A study of climbing-film flow characteristics of various liquids and air in an annular duct Public Deposited

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  • Momentum transfer in climbing-film flow in an annular duct was investigated. Air and various liquids, with a wide range of viscosities and surface tensions, were investigated. The two phases flowed co-currently upward, the liquid climbing as a thin film on the inner wail while the outer wall remained dry. The annulus was constructed of a three inch I. D. acrylic plastic tube and a one inch O. D. stainless steel rod. The dry, clear plastic outer wall made determination of pressure losses in the gas phase, radii of the point of maximum velocity, and outer wall shear stresses straightforward. High-speed motion picture photography of the liquid film was used to study the liquid film thickness and the wave character of the liquid surface. It was attempted to determine the effect of the liquid properties on the character of the liquid film and on the hydrodynamics of the air-liquid flow. The liquids used were water, Shellsolv, iso-octyl alcohol and light oil (viscosity ranging from 6.75x10⁻⁴ to 1.22x10⁻² lbm/ft sec and surface tension ranging from 24.4 to 72.75 dyne/cm at 68°F). The experimental investigation covered an air flow range of 7x10⁴<Re<2x10⁵ and a liquid flow range of 0.10 to 0.755 lbm/min. The results of this investigation are subsequently presented. 1. Friction factors for air flowing alone in the annulus were 2% to 13% larger than smooth tube values. The radius at the point of maximum velocity was in agreement with the values predicted by existing empirical correlations. 2. The two-phase pressure losses increased with increases in air and liquid flow rates. At the same liquid flow rate, the pressure losses were larger for the more viscous liquids. 3. The presence of the liquid film flowing on the inner wall caused only small changes in the outer wall shear stress from the single-phase values. Correlations were found for friction factor and Reynolds number using the region between the point of maximum velocity and the outer wall as the hydraulic flow region. In the region of low liquid entrainment the following empirical correlation was found f₂ = .201 Re₂⁻.³²⁶ 4. The radius of the point of maximum velocity increased as the liquid flow rate increased and was larger for the more viscous liquids. The following empirical correlation was found y* = .01 ReL.²⁴⁶μ*.⁴⁸ 5. The two-phase pressure losses were predicted well by the Lockhart and Martinelli correlation over the entire range of the data and for all liquids. 6. The laminar film model gives a reasonable approximation of the liquid film thickness for all the liquids investigated. The data indicate that the film thickness is proportional to the liquid viscosity to the .3 power. 7. Shear stresses at the air-liquid interface were 33% to 52% larger than the outer wall shear stresses and 7% to 130% larger than the inner wall shear stresses. The interfacial and the inner wall shear stresses approach each other at the high air flow rates, indicating the velocity profile in the liquid film is approaching linearity. 8. The frequency of the waves on the liquid surface increase as the air and liquid flow rates increase. The frequency was larger for the less viscous liquids and also for the liquids with the lower surface tensions. The wave celerity increases with increases in air and liquid flow rate and was 3% to 6% of the average air velocity. This compares well with the 5% to 10% reported for a liquid film climbing on a tube wall.
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