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Rules of Thumb for Mechanical Engineers 2010 Part 2

Tham khảo tài liệu 'rules of thumb for mechanical engineers 2010 part 2', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Fluids 15 Open-Channel Flow Measurements A weir is an obstruction in the flow path causing flow to back up behind it and then flow over or through it Figure 7 . Height of the upstream fluid is a function of the flow rate. Bernoulli s equation establishes the weir relationship Q c l 2gH3 CcLH15 where Cc is the contraction coefficient 3.33 in U.S. units and 1.84 in metric units L is the width of weir and h is the head of liquid above the weir. Usually a correction coefficient is multiplied to account for the velocity head. For a V-notch weir the equation may be written as Q Ctti 2gtan 0 2 H25 For a 90-degree V-notch weừ this equation may be approximated to Q CyH2-5 where Cv is 2.5 in U.S. units and 1.38 in metric units. Figure 7. Rectangular and V-notch weirs. Viscosity Measurements Three types of devices are used in viscosity measurements capillary tube viscometer Saybolt viscometer and rotating viscometer. In a capillary tube arrangement Figure 8 ĂpĩĩD4 128QL period. The Saybolt viscometer operates under the same principle. In the rotating viscometer Figure 9 two concentric cylinders of which one is stationary and the other is rotating at constant rpm are used. The torque transmitted from one to the other is measured through spring deflection. The reservofr level is maintained constant and Q is determined by measuring the volume of flow over a specific time Figure 9. Rotating viscometer. Figure 8. Capillary tube viscometer. 16 Rules of Thumb for Mechanical Engineers The shear stress T is a function of this torque T. Knowing Td shear stress the dynamic viscosity may be calculated from 27iR3hm Newton s law of viscosity. OTHER TOPICS Unsteady Flow Surge and Water Hammer Study of unsteady flow is essential in dealing with hydraulic transients that cause noise fatigue and wear. It deals with calculation of pressures and velocities. In closed circuits it involves the unsteady linear momentum equation along with the unsteady continuity equation. If the nonlinear friction