1 / 7

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

alika
Télécharger la présentation

FLOW MEASUREMENTS

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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

More Related