NASA NACA-ARR-L5C24-1945 Low-pressure boundary-layer control in diffusers and bends《扩散器和弯曲的低压边界层控制》.pdf

1、rjl. “ . ., -,i.u. ARRNo.LX24,-. NATIONAL A.DVISORYff _-fE FOR AERONAUTIW=2L+/(#WAlnlm Imw”rORIGINALLY ISSUEDi Al. 1945asAdvanceRestrictedReportL5C24f-1LOWPRESVREBUUNDARY-IJWERCONIROLINDITFKZRSARDBENIX3By WilliamJ. BiebelLangley. . . . . . .Wmmrlel.AeronauticalLaboratoryLangleyField,Va.NACA CN A C A

2、 LIBMRYWASHINGTON LANGLEY MEMOIWL AERONAUTICALLABORATORYLangleyField,Va.NACA WARTLME REPORTS arereprintsofpapersorigimllyissuedtoproviderapiddistributionofadvanceresearchresultstoan authorizedgrouprequiringthem forthewar effort.They were pre-viouslyheldundera securitystatusbutarenow unclassified.Som

3、e ofthesereportswere nottech-nicallyedited.Allhave been reproducedwithoutchangeinordertoexpeditegeneraldistribution.iL-84,.a.,Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-3 1176001878207 I*., -. , -.-. . .! - , - . ., -., -._. _. ., -, ,. . . . .

4、. ,. . -.Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-.-. .- . .NACA ARR NOO L5C24NIONAL ADVISORY COMMITTEE FOR AERONAUTICSe-,. . .ADvANCEREsTRx.C.mJ andthe pressure required to blow out the boundary layer wassmall relative to the pressures normal

5、ly available inairplane ducts. The slots in the diffuser.arrangementswere generally formed merely by cutting narrow strisfrom the two diverging walls of the diffuser. Not morethen one slot was used on each surface, and none wereusually required on the two parallel walls of the diffuser.Effective bou

6、ndary-layer control for the inner corners ofthe bends required a slot with a lip that prsjected intothe duct in order to help “peel off!lthe boundary layerand also required somewhat higher internal pressures thmwer6 used with the diffusers.INTRODUCTIONThe efficiency of airplane ducts has generally b

7、eenInmaired by the limitations of the sace available foridut installations. Rapid duct exp measurementswere made of the total-pressure losses and of the quantityof air lost through the boundary-layer-control slots.Becaus6 simplicity is desirable for any practicsl instal-lation, the arrangements tast

8、ed generally included notmore than one slot on each of the two divergent walls ofthe diffusers and one slot on ths inner WE1l of the 90bends. Boundary layers of different thiclmesses wereused at the duct inlets in an effort to slmulste differentoperating conditions. Since ths outlet condition affect

9、sthe flow and the total-pressure losses through a diffuser,three different outlet arrangements were tried: (i) anabrupt contraction to the finglmeasurement section, (2) along straight uniform section of ducting attached to thediffuser outlet, and () 8 rsslstsnca in the form of anintercooler at the d

10、iiussroutltit.SYMBOLSH totel ressure, pounds per square footAR total-pressurti loss in diffuser or bendP static pressure, pounds psr squars footq dynmic pressure, pounds par square footv volociby, fast p-m secorld-Provided by IHSNot for ResaleNo reproduction or networking permitted without license f

11、rom IHS-,-,-!NAOA ARR NO- L5c24 3Subscripts:b . . . . . .-,”,I.1 at.inleto atospheric conditions max maximumi .APPARATUS, BLOWZR,AND DUCT SYSTEMThe air flow was produoed by a centrifugal blowerdriven by an automobile engine. ti order to reduce theturbulence and improve the uniformity of the flow at

12、theinlet of the test duct, an expanded passage with astraightener was inserted between tne blower and the testduct (fig. 1). The straightener was an egg-crate!arrangement with layers of soreen across both the upstreamand downstream ends. Behind the straightener, the passagecontracted to a 5- by 1-in

13、ch rectangular section (fig. 1,section 1), which was the inlet for all the ducts tested,The outlet arrangement, which was common to all testsetups, consisted of a contracting passage (except for1the bends), a 5- by 12-inch measurement section (section 2),and a flapped exit. The purpose of the flaps

14、was topermit adjustment of the pressure in the systemThe diffusers were made with 15 and 30 Included,., angles (figs. 1 and 2, respectively). For the tests with-out the resistance, the large end of the diffuser wasI18 by 12$ inches, which corresponds to a two-dimensionalexpansion Of 3.6:1. A somewha

15、t larger expansion wasrequired for the tests with the resistance (fig. 3) sincethe duct had to be ftted to the 22- by 13-inch face of .the Airesearch intercooler that served as the resistance.The bends (fig. )4)were made with inner radii of 1 and2 inchbs and outer radii of,g and.y Inches, respective

16、ly.The aspect ratio of both bends was 2.5. The duct systemwas of sheet iron except for the side walls near thecritical sections, which were made of celluloid tofacilitate tuft observations of the internal flow.- .Provided by IHSNot for ResaleNo reproduction or networking permitted without license fr

17、om IHS-,-,-,MEASUREMENTS4 NACA ARR NO. L15CTotal-pressure end static-pressure measurements atthe inlet (section 1) and exit (section 2) permitteddetermination of the tatal-pressure losses within thesystem and of the quantity of alr lost through the slots.Flow quantities at sections 1 and 2 were obta

18、ined in mostcases from the arithmetic mean of the measured dynamicpressures. Tests without slots - that 1s, with no airloss - showed that the results from sections 1 and 2 agreedto about 1 percent; therefore, similar accuracy may beassumed for the slotted conditions. Simllsrly, for thedetermination

19、of the total-pressure losses In the system.,the arithmetic mean of the measured total pressures atsections 1 and 2 was used; however, when the flow was soirregular that an error of.over 1 percent was indicated,the total nressures were weighted according to the localvelocity. Some uncertainty existed

20、 concerning the best way todetermine the expansion losses in the diffuser-intercoolercombinations. Losses memured at the exit included thelarge pressure drop through the intercooler, whereas lossesmeasured at the face of the intercooler section 5) wouldbe considered inaccurate because of flow separa

21、tion in theregion of measurement. It was found, however, that theaverege of the total pressures at the face of the inter-cooler, obtained with shielded total-pressure tubes, alwaysdiffered from the.average total pressure at the exit bynearly the same amount - from 42 to 4.4times the meandynandc pres

22、sure at the intercoolar, which presumably is “the loss through the intercooler. Both methods thereforewould have given about the same results. The resultsraported were determined from the averages at the face ofthe Intercooler.In addition to the measurements obtsined with totel-pressure and static-p

23、ressure tubes distributed acrossinlet and outlet sreas at stations 1, 2, and 5, moredetailed meesuremnts were made, for several cases, of theboundary layars at the inlet mid at sevaral positionsalong th diffusers. Thea= measurements were made nearthe midpoints of eaeh of the walls at the sectionsdes

24、ignated 1, 3, and L in figures 2 and 3.Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,- . . . .- .NACA ARR NO* L5CDEgcRTQN.OF, TESTS-,. . . - .-w-,:- -. .Tuft Observations and Slot Arrangements.The looation and arrangement of the slots were chosenlar

25、gely from tuft observations of the flow in the diffuser,The main slot was usually plaoed slightly upstream of thepoint where the flow In the saledduct separated. Suohseparation always occurred on the surface on which the .Incoming boundary lgyer was thickest; but, when there wasno obstruction in the

26、 entrance cone so that the boundarylayer was about equally thick on both upper and lowersurfaces, the separation point would sometimes alternatebetween the two surfaces. In any case, when separation ofthe flow on the critice.1surface had been ellminted bythe slot, separation generally occurred on th

27、e oppcsltesurface at ebout the same section or perhaps slightlyfarther downstream. A slot on this surface tharefore wasalso desirable. Although the use of only one slot on eachof these two surfacesdid not prevent eventual separationfarther downstream, tufts skowad that the velocities neerthese senar

28、ated regions were very small so thet or,lyminortotal-pressure losses were associated with this eventualseparation.In cases in which separation was observed to start inthe oorners or near the middle of apparently, boundary layers can withstand more pressurerise along the parailel walls than along the

29、 divergingwalls,In the bends, separation occurred just downstream ofthe corner on the imer wallA slot formed by cutting astrip out ef the wall, which served satisfactorily for thediffusers, did not suffice to ramove the boundary layer inthe bends - probably because of the low static pressure at“the

30、inner corner. Accordingly, the slots for the bandshad to be designad to lead the boundary layer out of theduct, (See table VI.) App=ently, a bod=y layer must. -Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-6“ NAOA ARR No. L5C24be %eeled offn In thi

31、s wey if its total pressure, but notIts static pressura, exceeds the external static pressure;a boundary layer will flow out of a simple flush slot onlyif its static pressure exceeds the external static pressure.Inlet Boundary LayerSince the total-pressure losses through a diffuserend the point of s

32、eparstton are affected by the inlet flowconditions, the boundery layer whereas, with the slots installed, thereis no indication of separation at section )+.The 30 Diffuser with IntercoolerResults of tests with the intercoolor set at anglesof 00, 300, and ).+5 to the end of thediffuser are givenIn ta

33、bles III, IV, end V, respectively. No inlet obstruc-tions were used for thase tests, although some of thetests were mada with the 20-inch inlet passage betweenthe entrance cone and the diffuser inlet.Comparison of the results in tablas III to V showsthat incilnlng the lntercooler to the diffusm axis

34、gener however”;to “”“bleeabout12 percent of the alfi- a result that isprobably related to the high internal pressures, whichcould not be reduced becduse of the hi, resistance ofthe Inte.rcooler. Efforts to Use,marrowsr slots to reducethe air loss seemed to give less effective.boundary-layercontrol t

35、han was obtained for the “ diffuser without an.intercooler, which has broader slots and operates atsmaller pressures.Slots on only the upper and lower surfaces werefound to be less satisfactory with then without the 20-inchentrance passage, probably because the boundary layer onthe sides was thicker

36、 with the 20-inch entrance passage.A solution seemed to be to separate the flanges of the20-inch section and the diffuser inlet so as to provide-inch slot canpletelyaround the inlet.“4 A similararrangement also gavs a large reduction of the total-pressure losses for the cases without the 20-inch inl

37、etsection. The effectiveness of this slot around the inletis remarkably high. The effect is doubtless related tothat noted in reference 2 im which high duct efficiencieswere observed when the boundary layer at the iniet wasvery thin. An obvious contributing factor is the lossof air at the slot itsel

38、f, which causes the actual dynamicpressure just downstream of the slot to be only about 0.8of the value on which the values of AH/ql have beenbased. If the losses were calculated on the basis of thislower inlet dynamic pressure, this arrangement would, inmost of the cases given, show about the same

39、reduction Intotal-pressure losses as found with slots on only theupper and lower surfaces.The distribution of total-pressure loss at the faceof the intercooler is shown for a number of slotted andsealed conditions in figure 9. Large separated regionsat the upper and lower surfaces are shown for the

40、sealedconditions. The slots mostly eliminate these regions butsometimes develop a separated region on one of the sides.Velocity distributions across the horizontal and verticalcenter”linesof the ducts at sections.1, 3, mdL areshown in figure 10. This figure also chows how the slotsprevent the early

41、separation of the boundary layer.1.: .- . . . .- . . _ .Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-.,10BendsNACA ARR No. L5C24The duct bends were tested without obstructions inthe entrance cone and without the 20-inch inlet section.As has alread

42、y been noted, the Inner corners of the bendsrequired sore-eredesign in order to provide a slot thatpeeled off the boundary layer. The slotted conditionsaccordingly cantotbe compared witn corresponding sealedconditions, as wns done with the diffusers, In order toevaluate the reduction in total-pressu

43、re lossas. Table VI,which shows tineresults for the bends, therefore does notgive values for the reduction In AH/ql. The effectivenessof the slots is inalcated by comparison of the given valuesof AHql with the values for the three sealed conditionsshown, especially the arrangements with the 2-i.rchi

44、nnerradius for which tfiolosses were L!+ and la percent. AllIndicated total-pressure losses have been corrected forthe friction loss between the inlet and exit measurementsections.The total-pressure losses for the most efficient ofthe slotted conditions are of the order of 6 to 10 percentwith about

45、5 to 10 uercent loss of air. Most of thedesigns shown in table VI have a fairly large Inner radius,with the slot net very far beyond the end of the bend.The value of w must be of the order of 0.5 inorder that the pressure suffice to blow out the requtredamount of air,Figure 11, which shows the distr

46、ibution of dynsmicpressures at the exit of the bend for slotted and seledconditions, Indicates sn increased uniformity of flowfor the slotted condition.APPLICATIONPressures RequiredAs can be seen in tables I and II, adequate boundary-layer control In the diffusers was obtained with smallPIalues f +“

47、 The adateticm of this principle toL Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-NAOA LRR No. L5C4 11duet Installations on actual airplanes should r,b.e$easibl.g.sinoe-the average airp”lm”e”-diffuser ilet-shows static pressures greater than that

48、ofthe free stream;,that is,there is generally more than enough pressure to blow outthe boundary layer into the free stream. In a practicaldesign, however, the boundary layerwould probably bebloWn out, not directly into the frea stream but Intosome duct “especiallyprovided for the purpose or intosome space within the wing or fuselage from which,outlet would be provided. The pressure here would prob-ably be higher than that of the free stream; however, theease with which boundary-layer control was obtained evenwith small pressure diff