynamic quality, and explicitly includes the mean vapor–liquid velocity ratio, or the slip ratio. The core of the model is to predict the slip ratio profile and then to calculate the mixture enthalpy from the new heat balance and, finally, to derive the void fraction. In this work, it is shown that the continuity of the first derivative of the mixture specific volume just at saturation establishes a first equation to obtain the slip evolution. A second kinematic equation between the onset of nuclear boiling (ONB) and the vapor velocity is also found. Three additional parameters, related to the ONB and the point of net vapor generation, are needed for void fraction profiles predictions, which have been found by trial and error, searching the best fitting to the measured void fraction. The new model is compared to eight full axial void fraction profiles data for upward subcooled flow of water at high pressure.