Model
Digital Document
Publisher
Florida Atlantic University
Description
A complete understanding of the gas-liquid two phase flow in bubble columns is
required for the development of reliable models for scale-up of these multiphase reactors.
Although several models have been proposed to describe the hydrodynamics.
lack of adequate experimental data has hindered meaningful evaluation of model parameters
and model predictions. The Computer Automated Radioactive Particle
Tracking ( CARPT) facility that was implemented earlier has provided interesting
results on the recirculation patterns of the liquid phase. The technique has provided
quantitative information on liquid velocities and turbulence parameters as well. In
addition to these hydrodynamic parameters the measurement of void fraction is also
important. To complement the capabilities of CARPT a scanner for ;-ray Computed
Tomography (CT) was implemented to quantify the local void fraction and
its distribution in two phase flow systems. The automated scanner is capable of
imaging flows in test sections between 0.02.5 m and 0..15 m in diameter at different
elevations above the distributor. The scanner makes use of the same detectors used
in the CARPT facility and with the use of a specially designed moving collimator provides a spatial resolution of about 5 mm. A non-conventional algorithm based
on maximum likelihood principles called the E-M algorithm was used for imagw reconstruction.
Long scanning times are required leading to time averaged density
profiles. Although the system is only capable of providing time averaged void fraction
distributions. it can provide unique information concerning the structure of two
phase flow. The system performance was evaluated by identifying the sources of
errors in measurement and their bounds. The capabilities of the scanner for imaging
void fraction distribution was demonstrated both qualitatively and quantitatively.
Fse of existing radiation detectors and the associated signal processing and data
acquisition system helped in reducing the cost of the system.
The scanner was utilized to quantify the local void fraction and its distribution
in bubble columns of five diameters (0.10. 0.14, 0.19, 0.26. 0.30 m internal diameter)
and at four superficial gas velocities. The effect of various operating parameters
such as column diameter, superficial gas velocity. the type of distributor. the static
liquid height and some changes in the physical properties of the liquid phase were
studied. For the first time a comprehensive characterization of the void fraction in
an air water bubble column using a non-invasive technique was achieved.
The experimental data obtained using CARPT and CT under identical operating
conditions was used for developing a methodology for scale-up of bubble columns
using a one dimensional model for liquid recirculation. Successful scale-up of liquid
hydrodynamics using a one dimensional model requires an adequate closure scheme
for the Reynolds shear stress. The existing correlations for the prescription of the
eddy viscosity or the mixing length scale are demonstrated to be applicable only for a limited range of conditions. and consequently cannot be used for scale-up predictions.
A method for estimating the mixing length scale has been explored and an attempt
at unifying a wide range of data available in the literature within the pun·iew of
the method has been made. The futility of such an attempt is attributed to the
non-reproducibility of the flow in different laboratories and the conasequent lack of
data obtained under identical conditions. It is demonstrated, however, that scale-up
based on the mixing length distribution is possible when it is obtained from a
consistent set of data for liquid velocity and gas void fraction profiles. Using the
present method for prescribing the mixing length scale. model predictions for scale-up
compare satisfactorily for the data that was obtained as part of this research. The
achievement was that the turbulence length scale estimated in one column diameter
was successfully used in predicting the liquid velocities in larger diameter columns.
required for the development of reliable models for scale-up of these multiphase reactors.
Although several models have been proposed to describe the hydrodynamics.
lack of adequate experimental data has hindered meaningful evaluation of model parameters
and model predictions. The Computer Automated Radioactive Particle
Tracking ( CARPT) facility that was implemented earlier has provided interesting
results on the recirculation patterns of the liquid phase. The technique has provided
quantitative information on liquid velocities and turbulence parameters as well. In
addition to these hydrodynamic parameters the measurement of void fraction is also
important. To complement the capabilities of CARPT a scanner for ;-ray Computed
Tomography (CT) was implemented to quantify the local void fraction and
its distribution in two phase flow systems. The automated scanner is capable of
imaging flows in test sections between 0.02.5 m and 0..15 m in diameter at different
elevations above the distributor. The scanner makes use of the same detectors used
in the CARPT facility and with the use of a specially designed moving collimator provides a spatial resolution of about 5 mm. A non-conventional algorithm based
on maximum likelihood principles called the E-M algorithm was used for imagw reconstruction.
Long scanning times are required leading to time averaged density
profiles. Although the system is only capable of providing time averaged void fraction
distributions. it can provide unique information concerning the structure of two
phase flow. The system performance was evaluated by identifying the sources of
errors in measurement and their bounds. The capabilities of the scanner for imaging
void fraction distribution was demonstrated both qualitatively and quantitatively.
Fse of existing radiation detectors and the associated signal processing and data
acquisition system helped in reducing the cost of the system.
The scanner was utilized to quantify the local void fraction and its distribution
in bubble columns of five diameters (0.10. 0.14, 0.19, 0.26. 0.30 m internal diameter)
and at four superficial gas velocities. The effect of various operating parameters
such as column diameter, superficial gas velocity. the type of distributor. the static
liquid height and some changes in the physical properties of the liquid phase were
studied. For the first time a comprehensive characterization of the void fraction in
an air water bubble column using a non-invasive technique was achieved.
The experimental data obtained using CARPT and CT under identical operating
conditions was used for developing a methodology for scale-up of bubble columns
using a one dimensional model for liquid recirculation. Successful scale-up of liquid
hydrodynamics using a one dimensional model requires an adequate closure scheme
for the Reynolds shear stress. The existing correlations for the prescription of the
eddy viscosity or the mixing length scale are demonstrated to be applicable only for a limited range of conditions. and consequently cannot be used for scale-up predictions.
A method for estimating the mixing length scale has been explored and an attempt
at unifying a wide range of data available in the literature within the pun·iew of
the method has been made. The futility of such an attempt is attributed to the
non-reproducibility of the flow in different laboratories and the conasequent lack of
data obtained under identical conditions. It is demonstrated, however, that scale-up
based on the mixing length distribution is possible when it is obtained from a
consistent set of data for liquid velocity and gas void fraction profiles. Using the
present method for prescribing the mixing length scale. model predictions for scale-up
compare satisfactorily for the data that was obtained as part of this research. The
achievement was that the turbulence length scale estimated in one column diameter
was successfully used in predicting the liquid velocities in larger diameter columns.
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