NASA-TN-D-8151-1976 Fundamentals of fluid sealing《液体密封原理》.pdf

1、NASA0IZI,-Z,TECHNICAL NOTE NASATN D-8151FUNDAMENTALS OF FLUID SEALINGJohnLewis:_Cleveland,ZukResearch CenterOhio 44135NATIONAL AERONAUTICS AND_SPACEADMINISTRATIONq cqo_O4,%_. mz;, 6 _1cj-r_WASHINGTON, D. C. MARCH 1976Provided by IHSNot for ResaleNo reproduction or networking permitted without licens

2、e from IHS-,-,-Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-1. Report No, 2. Government Accession No.NASA TN D-81514. Title and SubtitleFUNDAMENTALS OF FLUID SEALING7. Author(s)John Zuk9. Performing Organization Name and AddressLewis Research Cent

3、erNational Aeronautics and Space AdministrationCleveland, Ohio 4413512, Sponsoring Agency Name and AddressNational Aeronautics and Space AdministrationWashington, D.C. 205463. Recipients Catalog No.5. Report DateMarch 19766, Performir_g Organization Code8. Performing Organization Report No.E-691010.

4、 Work Unit No,505-0411. Contract or Grant No.13. Type of Report and Period CoveredTechnical Note14. Sponsoring Agency Code15. Supplementary Notes16. AbstractThe fundamentals of fluid sealing, including seal operating regimes, are discussed. The generalfluid-flow equations for fluid sealing are devel

5、oped. Seal performance parameters such asleakage and power loss are presented. Included in the discussion are the effects of geometry,surface deformations, rotation, and both laminar and turbulent flows. The concept of pressurebalancing is presented, as are differences between liquid and gas sealing

6、. Also discussed aremechanisms of seal surface separation, fundamental friction and wear concepts applicable toseals, seal materials, and pressure-velocity (PV) criteria.17. Key Words (Suggested by Author(s)Seal; Face seal; Gas-film seals: Labyrinthseals; Lubrication; Incompresm%le flow;Compressible

7、 flow; Narrow slots; Noncontact-ing seals; Fluid-film seals; Hydrodynamicseals; Hydrostatic seals; Friction; Wear;Boundary lubrication; Seal materials19. Security Classif, (of this report)Unclassified18, Distribution StatementUnclassified- unlimitedSTAR Category 3/ (rev.)20. Security Classif. (of th

8、is page) 21. No. of Pages 22. Price*Unclassified 166 $6.25* For sale by the NationalTechnical InformationService,Spring_ield,Virginia 22161Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-Provided by IHSNot for ResaleNo reproduction or networking perm

9、itted without license from IHS-,-,-CONTENTSPageSUMMARY . 1INTRODUCTION . 1SEAL OPERATING REGIMES 2GENERAL EQUATIONS OF MOTION FOR VISCOUS FLUID FLOW . 4PHYSICAL QUANTITIES OF INTEREST . 9Leakage Flow Rate . 9Pressure Distribution . 10Opening Force . . . . . i0Frictional Horsepower Requirements . 10F

10、ilm Stiffness . 11Center of Pressure . IiFilm Temperature Rise iiSEAL PRESSURE-BALANCING FUNDAMENTALS . 12LAMINAR FLOW . 14Flow Between Parallel Plates inRelative Motion . 14Flow Between Flat Converging or Diverging Plates 20Axial Flow Between Annular Cylinders . 27TURBULENT FLOW . 29Turbulent Flow

11、Between Parallel Plates . 31Turbulent Flow with Small Linear Deformation (and Constant Width) . 32Turbulent Radial Flow . 32Turbulent Flow Between Concentric and Eccentric Cylinders . 33APPLICATION OF THEORY TO FACE-SEAL CONCEPTS . . . 34COMPRESSIBLE FLOW OF GASES . 34Compressible Flow Equations . 3

12、5Classical Viscous Compressible Flow Model . 36Pressure Distribution - Application Example . 43Approximate Models of Compressible Flow 43Geometry Effect on Leakage 48Opening Force . 49Center of Pressure . 49Continuum and Noncontinuum Flow . 50oo.lUProvided by IHSNot for ResaleNo reproduction or netw

13、orking permitted without license from IHS-,-,-Flow Through Porous Media 51INVISCID FLOW EQUATIONS . 51Gravitational-Head Orifice Flow 53Nozzle Flow . 53Flow Function Approach . 55Labyrinth Seals . 56ENTRANCE FLOWS AND LOSSES 57FLOW REGIMES 60MECHANISM OF FILM PRESSURE GENERATION BETWEEN RELATIVELYMO

14、VING SURFACES 64Hydrodynamic Pressure Development . 64Squeeze-Film Concept . 68Cavitation and Degasffication 74Pressure Development due to Microroughness . 75Pressure Generation due to “Wavy“ Surfaces . 76BOUNDARY LUBRICATING REGIMES 76Fundamental Friction Concepts . 76Fundamental Wear Concepts 78SE

15、AL MATERIALS . 82CONCLUDING REMARKS . 83APPENDIXESA - SYMBOLS 85B - VISCOSITY OF FLUIDS . 90C - COMPRESS_LE FLOW - AN ALTERNATE THEORETICAL APPROACH TOFINDING THE ORIFICE EXPANSION FUNCTION 92D - DERIVATION OF REYNOLDS LUBRICATION EQUATION . 94REFERENCES . 98ivProvided by IHSNot for ResaleNo reprodu

16、ction or networking permitted without license from IHS-,-,-FUNDAMENTALSOF FLUID SEALINGby John ZukLewis Research CenterSUMMARYThe fundamentals of fluid sealing, including seal operating regimes, are discussed.The general fluid-flow equations for fluid sealing are developed. Seal performance pa-ramet

17、ers such as leakage and power loss are presented. Included in the discussion arethe effects of geometry, surface deformations, rotation, and both laminar and turbulentflows. The concept of pressure balancing is presented, as are differences betweenliquid and gas sealing. Also discussed are mechanism

18、s of seal surface separation, fun-damental friction and wear concepts applicable to seals, seal materials, and pressure-velocity (PV) criteria.INTRODUCTIONIn fluid sealing as in every branch of engineering, one must draw on the experienceand knowledge of many other disciplines and fields. The engine

19、ering of seals can involvefluid mechanics, heat transfer, lubrication theory, structural and solid mechanics,thermodynamics, chemistry, physics, metallurgy, and dynamics, as well as otherfields. Seal problems may consist of a superposition of effects which can be interre-lated. Usually each effect c

20、an be analyzed by itself. Then integrated effects must beevaluated.Fluid sealing is generally the same as any other branch of engineering except forthe importance of small-scale effects. Seals are characterized by surfaces in relativemotion separated by a very narrow gap. In order to maintain proper

21、 operation, verysmall differences in the dimensions of seal parts must be maintained. Deformations ingeometry due to imposed thermal gradients, frictional heating, pressure gradients, andmechanical and inertial forces must be held to a minimum. Actually, in some cases,deformations must be no more th

22、an microvalues.The fundamentals of fluid flow are important to understanding the various sealingdevices. This presentation discusses those fundamentals as they apply to seals. BasicProvided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-principles of incompr

23、essible (liquid) and compressible (gas) sealing flows are studied.The mechanism of film pressure generation between relatively moving surfaces is de-scribed. Fundamental friction and wear concepts, various seal lubrication operatingregimes, and surface topography effects are also presented.Generally

24、, sealing flow is a direct opposite of conventional fluid flow through pipes,ducts, and nozzles, where it is desired to have as efficient a flow process as possible(low friction). In seals it can be desirable to have an inefficient flow process becausethis results in low leakage; however, a trade-of

25、f must be made with power loss.This presentation is intended to provide the background necessary for a fundamentalunderstanding of fluid sealing in general and of specific seal types in particular. Unfor-tunately, this discussion of the theory leaves much of the subject uncovered. Deriva-tions are o

26、mitted unless they are essential to the understanding or illustration of a con-cept. A remark, perhaps, should be made concerning the accuracy of the applicableequations contained herein. When we are dealing with fluids and seal configurations forwhich test data are available, design accuracy of per

27、haps 25 percent may be realized.When we are dealing with different configurations or using fluids that deviate from idealproperties, present knowledge is rated as fair or poor and testing becomes necessary inthe design.Fundamentals of fluid sealing can also be found in a textbook by Mayer (ref. 1).

28、Asurvey report (ref. 2) should be useful. Each topic covered includes, where possible,a reference wherein more information can be found. The seal nomenclature used isthat adapted by the ASLE Seals Technical Committee (ref. 3).In all references cited, the U.S. customary system of units was used. The

29、Inter-national System of Units (SI) has been added to the text and figures for this report only.SEAL OPERATING REGIMESSeals operate in many lubrication regimes depending on the type of seal, the sealedfluid, the application, and so forth. A plot of friction coefficient against seal duty pa-rameter i

30、llustrates the various seal operating (lubricating) regimes that can exist(fig. 1).For illustrative purposes, consider a lift-off type of seal that is in rubbing contactat startup and shutdown. The way this seal may change from one lubricating regime toanother within an application is shown in figur

31、e 1 for liquid lubrication. The figureshows the variation in friction coefficient a seal undergoes from startup under a load(e. g., spring force and pressure) in the boundary lubricating regime to the steady-stateoperating speed in the full-film lubricating regime. (The mechanism for achieving full-

32、film operation could be an external pressurization source, or it could be self-generatedby hydrodynamic lubrication. At startup the parts are in solid-to-solid contact and theProvided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-seal runner (rotating membe

33、r) begins to turn under essentially dry conditions and startsto follow the pathAB. If sufficient lubricant is available, the lubricant is ordinarilydrawn betweenthe sliding surfaces at once (by capillary action or a forced pumpingmechanism)and the seal immediately enters the thin-film (or mixed film

34、) regime, fol-lowing the path ABC. When the speed reaches the value corresponding to point C, theseal enters the full-film regime, in which it remains until coming to operating speed atpoint D. It can be seen in figure 1 that, if the lubricant were not present, the seal wouldbe forced to operate dry

35、 at a speed corresponding to point A. The resulting temperaturerise could be extreme because of the high friction.Now some of the details are examined more closely. The curves shown in figure 1were originally for journal bearings, but the same principles apply for a seal. Frictioncoefficient is plot

36、ted against a seal duty parameter pN/Pn, where # is the fluid vis-cosity, N is the rotational speed, and Pn is the net seal-face load. (All symbols aredefined in appendix A. ) To the right of the second dashed vertical line is the region offull-film fluid lubrication, that is, thick-film lubrication

37、, where the surface asperitiesare completely separated by an oil film of such thickness that no metal-to-metal contactcan occur (fig. 2(a). The friction coefficient for a hydrodynamic film can be calculatedfromTANet normal closing forcewhere TA is the traction force. Hydrodynamic lubrication theory

38、applies, and the flowis laminar. At sufficiently large values of the seal duty parameter, turbulent flow canoccur (transition occurs at point D and turbulent flow exists beyond point E of fig. 1).The friction here rises significantly and increases at a more rapid rate with speed thanin the laminar f

39、low regime. To the left of the dashed vertical lines are the regions ofboundary and thin-film lubrication. As noted in figure 2(c), the film thickness in bound-ary lubrication is so small that asperities make contact through the fluid film. The thin-film regime is one that combines hydrodynamic and

40、boundary lubrication. This is alsothe regime where elastohydrodynamic effects may be important. Friction coefficientsin the boundary and thin-film regimes are empirically determined.In full-film fluid lubrication, since the asperities do not contact, only bulk lubricantphysical properties are import

41、ant. In boundary and thin-film lubrication, the propertiesof the metals as well as surface physics and chemistry are of primary importance sincethere is metal-to-metal contact by asperities. Lubricant chemical properties can in-fluence the type of damage that occurs.The lubricating regimes can simil

42、arly be associated with seal-face loading andspeed. The three regimes are shown from this point of view in figure 2. That is, thefull-film lubricating regime is characterized by the film thickness being several timesProvided by IHSNot for ResaleNo reproduction or networking permitted without license

43、 from IHS-,-,-greater thanthe surface roughness. The thin-film regime has the film thickness of theorder of the surface roughness. In the boundary regime, asperity contact characterizesthe interface.The friction coefficient behavior is different for a gas (fig. 3). Since gases are poorboundary lubri

44、cants, the friction coefficient values are almost those obtained for a solidsliding on a solid at low seal duty parameters. Since a gas has a much lower viscositythan a liquid, friction forces can be one or two orders of magnitude less in the full-filmlubricating regime. For a gas film seal to opera

45、te in this regime, incorporation of alift geometry to the seal faces is usually required. Operating gaps are inherently smal-ler for gas film seals (due to the low viscosity); therefore, these seals are more sensi-tive to face distortions. This, coupled with the poor boundary lubricating properties

46、ofgases, means that stable self-induced hydrodynamic operation is unlikely in gas filmseals. The friction coefficient variation is qualitatively similar to that for liquid lubri-cation in the full-film regime until compressibility effects become significant. Gener-ally, compressibility effects becom

47、e important before turbulence occurs. However, forlarge gaps and sufficiently high pressures or speeds, turbulent flow may occur.GENERAL EQUATIONS OF MOTION FOR VISCOUS FLUID FLOWA brief derivation of the fluid-flow equation governing many seal flow situations willnow be given. More details can be f

48、ound in any fluid mechanics textbook, such as refer-ence 4. However, we will follow the development given in reference 5, which derivedthe equations for seal applications.Consider the elemental fluid particle dx dy dz moving in a pressure field, asshown in figure 4. The element dx dy dz is small eno

49、ugh that we can use differentialand integral calculus but large enough for a representative statistical average. That is,the density of the fluid varies smoothly. The general conditions for equilibrium will beformulated; no stipulation will be made as to the nature of the fluid other than that it isviscous and follows Newt