International Journal of Heat and Fluid Flow 32 (2011) 777–784

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International Journal of Heat and Fluid Flow

journal homepage: www.elsevier .com/ locate/ i jhf f

Pool boiling heat transfer of tandem tubes provided with thenovel microstructures q

Alexander Ustinov a,⇑, Victor Ustinov b, Jovan Mitrovic a

a University of Paderborn, 33095 Paderborn, Germanyb RWTH Aachen, 52062 Aachen, Germany

a r t i c l e i n f o a b s t r a c t

Article history:Received 10 September 2009Received in revised form 1 March 2011Accepted 6 April 2011Available online 8 June 2011

Keywords:BoilingHeat transfer enhancementMicrostructureThree phase lineTubeTandem

0142-727X/$ - see front matter � 2011 Elsevier Inc. Adoi:10.1016/j.ijheatfluidflow.2011.04.001

q This paper was partially presented at ECI InterTransfer and Fluid Flow in Microscale, held in WhistlerSeptember 2008.⇑ Corresponding author. Tel.: +49 1762 993 08 14.

E-mail address: alexander.ustinov@gmail.com (A.

The paper presents experimental data on pool boiling heat transfer of tandem tubes, arranged one abovethe other in the same vertical plane. The outer surface of the tubes is provided with the novel microstruc-tures. The structure elements are micropins created by electrolytic deposition of copper upon the tube,using a specially treated polycarbonate foil. By this technique the pins diameter can be varied from0.1 lm up to 25 lm, the pins height goes up to 100 lm at densities up to 1 � 109 pins/cm2 and pins incli-nation almost up to 180� regarding the base surface. Micropins with several different inclinations can becreated simultaneously on the same surface.

Experiments were conducted with two different microstructures using the refrigerant R134a and thehighly wetting Fluorinert liquid FC-3284 at pressures of 5–9 bar and 0.5–1.5 bar, respectively. The advan-tages of the novel microstructure regarding the boiling heat transfer for tandem tubes turned out to bepractically the same as for a single tube arrangement. Microstructured tubes have the superheat indepen-dent on the heat flux, they show a very low boiling inception superheats (below 2 K), are highly effectivein comparison with a technically smooth tubes, and operate stable over the long periods of time.

� 2011 Elsevier Inc. All rights reserved.

1. Introduction

Application of boiling process ranges from the heat transfer insteam power plants to the thermal decomposition of liquid mix-tures. Boiling has earned the attraction due to large heat transfercoefficients in comparison with single phase forced convection.One can enhance the performance of a heat exchanger operatedas boiler even more with a proper structuring of its surface. Suchstructure should reduce the driving temperature difference, whatusually is achieved by increasing the nucleation sites density. Orig-inally, structures were generated by the mechanical machining,creating different kinds of micro-fins and cavities in the heatingsurface, that along with the enhanced nucleation sites densityshould increase the bubble detachment frequency as well.

Since approximately 50 years re-entrant cavities of differentdimensions were generated and extensively tested (Griffith,1965). Other types of microstructures have been developed by em-ery paper treatment and sandblasting of the base surface (Gorenfloand Kotthoff, 2005; Luke, 2009), etching (Zhang and Lian, 2008), byproviding the heating surface with sintered or plasma-sprayedporous layers (see e.g. Ferjancic et al., 2006) and implementation

ll rights reserved.

national Conference on Heat, Canada, between 21 and 26

Ustinov).

of ions by bombardment of the surface by high energy particles(Mueller-Steinhagen and Zhao, 1997). Disadvantages of such sur-faces and processes of their creation have often economicalgrounds. Together with the tendency to dry out already at low heatfluxes, especially the multi layer structures, they demonstrateunexpectedly strong deterioration of boiling, and do not fit theneeds of industrial applications.

For the very first time a new boiling microstructure consistingof micropins with exceptionally high pin density has been devel-oped in the Institute of Thermal Process Engineering and PlantTechnology of University of Paderborn in a joint work with theInstitute for Nuclear Research from Dubna, Russia, and the com-pany ‘‘l-Technik’’ from Quedlinburg, Germany. The goals were tocreate a microstructure for evaporation processes, meeting the fol-lowing requirements:

– high heat transfer enhancement rates;– low boiling inception superheats;– high critical heat flux;– low variation of the surface superheat with varying applied heat

flux;– long time performance stability; and– low production costs.

Such microstructures has been developed and successfullytested earlier with different geometries under free convection pool

Nomenclature

d diameter, mf specific Gibbs energy, J/kgh q/DT, heat transfer coefficient, kW/(m2K)L distance between nucleation sites, mN density of micropins, 1/m2

q heat flux, kW/m2

S area, m2

TPL three phase lineDT wall-to-saturation temperature difference, KV volume, m3

Greek symbolsDU energy barrier for a nucleus, J

q density, kg/m3

r surface tension, J/m2

H contact (wetting) angle, deg

Sub- and superscriptsV vaporv vapor veinL liquidW wallLT lower tubeTT test tubePIR pressure sensorTIR temperature sensor

Inclined pore Polycarbonate foil

(a). Schematic view of a preprocessed polycarbonate foil with inclined pores.

Polycarbonate foil

Metallic specimen

(b). Foil attached to a specimen.

Metallic specimen

Growing structure

(c). Process of electrodeposition.

Metallic specimen

(d). Schematic view of the microstructure after foil stripping.

Fig. 1. Main stages of the microstructure generation.

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boiling of different liquids, namely R141b, R134a, and FC-3284. Forsome of these structures, experimental results of investigations(Mitrovic, 2006; Mitrovic and Ustinov, 2006a,b; Ustinov and Mitro-vic, 2007, 2008), have shown practically full independence of thesurface superheat on the applied heat flux. This quite unique fea-ture is attributed to the extremely large length of three-phase line(TPL) within the microstructure, formed on the micro-scale by thegreat number of pins, piercing growing vapor bubbles. The heattransfer enhancement rate up to 18 times in comparison with atechnically smooth surface has been observed for a single tubeconfiguration along with low boiling superheats (below 10 K),and long time performance stability, see Ustinov and Mitrovic(2007). As reported by Wei et al. (2011) boiling structures withmicropins are unlike other types of structures very effective alsounder microgravity boiling conditions. Boiling experiments onbasically similar structures have been reported by Im et al.(2010), Lee et al. (2010), Zhang and Lian (2008). For further refer-ences the reader is referred to the review paper by Wei and Xue(2011).

In these papers single tubes or small flat surfaces were used. Inpractice, however, single tubes are scarcely used in boiling equip-ments. Instead several tubes are arranged in a bundle and im-mersed in boiling liquid. Vapor generated on lower tubes passesthe bundle thereby establishing a two-phase flow which affectsthe heat transfer on individual tubes. Several papers in this areahave been conducted and the so-called bundle effect quantified(Hahne, 1983; Hahne and Müller 1983; Hahne et al., 1991; Liuand Qiu, 2006) using commercial tubes. Ribatski et al. (2008) rec-ommended correlation for bundle effect in wide ranges of systemparameters. As the authors are aware, no such experiments withtubes provided with a micropin-structure have been reported sofar.

This paper delivers experimental information on the boilingheat transfer from tandem tubes, arranged one above the otherin the same vertical plane. Experiments were conducted withtwo microstructured surfaces with different geometry using theRefrigerant R134a at pressures of 5 –9 bar and FC-3284 at pres-sures of 0.5 –1.5 bar. The article begins with a short explanationof the microstructure production technology, followed by thedescription of the experimental apparatus, the experimental re-sults and the conclusions.

2. Microstructure

Main steps of creation of the microstructure are illustrated inFig. 1. The very first step is the irradiation of a thin polycarbonatefoil (thickness 6100 lm) with heavy ions. Passing through the foil,ions break intermolecular bonds on their ways, leaving the traces

behind. These traces are widened to pores by combination of pro-cesses of ultraviolet irradiation and chemical etching (Fig. 1a). Den-sity, inclination and diameter of the pores can be altered byvariations of density and inclination of the ion beam(s) and thepost-processing duration, respectively. Preparation and processingof the polycarbonate foil was performed in the Laboratory for Nu-clear Reactions of Joint Institute for Nuclear Research in Russia. Tocontinue generation of the structure, prepared foil is attached to aspecimen (Fig. 1b), and the ensemble is subjected to an electrolyticprocess, in which the pores are filled by deposition of ions (Fig. 1c).In a further etching step, the foil is completely removed, leavingbehind pins metallically connected to the surface of the specimen(Fig. 1d). The height of the pins can be varied by the duration of the

Fig. 2. SEM-photographs of microstructured surfaces.

Table 1Geometry parameters of tested surfaces.

Structure Pinsheight(l)

Pinsdensity(cm�2)

Pinsdiameter(l)

Structureporosity (%)

Pinsinclinationangle deg.

A428 55 2.5 � 106 3.5 46 ±20A437 58 3.3 � 106 3.7 37 +40/�13A425 51 7.6 � 105 7.63 34 ±15

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electrodeposition, but it is limited to the foil thickness. Appropriatemanaging of the galvanic deposition may result in different shapesof tops of the pins. The whole process permits largely continuous

variation of the structure parameters. Microstructures with suchproperties can be generated in almost all electrochemically depos-itable materials. In our investigation both the base surface (tubes)and the generated structure were made of copper because of itshigh thermal conductivity and advantages, regarding theelectrodepositing.

As an example, Fig. 2 shows structures obtained in this way.These structures were used in experiments the results of whichare reported below. Table 1 lists geometry properties of the micro-structures A428 and A437 used for the upper tube in the tandemarrangement, while the microstructure A425 was used for the low-er tube.

3. Experimentals

The experiments were performed using a modified apparatusdescribed by Mitrovic and Fauser (2001) with a closed loop forthe test fluid, Fig. 3. Test tubes were arranged horizontally in thetest container A one above another (Fig. 4). Below the containerA, a pre-heater D was located, to ensure the saturation state of li-quid, leaving condenser B. To exclude the influence of vapor bub-bles generated in the pre-heated D on the boiling inceptionsuperheat of the test tube, the bubbles were channeled along theinside surface of container A. An additional evaporator C was em-ployed to change the system pressure and to reach the saturationstate in the system quickly, because of the large mass of the testrig. It is also used to regulate and keep the pressure in the systemtogether with variation of temperature and coolant flow throughthe condenser B. The apparatus, carefully evacuated, was filledwith the liquid by distillation under reduced pressure, created bycooling the condenser. During distillation, vapor from the evapora-tor F was condensed in the condenser B and the distillate is used astest fluid. Possible traces of remaining air in the apparatus were re-moved by venting the condenser at elevated pressure.

Two copper tubes of 18 mm outer diameter each were used inthe tests, Fig. 5. Microstructured tubes were stored in a dry airatmosphere below room temperature in a dark place, what insuresnon-oxidation conditions for copper and stable state of the surface.Holes for thermocouples in tubes (4 in each tube: on top, bottomand both sides) were generated by electrolytic deposition of a cop-per layer onto the tube surface that was correspondingly prepared.Tubes were provided with 2 kW heating cartridge coaxially placedinside. Connected to two separate DC power suppliers, thisarrangement allows independent regulation of the heating loadsfor each tube at heat fluxes up to 175 kW/m2.

In the experiments, the voltage on the cartridge heater in thetubes was first increased in steps of 20 V from 40 V up to 200 V(corresponds to maximum heat flux of 125 kW/m2), and then de-creased in steps of 50 V back to zero. In order to ensure the stea-dy-state, the voltage increase was suspended for a while afterthe boiling inception. To maintain this state during one experimen-tal run, the coolant temperature and/or the coolant flow rate in thecondenser was adjusted according to the variation of the tempera-ture. System pressure and temperature were varied by changingthe energy input (preheater D, heaters in tubes, evaporator C)

Fig. 3. Schema of the test apparatus.

Fig. 4. Photo of the test tubes arrangement.

Fig. 5. Construction of the test tube.

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and by changing the coolant temperature and/or its mass flow ratethrough the condenser B. The saturation temperature was keptconstant within ±0.05 K in each experimental run. The measure-ments were done after reaching the steady-state at the chosen heatflux.

The precision of measurements was analyzed by the method ofKline and McClintock (1953), and in more details is described by

Mitrovic and Fauser (2001). The experimental uncertainty of thetemperature measurements was estimated to be less than 1.50%,and 1.44% for the heat flux.

4. Results

4.1. Single tube results

Before presenting the results of the measurements with tandemtubes, the main conclusions drown from the single tube experi-ments shall be summarized. The superheat of the novel micro-structured surface during nucleate pool boiling remainsunaffected by the applied heat flux. This unique property is attrib-uted to the extremely large number of nucleation sites, formed be-tween the micropins, and very large lengths of the TPL, created bythe pins, piercing vapor bubbles, see Mitrovic (2006) and Mitrovicand Ustinov (2006a,b, 2007, 2008). The TPL acts as a very efficientheat sink, and it makes the surface temperature vastly independenton the heat flux. In several boiling modes the enlargement of theTPL with the increasing heat flux was so strong that the surfacesuperheat was reduces, what is in contradiction with the commonexperience; see Mitrovic and Ustinov (2006b, 2007, 2008), formore details.

To understand the reasons of this behavior, one should startwith the very first stage of a vapor bubble history, which is theappearance of a vapor nucleus. It is agreed among researches thatfirst nuclei are created in the liquid due to fluctuations of its den-sity. For this fluctuation process there is an energy barrier to besurmounted in order to create a vital vapor bubble. In the case ofinitial nucleation upon a solid surface this quantity was givene.g. by Labuntsov (1959) as:

DU ¼ ðfV � fLÞVqV þ rS� rSWð1� cos hÞ: ð1Þ

Eq. (1) is developed from the fundamental thermodynamic ap-proach that considers the difference of two energy states: right be-fore and just after the appearance of a nucleus, not considering theway of transition between them. A high of the energy barrier (1) isobserved for nucleation inside of the bulk liquid when no wall ispresent (h = 0), and the corresponding minimal diameter of a spher-ical nucleus, also called ‘‘critical’’, is given by:

dcr ¼4r

qV ðfV � fLÞ: ð2Þ

Only surface cavities with size deduced from Eq. (1) can affectthe initial nucleation upon a surface. In larger cavities the energybarrier DU reaches same values as for nucleation inside of the bulkliquid with the corresponding nucleus size expressed by Eq. (2). Inother words, Eq. (2) determines the upper margin for a size of acavity, where nucleation is preferable in comparison with the bulkliquid. For different liquids and pressures, Eq. (2) gives the criticalvapor bubble diameter in range of 1 � 10�6 m to 1 � 10�5 m, whatis confirmed experimentally; see for example Luke and Gorenflo(2000). Enhanced surfaces obtained with majority of existing tech-nologies provide cavities normally much larger than 10�6 m. Novelmicrostructured surfaces, investigated in this work, serve a greatnumber of cavities with the characteristic size, associated withthe distance L between neighboring pins:

L � 1=ffiffiffiffi

Np

; ð3Þ

where N is the number density of micropins. For microstructureA437 this value is equal to 5.5 � 10�6 m, a size, which is practicallyunreachable for enhanced surfaces for boiling, manufactured withother technologies.

The micropins are distributed homogeneously on the surface;therefore all the cavities formed by them have approximately the

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same size and the same activation probability at a given superheat.Their activation usually occurs at heat fluxes below 5 kW/m2. Dueto its unique properties, the microstructure demonstrates anothernon-conventional feature of the nucleation process: existence ofbubble chains, Fig. 6. Occurrence of vapor bubbles is accompaniedby the formation of a network of vapor veins, connecting the bub-bles centers. New vapor bubbles occur practically without thewaiting time, being fed with vapor by the veins, and almost themaximal possible number of active nucleation sites is reached al-ready at low heat fluxes. Further increase of the heat flux doesnot lead to significant increase in number of active nucleation sites,so the surface superheat remains constant. Bubble chains wereoriginally described first by Ustinov and Mitrovic (2007). Their for-mation may be viewed as a critical autonomous organization of thesystem thereby evolving in a manner that shall reduces the systemstresses. An increase of heat flux usually rises the driving temper-ature difference, which shifts the system further from the equilib-rium state. The microstructure, however, responses to thesechanges by increasing the TPL thereby tending to establish a homo-geneous net of vapor channels in the microstructure; as a conse-quence, the driving temperature difference caused by heat fluxrise decreases. The formation of bubble chains fit into the well-known Theorem of Moderation.

The microstructuring technology allows creation of surfaceswith intersecting inclined micropins forming the so-called ‘‘bi-cav-ity’’ patterns. Boiling upon such surfaces is accompanied at moder-ate heat fluxes by a sharp, step-like change of the surface

rb

rv

L

Micro pin

Fig. 6. Top: formation of bubble chains upon microstructured surface A437. Boilingof R134a at 5 bar, q = 32 kW/m2, DT = 1.2 K. Bottom: schema of two nuclei,connected with a vapor ‘‘vein’’.

superheat, which later remains unaffected by further heat flux in-crease. This process was named by Ustinov and Mitrovic (2007)‘‘boiling re-establishing’’. It seems to be caused by activation ofcavities of two characteristic sizes, formed by intersecting micro-pins. All structured tubes tested in this work have inclined micro-pins as structure elements.

To estimate the microstructure efficiency the enhancement fac-tor was calculated which is determined as a ratio of the experimen-tal heat transfer coefficient to the heat transfer coefficient,calculated in accordance with recommendations of Gorenflo(1993) for a smooth tube under the same conditions. As experi-mental the spatiotemporal average value of three heat transfercoefficients on the test tube circumference was used. Measure-ments were done for increasing and decreasing heat fluxes. Themicrostructured surface A437 was found by Ustinov and Mitrovic(2007) to be up to 18 times more efficient with boiling R134a incomparison with a technically smooth tube under idem conditions.The enhancement factor depends on the geometry of the micro-structured surface, system pressure and properties of the liquid.All microstructured surfaces were found to be more efficient atlow pressures, as the critical vapor bubble diameter becomes com-parable with the distance between micropins. The most effectiveboiling modes were accompanied by the boiling inception as atwo-phase front; see Mitrovic and Ustinov (2006a,b, 2007, 2008).The velocity of the front depends on the microstructure geometry.

4.2. Tandem tubes

The main purpose of the experiments with tandem tubes was toverify the conclusions of the single tube experiments for industryapplication, and to evaluate the influence of the lower tube onthe heat transfer on the upper one. Experiments were conductedwith two liquids, refrigerant R134a at pressures of 5 bar, 7.5 barand 9 bar, and highly wetting liquid FC-3284 at 0.5 bar, 1.0 barand 1.5 bar. Three types of measurements have been carried out:

(a) First type: The heat fluxes of both tubes were varied simul-taneously at an equal rate.

(b) Second type: The heat flux on the lower tube was kept con-stant while the upper tube experienced the whole range ofheat flux variation. The lower tube heat fluxes were: theminimal one required for the boiling inception, the maximalone for the fully developed boiling, and one in between.

(c) Third type: Opposite to the second type, one of three heatfluxes on the upper tube was kept constant while that onthe lower tube was varied.

Experiments with equal heat fluxes on both tubes modeled acommon practice of a stationary long time operation. Experimentsof second and third type modeled a situation, when a heat exchan-ger with microstructured tubes operates under non-stationarystart/stop conditions, or when the heat output of different tubesin a bundle is not equal.

4.3. Effect of lower tube

Fig. 7 presents results of the experiments of first type in form ofboiling curves of R134a at 5 bar. Test tube A437 demonstrates alow variation of the surface superheat from 0.25 K up to 3 K, whileheat fluxes on both tubes A437 and A425 vary simultaneously inrange from 5 kW/m2 to 125 kW/m2. Three curves in the figure cor-respond to three different positions of wall-thermocouples on top,side and bottom of the test tube (one side wall-thermocouple wasused for temperature control and shut-down the electric heating).These results confirm the main feature of the novel microstructureregarding the heat transfer during boiling – the practical indepen-

Fig. 7. Boiling curves of R134a at 5 bar with tandem A437 and A425. Equal heatfluxes on both tubes. Arrows show the direction of heat flux variation.

Fig. 8. Boiling curves of R134a at 5 bar with tandem A428 and A425 at constant, butdifferent heat fluxes on the lower tube. Arrows show the direction of heat fluxvariation.

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dency (or weak dependency) of the surface superheat on the heatflux. The surface superheat of the test tube in tandem arrangementis lower than that in single tube experiments; see Ustinov andMitrovic (2007). This happens due to two reasons: first, the turbu-lence near the test surface, created by the vertical bubbly flow fromthe lower tube, and second, the nucleation process from the vaporrest, captured by the microstructure of the test tube. The intensityof these processes drops with increasing tube spacing. Therefore,the temperature at the top of the test tube was normally slightlyhigher than that at the side or bottom of the tube.

Results of experiments of second type for R134a boiling at 5 barare depicted in Fig. 8. Measurements were conducted at constantheat fluxes on the lower tube A425 of 8 kW/m2, 50 kW/m2 and125 kW/m2, while the heat flux on the upper tube A428 was variedbetween 5 kW/m2 and 125 kW/m2. The spatial temperature gradi-ent can be observed upon the microstructured surface, arising fromdifferent both nucleation and bubble removal conditions on differ-ent areas of the tube. The gradient is suppressed by the increasingvelocity of the two phase flow, due to increasing lower tube’s heatflux. The temperature at the bottom of the test tube slightly rises,side temperature remains practically constant, and the tempera-ture at the top of the tube decreases, thus reducing the heterogene-ity of the surface temperature field.

The effect of temperature decrease on the top of the test tube isobserved in Fig. 8 as well as in single tube experiments. This effectis particularly evident at the heat flux of 50 kW/m2 on the lowertube. The system organizes the boiling process in a way, obeyingthe Theorem of Moderation – an external influence at a certainheat flux causes several processes inside of the microstructure tocompensate this influence. To such processes belongs the mecha-nism of extension of the TPL length, when greater numbers ofmicropins pierce larger vapor masses, and possible additionalbubble nucleation from the vapor rests, captured by themicrostructure.

The interaction of processes of vapor generation by the test tubeand the two-phase bubbly flow from the lower tube is not linear.Results of experiments of third type (constant heat flux on uppertube, varying on lower tube) with R134a boiling at 5 bar are de-picted in Fig. 9 for three different heat fluxes on the upper test tubeA428, with varying heat flux on the lower tube A425 in range be-tween 5 kW/m2 and 125 kW/m2. One can see, that the highest tem-perature is observed on the top of the upper test tube at heat fluxof 32 kW/m2. However, when the heat flux is further increased upto 125 kW/m2, the superheat of the upper tube reduces to 1.5 K.The maximal influence of the lower tube is observed when the heatfluxes of the test tube lies between 30 kW/m2 and 50 kW/m2 cor-responding to heat fluxes of the boiling re-establishing process,while at higher heat fluxes the compensational mechanisms inside

the microstructure of the test tube reduce its superheat. Obviously,the processes occurring on the micro-level within the microstruc-ture decisively affect the heat transfer enhancement.

4.4. Effect of system pressure

In earlier experiments with single microstructured tubes it wasobserved that the efficiency of the novel surface decreases withincreasing pressure, see Mitrovic and Ustinov (2006a,b, 2007).The same behavior was observed here in experiments with tandemtubes. It happens because the critical vapor bubble radius reduceswith rising pressure, what makes the microstructure too ‘‘rough’’to support the initial nucleation. At the same time, smooth tubeefficiency is increasing with rising pressure, because of activationof nucleation sites of sub-micron sizes, which are always presenton a real heat transfer surface. Fig. 10 presents the boiling curves

Fig. 9. Wall superheats of upper test tube A428 vs. heat flux on lower tube A425 forboiling R134a at 5 bar. Arrows show the direction of heat flux variation.

Fig. 10. Pressure effect on boiling characteristics of R134a with tandem A437 andA425 at equal heat fluxes on both tubes. Data are depicted for the increasing heatflux.

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of R134a with tandem A437 and A425 for a rising heat flux. Everypoint corresponds to a superheat, averaged on three simultaneousmeasurements on top, side and bottom of the test tube at a givenheat flux. The surface average superheat increases with the risingpressure at heat fluxes above 50 kW/m2, preceded by a non-lineardependency on pressure below this value. The highest averagesuperheat of the test tube was found to be below 3 K, showing veryhigh efficiency of the microstructure.

Fig. 11. Effect of the microstructure geometry on boiling curves of R134a at 7.5 bar.Arrows show the direction of heat flux variation.

4.5. Effect of microstructure geometry

Generally, it is well-known that the length of the TPL is a pivotalparameter in heat transfer nucleate boiling. After a vapor bubblewas created, further evaporation is governed by the heat and masstransport in a micro-region near the bubble base. The micropins ofthe investigated structure penetrate growing vapor bubbles, thus

increasing the TPL lengths greatly and subsequently rising theamount of heat energy consumed by the bubbles.

In earlier experiments with single tubes it was observed thatthe efficiency of the novel microstructure depends largely on thedensity of micropins, and structures with closer located pins dem-onstrate higher heat transfer coefficients and lower surface super-heat, see Ustinov and Mitrovic (2007). As example, Fig. 11compares boiling curves of R134a obtained with the microstruc-ture A437 and A428 at 7.5 bar for tested tandem. MicrostructureA437 has inclined micropins (Fig. 2) and a higher number densityof pins which in comparison to the structure A428 results in lowersurface superheat and, therefore, a higher efficiency. Surface super-heats shown in Fig. 11 are averaged on the tube circumference for agiven heat flux. The hysteresis of boiling curves depends on themicrostructure geometry as well.

4.6. Effect of liquid properties

Along with the refrigerant R134a, experiments have been car-ried out with a highly wetting liquid FC-3284 at pressures of0.5 bar, 1 bar and 1.5 bar. This single component liquid is a clear,colorless, thermally stable, fully-fluorinated substance, and itscomposition does not shift or fractionate with time. In earlierexperiments with single microstructured tubes the heat flux of70 kW/m2 was found to be critical for FC-3284, see Ustinov andMitrovic (2007, 2008), therefore, experiments with tandem tubeswere conducted at heat fluxes below this value. Results of thesemeasurements at pressure of 1 bar with tandem A428 and A425are presented in Fig. 12.

Fig. 12. Boiling curves of FC-3284 at 1 bar with tandem A428 and A425. Arrowsshow the direction of heat flux variation.

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Experiments with FC-3284 have confirmed all features of thenovel microstructured surface regarding the heat transfer duringboiling in tandem tube configuration, obtained earlier for R134a.Weak dependency of the surface superheat on the applied heat fluxis observed for this liquid as well, although boiling of highly wet-ting liquids is known to be quite unstable, especially at low andsubatmospheric pressures. Maximal superheat of the test tubewas below 6 K, and the temperature at the top of the test tubewas slightly decreasing with the rising heat flux.

5. Conclusions

Experimental investigation on boiling heat transfer of R134a atpressures of 5 –9 bar and FC-3284 at pressures of 0.5 –1.5 bar withtandem tubes provided with the novel microstructure has shown

practical independence of the surface superheat on applied heatflux. This feature is caused by a huge number of nucleation sitesaccompanied by an extremely long TPL formed by micropins pierc-ing the growing vapor bubbles.

Processes on micro-level inside of the test tube structure playthe defining role in enhancement of heat transfer in tandem boil-ing. The lower tube promotes the heat transfer on the upper one,creating the turbulence by the two-phase bubbly flow, and dueto additional bubbles, arising from the vapor rests, captured bythe upper tube. The joint action of these processes is complex,and the maximum of the test tube superheat is observed at mod-erate heat fluxes between 30 kW/m2 and 50 kW/m2.

The efficiency of the microstructured surface is larger at lowerpressure, when the critical vapor bubble diameter is comparablewith the distance between neighboring pins. Microstructured sur-faces with larger number of pins are more effective, as they providelarger lengths of the TPL.

The novel microstructures could be recommended for use inindustrial practice, as they demonstrate stable and highly effectiveoperation for different working liquids and pressures.

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