# Objectives Velocity and flow measurement

## Presentation on theme: "Objectives Velocity and flow measurement"— Presentation transcript:

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 R1 Vo + R2 - - 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 In all cases: Flow conditions are important Flow 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
ρ = 1.2 kg/m3 = 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 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 Temperature 3-D Constant Power
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

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

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