Closed Conduit: Measurement Techniques
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Closed Conduit Measurement Techniques Monroe L. Weber-Shirk School of Civil and Environmental Engineering Measurement Techniques Direct Volume or Weight measurements Velocity-Area Integration Pressure differential Pitot Tube Venturi Meter Orifice Elbow Meter Electromagnetic Flow Meter Turbine Flow Meter Vortex Flow Meter Displacement Meter Acoustic Doppler Laser Doppler Particle Tracking Some Simple Techniques... Direct Volume or Weight measurements Measure volume and time (bucket and stopwatch) Excellent for average flow measurements Velocity-Area Stream flow Pitot Tube Integration Venturi Meter 1797 - Venturi presented his work on the Venturi tube 1887 - first commercial Venturi tube produced by Clemens Herschel Minimal pressure loss V12 p2 V22 z1 z 2 hL 2g 2g p1 Yes! Bernoulli equation applicable?_______ 1 2 Venturi Meter Discharge Equation V22 V12 2g 2g 4 2 D2 p1 p2 V2 1 2 g D1 p1 V2 p2 2 g ( p1 p2 ) 1 D2 D1 4 Q Cv A2 1 D2 D1 4 Q K venturi A2 2 gh 2 V1D12 V2 D22 2 g ( p1 p2 ) 1 Cv is the coefficient of velocity. It corrects for viscous effects (energy losses) and velocity gradients (a). Kventuri is 1 for high Re and small D2/D1 ratios Orifice 8D 2.5 D h D Q K orifice Aorifice 2 gh Q K orifice Aorifice 2p The flow coefficient, Korifice, is a function of the ratio of orifice diameter to pipe diameter and is a weak function of ________ Reynolds number. Elbow Meter Centripetal force at the bend results in higher pressure at the outside of the bend Any elbow can be used as the meter Needs to be calibrated (no standard calibration curves are available) V2 Fc m r ? Q = Kelbow Aelbow 2 g D h Electromagnetic Flow Meter Conductor moving through a magnetic field generates an _______ electric field. Voltage is proportional to velocity Causes no “measurable” __________ resistance to flow High signal amplification is required magnet conductive fluid electrodes measure voltage here Turbine and Paddle Wheel Flow Meters Simply a turbine mounted in a pipe held in a stream The angular velocity of the turbine is related to the velocity of the fluid Can operate with relatively low head loss Needs to be calibrated Used to measure _________ ___ ____ volumetric flow rate or ___________ velocity Vortex Flow Meter Vortex shedding Strouhal number, S, is constant for Re between 104 and 106 Vortex shedding frequency (n) can be detected with pressure sensors L d nd S V0 L 4.3 d Displacement Meter Used extensively for measuring the quantity of water used by households and businesses Uses positive displacement of a piston or disc Each cycle of the piston corresponds to a known volume of water Designed to accurately measure slow leaks! Orifice Example Estimate the orifice diameter that will result in a 100 kPa pressure drop in a 6.35 mm I.D. pipe with a flow rate of 80 mL/s. The orifice coefficient (Korifice) is 0.6. What is the ratio of orifice diameter to pipe diameter? If the smallest pressure differential that can accurately be measured with the pressure sensor is 1 kPa, what is the smallest flow that can accurately be measured using this orifice? What are two ways of extending the range of measurement to lower flows? Acoustic Doppler Velocimetry The ADV measures the velocity of water using the Doppler effect. If a source of sound is moving relative to the receiver, the frequency of the sound at the receiver is shifted from the transmitted frequency. Fdoppler = -Fsource ( V / C ) V is the relative velocity between source and receiver, C is the speed of _______, sound Fdoppler is the change in frequency at the receiver, and Fsource is the transmitted frequency. Acoustic Doppler Velocimeter http://www.sontek.com/ Point measurement _______ Laser Doppler Velocimetry a single laser beam is split into two equal-intensity beams which are focused at a point in the flow field. An interference pattern is formed at the point where the beams intersect, defining the measuring volume. Particles moving through the measuring volume scatter light of varying intensity, some of which is collected by a photodetector. The resulting frequency of the photodetector output is related directly to particle velocity. http://www.tsi.com/ _______ Point measurement Particle Tracking Velocimetry Illuminate a slice of fluid (seeded with particles) with a laser sheet Take a high resolution picture with a digital camera Repeat a few milliseconds later Compare the two images to determine particle displacement Measures _______ velocity ______ field http://amy.me.tufts.edu/ Questions to Ponder Will an ADV need to be recalibrated if it is moved from freshwater to saltwater? A graduate student proposes to use an LDV in a wave tank that is stratified with freshwater on top of saltwater to measure turbulence from the breaking waves. What problem might arise? Orifice Solution Estimate the orifice diameter that will result in a 100 kPa pressure drop in a 6.35 mm I.D. pipe with a flow rate of 80 mL/s. The orifice coefficient (Korifice) is 0.6. Q K orifice Aorifice d= 4Q p K orifice 2D p r 2p p d 2 2D p Q = K orifice 4 r d= 4 ( 80 ґ 10 - 6 m3 / s ) p ( 0.6) 2 (100000 Pa ) 1000kg / m3 d = 3.46mm Orifice Solution What is the ratio of orifice diameter to pipe diameter? (0.546) If the smallest pressure differential that can accurately be measured with the pressure sensor is 1 kPa, what is the smallest flow that can accurately be measured using this orifice? p d 2 2D p Q = K orifice 4 r 8 mL/s What are two ways of extending the range of measurement to lower flows?
