Presentation on theme: "Objectives Velocity and flow measurement Lab tour and data acquisition use."— 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 Vo V EX R1 Our sensor R 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 4 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
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/m 3 = lb/m 3 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
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 15
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.