Cavitation erosion on aluminium in a venturi set-up

B. C. Syamala RAO *

N. S. Lakshmana RAO

* *

AND

K. SEETHARAMIAH ***

CAVITATION EROSION ON ALUMINIUM IN A VENTURI

SET-UP

Introduction

Exposure time

1/ Charac!erislie rate-time eU!,\'e accoding to Thiruvenga- dam [1].

Courbe caractéristique du taux d'érosion en jonction dll temps, d'après Th inwen(jl1l!am [1].

1

"

Attenuation zone

Accumule! tioll zone Incubation zone

... .. ..

g

ïiôo

..

ILl

c .2

1/1o

..

ILl

Although it had been noted for long, that the rate of material loss due ta cavitation is not uni- form with respect to time, some of its consequen- ces have received attention only recently. Thinl- vengadam and his co-workers [1J reported that characteristic erosion-lime curves could be des- cribed in tenus of four zones:

a) Incubation zone with no weight loss;

b) Accumulation zone wi th loss of rate increas- ing ta a peak;

c) Attenuation zone ,vith decreasing loss rate and IinalIy:

d) Steady-state zone with constant loss rate (Fig. 1). They suggested that the final zone should he used for comparison or correlation purposes of ditrerent materials. But Plesset and Devine [2J dis- puted the sequenlial location of the attenuation zone and remarked that the maximum erosion con- tinued for a certain period at a steady-state rate and then decreased (Fig. 2). The investigations of Hohbs [:3] also showed trends similar to those of Figure 2. AlI these investigations [1, 2, 3J were conducted using magnetostriction oscillators.

'Vorking with a venturi type of set-up, Hammitt [4] found that there were numerous peaks and valIeys in the erosion-time curve and it did not

Lecturer, Civil and Hydraulie Engineering Department, Indian Institute of Science, Bangalore, India.

Associate Professol', Civil and Hydraulie Engineering Department, Indian Institute of Science, Bangalore, India.

* * * Professor-in-eharge, Civil and Hydralllie Engineering Department, Indian Institute of Science, Bangalore, India.

Expozure time

2/ Chanlc!eristic rate-lime clll've acconling to Plesset and Devine [2J.

Courbe caractéristique du tallx d'érosion en j'onction dll temps, d'apJ'(~sP!esset et Devill:' [:lI.

713

Table 1 Mechanical Pl'opertïes of A11lminilll11

Aluminium test specimens, 1/8in. thick, were prepared to the desired size and their initial weight ,vas determined. The specimens were fixed in the test section of the water tunnel and subjected to cavitation action for 20 min. The tests were con- ducted at a flow velocity of 90 fps., Reynolds num- her of (L17 X 10" and an ambient pressure of 5G.0 psi. At this velocity and other test conditions, the erosion caused on specimens in venturi is fairly severe and the veloci ty falls in the critical range in which the volume eroded increased rapid- ly with velocity to an average exponent of 15 [7].

The specimens were removed after 20 min. test and their weight determined. The procedure was repeated with the same pair of specimens for eve- l'y 20 min. test up-to a cumulative lime of l 000 min.

by which time the specimens ,vere baclly damagecl and eroded. Before each weighing, the test speci- mens were washed in clean waler, dried with a hair drier and cooled in a desiccator. 1'0 check the variation in weight loss >vith test time and to verify the difl'erence in weight loss when the specimens were tested continuous.}y and intennittentlv, difIe- rent pairs of specimens "."vere also tested fOI: aIl the test periods up-to 440min. at intervals of 10min.

level oil' at any stage. Heymann [5], while pre- senting a critical review of the investigations on this subject proposed a statistical model for the prediction of some of the observed erosion-time curves.

Il is known that the cavitation erosion rate is highly accelerated in a magnetostriction equipment compared to the same that occurs in most hydrau- lic systems. The investigations presented in this paper were conducted in a two-dimensional water- tunnel, causing cavitation behind a circulaI' cylin- der. The erosion ,vas measured on alun1Ïnium whose mechanical properties are given in Table 1.

These investigations revcal that on an average the ratetime curve can be divided into the tluee zones shown in Figure 2. In the final zone, the rate of erosion increased further instead of decreasing as in Figure 2. An interesting finding of this study is that there are numerous peaks and vaIleys in each characteristic zone and is not a simple smooth C1IrVe as observed in the accelerated tests.

Il is generaIly agreed by aIl the investigators that the different characteristic zones of the rate- time Clll'Ve are influenced by the changes in surface roughness caused by cavitation on the test specimen [6'J. However, so far, no systematic measurements of the changes in surface roughness of the test spe- cimen \Vere measured. The present study includes such measurements at 20 minute intervals of test.

The formation and growth of pits caused by cavita- tion and the resulting erosion were also studied macrographicaIly at 5 X and a few macrographs are presented in the paper. Micrographs at 250 x, of the individual cavitation pits with respect to the grain structure of aluminium are also present- ed. These investigations help in explaining the progress of cavitation erosion through the diffe- rent characteristic zones as it occurs in most hy- draulic systems.

The paper also presents an empirical equation for the prediction of the rate of cumulative ero- sion and cumulative erosion up-to a period seven times the, incubation period.

Ultimate tensile strength ...

Fatigue strength .

Brinnell HarcJness .

Strain Energy .

Procedure.

7.0 tons/in² :3.:'') tons/in²

28.0 1,150.0 psi

Experimental equipment and procedure

Equipment.

The experiments were conducted in a two-dimen- sional water tunnel (Fig. 3). The tunnel consisted of a () in. dia pipe-Hne with a pressure regulating lnanifold, a by-pass needle valve, contraction cone, test-section and diffuser. The contraction cone was designed to transform the () in. dia circulaI' cross section to a 4 in. X 1/2in. rectangular sec- tion with a uniform change in pressure and velocity.

The details of the test section with the position of the test specimens and the cavitating body are shovm in Figure 4. Two test specimens were mounted on either wall with the cavitating body, 1/2in. thick, 3/4 in. dia, extending fully over the width of the test section. The front sicle of the test section >vas providecl with a transparent pers- pex coyer.

Analysis of the experimental data

Analysis of the weight loss measurements.

Figure 5 to 7 present the variations of the cumu- lative erosion in mg/m, rate of erosion in mg/min.

and the rate of cumulative erosion in mg/hour.

The rate of erosion in any interval is computed as the loss in weight of the specimens during that interval divided by the duration of the in- terval and is plotted at the end of that interval.

The rate of cumulative erosion is computed as the cumulative erosion divided by the cumulative time. Figure G shows that on an average the erosion rate-time curve can be divided into the tluee zones proposed by Plesset and Devine [2], Figure 2. Ho- weyer, ,vhen the erosion process is not accelerated as in the case of magnetostriction oscillators, each characteristic zone consists of a number of peaks and vaIleys. Also, in the present study, the final zone exhibited erosion at a higher rate than the steady-state rate unlike the decreasing rate report-

LA HOUILLE BLANCHE / N° 7-1969

4/ Sectiomll plan of the water tunnel test section.

Coupe de la section d'essai du tunnel hydraulique.

Contraction cone Diffu&er

Test section Dawn\ stre<.1M

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Test time: in hours 4 5

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of the two-dimensional water tunnel.

Schéma

dll tunnel hydraulique à écoulement plan.

5/^{Cumulalive cavitation}

erosion-lime curve.

Courbe cllmulée

de l'érosion de clwitation en fonction du temps.

6/ Hate of cavitation erosion-time curve.

Courbe du tall:r d'érosion de cIlvitation

en fonction du temps.

16 15 14 13 12 9 10 Il

in hour:!

6 7 a

Te:st time 4 5

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of eumulatiye cayitatioll erosioll-time CUI'Ye.

Courbe d'érosion de cavitation cllllllllée en fonction du temps.

Une on which surface profiles were meaSured

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used for the di al gage.

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Test specimen and eaYitatioll erosion.

Ecllllntillon d'essai et érosion de cavitation.

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« Profilomètre » de meSllre de la rllaosité sllperficielle des échantillons d'essai.

ed by other investigators (Fig. 2). This trend is believed to be the result of the loss in elastic properties of aIl the layers of the comparatively thin (1/8 in. thic1\:) specimens exposed to erosion in the present study. Figure 7 indicates that the rate of cumulative erosion-time relationship is a smooth curve l1nli1\:e that of rate of erosion-time.

Measurement of surface roughness.

GeneraIly, aIl the invesligators are in agreement that the different characteristic zones of the ero- sion rate-time curve are influenced by the changes in the hydrodynamic conditions caused by the va- riations in the surface roughness of the test speci- mens due to cavitation. Rowever, so far no syste- matie measurements of the variations in surface roughness of the test specimens were made by any investigator, mostly because, the roughness caused by cavitation is too large to be measured on com- mercial roughness measuring equipment. Rence, in order to gel detailed information on the varia- tions in surface ronghness caused by cavitation, a profilometer shown in Figure 8 was specially

designed and fabricated by the wrHers. The pro- filometer consisted of two micrometers to give lon- gitudinal and lateral movement to the test speci- men to an accl1racy of 0.001 in. and a dial gage to measure the surface profiles to an accuracy of 0.0001 in. The dimensions of the brass stylus used for the dial gage are shown in Figure 9. "The pro- filometer is hand operated and each measurement is time consumll1g. The surface profiles ,vere mea- sured on one specimen at the end of every 20 min.

cavitation test along a chosen line representing maximum damage characteristics shown in Fi- gure 10.

Figure 11 presents some of the representative surface profiles of the test specimen at different test dl1rations. Il may be seen that in the first 60 min. there are hardlv a few indentations and pHs have just started forming. Towards the end of the incubation stage and innnediately foIlmving lhat (100 min. lo 160 min.) the pits grow in size and depth rapidly. As the test duralion increases fur- ther (240 min. to 280 min.) a number of thin peaks and deep and wide valleys are formed. AIso, some

LA HOUILLE BLANCHE / N° 7-1969

of the peaks, A, 13, C, D and E that appear at any interval vanish in the succeeding interval and these con tribu te to the loss in weight of the specimen.

Figure 12 shows to an exaggerated scale a cross section of the aluminium lest specimen subjected to damage at a tesl lime of 400 min.

Macroscopic observations.

The formation and growth in size and number of pils caused by cavitation and their relative posi- tions on the test specimen was studied macrosco- pically at 5 x. Two such typical macrographs at test times of 170, and 200 min. are shown in Figures 13

and 14. After 30 min. test it was observed that the pits have just started forming at isolated loca- tions. At 170 min., Figure 13, the pits are very weil formed and there are a few over lappings of adj acent pits. As the test time increased (200 min.

and above) the pits increased considerably in size and there are many overlappings of adjacent pits.

From the study of the variation in loss of weight and the variation in surface profiles of the test spe- cimens and the macroscopic observations of the pitting it is possible to visualize the process of malerial erosion as the cavitation test duration in- creases. At the beginning of the test when the sur- face of the test specimen is smooth and fiat il of-

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r =

The rate of erosion during any test interval is found to he:

For aluminium used in the present studies Equa- tion (3) can be written as:

(6)

(1)

(5) (4) (2)

( 1

)IJ'"

y=a

a

(

1 ) ^0.76' rc=120

12Ü

,- (1 \

l

AW=_120 120) .• -.1 .. t

20)0.7G', - 1 _." 120(l 20)O.7G', - 'Il t,

'.~.::::_::'_'--=.!.- .~"::::":::'.' __ ..

t_{2 -} t₁

in which suhscripts 1 and 2 refer to quantities at the two instants of time. Substituting for Aw from Equation (4) in Equation (5) yields:

,- 1V'

l

Aw

=

=

(.0.) -

in which, a and b are coefficients; and x and y are the ahscissa and ordinate respectively.

The coefficientsa and b were determined by trial and error from the experimental data of Figure 7 and Equation (1) conforming to the présent study was found to be:

Equations (4) and (6) are shown in Figures!) and () respectively. Il may he seen that the cumulative erosion is predicted fairly weil hy Equation (4) up-to a periotl of 6-7 times the incubation period.

Figure 7 also presents the variation of the rate of cumulative erosion with test time for the expe- rimental data of Hohbs et aL [3J for brass and high tensile brass. It mav he seen that Equation (1) fits the data with reasonable accuracy. The values of the coefficients a and li in Equation (1) for these two materials are given in the figure. Figure 5 pre- sents the variation of cumulative erosion with time for these two materials also. A simi1ar extensive analysis by the wrHers of the experimental data of dicled. The writers have attempted to define the rate-time curve, Figure (j, through a series of sine functions and the rate of cumulative erosion-time curve, Figure 7, through an empirical equation.

The latter aUempt was more successful than the former and the shape of the rate of cumulative erosion-time curve was found to conform to that of the Gompertz Clll'Ve, generally used in statistics.

The curveis of the fOrIn:

1I1 which, r_c = rate of cumulalive erosion in mg/

hour; and t= test time in hours. Equalion (2) was found to he reasonahly accu rate lJl)-tO a period of 6-7 times the incuhation period. The cumula- tive erosion (Aw/mg) with test time is ohtained from Equation (1) as:

One of the im])ortant ilsnecfs of the investig:1tions of the nature ])resented in this napel' is to lmow whether the cavitation erosion of a given material under given hydrodynamic conditions ean he pre- Microscopie observations.

Tt is genera11v known thal cavitation nitting is influencec1 bv al'ain size fI. 41 of the sDecimen and the locntion of pHs is affecled hv the grain hou nd- aries r51. The writers made 1ll.icroscoDic ohserva- tions [d 100x ~T'rl ?sl)x of the IO("ltion s of the nits with resnpc1 io thp flT:lin houndaries at diffe- l'ent levels from the snpcimen surface. The speci- mens were T'olish('(] anf] etched with concentrated Kellpr's etch for Ihese observations. Figures 1!) and Hj' show some tvnicnl c'witation pits ohserver] at the samp 111no'niflc'1tion 'lt a denth of 6.2 m111 from the originnl sllrf?ce. The pit houndnrie<: are verv irrp'wlar nnd are located randomlv with respect to the grain stnlcturp of the specimen.

fers an unfavourable surface to the pressure waves associated with the collapse of cavities. As the spe- cimen surface is repeatedly attacked by the pres- sure waves it starts loosing i ts elastic properties and small indentations much similar to those fOrIll- ed by a hardness indentor are formed. However, at the very beginning of the lest there is a sm aIl weight loss due to the erosion of some of the weak spots on the surface of the specimen. Thereafter, theweight loss is almost zero for a certain period and this is termed as the "incubation period". Towards the end of the incubation zone the specimen sur- face looses much of its elastic properties and plas- tic flow of the material sets in rapidly resulting in the formation of pits with peaks and valleys.

At this stage the specimen ollers a very favourable surface for erosion to oCCUI' and hence the rate of erosion increases to a peak value over a length of test period. This is the accumulation zone. Artel' this, the formation of peaks and valleys and the erosion of the peaks proceeds simultaneously ,vith the weight loss of the specimen exhibiting more or less a steady rate with test time. If the tests are accelerated or if the observation of weight loss in specimen made at la l'gel' intervals of lime, a11 the three zones of the rate-lime curve would appear as smooth curve" ohserved by some investigators r2. 31. On the contrarv. if the tests are not accele- rated and the observations made at close intervals of time, aIl the three zones of the rate-lime curve, Figure 6, show a number of peaks and valleys indi- cating non-uniform erosion rate even in any cha- racteristic zone.

Also, from a studv of the surface profiles of the test specimen (Fia. 11), the material erosion in the present investiaations can he said to he laking place in the fo11üwing ,vay: when peal,s and vallevs are formed on the specimen surface. the peaks arp chipped off dne Lü the shearing action of the radinl outflow of thr liauic1 from the adïacent pits.

A similar ohserv:üion was reported hv Bowden and Brunton rS1 :llso while investigating the erosion caused by the high speed impact of liquids.

Prediction of c.avitation erosion

LA HOUILLE BLANCHE 1N° 7-1969

t~400mini.

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i5·078

.074 _

Length in inches

12/ Cross-section of the aluminium test specimen at 400 mins.

Section transversale de l'éclwntillon d'essai en aluminium au /Jout de 4-00mIl.

13/ l\lacl"ogl"aph of cavitation pitting at tcst timc of 170 mins.

Macrographie des cratères de cauilation au bout de 170mIl.

15/ l\liel"ogl'aph of individual cavitation pit (250 x, etched).

iHicro(frafJhie d'Ull cratère de cavitatioll (attaqué li l'acide, (frossi 250 fois).

14/ l\lacl"ogl"aph of cavitation pitting at test time of 200 mins.

Macrographie des cralères de cavitation au bout de 200^mIL

16/ :\licl'ogl"aph of individual cavitation pit (250 X, etched).

JJicro(fraphie d'un cratère de cavitatioJl (attaqué li l'acide, (frossi 250 fois).

719

olher invesligalors shows lhat Equalion (1) defines the l'ale of eumulative erosion with time with de- pendable accuracy up-to the end of the sleady-state zone. More experimental data to determine the dependcnce of lhe coefficients a and b in Equa- lion Cl) on the mechanieal properties of the mate- rials and the hydrodynamic condilions of the flow will be useful lo the profession.

Equation (7) can be used for predicting cavitation erosion under similar hydrodynamic conditions up- lo a period of G-7 times the incubation period.

Bibliography

Conclusions

]. The cavilalion erosion rate-test time curve of aluminium lesled in a venturi set-up exhibited lhrec characleristic zones, namely, incubation, ac- cumulation and steady-state zones. In each zone peaks and valleys sh(y\~ringnon-uniform rate of ero- sion were found unlike lhe smooth curves in the accelerated tests.

2. The measurements of lhe test specimen sur- face profiles and macroscopic observations of cavi- lalion pilting revealed that during the incubation

pl~riod the specimen surface exposed to cavitation

was damaged to a shape favourable for erosion.

In the acclllllulation zone the specimen lost its elaslic properties and plaslic fiow occurred rapidly with erosion rate increasing to a peak. In the sleady-slate zone plaslic fio"w of the material around pits and the erosion occurred simultaneously at more or less a constant rate.

:3. :Microscopic observation of individual cavita- lion pUs with respect to the grain boudaries of lhe specimen showed that they were randomly locat- cd and the boundaries of the pits were very irre- gular.

4. The variation of cumulative erosion \vith test Lime observed in the present investigations could be defined by an equaLion of the type:

[ ( 1 )^{0 7G'}

J

Aw= ]20 - . - 1 ... t

_ \120 . (7)

[1 J EISE;>;BEHG (P.l, PHEISEH (H .S.) and THll\UVE;>;GADAM (A.).

- On the lv[eehanism of Cavitation Damage and i\le- thod S of Protection. Presented at the Al1nual Mee.ting of the Soclety of Naval Arehiteets and Marine Engi-

neers (Novemller, 1B(5).

[2] PLESSET CILS.) and DEVI;>;E (ILE.). - Etreet of Exposure Time on Cavitation Damage. Journal of Basic En- gineering, Trans. ASi\lE, Vol. 88 D, No. 4 (Decemher 1HliG), pp. GBl-ïO.'5.

[:li HOBBs (.T.i\L), LAlIW (A.) (Miss) and Bn\J;>;To;>; (W.C.). - Laboratorv Evaluation of the Vibra tory Cavitation Erosion T·cst. National Engineering LahZn'atory, East Kilbride, Glasgow, Heport No. 2ï1 (.Tanuary l%ï).

[4.J IL"Dll'l''l' (F.G.), HOBl;>;SO;>; (M.J.), SIEBE1\T (C.A.) and

AYDl;>;~L\KI;>;E(F.A.). - Cavitation Damage eOlTelations for various Fluid-Material Comhinations. Heport No. O:J424-H-J, Dept. of Nuclear Engg. Univ. of Michi- gan (Octoher 1B(4).

[5i BEnIA;>;;>; (F ..J.). _ ..- On the Timc Dependence of the Hate of Erosion due to Impingement or Cavitation. Erosion Ily Cavitation 01' impingement, ASTM STP 408, (lB6ï), pp. ïO-lOB.

[6] SEE'l'HAHA:-IL\H (lU, LAKSH:-IA;>;A Iüo (N.S.) and SYAMALA Iho (B.C.). - Discussion of "Glen Canyon Dam Diver- sion Tunnel Outlets", hy '''.E. '''agner, Journlll of tile Hydmulics Division, ASCE, Vol. B4, No. HY 4, (.Tuly 1%8), pp. 1156-1158.

[ï] SYA~IALA HAO (B.C.) LAKSIDIA;>;A HAO (N.S.) and SEETHA- n.UIL\H (K). - "The effeet of Flow Veloeity on Cavi- tation Erosion". P"esented at the :J8th C.B.LP. AnnunI

Meeting held at Bangalore (.June IB(8).

[8] BOWIlE;>; (F.P.) and Bnu;>;T();>; (.T.H.). - The Deformation of Solids hy Liquid Impact at Supersonie Specds.

Proceedin{Js Royal Society of London, A, Vol. 2G:l (H)Gl), pp. 4:\:J-450.