ME301VenturiFlowLaboratoryManual

ME301VenturiFlowLaboratoryManual - 3. Venturi Flow Behavior...

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3. Venturi Flow Behavior 3.1. Purposes Measure the pressure profile of a venturi at different flow rates and calculate discharge coefficient performance for flow metering. Learn about Bernoulli’s equation and the interaction of static and dynamic pressures. 3.2. Background See pages 389-404 of the textbook. 1. Venturi Channel Shape A venturi is characterized by contracting and expanding areas separated by a throat of minimum flow area. Keeping the mass flow and density fixed along the axial direction, x , the mean axial flow velocity accelerates in the contracting section (decreasing diameter) up to a peak velocity. The mean axial flow velocity decreases as the area (diameter) increases in the expanding section according to constant mass flow 2. Geometric Parameter Using a factor for diameter relative to the inlet diameter, the variation in area, mean flow velocity and dynamic pressure, are found in terms of ratios for any axial location, j, The is a diameter ratio, 2 is an area ratio and 2 is a velocity ratio.
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3. Bernoulli’s Equation for Ideal Flow Bernoulli’s equation (see Bernoulli Equation derivation of §3.6. and The Story of g c §3.8.) for steady, inviscid, irrotational and incompressible flow predicts that the total pressure (a scaler) is constant between any two points Therefore, for constant elevation, z 2 =z 1 , and an increase in the mean flow velocity (due to Diameter decrease) is accompanied by a decrease in flow static pressure. This primary effect ignores the radial and azimuthal velocity components. 4. Wall Static Pressure Measurement Wall static pressure indicates the variation of flow static pressure, but with some error due to the velocity profile generated by wall friction. Then measurements used with theory contain four main non-ideal effects, 1. Frictional - total pressure to varies along the channel length 2. Profile - at fixed axial position, the near wall velocity is not the mean velocity 3. One-dimensional - the flow may have radial and angular motion 4. Unsteady - the flow fluctuates with time The static pressure quantity for each wall pressure tap is 5. Manometer Indication The wall static pressure translates into a manometer height (of the same fluid as the flow) that eliminates the elevation differences among wall pressure tap points. Evaluating the common free surface pressure of each manometer channel (the manometer reservoir), the manometer height, h, relative to the flow centerline is used It is assumed that the manometer fluid is static with no motion or flow to correctly
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indicate the wall static pressure. Then the manometer equation connected to the total pressure is Notice that when the flow in the venturi is ceased and the fluid is static, all manometer heights will be the same. 6. Manometer Differentials
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This note was uploaded on 02/07/2011 for the course AE/ME 301 taught by Professor Lafleur during the Spring '11 term at Clarkson University .

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ME301VenturiFlowLaboratoryManual - 3. Venturi Flow Behavior...

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