化工四大化学(专业)review234511Fluidmechanics(课堂讲义)

1、1 Review for Chapter 2,3,4,5 Fluid mechanics nI. Fluid statics and its application nII. Fluid dynamics and its application 2 I. Fluid statics and its application n1. Density n2. Pressure n3. Equation of hydrostatic equilibrium n4. Application of hydrostatic equilibrium 3 Density n = f (T, P) Usually

2、 solidliquidgas Unit of density: SI unit: kg/m3 cgs unit: g/cm3 fps unit: lb/ft3 4 2. Pressure Unit of pressure: SI unit: Pa (N/m2) others: atm, mmHg, mH2O, kgf/cm2, bar, at, psi (lbf/in2) Absolute pressure Gauge pressure Degree of vacuum(or vacuum) 5 3. Equation of hydrostatic equilibrium )( ba ab

3、ZZg pp (2.4) 0dZgdp(2.2) 6 4. Application of hydrostatic equilibrium n4.1 Barometer 0dZ RT gM p dp (2.6) RT ZZgM p p ab a b )( exp(2.7) 7 n4.2 Manometers (液压计) U-tube manometer Differential manometer Inclined manometer Series manometer 8 n4.3 Measurement of liquid level n4.4 Liquid seal of equipment

4、 9 II. Fluid dynamics and its application n1. Potential flow and viscous flow n2.Viscosity of fluid n3. Rheological properties of fluids n4. Laminar flow and turbulent flow n5. Boundary layer content of chapter 3 10 n6. Continuity equation n7. Bernoulli equation n8. Design of piping system content o

5、f chapter 4,5 11 1. Potential flow and viscous flow Potential flow 势流 Viscous flow 粘性流 12 2.Viscosity of fluid Viscosity: viscosity and kinematic viscosity Unit: Pa.s P, cP m2/s St, cSt Viscosities of liquids and gases 13 3. Rheological properties of fluids n了解牛顿型流体和非牛顿型流体的流变图 14 4. Laminar flow and

6、 turbulent flow nLaminar turbulence Re4000 Regular,parallel No lateral mixing or crosscurrent No eddy irregular, mixing Eddy and vortice criterion motion 15 Laminar turbulence Shear stress Velocity distribution Average velocity Boundary layer 16 Laminar turbulence Kinetic correction factor Momentum

7、correction factor Fanning friction factor 17 5. Boundary layer n(1). Concept of BL n(2). Development of boundary layer on a flat plate and in a pipe n(3). Separation of BL: wake formation, form friction 18 6. Continuity equation nthe important special case: nincompressible fluid flow in circular pip

8、e SVSVSVm bbbaaa 2 a b b a D D V V 19 7. Bernoulli equation * f bb b b p aa a a h V gZ p W V gZ p 22 22 20 (1). Choose datum plate. (2). Choose upstream station and downstream station. (3). The equation can be used for compressible fluid when the change of pressure is less than 20%. (4). The equatio

9、n can be used in unsteady state for any moment. (5). Hydrostatic equilibrium is a special case of Bernoulli equation. 21 (6). Each term in the Bernoulli equation has the same unit: J/kg (7). Static pressure energy, Potential energy, Kinetic energy(velocity head) (8). Energy delivered to the fluid: W

10、p (J/kg) power delivered to the fluid: m Wp (W) shaft work: Wp (9). Friction loss: hf (J/kg) pressure drop: ps (Pa) 22 nTotal friction = skin friction + form friction 2 4 2 V KKK D L fh fecf 23 nFanning friction factor f Laminar flow Re4000 Re 1616 VD f Hydraulically smooth pipe Hydraulically rough

11、pipe Complete turbulent flow Transition region 2100Re4000 Ref D k fRe, k f D 24 noncircular channels: equivalent diameter Deq hydraulic radius rH 25 nExpansion loss coefficient: Ke 2 1 b a e S S K For outlet (exit) Ke = 1 Contraction loss coefficient: Kc For turbulent flow, a b c S S K14 . 0 For Inl

12、et(entrance) Kc=0.5 26 nLoss factor for fitting: Kf 2 4 2 ae ff V D L fh Equivalent length: Le Table 5.1 2 2 a fff V Kh (5.71) 27 8. Design of piping system n(1). Compound pipes in series (a). Given diameter, length, velocity(or flow rate). Calculate pressure drop, energy delivered by pump (b). Give

13、n diameter, length, pressure drop. Calculate velocity.(trial and error) (c). Given length, flow rate, pressure drop. Calculate diameter (trial and error). 28 n(a) 2. Compound pipes in parallel 21ffBfA hhh 21 qqq J/kg 29 n(b) fC CC C C fB BB B B h V gZ p h V gZ p 22 22 CB qqq 30 n5.6. A liquid with a

14、 specific gravity of 2.6 and a viscosity of 2.0 cP flows through a smooth pipe of unknown diameter, resulting in a pressure drop of 0.183 lb f /in. 2 for 1.73 mi(英里，1km = 0.62137mi). What is the pipe diameter in inches if the mass rate of flow is 7,000 lb/h? 31 n5.7. Water flows through an 8-in. ste

15、el pipe at an average velocity of 6 ft/s. Downstream the pipe divides into an 8-in. main and a 2-in. bypass. The equivalent length of the bypass is 22 ft; the length of the 8-in. pipe in the bypassed section is 16 ft. Neglecting entrance and exit losses, what fraction of the total water flow passes

16、through the bypass? 32 n5.8. A steel pipe 2 ft in diameter carries water at about 15 ft/s. If the pipe has a roughness of 0.0003 ft, could the capacity be increased by inserting a smooth plastic liner that reduces the inside diameter to 1.9 ft? Calculate the change in pressure drop for the same flow

17、 and the change in capacity for a fixed pressure drop. nFor water, viscosity is 1cp, and density is 1000kg/m3. 33 n5.10. A reverse-osmosis(反渗透) unit for purifying brackish water has about 900,000 hollow fibers that permit the diffusion of water but reject most of the salt. The fibers are 85 m in outside diameter, 42m in inside diameter, and about 3 ft long. The average flow through the tubes is 2,000 gal of water every 24 h when the feed pressure is 400 psig. What is the pressure drop within an individual fiber from the feed end to the discharge end