Transcript CE/ARE 397 Indoor Air Quality: Field Measurements

Objectives
• Velocity and flow measurement
• Lab tour and data acquisition use
From the last class:
Wheatstone bridge
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Known resistor that we select based on R4
R1
Vo
+
R2
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VEX
Calculate R4
Our sensor
Converting Analog signal to
Digital signal
Analog-to-digital converter (ADC)
- electronic device that converts analog signals to an equivalent digital form
- heart of most data acquisition systems
Loss of information in conversion,
but no loss in transport and processing
Velocity and flow measurement
How to measure velocity?
Hot wire anemometer – rate of heat transfer
Propeller – rate of rotation, correlated with flow or velocity
Pitot tube – magnitude of velocity pressure
Laser – measure velocity of aerosol movement
Ultrasonic anemometer
Thermistor based –measure temperature
Other methods?
How to measure flow?
-Calibrated fan – magnitude of fan pressure
Flow hood – Capture flow in known area/measure velocity
Orifice – magnitude of pressure drop
Vortex flowmeter
Rotameters
Masflowmeters
Other methods?
In all cases:
Flow conditions are important
Flow disturbance is an issue
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Propeller
• Rotational speed is calibrated to flow rate
• Does this disturb flow?
• What flows are hard to measure?
• Example: Multifunction meter
Pitot Tube
• From Bernoulli Equation
v
2 pv

ρ = 1.2 kg/m3 = 0.075 lb/m3 at std. conditions
Ultrasonic Anemometer
- No moving parts
- Use ultrasonic sound waves to measure wind
speed and direction
- Good precision
- Relatively high frequency (up to 60Hz)
Several principle of operation
- Transmission (contrapropagating transit time) flowmeters
- Reflection (Doppler) flowmeters – for liquids
Transmission
Send sound pulses and measure transit time between
an ultrasonic pulse sent in the flow direction and an
ultrasound pulse sent opposite the flow direction.
RTD Temperature Based Velocity
Sensor
• Differential between two RTDs mounted on the sensor tube.
The upper sensor measures the ambient temperature of the gas
and continuously maintains the second RTD (near the tip of
the probe) at 60°F above ambient.
• The higher the gas velocity, the more current is required to
maintain the temperature differential.
• Good for high rangeability measurements of very low flows.
Hot Wire Anemometer (HWA)
• Issues
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Measures velocity at a single point
Omnidirectional
Directional (1D, 2D & 3D)
Minimal disturbance to flow
High frequency
Very Expensive
Fragile for field measurements
Require frequent calibration
Hot Wire Anemometer
3-D
Constant Power
- Constant Temperature
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Temperature control based on measured velocity
- Prevents overheating
Laser
LDV or LDA
Laser Doppler Velocimetry
- Non-intrusive 1D, 2D and 3D point
measurement of velocity and turbulence
distribution
- Requires particles seeded or from flow
- Ultra high precession
- High spatial and temporal resolution
- Very expensive
LDA (LDV)
As particles pass through the fringes, they reflect light (only from the regions of constructive interference) into a photodetector.
Since, the fringe spacing d is known (from calibration), the velocity can be calculated to be
u = f \times d
where f is the frequency of the signal received at the detector.
Laser
Particle Image Velocimetry (PIV)
Provide two- or three-dimensional velocity maps in flows using whole field
techniques based on imaging the light scattered by small particles in the flow
illuminated by a laser light sheet.
Is this CFD?
PIV
Properties similar to LDV
Schlieren flow visualization
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Flow Measurements
Flow hood
Orifice and Venturies tube
Rotameter
Orifice
• Pressure drop through a known (circular, sharp
edged) hole
• Flow is smoothed before entry (usually need ~10
diameters upstream)
• Q = C √ΔP
• C provided by manufacturer (sometimes √ too)
• Concerns/issues
• Example: Trueflow Plate
Thermistor Based Velocity Sensor
Thermistor
based
Vortex flowmeter
For given geometry
V~f
You measure sped of
pressure oscillations (frequency)
Flow with Pitot tube
Flow measurement Multiple measurements with pitot tube
Positioning of flow station / measuring
point
Gas Mass Flowmeter
The measuring gas is split.
Most goes through a bypass tube,
while a fraction goes through a sensor tube containing two
temperature coils. Heat flux is introduced at two sections
of the sensor tube by means of two wound coils. As gas flows
through the device, it carries heat from the upstream, to the downstream,
coils. The temperature differential, generates a proportional change in the resistance
of the sensor windings.
Bubble flow meter
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