Strojniški vestnik - Journal of Mechanical Engineering 53(2007)5, 270-284 UDK - UDC 536.7:66.045.5:621.311.22 Izvirni znanstveni članek - Original scientific paper (1.01) Izboljšanje termodinamičnih lastnosti hladilnih stolpov na naravni vlek Improvement of the Thermodynamic Properties in a Natural-Draft Cooling Tower Brane Širok - Maja Rotar - Marko Hočevar - Matevž Dular - Jure Smrekar - Tom Bajcar (Fakulteta za strojništvo, Ljubljana) Z uporabo metode CTP (Cooling Tower Profiler) v hladilnem stolpu na naravni vlek elektrarne Doel-3 (Belgija) smo določili hitrostna in temperaturna polja zraka nad izločevalniki kapljic. Meritve so pokazale, da so na obodu prečnega prereza stolpa področja z velikimi hitrostmi in nizkimi temperaturami zraka, kar ima za posledico manjšo učinkovitost v prenosu toplote in snovi. Postavljene pojavne povezave omogočajo rešitev problema s spremembo višine polnila in prerazporeditvijo masnega toka vode. Vpliv omenjenih parametrov smo preverili s simulacijo. Narejena je bila trirazsežna numerična simulacija enofaznega turbulentnega toka zraka. Lokalne izmerjene vrednosti hitrosti in temperature zraka so bile v model vključene prek izvirnih členov. Rezultati prikazujejo analizo vpliva lokalnih nepravilnosti na skupno značilko hladilnega stolpa. Predstavljeni so rezultati meritev, simulacija hitrostnega in temperaturnega polja po prerezu stolpa. Podani so ukrepi za odpravo nepravilnosti v delovanju stolpa, ki vključujejo povečanje višine polnil na obodu stolpa in ustrezno prerazporeditev celotnega masnega pretoka vode po prerezu stolpa. V numeričnem modelu smo ti dve spremenljivki opisali z lokalno spremembo izvirnih členov, kar vodi k ustreznejši porazdelitvi aerotermodinamičnih značilk in posledično k večji učinkovitosti hladilnega stolpa. Na primeru izbranega hladilnega stolpa Doel-3 je v prispevku predstavljena celovita diagnostična metoda lokalnih anomalij, ki temelji na eksperimentalnem in numeričnem modeliranju prenosnih pojavov v hladilnem stolpu. Opisana metoda omogoča povečevanje učinkovitosti delovanja hladilnih stolpov. Izračunana povprečna gostota toplotnega toka se je v tem dejanskem primeru povečala za 2,8 odstotka. © 2007 Strojniški vestnik. Vse pravice pridržane. (Ključne besede: hladilni stolpi, naravni vlek, termodinamične lastnosti, numerične analize) The velocity and temperature fields above droplet eliminators inside a natural-draft cooling tower of the Doel-3 powerplant (Belgium) were determined using the CTP (Cooling Tower Profiler) method. The measurements show regions of high velocities and low temperatures of air at the cooling tower circumference, leading to locally impaired heat and mass transfer. The established phenomenological relations enable the solution of this problem, which can be achieved by a variation of the fill height and the water mass-flow rate. The influence of these two parameters was analysed numerically. A 3D numerical simulation of a single-phase turbulent airflow was performed. Local values of the air velocity and air temperature were included in the numerical model through source terms. The numerical results present the analysis of local irregularities and their influence on the overall cooling-tower characteristics. Experimental and numerical results for the velocity and temperature fields in a cooling tower's transverse section are presented, followed by a procedure for reducing irregularities in the cooling tower's operation. This procedure includes the increase of the fill height and the rearrangement of the local cooling-water mass-flow rate in the cooling tower's transverse section. In the numerical model these two parameters were modelled by a local modification of the source terms. Modified source terms of the model lead to more uniform aero-thermodynamic properties in the tower and consequently to a higher cooling-tower efficiency. The paper presents a complete diagnostic method of local anomalies, based on the case of a representative Doel-3 cooling tower. The method is based on experimental and numerical modelling of the transport phenomena inside the cooling tower. It makes it possible to increase the efficiency of the cooling tower's operation. The calculated mean heat-flux density was increased by 2.8% in this particular case. © 2007 Journal of Mechanical Engineering. All rights reserved. (Keywords: cooling towers, natural draft, thermodynamic properties, numerical analysis) 270 Strojniški vestnik - Journal of Mechanical Engineering 53(2007)5, 270-284 OUVOD V termoenergetskih sistemih se v kondenzatorju prenaša toplota iz parnega krožnega postopka na hladilno sredstvo. Le-ta je običajno voda, ki se jo lahko črpa iz reke ali jezera oz. se jo zaradi okoljevarstvenih razlogov neprekinjeno uporablja v zaprtih sistemih -ohlajevanje vode in njena ponovna uporaba. V tem primeru toplo vodo, ki zapušča kondenzator, hladimo v hladilnem stolpu. Ker delovanje hladilnega stolpa vpliva na temperaturo vode na vstopu v kondenzator je njegov učinek bistvenega pomena za izkoristek celotnega sistema (delovanje kondenzatorja pri tem nižji temperaturi ima za posledico višji podtlak na parni strani, kar zagotavlja več pridobljenega dela iz turbine in večji izkoristek celotnega sistema). Hladilni stolp na naravni vlek deluje na podlagi prenosa toplote in snovi med vodo in zrakom, ki sta v neposrednem stiku. Voda se hladi tako v področju polnil kakor tudi v področju prhe. V protitočnem hladilnem stolpu je opazen protitok med vodo in zrakom. Zrak vstopa na vznožju stolpa in teče skozi polnila ter področje prhe. Voda, ki teče v nasprotni smeri, se z uporabo šob razprši in steče v obliki plasti skozi polnila. Bistvenega pomena za učinkovit prenos toplote in snovi v hladilnem stolpu je velika stična površina med zrakom in vodo ter velik količnik prenosa toplote in snovi. Prispevek prikazuje analizo delovanja hladilnega stolpa elektrarne Doel-3 z močjo 2040 MW. Možni problemi, ki se pojavljajo v hladilnih stolpih, so: omejen tlak prh, ki ga povzročajo odprti kanali, možnost prenapolnitve ob prehodnih pojavih - tveganje poškodbe polnil, poškodovanje polnil s peskom - zamašitev cevovodov in nalaganje nečistoč na polnilih, nerazprševanja hladilne vode zaradi odpadlih pršilnih glav, polomljene cevi in netesnosti razvodnih kanalov. Opravljene so bile standardne meritve celostnih parametrov hladilnega stolpa [1] in tudi dodatne meritve temperature in hitrosti zračnega toka z metodo CTP [2] nad izločevalniki kapljic. Raziskava lokalnega delovanja hladilnega stolpa je bila osredotočena na lokalni prenos toplote in snovi ter na lokalne količnike izgub v polnilih. Hitrost zraka nad izločevalniki kaplic je odvisna od geometrijske oblike hladilnega stolpa (na primer 0 INTRODUCTION In power-generation units (e.g., thermal and nuclear power plants) a circulating-water system supplies cooling water to a turbine condenser and thus acts as an instrument by which heat is extracted from the steam cycle to the environment. The turbine condenser is usually cooled with water from lakes or rivers, but often the use of cold, fresh water is limited for ecological reasons. Therefore, the continuous re-cooling and re-use of water in a closed system is necessary. In this case the warm water leaving the condenser is cooled in a cooling tower. The cooling tower’s operation influences the water temperature at the condenser inlet, so its performance is vital for the efficiency of the entire system, because a condenser operating at the lowest temperature possible results in a higher sub-pressure on the steam side, which in turn makes possible a higher turbine work output and overall cycle efficiency. The natural-draft cooling tower’s operation is based on a principle whereby energy is removed from hot water in direct contact with relatively cool and dry air. The water is cooled in both the fill and rain regions. In a counterflow cooling tower a gaseous phase (air) flows upwards and a liquid phase (water), in variously sized droplets, falls downwards. The airflow enters the cooling tower at the bottom and flows through the fill and the rain regions. The water is sprayed through nozzles and flows as a film down the sheets of the fill. The key factors required for intensive heat and mass transfer in the cooling tower are a large air-to-water interface area and high heat- and mass-transfer coefficients. In this study the operation of a Doel-3 (2040 MW) cooling tower was analysed. The problems that occur in the cooling tower are as follows: limited pressure of the sprayers caused by open channels, possible overflow during transients – risking fill damage and the fouling of fills by sand – clogging of the piping and deposit formation in fills, by-pass leaks due to lost end-caps, broken pipes, sprayers which have fallen off, and leaking baffles/distribution channels. Accordingly, in order to establish the operation of a particular cooling tower’s parts, standard integral measurements [1] as well as additional measurements of airflow temperature and velocity distribution above the droplet eliminators according to the CTP method [2] were performed. The investigation of the local operation of the cooling tower’s parts was focused on a determination of locally transferred heat between the water and the air and the local value of the fill-loss coefficient. The air velocities above the drift eliminators depend on the cooling tower’s geometry (e.g., the supporting walls, the water-distribution channels, etc.), on the local loss coefficient of the fill as well as on the difference between the air density inside and 271 Strojniški vestnik - Journal of Mechanical Engineering 53(2007)5, 270-284 sten, porazdelitvenih kanalov itn.), tlačnih izgub v polnilih ter od razlike gostot zraka v stolpu in okolici. Količnik izgub polnil je funkcija masnih tokov vode in zraka ter gostote in višine polnil. Lokalni količnik izgub je bil določen posredno iz padca statičnega tlaka in lokalne hitrosti nad izločevalniki kapljic. Temperatura zraka nad izločevalniki kapljic je odvisna od lokalnega prenosa toplote in snovi med vodo in zrakom. Lokalna vrednost prenesene toplote in snovi je bila izračunana iz masnega toka zraka in sprememb njegove temperature in vlažnosti. Glede na izmerjeni hitrostni in temperaturni polji zraka smo podali nekatere predloge za spremembe, ki bi se kazale v učinkovitejšem prenosu toplote in snovi med zrakom in vodo. Spremembe bi bilo mogoče doseči z zmanjšanjem tlačnih izgub v polnilih in z ustreznejšo porazdelitvijo vode po prerezu stolpa. Vpliv teh sprememb na lastnosti toka zraka je predmet tega prispevka. 1 PRENOS TOPLOTE IN SNOVI V HLADILNEM STOLPU Prenos toplote in snovi v določenem delu hladilnega stolpa je v veliki meri odvisen od lokalnih masnih tokov vode in zraka. Predstavljeni so osnovni modeli prenosa toplote in snovi v opazovanem sistemu, dobljene zakonitosti pa so vključene v enofazni trirazsežni model turbulentnega toka zraka skozi hladilni stolp na naravni vlek. Postopek hlajenja vode se odvija v območju pršil in polnil. Prenos toplote in snovi je v obeh primerih dosežen z neposrednim stikom med vodo in okolišnim zrakom. Mehanizem prenosa toplote in snovi je razlika delnim tlakov vodne pare v mejni plasti in obtekajočim zrakom ter v manj ši meri razlika temperatur med vodo in zrakom ([1] in [3]). Ker se večina toplote prenese v območju polnil ([4] in [5]), lahko območje pršil in polnil obravnavamo kot eno območje. Za ustaljene adiabatne pogoje zapišemo enačbo ohranitve energije v nadzorni prostornini dV [6\: mdadha = mwc Iz enačbe 1 vidimo, da je toplotni tok, ki ga prejme zrak, enak toplotnemu toku, ki ga odda voda. outside the cooling tower. For a particular fill the fill-loss coefficient is a function of the air and water mass-flow rates as well as of the density and the height of the fills. The airflow rate through the fill could be additionally obstructed because of broken or blocked fills and the mass-flow rate could be reduced because of sealed spray nozzles and broken or damaged splash-cups. The local loss coefficient was determined indirectly from the static pressure drop and the local velocity value above the droplet eliminators. The air temperatures above the droplet eliminators depend on the local transferred heat and mass between the water and the air. The local value of the transferred heat and mass was calculated from the air mass-flow rate and its temperature and humidity change. According to the obtained air velocity and temperature field some corrections are suggested to achieve a more efficient heat and mass transfer between the water and the air. This could be achieved by decreasing the air resistance in the fill system and by appropriately rearranging the water mass-flow rate in the cooling tower’s transverse section. The influence of these changes on the air-flow properties is reported in this paper. 1TRANSFER PHENOMENA IN A COOLING TOWER Transfer phenomena in particular segments of a cooling tower largely depend on the local water and air mass-flow rates. Basic models of heat and mass transfer in the observed system are presented later and the obtained relations are included in a single-phase 3D model of the turbulent airflow through the natural-draft cooling tower. The process of water cooling takes place in the cooling tower’s rain and fill region. In each of them the heat and mass transfer is accomplished by a direct contact between the water and the surrounding air. The heat and mass transfer is mostly driven by the difference between the partial pressures of the water vapour in the boundary layer and of the airflow, but the temperature difference between the water and the air also plays a role ([1] and [3]). Because the main heat exchange takes place in the fill region ([4] and [5]), the rain and fill regions are treated together as a fill system. For stationary adiabatic conditions the conser-vation-of-energy equation for the differential control volume, dV, can be written in the following form [6]: dTw + cpwTwd m&w (1). Eq. 1 states that the heat flux received by the air is equal to the heat flux delivered by the water. 272 Širok B. - Rotar M. - Hočevar M. - Dular M. - Smrekar J. - Bajcar T. Strojniški vestnik - Journal of Mechanical Engineering 53(2007)5, 270-284 Lokalno preneseni toplotni tok med vodo in zrakom The locally transferred heat flux between the water je odvisen od masnih tokov vode in zraka skozi and the air depends on the water and air mass flow določeno področje hladilnega stolpa ter od lokalnih through the particular segment of the cooling tower vrednosti količnikov prenosa toplote in snovi. and on the local values of the heat- and mass-trans- Krajevna masna tokova vode in zraka skozi hladilni fer coefficients. The water and airflow rates through stolp sta odvisna tudi od konstrukcijskih in okolišnih a cooling tower also depend on the structural and razmer. surrounding conditions. Različne predhodno navedene tehnične Various previously mentioned technical defects napake, ki se pojavijo po večletnem obratovanju that occur in a cooling tower after years of operation hladilnega stolpa, vodijo k neustrezni omočenosti lead to inappropriate fill-system moistening as well as to polnil ter k motenemu toku zraka skozi polnila. a disturbed airflow through the fill system. These anoma- Značilnost teh anomalij je ta, da se pojavljajo na lies can occur at various locations inside the cooling različnih mestih hladilnega stolpa in da jih je moč tower and can only be detected by the local measure- določiti le s krajevnimi meritvami temperaturnega in ments of velocity and temperature field of the cooling air hitrostnega polja hladilnega zraka v stolpu. Prenos across the cooling-tower area. The heat and mass trans- toplote in snovi ter tlačni padci v polnilih po prerezu fer as well as the pressure drop in the fills across the hladilnega stolpa so bili analizirani z numerično cooling tower’s transverse section were analysed nu- simulacijo. Izmerjene vrednosti padca tlaka v polnilih merically The measured values of the pressure drop in in lastnosti zraka nad izločevalniki kapljic so bile the fill region and the properties of the air above the uporabljene za izvirne člene v prenosnih in droplet eliminators were used as source terms in the energijskih enačbah numeričnega modela. transport and energy equations of the numerical model. Določitev prenosa toplote in snovi v The determination of heat and mass trans- določenem delu hladilnega stolpa temelji na port in a certain segment of the cooling tower is toplotnem toku, ki ga prejme zrak. Izračunamo ga po based on the heat flux received by the air. The latter enačbi [6]: is calculated using the following equation [6]: dQa=mdadha (2). Za rešitev enačbe (2) moramo poznati The solution of Eq. 2 requires the inlet and vstopno in izstopno temperaturo zraka, vlažnost zraka outlet air temperatures, the air humidity and the air in masni tok zraka. mass flow to be known. Vstopna temperatura zraka in njegova The inlet temperature and the air humidity were vlažnost sta bili izmerjeni v bližini stolpa - measured in the vicinity of the cooling tower, supposing predpostavili smo, da se vrednosti na vstopu v stolp that these parameters remain constant over the inlet area ne spreminjata. Na podlagi izkušenj prejšnjih of the cooling tower. It is also presumed that the relative meritev na različnih hladilnih stolpih [5] in priporočil humidity of the air that exits the fill system is 100%. This standarda DIN-1947 [5] smo privzeli tudi, da je presumption is based on experience and from experimen- relativna vlažnost zraka nad polnili 100 odstotna. tal results on various cooling towers [5], and is also in Temperatura zraka na izstopu iz polnil je bila accordance with the standard DIN-1947 [5]. The tempera- izmerjena. Masni tok vlažnega zraka na krajevni ture of the air exiting from the fill system was measured. ravni hladilnega stolpa je bil določen posredno z The local mass flow of the humid air through the respec- merjenjem hitrosti zraka. Entalpija vlažnega zraka je tive cooling-tower segment was determined indirectly by podana z vsoto entalpije suhega zraka in entalpije measuring the air velocity at the respective measurement vodne pare: points. The enthalpy of the humid air equals the sum of the dry-air enthalpy and the water-vapour enthalpy: ha=cpaTa+x(cpvTa+r) (3). Če uporabimo znane izraze mda = ma /(1 + x), Using the common relations mda = ma /(1 + x), ma = Vara in Va = S va, lahko gostoto toplotnega toka ma = Vara and Va = S va the heat-flux density from iz vode na zrak izrazimo z: water to air can be expressed as: Izboljšanje termodinamičnih lastnosti - Improvement of the Thermodynamic Properties 273 Strojniški vestnik - Journal of Mechanical Engineering 53(2007)5, 270-284 va ra (ha1 - ha2) 1 + x (4). Gostota vlažnega zraka je izračunana glede na izmerjeno temperaturo in predpostavko, da je relativna vlažnost zraka nad izločevalniki kapljic 100 odstotna. Izmerili smo lokalne vrednosti temperature zraka in njegove hitrosti, gostoto toplotnega toka v polnilih pa smo izračunali po enačbah (3) in (4). 2 KOLIČMK IZGUB V POLNILIH The density of humid air is calculated with respect to the measured temperature and the assumption that the relative humidity of the air above the droplet eliminators is equal to 100%. The measurements were performed to obtain local values of the heat-flux density in the fill system. The air temperatures and velocity were measured and the heat-flux density was calculated using Eqs. (3) and (4). 2 LOSS COEFFICIENT OF A FILL SYSTEM Količnik izgub v polnilih hladilnega stolpa je odvisen od padca tlaka v polnilih, ki ga lahko določimo s preizkusi. Količnik izgub je tudi odvisen od masnih tokov zraka in vode in je zato odvisen od vleka skozi hladilni stolp [7]. Padec statičnega tlaka v polnilih in količnik izgub sta povezana z [7]: The loss coefficient of the cooling tower’s fill system depends on the air-pressure drop across the fill, and this can be determined experimentally. The loss coefficient is also correlated with the air and water mass-flow rates and is therefore a function of the draft through the natural-draft cooling tower [7]. The static pressure drop through the fill system is coupled to the loss coefficient by the following relation [7]: Apfi =kfi pv (5). 3 POSTOPEK MERITEV 3 MEASURING PROCEDURE Hitrostna in temperaturna polja zračnega toka se v okviru metode CTP merijo z uporabo razvite daljinsko upravljane premične enote (si. 1), s katero je mogoče izmeriti hitrosti in temperature izstopnega In a CTP method, the velocity and temperature fields of the airflow are measured by a remotely controlled mobile unit (Fig. 1), developed to enable the air velocity and the temperature of the exit air mapping meas- HIIIHSVi : SI. 1. Premična enota s krilnim anemometrom in Pt-100 termometrom v hladilnem stolpu jedrske elektrarne Doel - Belgija Fig. 1. Mobile unit with a vane anemometer and a Pt-100 thermometer inside the Doel nuclear powerplant cooling tower (Belgium) 2 274 Širok B. - Rotar M. - Hočevar M. - Dular M. - Smrekar J. - Bajcar T. Strojniški vestnik - Journal of Mechanical Engineering 53(2007)5, 270-284 zračnega toka po celotni površini hladilnega stolpa urements over the entire cooling tower area at an arbi- v poljubni točki nad izločevalniki kapljic. trary measurement point above the droplet eliminators. Premična enota se giblje po površini The mobile unit (Fig. 2 (1)) moves gradually izločevalnikov kapljic (si. 2 (1)). Zaznavala so over the entire measuring plane - the droplet elimi- nameščena na premično enoto v skladu z zahtevami nator surface. A vane-anemometer, designed to op- standarda DIN 1947 [7]. Na premično enoto sta erate in 100% humidity, and Pt-100 thermometer sen- nameščena krilni anemometer, prirejen za delovanje sor are mounted on the mobile unit according to the v okolju nasičene vlažnosti ter Pt-100 temperaturno standard DIN 1947 [7]. The mobile unit measures zaznavalo. Premična enota izvaja meritve obeh both quantities simultaneously during its movement. parametrov med vožnjo. The external part of the equipment (Fig.2 (8)) com- Zunanji del (si. 2 (8)) sestavljajo: računalnik, ki prises a PC with the appropriate hardware for the commu- vsebuje strojno opremo za povezavo z računalnikom nications with the computer, mounted on the mobile unit premične enote (si. 2 (7)) in programska oprema za (Fig. 2 (7)), and the software for processing the acquired obdelavo dobljenih podatkov. Lega premične enote data The position of the mobile unit is determined by se določa z merjenjem radialne razdalje in kota od measuring the radial distance and the angle from a refer- referenčne točke v memi ravnini (si. 2 (2)). ence point in the measurement plane (Fig. 2 (2)). Hkrati z izvedbo zgoraj opisanih meritev se Simultaneously, measurements of the integral izvajajo meritve celostnih parametrov po standardu parameters are carried out according to the DIN 1947 DIN 1947 [7]. : vstopna in izstopna temperatura standard [14]: the inlet and outlet temperatures of the hladilne vode (si. 2 (4) in (5)), pretok hladilne vode, cooling water (Fig. 2 (4) and (5)), the cooling-water mass parametri okolice (si. 2 (6)) in izstopna moč flow, the parameters of the surroundings (Fig. 2 (6)) termoenergetskega sistema. and the output power of the thermo-energetic system. Z uporabo merjenih hitrostnih in The results of the velocity and temperature temperaturnih polj v memi ravnini hladilnega stolpa fields’ measurements in the measurement plane of SI. 2. Shema elementov meritev: (1) premična enota, (2) enota za merjenje položaja, (3) položaj merilnikov referenčnih meritev, (4) meritve temperature vstopne vode, (5) meritve temperature izstopne vode, (6) meritve parametrov okolice, (7) sistem za povezavo, (8) nadzorna meritev, obdelava podatkov in shranjevanje Fig. 2. Schematic view of the measurement elements: (1) mobile unit, (2) position-measurement unit, (3) position of the measurement equipment for reference measurements, (4) inlet-water temperature measurement, (5) outlet-water temperature measurement, (6) measurement of surroundings parameters, (7) communication system, (8) measurement control, data processing and saving Izboljšanje termodinamičnih lastnosti - Improvement of the Thermodynamic Properties 275 Strojniški vestnik - Journal of Mechanical Engineering 53(2007)5, 270-284 dobimo trirazsežne topološke porazdelitve hitrosti in temperature izstopajočega vlažnega zraka nad izločevalniki pri znanih celostnih parametrih elektrarne. Rezultati so osnova za določitev učinkovitosti prenosa toplote iz hladilne vode na okolišni zrak. Merilna negotovost merjenja temperature na premični enoti je znašala 1,5 odstotka, merilna negotovost anemometra pa 2 odstotka. Za merjenje lege premične enote pa je bila ključnega pomena merilna negotovost koračnega merilnika kota, ki je znašala 0,5° in je pri večjih oddaljenostih premične enote od izhodiščne točke značilno vplivala na določitev lege premične enote. Podrobnosti o merilni opremi, njeni kalibraciji in merilnem sistemu so opisane v [8]. Pretok hladilne vode je bil merjen z ultrazvočnim merilnikom. Vlažnost in temperaturo okolišnega zraka smo merili v bilžini hladilnega stolpa. Ker se obratovalni režim elektrarne med meritvami spreminja, smo hitrost in temperaturo zraka merili tudi v stalni točki. Te meritve so bila skupaj s celostnimi parametri, namenjene za popravo meritev na krajevni ravni. Sliki 3 in 4 prikazujeta rezultate meritev hitrosti in temperature vlažnega zraka. Iz diagramov lahko razberemo očitno nehomogenost hitrostnega in temperaturnega polja, kar kaže na to, da prenesena toplota po prerezu hladilnega stolpa ni nespremenljiva. Neenakomeren prenos toplote in snovi vodi k manjši učinkovitosti hladilnega stolpa in slabšemu izkoristku celotnega postrojenja [9]. Iz rezultatov meritev lahko sklepamo, da je ugodna rešitev težav prerazporeditev toplotnega toka v hladilnem stolpu. Povečana hitrost pretakanja hladilnega zraka na obodu stolpa (zunanje področje kolobarja s prekinjeno črto na sliki 3) in razmeroma nizka temperatura zračnega toka (zunanje področje kolobarja s prekinjeno črto na sliki 4) navaja na potrebo po povečani količini polnil odnosno na povišano plast polnil, ter hkrati povečanje dotoka vstopne hladilne vode v to področje. Nasprotno bi bilo treba dotok vstopne hladilne vode v osrednjem delu stolpa (notranje področje omejeno s sklenjeno neprekinjeno krivuljo s slik 3 in 4) sorazmerno zmanjšati ob hkratnemu zmanjšanju aerodinamičnega upora strujanja skozi osrednji del stolpa. Navedeni predlogi so v nadaljevanju prispevka ocenjeni z numeričnim modeliranjem, ki vključuje predstavljene the cooling tower provide essential data for obtaining 3D topological structures of the velocity and temperature distribution of the moist air above the droplet eliminators at known integral parameters of the powerplant. These results represent the basis for a determination of the heat-transfer efficiency between the cooling water and the surrounding air. The measurement uncertainty for the temperature measurements on the mobile unit was in the range of 1.5%, whereas the uncertainty of the anemometer amounted to 2%. The measurement uncertainty of the increment-angle measurement system was crucial for the measurement of the mobile unit’s position. Its value was 0.5o, and it significantly influenced the determination of the mobile unit’s position at greater distances from the origin point. The details of the equipment, its calibration and the measurement system operation are described in [8]. The cooling-water flow rate was measured with an ultra-sonic flowmeter. The air humidity and the air temperature were measured in the vicinity of the cooling tower. The power plant’s operating properties change during the measurements. For this reason the air velocity and the temperature at a stationary point were measured The stationary data, together with the integral parameters and the power plant’s operating data, serve as correction elements for the measurements on the local level. Figs. 3 and 4 show the results of the moist-air velocity and temperature measurements. From the diagrams in Figs. 3 and 4 the non-uniform velocity and temperature field is obvious, which suggests that the transferred heat over the area of the cooling tower is not uniform. The nonuniform field of the heat-flux density indicates that the cooling tower does not operate equally well over the entire area The non-uniformity of the heat and mass transfer leads to a lower efficiency of the cooling tower and thus to a lower efficiency of the entire powerplant [9]. It can be concluded from the measurement results that the problem of the mentioned non-uniformity could be solved by rearranging the heat flux inside the cooling tower. Higher cooling-air velocities at the cooling tower’s peripheral region (i.e., the region radially outwards of the dashed curve in Fig. 3) and the relatively low temperatures of the airflow (i.e., the region radially outwards of the dashed curve in Fig. 4) address the need to increase the fill height and to increase simultaneously the amount of the inlet mass flow of the cooling water in this specific region. The other solution is to decrease the amount of cooling water in the central region of the cooling tower (i.e., the region radially inwards of the solid closed curve in Figs. 3 and 4) as well as to simultaneously decrease the aerodinamic drag in this central region. These suggestions are assessed later on by 276 Širok B. - Rotar M. - Hočevar M. - Dular M. - Smrekar J. - Bajcar T. Strojniški vestnik - Journal of Mechanical Engineering 53(2007)5, 270-284 izmerjene vrednosti na slikah 3 in 4 kot dejanske robne pogoje v numerično shemo. V primeru drugih krajevnih anomalij, ki se kažejo kot naključne -krajevne nehomogenosti temperaturnega in hitrostnega polja na slikah 3 in 4, bi le-te morali odpraviti s krajevnim pregledom posameznih področij, npr. z uporabo pred kratkim razvite termovizijske metode za hitro odkrivanje anomalij [10], in ugotovitvijo dejanskih vzrokov, kakor so polomljeni razpršilniki kapljic, netesnosti razvodnih kanalov, zamašenih pretočnih kanalov polnil in izločevalnikov kapljic. JI* W*T* ^ •v M •# >> ^. M /^ N^^. W' -- .--------, NTV m-' s^ >*% // A • ^» * \ . 1/ / • V F/ 'i * * ' ^ - / »