1、!/I/iREFROOUCED BYi_llOl_L TECHNICALINFORMATION SERVICEU. S. DEPARTMENT OF COMMERCESPRINGFIELD, VA. 22151Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,
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3、ing permitted without license from IHS-,-,-Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-REPORT 949EFFECT OF SCREENS IN WIDE-ANGLE DIFFUSERSBy G. B. SCHUBAUER and W. G. SPANGENBERGNational Bureau of StandardsWashington, D. C.4.Provided by IHSNot fo
4、r ResaleNo reproduction or networking permitted without license from IHS-,-,-,/National Advisory Committee for AeronauticsHeadquarters, 1724 F Street NW., Washington 25, D. C.Created by act of Congress approved March 3, 1915, for the supervision and direction of the scientific studyof the problems o
5、f flight (U. S. Code, title 50, sec. !51). Its membership was increased from 12 to 15 by actapproved March 2, 1929, and to 17 by act approved May 25, 1948. The members are appointed by the President,and serve as such without compensation.JEROME C. HUNSAEER, SC. D., Massachusetts Institute of Technol
6、ogy, ChairmanALEXANDER WETMORE, Sc. D., Secretary, Smithsonian Institution, Vice ChairmanHoN. JO_N R. ALISON, Assistant Secretary of Commerce.DITLEV W. BRON E, PH.D., President, Johns Hopkins University.KARL T. COMPTO_, PH.D., Chairman, Research and DevelopmentBoard, Department of Defense.EDWARD U.
7、CONDON, PH. D., Director, National Bureau ofStandards.JAMES H. DOOLITTLE, Sc. D., Vice President, Shell Union OilCorp.R. M. IIAZZN, B. S., Director of Engineering, Allison Division,General Motors Corp.WILLIAM LITTLEWOOD, M. E., Vice President, Engineering,American Airlines, Inc.THEODORE C. LONNQUEST
8、, Rear Admiral, United States Navy,Deputy and Assistant Chief of the Bureau of Aeronautics.DONALD L. PUTT, Major General, United States Air Force,Director of Research and Development, Office of the Chief ofStaff, Materiel.JOHN D. PRICE, Vice Admiral, United States Navy, Vice Chief ofNaval Operations
9、.ARTHUR E. RAYMOND, SC. D., Vice President, Engineering,Douglas Aircraft Co., Inc.FRANCIS W. REICHELDERFER, SC. D., Chief, United StatesWeather Bureau.HON. DELOS W. RENTZEL, Administrator of Civil Aeronautics,Department of Commerce.HOYT S. VANDENBERO, General, Chief of Staf, United States AirForce.T
10、HEODORE P. WRIGHT, Sc. D., Vice President for Research,Cornell University.HUOR L. DRYDEN, PU. D., DirectorJOHN W. CROWLEY, JR., B. S., Associate Director for ResearchJOHN F. VICTORY, LL.M., Exec_ttive SecretaryE. H. CHAMRERLIN, Executive O_werHENRY J. REID, D. Eng., Director, Langley Aeronautical La
11、boratory, Langley Field, Va.SMITH J. DEFRANC_., B. S., Director, Ames Aeronautical Laboratory, Moffett Field, Calif.EDWARD R. SHARP, SU. D., Director, Lewis Flight Propulsion Laboratory, Cleveland Airport, Cleveland, OhioTECHNICAL COMMITTEESAERODYNAMICS OPERATING PROBLEMSPOWER PLANTS FOR AIRCRAFT IN
12、DUSTRY CONSULTINGAIRCRAFT CONSTRUCTIONCoordination of Research Needs of ,_lilitary and Civil AviationPreparation cf ReseaTch Pro07 antsAllocation of ProblemsPrevention of DuplicationConsideration of Invention.sLANGLEY AERONAUTICAL LABORATORY LEWIS FLIGHT PROPULSION LABORATORY AMES AERONAUTICAL LABOR
13、ATORYLangley Field, Va. Cleveland Airport, Cleveland, Ohio Moffett Field, Calif.Conduct, ur_der unified control, for all affeucies of scientific researdh on the fundamental problemn of flightOFFICE OF AERONAUTICAL INTELLIGENCEWashington, D. C.Collection, classification, compiloAion, and disseminatio
14、n oJ scientific and technical information on aeronauticsProvided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-REPORT 949EFFECT OF SCREENS IN WIDE-ANGLE DIFFUSERSBy G. B. SCHUBAUERand W. G. SPA_OENSEROSUMMARYAn experimental investigation at low airspeeds wa
15、s made ofthe filling effect observed when a screen or similar resistance isplaced across a diffuser. The filling effect is.found to be real inthat screens can prevent separation or restore separated flow indiffusers even o.f extreme divergence and to depend principally onscreen location and pressure
16、-drop coefftcient of the screen.Results are given .for three different diffusers of circular crosssection urith a variety of screen arrangements. Effects of singlescreens and multiple screens are shown. The mechanics of thefilling effect is explained, and possible eftciencies are discussed.Results o
17、f arrangements o. multiple screens in wide-anglediffusers are given to show a possible application to dampingscreens as used in wind tunnels to reduce turbulence.INTRODUCTIONAn investigation of diffuser-screen combinations was under-taken at the National Bureau of Standards under the spon-sorshil_ a
18、nd with the financial assistance of the NationalAdvisory Committee for Aeronautics in an effort to clarifythe so-called “filling effect“ commonly observed when ascreen or similar resistance is placed at the mouth of awide-angle subsonic diffuser.A wide-angle diffuser is defined herein as one in whic
19、h thecross-sectional area increases so rapidly in the direction offlow that separation is to be expected. Under ordinary condi-tions this would include all conical diffusers with wallsdiverging with a total included angle greater than about 8 .About the time that damping screens for reducing tur-bul
20、ence were found to be of use in the larger wind tunnels, theNACA adopted a rapidly expanding section just ahead of ascreen to reduce the loss through the screen. It appears tohave been this use of a wide-angle diffuser followed by ascreen that first aroused general curiosity and some skep-ticism abo
21、ut the possibility of filling diffusers by this means.Intuitively it could be seen that a screen would have atendency to spread the flow by its damming effect, but thedetails of the effect were not clear. As far as is known, thefirst quantitative study of tbp effect was made by McLellanand Nichols (
22、reference 1), who were concerned with thepractical advantages of wide-angle diffusers just ahead ofheat exchangers. They showed that the filling effect wasreal and that high diffuser efficiencies could be obtained, butthey did not study the flow phenomena in sufficient detail toexplain the effect. L
23、ater Squire and Hogg (reference 2)investigated several diffuser-screen combinations for reducingturbulence in wind tunnels, including cases when screens weredistributed through a diffuser. They demonstrated interest-ing and useful effects but did not explain the reason forthe observed effects.It was
24、 the purpose of the present work to investigate thephenomena of flow through diffusers containing screens insufficient detail to clarify the mechanics of the process and toshow how best advantage can be taken of the filling effectof screens or similar resistances. From the practical stand-point, int
25、erest is limited mainly to screens of low soliditywhere the pressure drop is of the order of the dynamicpressure. When the pressure drop is many times the dy-namic pressure, the flow through all pores of the screenis determined by the pressure drop and is nearly equalregardless of the condition of t
26、he approach flow. The investi-gation has therefore been restricted to screens of low solidity.Fine screens have been used to permit measurements close toa screen, and diffusers of circular cross section have been usedto avoid corners. The experiments were conducted with airat relatively low speeds a
27、t which compressibility can beneglected. It is hoped that the information obtained is ade-quate to indicate where diffuser-screen combinations can beused to advantage. The application treated in detail involvessuch combinations used with damping screens for thereduction of wind-tunnel turbulence.The
28、 authors wish to acknowledge the assistance of Messrs.I. A. Kenerson and M. J. Noble, who made many of theinstallations and obtained some of the data.SYMBOLSXrRDAVqq,PrPhpPKEEdistance along axis of duct or diffuserradial distance from axis of duct or diffusermaximum radius of duct or diffuserdiamete
29、r of duct or diffusercross-sectional area of duct or diffuseraxial component of velocityradial component of velocitydynamic pressurereference pressure; herein taken as pressure dropacross inlet nozzle of duct system (see fig. 1)reference static pressure (see fig. 1)static presurechange in static pre
30、ssure across a screen or betweentwo pointstotal flow of potential (pressure) energy per secondacross any section of duct or diffusertotal flow of kinetic energy per second across anysection of duct or diffuserefficiency of diffuser or diffuser-screen combinationefficiency of diffuser without taking
31、into accountenergy losses through screens1Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-2kSRNREPORT 949-1_ATIONAL ADVISORY COMMITTEE FOR AERONAUTICSpressure-drop coefficient of screen wheresolidity of screen, defined as ratio of closed area tototal
32、 areaReynolds numberangle between flow direction and axis of duct ordiffuser turbulence reduction factorSubscripts:Subscripts 0, 1, 2, . . . n refer to positions along the axisof duct or diffuser. They also designate a quantity in across section normal to the axis passing through the specifiedpositi
33、on. Position 0 refers to diffuser entrance and t refersto test section of wind tunnel.Examples of subscripts:Pressure pt is static pressure at section 1; EI._ is diffuserefficiency between sections 1 and 2. Symbols are sometimesused without subscripts when the meaning is clear-on_uJcves, for example
34、. Symbol E or E“ without subscriptsmeans diffuser efficiency between section 0 and some sectionat x.DEFINITION OF TERMSEFFICIENCYIn a diffuser the cross section of a stream increases and thevelocity decreases in the direction of flow. In an efficientdiffuser the loss in kinetic energy appears largel
35、y as potentialenergy in the form of a pressure rise. The customarydefinition of the efficiency of a diffuser, and the one usedherein, isEGain in potential energy- Loss in kinetic energyThere are various ways to expressgain in potential energyand loss in kinetic energy. For example, since p and q are
36、 thepotentia! and kinetic energy per unit volume, respectively,the efficiency between two points may be expressed asE_ _p2-pi1,2- qt-q2 (I)where point 2 is downstream from point 1. If p and q areconstant over cross sections 1 and 2, the diffuser efficiencybetween these two sections is given by equat
37、ion (1). Ifsection I is at the beginning and section 2 is at the end of adiffuser, equation (1) expresses the efficiency of the diffuser.Because of the effect of the shape of the walls, the presenceof a boundary layer, and possibly separation of the flow,p and q are never entirely constant over any
38、cross section.Consequently the ef_ciency between two sections of a diffusercan be expressed exactly only in terv-of the flow of potentialand kinetic energy through the two sections. Thus theexact expression for the efficiency isE_ Pt-Pt,t=K-_K2 (2)At AtP2-_-fo ptu2 dA Pl- fo ptu, dAKi-foA_,uldA K,=_
39、zq,u, dAIn theoretical derivations, equation (1) is often used inpreference to equation (2) because of the simplicity attendingthe use of p and q. In some cases equation (1) is a suffi-ciently close approximation for practical purposes, especiallyin narrow-angle diffusers and in cases when the effic
40、iency ishigh-say, 80 percent or greater.In the present investigation equation (2) was always usedto calculate efficiencies from experimentally determinedquantities. Values of P and K were determined by graphicalor numerical evaluation of the foregoing integrals. It wasfound necessary to sacrifice ac
41、curacy for convenience byusing the velocity corresponding to q in place of the axialvelocity u because of the difficulty of measuring u separately.Obviously this procedure involves an error when v is notzero, but the error is of the same order as the experimentalerror in the measurement of q.It is c
42、onvenient to make use of an efficiency E which doesnot include losses due to the screens themselves. If E isused when screens are present, it denotes the flow efficiencyof the diffuser as affected by screens. It is referred to as“flow efficiency.“FILLINGFilling is a term used rather loosely to denot
43、e that, eitherbecause of diffuser design or of the effect of a screen, the flowtakes place throughout all available volume in the diffuser.In this sense it means absence of separation. Although thepresence or absence of separation is an important flowcriterion, still another is the velocity distribu
44、tion. Inorder to include both of these, a filled condition might bedefined as one in which the velocity distribution at everysection is similar to that at the diffuser entrance. Thisdefinition has the objection that it ignores the effect of thegeometry of the diffuser on the flow pattern. The presen
45、tresults are given in such form that performance may bejudged either by the separation criterion or by the similaritycriterion.Use is made of charts called streamline diagrams, which consist of lines indicating the radial distances within which0.1, 0.2, 0.3, and so forth of the total observed flow o
46、ccur.In computing the total observed flow when separation waspresent, the reverse flow near the wall was neglected. Inthese cases the observed volume was generally a few percenttoo high, and the lines are not accurately streamlines.PJU_SSURE-DaOP COEFFICIENT kBy definition the pressure-drop coeffici
47、ent of a screen isk_APqProvided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-EFFECT OF SCREENS IN WIDE-ANGLE DIFFUSERS 3where q is the dynamic pressure of a uniform parallel flowaplSroaehing normal to the plane of the screen and hp is thestatic-pressure dr
48、op across the screen. The value of k isdetermined experimentally by measuring q and _p. Thecoefficient depends on the solidity S and on a Reynoldsnumber equal to the diameter of the wire times the velocitycorresponding to q, divided by the kinematic viscosity.The coefficient k is useful for calculat
49、ing _p when theflow is normal to the screen. The pressure drop may beabnormally high if the stream approaches the screen at aconsiderably large angle to the normal. In any case k isused as a parameter for connecting a given screen with itsaerodynamic effect, such as its effect on turbulence and onthe space distribution of velocity.APPA
