Objectives

• Velocity and flow measurement

• Lab tour and data acquisition use

From the last class: Wheatstone bridge

Known resistor that we select based on R4

Vo

VEX

R1

Our sensor

R2

+

-

+

-

Calculate R4

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

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How to measure velocity?Hot wire anemometer – rate of heat transferPropeller – rate of rotation, correlated with flow or velocityPitot tube – magnitude of velocity pressureLaser – measure velocity of aerosol movement Ultrasonic anemometerThermistor based –measure temperatureOther methods?

How to measure flow?- Calibrated fan – magnitude of fan pressure

Flow hood – Capture flow in known area/measure velocityOrifice – magnitude of pressure drop Vortex flowmeter Rotameters MasflowmetersOther methods?

In all cases:Flow conditions are importantFlow disturbance is an issue

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

vp

v2

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

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.

Transmission

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• 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

- Constant Power

- Constant Temperature

3-D

Temperature control based on measured velocity- Prevents overheating

LaserLDV 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 dwhere f is the frequency of the signal received at the detector.

LaserParticle 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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