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Wear, 75 (1982) 1 - 20 1

RUB ENERGETICS OF COMPRESSOR BLADE TIP SEALS*

W. F. LAVERTYPratt and Whit ney Ai rcraft, East Hartford, CT 06108 U.S.A.)(Received May 12,198l)

SummaryThe rub mechanics of the abradable blade tip seals of aircraft gas turbine

engine compressors were studied under simulated engine conditions. Intwelve statistically planned instrumented rub tests using single titaniumblades and fibermetal rubstrips, five basic rub parameters were varied todetermine their effects on the rub energy, the heat split between the blade,rubstrip surface and rub debris and the blade and seal wear. The rub energieswere found to be most significantly affected by the incursion rate while rubvelocity and blade thickness were of secondary importance. In five additionalrub tests using single nickel alloy blades and multiple titanium alloy blades,rub energy and wear effects were found to be similar for titanium and nickelalloy blades while rub energies increased for multiple blades relative to single-blade test results.

1. IntroductionIn modern aircraft turbine engines, effective gas-path sealing is a primary

factor in meeting objectives for superior performance and fuel economy. Themove to higher compressor pressure ratios to improve cycle efficiency hasalready made the current generation of large high bypass ratio turbofanengines extremely sensitive to changes in compressor blade tip operatingclearances. An increase in clearance as small as 0.25 mm can impose apenalty greater than 0.5% in fuel consumption in these engines. As pressureratios increase in the next generation of engines to meet objectives for energyefficiency, this effect will become even more pronounced [l] .

The ability to withstand rubs between the blade tips and the matingshrouds caused by normal operating deflections of the engine with a minimumincrease in the tip gap is essential to the achievement of minimum operatingclearances. To this end, the static shrouds in modem gas turbine enginesincorporate surface layers of abradable seal materials. In the past, develop-

*Paper presented at the International Conference on Wear of Materials 1981, SanFrancisco, CA, U.S.A., March 30 - April 1, 1981.0043-1648/82/0000-0000/ 02.50 0 Elsevier Sequoia/Printed in The Netherlands


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2ment of such materials has been a highly empirical process. Materials withreduced high temperature strength, such as epoxies, elastomers and lowmelting point alloys, and materials having porous structures, such as sinteredmetallic powders or fibers and plasma sprayed systems, have been employedto achieve favorable rubbing wear ratios (e.g. refs. 2 - 4). Since particulateerosion caused by ingestion of dirt during ground operation can cause severewear of low strength seal materials a careful balance between erosion resistanceand abradability must be designed into the seal material. In practice, metallicabradables which are used to meet the environmental temperatures in therear compressor stages have posed the most difficult development problem.

If abradable materials which will meet the aggressive performance goalsfor future energy efficient engines are to be developed it is clear that a basicunderstanding of compressor seal rub mechanics will be required to guide thedevelopment. Prior work provides some needed insight for the problem.Studies reported by Bill and Shiembob [ 51 and Bill and Wisander [ 61 showedthat for porous metallic seal systems the occurrence of low blade wear isaccompanied by clean cut abradable surfaces and no evidence of rub heatgeneration, whereas rubbing which produces high blade wear yields densifiedabradable surfaces and much evidence of heating. The desired materialremoval mechanism for these abradables is fracture of the bonds betweenparticles or fibers. When particle-to-particle bonds are not broken by the rubforces, the resulting deformation of the structure produces an increase in theheat generation within a very thin plastically deformed layer at the abradablesurface. Under some circumstances when repeated rubbing does not removethis material, a further increase in surface densification and heat generationwith attendant glazing occurs, as shown in Fig. 1. These observations haveled to the conclusion that small variations in the rub mechanism can lead to

Fig. 1. High pressure compressor rubstrip showing a glazed rub condition.


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3TABLE 1Range of parameters affecting compressor blade tip seal rubbing wearRub velocityInfusion rateDepth of rubCircumferent ial rub l engt hNumber of rubbi ng bl adesSpacing betw een rubbi ng bladesBlade thicknessEnvironmental tempemture

100 - 300 m 23-l0.0025 - 1.5mm 6-l0-2mm25 - 4000 mm1 - 0025 - 1000 mm0.25 - 1.5 mm20 - 600 C

Seal materials: epoxies, elastomers, sintered metallic fibers, sinteredand plasma-sprayed metallic particles; blade materials: nickel, steel andtitanium alloys.large differences in rub energy dissipation; this causes rub energy to be anexcellent measure of the mode of wear and indicator of incipient changefrom one wear mechanism to another.

While such work has provided a conceptual framework for analysis ofcompressor seal rub mechanics, additional data are needed to enable formula-tion of a comprehensive model. Ex~ination of the number of independentvariables involved and the wide range of conditions under which rubbing canoccur during engine operation provides an appreciation of the scope of workrequired in such an endeavor. A list of the pertinent variables together withrepresentative ranges for each are presented in Table 1. The combination ofvariables in a particular rubbing event depends on the type of incursion;some incursions are produced by shifts of the rotor into the case due toaircraft maneuvers, gust loads or gyroscopic moments and other incursionsresult mainly from radial growth of the rotor st~cture under the influenceof dynamic and thermal effects, In the face of so complex a problem it seemsevident that a systematic approach as advocated by Czichos { 71 will berequired if meaningful results are to be obtained. In the present work such astudy of the effects of the variables shown in Table 1 on rub energetics andseal rubbing wear was initiated.

2. Experimental approachIn establishing an experimental facility to investigate engine rub phe-

nomena, it was desired to model all the important rub variables. The testfacility selected for the investigation is a subscale abradability test rig; this isshown schematically in Fig. 2. In the operation of the rig a stationary rubspecimen which is mounted on a translating carriage is fed radially into amoving rub specimen mounted on the rotor. Cons~t rub velocities aremaintained automatically by a turbine governor system, controlled incursionrates are provided by carriage movement regulated by a variable motormicrometer feed system and, at the end of a test, automatic carriage with-


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Abradable Coated Disk

ncursionInstrumented

Turbine TorqueneterBearing PyrometersSupport

Fig. 2. Schematic of rub energetics test apparatus.

drawal, provided by a limit switch-air piston system, serves to terminaterubbing rapidly to prevent obscuring metallographic data with spark-outrub effects.

In the standard rig test set-up a static rubstrip segment is translated at acontrolled rate into a bladed rotor held at a constant surface speed. The rubthen occurs as a series of intermittent pulses as each blade contacts theshroud. Because of the high wheel speed and relatively short rub length, theduration of each blade contact is of the order of lo-* s. This situation makesenergy measurements, specifically those involving force and blade tempera-tures, very difficult to obtain. To overcome these problems the basic rigset-up for rub energy testing was inverted; the rubstrip material was appliedas a coating on the rim of a rotating disk 127 mm in diameter and the instru-mented blade or blades were fixed to a stationary holder.

In order to cover the desired range of test parameters the program wasdivided into two parts. An initial series of tests was conducted coveringvariations in rub velocity, incursion rate, incursion depth, blade thickness andabradable strength while in the second phase of the program the blade materialand the number of blades were added as variables. The blade materialsselected for the program were titanium 8-l-l (AMS 4916) as the primarymaterial and Incoloy 901; these are both commonly used in current-generationengines. For the abradable material the decision was made to employ onlyone well-defined material and to study the effect of abradable properties byvarying the strength of that material. Because of its uniformity and repeatabil-ity as well as its widespread use in current engines, Hastelloy X Feltmetal@fibermetal (Brunswick Corporation Technetics Division) was selected. In thefirst phase of the work a statistical test plan employing 12 tests was usedto establish the linear effect of each variable on rub energy and wear and toinvestigate possible interactive effects between variables. On the basis ofthese results test conditions were established for the second series of tests toexplore the effects of multiple blades and the additional blade material in aregion of observed high rub energy. The specific test conditions for each of


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5

L--Abradable Density :


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6Because the torquemeter which was used to measure the rub torque was

located in the drive shaft it was necessary to correct the torquemeter datafor inertia effects using a highly accurate rotational speed counter and toeliminate drive shaft windage and friction by pretest tare torque readings.The thermocouple array used for blade heating effects consisted of six(Pt-Pt)-lO%Rh thermocouples 0.08 mm in diameter arranged on the backsurface of the test blades as shown in Fig. 4. The pyrometers used for anindication of the abradable material heat-up were focused at two positionson the disk, at 30 from the rub zone in the direction of rotation and at 180from the rub zone. All instrumentation was read out continuously onoscillographs.

The data analysis for each test consisted of four distinct operations:review of the videotape record; wear analysis; metallographic analysis; rubenergy data reduction and analysis. The videotape review provided qualitativeinformation about the level of sparking and visible heating in the rub zoneand the variation in the intensity of heat release through the duration of thetest. A shadowgraph technique, supported by sectioning and photomicrog-raphy where necessary, was used to document post-test seal and blade wearincluding transfer of seal material to the blade tip. Metallographic analysiswas completed on selected blades and all abradable seals to identify changesin material structure and to determine the presence and constituents of anymaterial transfer.

Rub energy data reduction and analysis were carried out to determinetotal rub energy, interface temperature and the heat split between blade,abradable and wear debris. The total rub energy e was computed as the

0.08mm DIA.PTiPT 10 Rh

TIC

Fig. 4. Instrumented rub test blade.


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8the disk and that carried away by both blade and seal debris. The heat lost toblade debris was determined by calculating the amount of heat stored in thematerial that is worn from the blade tip:

tqbd = f JGCb Ti - To) dt

0Because the seal surface is porous and the seal material removal process

involves subsurface fracture of the fiber bonds, the instantaneous contactarea between the blade tip and the seal material is only a small fraction ofthe blade tip area and the bulk temperature of the seal debris is significantlyless than the interface temperature. For this reason, the approach selectedfor determining the heat to the disk and the heat to the seal debris employedboth a first-law analysis of the total system and a transient conductionanalysis of the disk surface. Using a one-dimensional semi-infinite bodytransient conduction analysis given by Carslaw and Jaeger [ 81 it was foundthat, because of the low thermal diffusivity of the abradable and the highdisk rotation speed, the periodic effects that occur once in each revolutionare felt only in the top 0.2 mm of the abradable layer. In addition, examina-tion of the decay of surface temperature with circumferential distance fromthe contact location using a moving heat source solution [9] showed thatthe periodic surface temperature effect drops to less than 15% of its maximumvalue in a half-rotation owing to only the conduction of heat into theabradable. On the basis of these estimates it was concluded that the averageheat into the disk could be calculated with adequate accuracy by treating thepyrometer data at the 180 position as if it were the result of uniform one-dimensional constant heating on the entire periphery of the disk. In thecalculation the thickness of the abradable was taken as its final value and thedisk under the abradable was assumed to remain at its initial temperature.On the basis of these assumptions, and with the appropriate solution givenby Carslaw and Jaeger [lo] , the average heat into the disk is obtained:

qa = T.T,) y 8 = 1 -a, 2n + 1)27r2t2 -Il- ;; nF :, 2n + 1)2 exp 4L4)

Convection of heat from the surface of the abradable was calculatedusing the equation

qc = Ws Us - To) 5)where the heat transfer coefficient was estimated from correlations given byKreith [ 111 for heat transfer from rotating disks and cylinders and theaverage surface temperature T, was again taken to be that recorded by thepyrometer focused on the 180 position of the disk. Finally, the heat lost inthe wear debris was estimated by summing the remaining component terms(heat to the blade and blade debris, conduction into the abradable and


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9

convection from the abradable surface) and subtracting the sum from thetotal rub energy:

Qad = e - (qb + qbd + Qa + a,) 6)and the corresponding debris temperature was calculated from the equation

tad = To + qad PaCaSaAs (7)

3. ResultsThe data collected for each of the tests fall into one of two categories,

quantitative data, such as rub forces and temperatures, and qualitative data,such as the visual test results and metallographic data. Because of the use ofa statistical test plan for the first 12 tests the approach taken to analyzethe data was to develop correlations of the behavior of the quantitative rubenergy parameters with the independent test parameters and then to use thequalitative data to develop phenomenological understanding of the observedeffects.Quantitative results for 11 of the first 12 tests are presented inTable 2; test 6 was eliminated owing to a problem which caused excessiverub velocity variation during the test. Representative data showing thevariation in the dependent rub parameters throughout one test are presentedin Fig. 5. While the minor peaks in the variables evident in the data weretypical of all the tests, the timing and duration of such peaks varied greatlyfrom test to test and, for the two higher incursion rates investigated, werenot dominant features in the data. However, at the lowest incursion rate(0.0025 mm s-l) the intermittent nature of the rub produced extremelyirregular data such as those shown in Fig. 6. In order to characterize both themagnitude of and variation in both types of data, it was decided to employtime-averaged values and peak values i.e. the maximum value which occurredduring the test) of the total rub energy and the heat to the blade as dependentvariables in the data correlations.

The energy quantities not shown in Table 2 are the heat loss to theblade debris, the heat conducted into the abradable and convected from theabradable surface and the heat loss to the abradable debris. The heat loss tothe blade debris, as determined using eqn. (3), was found to be less than 0.1%of the total rub heat except for test 9 for which it represented 1.2% of thetotal rub heat and test 12 for which it was 0.4% of the total. Calculation ofthe remaining three rub energy terms was only possible for the three tests inwhich the pyrometer at the 180 position on the -rub&rip exceeded itsthreshold temperature. For these tests (1, 3 and 9) the results obtained usingeqns. (4) - (7) and presented in Table 3 show that all three terms are sub-stantial when compared with the total rub energy. The negative values


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Fig. 5. Typical rub energy data for moderate and high incursion rates (test 5).computed for the heat loss to the abradable debris can only be explained bythe presence of an additional heat source. Because of the obvious pyro-technics accompanying these three tests, oxidation of the blade debris andseal debris was recognized as a likely source of heat. Estimates made of thepotential order of magnitude of exothermic oxidation showed that thetitanium blade debris, because of its small quantity, could account for nomore than 10 W for test 3 and 100 W for test 9 while oxidation of theabradable material could easily supply more than 1000 W for either of thesetests.

In developing correlation equations for each of the dependent param-eters shown in Table 2 with the independent test variables, linear regressionanalysis was employed using the principle of least squares. A linear depen-dence on the independent test parameters was found to be suitable for allthe variables except the incursion rate for which a logarithmic dependenceprovided better correlation. In order to facilitate comparison of the relativeimportance of the independent variables the independent correlation param-eters were normalized so as to represent the test range of each variable bythe band - 1 - +l . The resulting mathematical model is then given by theequation :


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?=Bo +B1 log i -log 0.025 6 - 0.75+B2 +B V-182.53 +log 10 0.25 30.5

b - 1.145 D - 17.5+& 0.635 +& 1.5 8)Regression analysis of the data was carried out for each of the rubenergy and wear parameters. The results of these analyses are presented in

terms of the regression coefficients in Table 4. The dependent wear variableadjusted blade wear is the measured blade wear normalized to a bladethickness of 0.5 mm on the basis of the volume of blade material lost. Thesecond (expanded) regression analysis shown in Table 4 for the rub energyand temperature terms was carried out because it was recognized thatcontinuous recording of these data made it possible to gain additional datapoints by treating the first half of a 1.0 mm incursion test as a 0.5 mmincursion test. In the tabulated regression coefficients presented in Table 4


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the asterisked (*) items indicate that these variables are statistically importantat the 95% confidence level. The question mark (?) terms are marginallyimportant at that confidence level. The remaining terms are of little impor-tance in explaining the variation in the dependent variables, at least for themodels selected.In addition to the regression coefficients, statistics for each predictionmodel were computed and are presented in Table 4. The coefficient r ofdetermination is a measure of the proportion of the variation in the depen-dent variable which is accounted for by the model. Since the standardtable values, based on 95% confidence, are 56.9% for 11 test points and30.6% for 17 test points it is evident that all the correlations were significant:total rub energy is the best, blade heating has an intermediate significanceand blade temperature and blade wear have the poorest significance. Thestandard error of the estimate (SEE) is a measure of the magnitude of theerror in predicting the dependent variable while the repeatability error factoru is a measure of the ability of the test result to be repeated in test replica-tions (test l-test 5 and test 2-test 10). In the average total rub energy andpeak heat to the blade correlations the high value of u relative to the SEEtogether with the high value of r indicate that most of the error is due toexperimental error. The lower relative values of u and/or lower r values forthe other two energy parameter correlations suggest that a greater number oftests might be useful for defining additional functional relationships toimprove the correlation. The comparable values of u and the SEE with lowervalues of r suggest that basic effects, such as stability factors which governthe occurrence of transfer, may be lacking from the model and the data.

In general, the results of the statistical test plan and regression modelsare seen to be very successful. Throughout the correlations, incursion rate isthe strongest factor in the rub phenomenon. Disk rim speed and bladethickness are substantially lower in influence but still show a significanteffect; the effect of disk speed on interface temperature and possibly bladewear is particularly noticeable. Incursion depth and abradable density werefound to have minimal effect except for the influence of abradable densityon blade wear. Of particular interest is the relatively minor change in thecoefficients which resulted from including the extra 0.5 mm data pointsfrom the deep rub tests. This rather large change in the sample size wouldhave had a significant impact on the equations had the data been of lowerquality. The examination of the interactive effects of two combined variableswas completed, and none of these two-factor correlations was found to besignificant. It should be noted, however, that the lack of importance of thesetwo-factor interactions may be due to an insufficient number of tests.

In support of these results, the metallographic data provide somenoteworthy features. In six of the tests a build-up of transferred materialoccurred on the blade tips. This material, a typical cross section of which isshown in Fig. 7, was identified by X-ray emission spectroscopy analysis to bea mixture of Hastelloy X fiber-metal seal material and particles of the cementused in instrumenting the blades. The presence of the cement, which was


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16

Fig. 7. Transfer material deposited on a portion of the blade from test 12

only employed on the back face of the blade, was indicative of transfer ofblade material onto the seal and then back to the blade. Examination of thedata shows that transfer occurred on none of the low incursion rate tests, onall the moderate incursion rate tests and on half the high incursion rate tests.One of the high incursion rate tests, test 12, exhibited transfer over onlyone-fourth of the blade tip with blade wear and no transfer over the remainingthree-fourths of the blade (a portion of this blade is shown in Fig, 7). Themetallographic analysis also showed the effect of high interface temperaturesin two of the high incursion rate tests, tests 9 and 12, as revealed by thepresence of a fine cy platelet grain structure at the rubbed surface of the blade(Fig. 8). Such a grain structure is indicative of metal temperatures muchgreater than 1300 K followed by rapid quenching. From these results aprogressive transition in the rub mechanism is postulated from fracture-dominated rubbing at the lowest incursion rate to plastic shearing in theinterface region for moderate incursion rates and finally to extensive plasticdeformation at high incursion rates with thermal effects dominating.

Visual inspection and metallographic analysis of the abradable rubsurfaces also support such a finding. In all the low incursion rate tests and ailbut one (test 11) of the moderate incursion rate tests, the rub surfaces werecleanly abraded and no densification of the rub surface was evident. In allthe high incursion rate tests the post-test abradable surface had a smeared orglazed appearance and the metallographic sections showed surface densifica-

Fig. 8. High rub interface temperatures revealed by grain structure in the blade (test 9).


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17

a) b)Fig. 9. (a) Abradable glazed and densified surface for high rub energy test (test 9) and(b) clean-cut surface for low rub energy test (test 10).

tion of the abradable. Representative photographs showing these differencesare presented in Fig. 9.

In addition to the 12 statistically planned tests, three tests wereconducted to investigate the effect of a change in blade material to a typicalcompressor nickel blade alloy (Incoloy 901) and two tests were conductedto investigate the influence of multiple titanium blades. The quantitativeresults for these tests are presented in Table 5. It can be seen by inspectionof Fig. 3 that the selection of test conditions for these tests makes possibledirect comparison with the single-titanium-blade tests. Several noteworthyeffects are seen from the comparison. Somewhat surprisingly, the occurrenceof transfer of seal material to the blade tip was not affected even by the verysignificant change in blade material composition. While temperatures recordedfor the nickel alloy tests were lower, corresponding to its lower meltingpoint, the rub energies were comparable for the two materials and the highwear rate experienced with the titanium blade in test 9 was repeated withthe nickel blade in test 17. For the multiple titanium blades the two testsshowed that distributing the rub between closely spaced blades (the cir-cumferential spacing was approximately 25 mm) does not reduce rub energyas might be expected from the extrapolation of single-blade results. Rubenergies were higher for the multiple-blade tests than for the corresponding


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19single-blade tests and blade temperatures and wear results were not signifi-cantly different.

4. ConclusionsA number of conclusions can be drawn from the work regarding both

the rub energetics of compressor seal systems and the test method employed.The statistical test approach was found to be very effective and permittedestablishment of the main linear effects. Incursion rate was found to be themost important independent variable, the rub velocity and blade thicknessshowed moderate importance and the incursion depth and abradable densityhad little effect. Three distinct wear modes were observed: a low energy lowblade wear mode at low incursion rates; a transfer mode in which sealmaterial transferred to the blade tips at moderate and high incursion rates; ahigh rub energy high blade wear mode which occurred at high incursion rates.A change in blade material from titanium alloy to nickel alloy did notproduce significant changes in either blade wear or the occurrence of transfer.Testing with multiple blades resulted in higher total rub energies and producedonly a moderate reduction in the heat load per blade compared with single-blade tests conducted under the same rub conditions.

AcknowledgmentsThis program was sponsored by the National Aeronautics and Space

Administration, NASA Lewis Research Center (Program Managers, Mr.Lawrence Ludwig and Dr. Robert Bill). The author is indebted to Mr. PaulDziomy and Mr. Gary ODell for conducting the test program and analysis ofthe data, to Mr. Frederick Dauser for statistical analysis, to Mr. Arnold Grotfor metallographic analysis and, in particular, to Mr. William Otfinoski foroverall program coordination.

Nomenclature

kLP

areablade thicknesscorrelation coefficientspecific heatabradable density (percentage solid)rub energymass flux of blade due to wearconvection heat transfer coefficientincursion ratethermal conductivityabradable thicknessperimeter of blade


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20

TV;Y;P

heat loadcross-sectional area of bladetimetemperaturerub velocitydistance from rub interfaceindependent rub variabledependent rub variablethermal diffusivityrub depthdensity

Subscripts;

abradableblade

Elconvectiondebris

i interfaceambient

S surface

References1 R. C. Kingcombe and S. W. Dunning, Design study for a fuel efficient engine, ASME

Paper 80-GT-141.2 Abradable seals for gas turbines, Tur bomach. I nt., 20 (March 1979) 53 - 60.3 Jet engine seal materials evaluated, Auiat. Week Space Technol ., 104 (June 7, 1976)

59 - 60.4 F. J. Hermanek, Jr., Coatings lengthen jet engine life, Met. Prog., 97 (March 1970)

104 - 106.5 R. C. Bill and L. T. Shiembob, Friction and wear of sintered fibermetal abradable seal

materials, J. Lubr. Technol., 99 1977) 421 - 427.6 R. C. Bill and D. W. Wisander, Friction and wear of several gas-path seal materials,

NASA Tech. Publ. 1128, January 1978.7 H. Czichos, A systems analysis data sheet for friction and wear tests and an outlinefor simulation testing, Wear, 41 (1977) 44 - 55.

8 H. S, Carslaw and J. C. Jaeger, Conduction ofH eat in Solids, Clarendon, Oxford,1959, p. 76.

9 H. S. Carslaw and J. C. Jaeger, Conduction of Heat in Solids, Clarendon, Oxford,1959, p. 269.10 H. S. Carslaw and J. C. Jaeger, Conduction of Heat in Solids, Clarendon, Oxford,1959, p. 113.11 F. Kreith, Pri nciples of Heat Transfer, International Textbook Company, Scranton,1958, pp. 324 - 327.

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