Ultrahigh CHF Prediction for Subcooled Flow Boiling Based on Homogenous Nucleation Mechanism

Abstract

Homogeneous nucleation, although being discounted as a mechanism for vapor formation for water in most conditions, is found to possibly occur under some extreme conditions in subcooled flow boiling. Under the conditions, vapor bubbles of molecular dimensions generated in the superheated liquid adjacent to channel wall from homogeneous nucleation due to the local temperature exceeds homogeneous nucleation temperature. The condition is called in this paper as homogeneous nucleation governed condition. Under the condition, conventional flow pattern for subcooled flow boiling, which is characterized by the existence of Net Vapor Generation (NVG) point and the followed bubble detachment, movement and coalescence processes, cannot be established. Critical heat flux (CHF) triggering mechanism so far proposed, which employs a premise assumption that the conventional flow pattern has been established, such as liquid sublayer dryout model, is no more appropriate for the homogeneous nucleation governed condition. In this paper, first, the existence of the homogeneous nucleation governed condition is indicated. In the following, a criterion is developed to judge a given working condition as the conventional one or the homogeneous nucleation governed one. With the criterion, subcooled flow boiling data are categorized and typical homogeneous nucleation governed datasets are listed. The homogeneous nucleation governed data are characterized by extreme working parameters, such as ultrahigh mass flux, ultralow ratio of heated length to channel diameter L/D or ultrahigh pressure. CHF triggering mechanism for the homogeneous nucleation governed condition is proposed and verified. Parametric trends of the CHF, in terms of mass flux, pressure, inlet subcooling, channel diameter, and the ratio of heated length to diameter are also studied.