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4. Validation of predictions
Experiment validation is crucial for the provision of
confidence in hydrocyclone flow-field predictions. For
meaningful validation, measurements are needed of
both the detailed internal flow-field, for confidence in
the fundamental prediction quality and of the hydrocyclone
separation performance. Therefore, two aspects
of validation are considered which are, low-solids, highdetail
velocity field measurements and the high-solids
distribution and flow-field. A number of key measurement
techniques are recommended for such measurements.
4.1. Low-solids flow-field
The hydrodynamics of the hydrocyclone features
high velocities of order 10 m/s with significant secondary
flows as well as significant asymmetric turbulence.
Therefore, to achieve the necessary high-accuracy
characterisation of the micro-scale phenomena requires
high resolution, short time-scale measurements. Capture
of a turbulence measure requires at least two sequential
measures over a short period. To achieve such measurements,
without intrusion into the flow-field, points
towards application of light dependent methods. Fully
three-dimensional measurement, i.e. without assumptions
of the out-of-plane velocities, requires the dualplanar
laser method. The success of this approach has
been well demonstrated for processes such as fuel
injection (Hu et al., 2001) and even for impinging water
jets (Koochesfahani and Nocera, 2001), although
application to hydraulic flows is less common.
The principal behind dual-planer laser-induced fluorescence
(Fig. 2) lies in the generation of a laser-induced
fluorescing grid within a fluorophore doped hydrocyclone
flow-field. Such a grid may be induced by means of
two orthogonal laser beams, each split into a number of
parallel, co-planar sub-beams, (grating or optics). The
fluorescence of the grid persists for a short time over
which two sequential images may be acquired. If each
image of the fluorescence is simultaneously grabbed by
two separate off-angle cameras, then the three-dimensional
velocity may be reconstructed. Provided sufficient
temporal resolution, then the turbulence may also be
reconstructed from sequential velocity measurements.
To date, time intervals of order 30 ls have been achieved
(Hu et al., 2001). Therefore, the dual-planar laser
method is appropriate for measurement of the hydrocyclone
flow-field and particularly for capture of detail
of the secondary flows.
4.2. High-solids flow-field
In general, measurement of high-solids flow precludes
non-intrusive application of laser methods due to light
extinction. Therefore, tomographic techniques are being
researched. Although there exists a wide range of
applicable techniques, such as X-ray, Positron-emission,
radioactive particle-tracking, ultrasound, nuclear magnetic
resonance and electrical impedance to mention a few, the present paper focuses upon the application
electrical impedance (EIT).
The principle behind EIT (Fig. 3) is the reconstruction
of the conductivity distribution from measured
boundary voltages, which result from boundary injections
of electric current. Knowledge of the inter-relation
between the conductivity and concentration fields permits
reconstruction of the solids concentration. Research
is underway to form constitutive relations
between the particle concentration and size distribution
and the bulk conductivity. Advantage may also be taken
of such a measure to provide velocity field and temporal
information for high-concentration flows. For example,
during the hydrocyclone start-up phase or when operating
with air-core instability, temporal conductivity
changes are induced by air-core motion. Modern statespace
methods as applied by authors such as Sepp€anen
et al. (2001) may be applied to increase the temporal
accuracy of reconstruction, potentially providing a
temporal resolution of order 0.003 s given current EIT
measurement rates. Therefore scope exists for the
application of averaging to optimise reconstruction
whilst maintaining a high temporal resolution. Greater
temporal resolution is also expected with the development
of more modern EIT systems. The above principle
may be extended through the use of flow follows (distinguishable
by means of their conductivity) in order to
provide a Lagrangian velocity measure, even within high
concentration, opaque flows.
5. Future
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