The effect of drop concentration on deposition in vertical annular two-phase flow
Hay, Kent James
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Permalink
https://hdl.handle.net/2142/20554
Description
Title
The effect of drop concentration on deposition in vertical annular two-phase flow
Author(s)
Hay, Kent James
Issue Date
1994
Doctoral Committee Chair(s)
Hanratty, Thomas J.
Department of Study
Chemical and Biomolecular Engineering
Discipline
Chemical Engineering
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Engineering, Chemical
Language
eng
Abstract
Deposition in annular two phase flow has been found to deviate from a linear rate law. At high liquid flows the rate constant is found to vary inversely with drop concentration (i.e. the rate of deposition is independent of concentration). This deviation causes concern when predicting dryout in two phase applications. To better understand this process, information on its drop size and on the behavior of drops in the turbulent gas flow are needed. Experiments were conducted in a 4.2 cm vertical pipe at an upward superficial gas velocity of 36 m/s and at liquid mass flow rates from 30 to 170 g/s. Photographic drop size measurements were obtained. Measurements of entrainment, gas velocity, slip ratio, and drop concentration were also obtained. An increase in mean drop diameters with increasing drop concentration was found.
Several explanations for the non-linearity of deposition are considered. These include gas phase turbulence dampening, the decrease in turbulence velocities with increasing drop size, and particle-particle interactions.
The theory of Reeks (1977) for the behavior of single particles in homogeneous isotropic turbulence and the data of Lee et al. (1989) were used to predict deposition rates based on the premise that increasing drop size decreases the turbulence of the particles. The results show that deposition is overpredicted at the higher concentrations and that a much larger increase in drop sizes, than observed, is needed for the measured rates to be predicted correctly.
By assuming a small velocity difference between particles of different sizes (terminal velocities due to gravity), calculations reveal that droplet-droplet interactions are important. Drop size measurements upstream indicate that only about ten percent of these interactions need to result in coalescence in order to predict the observed change in drop distribution. A theory is proposed that suggests that interactions between larger drops halt their radial velocities so that the time for the fluid turbulence to bring them back to speed is greater than the time between interactions. The theory correctly predicts the inverse proportionality of the deposition rate constant on drop concentration at large concentrations.
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