© Strojni{ki vestnik 50(2004)9,413-426 © Journal of Mechanical Engineering 50(2004)9,413-426 ISSN 0039-2480 ISSN 0039-2480 UDK 532.57:53.08:536.71 UDC 532.57:53.08:536.71 Izvirni znanstveni ~lanek (1.01) Original scientific paper (1.01) Meritve tokovnega polja okrog osamljenega mehurja pare nad umetno ustvarjenim zarodnim mestom s tehniko meritve hitrosti s sliko sledilnih delcev Velocity-Field Measurements Around an Isolated Vapour Bubble Over an Artificially Produced Nucleation Site Using the Particle Image Velocimetry Technique Sanib Ba{i~ - Jure Marn - Leopold [kerget V prispevku smo podali izsledke fizikalnih meritev hitrostnega polja v območju nad zarodnim mestom, ki je dejavno med potekom mehurčastega naravno konvektivnega vrenja. Po predstavitvi temeljnih mehanizmov prenosa v preučevanem režimu vrenja smo določili vodilne cilje potekajoče raziskave, opisali merilno progo ter podali nekatere osnovne značilnosti merilne tehnike MHSSD. Opisali smo potek meritev tokovnega polja, strnili rezultate preizkusa, nato pa povzeli sklepne ugotovitve. Delo prispeva h kolikostnem vrednotenju konvektivnih učinkov med oddaljevanjem parnih mehurjev od vrelne ploščice po ločitvi od zarodnega mesta. © 2004 Strojniški vestnik. Vse pravice pridržane. (Ključne besede: vrenje mehurčkasto, zakonitosti fizikalne, tehnika merilna, polja tokovna) In this paper measurements of the velocity field in the region over a nucleation site that was active during nucleate pool boiling are presented. After the presentation of the principal transport mechanisms in the investigated boiling regime the main aims of the research are defined. The experimental rig is presented and some basic features of the PIV measurement technique are described. A procedure for flow-field measurements is described, the results are presented and we conclude with final statements. The work contributes to a quantitative determination of the convective effects during the vapour-bubble removal process on heating a wafer after departure from a nucleation site. © 2004 Journal of Mechanical Engineering. All rights reserved. (Keywords: nucleate boiling, vapour bubbles, hydrodynamics, experimental techniques, flow fields) 0 UVOD Delo, ki ga predstavljamo v pričujočem prispevku, je posvečeno preučevanju fizikalnih zakonitosti v režimu delno razvitega mehurčastega naravno konvektivnega vrenja. Slednje je definirano kot heterogena fazna preobrazba iz kapljevitega v plinasto agregatno stanje, ki poteka ob intenzivnem nastajanju osamljenih parnih mehurjev v netekoči kapljevini. Gonilna sila številnih študij na tem področju izvira zlasti iz dveh dejstev, in sicer: (a) med potekom nukleacijskega vrenja je mogoče doseči zelo velike vrednosti gostote toplotnega toka ob razmeroma majhnih vrednostih stenskega pregretja, s čimer je omogočeno izjemno učinkovito hlajenje grelnih površin, ter (b) dedna lastnost pojava je zmožnost shranjevanja velikih količin eksergije v obliki latentne toplote parne faze. Prva značilnost pojava se izrablja v številnih napravah strojne (uparjalniki, povrelniki, 0 INTRODUCTION The aim of this paper is to provide some insight into the hydrodynamics of the bubbling process during the partly developed nucleate boiling regime. This can be defined as a heterogeneous phase transition from the liquid to the vapour state, accompanied by the intensive production of isolated bubbles in an initially quiescent liquid body. The main impetus of the numerous studies performed in this area arises primarilly from two facts: (a) very high heat fluxes at relatively low values of wall superheating can be achieved during the nucleate boiling process, and due to this a very effective cooling of heated surfaces is achieved employing this modus of heat transfer; and (b) an inherent feature of the boiling phenomenon is that a vast amount of energy can be compactly stored as the latent heat of vapour phase, and afterwards it can be readily gfin^OtJJlMlSCSD 04-9 stran 413 |^BSSITIMIGC Ba{i~ S., Marn J., [kerget L.: Meritve tokovnega polja - Velocity-field Mesurements hladilniki itn.) in elektronske (procesorske enote, tiskana vezja itn.) industrije, medtem ko je druga pogostokrat izkoriščena kot ena izmed stopenj pri spremembi energije v energetskih postrojih (parni kotli in uparjalniki termoelektrarn). Ključni motiv preučevanja različnih režimov vrenja je, v nasprotju z zapletenostjo in navidezno naključnostjo samega pojava, preprost. Smiselno bi bilo izpeljati takšne matematične izraze za gostoto toplotnega toka, ki bi znotraj sprejemljive natančnosti zadovoljili inženirsko prakso in bi ne bili odvisni od velikega števila geometrijskih in postopkovnih parametrov, kar se zelo pogosto dogaja ob uporabi znanih izkustvenih korelacij. Tudi dandanes ni redkost, da se vrednosti gostote toplotnega toka, ki je določena na podlagi več različnih, sicer uveljavljenih izkustvenih korelacij, razlikujejo celo za več ko 200 odstotkov glede na eksperimentalno ugotovljeno vrednost [1]. S pospešenim razvojem fizikalnih meritev in intenzivnim kopičenjem eksperimentalnih podatkov se pojavljajo prvi fenomenološki (mehanistični) modeli prenosa toplote v režimu mehurčastega vrenja. Kljub vloženim naporom znanosti še ni uspelo podati veljavnega analitičnega modela za vrednotenje prenosa toplote, celo v režimu osamljenih mehurčkov, brez uporabe empirično določenih stalnic, s čimer se uporabnost razvitih modelov močno omejuje. Različni mehanistični modeli so tako dopolnjeni z eksperimentalnimi stalnicami in predstavljajo bolj ali manj znane polempirične korelacije prenosa toplote. 1 VODILNI MEHANIZMI PRENOSA V REŽIMU MEHURČASTEGA VRENJA V splošnem obstaja okvirno soglasje, da so vodilni mehanizmi, ki prispevajo k prenosu toplote v režimu delno razvitega mehurčastega naravno konvektivnega vrenja: (a) mikrokonvektivni učinki med potekom rasti (sl. 1a), ločevanja (sl. 1b) in oddaljevanja mehurčkov (sl. 1c) [2]; (b) prehodni prevod toplote s spremembo “makro“ mejne plasti nad grelno površino, ki poteka med inkubacijsko dobo (sl. 1d); (c) izparevanje adsorbirane “mikro“’ plasti kapljevine pod rastočim mehurčkom in vzdolž medfazne površine (sl. 1e) in (d) naravna konvekcija, ki prevladuje v področjih zunaj vpliva posameznih zarodnih mest (sl. 1f) [3]. Štirje mehanizmi potekajo hkrati, njihov relativni pomen in prispevek k skupnemu toplotnemu toku pa je zelo odvisen od sistemskih in obratovalnih razmer. Pravilno vrednotenje pomembnosti posameznega izmed omenjenih mehanizmov ostaja še vedno odprto vprašanje. Uveljavljeni model prenosa toplote s štirimi vodilnimi mehanizmi je še vedno idealizacija dejanskih razmer, kajti ta velja le hipotetično ob mehurjenju z osamljenega zarodnega mesta. ^BSfiTTMlliC | stran 414 recovered in another process. The first feature is frequently used in apparatus in the process (evaporators, reboilers, coolers, etc.) and electronic (processor units, printed circuits, etc.) industries, while another one is often applied as one of the stages in energy-conversion systems (e.g., boilers and vapour generators in fossil-fuel power plants). In spite of the complexity of the boiling phenomenon, a leading motive, and also the main goal of investigations concerning different boiling regimes, is very simple. Namely, suitable mathematical expressions for the mean heat flux are needed with acceptable accuracy satisfy engineering practice in such a way that they should not depend on many geometrical and process parameters. Even today, it is not unusual for the values of the heat flux estimated on the basis of different well-known empirical correlations to differ by more than 200% from experimental values [1]. As a result of the fast development in the area of experimental techniques and intensive data accumulation the first phenomenological (mechanistic) models of heat transfer in the bubble boiling regime have been established. In spite of many efforts the scientific community has not yet achieved an appropriate analytical model for heat-transfer prediction that is free of empirical constants, even in the regime of isolated bubbles. With this limitation the usefulness of developed models is much restricted. Thus, the different mechanistic models use experimental constants and present more-or-less known semi-empirical correlations of heat transfer. 1 GOVERNING TRANSPORT MECHANISMS IN THE NUCLEATE BOILING REGIME It is well established that the governing transport mechanisms contributing to the heat transfer in the regime of partially developed nucleate boiling are: (a) micro-convection during the growth (Fig. 1.a), the departure (Fig. 1.b) and the rising (Fig. 1.c) of the bubbles [2], (b) transient heat conduction during the reformation of the macro-boundary layer immediately above the heating surface (Fig. 1.d), which occurs during the waiting period, (c) evaporation of the adsorbed liquid micro-layer underneath the growing bubble and around the bubble interface (Fig. 1.e), and (d) natural convection that is dominant out of the regions affected by the activity of nucleation sites (Fig. 1.f) [3]. Four mechanisms take place simultaneously and their relative contribution to the overall heat transfer depends to a great extent on the system and the operational parameters. The correct prediction for these particular mechanisms still remains an open question. The proposed heat-transfer model with four governing mechanisms can be viewed only as an idealised description of the real situation because of Ba{i~ S., Marn J., [kerget L.: Meritve tokovnega polja - Velocity-field Mesurements .1 ,1 1 /^^^C 1 1___,1 1f 1 \~^—~L /3 Dd (a) (b) 4 x Dd ' i i5 - 5 5 (e) Izoterma T3__. Izoterma T2 __. Izoterma_T1___. T3 ffg(m,n)=XXf(k,l)-g(k + m,l + n) C(s) =I k=-«l=-„ ^> I1(x)I2(x-s)dx Sl. 3. Osnove merilne tehnike MHSSD [10] (korelacijska podobmočja in algoritem obdelave posnetkov) Fig. 3. Basic principles of the PIV technique [10] (investigation areas and data processing) gfin^OtJJlMlSCSD 04-9 stran 419 |^BSSITIMIGC Ba{i~ S., Marn J., [kerget L.: Meritve tokovnega polja - Velocity-field Mesurements osemenjevalnih delcev ter njihova temperaturna sedimentation and agglomeration properties of the občutljivost so bistvene značilnosti, ki narekujejo seeding particles and their temperature sensitivity are the odmik dejanske od zaželene homogene porazdelitve main features that have to be considered when we want delcev. Velikost in gostota sledilnih delcev, njihova to achieve a homogeneous distribution of added particles odsevnost oz. razpršilni količnik, moč svetlobnega throughout the flow. Additional parameters concerning vira, trajanje laserskega sunka in časovni presledek the seeding particles, which are important for successful med sunkoma ter morebitni paralaktični učinki so data acquisition, are their size, density, and reflectivity, odločilnega pomena za uspešnost opravljenih the power of the illumination system, the duration and meritev. delay of the laser pulses and the parallactic effects. 5 POTEK MERITEV 5 MEASUREMENTS Meritve so potekale pri nasičenem vrenju All of the experimental results were achieved vode in atmosferskem tlaku. Površine vrelne posode during the saturated boiling of distilled water at smo pred začetkom preizkusa vsakokrat očistili z atmospheric pressure. Before the start of the experiment razredčenim etanolom, posušili in izprali z destilirano the internal surfaces of the boiling vessel were cleaned vodo. Prehodni pojav, od zagona s sobne temperature with diluted ethyl alcohol, dried and washed out in do navidezustaljenih razmer v režimu mehurčastega distilled water. The transition from cold state to the vrenja, je trajal 30 min. Z namenom razplinjevanja vode quasi-steady state condition in the nucleate boiling v posodi, je ta pred začetkom meritev vrela najmanj regime takes about 30 min., whereas the degasification 45 min. of the water before the data acquisition lasts 45 min. Uporabili smo vrelne ploščice iz valjane Fine polished boiling plates from rolled jeklene zlitine W.Nr.1.4301 (DIN) z izmero 120x120x1 stainless-steel alloy WNr.1.4301 (DIN) with dimensions mm s svetlo poliranim sijajem površine (sl. 4a). of 120x120x1 mm were used as the boiling surface (Fig. Aritmetični srednji odstopek hrapavosti površine 4.a). The average roughness, R, in the rolling and lateral vrelne ploščice R je v smeri valjanja znašal 0,08 directions was 0.08 and 0.1 mm, respectively. A single mm, v prečni smeri pa 0,1 mm. Umetno zarodno micro-cavity (Fig. 4.b) on the boiling surface was vdolbinico (sl. 4b) smo ustvarili z nadzorovanim produced by the mechanical impression of a needle nib, prodorom stožčaste konice iz karbidne trdnine s which was made from a hard metal. The conical peak of 30o kotom izteka (sl. 4c). Povprečni premer zarodne the impressing needle used here had a 30o angle at its votlinice je znašal 106 mm, ocenjena globina pa tip (Fig. 4.c). The mean diameter of the produced cavities okrog 90 mm. Pred začetkom meritev smo v segreto was 106 mm, whereas the estimated depth of the cavities vodo dodali približno 4 g/l osemenjevalnih delcev was around 90 mm. The concentration of the added najlona, povprečne velikosti 25 mm in specifične particles was approximately 4 g/l. The seeding particles teže 1,03 kg/l . were made from nylon, with a mean diameter of 25 mm V primeru, da se je temperatura vode v and a specific density of 1.03 kg/l. posodi znižala pod 98 oC, je termostat ponovno vklopil When the temperature of the water in the sekundarni električni grelnik. Temperaturo vrelne boiling vessel reaches 98oC, a secondary electrical ploščice smo ovrednotili posredno, preko temperature heater is turned on by a thermostat immersed in the oil (a) (b) (c) Sl. 4. Vtiskovanje zarodne vdolbinice: (a) fotografski posnetek vrelne ploščice s pripravo za vtiskovanje zarodne jamice (Fotona, d.d.), (b) mikroskopski posnetek zarodne jamice ob 20-kratni povečavi, (c) mikroskopski posnetek konice vtiskovalne igle ob 10-kratni povečavi (IJS - Reaktorski Center Brinje) Fig. 4. Nucleation-site preparation: (a) photos of the boiling plate with micro-cavity immersion tools (Fotona, d.d.), (b) microscopic photos of artificially produced micro-cavity at a magnification of 20 times, (c) conical end of pitting needle at a magnification of 20 times (IJS - Reactor Center Brinje) ^BSfiTTMlliC | stran 420 i V Ba{i~ S., Marn J., [kerget L.: Meritve tokovnega polja - Velocity-field Mesurements olja na odtoku in dotoku v oljno kopel. Temperaturno znižanje oljnega toka je pri sedanji osamitvi sistema znašal manj ko 1 oC. Izstopni optični modul laserja in vstopni filter objektiva kamere sta bila od vizualizacijskih odprtin vrelne posode oddaljena za 45 cm. Uporabili smo 1,3-kratno povečavo kamere, tako da smo iz dejanske velikosti opazovanega področja 76x61 mm dobili velikost posnetka 100x80 mm na 1280x1024 elementih zaslona kamere. Pri zbiranju podatkov je čas med dvema laserskima sunkoma znašal 1000 ms, svetlobna sunka sta trajala 0,01 m s, čas med zaporednima posnetkoma je bil 500 s, velikost korelacijskega podobmočja pa je znašala 64x64 celičnih elementov. 6 REZULTATI MERITEV Na slikah 5 do 8 so podani posnetki kamere in hitrostna polja opazovanega območja toka v štirih različnih časovnih trenutkih (t0 t1, t2, in t 3) med izvajanjem meritev Glede na vrisani koordinatni sistem so lege središč mehurjev podane s koordinatami: sl. 6 (v trenutku t1) x = 43,2 mm, y = 5,9 mm; sl. 7 (v trenutku t2) x = 41,5 mm, y = 16,1 mm; sl. 8 (v trenutku t3) x = 41,5 mm, y = 37,6 mm. Na sliki 8 je vidno nastajanje novega mehurja nad zarodnim mestom. Tokovno polje okrog tega mehurja ni zajeto v tukaj podanem prikazu rezultatov. Na slikah 9 do 11 so podani diagrami poteka navpične komponente hitrosti v režimu vrenja v časovnih trenutkih t1, t2 in t3 (polne krivulje). Vsakokrat smo za prikaz izbrali tri vodoravne prereze vzdolž hitrostnega polja, ki ustrezajo neposredni bližini mehurja na posnetku (a) bath. The temperature of the boiling plate is estimated indirectly using the inlet and outlet temperatures of the oil bath. The temperature drop of the circulating oil was less then 1oC between these two points. The light-sheet optics of the PIV laser and the green filter of the CCD camera objective are placed at a distance of 45 cm from the visualization openings of the boiling vessel. A magnification factor of 1.3 was used during the data acquisition. Thus, from 76x61 mm in physical space we obtained images with a size of 100x80 mm on the 1280x1024 elements of the light sensor. The time between two successive light pulses was 1000 ms, the duration of the light pulses was 0.01 m s and the time between successive images was 500 s. A more suitable size of the investigated area wass selected, i.e., 64x64 pixels. 6 RESULTS The PIV camera images and the resulting velocity fields for the selected visualization area are presented in Figures 5-8 for four different times (t„ t, t7 in t respectively) during the experiment. For the drawn co-ordinate system the central positions of the vapour bubbles on these pictures are defined as follows: Fig. 6 (at time t) x = 43.2 mm, y = 5.9 mm; Fig. 7 (at time t) x = 41.5 mm, y = 16.1 mm; Fig. 8 (at time t) x = 41.5 mm, y = 37.6 mm The formation of a new bubble over the nucleation site is also documented in Fig. 8. The velocity field around this bubble is not presented in the results analysed here. The vertical velocity component profiles in the nucleate boiling regime at times t, t in t (full lines) are shown by the diagrams in Fig. 9-11. To produce these diagrams we used three horizontal sections along the velocity field, which are situated immediately in the vicinity of the vapour bubble position on the camera image. On each of the y x (b) Sl. 5. Odsevi sledilnih delcev na posnetku digitalne video kamere (a) in pripadajoče hitrostno polje (b) v trenutku t, med potekom naravne konvekcije pred začetkom mehurjenja nad zarodnim mestom Fig. 5. Light reflection from seeding particles on the CCD camera image (a) and resulting velocity field (b) at time t0 during the natural convection before the onset of boiling over the nucleation site. stran 421 Ba{i~ S., Marn J., [kerget L.: Meritve tokovnega polja - Velocity-field Mesurements 1 V* y l i A \ - - I ' i\\ 0\N — ' / I (a) -M . / ¦ / i j i - ..----------.— ^-wv---'/ / ' , ^ - —.-—-v/// /.....—- '/J- / / / ' C ' =-^-^ , I I ] , - ; \ I J / / \\\ I f . '----,¦- '/' * 111 < t/ '-- - ; j j t > ' / / /'" :;;i;!;?J:ii1;,1-: x (b) Sl. 6. Odsevi sledilnih delcev na posnetku digitalne video kamere (a) in pripadajoče hitrostno polje (b) v trenutku t1, med ločitvijo mehurja po začetku mehurjenja nad zarodnim mestom Fig. 6. Light reflection from seeding particles on the CCD camera image (a) and resulting velocity field (b) at time t1 during the bubble departure after the onset of boiling over the nucleation site. ¦ ¦ ¦ ¦ ¦ ¦ • i i f * * y (a) x (b) Sl. 7. Odsevi sledilnih delcev na posnetku digitalne video kamere (a) in pripadajoče hitrostno polje (b) v trenutku t2, med oddaljevanjem mehurja proti prosti gladini Fig. 7. Light reflection from seeding particles on the CCD camera image (a) and resulting velocity field (b) at time t2 during the bubble rising towards the free surface. kamere. V vseh primerih prva izmed prerezov sekata tokovno brazdo oz. področje za mehurjem, medtem ko je tretji prerez pred mehurjem v smeri njegovega dviganja. Za primerjavo smo vsem diagramom dodali poteke navpične komponente hitrosti v polju naravne konvekcije (črtkane krivulje), ki smo jih izmerili v primerjalnem trenutku t0 pred začetkom mehurjenja. Višinsko se prerezi za prikaz navpične komponente hitrosti v polju naravne konvekcije ujemajo s prerezi, ki so bili izbrani za prikaz navpične komponente hitrosti v režimu vrenja. three presented sets of diagrams (a-c), the first two horizontal sections cross flow the field behind the bubbles, whereas the third one is placed immediately in front of the bubbles in the rising direction. Moreover, on each of the diagrams a vertical velocity component profiles in the natural convection, which were measured at time t0 (dashed lines) before the onset of boiling are also added to simplify the comparison. The heights of the selected horizontal sections for the velocity profiles’ presentation in the natural convection correspond to the heights of the sections chosen for the presentation of the velocity profiles extracted from the flow field during the nucleate boiling regime. 9 ssfiri°yöVJ][M]it|sc^[] fDTMlCC] stran 422 Ba{i~ S., Marn J., [kerget L.: Meritve tokovnega polja - Velocity-field Mesurements ------^ SS ¦ ¦' ' ' •' ' ¦ s I . ¦'.¦:: ,..., y =E: (a) ¦ ¦ ¦ -.--¦¦ -.-. ¦ ¦ — ... x v (b) Sl. 8. Odsevi sledilnih delcev na posnetku digitalne video kamere (a) in pripadajoče hitrostno polje (b) v trenutku t3, med oddaljevanjem prvega in rastjo naslednjega mehurja nad zarodnim mestom Fig. 8. Light reflection from seeding particles on the CCD camera image (a) and resulting velocity field (b) at time t3 during the rising of the first bubble and growth of another one over the nucleation site. 0,04 i 0,02 0----'..... •¦•....•¦•¦..,. -0,02 0, 04 -0,04 - -0,06 - -0,08 razdalja od koordinatnega izhodišča po x osi (mm) distance from origin in x direction (mm) & 2 0 .-*.-¦ *• " * * t. .. * * y (a) 0 ,0 4 -0,02 razdalja od koordinatnega izhodišča po x osi (mm) distance from origin in x direction (mm) (b) - 0,02 razdalja od koordinatnega izhodišča po x osi (mm) distance from origin in x direction (mm) (c) Sl. 9 (a do c). Diagrami navpične komponente hitrosti V med potekom vrenja v trenutku t1 (sl. 6)(polne krivulje) in pred začetkom vrenja v trenutku t0 (sl. 5)(črtkane krivulje) v izbranih vodoravnih prerezih merilnega polja. Višina prereza na sl. 9(a): y = 1,9 mm (področje za mehurjem); višina prereza na sliki 9(b): y = 3,8 mm (področje za mehurjem); višina prereza na sliki 9(c): y = 5,8 mm (področje pred mehurjem). Fig. 9 (a to c). Diagram of vertical velocity components V during the bubble boiling at time t1 (Fig. 6) full line) and before the onset of boiling at time t0 (sl. 5)(dashed line) along the selected horizontal sections of flow field. Height of reference plane on Fig. 9(a): y = 1.9 mm (region behind the bubble); height of reference plane on Fig. 9(b): y = 3.8 mm (region behind the bubble); height of reference plane on Fig. 9(c): y = 5.8 mm (region in front of bubble). | IgfinHŽslbJlIMlIgiCšD I stran 423 glTMDDC ¦ Ba{i~ S., Marn J., [kerget L.: Meritve tokovnega polja - Velocity-field Mesurements 0,06 0,08 0,04 0,02 0 -0,02 razdalja od koordinatnega izhodišča po x osi (mm) distance from origin in x direction (mm) (a) 0,08 0, 06 0, 04 0, 02 0 -0, 02 -0,02 razdalja od koordinatnega izhodišča po x osi (mm) distance from origin in x direction (mm) (b) razdalja od koordinatnega izhodišča po x osi (mm) distance from origin in x direction (mm) (c) Sl. 10 (a do c). Diagrami navpične komponente hitrosti V med potekom vrenja v trenutku t (sl. 7)(polne krivulje) in pred začetkom vrenja v trenutku t0 (sl. 5)(črtkane krivulje) v izbranih vodoravnih prerezih merilnega polja. Višina prereza na sl. 10(a): y = 13,5 mm (področje za mehurjem); višina prereza na sliki 10(b): y = 15,4 mm (področje za mehurjem); višina prereza na sliki 10(c): y = 17,3 mm (področje pred mehurjem). Fig. 10 (a to c). Diagram of vertical velocity components V during the bubble boiling at time t (Fig. 7)(full line) and before the onset of boiling at time t0 (Fig. 5)(dashed line) along the selected horizontal sections of flow field. Height of reference plane on Fig. 10(a): y = 13.5 mm (region behind the bubble); height of reference plane on Fig. 10(b): y = 15.4 mm (region behind the bubble); height of reference plane on Fig. 10(c): y = 17.3 mm (region in front of bubble). Potek hitrosti v režimu mehurčastega vrenja na diagramih (a) in (b) na sliki 9 nazorno prikazuje zalitje vrelne površine med ločevanjem mehurčka. Sesalni učinek in intenzivnost gibanja v tokovni brazdi za dvigajočim se mehurjem sta razvidna iz diagramov (a) in (b) na slikah 10 in 11. Izrazit hitrostni vrh v navpični smeri na diagramu (c) na sliki 11 je posledica pojemajoče tokovne brazde predhodnega mehurja. Primerjava navpičnih komponent vektorjev hitrosti kaže, da je intenzivnost gibanja kapljevine v navpični smeri v režimu vrenja za en velikostni razred večja od intenzivnost gibanja v isti smeri med potekom naravne konvekcije pred pričetkom mehurjenja. 7 SKLEPI Večina do sedaj opravljenih raziskav mehurčastega vrenja temelji na preučevanju temperaturnega polja vzdolž vrelne površine. Meritve hitrostnega polja so redke. Visoka frekvenca tvorbe parnih mehurjev, majhna razsežnostna skala pojava, veliko število zarodnih mest ter pomanjkljivosti preizkusnih metod vrednotenja From the vertical velocity component profiles shown by diagrams (a) and (b) in Figure 9 it is evident that an intensive immersion of the boiling surface occurs during the bubble departure over the nucleation site. The suction effect and the large velocity gradients in the flow wake behind the bubble are shown by diagrams (a) and (b) in Figures 10 and 11. The high-velocity peak in the vertical direction on the diagram (c) in Figure 11 appears as a result of the flow wake formed behind a previously released bubble. An analysis of the velocity distribution showed that the vertical velocity components during the bubbling process are one order of magnitude higher than in the regime of natural convection before the onset of boiling. 7 CONCLUSION A large number of the investigations of nucleate boiling were conducted using experimental data of the temperature field along and over the boiling surface. Velocity-field measurements are almost unknown. The high released frequency of the vapour bubbles, the low dimensional scale of the phenomenon, the large number of nucleation sites and the crucial restrictions of the VBgfFMK stran 424 Ba{i~ S., Marn J., [kerget L.: Meritve tokovnega polja - Velocity-field Mesurements 0, 14 0, 12 0,1 0, 08 0, 06 0, 04 0, 02 0 -0,02 -0,04 -0,06 0 ,1 8 0 ,1 6 0 ,1 4 0 ,1 2 0 ,1 0 ,0 8 0 ,0 6 0 ,0 4 0 ,0 2 0 razdalja od koordinatnega izhodišča po x osi (mm) distance from origin in x direction (mm) (a) 0 ,18, 0,16 0,14 0 ,1 2~ 0,1 0,08 0,06 0,04 0,02 0 -0, 04 razdalja od koordinatnega izhodišča po x osi (mm) distance from origin in x direction (mm) (b) -0,02 razdalja od koordinatnega izhodišča po x osi (mm) distance from origin in x direction (mm) (c) Sl. 11(a do c). Diagrami navpične komponente hitrosti V med potekom vrenja v trenutku t (sl. 8)(polne krivulje) in pred začetkom vrenja v trenutku t0 (sl. 5)(črtkane krivulje) v izbranih vodoravnih prerezih merilnega polja. Višina prereza na sl. 11(a): y = 34,6 mm (področje za mehurjem); višina prereza na sliki 11(b): y = 36,5 mm (področje za mehurjem); višina prereza na sliki 11(c): y = 38,5 mm (področje pred mehurjem). Fig. 11 (a to c). Diagram of vertical velocity components V during the bubble boiling at time t3 (Fig 8)(full line) and before the onset of boiling at time t0 (Fig. 5)(dashed line) along the selected horizontal sections of flow field. Height of reference plane on Fig. 11(a): y = 34.6 mm (region behind the bubble); height of reference plane on Fig. 11(b): y = 36.5 mm (region behind the bubble); height of reference plane on Fig. 11(c): y = 38.5 mm (region in front of bubble). hitrostnega polja v režimu vrenja so bile v preteklosti težko premostljiva ovira. Z razvojem laserskih merilnih tehnik MHSSD in LDA so se tudi na tem področju odprle nove možnosti. Kolikostno vrednotenje izsledkov preizkusnih raziskav hitrostnega polja, kjer bi bile zajete zapletene interakcije številnih zarodnih mest, njihova sprožitev in deaktivacija ter vpliv le-teh na prenos toplote z grelnika na tekočino, predstavlja zahteven fizikalno-matematični model. Tukaj smo ubrali bolj preprosto pot. Omejili smo se na analizo lokalnih tokovnih razmer v okolici osamljenega mehurčka, kar omogoča vpogled v temeljne fizikalne zakonitosti pojava. Doseženi rezultati potrjujejo pomen mikrokonvektivnih tokov kot enega izmed pomembnih mehanizmov prenosa med potekom vrenja. Določene izboljšave eksperimentalne naprave, vizualizacija in meritve tokovnega polja v neposredni okolici mehurja, združitev metode MHSSD s tehniko HDVK ter samodejna sinhronizacija merilne opreme z nastajanjem mehurjev na vrelni ploščici so naslednji koraki, ki bodo opravljeni v nadaljevanju raziskave. experimental techniques affected the successful imaging of the flow field in bubble boiling. Non-invasive PIV and LDA techniques give us some new possibilities to research the hydrodynamics of the boiling process. An investigation of the flow-field distribution, where it is needed to capture the mutual interactions between numerous nucleation sites, their activation and deactivation mechanisms and the impact of this process on the heat transfer, can be found as a very complex physical model. To avoid these difficulties we analysed the nucleation process only at one artificially produced nucleation site. A flow-field analysis in the vicinity of a single bubble allows us to obtain some insight into the fundamental features of the bubbling process. The results confirm micro-convection as one of the important transport mechanisms during nucleate boiling. Some improvements to the boiling apparatus, the visualization and the measurements of the velocity field immediately in the vicinity of vapour bubble interface, the combination of PIV and DHSVC techniques and the synchronisation of the data-acquisition process with bubble production on the boiling surface will be done in the future. | IgfinHŽslbJlIMlIgiCšD I stran 425 glTMDDC Ba{i~ S., Marn J., [kerget L.: Meritve tokovnega polja - Velocity-field Mesurements 8 LITERATURA 8 REFERENCES [1] Stephan, K. (1992) Heat transfer in condensation and boiling. Springer-Verlag, Berlin Heidelberg. [2] Beer, H. (1969) Contribution to heat transfer in boiling. Progr. Heat Mass Transfer 2, 311-370. [3] Kandlikar, S.G., M. Shoji, V.K. Dhir (1999) Handbook of phase change: Boiling and condensation. Taylor & Francis, Philadelphia. [4] Kenning, D.B.R., Y. Yan (1996) Pool boiling heat transfer on a thin plate: Features revealed by liquid crystal thermography. Int. J. Heat Mass Transfer, 30, 3117-3137. [5] Forster, D.E., R. Greif (1959) Heat transfer to a boiling liquid - Mechanism and correlation. J. Heat Transfer, 81, 43-53. [6] Mikic, B.B., W.M. Rohsenow (1969) A new correlation of pool boiling data, including the effect of heating surface characteristics. Journal of Heat Transfer, 9, 245-250. [7] Judd, RL., K.S. Hwang (1976) A comprehensive model for nucleate boiling heat transfer including microlayer evaporation. J. Heat Transfer, 98, 623-629. [8] FlowMap - Particle image velocimetry instrumentation. Dantec Measurement Technology, Denmark, 2000. [9] Durst, F (2002) Osebno sporočilo. Friedrich-Alexander Universität - Technische Fakultät, Erlangen- Nürnberg. [10] Raffel, M., C.E. Willert, J. Kompenhans (1998) Particle image velocimetry. Springer-Verlag Berlin Heidelberg. Naslov avtorjev: mag. Sanib Bašič prof dr. Jure Marn prof. dr. Leopold Škerget Univerza v Mariboru Fakulteta za strojništvo Smetanova 17 2000 Maribor sanib.basic@uni-mb.si jure.marn@uni-mb.si leo@uni-mb.si Authors’ Address: Mag. Sanib Bašič Prof. Dr. Jure Marn Prof. Dr. Leopold Škerget University of Maribor Faculty of Mechanical Eng. Smetanova 17 2000 Maribor, Slovenia sanib.basic@uni-mb.si jure.marn@uni-mb.si leo@uni-mb.si Prejeto: Received: 15.4.2004 Sprejeto: Accepted: 30.9.2004 Odprto za diskusijo: 1 leto Open for discussion: 1 year VH^tTPsDDIK stran 426