ME 120 Experimental Methods. Flow Measurement. BJ Furman 02MAY06

Transcription

1 ME 120 Experimental Methods Flow Measurement 02MAY06

2 Fluid Flow Measurement Classification of flow rate measurements Volume flow Industrial process control Heat transfer and cooling Mass flow Industrial process control Fluid velocity Experimental study of flow phenomena Flow visualization Image of overall flow field m t L 3 t 3 L = ρ t

3 Categories of Flow Measurement Primary or quantity methods Weight or volume tanks, burettes Flowmeters Obstruction meters Obstacle in flow path causes localized velocity changes Velocity is inferred by pressure change across obstacle Venturi meter Flow nozzle Orifices 2 1 V 2 + P1 + ρ gz 1 = 2 2 V 2 + P2 + ρ gz 2

4 Flowmeters, cont. Variable area meters SPECIFICATIONS Rotameters STANDARD ACCURACY ±2% FS (mm scales); ±5% FS (direct reading scales) CALIBRATED ACCURACY ±1% FS REPEATABILITY ±0.25% FS USEFUL FLOW RANGE 10:1 minimum with one float and better than 20:1 with combination of two floats installed in meters MAXIMUM OPERATING PRESSURE 200 psig (13.8 bars) MAXIMUM OPERATING TEMPERATURE 250 F (121 C) MATERIALS OF CONSTRUCTION FLOW TUBES heavy walled borosilicate glass FLOATS glass, sapphire, 316 stainless steel, carboloy and tantalum SIDE PANELS aluminum, black anodized FRONT SHIELD Lexan with longitudinal magnifier lens for enhanced reading resolution O-RING and PACKING Buna-n o-rings standard in aluminum and brass models; Viton o-rings standard in stainless steel models; optional Viton, PTFE/Kalrez, EPR CONNECTIONS 1/8 inch NPT female inlet and outlet connections; optional 1/4 inch FNPT, hose and compression fittings are available

5 Flowmeters, cont. Turbine and propeller meters

6 Flowmeters, cont. Magnetic flow meters Conductive fluids Accuracies to 0.5% Corrosive liquids Slurries E = B D V fluid

7 Flowmeters, cont. Vortex shedding meters Accuracy: 1% of reading over dynamic range of meter Repeatability: 0.5% of reading Vortex Shedding Meter Principle: An ultrasonic beam, located downstream from a small strut, is transmitted through the vortex pattern. As the vortices travel through the beam, they modulate its carrier wave. This signal is immediately processed electronically which provides an exact reading of the flow rate. Because the number of vortices formed is directly proportional to the rate of flow, it provides a highly accurate and extremely useful method to determine flow in a wide range of applications.

8 Velocity Probes Pressure Probes Pitot tubes

9 Velocity Probes, cont. Hot-wire and hot-film probes Pros: Excellent spatial resolution High frequency response, > 10 khz (up to 400 khz) Cons: Fragile, can be used only in clean gas flows. Needs to be recalibrated frequently due to dust accumulation (unless the flow is very clean) High cost.

10 Velocity Probes, cont. Scattering Probes Laser Doppler Anemometer (LDA) Ultrasonic Anemometer Non intrusive No calibration required Velocity range 0 to supersonic One, two or three velocity components simultaneously Measurement distance from centimeters to meters Flow reversals can be measured High spatial and temporal resolution Instantaneous and time averaged

11 Flow Visualization Smoke trails Dye injection Bubble Laser fluorescence Refractive index change

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