Incipient Cavitation of Freon-114 in a Tunnel Venturi

7/29/2019 Incipient Cavitation of Freon-114 in a Tunnel Venturi

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N A S A T E C H N I C A L N O T E

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INCIPIENT CAVITATION OFFREON-114 IN A TUNNEL VENTURIby Thomas F . Gelder, RoyceD. Moore,andRobert S. RzcggeriLewisResearch CenterCleveland Ohio

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NA TION A L AE RONA UTICS A ND SPACE A DM IN ISTR A TION WA SHINGTON, D . C. FEBRUARY 1 9 6


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TECH LIBRARY KAFB, NM

INCIP IENT CAV ITAT ION OF FREON-114 IN A TUNNEL VENTUR IBy Thomas F. Gelder, Royce D. M oore, and R obert S. Ruggeri

L ewi s R esearch C enterCleveland, Ohio

N A T I O N A L A E R O N A U T I C S A N D S P A C E A D M I N IS T R A T I O NFor ro l e by the Off ice of Tech nic al Serv ices, Deportment of Commerce,

Washington, D.C. 20230 -- Pr ico $1.00


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INCIP IENT CA VITATION OF FREON-114 I N A TUNNEL VENTURIby Tho m as F. Ge lder , Royce D. M o o r e , a n d Ro b e r t S. R u g g e r i

L ew i s R es e a r c h C e n t e rS U M M A R Y

Cavitation of liquid Freon- 114 (dichlorotetrafluoroethane) was induced on the walls ofaVenturi in aclosed-return hydrodynamic tunnel. The Venturi used aquarter round toprovide the transition from a 1.743-inch-diameter approach section to a 1.377-inch-diameter throat section. A t a fixed velocity, the incipient-cavitation parameter de-creased as much as 30percent when the liquid temperature was increased from 0 to80 F. (The incipient-cavitation parameter is the ratio of the difference between an up-stream reference pressure at incipient cavitation and the vapor pressure to the velocitypressure of the upstream flow. ) At a fixed temperature, the parameter increased asmuch as 50 percent when the approach velocity was increased from 20 to 47 feet per sec-ond. Values of the incipient-cavitation parameter indicated local effective tensions with-in the liquid that ranged from 4 to 20 feet of liquid, depending on temperature and veloc-ity.

I NTRO DUCT1ONCavitation may be described as the formation and subsequent collapse of vapor cav-

ities in a flowing liquid brought about by pressure changes resulting from changes inflow velocity. In visible systems, incipient cavitation usually denotes the first small butvisible rupture (bubble) in the liquid. Generally, at or just prior to incipient cavitation,the liquid is locally at apressure less than the vapor pressure corresponding to theliquid temperature (refs. 1 o 4). Thus, the liquid is superheated locally or is effectivelyin tension. Experimental studies have shown that the amount of tension at incipient cavi-tation is different for water than for nitrogen flowing in the same tunnel and Venturi testsection (refs. 1and 2). Although the existence, source, size, and exact role of nucleiin the cavitation process are not clearly understood (ref. 3), the effect of pure fluidproperties alone seems to be intimately involved in the cavitation process. At present,


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the role of any particular fluid property is insufficiently known, and abetter understand-ing of property effects is required to improve design of hydraulic equipment (ref. 5).

The purposes of this investigation were to determine the incipient-cavitation charac-teristics of Freon- 114 (dichlorotetrafluoroethane) flowing in the same tunnel and Venturiasused previously for water and nitrogen (refs. 1and 2) and thus to extend the range offluid properties investigated. The study was conducted at the NASA Lewis Research Center as part of ageneral program of cavitation research. Cavitation was induced on thewalls of atransparent Venturi test section in aclosed-return tunnel, which, for temper-ature control, was submerged in abath of antifreeze solution. The Venturi uses aquar-ter round (nominal radius, 0. 183 in. ) to provide the transition from a 1.743-inch-diameter approach section to a 1.377-inch-diameter throat section. The flow velocity inthe Venturi approach section was varied from 20to 47 feet per second, and the liquidtemperature was varied between 0 and 80' F. The ranges of velocity and temperaturewere determined by facility limitations.

A P P A R A T U SAside from the minor additions noted herein, the facil ity used in the present study

is the sameas that described in detail in references 1and 2. Briefly, the facil ity con-sists of a small closed-return hydrodynamic tunnel (capacity, approx 10U. S. gal) de-signed to ci rculate (by means of a 700 gal/min centrifugal pump) various liquids, includ-ing cryogenic liquids. The tunnel accommodates 12-inch-long test sections having max-imum inlet diameters of 1.743 inches. A liquid bath (capacity, approx 80U. S. gal) sur-rounds the tunnel to serve as a heat sink and also to control tunnel liquid temperature.For temperature control, a slowly circulated bath mixture of 60 percent ethylene glycoland water was provided. This mixture exchanged heat with a sump-mounted, single-tubecoil carrying either low-pressure steamor cold nitrogen gas (an addition to the facil ityof ref. 1). Absolute values of tunnel liquid temperature were measured with a cali-brated copper-constantan thermocouple (accuracy, 20. 5 F) mounted on the flow center-line 14. 5 inches upstream of the test section. Liquid pressure level in the tunnel wasvaried by gas pressurization of the ullage space above a butyl rubber diaphram (an addi-tion to the facil ity of ref. 1) in the expansion chamber. Pressures within the tunnel andtest section were measured by mercury manometers or by calibrated bleed-type preci-sion gages (accuracy, 20. 15 lb/sq in. ).

The transparent-plastic Venturi test section used with Freon- 114was the same oneasused in previous cavitation studies of water and nitrogen (refs. 1and 2). The Venturi(fig. 1)uses aslightly modified quarter round (nominal radius, 0. 183 in. ) to provide thetransition from a 1.743-inch-diameter approach section to a 1.377-inch-diameter throat2


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A - A '

Figure 1 - Sketch of hydrodynamic Venturi section showing dimensions and pressure instrumentation. (Dimensions in inches. )

section. The 0.75-inch-long, constant-diameter throat is followed by a conical diffuser.The precise contour of the modified quarter-round region is given in reference 1.

The firstappearance of vapor cavities was photographed by a 4- by 5-inch still cam-era in conjunction with a 0. 5-microsecond, high-intensity flash unit.

PROCEDURETes t Liquid

A commercial grade of Freon- 114 (dichlorotetrafluoroethane) was used as the testliquid. It is a clear, colorless liquid with a normal boiling point of 38.8' F. Some liq-uid Freon-114 properties (refs. 6 to 8) are presented for convenience in table I.

One chargewas direct f rom the cylinder as received from the manufacturer, who specifiesa maxi-mum noncondensable gas content (assumed to be air) of 1.5 percent by volume in the va-por phase. This is equivalent to an air content of 20 parts per million (mg air/kg liquid).Because the original contents of another Freon- 114 cyl inder had been inadvertently pres-surized with nitrogen gas, this liquid was reprocessed to remove or reduce the gas con-tent before it was used in the tunnel.110 F in aclosed tank; then, with heat off, the tank was vented to the atmosphere untilabout 5 percent by weight was boiled off. The gas content of this reprocessed charge isunknown.in water and, like that for water, increases with decreasing temperature.

An evacuated and subcooled tunnel was twice charged with Freon-114.

The liquid was reprocessed by being heated to

The solubility coefficient for air in Freon-114 is much higher than that for ai rFor instance,

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T A B L E I . - PROPERT IES O F LIQUID FREON-114I T emperature

i1001020' i0

aRef. 6.bRef. 7.'Ref. 8.

Vaporiressure

ft ofFreon-111( 46. 638.70

11.2714.4318.2922.9328.4835.0742.8351.9162.47

Zpecifiweigh1h/cu f

( 499.4298.5097.5796.6395.6794.6993.7092.6991.6590.5989.51

Absoluteviscosity,

lb-sec)/sq f

(b)13. 7X10-612.511.5510.7710.099.488.948.458.037.677.30

Surfacetension,

lb/ft

(c)12.3&10-'11.8511.37LO. 90LO. 419.939.469.008. 538.107. 64

at a total pressure (airplus vapor) o1atmosphere, the manufacturer in-dicates that air-saturated Freon-114contains about 140 parts per millionat a temperature of 32' F, and abou1000 parts per million at 0 F.

C riteria and A ppearance ofIn cip ien t Cavitation

Cavitation was induced on thewalls of aVenturi and controlled byvarying the overall liquid pressurelevel in the tunnel while maintainingfixed approach velocity and a constantemperature (within20. 5 F). Theoperating condition atwhich the for-

mation and collapse of vapor bubbles near the Venturi surface is just detectableby eye isdefined herein as incipient cavitation. At this condition, free-stream values of staticpressure ho, velocity Vo, and temperature (for vapor pressure hv) were measured forsubsequent determination of the incipient cavitation parameter Ki. (Symbols are definedin the appendix. ) All free-stream values refer to an axial location in the approach sectiothat is about 3/4 inch upstreamof the start of the quarter round (i.e., at tap 1, fig. 1).Typical photographs of incipient cavitation of Freon-114 are shown in figure 2. In gen-eral, incipient cavitation at all conditions studied was evidenced by intermittent (approx5/sec) bursts of vapor cavities (minimum length, 1/8 in.) with leading edges located inthe region of minimum pressure on the quarter round. These bursts occurred in a ran-dom manner about the periphery of the Venturi, each burst lasting but a few milliseconds.Incipient-cavity size generally decreased, and frequency of occurrence increased with in-creasing speed. Increased temperature appears to increase the size of the incipient cav-ity of Freon-114, as shown in figure 2. Therewere no hysteresis effects; that is, nomeasurable differences in incipient conditions were observed whether the incipient statewas approached from initially noncavitating flow by decreasing pressure, or froman ini-tially cavitated state by increasing pressure.

No ncavitating P ressure Dis tributionThe noncavitating wall pressure distribution in the cri tical low-pressure region of the

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(a ) Free-stream temperatur e, - 1. 3" F; free-stream velocity, 25.9 feet per second; incipient-cavitationparameter, 2.80.

( b l Free-stream temperature, ho.7" F; free-stream velocity, 26.8 feet per second; incipient-cavitationparameter, 2.90.Figure 2, - Appearance of incipient cavitation of Freon-114 at two free-stream temperatures.

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1

00c -1.-VW0a,

.-c

g -2LnInWQ

-3

-42.4 2.5

I I I I IFluid Free-streamdiameter, .

~ A ir 8.030 --i-2.6

111-114

t.7.~ ~.. 43-I

2.8Ratio of axial distance from test-section entrance tofree-stream diameter, xlDFigure 3. - Noncavitating pressure distribution forFreon-114 Venturi and aerodynamic Venturi.

2.9

Venturi is used in evaluating cavitation data.This pressure distribution is obtained primar-ily from previous aerodynamic studies (refs. 1and 2) and ispresented in figure 3. The aero-dynamic data are from the accurately scaledwind tunnel model of the Venturi with free-stream diameter of 8.030 inches described inreference 2. Theair data areslightly differ-ent from those previously reported in that animproved method is now used to correct forcompressibility. The locally measured airpressures obtained at numerous levels of free-stream Mach number are extrapolated to aMach number of zero (the incompressiblecase). Previously (refs. 1and 2), compres-sibility factors obtained from oversimplifiedcorrections to the incompressible pitot equa-tion were used. Figure 3 shows good agree-ment between pressure coefficients for air and

the Freon-114 studied herein. A study of available hydrodynamic pressure data in thesame Venturi indicates a critical Reynolds number (based on the approach-section diam-6eter of 1.743 in., the free-stream velocity, and the liquid properties) near 0.4xlO ,above which all local pressure coefficients remain constant. The minimum Reynolds

6number for the Freon-114 data herein is about 0.7X10 .unchanging pressure distribution for Freon- 114, and in particular, avalue of Cof -3.35 at an x/D location of 2.471. This constant value of -3.35 for CReynolds numbers above 0.4X10 differs slightly from that previously reported m refer-ences 1and 2 (wherein Cp, min varied from -3.28 to -3.62 as Reynolds numbers in-creased from 0.4X10 to 4x10 ) because of the improved compressibility correctiondescribed herein. The new Cp, min value of -3.35 should replace all Cp, min valuesof references 1and 2; this results in only small changes in the tension values and in nochange in the values of the incipient-cavitation parameter K..

Thus, figure 3 represents theP, minat all6 P, mi?

6 6

1

RESULTS A ND D I S C U S S I O NThe conventional incipient-cavitation parameter Ki is the ratio of the difference

between a free-stream reference pressure at incipient cavitation h and the vaporpressure hv to the velocity pressure or head of the free-stream flow V0/2g. The26


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S

n

0m

"mVmn.-c" 4 05wLcw 30

20

10-20

Plain symbols denote.as-received liquid(air content, 20 mg.airlkg Freon-114)Tailed symbols denote.reprocessed liquid(air content un- .

0 40

B

7

c-

t -

-II

60Free-stream liquid emperature,

Free-stream

ftlsec0 20.026.10 33.30 40.00 46.7

k r

80 100' F

vn)

Figure 4. - Free-stream static pressure above vapor pressure at incipient cavitationas function of free-stream liquid temperature for several free-stream velocities.basic data are given in figure 4, where the numerator of Ki, ho - hv, is shownas afunction of liquid temperature for the five free-stream velocities and the two loadings ofliquid.increases with increasing velocity and decreases with increasing temperature. The re-processed liquid with unknown gas content yields the same results as the as-receivedliquid with an air content of 20parts per million.sented by the faired lines drawn for each velocity, and the values on these lines are usedto calculate Ki at constant temperature over the range of free-stream velocities tested.Thus, Ki as a function of free-stream velocity for liquid temperatures of Oo , 40, and80 F is given in figure 5.-Cp, min is also shown for reference.with increasing velocity, the curves tending to parallel the -Cp, min value at the higher

Free-stream static pressure above vapor pressure at incipient cavitation ho - hv

The data of figure4 arewell repre-

The negative noncavitating minimum-pressure coefficientThe incipient-cavitation parameter y increases

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.-Y

LWc5L

cLm

n.-.->mYLcV

.-n.--10 20 30 40Free-stream velocity, Vo, ftlsec

Figure 5. - Incipient-cavitation parameter(from data fairing of fig. 4) for Freon-114and negative noncavitating minimumpressure coefficient as functions of free-stream velocity and liquid temperature.

Free-stream -liquid

I20-10

010 20 30 40 50Free-stream velocity, Vo, ftlsec-1 I I I200 150 loo 75 50 40Exposure time, psec

Figure 6. - Effective liquid tension and effec-tive-tension parameter (from data fairing offig. 4) for Freon-114 based on incipientcavitation. Exposure time scale for 4@ Fdata only.

velocities. The velocity effect on K., is greatest at the highest temperature (80 F), Yincreasing about 50percent as velocity increases from 20 to 47 feet per second. As previously indicated, Ki decreases with increasing temperature, and figure 5 shows thegreatest effect at the lowest velocity. At avalue of Vo of 20 feet per second, 5 de-creases about 30 percent with an increase in liquid temperature from0 to 80 F.incipient cavitation the minimum pressure on the Venturi surface hdn is always lessthan the free-stream vapor pressure hv. With hmin


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The tension values of figure 6 were the maximum values experienced by the liquid ator just prior to incipient cavitation because they weredetermined by comparing % withthe minimum-pressure coefficient Cpressure profile of figure3, it experienced, just prior to incipient conditions, a range oftensions or pressure decrements relative to vapor pressure that varied from zero whenK; and -Cagain.(ref. 2). Exposure times for the 40 F data only are indicatedat the bottom of figure 6for reference. For example, at 40 F exposure time decreased from about 200 to 50microseconds for avelocity increase from 20 to 40 feet per second.

However, as the fluid flowed through theP ,a'

were first equal, to the maximum values of figure6and then back to zeroPThe total time the fluid i s below the vapor pressure is called the exposure time

S U M M A R Y OF RESULTSExperimental studies of incipient cavitation induced on the walls of aVenturi

(quarter-round transition section) in asmall closed-return tunnel using liquid Freon- 114(dichlorotetrafluoroethane) over a range of temperatures and velocities yielded the fol-lowing principal results:

1. A t a fixed velocity, the incipient-cavitation parameter Ki decreased as much as30 percent when the liquid temperature was increased from 0 to 80 F, and at a fixedtemperature, K i increased as much as 50percent when the free-stream (approach sec-tion) velocity was increased from 20 to 47 feetper second.

2. Absolute values of the incipient-cavitation parameter indicated local minimumwall pressures less than the vapor pressure corresponding to the liquid temperature.The maximum pressure decrement (effective liquid tension) occurred at 80 F and rangedfrom 9 feet of liquid Freon-114 at a free-stream (approach section) velocity of 20 feetper second to 20 feetof liquid at 47 feet per second. At 0 F these pressure decrementswere about halved.Lewis Research Center,

National Aeronautics and Space Administration,Cleveland, Ohio, December 3, 1964.

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II11IIII I1 I

A P P E N D I X - SYMBOLS2C

p minD free-stream (approach section) diameter, 1.743 in. for cavitation model,

noncavitating pressure coefficient, (hx - ho)/(Vo/2g)nonc avitat ng minimum- pressure coefficient, (hm n - ho)/ V0/2g)P 2

8.03 in. for aerodynamic model2g

hmin%%hO

K ivO

acceleration due to gravity, 32.2 ft/secminimum static pressure, f t of liquid Freon-114 absvapor pressure corresponding to free-stream liquid temperature, f t of liquid

Freon- 114 absstatic pressure at x/D, f t of liquid Freon-114 absfree-stream static pressure at x/D of 1.98 (approach section), f t of liquidinc pient -cavitation parameter, [ho - hv) (Vi/2g)] incipientfree-stream velocity at x/D of 1.98 (approach section), ft/sec

Freon- 114 abs

X axial distance from test-section inlet, in. (see fig. 1)

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REFERENCES1. Ruggeri, Robert S. ; and Gelder, Thomas F. : Cavitation and Effective Liquid Ten-

sion of Nitrogen in aTunnel Venturi. NASA TN D-2088, 1964.2. Ruggeri, Robert S. ; and Gelder, Thomas F. : Effects of Air Content andWater Pur -ity on Liquid Tension at Incipient Cavitation in Venturi Flow. NASA TN D-1459,

1963.3. Holl, J . William; and Wislicenus, George F. : Scale Effects on Cavitation. J our.

Basic Eng. (Trans. ASME), ser. D, vol. 83, no. 3, Sept. 1961, pp. 385-398.4. Parkin, B. R. ; and Kermeen, R. W. : The Roles of Convective A ir Diffusion and

Liquid and Tensile Stresses During Cavitation Inception. IAHR Symposium on Cav-itation and Hydraulic Machinery, Sendai (Japan), Sept. 3-8, 1962.

5. Jacobs, R. B. : Prediction of Symptoms of Cavitation. J our. Res. Nat. Bur. Stan-dards, sec. C, vol. 65, no. 3, July-Sept. 1961, pp. 147-156.

6. Van Wie, Nelson H. ;and Ebel, Robert A. : Some Thermodynamic Properties ofFreon-114. Vol. 1- 40 F to the Cri tical Temperature. Rep. K-1430, AEC,1959.

7. Anon. : "Freon. ? ' Products Div. Bull. T114B; E. I. DuPont De Nemours andco., Inc.

8. Anon. : "Freon. f t Products Div. Bull. D-27; E. I. DuPont De Nemours and Co.,Inc.

NASA-L angley, 1965 E-2846 11


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Incipient Cavitation of Freon-114 in a Tunnel Venturi