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Objectives

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. -. -. V EX. Calculate R4. Our sensor. Converting Analog signal to Digital signal .

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Objectives

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  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 R4 R1 + Vo R2 - - VEX Calculate R4 Our 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 systems Loss 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 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

  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 = 0.075 lb/m3at std. conditions

  7. 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.

  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) • 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

  10. Hot Wire Anemometer 3-D • Constant Power • Constant Temperature 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 distribution • Requires particles seeded or from flow • Ultra high precession • High spatial and temporal resolution • Very 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 be u = f \times d where 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 field techniques based on imaging the light scattered by small particles in the flow illuminated by a laser light sheet. Is this CFD?

  14. PIV Properties similar to LDV

  15. Schlieren flow visualization

  16. Flow Measurements Rotameter Flow hood Orifice and Venturies tube

  17. 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

  18. Thermistor Based Velocity Sensor Thermistor based

  19. Vortex flowmeter For given geometry V~f You measure sped of pressure oscillations (frequency)

  20. Flow with Pitot tube Flow measurement Multiple measurements with pitot tube

  21. Positioning of flow station / measuring point

  22. 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.

  23. Bubble flow meter

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