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Unformatted text preview: Introduction 1 No industrial measurement is more important than the accurate detection of the flow rates of gases, liquids, and solids. The rate at which fluid flows through a closed pipe can be quantified by either measuring the mass flow rate or measuring the volume flow rate. Of these alternatives, mass flow measurement is more accurate, since mass, unlike volume, is invariant. In the case of the flow of solids, the choice is simpler, since only mass flow measurement is appropriate. Basic Terms 2 Velocity, is a measure of speed and direction of an object. When related to fluids it is the rate of flow of fluid particles in a pipe. The speed of particles in a fluid flow varies across the flow, i.e., where the fluid is in contact with the constraining walls (the boundary layer) the velocity of the liquid is virtually zero; in the center of the flow the liquid will have the maximum velocity. Thus, the average rate of flow is used in flow calculations. Basic Terms 3 Laminar flow of a liquid occurs when its average velocity is comparatively low and the fluid particles tend to move smoothly in layers, as shown in figure. The velocity of the particles across the liquid takes a parabolic shape. Basic Terms 4 Turbulent flow occurs when the flow velocity is high and the particles no longer flow smoothly in layers and turbulence or a rolling effect occurs. Basic Terms 5 Viscosity is a property of a gas or liquid that is a measure of its resistance to motion or flow. A viscous liquid such as syrup has a much higher viscosity than water and water has a higher viscosity than air. Syrup, because of its high viscosity, flows very slowly and it is very hard to move an object through it. Viscosity (dynamic) can be measured in poise or centipoise, whereas kinematic viscosity (without force) is measured in stokes or centistokes. Dynamic or absolute viscosity is used in the Reynolds and flow equations. Typically the viscosity of a liquid decreases as temperature increases. Flow Measurement Instruments 6 Flow measurements are normally indirect measurements using differential pressures to measure the flow rate. The choice of the measuring device will depend on the required accuracy and fluid characteristics (gas, liquid, suspended particulates, temperature, viscosity, and so on.) Flow rate Differential pressure measurements can be made for flow rate determination when a fluid flows through a restriction. The restriction produces an increase in pressure which can be directly related to flow rate. Flow rate Type of constrictions 7 Venturi tube Flow nozzle Dall tube Orifice plate Flow rate Orifice plate 8 The orifice plate is normally a simple metal diaphragm with a constricting hole. The diaphragm is metal diaphragm with a constricting hole....
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This note was uploaded on 12/27/2009 for the course MINING ENG MAD 413 taught by Professor Erencanerorhan during the Fall '09 term at Hacettepe Üniversitesi.
 Fall '09
 ErenCanerOrhan

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