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FLOW MEASUREMENTS

FLOW MEASUREMENTS. PITOT TUBES VENTURI METER ORIFICES FLOW METERS. PITOT & PIEZO TUBES. L shaped tubes inserted in fluid to measure total pressure at point (SEE FIG. 13.1) Useful for measuring flow of liquids in open channels and of gases in closed ducting/pipes

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FLOW MEASUREMENTS

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  1. FLOW MEASUREMENTS PITOT TUBES VENTURI METER ORIFICES FLOW METERS KK's FLM 221: Week13: FLOW MEASUREMENTS

  2. PITOT & PIEZO TUBES • L shaped tubes inserted in fluid to measure total pressure at point (SEE FIG. 13.1) • Useful for measuring flow of liquids in open channels and of gases in closed ducting/pipes • Total pressure = Static + dynamic pressure (13.1) Open channel flow measurements: so that (13.2) Gas flow: Manometer used to separate static pressure from dynamic pressure: (13.3) Also by integrating over entire ducting cross section, we get Q as in (14.2) above • Liquid flow in pipes is best determined using the Venturi meter – discussed below: FIGURE 13.1: PITOT / PIEZO TUBES Open channel flow measurement Gas flow measurement in ducting KK's FLM 221: Week13: FLOW MEASUREMENTS

  3. VENTURI METER • Consists of converging section connected to a diverging section via a narrow uniform circular section called a throat (SEE FIG. 13.2) • There are provisions for connection of a manometer at the throat and inlet section so that pressure differences can be measured, and flow rate deduced using the loss corrected Bernoulli equation For m = A1/At; H = (P1-Pt)/ρg – (zt –z1); we have: (13.4) And (13.5) These are theoretical equations. In practice, we use a discharge factor Cdto correct for losses. Then: (13.6) Also, we normally measure the height difference h instead of H. Then we get: so that (13.7) KK's FLM 221: Week13: FLOW MEASUREMENTS

  4. FIGURE 13.2: THE VENTURI METER – FOR FLOW OF LIQUIDS IN PIPES KK's FLM 221: Week13: FLOW MEASUREMENTS

  5. ORIFICES • Liquid flow can be determined using an orifice plate instead of a venturi meter because the latter is expensive. The orifice forms a VENA-CONTRACTAwhich is smaller than the orifice area by a factor Cc, called COEFFICIENT OF CONTRACTION (13.8) • The velocity through the vena contractavc is also less than the theoretical (because of friction) by a factor Cv, called COEFFICIENT OF VELOCITY. Then: (13.9) • The discharge rate Q is given as: (13.10) Normally Cd is between 0.6 and 0.65! EXERCISE: Wine of density 800 kg/m3 and viscosity 0.6 mPas flowing in a 50 mm pipe. A 25 mm orifice plate in the flow yields a 100 mm mercury manometer reading. If Cd=0.61, determine the flow rate and Reynold’s No. KK's FLM 221: Week13: FLOW MEASUREMENTS

  6. FIGURE 13.3: ORIFICE PLATE AND FORMATION OF VENA CONTRACTA KK's FLM 221: Week13: FLOW MEASUREMENTS

  7. FLOW METERS • Instruments used to measure flow rates of fluids. • Most common ones use pressure differentials in a transducer to give the fluid flow velocity and flow rate. Thus we have: pitot based; venturi based (called nozzle meters) and orifice ones. • Others are positive displacement based -: where flowing fluid actuates mechanical transducers e.g. lobe impeller meters, nutating disk meters, turbine flow meters, paddle meters etc. • Some use drag force to measure flow rate – egRotameters – as used in labs: accuracy up to 5% HOMEWORK EXERCISE: Read about others eg ultrasonic, electromagnetic, vortex etc. Expect a question about them in either Test 2 or the Exam or BOTH!: KK's FLM 221: Week13: FLOW MEASUREMENTS

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