Droplet Motion and Deposition in Vertical Turbulent Pipe Flow
Lee, Michael M.
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https://hdl.handle.net/2142/69786
Description
Title
Droplet Motion and Deposition in Vertical Turbulent Pipe Flow
Author(s)
Lee, Michael M.
Issue Date
1987
Doctoral Committee Chair(s)
Hanratty, Thomas J.
Adrian, Ronald J.
Department of Study
Chemical Engineering
Discipline
Chemical Engineering
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Engineering, Chemical
Abstract
The goals of this investigation were to study the motion of droplets injected in a turbulent air flow and the rate of deposition on surrounding boundaries.
A new axial viewing photographic technique was developed to measure directly the turbulent characteristics of the droplets in planes perpendicular to the mean flow in a 2-inch i.d. pipe carrying a downward flow of air. The size of the centrally injected droplets were 50 $\mu$m, 90 $\mu$m and 150 $\mu$m in diameter, and the Reynolds numbers of the air flow were 36,000 and 52,000.
Direct measurements were made of the particle eddy diffusivity, the radial and tangential turbulent intensities of the droplets, the characteristics of droplet motion, the mass flux and the concentration profiles. It was found that the droplets considered in this investigation do not follow the fluid motion completely. A design equation was developed to predict the radial intensity of the droplets as a function of $\beta\tau\sb{\rm f}$, where 1/$\beta$ is the characteristic time of the droplets and $\tau\sb{\rm f}$ is the characteristic time of the fluid.
Deposition experiments were conducted under the same flow condition as those used in the optical experiments by tagging the droplets with black ink. The cumulative fraction of the initially injected droplets deposited, F$\sb{\rm D}$, at various times were determined from the total amount of ink collected from the pipe wall up to that time. Dimensionless deposition constants, k$\sb{\rm D}$/u*, were also calculated, where u* is the friction velocity. These values for k$\sb{\rm D}$/u* are in the same range as those obtained under the actual two-phase flow condition by other researchers.
A theoretical deposition model is developed to relate deposition results to measurements of turbulent characteristics of the droplets. This model, which solved the diffusion equation with a non-zero droplet concentration at the wall, is applicable to large droplets for which the dimensionless stopping distance is greater than 30. Excellent agreement was found between the model and the experimental data.
A two-resistance theory is also presented to interpret droplet mass transfer across the pipe. This theory allowed one to predict whether diffusion to the wall or free flight to the wall is controlling the mass transfer process.
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