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Capabilities and Limits for ADV
Measurements in Bubbly Flows
Kevin D. Nielsen, Larry J. Weber, and Marian Muste
Iowa
Institute of Hydraulic Research, 300 S. Riverside Drive
University
of Iowa, Iowa City, IA 52242, USA
Telephone:
(319) 335-5597
Fax:
(319) 335-5238
Email:
larry-weber@uiowa.edu
This
report presents the experimental method, results, and conclusions for a study
of the capabilities and limits for Acoustic Doppler Velocimeter (ADV)
measurements in bubbly flows. The
purpose of this project was to investigate procedures to separate the
measurements of bubbles from those of water so that the water velocity could be
extracted from the ADV time series measurements. A controlled experimental setup was constructed to test the phase
discrimination procedures for the ADV.
Several water and air bubble combination scenarios are presented. This project demonstrated that for flows
with low concentrations of air bubbles it is possible to separate ADV
velocities measured on water from those of air bubbles based on the ADV signal
strength. The project also demonstrated
that air bubbles in the signal path (the space between the acoustic transmitter
and the sampling volume) significantly increase the noise of the raw signal
leading to errors in the measurement of turbulence intensities.
Keywords:
ADV, velocity measurements, bubbly flows, two-phase flow
Acoustic
Doppler Velocimeters (ADV) are an established method of measuring three
dimensional velocity components.
However, when bubbles are present in the measurement area the quality of
the measurement decreases rapidly. At
high concentrations of bubbles, the ADV is mainly measuring the bubble velocity
rather than the velocity of the water.
The purpose of this project was to investigate procedures to separate
the measurements of bubbles from those of water so that the water velocity
could be extracted from the ADV time series measurements. An additional purpose was to gain a better
understanding of the effects of air bubbles on ADV velocity measurements. These objectives were accomplished by
constructing an experimental setup where air and water flow in a pipe could be
controlled. ADV signal strengths and
velocity measurements were compared for various combinations of air concentrations
and water flow discharges. The ADV
signal strength provided a way to distinguish between measurements of water
from those of air bubbles. Comparison
of velocity measurements provided an assessment of the impact of bubbles on ADV
velocity measurements.
A
controlled experimental setup was constructed to test phase discrimination
procedures for the ADV. An ADV probe
was installed in a vertical plexiglass pipe (see Figure 1). The flow of water in the pipe was controlled
with a variable speed pump and measured with an elbow meter. Air was introduced into the pipe through a
small diffuser upstream of the ADV probe.
The location of bubbles was controlled by moving the location of the air
diffuser outlet. The timing of the
bubbles was controlled by adjusting the flow rate of air into the diffuser with
manually controlled valves.
A no water
flow, small water flow, and high water flow condition were measured. The small water flow and high water flow
conditions had an average pipe water velocity of 0.085 m/s and 0.338 m/s,
respectively. Air bubbles were released
in the sampling volume only, in the signal path only, and in both the sampling
volume and signal path. Bubble timing
alternatives included bubble burst, bubble train, and bubble-water series. A bubble burst consisted of a large
conglomeration of bubbles of various sizes released every 10 seconds. A bubble train consisted of a series of
relatively uniform bubbles closely located so that very little water was
present between the bubbles in the train.
A bubble-water series consisted of a series of relatively uniform
bubbles spaced to provide significant volumes of water between each
bubble. Figures 1 and 2 illustrate the
spatial location and distribution of the above described alternatives. Measurements with no air bubbles for each
test condition were obtained as a reference.
A 3D down
looking ADV probe was used for the experiment.
The x-component was aligned to measure the velocity along the axis of
the pipe. Velocity comparisons were
performed only on the x-component of velocity because the rising velocities of
air bubbles are along this direction.
Only average signal strength is available from the data processing
provided by the manufacturer.
Therefore, signal strength comparisons were performed using the average
signal strength from the three measured components. The sampling rate was set at 25 Hz and the velocity range was set
at 2.438 m/s. A two minute time series
of ADV measurements were collected for each scenario. Each measurement was repeated 3 times to verify consistency and
repeatability. It was confirmed that
the measurements were consistent and repeatable for all tested conditions. Therefore, average values of velocity and
signal strength were computed for a 60 second time series and plots were
compared for a 30 second time series for greater resolution of the plots.
Figure 3
is an example signal strength time series plot that illustrates impacts of
bubbles in the sample volume. Figure 3
demonstrates that the signal strength is significantly higher for air bubbles
than for small particles tracing the water.
This is most clearly illustrated by the center plot of the bubble bursts
every 10 seconds. Analysis on these
results led to the selection of a threshold of signal amplitude equal to 100
Internal Unit Counts (IUC) to distinguish between measurements on water and
measurements on air. The signal
amplitude for measurements on water are generally below 100 IUC while signal
amplitude for measurements on air are significantly above 100 IUC. Associated velocity measurements
demonstrated that the measured velocity difference between the rising air
bubbles and water are consistent with the timing of the corresponding signal
amplitude.
Results
demonstrated that bubbles in the signal path have only a small impact on the
signal strength. Though the variability
of the signal strength is increased with air bubbles in the signal path, the
amplitude generally remains below or close to 100 IUC. Therefore, the signal amplitude can still be
used as the phase discrimination criterion.
However, instantaneous velocities are affected by the bubbles present in
the signal path. The average velocity
is the same as the base condition but higher variability of the instantaneous velocity
readings around the mean is recorded when air bubbles are in the signal
path. It is not clear yet if this
effect is due to noise in the recorded signal or if it is the consequence of
flow conditions generated by the bubbles present in the flows. Bubbles in both the sampling volume and
signal path have a combined effect of the results discussed previously for
bubbles only in the sampling volume and for bubbles only in the signal path.
It is
possible to distinguish ADV readings on water from those on air bubbles based
on the signal strength. Air bubbles in
the signal path do not increase the signal amplitude above the established
threshold value. Therefore, velocity
readings on water can be identified using the signal strength as phase discrimination
criterion. However, air bubbles in the
signal path significantly increase the variability of the instantaneous
velocities around the mean velocity increasing the turbulence intensities
measured by the ADV. As the bubble
concentration increases, there is limited opportunity for the sampling volume
to be occupied by water. Therefore, the
ADV is mainly measuring the velocity of air bubbles. It can be concluded that the ADV is capable of reliably providing
separate mean velocity measurements on water and air bubbles in a bubbly
flow. Further study is needed to
investigate the origin and characteristics of the increased turbulence
intensity when bubbles are present in the signal path.
Figure 1
Experimental Setup and Bubble Burst Test Conditions
Figure 2
Bubble Train and Bubble - Water Series Test Conditions
Figure 3
Signal Amplitude for Bubbles in Sample Volume