Investigation of boiling burnout and flow stability for water flowing in tubes

Warren H. Lowdermilk, Chester D. Lanzo, Byron L. Siegel
naca-tn-4382
Sep 1958


Boiling-burnout heat-transfer rates were measured with water flowing vertically upward in electrically heated tubes. Flow stability in the experimental test section during burnout was affected by the amount of pressure drop across a throttling valve located upstream. As the restriction pressure drop across the valve was increased, the magnitude of the flow fluctuations in the section decreased and the burnout heat flux increased until the pressure drop exceeded a critical value. For further increases in pressure drop, the flow was steady and the burnout flux was independent of the pressure drop across the throttling valve. The minimum restriction values required to stabilize the flow varied nearly linearly from 5 to 100 pounds per square inch with an increase in velocity from 0.5 to 40 feet per second; these values were independent of the pressure drop across the test section. A compressible volume introduced in the flow system between the throttling valve and the test section resulted in unsteady flow during burnout. The larger the volume, the greater were the flow fluctuations and the lower were the attendant burnout heat-transfer rates. Burnout heat-transfer rates were measured in the stable-flow region for a range of velocity from 0.1 to 98 feet per second, pressure ranging from atmospheric to 100 pounds per square inch, inlet subcooling from 0 to 140 degrees F, tube diameters from 0.051 to 0.188 inch, and length-to-diameter ratios from 25 to 250. The resulting burnout heat fluxes ranged from 0.9 to 13.2 x 10 to the 6 Btu per hour per square foot. Net steam was generated for all stable-flow conditions, and two regimes of burnout were obtained.

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