Flow excursion instability in downward flow systems part II: two-phase instability

A mechanistic model is presented in this paper to predict the onset of flow excursion in downward flows at low-pressure conditions. The model is represented in a graphical form on the subcooling number versus the Zuber (phase-change) number plane. The subcooling number and the Zuber number are measures of integral system parameter such as fluid properties, inlet subcooling, heat flux, channel geometry and coolant flow rate (all measurable quantities) resulting in an easy extrapolation of the predictive correlation to the physical system. The model addresses the distinction between the point of significant void and the onset of flow excursion and provides an analytical method for its evaluation. The model is compared with flow excursion data for downward flows and the agreement is satisfactory.