Robert Cole

BUBBLE GROWTH RATES AT HIGH JAKOB NUMBERS

Abstract Bubble growth rates were investigated experimentally to determine the effect of high Jakob number conditions. Comparison of the experimental data with existing theory for Jakob numbers ranging from 24 through 792 indicates that the shape of the bubble growth curve over the entire range investigated is best described by the t 1 2 variation predicted by the uniform superheat model. Reasonable agreement with the magnitude of the bubble growth data was obtained only for Jakob numbers less than 100. Above this value, the discrepancy between existing theory and experiment becomes increasingly greater. At a Jakob number of 792, theory predicts bubble diameters almost an order of magnitude greater than those found experimentally. The growth data over the entire range of Jakob numbers investigated were correlated by the expression D = 5 N 1 2 ja √(αt) . It is apparent however that serious shortcomings exist in current bubble growth theory and it is recommended that the theory be re-examined to determine the relative importance of dynamic effects, particularly at high Jakob numbers.

Optical temperature gradient measurements using speckle photography

Abstract The grainy appearance of a diffusing object illuminated by a monochromatic source such as laser light is known as ‘laser speckle’. This paper presents an optical method for measuring temperature gradients in liquids, based on this phenomena. Double exposure speckle photography is used to measure temperature gradients in liquids heated from below, in the Rayleigh number range 1.5–276 where the heat transfer is by conduction. Comparison with thermocouple measurements yields reasonable agreement. A few experiments illustrating the feasibility of using ‘speckle’ to study convection problems in the Rayleigh number range 4600–8957 are included.

Bubble growth rates during nucleate boiling at high Jakob numbers

Abstract Bubble growth rates were investigated experimentally to determine further, the effect of high Jakob number conditions. Comparison of the experimental data with existing theory for Jakob numbers from 955 to 1112 show clearly that the discrepancies reported in earlier work [1] were primarily due to neglect of the effect of liquid inertia.

Thermocapillary migration of a fluid droplet inside a drop in a space laboratory

Abstract The thermocapillary migration of a fluid droplet located inside a liquid drop in a space laboratory is analyzed. The quasi-static momentum and energy equations are solved at the instant when the droplet passes the center of the drop. Results are presented for prescribed axisymmetric distributions of temperature on the drop surface.

Thermocapillary convection in a liquid bridge

Abstract Results from experiments on thermocapillary flow in a vertical liquid bridge are reported. Data on surface velocities are presented for four different silicone oils at several values of the temperature difference across the liquid bridge, and for three different average temperatures. Also, axial velocity measured as a function of radial position in the bridge is reported. A theoretical model of the velocity and temperature fields in the system is formulated and solved by finite differences. The experimental data on surface velocities are nondimensionalized, and are found to collapse on a single line when plotted against the Marangoni number. Scaled velocities are relatively insensitive to changes in the value of the Marangoni number from 20 to 290, and the Prandtl number from 1460 to 7120. The data are in reasonable accord with predictions from theory.

Combined thermocapillary and buoyant flow in a drop in a space laboratory

Abstract The problem of axially symmetric motion in a levitated drop due to a prescribed surface temperature distribution is treated. The effects of thermocapillarity as well as buoyancy due to a residual steady gravity vector aligned parallel to the axis of symmetry are included. The relevant equations are solved in the creeping flow limit wherein the convective transport of momentum as well as that of energy is neglected. Also, the solution for axially symmetric buoyant flow in a spherical container is reported in the creeping flow limit.

Thermocapillary Motion of Bubbles Inside Drops

A quasi-static analysis is performed for the thermocapillary motion of a bubble located inside a drop in free fall, with arbitrary axisymmetric temperature fields prescribed on the drop surface. It is shown that in the case of an axially symmetric temperature field, the bubble moves along the axis of symmetry toward the nearest warm pole. The bubble velocity as well as the velocity and temperature fields in the drop can be predicted on the basis of the quasi-static assumptions. An approximation is presented which adequately describes bubble migration velocities in the case where the ratio of the bubble radius to the drop radius is relatively small.