流体力学与传热（伍钦）08-9chapter1-7.pptVIP

Water mass flow rate, in kg/s ，when the reading R of U tube manometer in orifice meter is 600 mm Hg? (?H2O =1000 kg/m3, ?Hg =13600 kg/m3) Developed head H of pump, in m H2O? The equation for the orifice is similar to equation for the venturi where V2 is the velocity in the orifice, D0 is the orifice diameter, and C0 is the orifice coefficient and always determined experimentally. 1.6-12 Co is almost constant and independent of D0/D1 provided Re is greater than about 20000 and D0/D1 is less than about 0.5. It varies considerably with changes in D0/D1 and with Reynolds number at the orifice. Under these conditions Co may be taken as 0.61 for both flange taps and vena-contracta taps. Furthermore, if D0/D1 is less than 0.25, the term differs negligibly from unity, and Eq.(1.6-12 ) becomes The mass flow rate is given by Unless considerable precision is desired, equation is adequate for orifice design . A check on the value of the Re should be made, however, since the coefficient 0.61 is not accurate when Re is less than about 20000. It is especially important that enough straight pipe both above and below the orifice to ensure a flow pattern that is normal and undisturbed by fittings, valves, or other equipment. Pressure recovery The pressure recovery in an orifice is poor, which is one disadvantage of the orifice meter. The fraction of orifice differential that is permanently lost depends on the value of the β. 2.12D. Flow-Nozzle Meter A typical flow nozzle is shown in figure. It is essentially a short cylinder with the approach being elliptical in shape. This meter has characteristics similar to those of the venturi meter but is shorter and much less expensive. The length of the straight portion of the throat is about one-half the diameter of the throat, D2. The upstream pressure tap p1 is 1 pipe diameter from the inlet-nozzle face, and the downstream tap p2 is 1/2 pipe diameter from the inlet-nozzle face. The equation for th