For verification of ultrasonic measurements described in section 3.4.3, images for Particle Image Velocimetry (PIV) measurement have been recorded by Pixelfly high speed camera. Sheet of green laser light was illuminating two cross-sections: 1) axial, along the flow channel circumference and in the mid channel width and 2) transverse, across the flow channel width. Examples can be seen in Figs. 77-78, where particle suspension and water with seeding particles are presented. Particle suspension consisted of glass beads of sizes 180-250 µm and concentration 35.5 %wt, flowing in
water. Seeding particles are glass beads of sizes 20-40 µm, added to water in the amount of 10 g.
Initial processing of recorded images has revealed that it is possible to cross- correlate acquired images using the PIV method. Processing has been performed using MatPIV 1.6.1 toolbox for PIV in Matlab software. In Fig. 79 an example of calculated velocity vectors for the particle suspension is shown. Instantaneous and local velocity profile has been determined (at chosen position x=440, along the channel height), see in Fig. 80, based on two images acquired one after another, separated by 2 ms time interval. Calculated velocity is related to movement of glass particles. Estimated velocity varies between 0.3 and 0.7 m/s in the upper part of the cell where particles flow (from 1 to 7 cm of flow channel height), whereas is 0 m/s for particles lying on the cell bottom (from 0 to 1 cm). These values correspond in some degree to velocity obtained using the ultrasonic TTD method, for particle suspension as in Fig. 73 a and are prone to reject theoretically calculated values in Fig. 73 c.
There is a need to obtain an average velocity profile, based on large number of images and applying advanced image averaging techniques, in order to represent well velocity. Also, to describe complex flow patterns of this very dynamical suspension. Additionally, compare with reference for water with seeding particles. This type of study has not been performed as part of this Ph.D. work due to longer time for data
processing than anticipated, however it is recommended to be carried on in future research.
Fig. 77 High speed camera images for particle suspension in the small scale rotational flow cell.
Fig. 78 High speed camera images for water with small glass seeding particles in the small scale rotational flow cell.
Fig. 79 Velocity vectors of particle suspension, along axial cross-section of the small scale rotational flow cell, as shown in Fig. 77 left, calculated using PIV method.
Fig. 80 Instantaneous and local (for x=440 px) velocity profile of particle suspension in the small scale rotational flow cell, at 250 RPM rotor speed.
3.4.8 Conclusions
Mean axial flow speed measurement for water-particle and water flow at rotor speed 250 RPM (3.14 m/s) was performed, by the ultrasonic Transit Time Difference (TTD) method in the annular flow channel.
In addition, high speed camera images have been acquired for flow velocity determination using Particle Image Velocimetry (PIV) method, in order to verify ultrasonic results.
Theoretical flow speed pure water for the simple Couette flow ranges from 3.14 m/s at the cell top (measured at 8.6 cm from the cell bottom) to 0 m/s at the cell bottom (at 0 cm), Fig. 68d. Measured speeds of particle suspension vary between 0.32 m/s (at 7 cm from the cell bottom) and 0.1 m/s (at 1 cm
from the cell bottom), Fig. 73 a. Similar values have been obtain for pure water, here however, values doe not clearly drop towards the cell bottom. Measured by ultrasound lower flow speed values are probably due to the secondary flow slowing down the axial flow and the gas ring under the rotor, reducing transferred dragging force from the rotor into fluid.
Instantaneous, local axial velocity profile for particle suspension, obtained using PIV method, confirms at least to some degree, flow velocity values determined by ultrasonic method. Estimated particle velocities oscillate between 0.3 and 0.7 m/s, within the cell height range 1 and 7 cm (the same as for ultrasonic testing), Fig. 80.
However, one need to keep in mind that more advanced case studies and mean axial velocity profiles are necessary to describe accurately and in detail this dynamic, heterogeneous particle suspension using PIV method.
Ultrasonic testing resulted in tendency of linear drop of flow velocity values towards the cell bottom (as theoretically predicted), in case of water-particle flow; however, no clear tendency is seen for water flow. This is in contrary to what is expected, since presence of particles in principle should encourage scattering of data points (flow fluctuations induced by particle inertia force) and not pure water flow. This can be explained by turbulent flow structures created with presence of the secondary flow. A hypothesis is that the secondary flow is more pronounced for pure water than for water-particle flow. Particle bed on the cell bottom damps and shifts upwards kinetic energy.
Ultrasound TTD system in the rotational test cell is sensitive to various rotor speeds and therefore flow velocity. Minimum detected speed change was 5 RPM (0.06 m/s). Preferable position for testing the sensitivity is in the upper part of the cell, due to suspected presence of secondary flow structures in the lower part, as results showed.
Measured by ultrasonic system flow fluctuations at fixed transducer position sending sound beam parallel to the flow direction are small ~2·10-8 s (0.054 m/s), comparing to scattering of data acquired for water flow, from 3·10-8 to 8·10-8 s (from 0.1 to 0.25 m/s). This indicates that the secondary flow structures were captured by ultrasound in water flow, which results in dispersion of average flow velocity points.
Risk of introduction of error into sound speed measurements by temperature change of a system has been minimized by acquisition of signal with and against the flow within possible shortest time interval. However, to avoid the temperature effect, simultaneous measurements with and against the flow should be performed.