Proceedings MTP-011

Proceedings of the Microgravity Transport Processes inFluids, Thermal, Biological and Material Sciences II

Banff, Alberta, CanadaSeptember 30 to October 5, 2001

UEF: MTP- 01- 45

POOL FILM BOILING EXPERIMENTS ON A WIRE IN LOW GRAVITY:PRELIMINARY RESULTSP. Di Marco, W. Grassi, F. Trentavizi

LOTHAR – Dipartimento di Energetica – Università di PisaVia Diotisalvi, 2 – 56126 Pisa – Italy

Phone +39.050.569646, Fax +39.050.830116, w.grassi@ing.unipi.it

ABSTRACT

The paper reports the preliminary results about pool filmboiling on a wire immersed in almost saturated FC72,recently obtained during an experimental campaignperformed in low gravity on the European Space AgencyZero-G airplane, (reduced gravity level 10-2). This is partof a long-term research on the effect of gravitational andelectric forces on boiling. The reported data set refers toexperiments performed in the following conditionsa) earth gravity without electric field,b) earth gravity with electric field,c) low gravity without electric field,d) low gravity with electric field.While a decrease of gravity causes a heat transferdegradation, the electric field remarkably improves theheat exchange. This improvement is so effective that,beyond a certain field value, the heat flux is no longersensitive to gravity. Two main different film boilingregimes have been identified both in normal and in lowgravity: one affected by the electric field and the otherone practically insensitive to the field influence.

INTRODUCTION

In saturated film boiling on a horizontal cylinder a stablevapour film entirely surrounds the cylinder separating theheater surface from the surrounding liquid. On groundthe vapour layer has a different thickness increasing frombottom to top of the cylinder. Contacts between liquidand heating wall are prevented and the vapour-liquidinterface oscillates. Equally spaced vapour bubblesdetach from the above interface and rise into the liquid.Conduction, convection and radiation across the vapourlayer are the involved heat exchange mechanisms. Theirrelative importance depends on the local vapour fluid

dynamics (e.g. heater geometry and orientation),thermophysical properties and wall superheat. Inparticular radiation starts playing a significant role athigh wall superheats.The first theoretical model of film boiling on a horizontalcylinder was proposed by Bromley (1950, 1952). Severalfurther models, all based on refinements of Bromley’sone, were proposed ever since. A most comprehensivecorrelation for film boiling on wires was developed bySakurai (Sakurai et al., 1990a, 1990b). This correlationfitted a large amount of experimental data, obtained onearth, both in saturated and in subcooled conditions. Inaddition, Sakurai et al. (1984) collected data on interfacewavelength, bubble detachment diameter and frequencyin film boiling.A comparison among the correlations supplied by mostof the existing models is provided in (Cipriani, 1999),with reference to the heater geometry investigated herein.Large discrepancies (up to 350%) among theirpredictions have been pointed out in the above quotedreport.Apart from this very disappointing situation regardingcorrelations, there is a general agreement on thefundamental role played by the wavelength of the liquid-vapour interface oscillations, and in particular by the socalled most dangerous wavelength. In general thiswavelength is recognised to be of the form

)'(32 RFl Ld π=λ (1)

where lL is the capillary Laplace length and F(R’) is afunction of the dimensionless wire radius R’, with

Ll

RR =' (2)

Proceedings MTP-012

i.e., square root of the Bond number, Bo. Differentrelations have been proposed for F(R’), among which wecan recall that by Lienhard and Wong (1964)

'/21

1)'(R

RF L+

= (3)

and the one by Sakurai et al. (1984)

+=

85.0'2

'2)'(R

RRFS (4)

In a recent paper (Cipriani et al. 2000) the present authorsobtained a good fitting of experimental results bymodifying the Sakurai correlation, to account for thevapour layer thickness, a. This thickness was estimatedsimply by assuming a conductive heat transfermechanism within the vapour film. Thus, in Eq.(4), R’was replaced by R’corr, given by

−=+= α 1,' R

k

L

corr

v

eRal

aRR (5)

What has been shortly outlined above clearly shows howthe gravitational force play an essential role in thisboiling regime, as it exerts a destabilising action on theliquid – vapour interface. According to some theoreticalapproaches (Lienhard’s modified Hydrodynamic Theoryand Katto’s theory accounting for the HydrodynamicInstability of vapour stems) this force also exerts a keyinfluence on critical heat flux, CHF. On this basis, abetter understanding of film boiling mechanisms mightshed light on the physics of CHF.But gravity is not the only force affecting the interfacebehaviour. In particular electric forces heavily influencethis behaviour, due to the different electric permittivitiesof the two phases (liquid and vapour) as, for example,shown in (Di Marco and Grassi, 1993, 1994 and Ciprianiet al., 2000). They too tend to destabilise the interface.In general an increase in both these forces (gravitationaland electric) causes a decrease in the oscillationwavelength and a corresponding increase in thedetachment frequency of bubbles from the interface.The effect of an electric field on film boiling has beenstudied for more than forty years, but only more recentlythis research has become more systematic and aware ofsome physical aspects, often not taken into account in thepast (e.g. relaxation time, electric field frequency, heatersize). In addition the present authors are the firstperforming experiments with an electric field in lowgravity. Here we will refrain from examining in somedetail the existing literature, and we will only shortlydescribe some previous works, closely related to thepresent paper.The classical paper by Baboi et al. (1968), investigatingpool boiling of toluene and benzene on a wire heater andapplying an electric field between the heater and aparallel wire, already referred an exponential increase of

the heat transfer coefficient with the applied field. Thefew measurements of the film wavelength reported showa wavelength decrease with increasing field intensity.Film boiling regime was reported to disappear for a fieldintensity higher than 50 MV/m, with bubbles departingalso from the lower side of the heater.Jones and Schaeffer. (1976) and Jones and Hallock(1978) performed boiling experiments with R113 in anapparatus quite similar to the present one. They alsoobtained some film boiling data showing a clear heattransfer enhancement in this regime, as well as a decreaseof the oscillation wavelength, both due to the electricfield.Berghmans (1978) improved the previous theoreticalmodels, also accounting for a two-dimensional wavepattern around the wire.Recently, Verplaetsen and Berghmans (1999) modelledthe effect of electric field on pool film boiling on a planeheater, developing a correlation in good agreement withexperiments.Carrica et al. (1996) performed experiments of filmboiling of R113 on a platinum wire, 0.2 and 0.3 mm indiameter. For the first time, it was clearly demonstratedthat the electro-hydro-dynamic (EHD) enhanced filmboiling regime cannot be sustained indefinitely in thisconfiguration. In fact a transition takes place at highenough wire superheat, bringing the system back to afilm boiling regime which is practically unaffected by thepresence of the field. Later on, a confirmation that theapplication of an electric field has a poor influence onfilm boiling performance at high wall superheats wasfound by Di Marco et al. (1997), who used R113 andVertrel XF (Dupont) as testing fluids. Quite recentlyCipriani et al. (2000) proposed a simplified model forthis transition also accounting for the vapour filmthickness.The aim of the present paper is to investigate in somedetail the effect of electric field on pool film boiling onwires in FC-72, within a broader research programme onthe effect of force fields on boiling. Electric andgravitational forces are taken into account at the presentstage and will be dealt with in the following. The choiceof wires as heaters is imposed mainly by the requirementof using test samples with low thermal inertia, owing tothe short time available (20 seconds) during eachmicrogravity test on the airplane.

EXPERIMENTAL APPARATUS

The experimental apparatus is shown in figure 1. Itconsisted of an aluminium parallelepiped vesselcontaining the test section. A bellows, connected to thisvessel and operated by pressurised nitrogen in thesecondary side, compensated for volume variations dueto vapour production and thermal dilatation of the liquid.The vessel had two windows to allow for visualisation ofthe phenomenon by means of a commercial and a high-

Proceedings MTP-013

speed video camera, with appropriate back-illumination.The two cameras could record the occurring phenomenathrough the same window thanks to the use of a beamsplitter. The high speed camera was a Phantom V 4.0with a memory of 1024 Mbytes (4096 frames, equal to 4seconds of recording at 1000 f.p.s. and full resolution of512x512 pixels) and connected to a dedicated laptop(Sony Pentium III). The camera was operated at 500f.p.s. during most of the experiments. The light wasprovided by a flat illuminating set (surface size 85.6X108mm, made by Dolan-Jenner mod. QVABL-48, 150W)An external heating-cooling system, governed by a PIDcontroller, kept the fluid temperature constant within0.5K. A much more detailed description of the systemcan be found in (Trentavizi, 2001).Experiments were carried out using a horizontal platinumwire of 0.2 mm diameter and 45 mm length, heated byJoule effect (d.c. current) which served as both aresistance heater and a resistance thermometer. Theheater (see figure 1) was made by brazing the platinumwire (the active heater) coaxial to two copper capillarytubes (1 mm O.D., 0.2 mm ID), designed to work at lessthan 1 K superheat at the maximum current rate in theexperiments. Two further thin insulated wires (0.08 mmdiameter) were passed inside the copper tubes and brazedto the copper-platinum junctions for direct voltagesensing and measurement. This design was chosen toeliminate the distortion effects on the electric field thatmight arise in the presence of external sensing wires atthe junctions.The fluid adopted in the tests was FC-72 (C6F14) afluoroinert liquid, trademark by 3M, slightly subcooled(1-3K) at a pressure of 115 kPa.

The electric field was produced applying a d.c. highvoltage (up to 7 kV) between the wire and a coaxial 8-rodcylindrical “squirrel cage”, of 60 mm diameter and 200mm length. The electric field was thoroughlycharacterised via finite element and finite differenceanalysis (Di Marco and Grassi, 1996), assuming that thefluid was single-phase and homogeneous in theinvestigated domain. It was shown that the length of thecage is sufficient to avoid side effects along the activepart of the heater. For the 0.2 mm wire, the analysisshowed that the field obeyed the law E = C V/r (Eelectric field intensity at a distance r from the wire axis,V the applied high voltage and C = 0.1034 adimensionless constant) up to 10 mm away from the wireaxis. For a solid cylindrical electrode of the samediameter the constant can be calculated analytically andits value is C = 0.1735.Two main types of tests were performed in film boilingduring the flight.• Constant heat flux tests – the wall heat flux was kept at

the same value during the whole parabola, starting atg/g0=1, before the pull-up, and ending at g/g0=1, afterthe pull-out. These tests provided direct results on theinfluence of acceleration on the involved phenomenaand on the related physical quantities.

• Variable heat flux tests – stable film boiling wasestablished during the levelled flight (g/g0=1). At thebeginning of the microgravity phase the heat fluxstarted increasing linearly according to a computeraided procedure. This allowed for obtaining differentfilm boiling curves in correspondence of the sameacceleration value.

Table 1 – Measurement range and maximum accidentalerror of the main measured quantities.

Quantity Measurement range Maximumerror

Heating current I 4 < I < 12 A 0.085% at 4AHeater voltage Vs 1.5 < V < 5 V 1.25mVLiquid temp. T 38.4 < T < 70.2 °C 0.3 KPressure p 115 kPa 0.2%High voltage V 0 < V < 7 kV 100 VWire temp. Tw (*) 200 < Tw < 900 °C 5.7%

at 900 °CWire superheatTw-Tsat (*)

150 < Tw-Tsat < 850 K 6% atTw=900°C

Heat flux q 140 < q < 900 kW/m2 5.46%(*) Obtained by numerical elaboration

During the low gravity phase the mean value of g/g0 hasbeen found to be 0.01 with a standard deviation of 0.02.

I

Vs

P

T1

T2

Peek Insulating Frame

Heater

High voltage rods (squirrel cage)

Pressurecompens.bellows

N2 Exhaust valve

N2 Injection valve

TEST SECTIONSIDE VIEW

Drain/fill valve

heater/cooler

Heater low voltage supply (5V, 20 A)

Sense wires

HEATER DETAIL0.1 / 0.2 mm Pt wire

Copper capillary pipe 1 mm O.D.Insulated sense wire 45 mm

Cage diameter:60 mm

Cage high voltage supply (0-10 kV)

200 mm

Fig. 1 - Experimental apparatus.

TSdApotraemTa

E

Af

detailed discussion of the matter of this article. Theclassical pattern on earth, for small cylinders or wires, isconstituted by a vapour layer surrounding the cylinder,with a varying circumferential thickness: smaller on thelower side and thicker on the upper one. Bubbles detachfrom the interface on the latter side and are regularlyspaced along a co-ordinate parallel to the cylinder axis.In other words no waves exist along the cylindercircumference. In this case we can refer to a one-dimensional wave distribution. If the cylinder diameter islarge enough it can also accommodate waves along itscircumferential periphery. In this case the wave patternbecomes two-dimensional, similarly to what happens ona flat surface. It is very easy to infer that this shoulddepend at least on the cylinder Bond number. Moreprecisely the comparison should be done between theactual interface wavelength and the cylinder diameter.For sake of clarity and shortness we will name the one-dimensional wave pattern as regime A and the two-dimensional one as regime B. At very low gravity levelthere is no reason why vapour thickness, in regime A,should vary, except for the effect of g-jitter. Thus, strictlyspeaking, we should expect a sort of “modified” regimeA, with an increased size of detaching bubbles. Theeffect exerted by an electric field is the reduction of thewavelength value according to a relation of the form(Cipriani et al. 2000)

3

32** ++

λ=λElEl

ddE (6)

where the electric influence number El* is given by

g

ElEEl

vl

veqLveq

σρ−ρ

⋅ε=

σ

⋅⋅ε=

)(

20

20* (7)

The equivalent electric permittivity for two dielectricmaterials, assuming that the liquid layer has a thicknessfar larger that the one of the vapour, a, has the form (Di

Fig.2 - Regime A: mono-dimensional wave pattern;p = 115 kPa; d = 0.2 mm; V = 0 kV; q = 400 kW/m2;∆Tsat = 449 K.

Fig.3 - Regime B: two-dimensional wave pattern dueto the presence of electric field ; p = 115 kPa; d = 0.2mm; V = 7 kV; q = 400 kW/m2; ∆Tsat = 201 K.

Fig. 4 – Regime C: independent of applied voltage;one-dimensional oscillatory pattern; p = 115 kPa; d =0.2 mm; V = 7 kV; q” = 850 kW/m2; ∆Tsat = 781 K.

Proceedings MTP-014

he signals were acquired by means of a Toshibaatellite notebook computer equipped a 12-bit PCMCIAata acquisition card (National Instruments DAQ CardI-16E-4) and conditioned according to a well assessedrocedure (Cipriani et al, 2000, Trentavizi, 2001) tobtain the data listed in Tab.1. In particular, the wireemperature was derived from the platinum temperature-esistance curve and it was necessary to correct itnalytically to account for the effect of the colder side-nds of the heater. Table 1 reports the uncertainty of theain measurements performed.ests at normal gravity were also performed before andfter each flight for comparison.

XPERIMENTAL RESULTS AND DISCUSSION

few words are worth saying about the vapour pattern inilm boiling on a horizontal cylinder, before going into a

Marco and Grassi, 1994)

])(th[

)( 20

vlv

vlleq

ak ε+εε

ε−εεε=ε (8)

And the electric field is evaluated at the interface,supposed cylindrical with a radius R+a,

1

2lnln

−

+ε

ε++

+=

aR

R

R

aR

aR

VEl

vI (9)

in agreement with what the present authors alwaysstressed (Di Marco and Grassi, 1994) about theimportance of considering the value of the electric fieldat the liquid – vapour interface in the study of theinstability of this interface.

AaBOpweacatCiaciaiba

Fakg

4.0

5.0

6.0

7.0

)

Sakurai correlation

Experimental data

Table 2 – Film boiling regimes at normal gravity

Regimes Possible sub-regimesRegime A (1-dimensional)-First film boiling regime

(effect of electric field) Regime B (2-dimensional)Second film boiling regime Regime C (1-dimensional)

Proceedings MTP-015

s already mentioned in the introduction the presentuthors, on ground, detected both regime A and regime, depending on the value of the applied electric field.ne more regime (regime C also named in previousapers “second film boiling regime”) was discoveredhere the process proved to be almost insensitive to the

lectric field beyond a certain wall superheat, againssuming a one-dimensional wave pattern. Theorresponding vapour patterns are shown in figures 2, 3nd 4, at normal gravity. One possible explanation for theransition from regime B to regime C was suggested inipriani et al. (2000) as follows. The vapour thickness

ncreases with increasing wall superheat. This fact causes weakening of electric field on the interface, due to itsylindrical distribution. Consequently interface behaviours only poorly affected by the electric forces. A similarpproach might be also adopted to explain the very littlenfluence of an electric field on fully developed nucleateoiling in this same geometry. We can summarise whatbove mentioned about the different regimes in table 2.

Tests at different gravity levels

In a parabolic flight different values of acceleration acton the process (see Di Marco and Grassi, 1996, for awider description of procedure):• g/g0=1 during the levelled (horizontal) flight;• g/g0=1.8 in the pull-up phase;• g/g0=0.01 during the low gravity stage (parabola);• g/g0=1.6 in the pull-out phase.Thus data corresponding to these acceleration levelscould be obtained.Figure 5 clearly show the effect of gravity on bubblespacing (wavelength), with the expected increase withreducing gravity. A comparison between the wavelengthexperimental data set and correlations has beenperformed. The modified correlation by Sakurai et al.(1984) has been found to be the most reliable to fit thedata, as shown in figure 6 for one of the parabolas. Thiscorrelation is given (see Eqs. 3 and 4) by

+π=λ

85.0'2

'232corr

corrLd

R

Rl (10)

In connection with wavelength measurements the heattransfer coefficient occurring during the different flightshas been detected. Such a trend is reported in figure 7.The curve of g/g0 (values on the right ordinate axis in thegraph) clearly shows the different flight stages. The heattransfer coefficient closely follows the acceleration trend.Thus, in this case, the exchange is strictly connected tothe wave pattern, that is modified by the gravity value asshown in figure 5.

Influence of an imposed electric fieldThe effect of an electric field on the vapour pattern hasalready been shown in figures 2 and 3. Its effect on the

(a)

(b)

(c)

ig. 5 – Effect of acceleration on wavelength in thebsence of electric fields. Wall heat flux q = 190W/m2. Photos: (a) g/g0=1.8, (b) g/g0=1.0, (c)/g0=0.01.

0.0 0.5 1.0 1.5 2.0g / g 0

0.0

1.0

2.0

3.0λ (m

m

Fig. 6 – Wavelength versus gravity: comparison of theexperimental data with Sakurai et al. (1984) correlation,Eq.(10).

(no electric field effect)

Proceedings MTP-016

heat transfer coefficient during a flight test is reported infigures 7 and 8, for different values of the high voltage.The corresponding acceleration trend is plotted in figure7, with micro-g phase spanning from 0 to about 20 s.The graphs of figure 8, compared with the one in figure7, show that the electric field enhances the heat exchangein the whole range of tested gravity levels, and beyond ahigh voltage of 2 kV this coefficient does not exhibit anyappreciable sensitivity to the acceleration. This isobviously true for the regime we called “first film boilingregime”.To provide a better evidence of the above matter, the filmboiling curves (wall heat flux vs. wall superheat) in thedifferent conditions have been obtained and are shown infigure 9. Each curve is countersigned with the level ofgravity (earth or p.nnn for micro-g, where nnn is thenumber of the parabola) and the value of the applied highvoltage (V).In the absence of electric field the influence of gravity onthe heat exchange can be easily detected by comparingthe curve obtained on ground (Earth V=0) with the one atlow gravity (p.309 V=0). As expected a decrease in theacceleration leads to a decrease in the heat exchange.Therefore a close connection between the wave patternand the heat exchange is proved to exist, what does nothold for fully developed nucleate boiling in this geometry(Di Marco and Grassi, 2001).The presence of the electric field improves the heattransfer both on earth and in microgravity. In particular itis quite evident from the figure that, above a given fieldthreshold (2kV in the present case), boiling curvesobtained on ground and in low gravity, with the sameelectric field, are indistinguishable. Of course this wasalready clear from the plots of figure 8. Corresponding tothe same high voltage value a change from regime A to

-40 -20 0 20 40 time ( s )

200

300

400

500

600

700

800

900α

(W/m

2 K )

-1

0

1

2

3

g / g

0

gravity level

heat transfer coeff.

Par. 225, V = 0, q" = 190 kW/m2

Fig. 7 – Trends of acceleration and corresponding heattransfer coefficient vs. time, during a parabola. The lowgravity phase starts a little before zero and ends around20 seconds.

-40 -20 0 20 40 time ( s )

500

600

700

800

900

1000

1100

1200

α (W

/m2 K

)

Par. 118, q" = 250 kW/m2

V = 2 kV

-40 -20 0 20 40 time ( s )

1300

1400

1500

1600

1700

1800

1900

2000

α (W

/m2 K

)

Par. 114, q" = 250 kW/m2

Par. 115, q" = 250 kW/m2

Par. 123, q" = 350 kW/m2

Par. 124, q" = 350 kW/m2

V = 5 kV

-40 -20 0 20 40 time ( s )

2000

2100

2200

2300

2400

2500

2600

2700

α (W

/m2 K

)

Par. 121, q" = 350 kW/m2

Par. 122, q" = 350 kW/m2

V = 7 kV

Fig. 8 – Trends of the film boiling heat transfercoefficient vs. time during several flight tests. The highvoltage value (HV) and the heat flux (q) adopted areindicated in the plots.

Proceedings MTP-017

regime B seems to take place. At the present stage it ispossible to infer that above this threshold the vapour-liquid interface wavelength is essentially controlled bythe electric field, as the electric forces become absolutelyprevailing on the gravitational force. This should beconsidered as a further proof of the fundamentalinfluence of the interface fluid-dynamics on film boilingheat transfer.

It is quite evident how a conclusion like this might givean essential contribution also on the role played by theinterface behaviour on CHF, at least for small cylindricalheaters.So far we have shortly discussed the experimental dataobtained with reference to what we named “first filmboiling regime”. The following paragraph will bededicated to the second regime.

200 300 400 500 600 700 800∆ T sat ( K )

200

300

400

500

600

700

q" (k

W/m

2 )

Earth, V = 5 kV

p.321, V = 5 kV

Earth, V = 4 kV

p.318, V = 4 kV

Earth, V = 3 kV

p.316, V = 3 kV

Earth, V = 2 kV

p.127, V = 2 kV

Earth, V = 1 kV

p.311, V = 1 kV

Earth, V = 0 kV

p.309, V = 0 kV

D = 0.2 mmp = 115 kPa

Fig.9 - Wall heat flux versus wall superheat for the first film boiling regime: effect of gravity and electric field

0 200 400 600 800 1000∆ T sat ( K )

0

500

1000

1500

2000

q" (k

W/m

2 )

V = 0 kV

V = 2 kV

V = 5 kV

V = 7 kV

V = 10 kV

D = 0.2 mmp = 115 kPa

Fig.10 - Film boiling curves on earth for FC72 (Ciprianiet al., 2000). Regime C is located beyond a wallsuperheat of around 600°C.

0 200 400 600 800 1000∆ T sat ( K )

0

100

200

300

400

500

600

700

q" (k

W/m

2 )

Par. 328, V = 6 kV

Par. 322, V = 5 kV

Par. 326, V = 4 kV

D = 0.2 mmp = 115 kPa

A

B

Fig. 11 – Transition from regime A to regime C in lowgravity. The dotted line represents the final curve onwhich the curves at different electric field collapse.

Proceedings MTP-018

Second film boiling regime (regime C)

Figure 10 shows the film boiling curves that can beobtained on earth at different values of the appliedelectric field. Regimes A, B and C can be clearlyidentified, with the collapse of the different curves into asingle one.We will dedicate the following discussion to thistransition, comparing the results obtained on earth withthose in low gravity.The results obtained in three different parabolas arereported in figure 11. The dotted line, marked with AB,represents (only in a qualitative manner) the commonfilm boiling curve on which the different curves of thefirst film boiling regime collapse. Even at a first glance itclearly appears that the wall superheat corresponding tothe end of the first boiling regime in microgravity isabout 200 K lower than on earth. A direct comparisonbetween on-ground and low gravity data, at the sameelectric field, has thus been done to gain a better insightabout this point. This comparison is shown in figure 12.Unfortunately, at the present stage, we have just a fewdata (three parabolas) about this transition from the firstto the second boiling regime in low gravity, to draw anygeneral conclusion. Therefore some further investigationis needed.

Table 3 – Heat flux q* and wall superheat ∆T* at thebeginning of transitionHigh voltage (V) q*(kW/m2) ∆∆∆∆T*(K)

Microgravity4000 475 3685000 587 3646000 642 353

On earth4000 701 5585000 874 5906000 1010 580

Nevertheless the results we have, reported in table 3,supply very clear indications about the reduction of theabove mentioned wall superheat. This might be explainedas follows. In microgravity the vapour volumesurrounding the wire is larger than on ground. Therefore,on an average basis, the liquid-vapour interface is fartherfrom the heater and the corresponding electric field,acting on the interface with the same applied highvoltage, is weaker. In addition the same table 2 singlesout how this transition occurs at the same wall superheatfor each level of gravity. Thus this superheat (and not thewall heat flux) seems to be the mechanism that controlsthe phenomenon.The availability of the high-speed video camera allowedfor obtaining a detailed visual documentation of thetransition mechanism in low gravity. A complete set ofimages is reported in figure 13.

The photos in figure 13 show a much less regular vapourstructure with respect to that of figure 5c. This isessentially related to the different value of heat flux (190kW/m2 for figure 5c and 642 kW/m2 for figure 13) andwall temperature. The random direction of bubblesleaving the heater is attributable to g-jitter. This has noeffect on the heat transfer thanks to the cylindricalgeometry of the heater that allows bubbles to escape fromthe test sample in any direction. The same should not beexpected for a plane surface, where an upward directedacceleration (boiling on the upper wall of the heater)would push bubbles onto the surface.A large level of horizontal bubble coalescence isdetectable from the photo (the most evident are indicatedwith a circle) with the formation of very big vapourmasses. This notwithstanding a sort of regular structure(regular bubble spacing on the heater) can be observed tooccur on the heater. Quite interesting is the way how thetransition takes place: i.e. through a “vapour frontpropagation mechanism” resembling the mechanismreferred to by Lienhard and Dhir (1973) for CHF at verylow Bond numbers. It is in fact very easy to identify inthe set of photos of figure 13 a movement of the largebubbles front from right to left.

0 200 400 600 800 1000∆ T sat ( K )

0

200

400

600

800

1000

q" (k

W/m

2 )

Par. 326, V = 4 kV

Ground, V = 4 kV

(a)

0 200 400 600 800 1000∆ T sat ( K )

0

200

400

600

800

1000

q" (k

W/m

2 )

Par. 328, V = 6 kV

Ground, V = 6 kV

(b)

Fig. 12 – Comparison between transitions on earthand in low gravity, with an applied high voltage: (a)4kV, (b) 6 kV.

Proceedings MTP-019

Fig.13 – Vapour pattern evolution during transition fromfirst to second film boiling regime in low gravity with a

high voltage of 6kV. Time elapsed from the beginning oftransition is reported below each photo.

t = 0 s

t = 0.1 s

t = 0.2 s

t = 0.3 s

t = 0.4 s

t = 0.5 s

t = 0.75 s

t = 1.0 s

t = 1.5 s

t = 2.0 s

t = 2.5 s

t = 3.0 s

Proceedings MTP-0110

For the sake of comparison a photo of the transition atnormal gravity is shown in figure 14. In this case tooregime C starts from a region of the heater, thenspreading out over the whole surface. Except for theobvious difference in the occurring vapour pattern, thetransition to this regime seems to take place according toa similar mechanism (vapour front propagation) both inreduced gravity and at normal one.

CONCLUSIONS

The results obtained about the influence of thegravitational and electrical fields on pool film boiling ofFC72 on a heating wire have been presented in thispaper. The following experimental conditions have beeninvestigated: a) earth gravity without electric field, b)earth gravity with electric field, c) low gravity withoutelectric field, d) low gravity with electric field. The mainresults discussed in the paper can be shortly summariseas follows.• A very close connection between the vapour

morphology on the heating wall, and in particular thewave structure of the liquid-vapour interface, and theheat exchange has been proven to exist. A reductionin gravity causes an increase in wavelength and adecrease in heat transfer. On the other hand theeffect of increasing the value of the applied electricfield is a wavelength reduction with a correspondingheat transfer augmentation.

• The heat transfer improvement due to the electricfield takes place on earth and in low gravity, so thatthe heat transfer coefficient, beyond a certain fieldthreshold, becomes insensitive to gravity changes.

• Film boiling showed both a one-dimensional and atwo dimensional wave pattern also under reducedgravity conditions. This confirms previous resultsfound on ground. Thus two main film boilingregimes (first and second regimes) can be identified.The first one is heavily affected by the electric field

(no matter of the gravity value), and can be dividedin two sub-regimes: regime A, with a one-dimensional wave pattern and regime B, with a two-dimensional wave pattern. The transition betweenthe two sub-regimes is controlled by the electricfield. The second film boiling regime (also namedregime C) implies a one-dimensional wave structure,no matter of the electric filed value. The transition tothis regime is controlled by the wall superheat. Thissuperheat value, on turn, depends on the value ofgravity.

• The importance of accounting for the vapour filmlayer thickness has been widely stressed in the paper.

• A mechanism of vapour film propagation has beenclearly evidenced to lead from the first to the secondfilm boiling regime.

The reported data show how experimentation inmicrogravity may shed light on the fundamental physicalmechanisms of film boiling and also critical heat flux.This is the reason why the present authors alreadyrequested to ESA (European Space Agency) toparticipate to the next parabolic flight campaign to beheld next autumn (2001). In particular different levels ofgravity should be available during the above campaign.

NOMENCLATURE

a vapour layer thickness (m)El* electrical influence numberg gravity acceleration (m/s2)I current intensity (A)k wave number (m-1)lL Laplace length σ1/2/(ρl-ρv)1/2g1/2 (m)p pressure (Pa)q heat flux (W/m2)R radius of the wire (m)R’ dimensionless radius, R/lLT temperature (K)V applied high voltage (V)

Fig. 14 – Transition from the first to the second film boiling transition (regime B to regime C) at normal gravity.

Proceedings MTP-0111

α heat transfer coefficient (W/m2 K)∆Tsat wire superheat (K)ε relative dielectric permittivityε0 vacuum dielectric permittivity (F/m)λd oscillation wavelength (kg/m3)ρ density (kg/m3)σ surface tension (N/m)

Suffixescorr correctedcyl cylinderE in the presence of the electric fieldl liquidsat saturatedv vaporw heated wall* transition from first to second regime

ACKNOWLEDGEMENTS

The apparatus was designed and built in the laboratoriesof the University of Pisa, with the assistance of KayserItalia. Mr. R. Manetti designed and assembled theelectronics. Dr. G. Memoli gave his assistance in settingup the optical system. Both Mr. Manetti and Dr. Memoliactively participated to the research campaign inBordeaux and joined the first two authors on board theA-300 “Zero-G”.This work was funded by ASI (Italian Space Agency)and by ESA (European Space Agency). The authors areindebted to ESA personnel and in particular to the wholecrew on board the aircraft and to the people fromNovespace who did a wonderful job. A very particularthanks goes to Dr. Vladimir Pletser, from ESA, a gentleand most effective scientist who did an enormous amountof work to make things go smoothly and made thiscampaign a real success.

REFERENCES

Baboi N.F., Bologa M.K. and Klyukanov A.A., 1968,Some Features of Ebullition in an Electric Field, Appl.Electr. Phenom., 2 (20), pp.57-70.

Berghmans J., 1978, Prediction of the Effect of ElectricField on the Most Unstable Wavelength During FilmBoiling on Small Wires, J. of Electrostatics, Vol.5,pp.265-272.

Bromley L. A., 1950, Heat Transfer in Stable FilmBoiling, Chem. Eng. Prog., 58, pp.67-72.

Bromley A. L., 1952, Effect of Capacity of Condensate,Ind. Eng. Chem., vol.44, p. 2966.

Carrica P., Di Marco P., Grassi W., 1996, Electric FieldEffects on Film Boiling on a Wire, Experimental HeatTransfer, vol.9, pp.11-27.

Cipriani M., 1999, Experimental Study on the Influenceof the Electric and Gravitational Fields on Pool FilmBoiling (in Italian), Degree Thesis, University of Pisa.

Cipriani M., Di Marco P., Grassi W., 2000, Effect of anExternally Applied Electric Field on Pool Film Boilingof FC72, Proc. 18th UIT National Conference,Cernobbio, Como (I), pp.703-714.

Di Marco P. and Grassi W., 1993, Saturated Pool BoilingEnhancement by means of an Electric Field, EnhancedHeat Transfer, vol.1, n.1, pp.99-114.

Di Marco P. and Grassi, 1994, W. Gas-Liquid InterfaceStability in Presence of an Imposed Electric Field,Proc. 12th UIT National Conference, L’Aquila (I),pp.299-310.

Di Marco P., Grassi W., 1996, Nucleate Pool Boiling inthe Presence of an Electric Field and in a VariableGravity Field: Results of Experiments in ParabolicFlight, Proc. of Eurotherm Seminar n.48, ed. by D.Gorenflo, D. Kenning, C. Marvillet, Paderborn, D,pp.255-264.

Di Marco P., Grassi W., Iakovlev I., 1997,Single andTwo-Phase EHD Enhanced Heat Transfer - A Reviewof Experimental Results, 48th InternationalAstronautical Conference, Torino (I), paper IAF-97-J.1.04.

Di Marco P., Grassi W., 2001, Motivation and Results ofa Long-Term Research on Pool Boiling Heat transfer inLow Gravity, Keynote Lecture, Proc. of 5th Conferenceon Experimental Heat Transfer, Fluid Mechanics andThermodynamics (ExHFT-5), ed. by G.P. Celata, P. DiMarco, A. Goulas and A. Mariani, Thessaloniki (Gr),pp. 33-52.

Jones T.B. and Hallock K.R., 1978, Surface Wave Modelof Electrohydrodynamically Coupled Minimum FilmBoiling, J. of Electrostatics, vol.5, pp.273-284.

Jones T.B. and Schaeffer R.C., 1976,Electrohydrodynamically Coupled Minimum FilmBoiling in Dielectric Liquids, AIAA J., vol.14, pp.1759-1765.

Lienhard J.H. and Dhir V., 1973, ExtendedHydrodynamic Theory of the Peak and Minimum PoolBoiling Heat Fluxes, NASA Contractor Report 2270.

Lienhard J.H. and Wong P.T.Y., 1964, The DominantUnstable Wavelength and Minimum Heat Flux duringFilm Boiling on an Horizontal Cylinder, J. HeatTransfer, Trans. ASME, Vol.86, pp.220-226.

Sakurai A., Shiotsu M. and Hata K., 1984, Effect ofSystem Pressure on Film Boiling Heat Transfer,Minimum Heat Flux and Minimum Temperature, Nuc.Sci. Eng., 88, pp. 321-330.

Sakurai A., Shiotsu M. and Hata K., 1990a, A GeneralCorrelation for Film Boiling Heat Transfer fromHorizontal Cylinder to Subcooled Liquid: Part 1 – ATheoretical Pool Film Boiling Heat Transfer Model

Proceedings MTP-0112

Including Radiation Contribution and its AnalyticalSolution, J. Heat Transfer, Trans ASME, vol.112, pp.430-440.

Sakurai A., Shiotsu M. and Hata K., 1990b, A GeneralCorrelation for Film Boiling Heat Transfer fromHorizontal Cylinder to Subcooled Liquid: Part 2 –Experimental Data for Various Liquids and itsCorrelation, J. Heat Transfer, Trans ASME, vol.112,pp. 441-450.

Trentavizi F., 2001, Pool Film Boiling Heat TransferExperiments in Reduced Gravity Conditions (inItalian), Degree Thesis, University of Pisa.

Verplaetsen, F.M.; Berghmans, J.A., 1999, Film Boilingof an Electrically Insulating Fluid in the Presence of anElectric Field, Heat and Mass Transfer/Waerme- undStoffuebertragung vol. 35, n. 3, pp. 235-241.