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1、2023/3/22,1,东北石油大学,化 学 化工 学 院,版权所有：大庆石油学院化学化工学院 Copyright 2004 Daqing Petroleum Institute。All rights reserved。,电子教案,化工流体流动与传热,2023/3/22,2,主讲：余兰兰,化工流体流动与传热(第二章),2023/3/22,3,New Words And Expressions,fluidshear stressesincompressiblecompressiblesolid boundariespotential flowirrotational flowfrictionme
2、chanical energyboundary layervelocity-gradient shear-stress fields,流体剪应力不可压缩的可压缩的固体壁面势流无旋流摩擦机械能边界层速度梯度剪应力场,2023/3/22,4,turbulencevelocity fieldsteadyone-dimensional flowvectorcomponents scalareddieslaminar flowrheological behavior of fluidsNewtonian fluidsnon-Newtonian fluids,湍流速度场稳定的一维流动矢量分量标量漩涡 层流
3、流体流变性质牛顿型流体非牛顿型流体,2023/3/22,5,threshold shear stressBingham plasticspseudoplastic fluiddilatant fluidshear-rate-thinningshear-rate-thickeningtime-dependent flowthixotropic liquidsrheopectic substancesviscositycritical velocityturbulent flow,剪切应力的临界值宾哈姆塑性流体假塑性流体涨塑性流体剪切速率递减剪切速率递增时变流摇溶性液体震凝性物质粘度临界速度湍流,
4、2023/3/22,6,the diameter of the tubedensityaverage linear velocitygroupReynolds numbervelocity fluctuationsqualitative and quantitativewall turbulencefree turbulencediameterpotential energyboundary layersthe bulk fluid velocitythe viscous sublayer,管径密度平均线速率群组雷诺准数脉动速度定性和定量壁面湍流自由湍流直径势能边界层主流体速度粘滞底层,202
5、3/3/22,7,缓冲层湍流层过渡湍流强度过渡长度二维的速度梯度动量 漩涡尾流剪应力压力损失边界层分离,the buffer layerthe turbulent zonetransitionintensity of turbulencetransition lengthtwo-dimensionalvelocity gradientsmomentumvorticeswakeshear stressespressure loss inboundary-layer separation,2023/3/22,8,the mass-balancecontinuity equationsthe lin
6、ear-and angular-momentum-balance the mechanical-energy balance vectortensorstreamlines,质量守恒连续性方程式线性动量守恒和角动量守恒机械能守恒矢量张量流线,2023/3/22,9,Chapter 2 Fluid Flow,2.1 Fluid Statics and Its Applications,2023/3/22,10,Nature of Fluids,An attempt to change the shape of a mass of fluid results in layers of fluid
7、sliding over one another until a new shape is attained,流体是不能永久抵抗形变的物质。,若想改变一定质量流体的形状，将导致流体中层与层之间相互滑动直至获得新的形状。,A fluid is a substance that does not permanently resist distortion,2023/3/22,11,During the change in shape，shear stresses exist，the magnitudes of which depend upon the viscosity of the fluid
8、 and the rate of sliding，but when a final shape has been reached，all shear stresses will have disappeared,在变形时，存在剪应力，其大小取决于流体的黏度和滑动的速率，当最终形状确定后，剪应力消失。,2023/3/22,12,处于平衡的流体与剪应力无关。,A fluid in equilibrium is free from shear stresses,在给定的温度和压力下，流体的密度是常数，在工程上通常以磅/立方英尺或公斤/立方米来测量。,At a given temperature an
9、d pressure，a fluid possesses a definite density，which in engineering practice is usually measured in pounds per cubic foot or kilograms per cubic meter,2023/3/22,13,If the density is but little affected by moderate changes in temperature and pressure，the fluid is said to be incompressible，and if the
10、 density is sensitive to changes in these variables，the fluid is said to be compressible,如果温度和压力轻度改变时对密度的影响很小，将该流体称为不可压缩的；若密度受这些变量的影响很大，则称该流体为可压缩的。,2023/3/22,14,2.2 Fluid-Flow Phenomena,流动流体的行为在很大程度上取决于流体是否受固体边界的影响。,The behavior of a flowing fluid depends strongly on whether or not the fluid is unde
11、r the influence of solid boundaries.,1.Potential flow,2.2.1 Fluid flow,2023/3/22,15,A moving fluid uninfluenced by stationary solid walls is not subject to shear,and shear stresses do not exist within it.,不受固定固体壁面影响的流动流体就不受剪切力作用，且其内部不存在剪切应力。,2023/3/22,16,The flow of incompressible fluids with no she
12、ar is called potential flow and is completely described by the principles of Newtonian mechanics and conservation of mass.,无剪切力的不可压缩流体的流动称为“势流”，完全由牛顿力学和质量守恒原理来描述。,The mathematical theory of potential flow is highly developed but is outside the scope of this book势流的数学理论已被深入研究，但不属于本书的范围。,2023/3/22,17,
13、Potential flow has two important characteristics:,(1)neither circulations nor eddies can form within the stream,so that potential flow is also called irrotational flow,and,(2)friction cannot develop,so that there is no dissipation of mechanical energy into heat.,势流有两个重要的特征：,（2）没有摩擦，因此没有机械能损耗变热的情况。,（
14、1）在流动流体内不能生成漩涡，因此势流也称为无旋流,2023/3/22,18,A fundamental principle of fluid mechanics,originally stated by Prandtl in 1904,is that except for fluids moving at low velocities or possessing high viscosities,the effect of the solid boundary on the flow is confined to a layer of the fluid immediately adjace
15、nt to the solid wall.,Prandtl 在1904年最先提出的流体力学基本原理中说，除低速流动或黏度很大的流体外，固体边界对流动的影响仅限于紧挨固体壁面的流体层内。,2.Boundary layer,2023/3/22,19,此流体层称为边界层，剪切力仅存在于这部分流体中。在边界层之外，则是势流。,This layer is called the boundary layer,and shear forces are confined to this part of the fluid.Outside the boundary layer,potential flow su
16、rvives.,Most technical flow processes are best studied by considering the fluid stream as two parts,the boundary layer and the remaining fluid.,对于大多数工艺流动过程，是将流体看作两部分边界层及其以外的流体，进行了深入的研究。,2023/3/22,20,In some situations,the boundary layer may be neglected,and in others,such as flow through pipes,the b
17、oundary layer fills the entire channel,and there is no potential flow.,在某些情况下可以忽略边界层，而在另外一些时候边界层充满整个通道（像通过管道的流动），就没有势流。,Within the current of an incompressible fluid under the influence of solid boundaries,four important effects appear:,在受固体边界影响的不可压缩流体的流动中，有四个重要的影响因素：,2023/3/22,21,(1)the coupling of
18、 velocity-gradient and shear-stress fields,（1）速度梯度与剪应力场的关系；,（2）湍流的开始；,（3）边界层的生成与发展；,（4）边界层的分离。,(2)the onset of turbulence,(3)the formation and growth of boundary layers,and,(4)the separation of boundary layers from contact with the solid boundary.,2023/3/22,22,3.The Velocity Field,When a stream of f
19、luid is flowing in bulk past a solid wall，the fluid adheres to the solid at the actual interface between solid and fluid,当流体大量流过固体壁面时，与固体紧挨着的流体粘附在固体表面上。,2023/3/22,23,The adhesion is a result of the force fields at the boundary，which are also responsible for the interfacial tension between solid and
20、fluid,这个粘附是力场在边界作用的结果，也与固体和流体间的界面拉力有关。,If，therefore，the wall is at rest in the reference frame chosen for the solid-fluid system，the velocity of the fluid at the interface is zero,因此，若认为固体壁面相对于流体是静止的，则在界面上流体的速度为零。,2023/3/22,24,Since at distances away from the solid the velocity is finite，there must
21、be variations in velocity from point to point in the flowing stream,因为距固体壁面一定距离处的速度是一定值，所以在流体中的速度逐点变化。,Therefore，the velocity at any point is a function of the space coordinates of that point，and a velocity field exists in the space occupied by the fluid,因此，任一点的流速是该点空间坐标的函数，且在流体中存在速度场。,2023/3/22,25,
22、4.Laminar Flow,At low velocities fluids tend to flow without lateral mixing，and adjacent layers slide past one another like playing cards,流体在低速流动时，没有横向混合，相邻层间就像扑克牌那样互相滑过。,2023/3/22,26,There are neither cross-currents nor eddies既没有横向流也没有漩涡。This regime is called laminar flow这种情况称为层流。,At higher velocit
23、ies turbulence appears，and eddies form，which，as discussed later，lead to lateral mixing,在较高流速下，出现湍流，形成引起横向混合的漩涡，将在后面讨论。,2023/3/22,27,2.2.2 Newtonian and non-Newtonian Fluids,The fact that at each point in a flowing fluid both a rate of shear and a shear stress exist suggests that these quantities may
24、 be related.,在流动流体中任一点上同时存在速度梯度和剪切应力这个事实提醒我们这些量之间可能是有联系的。,1.Newtonian and non-Newtonian Fluids,2023/3/22,28,In fact they are,the science of rheology deals with their coupling and the complex relations between them.,事实上的确如此，流变学论述它们之间的联系以及复杂关系。,Figure 2-1 shows several examples of the rheological beha
25、vior of fluids.图2-1示出流体流变性质的几个例子。,2023/3/22,29,Figure 2-1 Shear stress versus velocity gradient for Newtonian and non-Newtonian fluids,2023/3/22,30,The curves are plots of shear stress vs.rate of shear and apply at constant temperature and pressure.这些曲线是剪切应力对速度梯度作图的结果，适用于恒温恒压条件。,The simplest behavio
26、r is that shown by curve A,which is a straight line passing through the origin.最简单的特性如曲线A所示，这是一条过原点的直线,2023/3/22,31,Fluids following this simple linearity are called Newtonian fluids.,Gases,true solutions,and noncolloidal liquids are Newtonian.,气体、实际溶液以及非胶体液体都是牛顿型的。,遵循这个简单直线的流体称为牛顿型流体。,2023/3/22,32,
27、图2-1中其他曲线表示非牛顿型流体的流变性质。,The other curves shown in Fig.2-1 represent the rheological behavior of liquids called non-Newtonian.,Some liquids,e.g.,sewage sludge,do not flow at all until a threshold shear stress,denoted by,is attained and then flow linearly at shear stresses greater than.,某些液体，如污泥浆，在其剪切
28、应力达到临界值（记作0）以前根本不流动，超过此值，流动中剪切应力则从0起线性增长。,2023/3/22,33,Curve B is an example of this relation.Liquids acting this way are called Bingham plastics.曲线B是这个关系的例子。这类液体称为宾哈姆塑性流体。,Line C represents a pseudoplastic fluid.C线表示假塑性（或拟塑性）流体。The curve passes through the origin,is concave downward at low shears,a
29、nd becomes linear at high shears.该线通过原点，在低剪切力时凹边向下，高剪切力时则成直线。,2023/3/22,34,Rubber latex is an example of such a fluid.乳胶就是这样一种流体。Curve D represents a dilatant fluid.The curve is concave upward at low shears and becomes linear at high shears.曲线D代表涨塑性流体，在低剪切力时凹边向上，高剪切力时也成直线。,2023/3/22,35,Quicksand and
30、 some sand-filled emulsions show this behavior.流沙以及某些充满沙子的乳浊液表现出这种性质。,Pseudo plastics are said to be shear-rate-thinning and dilatant fluids shear-rate-thickening.假塑性流体称为剪切速率递减型流体，涨塑性流体称为剪切速率递增型流体。,2023/3/22,36,2.Viscosity,In a Newtonian fluid the shear rate obviously is proportional to the shear st
31、ress.,在牛顿型流体中，剪切速率显然是与剪切应力成正比的。,2023/3/22,37,The proportionality constant is called the viscosity and is defined by the equation,比例常数称为黏度，其定义由方程（2-1）给出：,（2-1）,2023/3/22,38,3.Turbulence,The distinction between laminar and turbulent flow was first demonstrated in a classic experiment by Osborne Reynol
32、ds,reported in 1883.,层流与湍流的区别由Osborne Reynolds在1883年首先从试验中发现。,The equipment used by Reynolds is shown in Fig.2-2.Reynolds所用的设备示于图2-2。,(1)Turbulence,2023/3/22,39,Figure 2-2 Reynolds experiment,2023/3/22,40,Reynolds found that,at low flow rates,the jet of colored water flowed intact along with the mai
33、nstream and no cross mixing occurred.,Reynolds发现，在低流速下，染色水流与主流体一起流动，不受任何影响，不发生横向混合。,The behavior of the color band showed clearly that the water was flowing in parallel straight lines and that the flow was laminar.,染色水的特征表明水是以平行直线的方式流动，即层流。,2023/3/22,41,When the flow rate was increased,a velocity,ca
34、lled the critical velocity was reached at which the thread of color disappeared and the color diffused uniformly throughout the entire cross section of the stream of water.,当流速增加，达到一个临界速度时，染色水流消失，而颜色在整个水流中均匀地扩散。,2023/3/22,42,This behavior of the colored water showed that the water no longer flowed i
35、n laminar motion but moved erratically in the form of crosscurrents and eddies.,染色水流的这个特征表明水不再是层流流动，而是以涡流和漩涡的形式不稳定地移动。,This type of motion is turbulent flow.,这个运动类型称为湍流。,2023/3/22,43,(2)Reynolds Number and Transition from Laminar to Turbulent Flow,2023/3/22,44,Reynolds studied the conditions under w
36、hich one type of flow changes into the other and found that the critical velocity,at which laminar flow changes into turbulent flow,depends on four quantities:,Reynolds研究了流动类型转变的条件，并发现了临界速度，在此值下层流变为湍流。,该值与4个量有关：管径、液体的黏度、密度和平均线速度。,the diameter of the tube,and the viscosity,density,and average linear
37、velocity of the liquid.,2023/3/22,45,Furthermore,he found that these four factors can be combined into one group and that the change in kind of flow occurs at a definite value of the group.,进而，他又发现这4个因素可被组成一个数群，该数群在某一定值时，流动的类型发生变化。,The grouping of variables so found was这些变量的组合形式为,2023/3/22,46,（2-2）,
38、where d=diameter of tube u=average velocity of liquid=density of liquid=viscosity of liquid,2023/3/22,47,The dimensionless group of variables defined by Eq.(2-2)is called the Reynolds number，Re.由方程（2-2）定义的这些变量的无因次数群称为雷诺准数。It is one of the named dimensionless groups listed in Appendix 5.它是附录5中所列的无因次数
39、群之一。,2023/3/22,48,Its magnitude is independent of the units used,provided the units are consistent.它的值与所使用的单位制无关，只要单位一致即可。,Additional observations have shown that the transition from laminar to turbulent flow actually may occur over a wide range of Reynolds numbers.,另外的观察表明，由层流到湍流的过渡可以发生在一个较大的雷诺数范围内
40、。,2023/3/22,49,Laminar flow is always encountered at Reynolds numbers below 2,100,but it can persist up to Reynolds numbers of several thousand under special conditions of well-rounded tube entrance and very quiet liquid in the tank.,雷诺数低于2100时，总是层流流动，但在管口很圆，箱内液体很平静的特殊条件下，在雷诺数达到几千时仍可保持层流。,2023/3/22,
41、50,Under ordinary conditions of flow,the flow is turbulent at Reynolds numbers above about 4,000.,在通常的流动条件下，雷诺数大于4000时就是湍流流动。,Between 2,100 and 4,000 a transition,or dip,region is found,where the type of flow may be either laminar or turbulent,depending upon conditions at the entrance of the tube an
42、d on the distance from the entrance.,在2100和4000之间是过渡区，流动类型不是层流就是湍流，取决于管口的条件以及到管口的距离。,2023/3/22,51,Reynolds experiments were conducted with water,but they have long since been shown to apply to other liquids and also to gases.,雷诺实验中用的是水，但也适用于其他液体和气体。,2023/3/22,52,(3)Nature of Turbulence,Because of it
43、s importance in many branches of engineering,turbulent flow has been extensively investigated in recent years,and a large literature has accumulated on this subject.,由于湍流在许多工程学科中的重要性，近些年来人们已对其进行了广泛的研究，发表了许多文章。,2023/3/22,53,The first kind of turbulence is called wall turbulence and the second kind fr
44、ee turbulence.,Wall turbulence appears when the fluid flows through closed or open channels or past solid shapes immersed in the stream.,前一种湍流称为壁面湍流，后一种称为自由湍流。,流体在封闭或敞开的流道中流动时，或流过浸没在流体中的固体时出现壁面湍流。,()Types of Turbulence,2023/3/22,54,Free turbulence appears in the flow of a jet into a mass of stagnant
45、 fluid or when a boundary layer separates from a solid wall and flows through the bulk of the fluid.在一射流流入大量停滞流体内，或一边界层从固体壁面分离流入流体主流中时，发生自由湍流。,2023/3/22,55,Large eddies are continually formed.They break down into smaller eddies,which in turn evolve still smaller ones.Finally,the smallest eddies disa
46、ppear.大漩涡不断形成，破碎成较小的漩涡，又依次变成更小的。最终，最小的漩涡消失,Turbulent flow consists of a mass of eddies of various sizes coexisting in the flowing stream.共存于流动流体中的大量大大小小的漩涡构成了湍流流动。,()Eddies in Turbulence,2023/3/22,56,Any given eddy possesses a definite amount of mechanical energy,much like that of a small spinning t
47、op.,任何指定的漩涡都具有一定量的机械能，很像旋转的小陀螺。,The energy of the largest eddies is supplied by the potential energy of the bulk flow of the fluid.,最大漩涡的能量来自流体主流中的势能。,()Energy in Turbulence,2023/3/22,57,2.2.3 Flow in Boundary Layers,A boundary layer is defined as that part of a moving fluid in which the fluid motio
48、n is influenced by the presence of a solid boundary.,所谓边界层，就是流动流体中受固体边界影响的部分。,1.Boundary layer,2023/3/22,58,图2-3中所示为平行于一薄板流动的流体形成边界层的示例。,As a specific example of boundary-layer formation consider the flow of fluid parallel with a thin plate,as shown in Fig.2-3.,2023/3/22,59,Figure 2-3 Prandtl bounda
49、ry layer,2023/3/22,60,In Fig.2-3 the dotted line OL is so drawn that the velocity changes are confined between this line and the trace of the wall.,在图2-3中的虚线OL表明速度仅在该线与壁面之间变化。,Because the velocity lines are asymptotic with respect to distance from the plate,it is assumed,in order to locate the dotte
50、d line definitely,that the line passes through all points where the velocity is 99 percent of the bulk fluid velocity u.,由于速度曲线相对遇到平板的距离呈渐近线变化，为了明确虚线的位置，假定该线上各点的速度均等于主流体速度u 的99%。,2023/3/22,61,Line OL represents an imaginary surface which separates the fluid stream into two parts:,one in which the fl
