Transcript Closed Conduit: Measurement Techniques

Closed Conduit Measurement
Techniques
Monroe L. Weber-Shirk
School of Civil and
Environmental Engineering
Measurement Techniques
Direct Volume or
Weight measurements
 Velocity-Area
Integration
 Pressure differential

Pitot Tube
 Venturi Meter
 Orifice
 Elbow Meter

Electromagnetic Flow
Meter
 Turbine Flow Meter
 Vortex Flow Meter
 Displacement Meter
 Acoustic Doppler
 Laser Doppler
 Particle Tracking

Some Simple Techniques...
 Direct Volume
or Weight measurements
 Measure
volume and time (bucket and
stopwatch)
 Excellent for average flow measurements
 Velocity-Area
Stream flow
 Pitot
Tube
Integration
Venturi Meter
 1797
- Venturi presented his work on the
Venturi tube
 1887 - first commercial Venturi tube
produced by Clemens Herschel
 Minimal pressure loss
V12
p2 V22

 z1  
 z 2  hL
 2g
 2g
p1
Yes!
Bernoulli equation applicable?_______
1
2
Venturi Meter Discharge
Equation
V22 V12



  2g 2g
4
2 
 D2  
p1 p2 V2


1    
  2 g   D1  
p1
V2 
p2
2 g ( p1  p2 )

 1   D2 D1 4
Q  Cv A2
 1   D2 D1 4
Q  K venturi A2 2 gh
2
V1D12  V2 D22

2 g ( p1  p2 )

1

Cv is the coefficient of velocity. It
corrects for viscous effects
(energy losses) and velocity
gradients (a).
Kventuri is 1 for high Re and small
D2/D1 ratios
Orifice
8D
2.5 D
h
D
Q  K orifice Aorifice 2 gh
Q  K orifice Aorifice
2p

The flow coefficient, Korifice, is a
function of the ratio of orifice
diameter to pipe diameter and is a
weak function of ________
Reynolds number.
Elbow Meter
Centripetal force at the bend
results in higher pressure at
the outside of the bend
 Any elbow can be used as the
meter
 Needs to be calibrated (no
standard calibration curves
are available)

V2
Fc  m
r
?
Q = Kelbow Aelbow 2 g Dh
Electromagnetic Flow Meter
Conductor moving
through a magnetic field
generates an _______
electric
field.
 Voltage is proportional to
velocity
 Causes no “measurable”
__________
resistance to flow
 High signal amplification
is required

magnet
conductive fluid
electrodes
measure voltage here
Turbine and Paddle Wheel Flow
Meters
 Simply
a turbine mounted in a pipe held in
a stream
 The angular velocity of the turbine is
related to the velocity of the fluid
 Can operate with relatively
low head loss
 Needs to be calibrated
 Used to measure
_________
___ ____
volumetric flow
rate or
___________
velocity
Vortex Flow Meter
Vortex shedding
 Strouhal number, S, is constant
for Re between 104 and 106
 Vortex shedding frequency (n)
can be detected with pressure
sensors

L
d
nd
S
V0
L  4.3d
Displacement Meter
 Used
extensively for measuring the quantity
of water used by households and businesses
 Uses positive displacement of a piston or
disc
 Each cycle of the piston corresponds to a
known volume of water
 Designed to accurately measure
slow leaks!
Orifice Example
Estimate the orifice diameter that will result in a 100 kPa
pressure drop in a 6.35 mm I.D. pipe with a flow rate of
80 mL/s. The orifice coefficient (Korifice) is 0.6.
 What is  the ratio of orifice diameter to pipe diameter?
 If the smallest pressure differential that can accurately be
measured with the pressure sensor is 1 kPa, what is the
smallest flow that can accurately be measured using this
orifice?
 What are two ways of extending the range of
measurement to lower flows?

Acoustic Doppler Velocimetry

The ADV measures the velocity of water using the
Doppler effect. If a source of sound is moving
relative to the receiver, the frequency of the sound
at the receiver is shifted from the transmitted
frequency.

Fdoppler = -Fsource ( V / C )

V is the relative velocity between source and
receiver, C is the speed of _______,
sound Fdoppler is the
change in frequency at the receiver, and Fsource is
the transmitted frequency.
Acoustic Doppler Velocimeter
http://www.sontek.com/
Point measurement
_______
Laser Doppler Velocimetry

a single laser beam is split into two equal-intensity
beams which are focused at a point in the flow
field.

An interference pattern is formed at the point where
the beams intersect, defining the measuring volume.

Particles moving through the measuring volume
scatter light of varying intensity, some of which is
collected by a photodetector.

The resulting frequency of the photodetector output
is related directly to particle velocity.
http://www.tsi.com/
_______
Point measurement

Particle Tracking Velocimetry
Illuminate a slice of fluid
(seeded with particles) with a
laser sheet
 Take a high resolution picture
with a digital camera
 Repeat a few milliseconds later
 Compare the two images to
determine particle displacement
 Measures _______
velocity ______
field

http://amy.me.tufts.edu/
Questions to Ponder
 Will
an ADV need to be recalibrated if it is
moved from freshwater to saltwater?
 A graduate student proposes to use an LDV
in a wave tank that is stratified with
freshwater on top of saltwater to measure
turbulence from the breaking waves. What
problem might arise?
Orifice Solution
 Estimate
the orifice diameter that will result in a
100 kPa pressure drop in a 6.35 mm I.D. pipe
with a flow rate of 80 mL/s. The orifice
coefficient (Korifice) is 0.6.
Q  K orifice Aorifice
d=
4Q
p K orifice
2p

d=
2 Dp
r
pd2
Q = K orifice
4
2Dp
r
4 (80 ´ 10 - 6 m3 / s )
p (0.6)
2 (100000 Pa )
1000kg / m3
d = 3.46mm
Orifice Solution
What is  the ratio of orifice diameter to pipe
diameter? (0.546)
 If the smallest pressure differential that can
accurately be measured with the pressure sensor is
1 kPa, what is the smallest flow that can
accurately be measured using this orifice?

pd2
Q = K orifice
4

2Dp
r
8 mL/s
What are two ways of extending the range of
measurement to lower flows?