STRUCTURAL, THERMOPHYSICAL AND MECHANICAL CHARACTERISTICS OF METAL FELT WICKS OF MODERN HEAT PIPES

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1 7th International Heat Pipe Conerence (7 th IHPC) Kanpur, India, October 3-7, 3 STRUCTURAL, THERMOPHYSICAL AND MECHANICAL CHARACTERISTICS OF METAL FELT WICKS OF MODERN HEAT PIPES Gershuni A, Zaripov V National Technical University o Ukraine Kyiv Polytechnic Institute, Heat Pipe Lab 37 Prospect Peremogy, Kiev 356, Ukraine Phone: (+38) , Fax: (+38) rial@ukrpostua, zaripo@ukrnet Baturkin V National Technical University o Ukraine Kyiv Polytechnic Institute /Institute o Space Systems, Bremen, Germany Robert-Hooke-Str 7, 8359 Bremen, Germany Phone: (+49)-4-446, Fax: (+49) baturkinvm@mailru ABSTRACT The article relects the results o many years researches, begun in 97, o structural, hydrodynamic, thermophysical and mechanical properties o metal elt capillary structures (MFCSs) o heat pipes (HPs) These structures were made rom the discrete sintered ibres with length - m by a diameter rom μm to 7 μm o a copper, stainless steel, or nickel They were intended or HPs with shells o a copper, stainless steel, nickel, titan, nickel silver, ceramics and heat carriers: nitrogen, ammonia, water, ethanol, methanol, acetone, sodium and others HPs worked in the range o temperatures rom a cryogenic level (- 9 о С) to high temperatures (+ 8 о С) Analytical and experimental researches are executed by authors rom the National Technical University o Ukraine Kyiv Polytechnic Institute (Kyiv, Ukraine) under the leadership o Dr, Pro Semena M On the basis o the conducted researches the unctional dependences are obtained or implementation into engineering calculations o heat transer characteristics o HPs The proposed methods o the rational HP design with the use o MFCSs and elaborated technology o their abrication allowed to create and implement into practice a variety o heat pipes and systems on their basis or dierent ields o application KEY WORDS: heat pipe, metal ibres, capillary structure, characteristics INTRODUCTION Application o MFCSs as capillary structures (CSs) o HPs allows creating o such designs o HPs, which are oten impossible to realize on the basis o other types o CSs (rom metallic screens, powders, grooves and others) It is especially important at making some speciic demands on HPs: operation against gravity, necessity o applying o CS to the suraces o complex shape, necessity o deormation o HP ater its abrication and others Advantages o CS rom discrete metal ibres over other types o CS are conditioned by a number o their unique properties So, MFCSs have a very wide range o eective porosity: rom 5 (and less, i necessary) to 96 (and higher) They are characterized by the wide range o diameters o pores: rom minimum (5 μm and less) to maximal (3 μm and more), and have no closed pores (ig ) This allows to reach the optimum combination o basic hydraulic characteristics o MFCS, namely, high coeicient o permeability and high available capillary head Optimum hydraulic characteristics o MFCSs in combination with their high enough rame heat conductivity (rom 5 to 5 W/mK) provide higheicient heat engineering characteristics o HPs: low thermal resistance R hp and high maximal heat transer ability Q max a) b) Figure : Appearance o metal elt capillary structures (magniication in 75 times): ibres rom a copper by a diameter o μm (a), rom stainless steel, 3 μm (b) For example, or low temperature HPs the values o R hp are, as a rule, within the range (3 3) K/W, and the value o Q max makes tens watts

2 during operation against gravity and hundreds watts in the absence o its inluence It is possible to ind in-depth inormation about characteristics o HPs with MFCS and results o their practical application in work [Semena et al, 984], and additional inormation about properties o metal elt materials in work [Kostornov, ] RESEARCH OF STRUCTURAL CHARACTERISTICS OF MFCS Among initial structural characteristics o MFCS there are the ollowing: diameter o ibre d, length o ibre l, average porosity ε, maximal D max, middle (eective) D mid and minimum D min diameters o pores, and also pore size distribution Usually or HPs application the discrete ibres by a diameter rom μm to 5 μm are used, and lately, ultraine ibres rom 4 μm to 8 μm The length o separate ibre is rom to 3 m (and more, i necessary) Average volume porosity o CS is within the range rom 6 to 96, and at the use o micro ibres one can attain extremely high porosity rom 98 to 995 [Adkins et al, 994] Obtainable (maximal) porosity o MFCS can be deined by empiric dependence: ε max = exp ( - 6 d,/ l ) () MFCSs are nonuniorm structures, thereore they are characterized by the dierential or integral (cumulative) pore size distribution (ig ) permeability in combination with the model o ideal porous medium and method o extruding o liquid rom pores (method o Barus-Bechgold) On the basis o processing o experimental data, got by the indicated methods, authors obtained the ollowing dependences or calculations o MFCS parameters 7 3 max 5 ( ε) ε ( ε) 5 ε( ε) D = d l ε ; mid 4 D = d l 7 3 min 5 D = d l () 3 HYDRODYNAMIC CHARACTERISTICS OF MFCS The considered structural characteristics o MFCS determine capillary-transport properties: capillary pressure P cap, coeicients o liquid permeability K and K (permeability at liquid movement in two orthogonal directions), limiting wetting angle, characteristics o capillary absorption o heat carrier into the dry structure Hydrodynamic characteristics o MFCS were investigated on samples with porosity rom 58 to 96, rom copper and stainless steel ibres by a diameter o, 3, 4, 55 and 7 μm, and the length o 3 mm, with water, acetone, ethanol and benzyl alcohol as workings luids As a result o experimental data processing and modelling authors have obtained the ollowing dependences: - or determination o capillary pressure: P = σ cos θ( ε )d ε, (3) cap 35 max - or determination o permeability across the axis o ibres (across the plane o elting): Figure : Curve o cumulative distribution o relative volume o pores over pore diameters Volume porosity ε = 85, d = 3 μm, l = 3 m Such distributions can be obtained by mercury intrusion porometry From cumulative distribution curves it is possible to extract inormation about the maximal, middle and minimum sizes o pores (ig ) Typical ranges o sizes o MFCS pores: D max = 5 3 μm; D mid = 5 μm; D min = 5 5 μm More correct values o sizes o MFCS pores (especially, high-porous) are obtained by other methods o their determination: method o gas ε ln ( ε ) ( ε ) ( ε),7 ε ( ε ) d + ε 6 + εmax K = ( max ) exp 45( ), (4) - or determination o permeability along the axis o ibres (along the plane o elting): K = (D ) ( ε ) (5) 43 mid On igure 3 the generalizing dependences o coeicients o MFCS liquid permeability with the diameter o ibres o 3 μm (curves and ) and 55 μm (curves 3 and 4) on porosity, calculated by

3 ormula (5) or curves and 3 and by ormula (4) or curves and 4 are resulted Coeicients o MFCS permeability across and along ibres are approximately identical in the wide enough range o porosity, up to the value ε/ε max 9 It is explained by homogeneity and regularity o structure in various directions In MFCSs the volume porosity o which is approached to maximally possible, the isotropy o their structure is violated, and permeability in direction along ibres considerably exceeds permeability across ibres K μm ε Figure 3: Dependences o coeicient o MFCS liquid permeability on volume porosity, diameter o ibres and direction o liquid iltration Limiting angle o wetting Θ o metals with liquids, as a rule, diers rom ideal, that results in diminishing o values o capillary pressure and, accordingly, maximal heat lux, transerred by HP According to authors data the values o Θ, measured by the method o lying drop, or a pair copper-water are 7 ± о, or a pair copper ethanol 5 ± 5 о, and at the use o acetone, methanol, methylal and reons Θ = о However the use o method o the capillary rise o liquid in dry MFCS gives other results: copper ibres - acetone 4 ± о, stainless steel ibres - acetone 395 о, copper - ethyl alcohol 45 ± о, copper ethanol 4 ± о, and water in general is not absorbed in dry MFCS Thermal oxidation o CS rom the ibres o copper, steel, and nickel sharply improves their wettability with organic heat carriers, practically to Θ = о, and with water to 8 3 о Results o researches o the available capillary pressure o P cap, coeicients o permeability K and K and limiting wetting angles Θ were approved and conirmed at the study o characteristics o capillary absorption organic liquids and water into the dry MFCS samples as plates 5 m long, 45 m wide and m thick Porosity o samples was within the range rom 6 to 95, material o ibres was copper and 3 4 stainless steel, their diameter, 3, 4 and 7 μm The conducted researches have shown a high capillary-transport ability o MFCS not only at horizontal orientation but also at operation against gravity Rate o saturation o MFCS with working luids was ound ar above, than o CS rom screens and abrics 4 HEAT CONDUCTIVITY OF MFCS Experimental data on heat conductivity o MFCS available in reerences dier considerably rom the results o calculations by the theoretical and semiempiric ormulas o heat conductivity o porous bodies It is explained, mainly, by absence o consideration o thermal resistance o heat conductivity o contacts between dispersible discrete elements orming a porous structure Technology o MFCS abrication, including the processes o pressing and sintering, provides a good thermal contact between separate ibres However, as ollows rom the results o metallographic analysis, nevertheless, contacts are not ideal Equivalent diameter o spot o contact between the sintered ibres is less than ibre diameter, ie ratio o these two magnitude (y), as a rule, is within < у The value o у ormed due to the processes o sintering (у ) and pressing (у ), can be calculated by dependences, obtained by processing o experimental data: 5 у = у +у,; y = c ε (l / d ), < у < ; y = ( + c )d ε, у <, (6), 3 where d and l are in micrometres, and coeicient c depends on material o ibres and technological parameters; it is equal: or copper 3, or stainless steel 5, or nickel 65, or nichrome 8 For the calculation o eective heat conductivity o porous materials rom discrete ibres across a plane o elting authors oer the ollowing analytical dependence: ( ε ) 4βε λe = λ ( ε) M + ε β + + β, (7) where β = λ s /λ ; λ s, λ coeicients o heat conductivity o material o ibres and liquid The parameter M takes into account an inluence o imperection o contacts between ibres on heat transer in contact area:

4 ( ) ( ) M = y+ A y y A y + y [ ]/[ ] ( ) A = πβ( β) ( β) ln β (8) One can use the same ormulas or the calculation o rame heat conductivity o MFCS (λ s = and β = λ s /λ = ) Figure 4 illustrates the authors experimental data and data o other researchers as sings, and also curves, calculated by ormulas (7) and (8), or eective heat conductivity o MFCS rom stainless steel (curve I), nickel (curve II) and copper (curves III VIII), and namely, or ibres: by diameter o 3 μm, 3 mm long (curve I), 5 μm, 3 mm (curve II), 7 μm, 3 mm (curve III), 4 μm, 3 mm (curve IV), 7 μm, mm (curve V), μm, 3 mm (curve VI), 4 μm, mm (curve VII), μm, mm (curve VIII) λ e W/mK V VII VI II IV I III VIII ε Figure 4: Dependences o coeicients o eective heat conductivity o MFCS saturated with water on porosity The increase o CS porosity, shortening o discrete ibres and increase o their diameter reduce both eective and rame heat conductivity o MFCS Substantial distinction o experimental and calculation data is observed only or CS with porosity close to highest possible, where the structure becomes nonisotropic 5 RESEARCH OF MECHANICAL PROPERTIES OF MFCS Except structural, hydrodynamic, thermophysical characteristics o MFCS and their capillarytransport properties, in a number o cases, knowledge o mechanical properties o porous structures is needed, or example, or structures used in ull scale solar receivers with sodium as heat carrier Authors investigated mechanical properties o CS with porosity rom 8 to 99, thickness rom to 5 mm rom discrete ibres o stainless steel by a diameter rom 4 to 55 μm, 3 mm and mm long Fibres were sintered with each other and with substrates or dome o receiver o Haynes Alloy 3 Researches were conducted at the temperature o о С and 75 о С on test benches, described in works [Baturkin et al,, 5] Dependences o some mechanical properties o MFCS on their porosity ε (a) and relative porosity ε/ε max (b) are resulted on igure 5 The values o proportional limit o MFCSs at elongation are substantially reduced not only at the increase o their porosity and decrease o ibre diameter, but also at the rise o temperature o exploitation (reduction approximately in 5 times) 6 σ 4 4 MPa ε, % ε a) MPa b) 8 σ p ε/εlim 9 96 Figure 5: Dependences o proportional limits at elongation o MFCS (a) and pullout orces rom substrate (b) on porosity Lines and 3 ibre diameter o 3 μm, length 3 mm; line 8 μm, mm Temperature o tests: lines and о С, line 3 75 о С The proportional limits at the compression o MFCS in the conditions o high temperature (75 о С) have similar dependences and or the structures rom ibres with diameter o 8 μm are equal: 3 kpa (porosity 993) and 5 7 kpa (985), rom 3-μm ibres 3 5 kpa (9) and 5 8 kpa (85), rom 55-μm ibres 9 kpa (85) and 3 kpa (8) The conducted researches conirmed possibility o application o MFCS even or the high temperature heat pipes o large sizes However not all CS designs were suitable or this application So structures rom ultraine ibres (by a diameter o 4 and 8 μm) do not possess suicient mechanical properties, necessary or their using at the temperatures o 8 o С in ull scale high temperature solar receivers 6 APPLICATION OF HEAT PIPES WITH MFCS The ield o application o MFCS in HP is determined by the ollowing eatures Technology o abrication o capillary structures allows to get CS with the given structural parameters, which can be repeatedly reproduced, to realize the reliable

5 astening and contact o CS with the HP case, to apply CS to the surace o complex shape, and to give possibility to deorm the cases with CS to put HP into proper shape Hydrodynamic, capillary-transport and thermophysical characteristics and properties o MFCS can be presented by the clear enough unctions o initial structural parameters Above mentioned advantages are the necessary terms o receipt o reliable unctional dependences between the structural parameters o CS and heat mass transer characteristics o HP, which do not variate during HP operation and allow to conduct their calculations and optimization High capillary-transport properties o MFCS in comparison with other types o inserted CS in the conditions o presence or absence o counteractive inluence o mass orces are explained by the speciic o ormation o MFCS, consisting o high limiting porosity, absence o the closed and oneside open pores, and low convolution o pore channels Heat pipes with MFCS possess high heat transer ability and low thermal resistance in the conditions o presence (absence) o counteractive inluence o mass orces On the basis o such CSs a number o HP modiications are created, such as gas regulated heat pipes, thermal diodes, thermal siphons, and loop heat pipes 7 CONCLUSIONS The conducted researches o structural, hydrodynamic, thermophysical and mechanical properties o metal elt capillary structures have shown appropriateness o their use as wicks o heat pipes Technology allows to make HP o any orm, to create complex capillary structures with reliable hydraulic and mechanical connection, to attain the reiteration o CS properties, which can be preliminary calculated, to deorm the HP case at its mounting without wick damage or its perormance degradation The aoresaid is conirmed by long experience o application o HPs with MFCS in such ields, as cooling, temperature control and thermostatting o various electronics, cooling o devices o aircrats, including space vehicles, providing o the thermal conditions o optical, analytical and precision instruments, including lasers, cooling o mechanical and electrotechnical equipment, providing o technological treatment o materials, medical engineering and so orth NOMENCLATURE R hp λ e λ s /λ ε max Q max heat pipe thermal resistance (K/W) heat conductivity o capillary structure, saturated with liquid (W/(mK) ratio o ibre material heat conductivity and liquid conductivity (-) maximal obtainable porosity o capillary structure (-) maximal heat transer o heat pipe (W) l length o ibre (mm, m) d diameter o ibre (μm, m) Θ limiting angle o wetting (grad) ratio o volume o pores /to total volume o V/V capillary structure D pore diameter (μm, m) P capillary pressure (N/m ) cap liquid permeability o capillary structure, K saturated with liquid (m, μm ) ACKNOWLEDGEMENT Participation in conerence was supported by grant o US Department o Energy Ministry on topic Advanced Wicks or Heat Pipes in rame o partner project agreement P577 between Argonne National Laboratory, the Science and Technology Center in Ukraine and National Technical University o Ukraine "Kyiv Polytechnic Institute" REFERENCES Adkins DR, Moss TA, Andraka CE, Andreas NH, Cole HM An examination o metal elt wicks or heatpipe applications - Sandia National Laboratories, SAND C, р Baturkin V, Zaripov V, Kravetz V, Nizhilk A, Moreno A Research o structural and mechanical properties o metal iber capillary structures, intended or high temperature heat pipes-solar absorbers J Energetica, economica, technologia, ecologia Kyiv,,, p 4-46 (in Russian) Baturkin V, Zaripov V, Andraka C Technological peculiarities and calculation o capillary structure parameters or heat pipe absorbers o solar power plants J Energetica, economica, technologia, ecologia Kyiv,, 5, p 7-34 (in Russian) Kostornov AG Materials science o dispersed and porous metals and alloys (in two volumes) - Naukova dumka, Kyiv,, vol, 568 p (in Russian) Semena MG, Gershuni AN, Zaripov VK Heat pipes with metal iber capillary structures - Vyscha shkola, Kyiv, 984, 5 p (in Russian)