Modeling of Upward Subcooled Flow Boiling of Refrigerant-113 in a Vertical Annulus

Subcooled flow boiling can be found in many practical applications, such as heat exchangers, steam generators, refrigeration systems and it is especially important in water-cooled nuclear power reactors, where the presence of vapor bubbles in the core influences the reactor behavior at operating and accident conditions. At high heat-flux densities, vaporization may occur at the heated surface despite the fact that the mean temperature of the cooling liquid has not yet reached the saturation point. This phenomenon is called “subcooled boiling,” which is caused by a thermodynamic nonequilibrium in the liquid. There is a superheated liquid in the boundary layer near the heated wall, while the bulk temperature is still fairly subcooled. In subcooled boiling flow in a vertical channel, vapor distribution not only is uneven over the channel cross section but also evolves along the flow, as both the void fraction and the width of the two-phase layer near the heated surface gradually increase. This non-uniform distribution of vapor enormously influences hydrodynamic and thermal processes, including heat transfer. Although a significant amount of literature deals with the cross-sectional distribution of the gas phase in adiabatic bubbly flow, investigations of analogous phenomena in boiling flow have been much less common. Among multidimensional theoretical analysis of subcooled boiling flow, the most widely used approach so far appears to be two-fluid modeling [1]. This model treats the general case of modeling each phase or component as a separate fluid with its own set of governing