Proveden

1
Experimental methods E181101
EXM5
Flowrate
Some pictures and texts were copied from
www.wikipedia.com
Rudolf Žitný, Ústav procesní a zpracovatelské
techniky CVUT FS 2010
2
Flowrate measurement
EXM5

• Rotameter
• Turbine
• Vortex flowmeters
• Nozzles Venturi, orifice
• Pitot tube (multihole tube)
• Ultrasound
• Coriolis
• Thermal
• Laser Doppler

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Rotameter, Turbine
EXM5

• Rotameter, floater and conical glass pipe.
  Position of floater is determined by the balance
  of forces weight of floater fluid forces. The
  higher is floater, the wider is the gap,
  therefore the lower are velocities and viscous
  friction. This flowmeter can be used not only for
  liquids, but also for gases (or inviscid fluids).
  In this case the fluid forces are not viscous,
  but inertial and can be derived from Bernoullis
  equation.
• Turbine (detector of pulses flowrate
  proportional to frequency)

What do you think is the purpose of these
inclined grooves?
f-frequency. g-viscous correction
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Vortex, orifice
EXM5

• Vortex flowmeters utilize the vortex shedding
  principle. The fluid strikes a bluff body,
  generating vortices (eddies) that move
  downstream. The vortices form alternately, from
  one side to the other. A piezoelectric sensor
  housed in a sensor tube directly downstream of
  the bluff senses the pressure zones created by
  the vortices. The sensor generates a frequency
  directly proportional to the vortices (flow).
• Look at more details about von Karman vortex
  street
• Nozzles, Venturi, orifice use the Bernoulli
  Equation to calculate the fluid flow rate by
  measuring the pressure difference through
  obstructions in the flow

Strouhals number Sr0.21 for cylinder of
diameter D (holds for Regt10000)
1 accuracy, low pressure drop 2, pressure
drop large (vena contracta)
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Ultrasound flowmeters
EXM5
Transit time (without particles) expensive,
accurate
Measurements are made by sending bursts of
signals through a pipe. Sound waves travelling in
the direction of flow of the fluid require less
time than when travelling in the opposite
direction. The difference in transit times of the
ultrasonic signals is an indication for the flow
rate of the fluid. Since ultrasonic signals can
also penetrate solid materials, the transducers
can be mounted onto the outside of the pipe.
?
L-length of beam, u-flow velocity, c-speed of
sound (1500 m/s in water)
Doppler effect (reflected wave by particles).
Doppler frequency shift
Doppler ultrasonic flowmeters operate on the
Doppler shift principle, whereby the transmitted
frequency is altered linearly by being reflected
from particles and bubbles in the fluid. The net
result is a frequency shift between transmitter
and receiver frequencies that can be directly
related to the flow velocity. Doppler meters
require a minimum amount of solid particles or
air in the line to achieve measurements.
?
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UVP monitor (Ultrasound Velocity Profile)
EXM5
Ultrasound Doppler effect for measurement
velocity profiles

1. Piezotransducer is transmitter as well as
   receiver of US pressure waves operating at
   frequency 4 or 8 MHz.
2. Short pulse of few (?10) US waves is transmitted
   (repetition frequency 244Hz and more) and crystal
   starts listening received frequency reflected
   from particles in fluid.
3. Time delay of sampling (flight time) is directly
   proportional to the distance between the
   transducer and the reflecting particle moving
   with the same velocity as liquid.
4. Received frequency differs from the transmitted
   frequency by Doppler shift ?f, that is
   proportional to the component of particle
   velocity in the direction of transducer axis.

http//biomechanika.cz
PROBLEMS

1. What is spatial resolution of velocity, knowing
   speed of sound in water (1400 m/s) and sampling
   frequency 8 MHz ?
2. Calculate flowrate in a circular pipe from
   recorded velocity profile (given angle ?)

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Electromagnetic flowmeters
EXM5
Magnetic flowmeters (electromagnetic or induction
flowmeters), obtain the flow velocity by
measuring the changes of induced voltage of the
conductive fluid passing across a magnetic field.
A typical magnetic flowmeter places electric
coils around the pipe of the flow to be measured
and sets up a pair of electrodes across the pipe
wall. If the targeted fluid is electrically
conductive, i.e., a conductor, its passing
through the pipe is equivalent to a conductor
cutting across the magnetic field. This induces
changes in voltage reading between the
electrodes. The higher the flow speed, the higher
the voltage.
Lorentz force acting on ion (charge q) moving
with velocity u in magnetic field (magnetic
induction B).
Lorentz force is in balance with electromotive
force
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Coriolis flowmeters
EXM5
Coriolis force in a rotating pipe
Rotation is substituted by vibration in an actual
design of flowmeter.
?-phase shift
Detectors of position
kg/m3 density from eigenfrequency
kg/s f-frequency of oscillation
Elmag.induced oscillations
The flow is guided into the U-shaped tube. When
an osillating excitation force is applied to the
tube causing it to vibrate, the fluid flowing
through the tube will induce a rotation or twist
to the tube because of the Coriolis acceleration
acting in opposite directions on either side of
the applied force. This twist results in a phase
difference (time lag) between the inlet side and
the outlet side and this phase difference is
directly affected by the mass passing through the
tube. A more recent single straight tube design
is available to measure some dirty and/or
abrasive liquids. Vibration of Coriolis
flowmeters has very samll amplitude, usually less
than 2.5 mm, and the frequency is near the
natural frequency of the device, usually around
80 Hz. The vibration is commonly introduced by
electric coils and measured by magnetic sensors.
Resonant frequency depends upon density
therefore not only flowrate but also density is
measured.
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Thermal flowmeters
EXM5
Thermal mass-flowmeters heated wire/hot film
anemometers electr.current is controlled so that
the temperature of wire (resistance) is constant.

Power necessary to maintain constant temperature
of heated wire depends upon flow velocity
i-current is adjusted so that the Rm will be the
same as the fixed resistors in the bridge.
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Thermal flowmeters
EXM5
Hot wire anemometer simplified theory
Heat generated in wire
Heat transferred to fluid by convection
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Thermal flowmeters
EXM5
Differential anemometers heater 2
thermocouples symmetrically located
Heater
T1
T2
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Correlation flowmeters
EXM5
Cross-correlation of stimulated or random signal
detected at two locations (technically it can be
a heater and thermocouples)
Heater
T1
T2
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Correlation flowmeters
EXM5
Example calculated by MATLAB
Random signal shifted by 100 time steps