BEHAVIOR OF LIQUID-FILMS AND DROPLETS IN THE NONEQUILIBRIUM REGION OF A DOWNWARD ANNULAR MIST FLOW (COMPARISON OF POROUS AND CENTRAL NOZZLE MIXING METHODS)

Experiments were conducted on the behavior of liquid films and droplets in the non-equilibrium region of a downward annular mist flow, using a vertical pipe of about 30 mm dia and 6.5 m long, under two extreme conditions of air-water mixing with either a porous wall or a central nozzle to feed the water. The onset of large disturbance waves was influenced remarkably by the mixing method. Axial changes in the film thickness, wave velocity and frequency were obtained by the electrical conductivity method. The water droplet mass flux distributions were measured by an isokinetic sampling probe. In porous wall mixing, large disturbance waves occurred farther upstream than in nozzle mixing. The fraction of liquid-entrained E axially increased monotonously and attained a certain constant value, whereas in nozzle mixing it decreased monotonously and became constant. For high air velocity and high water velocity, the non-equilibrium length (z/d)E was 70-140. The critical liquid film thickness depended heavily on the gas velocity in a downward annular mist flow in contrast with an upward flow.