PERFORMANCE ANALYSIS OF HEAT TRANSFER PROCESSES FROM WET AND DRY SURFACES: COOLING TOWERS AND HEAT EXCHANGERS REPORT A10

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1 Teknillinen korkekoulu. Konetekniikn ossto. LVI-tekniikn lbortorio. A Helsinki University of Technology. Deprtment of Mechnicl Engineering. Lbortory of Heting, Ventilting nd Air Conditioning. A Espoo 2005 PERFORMANCE ANALYSIS OF HEAT TRANSFER PROCESSES FROM WET AND DRY SURFACES: COOLING TOWERS AND HEAT EXCHANGERS REPORT A10 Al Ali Hsn Disserttion for the degree of Doctor of Science in Technology to be presented with due permission of the Deprtment of Mechnicl Engineering, Helsinki University of Technology for public exmintion nd debte in Auditorium E t Helsinki University of Technology (Espoo, Finlnd) on the 6th of My, 2005, t 12 noon. Helsinki University of Technology Deprtment of Mechnicl Engineering Lbortory of Heting, Ventilting nd Air Conditioning

2 Distribution: Helsinki University of Technology HVAC-Librry P.O. Box 4100 FI TKK Tel Fx Author s ddress: Helsinki University of Technology Lbortory of Heting, Ventilting nd Air Conditioning P.O. Box 4400 FI TKK Tel Fx E-mil: l.hsn@tkk.fi Supervisor: Professor Ki Sirén Helsinki University of Technology Reviewers: Associte Professor Dvid S-K Ting University of Windsor, Cnd Deprtment of Mechnicl, Automotive nd Mterils Engineering Dr. Pertti Heikkilä Metso Pper Inc., Finlnd Air Systems Opponents: Professor Mts Westermrk Royl Institute of Technology, Sweden Deprtment of Chemicl Engineering nd Technology Dr. Pertti Heikkilä Metso Pper Inc., Finlnd Air Systems ISBN (PDF) ISBN X (printed) ISSN Otmedi Oy 2005

3 ABSTRACT The objective of this work is to study the therml nd hydrulic performnce of evportively cooled het exchngers, including closed wet cooling towers, nd dry tube het exchngers with vrious geometries. Applictions utilising such equipment exist in lmost every therml process. The investigtion includes theoreticl nlysis, computtionl pproches, nd experimentl mesurements. In this work, computtionl model is presented for the therml performnce of closed wet cooling towers intended for use in conjunction with chilled ceilings in cooling of buildings. A vrible spry wter temperture inside the tower is ssumed. A prototype tower ws subjected to experimentl mesurements to find its chrcteristics. Optimistion of the tower geometry nd flow rtes for specified design conditions is crried out in order to chieve high vlue of the coefficient of performnce (COP). Results from globl simultion progrm (including the tower model, trnsient building model, chilled ceiling model, system control etc.) show tht closed wet cooling towers cn be used with chilled ceilings to chieve cceptble indoor ir tempertures in loctions hving suitble climtic conditions. This is supported by published results from performnce test of n office building using this method of cooling. Simplifiction of the model is obtined by ssuming constnt temperture for the spry wter. The tower performnce predicted by the simplified model nd the computtionl model shows comprble results. The results of the simplified model re then incorported with Computtionl Fluid Dynmics (CFD) to ssess the temperture distribution inside the tower. It is shown tht CFD cn be implemented to study the effect of ir distribution inside the tower on its performnce. The effect of introducing plte fins in evportively cooled plin circulr tubes is experimentlly studied. The mesurement results show 92% to 140% increse in the mount of het trnsfer for the finned tubes. This is ccompnied by n increse in the pressure drop, so tht n indiction of the combined therml hydrulic performnce is found to be close for the two geometries. However, it shows higher het trnsfer rtes per volume for the finned tubes. The performnce of ovl tubes in the evportively cooled het exchnger is then experimentlly investigted. The mesurement results for the ovl tubes show good het nd mss trnsfer chrcteristics; its verge mss trnsfer Colburn fctor is 89% of tht for the circulr tubes. Furthermore, it shows low friction fctor for the ir flow, which is 46% of tht for the circulr tubes. It is concluded tht the tested ovl tube is better thn the circulr tubes in combined therml hydrulic performnce. The fetures of ovl tubes pper clerer in dry het trnsfer process. Five shpes of dry ovl tubes re experimentlly investigted in cross-flow of ir. The mesurement results for the ovl tubes re compred with those for n equivlent circulr tube. It is found tht the Nusselt numbers Nu D for the studied tubes re close for Reynolds numbers Re D < While for higher Re D, the Nu D decreses with the increse of the 1

4 ovl tube xis rtio. The drg mesurements indicte lower drg coefficients C d vg for the ovl tubes. It is reveled tht the investigted ovl tubes hve fvourble combined therml-hydrulic performnce, which is expressed in terms of (Nu D /C d vg ). The rtio of (Nu D /C d vg ) for the ovl tubes to tht for the circulr tube is from 1.3 to

5 PREFACE The highest rewrd for person's toil is not wht they get for it, but wht they become by it John Ruskin, English poet ( ) This thesis is bsed on reserch work crried out t the Lbortory of Heting Ventilting nd Air Conditioning, Helsinki University of Technology during the period The work on the closed wet cooling tower reltes to the ECOCOOL project (Ecologicl cooling for buildings by combining closed wet cooling tower with chilled ceilings), which ws prtilly funded by the Commission of the Europen Union (DGXII)- Joule IV progrmme. The work on dry ovl tubes is prt of the ovl tube project, which ws funded by the Finnish Technology Agency (TEKES) nd the compnies: Cetetherm Oy, Ekocoil Oy nd Koj Oy. Additionlly, I received grnts from the L.V.Y Foundtion nd K. V. Lindholm Foundtion. I would like to grtefully cknowledge ll those prties for their funding. I wish to express my deepest grtitude to my supervisor Professor Ki Sirén, who gve me the chnce to come nd work in the Lbortory nd pursue my postgrdute study. I m grteful to him for ll of his invluble support during my work with him, nd I would like lso to thnk him very much for his supervision nd guidnce through the work of this thesis. I would like to sincerely thnk the hed of the HVAC Lbortory, Professor Olli Seppänen, for his support of my work nd study in the Lbortory. I would like to express my grtitude to pre-exminers Associte Professor Dvid S-K Ting nd Dr. Pertti Heikkilä for reviewing this thesis nd for their vluble comments. I would lso like to thnk Professor Mts Westermrk for cting s opponent during the discussion of this thesis. I would like to thnk the mnger of the lbortory, Mrkku Sivukri, nd the lbortory technicin, Petteri Kivivuori, for their help in the experimentl test fcilities. My thnks go to co-uthor Dr. Guohui Gn for his fruitful coopertion. Mny thnks go to ll of my collegues in the HVAC Lbortory. Specil thnks to Tpio Helenius for his support nd guidnce during my first dys in Finlnd nd to Mikko Suoks for his coopertion in the ECOCOOL project. Thnks to the stff of the Aerodynmics Lbortory who helped me in the turbulence intensity mesurements. Additionlly, my specil thnks re to my collegues Juh Jokislo, Runo Holopinen nd Loy Seed. No words re ble to express how grteful I m to my wife Khloud for ll of her love nd cre nd for her continuous encourgement, without which this work would not hve been chievble. I would like lso to mention my children, Mohmmed, Hussin nd Abdull; with them this work ws difficult to ccomplish, but without them there would be no mening in life. 3

6 I dedicte this work to the memory of my fther, who would hve been hppy to see me on the stge with my thesis, nd to my der mother. Finlly, nd before ll, prise be to God, the most compssionte, the most merciful. Al 4

7 NOMENCLATURE ABBREVIATIONS COP CWCT ECHE coefficient of performnce closed wet cooling tower evportively cooled het exchnger SYMBOLS A re (m 2 ) A f fin re (m 2 ) A i inner re of tube (m 2 ) A o outer re of tube (m 2 ) c outer mjor xis or chord (m) 2 C d drg coefficient, C d = F d / (0.5 V T A F ) C H specific het cpcity of moist ir (J /(kg dry ir K)) C w specific het cpcity of wter (J kg 1 K 1 ) d tube inside dimeter (m) D tube outside dimeter (m) Dif diffusivity (m 2 s 1 ) D o outside dimeter for circulr tube, for n ovl tube it is the outside dimeter of circulr tube hving equivlent perimeter, (m) f friction fctor, f = p / (0.5 v 2 mx ) F d drg force (N) G ir mss velocity (kg dry ir s 1 m 2 ) h enthlpy of moist ir (J /kg dry ir) h enthlpy of sturted ir (J /kg dry ir) h fg ltent het of evportion of wter (J kg 1 ) h i enthlpy of sturted ir t the interfce temperture t i (J /kg dry ir) h s enthlpy of sturted ir t the spry wter temperture t s (J /kg dry ir) H humidity rtio of moist ir (kg wter/kg dry ir) H humidity rtio of sturted ir t the interfce temperture t i (kg wter/kg i H s dry ir) humidity rtio of sturted ir t the spry wter temperture t s (kg wter/kg dry ir) j m mss trnsfer Colburn fctor, j m = K Sc 2/3 / ( v mx ) k therml conductivity of ir (W m 1 K 1 ) k w therml conductivity of tube wll (W m 1 K 1 ) k x convective mss trnsfer coefficient (m s 1 ) K mss trnsfer coefficient (kg wter s 1 m 2 (kg wter/kg dry ir) 1 ) L tube length (m) m ir mss flow rte (kg dry ir /s) m c cooling wter mss flow rte (kg s 1 ) m h hot wter mss flow rte (kg s 1 ) m s spry wter mss flow rte (kg s 1 ) Nu Nu D Nusselt number, Nu = α D / k men Nusselt number for tube, Nu D = α D o / k 5

8 q rte of het trnsfer for ir (W) q c rte of het trnsfer for cooling wter (W) q f rte of het trnsfer from fin (W) q h rte of het trnsfer for hot wter (W) q s rte of het trnsfer for spry wter (W) R nominl xis rtio for n ovl tube Re Reynolds number, Re = v D / Re c Reynolds number, Re c = V T c / Re D Reynolds number, Re D = V f D o / Sc Schmidt number, Sc = / ( Dif) Sh Sherwood number, Sh = k x D / Dif t ir temperture ( C) t c cooling wter temperture ( C) t h hot wter temperture ( C) t i ir-spry wter interfce temperture ( C) t s spry wter temperture ( C) Tu turbulence intensity U o overll het trnsfer coefficient (W m 2 K 1 ) v mx velocity of ir t minimum flow section (m s 1 ) V f free strem velocity of ir (m s 1 ) V T upstrem velocity of ir to test section (m s 1 ) y outer minor xis of n ovl tube (m) α ir side convective het trnsfer coefficient (W m 2 K 1 ) α c convective het trnsfer coefficient between cooling wter nd tube wll (W m 2 K 1 ) α h convective het trnsfer coefficient between hot wter nd tube wll (W m 2 K 1 ) α i convective het trnsfer coefficient for the irside of the interfce (W m 2 K 1 ) α s convective het trnsfer coefficient between spry wter nd tube wll (W m 2 K 1 ) dynmic viscosity (kg s 1 m 1 ) density (kg m 3 ) Subscripts ir c cooling wter h hot wter lm log-men-difference s spry wter 1 inlet to tower 2 outlet of tower Superscript sturted condition 6

9 TABLE OF CONTENTS 1 INTRODUCTION BACKGROUND OBJECTIVES CONTENTS THEORETICAL AND COMPUTATIONAL ANALYSIS OF CLOSED WET COOLING TOWERS AND ITS APPLICATIONS IN COOLING OF BUILDINGS THEORETICAL BACKGROUND Energy blnce Mss blnce COMPUTATIONAL MODEL SOLUTION RESULTS AND DISCUSSION SIMPLIFICATION OF ANALYTICAL MODELS AND INCORPORATION WITH CFD FOR THE PERFORMANCE PREDICTION OF CLOSED WET COOLING TOWERS SIMPLIFICATION OF ANALYTICAL MODELS Simple models Results of the simplified models COMPUTATIONAL FLUID DYNAMICS (CFD) SIMULATION CFD model Results of fluid flow nd therml performnce PERFORMANCE INVESTIGATION OF EVAPORATIVELY COOLED HEAT EXCHANGERS PLAIN AND FINNED CIRCULAR TUBES Theoreticl bckground Experimentl work Results nd discussion PLAIN CIRCULAR AND OVAL TUBES Experimentl work Results nd discussion THERMAL-HYDRAULIC PERFORMANCE OF OVAL TUBES IN A CROSS-FLOW OF AIR EXPERIMENTAL WORK RESULTS AND DISCUSSION Therml mesurements Hydrulic mesurements Combined therml-hydrulic performnce of the tubes CONCLUSIONS REFERENCES

10 LIST OF PUBLICATIONS I II III IV V Hsn A, Sirén K (2002). Theoreticl nd computtionl nlysis of closed wet cooling towers nd its pplictions in cooling of buildings, Energy nd Buildings 34 (5): Hsn A, Gn G (2002). Simplifiction of nlyticl models nd incorportion with CFD for the performnce prediction of closed wet cooling towers, Interntionl Journl of Energy Reserch 26 (13): Hsn A, Sirén K (2003). Performnce investigtion of plin nd finned tube evportively cooled het exchngers, Applied Therml Engineering 23 (3): Hsn A, Sirén K (2004). Performnce investigtion of plin circulr nd ovl tube evportively cooled het exchngers, Applied Therml Engineering 24 (5-6): Hsn A (2004). Therml-hydrulic performnce of ovl tubes in crossflow of ir, ccepted for publiction in Het nd Mss Trnsfer. The disputnt is the principl uthor of the five publictions. In Publiction II, the uthor crried out the work of simplifying the nlyticl models nd compring the results of the prototype tower with those from the computtionl model. The co-uthor, Dr. Guohui Gn, crried out the CFD simultions. The nlysis of the results nd the conclusions were worked out together. The TRNSYS simultion in Publiction I ws crried out by Mikko Suoks s prt of the Lbortory contribution within the ECOCOOL project. 8

11 1 INTRODUCTION Evportively cooled het exchngers (ECHEs) hve mny pplictions in the fields of ir-conditioning, refrigertion nd power plnts. They cn chieve higher het trnsfer rtes thn dry het exchngers. Het trnsfer tkes plce from hot fluid flowing inside the tubes of the het exchnger to ir through wter film which is formed by sprying wter onto the het exchnger surfce. Closed wet cooling towers (CWCTs), evportive fluid coolers nd evportive condensers re exmples of this ppliction. Evportive fluid coolers usully operte t higher temperture levels thn closed wet cooling towers (CWCTs); however the therml nlysis is similr. Evportive condensers work t constnt condensing temperture. In cses in which the lod is reltively low, dry opertion of n evportively cooled het exchnger could be sufficient to chieve the therml duty. Additionlly, cross-flow dry-surfce het exchngers re widely used in numerous pplictions. The rte of het trnsfer could be incresed from dry nd wet surfce het exchngers by improving the geometry of the surfces. An exmple is the use of fins or noncirculr tubes, which increses the compctnce of the exchnger. With higher contct surfces, the ccompnied energy required to move ir cross the surfces will typiclly increse. Thus, the energy efficiency of the het trnsfer process is n importnt prmeter tht hs to be studied too. 1.1 Bckground Evportively cooled het exchngers Due to their lrge surfce re, chilled ceilings cn work with low temperture difference between room ir nd the surfce of the cooling pnel. This results in lower ir velocities in the room, so high level of indoor ir comfort cn be provided for the occupnts. When compred to conventionl systems, chilled ceilings require smller volume for the distribution system; besides this, the ventiltion rte cn be reduced to lower levels. The use of wter s the therml crrier medium insted of ir results in reduction in the energy demnded for the fluid pumping. For this reson, energy required to pump wter in wter-bsed cooling system is lower thn tht required to blow ir in n ir-bsed system. With chilled ceilings, the cooling effect could be chieved by reltively high cooling wter temperture. A cooling wter supply temperture of 18 C (Sprecher et l., 2000) or bout C (Koschenz, 1995; Fcão nd Oliveir, 2000) could be used; this could be supplied from CWCT. Three fluids flow inside CWCT: cooling wter, spry wter, nd ir. The cooling wter comes from the chilled ceiling nd flows inside tubes rrnged in rows inside the tower. Spry wter is injected onto the tube surfces nd is recirculted in closed circuit. It is n intermedite fluid in the het trnsfer process. Het crried wy from the building by the cooling wter trnsfers to the spry wter through the tube wlls. 9

12 From the spry wter, it trnsfers to ir by both sensible het nd ltent het. The ltter mkes the mjor contribution nd is cused by the evportion of smll mount of the spry wter into the ir strem. The use of the closed type of cooling towers, which is indirect contct equipment, permits high level of clenliness in the piping, resulting in effective internl het trnsfer surfces, reduced mintennce costs, nd longer opertionl life. A system consisting of closed wet cooling tower nd chilled ceilings when used for the cooling of buildings will result in Chlorofluorocrbon (CFC) free nd environmentlly clen system. The initil nd running costs of the system re low when compred to trditionl vpour-compression cooling systems. Figure 1.1 shows the bsic components of the cooling system, while Figure 1.2 shows the CWCT schemtic. Mking use of the nturl cooling effect, the CWCT brings down the cooling wter temperture to lower levels, which depend on the outdoor ir conditions nd the effectiveness of the het trnsfer process. The wet bulb temperture is, theoreticlly, the lowest temperture for ny ir-wter contct opertion, which could be reched only under the dibtic condition dibtic sturtion. The wet bulb temperture for mny loctions in Europe my permit use of the proposed cooling system. An exmple is given in Fig. 1.3 by showing the percentge occurrence of wet bulb temperture in Zurich (CH), which is bsed on hourly test-reference-yer dt. The percentge occurrence is defined s the percentge of the totl nnul hours (8760 h) during which temperture t or below prticulr temperture occurs. CWCTs re conventionlly used for industril pplictions in temperture rnge of C. Towers sized for industril pplictions will cuse overpowering in ir nd spry wter flow when used with chilled ceilings in cooling of buildings (Sprecher et l., 1996). This implies tht CWCT intended for such n ppliction should be designed ccording to empiricl dt obtined in the rnge of the operting tempertures. ir out fn chilled ceiling chilled ceiling ir in cooling wter pump spry wter pump Cooling tower Building Fig The components of the cooling system. 10

13 ir out H 2 h 2 Fn spry wter in m s t s1 cooling wter in m c t c1 cooling wter out spry wter out t c2 t s2 spry wter pump m H 1 h 1 ir in Fig A closed wet cooling tower (CWCT) Annul Occurrence (%) Wet Bulb Temperture ( o C) Fig Percentge nnul occurrence of wet bulb temperture in Zurich (CH). Fluid flow in n ECHE involves simultneous het nd mss trnsfer tht is regulted by the relevnt het nd mss blnce equtions. Prker nd Treybl (1962) presented n nlyticl method for evportive liquid coolers. The spry wter temperture vrition long the het exchnger ws considered. The solution of the governing differentil equtions ws chieved by ssuming liner reltion between the spry wter temperture nd enthlpy of the sturted ir-wter mixture. The het nd mss trnsfer coefficients were needed for the solution of the temperture nd humidity distribution. The trnsfer coefficients were found from experimentl mesurements. 11

14 Mizushin et l. (1967) conducted tests on the cores of n evportive cooler with three different tube dimeters. An ssumption of constnt spry wter temperture inside the tower ws pplied to evlute the empiricl het nd mss trnsfer correltions. The results of the mss trnsfer coefficient were presented in terms of the ir nd spry wter Reynolds numbers. In nother pper, Mizushin et l. (1968) set the design limittions for evportive liquid coolers where the generl cse of vrible spry wter temperture inside the tower ws considered. The sturtion enthlpy of ir ws ssumed s liner reltion with the spry wter temperture, while the evportion of spry wter ws ignored. Niitsu et l. (1969) tested bnks of plin nd finned tubes. The tubes were in stggered rrngement. Correltions of the plin tubes nd the finned tubes were presented. Peterson et l. (1988) pplied trnsfer coefficients estimted ccording to the Prker nd Treybl s correltion (1962) to determine the performnce of plin tubes. Field tests on n evportive condenser were performed, nd the predicted nd mesured results were compred, showing 30% underprediction in the het lod which ws ttributed to low estimted vlues of the overll het trnsfer coefficient. Dreyer nd Erens (1990) crried out experimentl work on plin tubes in cross-flow rrngement nd compred the het nd mss trnsfer coefficients with counter-flow rrngement. For CWCTs, counter-flow rrngement refers to multiple psses of the sme tube cross the het exchnger where ir flows cross the tubes, which is not purely counter flow. Koschenz (1995) presented model for CWCTs to be used with chilled ceilings. Two ssumptions were implemented: first, constnt spry wter temperture long the tower; second, the spry wter temperture ws equl to the outlet cooling wter temperture. He clculted the outlet cooling wter temperture ccording to his model nd compred it with dt from mnufcturer s ctlogue. He indicted tht the greement between the results ws fully stisfctory for prcticl pplictions. Zlewski nd Gryglszewski (1997) presented mthemticl nlyticl model for the het nd mss trnsfer equtions for evportive fluid coolers. The nlogy between het nd mss trnsfer ws pplied to find the mss trnsfer coefficient from het trnsfer correltions of fluid flow cross tube bundles. A correction for the mss trnsfer coefficient ws suggested s function of inlet ir wet bulb temperture to improve greement between clculted dt nd experimentl results for n evportive cooler. Jng nd Wng (2001) presented numericl model for closed-type cooling towers with bre tube bnks. The het nd mss trnsfer coefficients were clculted ccording to correltions from other works. Experimentl work with 60 C inlet wter temperture ws crried out. The results show tht counter flow of ir nd spry wter hs 10% higher overll het trnsfer coefficient thn tht of prllel flow. The disgreement between the numericl predictions nd experimentl dt ws 20-25%. 12

15 Computtionl fluid dynmics (CFD) hs been implemented in the nlysis of the opertion of cooling towers. Milosvljevic nd Heikkilä (2001) implemented CFD to predict the ir flow pttern inside counter flow open type cooling tower sized for industril pplictions. The differences in ir velocity upstrem of the tower s filling mteril were found to be < 5%. Furthermore, CFD ws implemented to ssess the effect of externl ir flow round the tower nd the bckflow in different wether conditions. CFD hs lso been implemented to study the temperture nd ir flow fields in CWCTs. Boundry conditions, such s het flux distribution long the cooling tower, hve significnt influence on the prediction of the therml performnce. Gn nd Rifft (1999) pplied CFD to predict the performnce of CWCT ssuming uniform volumetric het flux in the tube coils. The results showed n increse of the cooling wter temperture for the lower tube rows, which ws ttributed to the ssumption of uniform het flux genertion. In relity, the het flux is higher for the upper rows becuse of the higher inlet cooling wter temperture. In nother pper, Gn et l. (2001) ssumed liner het flux distribution long the tower, which ws twice s high for the upper rows s for the bottom rows. In conclusion of the nlysis for the pressure field, the predicted pressure losses for single-phse flow of ir over the tubes were in good greement with estimted vlues from empiricl equtions. For the therml performnce of the tower, it ws concluded tht ny ssumed het flux distribution would yield inccurte results. Therefore, CFD needs to be incorported with therml models describing the het flux distribution inside the tower. Finned tube evportively cooled het exchngers Niitsu et l. (1969) tested bnks of finned tubes in evportive liquid coolers. The tubes were 16 mm o. d. in stggered rrngement with trnsversl tube spcing of 37.5 mm nd longitudinl tube spcing of 38.1 mm. The fins were circulr, 42.6 mm dimeter. Two fin spcings were tested (6.1 mm nd 11 mm). They concluded tht the finned bnk hd much lower het nd mss trnsfer coefficients for the spry wter side compred to the plin bnk. This ws ttributed to possible wter hold up between the fins nd low fin efficiency for the wet fins (Finly nd Hrris, 1984). Kried et l. (1978) proposed theoreticl model for deluged (flooded) finned het exchngers. By introducing pproprite prmeters, they trnsformed wet-surfce het trnsfer equtions to pproximted equtions tht were in nlogy to dry het trnsfer equtions. Leidenfrost nd Korenic (1986) presented mthemticl model for finned tube evportive condensers bsed on grphicl procedure, which ws executed by computer progrm in stepwise integrtion. Mss trnsfer coefficients were estimted from dry het trnsfer coefficients. 13

16 Erens (1988) numericlly compred three different designs for n evportive liquid cooler. The first design with plin tubes, the second with the tubes plced integrlly in plstic fill, nd the third with the plstic fill plced below the tube bnk. The clcultion results indicted tht the performnce of the plin tube cooler could be considerbly enhnced by the utilistion of the fill mteril. To the disputnt s knowledge, only Niitsu et l. (1969) hs presented comprison of the performnce of plin circulr tubes nd these tubes fter being finned. The literture lcks more dt for such comprisons. Ovl tube het exchngers If het exchnger is constructed from ovl tubes (where the mjor xis is prllel to ir flow), the expected pressure drop of ir will be low. This is due to the slender shpe of n ovl tube. The lower pressure drop will result in decrese in the pumping power required to move ir cross the tubes. The disputnt couldn t find ny publiction in the open literture bout the utilistion of ovl tubes in evportively cooled het exchngers. However, ctlogue of Evpco Inc. ( mnufcturer of cooling towers, evportive liquid coolers nd evportive condensers shows tht some of the compny s products re mnufctured ccording to ptented Therml-Pk coil design in which ellipticl tubes re utilised. Enough informtion is not vilble bout this design. Dry opertion of n ECHE could bring the temperture of the process fluid to the required level when the therml lod is reltively low. In ddition, dry het exchngers hve plenty of pplictions in vrious fields. Dry ovl tubes hve been subjected to experiments for long time. Mybe the oldest work on single ellipticl cylinder mentioned in the literture is tht by Reiher (1925), s quoted by Ot et l. (1983), who reported the men het trnsfer coefficient for n ellipticl cylinder whose configurtion ws obscure. Ot et l. (1983) investigted experimentlly the therml performnce of single ellipticl cylinder with n xis rtio (mjor xis to minor xis) of 2. The Reynolds number (Re c ) ws 5000 to (where Re c is the ir Reynolds number bsed on the mjor xis c) nd the ngle of ttck ws 0 to 90. For ir flow prllel to the mjor xis, they indicted tht the Nusselt number for the ellipticl cylinder ws higher thn tht obtined for circulr cylinder from n empiricl correltion by Hilpert (1933). Ot et l. (1984) tested n ellipticl cylinder with n xis rtio of 3, with Re c from 8000 to They indicted tht the mesurements for the Nusselt number were higher thn those for circulr cylinder from Hilpert s correltion. Their comprison of the results with those from the previous work on the ellipticl cylinder with xis rtio 2 (Ot et l., 1983) showed smll increse in het trnsfer. 14

17 Kondjoyn nd Dudin (1995) studied experimentlly the effect of vrition in the free strem turbulence intensity Tu from 1.5% to 40% on the het trnsfer from circulr cylinder nd n ellipticl cylinder (xis rtio 4). The ir Reynolds number Re D ws between 3000 nd 40000, where Re D is bsed on the dimeter of n equivlent circulr cylinder. Their conclusion ws tht turbulence intensity effect is s importnt s the ir velocity effect. However, they indicted tht the Nusselt number for the ellipticl cylinder ws bout 14% lower thn tht for the equivlent circulr cylinder. For the flow round n ellipticl cylinder, Schubuer (1936) mde mesurements of the velocity distribution inside the lminr boundry lyer. Hoerner (1965) showed the drg coefficient s function of the xis rtio. For more thn one tube or for bnk of tubes, Merker nd Hnke (1986) crried out experimentl work for the het trnsfer nd pressure drop of stggered ovl tube bnks with different trnsversl nd longitudinl spcings. The xis rtio of the ovl tube ws They indicted tht ovl tubes in het exchnger will hve smller frontl re on the shell-side compred to circulr tubes. Ot nd Nishiym (1986) experimentlly studied the flow round two ellipticl cylinders (xis rtio 3) which were in tndem rrngement. The sttic pressure distribution on the surfce ws mesured nd the drg, lift, nd moment coefficients were evluted for rnge of ngles of ttck nd cylinder spcings. They concluded tht the flow chrcteristics vry drsticlly with the ngle of ttck nd cylinder spcing. Nishiym et l. (1987) investigted the het trnsfer round four ellipticl cylinders (xis rtio 2) tht were plced in tndem rrngement with ir Reynolds number Re c from to They showed tht the therml performnce of the ellipticl cylinders ws comprble to tht of in-line circulr cylinders t nrrower cylinder spcings nd t smller ngles of ttck. Slzr et l. (1997) mesured the het trnsfer from bnk of ellipticl tubes in cross flow. The xis rtio of the ellipticl tube ws 1.054, 1.26, nd The results were presented in terms of Reynolds number nd Nusselt number where the ellipticl tube minor xis ws considered s the chrcteristic length. The results indicted tht correltions of circulr tubes were slightly higher thn their mesurements for the ellipticl tubes. It could be noted tht the chrcteristic length in the non-dimensionl prmeters (e.g. Re nd Nu) hs been selected in different wys by the reserchers worked on ovl tubes, s it ws tken s the mjor xis c, the minor xis y or the dimeter of the equivlent circulr cylinder D o. For colder ellipticl tubes tht cool wrmer ir flowing norml to the tubes, Liu et l. (2003) nd Khn et l. (2004) exmined experimentlly the performnce of n rry of 18 ellipticl tubes, where the tube xis rtio ws Correltions for the Nusselt 15

18 number on the wter side nd ir side were presented in terms of the relevnt Reynolds numbers nd the dimensionless pressure drop fctor on the ir side ws evluted. For finned ellipticl tubes there re severl experimentl works: Bruer (1964) nd Schulenberg (1966) showed better het trnsfer for finned ellipticl tubes thn for finned circulr tubes, nd Sboy nd Sboy (2001) indicted no mjor differences, while Jng nd Yng (1998) indicted lower het trnsfer performnce for finned ellipticl tubes. As becme pprent, the vilble literture seemed to be inconclusive concerning the expected therml performnce of ovl tubes reltive to circulr tubes. While some works refer to better performnce, others indicte the reverse. This hs lso been concluded by Cstigli et l. (2001). However, their experimentl nd numericl study for widely spced rry of elliptic cylinders (xis rtio 2) covered only the flow field. Their results show tht the flow ws chrcterized by low turbulence levels nd poor lterl mixing. From the flow field conclusion, they expect tht the studied elliptic cylinders would not increse the rte of het trnsfer when compred to circulr cylinders. Ovl tubes in cross-flow of ir would exhibit lower ir pressure drop thn circulr tubes. The operting costs in cross-flow het exchngers re minly due to the energy required to move ir cross the tubes. While the dvntge gined from their hydrulic performnce is cler, no specil conclusions could be drwn from the vilble literture concerning the expected therml performnce of ovl tubes reltive to circulr tubes. 1.2 Objectives The generl objective of this work is to study the het trnsfer process from dry nd wet surfces for pplictions of evportively cooled het exchngers, including closed wet cooling towers, nd dry het exchngers. The study shows the effect of improving the surfce geometry on the het trnsfer nd investigtes the fesibility of such processes in terms of energy efficiency. Specific objectives of this study re: To present generl model for the therml performnce of CWCTs tht tkes into considertion the vrition of the spry wter temperture long the tower To develop procedure for the optimistion of the CWCT dimensions nd flow rtes to chieve specified therml duty with high COP vlue To simulte building performnce by globl building progrm tht includes the CWCT s model To implement simplifictions to the CWCT nlyticl model nd compre the results with those obtined from the generl model nd experimentl dt 16

19 To investigte CFD simultion of the tower s performnce by incorporting the simple tower model nd to study the effect of ir flow distribution inside the tower on the performnce To experimentlly investigte the performnce of plin tubes nd finned circulr tubes in evportively cooled het exchngers nd compre their therml nd hydrulic performnce To experimentlly study the therml nd hydrulic performnce of plin ovl tubes in evportively cooled het exchngers nd compre the performnce with tht of the plin circulr tubes To test severl shpes of ovl tubes nd circulr tube for dry het trnsfer in cross-flow of ir, nd compre their therml nd hydrulic chrcteristics 1.3 Contents This thesis comprises five ppers published in interntionl scientific journls. Ppers I to IV del with het nd mss trnsfer in ECHEs, including CWCTs, while pper V covers n investigtion into dry het trnsfer from severl shpes of ovl tubes. Pper I covers theoreticl nd computtionl study on CWCTs, nd investigtes its pplictions in cooling of office buildings in combintion with chilled ceilings. This is done by developing generl model for the therml performnce of CWCTs with vrible spry wter temperture long the tower. Bsed on this, computtionl model is developed. Experimentl mesurements for the opertion of prototype tower re considered to optimise the tower dimensions nd flow rtes for required cooling lod nd to chieve high COP vlues. The tower model is integrted into globl system simultion progrm which includes the trnsient building performnce progrm nd models for other components of the cooling system. The globl model simultes the system performnce for different loctions with different operting conditions nd studies its energy performnce nd indoor ir tempertures. Pper II studies the effects of simplifictions of the generl model by considering constnt spry wter temperture. The results re compred to those from the computtionl model nd from experimentl dt. The simplified model is incorported with CFD to ssess the effects of ir flow distribution inside the tower on the therml performnce of the tower. Pper III investigtes, experimentlly, the performnce of plin nd finned circulr tubes in n evportively cooled het exchnger. The work is extended in Pper IV to cover the performnce of plin ovl tubes. The therml nd hydrulic performnce is studied, which refers to the mount of het trnsfer from the tubes nd the ccompnied pressure drop in the ir flow cross the tubes. Pper V includes n experimentl investigtion of dry het trnsfer in cross-flow of ir. Five ovl tube shpes re studied nd the therml nd hydrulic chrcteristics re compred with tht for circulr tube hving n equivlent surfce re. The 17

20 investigtion covers three plin ovl tubes nd two ovl tubes with specil configurtions: tube with two soldered wires, nd cut-ovl tube. The energy efficiency for ech het trnsfer process hs been expressed in terms of pproprite physicl prmeters which refer to the ccomplished therml duty nd the relted energy demnd. New cquired knowledge from this thesis is bout: Appliction of CWCTs, in combintion with chilled ceilings, in cooling of office buildings: Such n ppliction hs not been investigted before Simplifictions of CWCT models nd their incorportion with CFD to study the effects on the therml performnce: Published literture for the implementtion of CFD in the therml nd hydrulic design of CWCTs does not include incorportion of simple nlyticl models Effects of introducing plte fins in evportively cooled plin circulr tubes on its therml-hydrulic performnce: There is only one comprble work (Niitsu et l., 1969) which indictes much lower het nd mss trnsfer coefficients for the spry wter side nd ws ttributed to possible wter hold up between the fins Therml-hydrulic performnce of ovl tubes in n evportively cooled het exchnger: A similr work hs not been found in the open literture Comprtive therml-hydrulic chrcteristics of five ovl tubes nd one circulr tube in cross-flow of ir for dry het trnsfer pplictions: Literture re inconclusive concerning the therml performnce of ovl tubes in reltion to circulr tubes 18

21 2 THEORETICAL AND COMPUTATIONAL ANALYSIS OF CLOSED WET COOLING TOWERS AND ITS APPLICATIONS IN COOLING OF BUILDINGS CWCTs could be used to provide cooling wter to chilled ceilings in the cooling of buildings. The cooling wter temperture chieved by the CWCT will depend on the temperture pproch to the previling wet bulb temperture in the loction. A cooling tower mking use of the free cooling effect would show vlues of the coefficient of performnce (COP) higher thn tht for vpour compression mchine. The objectives of pper I re to develop generl model for the therml performnce of CWCTs nd to investigte the ppliction of CWCTS with chilled ceilings in cooling of office buildings. The model could be used for the optimistion of the tower geometry nd flow rtes nd for higher COP vlues. The model could be prt of cooling system simultion progrm which could include trnsient building model. 2.1 Theoreticl Bckground As shown in Fig. 2.1, three fluids flow inside CWCT (cooling wter, spry wter, nd ir). The cooling wter, coming from chilled ceilings, crries the het from the building. The het trnsfers from the cooling wter to the spry wter, from which it trnsfers to ir. Tking n element of tube, which hs length of dl nd surfce re of da, Fig. 2.1b shows the flow direction for the three strems inside the element. A one-dimensionl stedy-stte nlysis is ssumed. ir out h 2 spry wter in t s1 element 1 cooling wter in t c1 tube h out H out spry wter m s t s in t c out dl cooling wter m c t c in element N cooling wter out t c2 t s out element M spry wter out t s2 m h in H in ir () ir in h 1 (b) Fig Flow strems inside the tower 19

22 2.1.1 Energy blnce Het trnsfer tkes plce from the cooling wter through the tube wll to the spry wter s result of the temperture grdient. The rte of het trnsfer from the cooling wter dq c is dq c = m C dt = U ( t t ) da (2.1) c w c o c s U o is the overll het trnsfer coefficient bsed on the outer re of the tube. It ccounts for the het trnsfer coefficient between the cooling wter nd the internl surfce of the tube α c, the tube wll therml conductivity k w, nd the het trnsfer coefficient between the externl surfce of the tube nd the spry wter bulk α s 1 U o 1 D D D 1 = + ln + (2.2) d 2k d c w s α c is tken from empiricl correltion in literture. When α s is considered s n input, it could be estimted from literture. Het gined by the ir strem dq is due to het trnsferred from the ir-wter interfce, which results in n enthlpy rise of dh. dq consists of sensible het dq sn nd ltent het dq L. dq = m dh = dq + dq (2.3) sn L Substituting for the sensible nd ltent hets dq = m dh = ( t t )da + K( H H ) h da (2.4) i i i fg where α i is the het trnsfer coefficient for the irside of the interfce, K is the mss trnsfer coefficient, h fg is the ltent het of evportion of wter, nd H i is the humidity rtio of sturted moist ir t the interfce temperture t i. The enthlpy of moist ir is h h = C t + h H (2.5) H fg where C H is the specific het cpcity of humid ir. C H = C + H C wv, where C nd C wv re the specific het cpcity of ir nd wter vpour, respectively (ASHRAE, 1997). Substituting for t nd t i from Eq. (2.5) in Eq. (2.4) nd rerrnging the terms yields 20

23 m dh i i = ( h i h ) + hfg K(1 )( H i H ) da (2.6) CH KCH where h i is the enthlpy of sturted ir t the interfce temperture t i. Noting tht the Lewis reltion ( i K CH ) ppers on the right hnd side of Eq. (2.6). The Lewis reltion cn be obtined from the Reynolds nlogy, which gives ( i K CH ) = (Sc/Pr) 2/3, where (Sc/Pr) is the Lewis number. For ir-wter vpour mixtures, (Sc/Pr) 2/3 1. However, the pproximtion to unity involves only smll error (ASHRAE, 1997). Hence, Eq. (2.6) cn be reduced to m dh = K( h h ) da (2.7) i The therml resistnce of the liquid side of the interfce could be considered negligible (ASHRAE, 1992), so tht t i = t s. Therefore, the interfce enthlpy h i in Eq. (2.7) equls h s, the sturted ir enthlpy t the spry wter temperture t s. Thus dq = m dh = K( h h ) da (2.8) s Eqution (2.8) is the Merkel eqution (Merkel, 1925). It shows tht the energy trnsfer could be represented by n overll process bsed on the enthlpy potentil difference, between the ir-wter interfce nd bulk ir, s the driving force. The energy blnce for the three strems flowing inside the element shown by Fig. 2.1b gives dq + dq + dq 0 (2.9) c s = The mount of vrition in the spry wter flow rte m s, due to evportion, is so smll tht m s could be considered constnt (ASHRAE, 1992). Hence, Eq. (2.9) becomes m C dt + m dh + m C dt 0 (2.10) c w c s w s = Spry wter temperture vries inside the tower ccording to the height of the element. If the het loss from the spry wter piping outside the tower is considered negligible, the inlet spry wter temperture t s1 will equl the outlet spry wter temperture t s2. t s1 = t s2 (2.11) 21

24 When pplied to the whole tower, the term refering to dq s will dispper from Eqs. (2.9 nd 2.10) Mss blnce The mss blnce for the element gives the rte of spry wter evportion m e (kg s 1 ) m e = m dh = K( H H ) da (2.12) s Equtions (2.1, 2.8, 2.10, 2.11, nd 2.12) govern the het nd mss trnsfer process in the tower. The Merkel eqution (Eq. 2.8) is result of the ssumption of Lewis number = 1. Another generl pproch by Poppe (1973) tkes the Lewis number in Eq. (2.6) without such n pproximtion. Erens nd Dreyer (1988) implemented the two methods on cross-flow wet liquid cooler. Their clcultions for 640 kw liquid cooler showed insignificnt differences between the results obtined by these two methods. This ws confirmed by them lter (Dreyer nd Erens, 1990) ccording to mesurement results from n evportive liquid cooler. 2.2 Computtionl model solution The solution involves dividing the tower into elementry volumes where the properties of cooling wter, spry wter, nd ir re defined on the surfce boundries of the elements, s shown in Fig. 2.1b. The exchnged dt between the djcent elements depend on the direction of the flow: horizontlly for the cooling wter, nd verticlly for the spry wter (downwrd) nd the ir (upwrd). The solution strts by tking the first element uptower, surrounding the tube t the cooling wter inlet, nd proceeds in the direction of the cooling wter flow. Itertive methods should be implemented becuse of the sctter of the tower s inlet prmeters uptower nd downtower. Figure 2.2 shows the flowchrt digrm for the temperture field solution. Finite difference representtion of Eqs. (2.1, 2.8, nd 2.10), together with Eq. (2.11), is implemented. Successive itertions re needed for the vlues of the outlet ir enthlpy h 2 nd the inlet spry wter temperture t s1 to find the temperture for ll the elements. Subsequently, the solution for the humidity distribution cn be found from Eq. (2.12) strting from the lst downtower element upwrds. 2.3 Results nd discussion A prototype tower with design cooling power of 10 kw ws mnufctured by Sulzer- Escher Wyss GmbH Lindu (DE) to operte with chilled ceilings for the purpose of 22

25 cooling of buildings s prt of the ctivities of the ECOCOOL project ( Dimensions of the prototype tower re shown in Fig Strt Input: m c, m, m s, t c1, h 1, K, α s, α c Guess vlue for h 2 Guess vlue for t s1 For elements N = 1 to M Solve for t c out, t s out, h in Next element NO Is t s1 = t s2? YES Is h 1(cl) = h 1? YES NO t c2, t s1, h 2 End Fig Flow chrt digrm for the temperture distribution clcultion Fig The prototype tower dimensions (in mm). 23

26 The tower consists of 19 tubes of 10 mm outside dimeter rrnged in 12 rows in stggered rrngement with tower width of 0.6 m. Ech row is 1.2 m in the horizontl direction. The longitudinl nd trnsversl spcing of the tubes re 0.02 m nd 0.06 m, respectively. Nominl operting conditions for the tower re: ir flow rte m = 3.0 kg s 1, cooling wter flow rte m c = 0.8 kg s 1, spry wter flow rte m s = 1.37 kg s 1, inlet cooling wter temperture t c1 = 21 C, nd inlet wet bulb temperture t wb1 = 16 C. Mesurements of the prototype tower opertion with vrious operting conditions were crried out by the Fculty of Engineering - University of Porto (Fcão nd Oliveir, 1999). The test operting conditions were: 0.58 m 1.7 kg s 1, 0.4 m c 0.8 kg s 1, 0.20 m s 1.39 kg s 1, 15 t c1 28 C, nd 10 t wb1 20 C. Mesurement results re described in (Fcão nd Oliveir, 2000). They studied the effects of the operting prmeters on the therml efficiency of the tower defined s t t c1 c2 = (2.13) c1 t t wb1 Their observtions were tht: t c1 hd very little influence on, increse of m s resulted in n increse in until complete wetting of the surfces occurred t bout 1 kg s 1 beyond tht it mde no significnt improvement, nd incresed with the increse of m nd decresed with the increse of m c. Model results Dt from the mesurements re fed to the model s input dt. To find the mss trnsfer coefficient K, the outlet cooling wter temperture t c2 from the prototype tower mesurements is tken s input to the model. The correltion eqution for the mss trnsfer coefficient concluded from 60 sets of mesurement dt is K = 0.065G 0.96 < G (kg s 1 m 2 ) < 2.76 (2.14) where G is the ir mss velocity t the minimum flow section between the tubes. This correltion is shown in Fig. 2.4 on logrithmic scle, together with correltions from other works (Prker nd Treybl, 1962; Mizushin et l. 1967; Niitsu et l., 1969). Figure 2.4 shows tht Eq. (2.14) flls within the rnge of similr correltions. Estblishing the trnsfer coefficients, K from Eq. (2.14) nd α s from literture (e.g. Prker nd Treybl, 1962), the model cn be used to predict the performnce of the tower with vrible flow rtes nd outdoor ir conditions. The prototype tower tests didn t include the tower opertion t the nominl irflow of 3.0 kg s 1. However, it is possible to predict the tower performnce for this irflow rte. Figure 2.5 shows the predicted temperture nd enthlpy distributions for the mid-row elements long the tower for the nominl conditions. Row number 1 refers to the lowest row in the tower. 24

27 1 Mss trnsfer coefficient K (kg/s m 2 ) Prototype tower: K = G Air mss velocity G (kg/s m 2 ) Fig Mss trnsfer coefficient from the experimentl mesurements: (1) Niitsu et l. (1969), (2) Prototype tower; (3) Prker nd Treybl (1962), (4) Mizushin et l. (1967). Row Number t s t t c Temperture ( C) () Air enthlpy (kj/kg) (b) Fig Clculted temperture () nd enthlpy (b) for the mid-row elements long the tower. For the upper rows, nd s result of the reltively high cooling wter temperture nd ir enthlpy, prt of the het lost by the cooling wter is retined in the spry wter, resulting in n increse of t s. While for the lower rows, the spry wter loses its het to ir t n incresed rte due to the low ir enthlpy, resulting in decrese of t s. This is shown by Fig. 2.5, which lso shows cooling wter temperture decrese nd ir dry bulb temperture decrese long the corresponding flow pths (downwrds for cooling wter nd upwrds for ir). The spry wter is directly ffected by the evportive cooling, which mkes it in closer pproch to the wet bulb temperture. The inlet ir dry bulb temperture is tken to be 20 C, nd s it ppers from Fig. 2.5, the ir dry bulb temperture is higher thn spry wter temperture. Therefore the direction of sensible het trnsfer is from ir to spry wter which results in decrese of ir dry bulb temperture. However, this behviour depends on the vlue of the inlet ir temperture. Although ir is losing het s sensible het, the enthlpy of ir is rising due to ltent het gin from the evportion of spry wter t the sturted ir-wter interfce. The increse of ir enthlpy is shown in Fig. 2.5b. h 25

28 Optimistion of flow rtes nd number of tubes nd rows Model output shows tht for the nominl tower operting conditions, the predicted rejected het q c is 9250 W nd the coefficient of performnce COP is 4.6. Bsed on the fn nd pump mesurements for other operting conditions, the estimted totl power consumption for the nominl cse is 1990 W, where the fn power is 1850 W. The COP is defined s q COP = c (2.15) W tot where W tot is the totl power consumption for the cooling tower (sum of the fn power nd the spry wter pump power). For specific geometry, the totl power consumption W tot is function of the ir velocity inside the tower. As the ltter increses, the rejected het q c will increse, but the COP will decrese. If the longitudinl nd trnsversl tube spcing is kept similr to tht of the prototype tower, study of the optimum overll tower geometry (in terms of the number of tubes nd rows) cn be crried out. This cn be chieved by considering the pressure drop dt from the test mesurements s bsis to estimte the pressure drop for vrious ir nd wter flow rtes. The cooling tower model cn be implemented to estimte the tower therml performnce where the mss trnsfer coefficient is function of the ir velocity (Eq. (2.14)). Therefore, the tower performnce cn be found for vrible ir velocities, number of tubes, nd number of rows. The rejected het increses with the increse of the contct re. Furthermore, the power consumption for lrge tower with low ir velocity is lower compred to smll tower with high ir velocity. Therefore, it could be concluded tht better COP is obtined when operting with low ir velocity nd high contct re. Model output dt for the COP nd the rejected het cn be clculted for different vlues of number of tubes nd number of rows, from which, n optimum selection of the tower geometry cn be mde. Performing such nlysis with the nominl operting tempertures nd cooling wter flow rte, it could be concluded tht when the number of tubes is 24 nd the number of rows is 18, the rejected het is 10 kw nd the COP 11.4, for which: m = 2.23 kg s 1, m s = 1.73 kg s 1, W tot = 875 W, nd fn power = 700 W. Cooling System Simultion The model developed for the CWCT is integrted into globl system performnce simultion progrm, which lso includes the trnsient building simultion model TRNSYS (1996), chilled ceiling model, night cooling strtegy, system control, nd models for other components. 26