Figure 2: Schematic of experimental setup.
A mixing tank is used to generate a mixture of bentonite powder with
known concentration in water. Dried calcium bentonite powder was
dispersed in water with the assistance of a mechanical stirrer. The
bentonite powder was added at a ratio according to Eq (15) in section
3.2. Before the experiments, the suspension was stirred for 15 minutes.
The homogeneous fluid mixture was then re-circulated by a progressive
cavity pump in a closed loop for ultrasonic flow measurement. The pump
motor speed was regulated by a variable frequency drive (VFD) via a
speed potentiometer. A bellow was installed at the pump discharge side
to minimize the piping vibration which may affect the meter accuracy.
The single path transit-time ultrasonic flowmeter utilized two
piezoelectric transducers, which were clamped on the outside of the
closed pipes at a specified distance from each other. The flowmeter
measured fluid velocity by transmitting acoustic signals between the two
transducers alternatively, first in the opposing direction of fluid
flow, and then in the direction of flow. The differences between the
transit times of the signals were directly proportional to fluid
velocity. The flow output meter signal was then fed to the PC after
being converted to an appropriate digital signal. The mud fluid was
slowly pumped into 2-in (54.78 mm ID) stainless steel test section,
which was controlled by the manual operated ball valve.
Experimentally, the output of transit-time ultrasonic flowmeter alone
was insufficient, and a joint measurement with another reference method
was normally required. Therefore, the desired flow was drained to an
overhead tank for volumetric measurement. Video was recorded and the
video images were processed by an algorithm to acquire the time series
flow rate for comparison with the transit-time ultrasonic flowmeter
readings.
After completion of all measurements, The experimental data were
recorded in room temperature (27~33°C) were analyzed to
obtain relationship between signal patterns in the ultrasonic sensors
and bentonite concentration, flow speed and Reynolds number. Each
experiment was conducted at unsteady flowing condition with flow
accelerating from rest to 1.2 ~1.5 m/s, and next,
decelerated to rest again, within a duration of 25~30
seconds.