ASHRAE FUNDAMENTALS SI CH 3-2017 Fluid Flow.pdf

1、3.1CHAPTER 3FLUID FLOWFluid Properties 3.1Basic Relations of Fluid Dynamics . 3.2Basic Flow Processes 3.3Flow Analysis 3.6Noise in Fluid Flow. 3.14Symbols . 3.14LOWING fluids in HVAC gasesmay range from compressible to nearly incompressible. Liquidshave unbalanced molecular cohesive forces at or nea

2、r the surface(interface), so the liquid surface tends to contract and has propertiessimilar to a stretched elastic membrane. A liquid surface, therefore,is under tension (surface tension).Fluid motion can be described by several simplified models. Thesimplest is the ideal-fluid model, which assumes

3、that the fluid hasno resistance to shearing. Ideal fluid flow analysis is well developede.g., Schlichting (1979), and may be valid for a wide range ofapplications.Viscosity is a measure of a fluids resistance to shear. Viscouseffects are taken into account by categorizing a fluid as either New-tonia

4、n or non-Newtonian. In Newtonian fluids, the rate of deforma-tion is directly proportional to the shearing stress; most fluids in theHVAC industry (e.g., water, air, most refrigerants) can be treated asNewtonian. In non-Newtonian fluids, the relationship between therate of deformation and shear stre

5、ss is more complicated.DensityThe density of a fluid is its mass per unit volume. The densitiesof air and water (Fox et al. 2004) at standard indoor conditions of20C and 101.325 kPa (sea-level atmospheric pressure) arewater= 998 kg/m3air= 1.21 kg/m3ViscosityViscosity is the resistance of adjacent fl

6、uid layers to shear. A clas-sic example of shear is shown in Figure 1, where a fluid is betweentwo parallel plates, each of area A separated by distance Y. The bot-tom plate is fixed and the top plate is moving, which induces a shear-ing force in the fluid. For a Newtonian fluid, the tangential forc

7、e Fper unit area required to slide one plate with velocity V parallel to theother is proportional to V/Y:F /A = (V /Y) (1)where the proportionality factor is the absolute or dynamic vis-cosity of the fluid. The ratio of F to A is the shearing stress , andV /Y is the lateral velocity gradient (Figure

8、 1A). In complex flows,velocity and shear stress may vary across the flow field; this isexpressed by(2)The velocity gradient associated with viscous shear for a simple caseinvolving flow velocity in the x direction but of varying magnitude inthe y direction is shown in Figure 1B.Absolute viscosity d

9、epends primarily on temperature. For gases(except near the critical point), viscosity increases with the squareroot of the absolute temperature, as predicted by the kinetic theory ofgases. In contrast, a liquids viscosity decreases as temperatureincreases. Absolute viscosities of various fluids are

10、given in Chapter33.Absolute viscosity has dimensions of force time/length2. Atstandard indoor conditions, the absolute viscosities of water and dryair (Fox et al. 2004) arewater= 1.01 (mNs)/m2air= 18.1 (Ns)/m2Another common unit of viscosity is the centipoise 1 centipoise =1 g/(sm) = 1 mPas. At stan

11、dard conditions, water has a viscosityclose to 1.0 centipoise.In fluid dynamics, kinematic viscosity is sometimes used inlieu of absolute or dynamic viscosity. Kinematic viscosity is the ratioof absolute viscosity to density: = /The preparation of this chapter is assigned to TC 1.3, Heat Transfer an

12、dFluid Flow.Fig. 1 Velocity Profiles and Gradients in Shear Flowsdvdy-=3.2 2017 ASHRAE HandbookFundamentals (SI)At standard indoor conditions, the kinematic viscosities of waterand dry air (Fox et al. 2004) arewater= 1.01 mm2/sair= 15.0 mm2/sThe stoke (1 cm2/s) and centistoke (1 mm2/s) are common un

13、itsfor kinematic viscosity.2. BASIC RELATIONS OF FLUID DYNAMICSThis section discusses fundamental principles of fluid flow forconstant-property, homogeneous, incompressible fluids and intro-duces fluid dynamic considerations used in most analyses.Continuity in a Pipe or DuctConservation of mass appl

14、ied to fluid flow in a conduit requiresthat mass not be created or destroyed. Specifically, the mass flowrate into a section of pipe must equal the mass flow rate out of thatsection of pipe if no mass is accumulated or lost (e.g., from leak-age). This requires thatdA = constant (3)where is mass flow

15、 rate across the area normal to flow, v is fluidvelocity normal to differential area dA, and is fluid density. Both and v may vary over the cross section A of the conduit. When flowis effectively incompressible ( = constant) in a pipe or duct flowanalysis, the average velocity is then V = (1/A)vdA,

16、and the massflow rate can be written as= VA (4)orQ = = AV (5)where Q is volumetric flow rate.Bernoulli Equation and Pressure Variation in Flow DirectionThe Bernoulli equation is a fundamental principle of fluid flowanalysis. It involves the conservation of momentum and energyalong a streamline; it i

17、s not generally applicable across streamlines.Development is fairly straightforward. The first law of thermody-namics can apply to both mechanical flow energies (kinetic andpotential energy) and thermal energies.The change in energy content E per unit mass of flowing fluidis a result of the work per

18、 unit mass w done on the system plus theheat per unit mass q absorbed or rejected:E = w + q (6)Fluid energy is composed of kinetic, potential (because of elevationz), and internal (u) energies. Per unit mass of fluid, the energychange relation between two sections of the system is = EM + q (7)where

19、the work terms are (1) external work EMfrom a fluidmachine (EMis positive for a pump or blower) and (2) flow workp/ (where p = pressure), and g is the gravitational constant.Rearranging, the energy equation can be written as the generalizedBernoulli equation: = EM+ q (8)The expression in parentheses

20、 in Equation (8) is the sum of thekinetic energy, potential energy, internal energy, and flow work perunit mass flow rate. In cases with no work interaction, no heat trans-fer, and no viscous frictional forces that convert mechanical energyinto internal energy, this expression is constant and is kno

21、wn as theBernoulli constant B:+ gz + = B (9)Alternative forms of this relation are obtained through multiplica-tion by or division by g:p + + gz = B (10)(11)where = g is the weight density ( = weight/volume versus =mass/volume). Note that Equations (9) to (11) assume no frictionallosses.The units in

22、 the first form of the Bernoulli equation Equation(9) are energy per unit mass; in Equation (10), energy per unit vol-ume; in Equation (11), energy per unit weight, usually called head.Note that the units for head reduce to just length i.e., (Nm)/Nto m. In gas flow analysis, Equation (10) is often u

23、sed, and gz isnegligible. Equation (10) should be used when density variationsoccur. For liquid flows, Equation (11) is commonly used. Identicalresults are obtained with the three forms if the units are consistentand fluids are homogeneous.Many systems of pipes, ducts, pumps, and blowers can be con-

24、sidered as one-dimensional flow along a streamline (i.e., variationin velocity across the pipe or duct is ignored, and local velocity v =average velocity V ). When v varies significantly across the crosssection, the kinetic energy term in the Bernoulli constant B isexpressed as V2/2, where the kinet

25、ic energy factor ( 1)expresses the ratio of the true kinetic energy of the velocity profileto that of the average velocity. For laminar flow in a wide rectangu-lar channel, = 1.54, and in a pipe, = 2.0. For turbulent flow in aduct, 1.Heat transfer q may often be ignored. Conversion of mechanicalener

26、gy to internal energy u may be expressed as a loss EL. Thechange in the Bernoulli constant (B = B2 B1) between stations 1and 2 along the conduit can be expressed as+ EM EL= (12)or, by dividing by g, in the form+ HM HL= (13)Note that Equation (12) has units of energy per mass, whereaseach term in Equ

27、ation (13) has units of energy per weight, or head.The terms EMand ELare defined as positive, where gHM= EMrepresents energy added to the conduit flow by pumps or blowers. Aturbine or fluid motor thus has a negative HMor EM. Note thesimplicity of Equation (13); the total head at station 1 (pressure

28、headplus velocity head plus elevation head) plus the head added by amv =mmmv22- gz u+ p-v22- gz up-+v22-p-v22-p-v22g- z+Bg-=p- V22- gz+1p- V22- gz+2p- V22g- z+1p- V22g- z+2Fluid Flow 3.3pump (HM) minus the head lost through friction (HL) is the totalhead at station 2.Laminar FlowWhen real-fluid effe

29、cts of viscosity or turbulence are included,the continuity relation in Equation (5) is not changed, but V must beevaluated from the integral of the velocity profile, using local veloc-ities. In fluid flow past fixed boundaries, velocity at the boundary iszero, velocity gradients exist, and shear str

30、esses are produced. Theequations of motion then become complex, and exact solutions aredifficult to find except in simple cases for laminar flow between flatplates, between rotating cylinders, or within a pipe or tube.For steady, fully developed laminar flow between two parallelplates (Figure 2), sh

31、ear stress varies linearly with distance y fromthe centerline (transverse to the flow; y = 0 in the center of the chan-nel). For a wide rectangular channel 2b tall, can be written as = w= (14)where wis wall shear stress b(dp/ds), and s is flow direction.Because velocity is zero at the wall ( y = b),

32、 Equation (14) can beintegrated to yieldv = (15)The resulting parabolic velocity profile in a wide rectangularchannel is commonly called Poiseuille flow. Maximum velocityoccurs at the centerline (y = 0), and the average velocity V is 2/3 ofthe maximum velocity. From this, the longitudinal pressure d

33、rop interms of V can be written as(16)A parabolic velocity profile can also be derived for a pipe ofradius R. V is 1/2 of the maximum velocity, and the pressure dropcan be written as(17)TurbulenceFluid flows are generally turbulent, involving random perturba-tions or fluctuations of the flow (veloci

34、ty and pressure), character-ized by an extensive hierarchy of scales or frequencies (Robertson1963). Flow disturbances that are not chaotic but have some degreeof periodicity (e.g., the oscillating vortex trail behind bodies) havebeen erroneously identified as turbulence. Only flows involving ran-do

35、m perturbations without any order or periodicity are turbulent;velocity in such a flow varies with time or locale of measurement(Figure 3).Turbulence can be quantified statistically. The velocity mostoften used is the time-averaged velocity. The strength of turbulenceis characterized by the root mea

36、n square (RMS) of the instantaneousvariation in velocity about this mean. Turbulence causes the fluid totransfer momentum, heat, and mass very rapidly across the flow.Laminar and turbulent flows can be differentiated using theReynolds number Re, which is a dimensionless relative ratio ofinertial for

37、ces to viscous forces:ReL= VL/ (18)where L is the characteristic length scale and is the kinematic vis-cosity of the fluid. In flow through pipes, tubes, and ducts, the char-acteristic length scale is the hydraulic diameter Dh, given byDh= 4A/Pw(19)where A is the cross-sectional area of the pipe, du

38、ct, or tube, and Pwis the wetted perimeter.For a round pipe, Dhequals the pipe diameter. In general, laminarflow in pipes or ducts exists when the Reynolds number (based onDh) is less than 2300. Fully turbulent flow exists when ReDh10 000. For 2300 3 105Sphere 0.36 to 0.47 0.1Disk 1.12 1.12Streamlin

39、ed strut 0.1 to 0.3 0), flow decays exponentially as e.Turbulent flow analysis of Equation (42) also must be based onthe quasi-steady approximation, with less justification. Daily et al.(1956) indicate that frictional resistance is slightly greater than theCd1 4-ReFig. 19 Flowmeter Coefficientsd24-R

40、2D- 2 ghd24-2 P1P2 1 4-dVd-1-dpds-fV22D-+dVd-pL-fV22D-=dVd-pL-32VD2-=dVdABV-1B-=pL-D232- 1Lp-32D2-exppL-D232- pL-R28- =1Lp-fV2D-exp64VD-3.12 2017 ASHRAE HandbookFundamentals (SI)steady-state result for accelerating flows, but appreciably less fordecelerating flows. If the friction factor is approxim

41、ated as constant,= A BV2(50)and for the accelerating flow, = (51)orV = (52)Because the hyperbolic tangent is zero when the independentvariable is zero and unity when the variable is infinity, the initial(V = 0 at = 0) and final conditions are verified. Thus, for long times(),V= (53)which is in accor

42、d with Equation (30) when f is constant (the flowregime is the fully rough one of Figure 13). The temporal velocityvariation is thenV = Vtanh ( fV/2D) (54)In Figure 20, the turbulent velocity start-up result is compared withthe laminar one, where initially the turbulent is steeper but of thesame gen

43、eral form, increasing rapidly at the start but reaching Vasymptotically.CompressibilityAll fluids are compressible to some degree; their density dependssomewhat on the pressure. Steady liquid flow may ordinarily betreated as incompressible, and incompressible flow analysis issatisfactory for gases a

44、nd vapors at velocities below about 20 to40 m/s, except in long conduits.For liquids in pipelines, a severe pressure surge or water hammermay be produced if flow is suddenly stopped. This pressure surgetravels along the pipe at the speed of sound in the liquid, alternatelycompressing and decompressi

45、ng the liquid. For steady gas flows inlong conduits, the pressure drop along the conduit can reduce gasdensity enough to increase the velocity. If the conduit is longenough, the velocity may reach the speed of sound, and the Machnumber (ratio of flow velocity to the speed of sound) must be con-sider

46、ed.Some compressible flows occur without heat gain or loss (adia-batically). If there is no friction (conversion of flow mechanicalenergy into internal energy), the process is reversible (isentropic)and follows the relationshipp/k= constantk = cp/cvwhere k, the ratio of specific heats at constant pr

47、essure and volume,is 1.4 for air and diatomic gases.When the elevation term gz is neglected, as it is in most com-pressible flow analyses, the Bernoulli equation of steady flow,Equation (21), becomes= constant (55)For a frictionless adiabatic process,(56)Integrating between upstream station 1 and downstream station 2gives= 0 (57)Equation (57) replaces the Bernoulli equation for compressibleflows. If station 2 is the stagnation point at the front of a body, V2=0, and solving Equation (57) for p2 givesps= p2= p1(58)where ps is the stagnation pressure.Because the speed of sound of the gas is