A Visual and Frictional Pressure-Drop Study of Natural-Circulation Single-Component Two-Phase Flow at Low Pressures

A visual and frictional pressure-drop study of low-pressure high-void-fraction two-phase flow has been performed in a boiling-water natural-circulation system with heat addition. Heat was added uniformly by four tubular electrical resistance elements placed parallel to the flow, simulating cylindrical nuclear fuel elements. A 6-ft vertical test channel, 1.25-in. i.d. was used. It contained six opposite pairs of observing windows permitting high-speed motion pictures of the flow to be taken at different operating conditions. Experimental two-phase pressure-drop data at various flow rates were conducted at pressures of 25, 35, and 50 psia, and steam qualities ranging from 0.7 to 7.8% corresponding to void fractions of 63 to 94.5%. Bubbly and transition from bubbly to slug flow regimes were observed. Strong pulsations, inherent in natural-circulation systems with internal heat addition, were also observed. Frictional pressure-drop data were obtained as a function of both quality and mass flow rate. Under the conditions of the investigation, no discontinuities in flow regime or frictional pressure drop were observed and the Martinelli-Nelson correlation for the friction multiplier was found to greatly underestimate the value of the multiplier. A motion-picture film of flow is available as a supplement to this paper.