Presentation on theme: "Objectives Velocity and flow measurement"— Presentation transcript:
1 Objectives Velocity and flow measurement Lab tour and data acquisition use
2 From the last class: Wheatstone bridge +Known resistor that we select based on R4R1Vo+R2--VEXCalculate R4Our sensor
3 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 systemsLoss of information in conversion,but no loss in transport and processing
4 Velocity and flow measurement 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 movementUltrasonic anemometerThermistor based –measure temperatureOther methods?How to measure flow?Calibrated fan – magnitude of fan pressureFlow hood – Capture flow in known area/measure velocityOrifice – magnitude of pressure dropVortex flowmeterRotametersMasflowmetersIn all cases:Flow conditions are importantFlow disturbance is an issue
5 Propeller Rotational speed is calibrated to flow rate Does this disturb flow?What flows are hard to measure?Example: Multifunction meter
6 Pitot Tube From Bernoulli Equation ρ = 1.2 kg/m3 = lb/m3 at std. conditions
7 Ultrasonic Anemometer - No moving parts- Use ultrasonic sound waves to measure windspeed and direction- Good precisionRelatively high frequency (up to 60Hz)Several principle of operation- Transmission (contrapropagating transit time) flowmeters- Reflection (Doppler) flowmeters – for liquidsTransmissionSend sound pulses and measure transit time betweenan ultrasonic pulse sent in the flow direction and anultrasound pulse sent opposite the flow direction.
8 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.
9 Hot Wire Anemometer (HWA) IssuesMeasures velocity at a single pointOmnidirectionalDirectional (1D, 2D & 3D)Minimal disturbance to flowHigh frequencyVery ExpensiveFragile for field measurementsRequire frequent calibration
10 Hot Wire Anemometer Constant Temperature 3-D Constant Power Temperature control based on measured velocity- Prevents overheating
11 Laser LDV or LDA Laser Doppler Velocimetry Non-intrusive 1D, 2D and 3D point measurement of velocity and turbulence distributionRequires particles seeded or from flowUltra high precessionHigh spatial and temporal resolutionVery expensive
12 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 beu = f \times dwhere f is the frequency of the signal received at the detector.
13 Laser Particle Image Velocimetry (PIV) Provide two- or three-dimensional velocity maps in flows using whole fieldtechniques based on imaging the light scattered by small particles in the flowilluminated by a laser light sheet.Is this CFD?
16 Flow MeasurementsFlow hoodOrifice and Venturies tubeRotameter
17 Orifice Pressure drop through a known (circular, sharp edged) hole Flow is smoothed before entry (usually need ~10 diameters upstream)Q = C √ΔPC provided by manufacturer (sometimes √ too)Concerns/issuesExample: Trueflow Plate
22 Gas Mass Flowmeter The measuring gas is split. Most goes through a bypass tube,while a fraction goes through a sensor tube containing twotemperature coils. Heat flux is introduced at two sectionsof the sensor tube by means of two wound coils. As gas flowsthrough the device, it carries heat from the upstream, to the downstream,coils. The temperature differential, generates a proportional change in the resistanceof the sensor windings.