Pressure drop with surface boiling in small-diameter tubes

Abstract

Pressure drop for water flowing in small-diameter tubes under isothermal, nonboiling, and surface-boiling conditions was investigated. Experimental results for local pressure gradient and heattransfer coefficients are presented. Heat-transfer results for nonboiling and surface boiling are in agreement with previous investigations. Isothermal friction factors compare favorably with conventional smooth-tube data. Nonboiling friction factors were well correlated with a wall-to-bulk fluid viscosity ratio. It is concluded that boiling pressure gradients cannot be correlated on the basis of local conditions alone. The axial build up of nonequilibrium vapor in the tube produces an increase in pressure gradient even when all other local parameters are constant. The heat-transfer - pressure-gradient analogy was investigated in the boiling region. For the chosen boiling-to-nonboiling ratios, the analogy was found to be valid only under limited conditions. Over-all pressure-drop data are presented for numerous geometries and a range of flow conditions. Diameters of 0.062 to 0.180 in. and L/D's of 25 to 200 were considered. Exit pressures ranged from 30 to 80 psia and velocities ranged from 5 to 50 ft/sec. The majority of the data was taken for an inlet temperature of 80 OF. Heat fluxes were increased from zero to near the burnout condition unless the saturation condition was reached first. These results were correlated by a relation which is independent of all parameters except geometry. This correlation is presented graphically for all the geometries used. Either this plot or the original data plots can be readily used for design purposes