EFFECT OF SURFACE FINISH ON HEAT TRANSFER PERFORMANCE OF PLATE HEAT EXCHANGER

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1 HEFAT202 9 t International Conferene on Heat Transfer, Fluid Meanis and Termodynamis 6 8 July 202 Malta EFFECT OF SURFACE FINISH ON HEAT TRANSFER PERFORMANCE OF PLATE HEAT EXCHANGER Wajs J.,2 * and Mikielewiz D.,2 *Autor for orrespondene Gdansk University of Tenology, Faulty of Meanial Engineering, Department of Energy and Industrial Apparatus, ul. Narutowiza /2, Gdansk, Poland jwajs@me.pg.gda.pl 2 Te Szewalski Institute of Fluid-Flow Mainery PAS, ul. Fiszera 4, Gdansk, Poland ABSTRACT In te paper an experimental analysis of passive eat transfer intensifiation tenique employed in te ase of plate eat exanger is presented. Te passive intensifiation was obtained by a modifiation of te eat transfer surfae. Te rougness of surfae was inreased by a usage of glass mirobeads. Single-pase onvetive eat transfer in te water-water system was studied. Te experiment was aomplised in two stages. In te first stage te ommerial plate eat exanger was investigated, wile in te seond one te idential eat exanger but wit te eat transfer surfae. Te diret omparison of termal and flow arateristis between bot devies was possible due to te assurane of equivalent onditions during te experiment. Equivalent onditions mean te same volumetri flow rates and te same media s temperatures at te inlet of eat exangers in te orresponding measurements series. Due to tis te systemati experimental data sow tat larger rougness of eat transfer surfae leads to an inrease of eat transfer oeffiient on te side of ooling water (inrease by about 30 35%) and simultaneously to an inrease of flow resistane (up to 30% wen te volumetri flow rate is equal to 500 l/). On te side of eating water it was found tat te eat transfer oeffiient inreased by about 25%, wile te flow resistane by about 22% (te volumetri flow rate of 500 l/). INTRODUCTION Te teniques of eat transfer improvement (intensifiation) in onventional appliations ave been under srutiny in literature for more tan entury and a large number of information was gatered up to now []. Generally speaking, te intensifiation metods an be lassified as passive (no additional energy ave to be supplied) and ative (an additional energy is required). Effiieny of su metods strongly depends on eat transfer onditions and meanisms wi an ange from single pase onvetive eat transfer to te flow boiling. Wit te prospets of energy effiieny, miniaturization, produt reliability, and te potentially large eonomi advantages, an extensive resear and development effort as been undertaken in te area of enaned eat transfer over te past ouple of deades [ 4]. NOMENCLATURE A [m 2 ] surfae C,C,C 3 [-] onstants of linear regression [(J/kg K)] entalpy k [W/(m 2 K)] overall eat transfer oeffiient m& [kg/s] mass flux P [Pa] pressure Q & R [W] [mm] rate of eat rougness parameter T [K] temperature V & [m 3 /s] volumetri flow rate w x y [m/s] veloity Cartesian axis diretion Cartesian axis diretion Greek letters α [W/(m 2 K)] onvetive eat transfer oeffiient d [m] wall tikness l [W/(m K)] termal ondutivity Subsripts in out old ot inlet outlet Over te past 20 years, te ompat plate eat exangers ave replaed te traditional sell-and-tube eat exangers, sine te former are more energy and spae effiient and are eaper to produe. Despite tis trend, only a few attempts to enane ompat plate eat exangers wit ig performing mirosized, or smaller, enanement strutures ave been 454

reported. Nowadays most of te ativities in tat area relate to enanement of eat transfer in boiling range.

Hillis and Tomas [5], as part of an evaluation of eat exangers for a large 40 MW oean termal energy onversion pilot plant in Hawaii, tested te performane a small-frame plate eat exanger wit ammonia

Boiling eat transfer oeffiients of about 30 kw/(m 2 K) were reorded at a eat flux of 26 kw/m 2, equivalent to a fivefold improvement ompared to unoated surfae.

2 reported. Nowadays most of te ativities in tat area relate to enanement of eat transfer in boiling range. Presented below is a survey of some metods wi lead to enanement of pool boiling in plate eat exangers. Hillis and Tomas [5], as part of an evaluation of eat exangers for a large 40 MW oean termal energy onversion pilot plant in Hawaii, tested te performane a small-frame plate eat exanger wit ammonia as refrigerant. Te eat exanger plates featured a 60 deg evron angle orrugation pattern and were oated in Linde s ig-flux surfae: a porous aluminum partile layer. Boiling eat transfer oeffiients of about 30 kw/(m 2 K) were reorded at a eat flux of 26 kw/m 2, equivalent to a fivefold improvement ompared to unoated surfae. Müller-Steinagen [6] vauum plasma sprayed a 250 µm tik layer of sperially saped Inonel 625 partiles on to a plate and frame eat exanger surfae. Te partiles ad a diameter of µm and enaned te boiling eat transfer oeffiient of R34a wit up to 00%. Matsusima and Uida [7] tested a brazed plate eat exanger wit a novel pyramid-like struture in R22. Te strutural features were.5 mm in eigt, ene, not in te mirosized region, but te evaporation eat transfer oeffiients were estimated to be.5 2 times iger tan tose of regular erringbone-type plates. Longo et al. [8] applied µm sized, pyramid-like surfae features to a plate eat exanger wit erringbone marosale orrugation, resulting in a 40% inrease in te boiling eat transfer oeffiient of R22. Tis enanement was larger tan te inrease in eat transfer area, suggesting a real improvement in te boiling eat transfer meanism. Reent developments witin nano- and mirotenologies ave made possible te reation of well defined treedimensional onneted porous network strutures, Davis [9], wi, so far, ave mainly been used in appliations su as atalysis moleular sieves, fuel ells, sorption, and separation [0]. Tese developments ave opened up new possibilities to struture ig performing boiling surfaes wit well defined miro- and submiron topology, using metods more preise tan meanial deformation. A novel nano- and miroporous struture was reently sown to enane pool boiling eat transfer in 34a wit over one order of magnitude ompared to a plain mained opper surfae by Furberg et al. []. He presented an experimental study of te performane of a plate eat exanger evaporator wit and witout tis novel enanement struture applied to te refrigerant annel. Te flow boiling tests were onduted for 34a wit eat fluxes ranging from 4.5 kw/m 2 to 7 kw/m 2. Various distane frames were also used to widen te refrigerant annel in order to isolate te influene of refrigerant mass flux and tereby gain a better understanding of boiling eat transfer in plate eat exangers in general and te enaned surfae in partiular. In tis paper, for te purpose of investigating Organi Rankine Cyle evaporators and future tenial appliations, an analysis of passive eat transfer intensifiation in te ase of plate eat exanger was done. A new tenique of inreasing te surfae rougness is proposed, namely troug abrasive blasting wit te utilization of glass miro-beads. Su tenique is relatively not expensive and produes te enanement effet. Experimental data were olleted for water-water ase, were te eat transfer oeffiient was alulated using te Wilson metod. Tat metod seems to be, in te autors opinion, te only one for finding te eat transfer oeffiient for su a omplex eat exanger struture. In te near future it is planned to perform experiments on boiling wit low boiling point fluids to sow te apaity of proposed metod of surfae modifiation. PLATE HEAT EXCHANGER Te twisted plate eat exanger offered at te ome/world market by Sondex was te subjet of presented investigations. In tis kind of eat exanger te eat is transferred in one pass. It was made of stainless steel (no. 36 aording to AISI standard) and onsisted of twelve plates, wose tikness was 0.5 mm. Te total lengt of eat exanger was 270 mm, its apaity was 2 l and its weigt was 9.5 kg. Te overall eat transfer area was equal to m 2. Te distane between te plates was kept onstant and te EPDM seal was fixed in te system ang on. Permissible working pressure was equal to 6 bar. Te semati view of eat exanger is presented in Figure, wile its main features are listed in Table. Te individual plate is sown in Figure 2. Table Main features of ommerial eat exanger plate dimensions lengt/widt [mm] apaity of one annel [l] number of plates rougness of plate surfae [mm] 45 / R a =0.46, R z =3.34 ot in ot out old out old in Figure Seme of plate eat exanger Figure 2 View of a single plate 455

3 For te purpose of investigations te eat exanger plates were subjeted to a abrasive blasting wit te utilization of glass miro-beads. Granulation of te beads was of μm, wile te density of glass was 2.5 g/m 3 and its ardness was 6 aording to te Mosa s sale. Te rougness anges of eat exanger plate surfae, as a result of abrasive blasting, were examined by te Sip Design and Resear Center in Gdansk. Te measurements were done wit te Surftest 2 (Mitutoyo). At first te measurement devie was alibrated wit appliation of te rougness standard delivered by Mitutoyo ompany. Te flat parts of eat exanger plates were examined, wile te sampling lengt was 0.25 mm. Te results are presented in Table 2 and applied designations are as follows: A primary surfae state, B surfae after te abrasive blasting. Te meanial working abrasive blasting aused six times iger rougness expressed by parameter R a and about five times iger rougness expressed by parameter R z. EXPERIMENT Te experimental investigations of plate eat exangers were arried out on a dediated faility for testing of eat exangers, Figure 5. Table 2 Te measurement results of surfae rougness sample measured parameter average value [μm] A B R a 0.46 R z 3.34 R a 2.8 R z 5.9 Parameter R a is an average aritmetial rougness in te range of sampling lengt l (Figure 3). Parameter R z is an aritmeti average of absolute eigt of five te igest rougness peaks and eigt of five te deepest valleys in te range of sampling lengt l (Figure 4). R = ò l y( x) dx» å a y i l 0 n i= Figure 3 Grapial interpretation of parameter R a 5 5 æ ö RZ = çå y pi + å yvi 5 è i= i= ø Figure 4 Grapial interpretation of parameter R z n Figure 5 Seme of experimental faility Te test stand enabled te eat transfer by onvetion between te ot and old water. Te ot water was irulating in te system wit an eletri flow eater, wile te old water was a tap water. In bot iruits fine filters were installed. Te eat was transferred due to te o-urrent flow of working media. Te fluid flow rates were measured by rotameters wit te auray of ±3 l/. Te eater was ontrolled by te power supply in te range from 0% to 00% of eating power. As a variable parameter te input temperature of eat exanger was taken. Te pressure drop was measured by merury manometers wit auray of ±2 mmhg. Termoouples of J- type were used to measure temperature in four points i.e. at te inlet and outlet of eat exanger s old side and at te inlet and outlet of eat exanger s ot side. Prior to experiments all termoouples were alibrated to yield te auray of measurements of ± 0.5 o C. Te referene temperature for termoouples measurements was equal to 0 o C. During experiments te following parameters were measured: te ot water temperature at te inlet (T -in ) and at te outlet (T -out ) of eat exanger, te old water temperature at te inlet (T -in ) and at te outlet (T -out ) of eat exanger, te pressure drop onneted wit te ot water flow (ΔP ), te pressure drop onneted wit te old water flow (ΔP ), te volumetri flow rate of ot water ( V & ) and te volumetri flow rate of old water ( V & ). Te volumetri flow rate of ot/old 456

4 water was varied in te range from 00 to 500 l/. Te water supply pressure was 4 bar. On te basis of measurement results te eat flux (q), te Logaritmi Mean Temperature Differene in te eat exanger (LMTD) and te overall eat transfer oeffiient (k) were alulated. Te overall eat transfer oeffiient was determined wit te Pelet law based on te rate of eat transfer, taken up by ot water and te eat transfer area equal to m 2. DETERMINATION OF HEAT TRANSFER DATA Te experimental investigations of eat exangers require determination of mean eat transfer oeffiients on bot sides of te wall separating fluids exanging eat. Usually tat requires installation of termoouples for measurements of wall temperature separating two fluids. If te eat exanger as a large number of tubes and a omplex surfae geometry ten aurate measurement of te mean surfae temperature faes signifiant diffiulties for example in te ourse of disassembling installation a large number of termoouples must be attaed and subsequently everyting must be put up togeter. Su diffiulties an be alleviated if te Wilson s metod [2] is applied. Te metod is very simple and an be applied to te analysis of different types of eat exangers [3]. A simple and effiient Wilson metod in a version similar to te original one was applied in determination of eat transfer oeffiient. Te lassial Wilson metod, as well as its modifiations, requires only determination of te overall termal resistane in te eat exanger. From te Wilson s metod an aurate energy balane, based on measurement of flow rates of fluids exanging eat and teir mean temperatures at inlet and outlet of te eat exanger is obtained. Te termal balane of eat exanger an be presented in te form: Q& = k LMTD A = m& D = m& D () were: LMTD - logaritmi mean temperature differene, A eat transfer surfae, wereas overall eat transfer oeffiient an be desribed as: - æ d ö k = ç + + (2) è a l a ø were: a and a are eat transfer oeffiients for respetive mass flow rates; δ is a tikness of a wall separating two fluids, wereas λ its termal ondutivity. Te mean wall temperature an be determined from a relation: ( T _ in - T _ out )- ( T _ out - T _ in ) ( T _ in - T _ out ) ln ( T - T ) LMTD = (3) _ out _ in Tat is espeially important in te ase of finned tubes were determination of a mean value of wall temperature is diffiult basing on loal measurements. Assuming tat eat transfer is primarily governed by flow veloities of bot fluids, ten simple relations an be written: for m& = onst. and m& = var tere is: a = onst, n a = C w (4) for m& = onst. and m& = var tere is: a = onst, n a = C w (5) were w and w are respetive flow veloities; n is oeffiient depending on te arater of eat transfer, for example in ase of turbulent flow inside tubes n=0.8, wereas in ase of a laminar one, n=0.5. For eating medium following relation an be formulated: or: æ = ç k è k a = 3 d ö + + C l ø C + C w -n w - n were: = d C a + 3 (8) l for a series were m& = onst. Assuming new variables, i.e. -n x = and y = / k a linear relation is obtained: w (6) (7) y = C 3 + C x (9) For ooling side analogial relations an be derived. Te eat transfer oeffiient alulations by Wilson s metod were onduted for te plate tikness of 0.5 mm. Te plate material (te stainless steel) as te termal ondutivity λ equal to 5 W/(m K). For te ot fluid te straigt line was plotted and its equation as form y = C 3 + C x (Figure 6), were C = and C 3 = () and C = and C 3 = (ommerial). /k [m 2 K/W],x0-3,0x0-3 ommerial 9,0x0-4 8,0x0-4 7,0x0-4 6,0x0-4 5,0x0-4 4,0x0-4 3,0x0-4 2,0x0-4 ) y = x + 0,00029,0x0-4 2) y = x + 0, , w -0.8 Figure 6 Experimental points and linear regression for V & = 500 l/ ) 2) 457

5 In Tables 3 and 4 te summary of eat transfer oeffiient values obtained in te old and ot passes are presented. Table 3 Heat transfer oeffiient on te old side 30 ommerial ommerial V & [l/] V & [l/] a [W/(m 2 K)] Table 4 Heat transfer oeffiient on te ot side V & [l/] LMTD [K] T in = 80 C T in = 0 C V = 500 l/ V [l/] Figure 7 LMTD versus volumetri flow rate of ot water ommerial V & [l/] a [W/(m 2 K)] RESULTS OF MEASUREMENTS Te exemplary omparison of studied eat exangers termal arateristis are sown below. Te diret omparison of termal and flow arateristis between bot devies was possible due to te assurane of equivalent onditions during te experiment. Equivalent onditions mean te same volumetri flow rates and te same media temperatures at te inlet of eat exangers in te orresponding measurements series. Te presented below graps were onstruted at following onditions: temperature of ot water at te eat exanger inlet was 80 C, temperature of old water at te eat exanger s inlet was 0.5 C, volumetri flow rate of old water was equal to 500 l/, volumetri flow rate of ot water was varied in te range from 00 to 500 l/. Distribution of LMTD versus te volumetri flow rate of ot water (eating medium) is presented in Figure 7. Te pressure drop as a funtion of volumetri flow rate applied in te experiment is presented for ot and old passes respetively in Figure 8 and Figure 9. Analysis of presented flow and termal arateristis sows tat in te eat exanger wit surfae (larger rougness) inreasing volume flow rate of working fluids auses distint inrease of te LMTD and simultaneous of te flow resistane inrease. Dp [kpa] Dp [kpa] 5,0 4,5 4,0 3,5 3,0 2,5 2,0,5,0 0,5 ommerial 0, ,0 4,5 4,0 3,5 3,0 2,5 2,0,5,0 0,5 0,0 V [l/] Figure 8 Flow arateristis of ot iruit ommerial V [l/] Figure 9 Flow arateristis of old iruit 458

6 k [W/(m 2 K)] q [kw/m 2 ] ommerial T in = 80 C T in = 0 C V = 500 l/ V = l/ LMTD [K] Figure 0 Overall eat transfer oeffiient versus LMTD ommerial T in = 80 C T in = 0 C V = 500 l/ V = l/ LMTD [K] Figure Heat flux density versus LMTD In Figure 0 te distribution of overall eat transfer oeffiient of te eat exangers in relation to te LMTD is presented. It an be seen tat in te eat exanger wit surfae it was possible to obtain iger values of overall eat transfer oeffiients. In Figure te arateristis of eat flux density versus te Logaritmi Mean Temperature Differene (LMTD) for two kinds of eat exanger are sown. Te inrease of eat flux density wit te inrease of LMTD an be observed. Te iger values of eat transfer oeffiient indiates tat te iger values of eat flux ould be transferred by te eat exanger in omparison wit te ommerially available one. CONCLUSIONS Te systemati experimental investigations of two eat exangers: normal one and wit eat transfer surfae were desribed. On te basis of presented in te paper results it an be seen tat larger rougness of eat transfer surfae leads to an inrease of eat transfer oeffiient on te side of ooling water (inrease about 30 35%) and simultaneously to an inrease of flow resistane (to 30% wen te volumetri flow rate is equal to 500 l/). On te side of eating water it was found tat te eat transfer oeffiient inreased about 25%, wile te flow resistane of about 22% (te volumetri flow rate of 500 l/). Tese values indiate tat te iger values of eat flux ould be transferred by te eat exanger wit eat transfer surfae. Presented results are promising and sow te perspetive for furter eat transfer enanement. Desribed model of eat exanger will be submitted to te patent proedure. ACKNOWLEDGEMENTS Te results presented in tis paper were obtained from resear work o-finaned by te National Centre of Resear and Development in te framework of Contrat SP/E//67484/0 Strategi Resear Programme Advaned Tenologies for obtaining energy: Development of a tenology for igly effiient zero-emission oal-fired Power units integrated wit CO2 apture. REFERENCES [] Tome, J., 990, Enaned Boiling Heat Transfer, Hemispere, New York. [2] Bergles, A. E., 997, Enanement of Pool Boiling, Int. J. Refrigeration, Vol. 20, No 8, pp [3]. Webb, R. L., 994, Priniples of Enaned Heat Transfer, Wiley- Intersiene, New York. [4] Webb, R. L., 2004, Odessey of te Enaned Boiling Surfae, ASME J. Heat Transfer, Vol. 26, pp [5] Hillis, D., and Tomas, A., 98, Test and Development Program for Closed-Cyle OTEC Heat Exangers, Oeans, Vol. 3, pp [6] Müller-Steinagen, H., 2008, Smart Surfaes for Improved Heat Exangers, HTRI e-ubliations, [7] Matsusima, H., and Uida, M., 2002, Evaporation Performane of a Plate Heat Exanger Embossed Wit Pyramid-Like Strutures, J. Enaned Heat Transfer, Vol. 9, pp [8] Longo, G. A., Gasparella, A., and Sartori, R., 2004, Experimental Heat Transfer Coeffiients During Refrigerant Vaporisation and Condensation Inside Herringbone-Type Plate Heat Exangers Wit Enaned Surfaes, Int. J. Heat Mass Transfer, Vol. 47, pp [9] Davis, M. E., 2002, Ordered Porous Materials for Emerging Appliations, Nature, Vol. 47, pp [0] Stein, A., 2003, Advanes in Miroporous and Mesoporous Solids Higligts of Reent Progress, Advaned Materials, Vol. 5, pp [] Furberg R., Palm, B., Li S., Toprak M., Muammed M., 2009, Te Use of a Nano- and Miroporous Surfae Layer to Enane Boiling in a Plate Heat Exanger, Journal of Heat Transfer, Vol. 3, No. 0, [2] Wilson E.E., 95, A basis for rational design of eat transfer apparatus, Trans. ASME, Vol. 37, No. 47. [3] Sa R.K., Assessment of Wilson plot teniques for obtaining eat exanger design data, Proeedings of te 9 t International Heat Transfer Conferene, Jerusalem,

Natural Convection Experiment Measurements from a Vertical Surface

Natural Convection Experiment Measurements from a Vertical Surface OBJECTIVE Natural Convetion Experiment Measurements from a Vertial Surfae 1. To demonstrate te basi priniples of natural onvetion eat transfer inluding determination of te onvetive eat transfer oeffiient.