流体力学与 及传热课件1.6 Metering of Fluids.pptVIP

1.6 Metering of Fluids ;Most meters operate on all the fluid in the pipe or channel and are known as full-bore meter.;1.6.1 Insertion Meters ; Pitot Tube ;The impact tube has its opening normal to the direction of flow; The coefficient Cp is used to take into account deviations, which generally varies between about 0.98 and 1.0. ; This equation applies to incompressible fluids but can be used to approximate the flow of gases at moderate velocities and pressure changes of about 10% or less of the total pressure. ;The value of the pressure drop Δp is related to R, the reading on the manometer: ;Since the pitot tube measures velocity at only one point, several methods can be used to obtain the average velocity in the pipe. ; Care should be taken to have the pitot tube at least 100 diameters downstream from any pipe obstruction. ;1.6.2 Full-Bore Meters ;1. Venturi Meter;At the junction of the cylindrical and conical portion, an annular chamber is provided, and a number of small holes drilled from the inside of the tube to the annular chamber. ; A second piezometer ring is formed in the throat section. ;In the venturi meter , the velocity is increased, and the pressure decreased in the upstream cone. The pressure drop in the upstream cone is used to measure the rate of flow through the instrument.; To derive the equation for the venturi meter, friction is neglected and the pipe is assumed horizontal. Assuming turbulent flow and writing the mechanical-energy-balance equation between points 1 and 2 for an incompressible fluid,;The continuity equation for constant ρ is ; V2 is estimated from equations;Venturi coefficient;When d2 is less than d1/4, the approach velocity and the term can be neglected, since the error is less than 0.2 percent.;Flow rate;Pressure recovery; 2.12C. Orifice Meter;For a given meter and definite manometer system, the maximum measurable flow rate is fixed, so if the flow range is changed, the throat diameter may be too l