Natural and forced-circulation test data for a closed-loop system are presented and analyzed. The data were obtained at pressures of 800, 1200, 1600, and 2000 psia from the natural-circulation loop at the Bettis Laboratory, using single rectangular channel test sections (0.100 in. × 1.0 in. × 27.0 in. long, 0.200 in. × 1.0 in. × 27.0 in. long, and 0.250 in. × 1.0 in. × 27.0 in. long). Heat fluxes ranged from 50,000 Btu/hr-sq ft to burnout with inlet subcoolings of 20, 70, and 100 deg F. The results showed that single and two-phase pressure drop, burnout heat flux, and riser density measured under natural-circulation operation are no different from those measured with forced circulation at the same thermal and fluid flow conditions. For the loop studied, it was shown that natural-circulation-loop flow rates can be predicted to within 10 per cent for both single and two-phase flow. Some data for slip ratios at liquid velocities less than 1/2 fps and for two-phase exit losses were obtained. Flow fluctuations were noted during some of the natural-circulation runs; these occurred before burnout heat flux was reached. In some instances these fluctuations were severe enough to cause a premature burnout.

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