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VOL. 1, NO. 5, MAY1963 AIAA J O U R N A L 1043

irCondensation in a HypersonicWindTunnelFRED L. D AU M

AeronauticalResearch Laboratories Wright-PattersonAir ForceBase Ohio

ircondensation studieswereconductedin a3-in. hypersonicwindtunnel wherethe flowMach numbers ranged from 9.5 to 17.0. The stream pitotand static pressuresboth werefound tobeaffected stronglyby thepresenceofcondensation. Largeamounts ofapparentsupersaturation which increased with increasing Mach number were found to exist. Itwas shownthathypersonic wind tunnels may beoperated with stagnation temperaturesmuchlowerthan those temperatures required for avoiding saturation conditions in the airstream andsingle-phaseflowstillmay bemaintained.

C^OR the past two decades the problems arising from the-- occurrence of air condensation in hypersonic nozzleshave beenofgreat concernto the operatorsand designersofhypersonic wind tunnels. When condensation occurs,isentropicity is lost, and meaningful data no longer are ob-tained. This phenomenon of air condensation may occurwhen the decreasing pressure and temperature of the ex-panding flow reachor exceedthe air saturation point condi-tions. Atthispoint the air may begin to condense or it maycontinue to expand into a supersaturated state. To avoidcondensation, the air may beheated to stagnation tempera-turessufficientlyhighthatthestaticair temperatures through-out theexpansionaregreaterthanthesaturationtemperature.However, at high Mach numbers thisapproach becomes im-practical because of the prohibitively high stagnation tem-peratures required.A number of early studies19of air condensation in windtunnels were madein theperiodof1948to 1953in the Machnumber rangeofabout5 to 7. Mostofthese studies showedthat condensation occurred at, or very soon after, the pointwhere the stream reached saturation conditions. Morerecently,in1960, Dayman10at the JetPropulsion Laboratoryreported thatappreciable levels ofsupersaturation couldbeachieved at Mach numbers up to9.5. Subsequently, Leeand Gregorek at Ohio State University showed with un-published data that, at Mach numbers greater than 10,large, significant amounts of supersaturation apparentlycould be obtained. The present study was undertakenfo r th e purpose of extending an d gaining more completeknowledge of the phenomenonof air condensation. Inpar-ticular, the objectives wereto determinethe influence of aircondensation on the pressure characteristics of the flow andto establish the operational limits of the facility for main-taining condensation-freeflow in terms ofMach number an dstagnation pressure and temperature.

ExperimentalTestFacilityThe presentexperimental study wascarriedout in ablow-

down type of wind tunnel which exhausts dried air f rom astorage system having a maximum pressure of 3000 psithrough the tunnel and into a 35,000-ft3 vacuum spherehaving a minimum vacuum pressure of about 5 mm HgPresentedat theIAS31stAnnualMeeting, NewYork,January

21-23,1963;revision receivedFebruary 14,1963. Theauthorwishes to acknowledge gratefully the encouragement an dvalu-ablesuggestionsafforded byHenryT.Nagamatsuof theGeneralElectric ResearchLabora toryduring this study. Aeronautical Research Engineer, Hypersonic ResearchLaboratory. MemberA I A A .

absolute. The air is dried to an average dewpointtem-perature of 65F , corresponding to 25 parts of water per106parts of air byvolume. Aresistance-type, 25-kw electricheater capable ofdelivering air at about 2200R stagnationtemperature is used. The tunnel uses a free-jet type oftestsection and is equipped with four interchangeable conicalnozzleshaving geometrical area ratios corresponding to Machnumbers 10, 12, 15, and 17. These nozzlesdeliver nominalMach numbersof9.4,10.5 13, and 16, respectively, on thecenterline at the nozzle exit. For convenience, the nozzleshereafter are identified by the geometrical Mach number.The nozzles each were equipped with several wall staticorifices in the downstream end which were spaced at 1-in.intervals. The orificediameter usedwas0.047in. Theaxialand lateral Mach number gradients ranged from 0.4 to 0.8Mach numberM/m. and from0.2 to 0.8M/in., respectively,for the M 10 through the M 17 nozzles. The Reynoldsnumbers covered in the study ranged from 0.175 X 106 to0.600 X 106/ft.

InstrumentationTotalpressure measurementsweremade with Wallace andTiernan gages of either 50 or 100 mm Hg full scale range;staticpressuresweremeasured withAceMcLeod gages. Thestagnation temperature wassensed by a platinum-rhodium-platinum thermocouple and recorded on a Brown potenti-ometer. Stagnation temperature readily was controlled towithin 5R. The stagnation pressure was indicated andrecorded by a Brown potentiometer and was controlled

manually to within about 1%of the indicated value.The pitotpressure probes were J in. in diameter with flatends having an opening 0.078 in. in diameter. All probesand orifices wereshort-coupledtominimize pressure stabiliza-tion times.

TestProcedureThe primary measurements made consisted ofpitot andstaticpressures measured at fixed locations near the nozzleexitas the stagnationtemperaturewasvaried. It wasfoundthat,unless a considerable dwelling time was allowed at eachtemperature, undesirable hysteresis effects appeared. Tominimizetheseeffects, theprocedure usedwasfirsttoincreasethe stagnation temperature to a stable maximum desiredtemperature and then to stabilize at discreet values of suc-cessivelylower temperatures. In reducing dataobtained atstagnationtemperatures greater than 1000 R,thecustomary

corrections for caloric imperfections were made using theprocedures ofRef. 11.