流體力學與傳熱課件Unit Operations of Chemical Engineering

1、流體力學與傳熱課件流體力學與傳熱課件Unit Unit Operations of Chemical Operations of Chemical EngineeringEngineering華南理工大學 化工學院 伍欽 , TelThe principles of momentum, heat, and mass transport and the separation processes are widely used in material processing fields. In this text the principles of the transpo

2、rt processes are described first, and then the separation processes (unit operations). 華南理工大學 化工學院 伍欽 , TelPART 1: Transport Processes: Momentum and Heat transfer PART 2: Separation Process Principles (Includes Unit Operations) To accomplish this, the text is divided into two main parts

3、. 華南理工大學 化工學院 伍欽 , Telhapter 1 Fluid Flow The behavior of fluids is the most important to process engineering, so , an understanding of fluids is essential 華南理工大學 化工學院 伍欽 , TelFluids which include liquids, gases, and vapors Fluid mechanics has two branches important to the

4、study of unit operations: fluid statics and fluid dynamics華南理工大學 化工學院 伍欽 , Tel南理工大學 化工學院 伍欽 , Tel:135387043151.1.1 Nature of fluids Fluid does not permanently resist distortion. During the change in shape, shear stresses exist, the magnitudes of which depend upon the viscosity of the fl

5、uid and the rate of sliding. 華南理工大學 化工學院 伍欽 , Telncompressible fluidThe density is but little affected by moderate changes in temperature and pressure.Compressible fluidThe density is sensitive to changes in these variables.華南理工大學 化工學院 伍欽 , TelIf the density changes only sl

6、ightly with changes in T and P, the fluid is said to be incompressible; If the changes in density are significant, the fluid is said to be compressible華南理工大學 化工學院 伍欽 , TelPressure concept Pressure is defined as force exerted per unit area. The pressure at any point in the fluid is indep

7、endent of direction. Pressure is a scalar quantity華南理工大學 化工學院 伍欽 , TelPressure units Pressures are given in many different sets of units. SI, standard atmospherePressure referencegauge pressure absolute pressure華南理工大學 化工學院 伍欽 , Teln order to understand pressure, we must fir

8、st discuss a basic law of Newtons. mgF The force exerted by a mass under the influence of gravity by a mass of fluid on a supporting area. gAhmgF華南理工大學 化工學院 伍欽 , TelSghgSShSFP The force exerted per unit area is defined as pressure華南理工大學 化工學院 伍欽 , Tel:135387043151.1.2. Hydrostatic Equili

9、briumIn a stationary mass of a single static fluid, the pressure is constant in any cross section parallel to the earths surface but varies from height to height. Consider the vertical column of fluid shown in Fig. 1.1 華南理工大學 化工學院 伍欽 , Telhree vertical forces are acting on this volume:F

10、igure1.1 Hydrostatic equilibriumpp +dpg(1) the force from pressure p acting in an upward direction, Which is pS;(2) the force from pressure p +dp acting in a downward direction,which is (p+dp)S;(3) the force of gravity acting downward, which is gSdz 華南理工大學 化工學院 伍欽 , Telhen SdzgSdpppS0dz

11、gdp1.1-2After simplification and division by S, equation becomesIntegration of this equation on assumption that is constant gives華南理工大學 化工學院 伍欽 , Telbetween the two definite heights Za and Zb shown in figurebaabZZgPP1.1-4 constgzpZaZbFigure2 Hydrostatic equilibrium華南理工大學 化工學院 伍欽 , Tel:1

12、3538704315 Since it is the vertical height of a fluid that determines the pressure in a fluid, the shape of the vessel does not affect the pressure. Also, it can be shown that the pressures at any given point in a static fluid must be the same in all directions.For a fluid at rest, pressure is the s

13、ame at all points with the same elevation. 華南理工大學 化工學院 伍欽 , TelHead of a Fluid Pressures are given in many different sets of units. A common method of expressing pressures is in terms of head of a particular fluid. The head h of a fluid is related to pressure P()Ph headmg華南理工大學 化工學院 伍欽

14、, Tel:135387043151.1.3 Applications of fluid statics華南理工大學 化工學院 伍欽 , Tel:135387043151.Barometric equation For an ideal gas the density and pressure are related by equation RTpM0dZRTgMpdpsubstitution the density from this equation to the differential equation of hydrostatics gives華南理工大學 化工學院 伍欽 , Tel

15、ntegration of the equation between levels a and b, on the assumption that T is constant, givesababZZRTgMpplnMethods are available for estimating the pressure distribution in situation, for example, a deep gas well, in which the temperature is no constant.華南理工大學 化工學院 伍欽 , Tel/p>

16、lit is often important to measure and control the pressure in a vessel or process 華南理工大學 化工學院 伍欽 , Tel南理工大學 化工學院 伍欽 , Tel tube manometerThe manometer is an important device for measuring pressure differences. 華南理工大學 化工學院 伍欽 , Teligure shows the simplest form of

17、 manometer.papbRm132BAzm4A pressure pa is exerted in one arm of U tubeand a pressure pb in the other.華南理工大學 化工學院 伍欽 , Tels a result of the difference in pressure, the meniscus in one branch of the tube is higher than in the other.Vertical distance between the two meniscuses Rm may be us

18、ed to measure the difference in pressure.華南理工大學 化工學院 伍欽 , Telhe pressure at the point 1 isBmmaRzgpp)(1lThe pressure at the point 2 isAmBmbgRgzpp2p1 is equal to p2 for the continuous fluid at the same level, thusAmBmbBmmagRgzpRzgp)(華南理工大學 化工學院 伍欽 , Telimplification of this e

19、quation gives）（BAmbagRpp(1.1-9 )Note that this relationship is independent of the distance zm, and of the dimensions of the tube, provided that pressure pa and pb are measured in the same horizontal plane.If fluid B is a gas, B is usually negligible compared to A and may be omitted from Eq. (1.1-9)華

20、南理工大學 化工學院 伍欽 , Tel:135387043153. Two-fluid U tube A two-fluid U tube is shown, which is a sensitive device for measuring small pressure differences. Let A be the cross-sectional area of each of the large reservoirs and a be the cross-sectional area of each of the tubes forming the U. a/A is made su

21、fficiently small as to be negligible ()abABPPRg華南理工大學 化工學院 伍欽 , Tel4. Inclined manometerFigure1.4 Inclined manometeraRmR1papbpressurepressure華南理工大學 化工學院 伍欽 , TelBy making small, the magnitude of Rm is multiplied into a long distance R1, and large reading becomes equivalent

22、to a small pressure differencesin1RgppBAba華南理工大學 化工學院 伍欽 , Tel:135387043155. Continuous gravity separator Decanter is used for the separation of two immiscible liquids A and B . The feed mixture enters at one end of the separator vessel, and the liquids flow slowly to the other end and separate into

23、 two distinct layers. Each liquid flows through a separate overflow line. 華南理工大學 化工學院 伍欽 , TelThe depth of the layer A is hA1 and that of B is hB. The total depth hT = hA1 + hB and is fixed by position of the overflow line. The A discharges through an overflow leg hA2 above the vessel b

24、ottom. The vessel and the overflow lines are vented to the atmosphere. A hydrostatic balance givesghghghAAAABB21Substituting hB = hT - hA1 into above and solving for hA1 華南理工大學 化工學院 伍欽 , Telroblem 11. The shear stress depends upon the ( ) and ( )2. Pressure is a ( ) quantity; at any giv

25、en point its magnitude is ( ) in all directions.A) vector , B) scalar, C) different , D) the same華南理工大學 化工學院 伍欽 , Tel:135387043153. Shown in Figure, the pressure at point 1 is 1.5 atm abs, Pa is 101.33103Pa. If the pressure Pa at the surface of liquid increases by 1 times, the pressure at this point

26、 increases by ( ), and is ( )times original pressure.Pa4. Incompressible fluid means that ( );compressible fluid means that( )華南理工大學 化工學院 伍欽 , TelA manometer of the type shown in Fig1.33. is used to measure the pressure drop across the orifice. Liquid A is mercury and fluid B, flowing t

27、hrough the orifice and filling the manometer leads, is brine (density 1260 kg/m3). When the pressure at the taps are equal, the level of the mercury in the manometer is 0.9m below the orifice taps. Under operating condition, the gauge pressure at upstream tap is 0.14bar, the pressure at the downstream tap 250mmHg below atmospheric. What is the reading of the manometer in millimeters?Problem 2華南理工大學 化工學院 伍欽 , Telolution: 0.14bar=14000Pa華南理工大學 化工學院 伍欽 , Tel:1353870