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|Title: ||Investigation of turbulence modulation in solid-liquid suspensions using FPIV and micromixing experiments|
|Authors: ||Unadkat, Heema|
|Keywords: ||Multiphase flows|
Particle image velocimetry
|Issue Date: ||2010|
|Publisher: ||© Heema Unadkat|
|Abstract: ||The focus of this thesis is the study of turbulent solid-liquid stirred suspensions, which are
involved in many common unit operations in the chemical, pharmaceutical and food industries.
The studies of two-phase flows present a big challenge to researchers due to the complexity of
experiments; hence there is a lack of quantitative solid and liquid hydrodynamic measurements.
Therefore, an investigation of turbulence modulation by dispersed particles on the surrounding
fluid in stirred vessels has been carried out, via two-phase fluorescent Particle Image
Velocimetry (FPIV) and micromixing experiments. The main property of interest has been the
local dissipation rate, as well as root-mean-square (rms) velocities and turbulent kinetic energy
(TKE) of the fluid.
Initially a single-phase PIV study was conducted to investigate the flow field generated by a
sawtooth (EkatoMizer) impeller. The purpose of this study was to gain insight into various PIV
techniques before moving on to more complex two-phase flows. Subsequently stereo-, highspeed
and angle-resolved measurements were obtained. The EkatoMizer formed a good case
study as information regarding its hydrodynamics is not readily available in literature, hence
knowledge has been extended in this area.
An analysis of the mean flow field elucidated the general structure of fluid drawn into the
impeller region axially and discharged radially; the latter characterised the impeller stream. The
radial rms velocity was considered to represent best the system turbulence, even though the
tangential rms velocity was greater close to the blade; however the radial component was more
prevalent in the discharge stream. Due to differences in rms velocities, TKE estimates obtained
from two and three velocity components deviated, being greater in the latter case. Integral (1-D
and 2-D) length scales were overestimated by the quantity W / 2 in the impeller region. Ratios
of longitudinal-to-lateral length scales also indicated flow anisotropy (as they deviated from 2:1).
The anisotropy tensor showed that the flow was anisotropic close to the blade, and returned to
isotropy further away from the impeller. Instantaneous vector plots revealed vortices in the
discharge stream, but these were not associated with flow periodicity. Alternatively, the vortex
structures were interpreted as low frequency phenomena between 0-200 Hz; macro-instabilities
were found to have a high probability of occurrence in the discharge stream.
Dissipation is the turbulent property of most interest as it directly influences micromixing
processes, and its calculation is also the most difficult to achieve. Its direct determination from
definition requires highly resolved data. Alternative methods have been proposed in the
literature, namely dimensional analysis, large eddy simulation (LES) analogy and deduction
from the TKE balance. All methods were employed using 2-D and 3-D approximations from
stereo-PIV data. The LES analogy was deemed to provide the best estimate, since it accounts
for three-dimensionality of the flow and models turbulence at the smallest scales using a subgrid
scale model. (Continues...).|
|Description: ||A Doctoral Thesis. Submitted in partial fulfillment of the requirements for the award of Doctor of Philosophy of Loughborough University.|
|Appears in Collections:||PhD Theses (Chemical Engineering)|
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