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Nuclear Engineering and Design 240 (2010) 3949–3955

Contents lists available at ScienceDirect

Nuclear Engineering and Design

journa l homepage: www.e lsev ier .com/ locate /nucengdes

ependence of bubble behavior in subcooled boiling on surface wettability

akahiro Harada ∗, Hiroshi Nagakura, Tomio Okawaepartment of Mechanical Engineering, Osaka University, 2-1, Yamadaoka, Suita-shi, Osaka 565-0871, Japan

r t i c l e i n f o

rticle history:eceived 24 August 2009eceived in revised form 11 February 2010ccepted 7 March 2010

a b s t r a c t

This paper presents the results of visualization experiments that were carried out to investigate thedynamics of vapor bubbles generated in water pool boiling. In the experiments, vapor bubbles weregenerated on a vertical circular surface of a copper block containing nine cartridge heaters, and the contactangle of the heated surface was used as a main experimental parameter. The experiments were performedunder subcooled as well as nearly saturated conditions. To enable clear observation of individual bubbleswith a high speed camera, the heat flux was kept low enough to eliminate significant overlapping of

bubbles. When the contact angle was small, the bubbles were lifted-off the vertical heated surface withina short period of time after the nucleation. On the other hand, when the contact angle was large, they slidup the vertical surface for a long distance. When bubbles were lifted-off the heated surface in subcooledliquid, bubble life-time was significantly shortened since bubbles collapsed rapidly due to condensation.It was shown that this distinct difference in bubble dynamics could be attributed to the effects of surface tension force.

. Introduction

.1. Literature survey

In the subcooled boiling region in an evaporation tube, the ther-odynamic quality is negative but vapor bubbles exist since the

emperature of the heated wall exceeds the local saturation tem-erature. Accurate estimation of the void fraction distribution inhe subcooled boiling region is of considerable practical interestecause the presence of vapor bubbles affects the fuel burnup ofuclear reactor core as well as the steady state, transient responsend the inception of flow instability of many industrial plants usingoiling heat transfer. If a subcooled liquid flows in a heated channel,he wall temperature rises gradually in the single-phase region, andhe first bubbles appear on the wall at which the wall temperatureecomes sufficiently higher than the local saturation temperature.his location is identified as the point of the onset of nucleateoiling, or PONB. However, significant increase in the vapor voidraction does not occur immediately downstream of PONB due toigh subcooling of bulk liquid. Further downstream, the liquid sub-ooling becomes low enough and the thermal-hydraulic condition

s such that a rapid increase in the void fraction can be initiated. Thisocation is commonly regarded as the point of net vapor genera-ion, or PNVG, since the cross-sectional area-averaged void fractions negligibly small between PONB and PNVG in many practical

∗ Corresponding author.E-mail address: kannkanngaku@yahoo.co.jp (T. Harada).

029-5493/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.nucengdes.2010.03.016

© 2010 Elsevier B.V. All rights reserved.

situations. It is known that accurate determination of PNVG is a par-ticularly important step in predicting the void fraction distributionin the subcooled boiling region.

In many available models for the void fraction in the subcooledflow boiling, the bubble detachment from a heated wall is regardedas the triggering mechanism of the net vapor generation (Collierand Thome, 1994; Kandlikar and Nariai, 1999). Bowring (1962)developed an empirical correlation for PNVG. This correlation isfairly good agreement with experimental data at low velocitiesbut underestimates the liquid subcooling at PNVG at higher massvelocities (Staub, 1968). Based on the concept of a force balance,Levy (1967) developed a more mechanistic model to predict PNVG.The main assumption of this model is that the frictional drag forceattempting to remove the bubble from the wall exceeds the surfacetension force attempting to hold it at PNVG. This model is reason-ably successful in correlating many experimental data, though itspredicting performance deteriorates at low mass velocities (Sahaand Zuber, 1974). Saha and Zuber (1974) proposed a simple corre-lation to calculate PNVG. In this correlation, the Nusselt number andthe Stanton number are assumed to be constant at PNVG under lowand high mass velocity conditions, respectively. They interpretedthe resulting correlation as meaning that not the bubble detach-ment but a sufficient decrease in the local subcooling initiates thenet vapor generation at low mass velocity conditions.

Extensive experimental studies have also been carried out toobtain detailed information on the bubble dynamics in subcooledflow boiling. Kandlikar and Stumm (1995) measured the bubble sizeat the departure from a nucleation site using a rectangular horizon-tal channel as the test section. They emphasized the importance

3950 T. Harada et al. / Nuclear Engineering a

Nomenclature

db bubble diameter (m)g gravitational acceleration (m/s2)qw wall heat flux (W/m2)RA aspect ratioT half cycle of the shape oscillation (s)Tw wall temperature (K)Vb bubble velocity in the lateral direction (m/s)

Greek letters�Tsub liquid subcooling (K)�Tw wall superheat (K)�x lateral displacement of bubble centroid (m)� contact angle (◦)� density (kg/m3)� surface tension (N/m)

Subscripts0 initialb bubbledept departure

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fluid temperature at the center of the surfaces that was measuredusing a type-K thermocouple. In the present study, the behavior ofvapor bubbles generated on the end surface of the copper block wastaken as the subject of investigation. Hence, to eliminate the effectof bubbles generated at the boundary between the copper block

l liquidlift lift-off

f the upstream and downstream contact angles in determininghe departure bubble diameter. In the experiments performed byibeau and Salcudean (1994), bubbles departed from the nucleationites and started to slide along the vertical heated surface immedi-tely after ONB. Bubbles were then lifted-off the heated wall andjected into the main flow. This observation appears contradictoryo the bubble detachment model since this model does not per-

it the bubble departure from the nucleation site between PONBnd PNVG. However, experimental data reported by several otheresearchers may also suggest the presences of the bubble depar-ure from the nucleation site and the bubble detachment from theeated wall prior to the onset of NVG (Unal, 1975; Thorncroft et al.,998; Okawa et al., 2005; Situ et al., 2005). Bibeau and Salcudean1994) also mentioned the difference of their results from those ofunther (1951) who observed that bubbles grew and collapsed onvertical heated surface.

.2. Objectives

From the literature survey shown in the previous section, it isonsidered that the fundamental assumption adopted in the widelysed bubble detachment models for predicting the void fraction

n subcooled flow boiling is not fully consistent with the resultsf some photographic studies reported in literature. Therefore, toevelop a sophisticated subcooled boiling model, it would be nec-ssary to understand the bubble dynamics in more depth. In spite ofhe numerous experimental works, the effects of some parametersuch as the cavity geometry, surface roughness and surface wetta-ility on bubble behavior remain to be fully elucidated (Shoji andakagi, 2001; Kudritskiy, 1995; Oka et al., 1996). In these parame-ers, the effect of the surface wettability is considered of particularmportance since the contact angle is included as a main param-ter in many models for bubble behavior (Fritz, 1935; Al-Hayesnd Winterton, 1981; Thorncroft et al., 2001). In addition, it is

nown experimentally that the surface wettability is influential inhe heat transfer rate and critical heat flux in pool boiling (Liawnd Dhir, 1989; Takata et al., 2005; H.D. Kim et al., 2007; S.J. Kimt al., 2007). To authors’ knowledge, however, the surface wetta-ility is not used as a main experimental parameter in available

nd Design 240 (2010) 3949–3955

experiments to reveal the bubble dynamics. In view of these facts,a photographic study of subcooled boiling is performed in this workto provide experimental information on the effect of surface wetta-bility on the bubble dynamics. In the case of flow boiling, however,the main flow complicates the temperature distribution near thewall and the interfacial force acting on bubbles. Therefore, althoughthe ultimate goal is to elucidate the mechanisms to initiate thenet vapor generation in subcooled flow boiling, the present exper-iments are carried out under pool boiling conditions to highlightthe role of surface property by eliminating the additional effectsexerted by the main flow.

2. Experimental apparatus

2.1. Test vessel

Depicted in Fig. 1 is a schematic diagram of the experimentalapparatus. The rectangular test vessel was 70 mm wide, 140 mmlong and 260 mm high and its top was open to the atmosphere. Themain body of the vessel was made of stainless steel, but transpar-ent polycarbonate plates were used for the front and rear faces toenable visualization of bubbles. The one end of a copper block wasmachined into a cylindrical shape of 20 mm diameter. Its end sur-face was used as a heated surface. Nine cartridge heaters rated at70 W were embedded in the other end of copper block. Three type-Kthermocouples were positioned along the central axis of the cop-per cylinder to calculate the heat flux qw and the wall temperatureTw. The copper block was put in a stainless steel jacket to reducethe heat loss from the side wall; the block and the jacket weresilver-soldered to avoid water leakage. As shown schematically inFig. 1, the heating block was fixed on a side face of the test vesseland cooling box made of a thin copper plate was mounted on theopposite face. The distance between the heating and cooling sur-faces was 20 mm. The liquid subcooling �Tsub was defined from the

Fig. 1. Schematic diagram of the experimental apparatus.

T. Harada et al. / Nuclear Engineering an

Table 1Main experimental conditions.

Run No. Surface � (◦)a �Tsub (K) qw (kW/m2)b

1 A (Bare) 79 (84/74) 0.6 ± 0.2 20–242 A (Bare) 72 (81/64) 5.0 ± 0.1 26–453 A (Bare) 77 (75/79) 9.1 ± 0.1 42–524 B (Oxide) 60 (65/56) 0.2 ± 0.2 44–585 B (Oxide) 50 (56/45) 5.2 ± 0.4 53–886 B (Oxide) 52 (54/49) 9.8 ± 0.0 44–577 B (Oxide) 44 (43/44) 9.8 ± 0.5 51–828 C (TiO2) 71 (71/70) 0.1 ± 0.1 31–459 C (TiO2) 56 (57/55) 4.8 ± 0.1 22–4110 C (TiO2) 43 (43/43) 9.2 ± 0.1 48–6411 D (TiO2) 0 (0/0) 0.0 ± 0.0 17–1712 D (TiO2) 0 (0/0) 4.8 ± 0.2 38–6313 D (TiO2) 0 (0/0) 10.2 ± 0.1 41–52

a Average for 10 measurements (average for 5 measurements before boilinge

c

af

2

tspapwAaptfIdeistubiss±mTamTs2doesa

lAfib

on � (Yang and Kim, 1988), the change of nucleation site den-sity was not influential in the bubble dynamics observed in thiswork. In the visualization, the frame rate and shutter speed wereset to 8000 frame/s and 0.033 ms, respectively; each image con-sisted of 256 × 512 pixels, and the spatial resolution was about

xperiments/average for 5 measurements after boiling experiments).b Uniform distribution of qw was assumed for the end and side faces of copper

ylinder that were exposed to water.

nd the stainless steel jacket, the tip of copper block was protrudedrom the jacket for 1.5 mm.

.2. Procedures

Main experimental parameters of the present experiments werehe static contact angle of the heated surface � and the liquidubcooling �Tsub. Each experimental run began with the surfacereparation to control �. The surface was first polished using #400nd #2000 emery papers (100 strokes each) and metal polishingaste. This was followed by the surface cleaning using distilledater and acetone. These were all the procedures to obtain a Type-surface. To reduce the contact angle, two methods were adopted

fter the above-mentioned procedures. In the first method, the cop-er block was heated in air to oxidize the heated surface in referenceo Liaw and Dhir (1989). The block temperature was kept at 510 Kor 2 h. This surface is identified as a Type-B surface in this work.n the second method, 0.005 or 0.01 g of TiO2 nano-particles wereispersed in distilled water by means of ultrasonic excitation in ref-rence to H.D. Kim et al. (2007). The heated surface was immersedn these nano-fluids and heat flux of 110 kW/m2 was applied to theurface for 2 h to obtain Type-C and Type-D surfaces, depending onhe particle concentration. It is noted that a different vessel wassed in these procedures to avoid contamination of the test vessely nano-particles. Before and after a series of photographic exper-

ments, a water droplet of 0.5 mm3 in volume was placed on theurface 5 times. The static contact angle � was determined from theide views of droplets. The measurement accuracy of � was within1◦. Table 1 summarizes the main experimental conditions. Theean value of � for the 10 measurements was within 72–79◦ for

ype-A surface, and reduced to 44–60◦, 43–71◦ and 0◦ for Type-B, Cnd D surfaces, respectively. The mean values of � for the measure-ents before and after the boiling experiments are also listed in

able 1. The difference between the two mean values of � is ratherignificant in some cases; the maximum difference is 17◦ (Run No.). This would be attributed to the damage exerted on the surfaceuring nucleate boiling. Therefore, in the following chapter, notnly the mean value but also the range of � will be indicated in thexperimental results. The photos of droplets placed on the heatedurface are displayed in Figs. 2a–d. It can be confirmed that thebove described procedures are effective to change �.

Following the surface preparation, distilled water used as a test

iquid was kept boiling in a separate vessel for 2 h for degassing.fter placing the heating block on the test vessel, the vessel waslled with the test liquid. Oil at 400 K was circulated in the coolingox for 1 h for further degassing (the cooling box was used to heat

d Design 240 (2010) 3949–3955 3951

the liquid at this stage). The oil temperature was then decreasedto adjust �Tsub at a prescribed value. The electric power appliedto the cartridge heaters was then increased gradually to gener-ate vapor bubbles, while controlling the temperature of cooling oilappropriately to maintain �Tsub. After the steady state was estab-lished, behavior of bubbles generated on the heated surface wasrecorded using a high speed camera at several heat fluxes. To obtainclear bubble images, however, the process of bubble nucleation wasimaged only in isolated bubble regime near ONB. It was thereforeassumed that, although the nucleation site density is dependent

Fig. 2. Side views of water drops placed on the heated surface: (a) bare surface,(b) oxidized surface, (c) TiO2 deposited surface (low concentration), and (d) TiO2

deposited surface (high concentration).

3952 T. Harada et al. / Nuclear Engineering and Design 240 (2010) 3949–3955

/m2)

0l

rlawwbsanri

3

3

tintfhrf

Fig. 3. Sequential images of a sliding bubble (Run No. 1; qw = 20 kW

.02 mm/pixel. The entire process was backlit using a metal halideamp.

An analog-to-digital converter attached to a personal computerecorded the temperatures measured in the copper block and testiquid. The thermocouples used in the present experiment wereccurate within ±1.5 K. The liquid subcooling �Tsub was variedithin 0–10 K. The wall heat flux qw and the wall superheat �Tw

ere calculated from the temperatures measured in the copperlock, assuming that qw and �Tw were constant on the copper blockurface exposed to the test liquid. The experimental ranges of �Tsubnd qw are also listed in Table 1. Although the value of �Tsub couldot be kept constant for different values of qw in each experimentalun, the deviation from the mean value was within 0.5 K as shownn the table.

. Results

.1. Bubble departure from nucleation site

Depicted in Figs. 3 and 4 are the two types of bubble behaviorhat were typically observed in this work. In both cases, a rapidncrease in the bubble size was observed immediately after theucleation; bubbles first adhered to nucleation sites for a certain

ime period and then started to slide up the vertical heated sur-ace. This observation may be consistent with Levy (1967) whoypothesized that buoyant force and frictional force attempt toemove bubbles from their nucleation site while surface tensionorce attempts to hold them in subcooled flow boiling. Note that

; the horizontal dashed line denotes the height of nucleation site.

Levy assumed that buoyancy plays a minor role in flow boiling.However, in the present experiment performed under pool boil-ing conditions, it is considered that the frictional drag force actingon a bubble was much smaller than that exerted in flow boilingand buoyancy was a main force to move bubbles from their nucle-ation site. The sliding motion of bubbles depicted in Figs. 3 and 4may hence be interpreted as meaning that, although other forcesincluding the growth force would also play a role (Thorncroft etal., 2001), the bubbles moved upward to depart their nucleationsite when the bubble size became large enough for buoyancy toovercome the surface tension force.

The measured departure diameters db,dept (bubble diameters atthe onset of sliding) are plotted against � in Fig. 5. It should benoted that accurate determination of db,dept was not possible sincethe size and shape of bubbles were changing at the onset of sliding.Hence, the onset of sliding was detected when the midpoint of theupper and lower limits of a bubble moved upward for a certaindistance from a nucleation site. The symbols in Fig. 5 are the resultswhen the critical distance was set to 0.1 mm; the vertical error barsindicate the range of db,dept when the critical distance was varied to0.05 and 0.15 mm, and the horizontal bars indicate the minimumand maximum values in the 10 measurements of �. It should alsobe noted that some bubbles were detached from the heated surfacebefore sliding up for 0.15 mm. The data for these bubbles were not

included in the figure. Eliminating the frictional force, the forcebalance proposed by Levy (1967) is transformed to

Cb�l

�d3dept

6g = Cs��ddept (1)

T. Harada et al. / Nuclear Engineering and Design 240 (2010) 3949–3955 3953

/m2)

wtf

d

Ff

Fig. 4. Sequential images of bubble lift-off (Run No. 10; qw = 48 kW

here �l is the liquid density, g is the gravitational acceleration, � ishe surface tension coefficient, and Cb and Cs are the proportionality

actors. The departure diameter is hence calculated by

dept =√

Cs

Cb

6�

�lg(2)

ig. 5. Relation between the contact angle � and the bubble size at the departurerom a nucleation site db,dept.

; the horizontal dashed line denotes the height of nucleation site.

The value of Cb may be set to unity as the first approximation,but the information on the contact area between the bubble andwall and the spatial distribution of the contact angle along the con-tact line is needed in evaluating Cs. The horizontal dashed line inFig. 5 indicates that the value of db,dept calculated by Eq. (2) coin-cides with the mean experimental value if Cb and Cs are set to 1 and0.08, respectively. The diameter of the contact area was indetermi-nate but smaller than the bubble diameter (see Figs. 3 and 4), andthe surface tension force does not necessarily act in the downwarddirection. The value of Cs is hence considered to be substantiallysmaller than 1. The results shown in Fig. 5 would therefore sup-port the assumption that the relation between the buoyant andsurface tension forces is of importance for bubbles to start slidingand depart from their nucleation site.

3.2. Sliding and lift-off of bubbles

After the rapid growth in the initial stage, two distinctly dif-ferent types of bubble behavior were observed as delineated inFigs. 3 and 4. Fig. 6 indicates the experimental values of � and �Tsubat which each bubble behavior was observed.

When the hydrophobic surface of � ≥ 72◦ was used as the heatedsurface, bubbles slid up the vertical surface for a long distance with-out being detached from the wall. Although the bubble was onceelongated in the nearly perpendicular direction to the wall (1–4 msin Fig. 3), it appears that the surface tension force acting on the con-

tact line prevented the bubble from being detached from the wall.Then, the bubble returned to nearly spherical or slightly flattenedshape (5–7 ms), and slid up the wall. It can also be seen that the bub-ble size gradually increased due to the heat applied from the wall.In Fig. 3, change in bubble shape is not significant for 15–40 ms. In

3954 T. Harada et al. / Nuclear Engineering and Design 240 (2010) 3949–3955

F

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3

tbreectflo

T = �d3

b,lift�l

96�(4)

�x = 12

db,lift(1 − R2/3A0 ) (5)

ig. 6. Effects of contact angle � and liquid subcooling �Tsub on bubble behavior.

he experiments performed under the subcooled conditions (Runos. 2 and 3), however, the oscillation of bubble shape was repeateduring the sliding stage.

Fig. 4 displays the typical bubble behavior observed on theydrophilic surface of � ≤ 71◦. In sharp contrast to the results

or the hydrophobic surface, the bubble was detached from theeated surface within a short period of time after the inceptiont the nucleation site. Prior to the lift-off, the bubble centroidoved not only parallel but also normal to the surface (4–10 ms).lthough this behavior is similar to that depicted in Fig. 3, it appears

hat the surface tension force was not sufficient to prevent theubble detachment. As a result, the bubble was lifted-off the sur-ace at 11 ms and then propelled into the subcooled bulk liquid12–19 ms), where it collapsed rapidly due to condensation. There-ore, the bubble life-time was significantly shortened if the bubbleift-off took place in subcooled liquid. Although the bubble was usu-lly flattened along the heated surface in the initial growth period,ts shape then became more rounded as delineated in Fig. 4. At theame time, the wall side bubble interface converged to central nor-al axis of the bubble until the bubble was eventually lifted-off

he vertical heated surface. These observations are similar to theumerical results reported by Okawa et al. (2006, 2008), who con-luded that the change in initially spheroidal bubble shape inducedy surface tension force is one of the primal causes of the bubble

ift-off from the heated surface. Hence, the present experimentalbservation supports the hypothesis that surface tension plays anmportant role to remove the bubble from the hydrophilic heatedurface.

.3. Bubble lift-off velocity from a hydrophilic surface

In the present experiments, surface tension force tended to holdhe bubble to the heated surface when � was large, while lift theubble off the surface when � was small. In the case of nucleareactor core, � is expected to be small due to the high-temperaturenvironment and the high-intensity radiation. It is hence consid-red that the bubble lift-off is not a rare event in the nuclear reactor

ore, although the surface tension effect to detach bubbles fromhe heated wall is not accounted in most models for subcooledow boiling. In view of these, the characteristics of bubble lift-offbserved for hydrophilic surfaces are investigated in more detail.

Fig. 7. Dependence of the bubble lift-off velocity Vb,lift on the bubble lift-off diameterdb,lift .

Dependence of the bubble migration velocity in the lateral direc-tion at lift-off Vb,lift on the bubble lift-off diameter db,lift is shownin Fig. 7. Here, since accurate measurement of Vb,lift from two suc-cessive bubble images was difficult, Vb,lift was estimated from thelateral displacement of bubble centroid within 2 ms prior to the lift-off. Fig. 7 indicates that Vb,lift decreases with an increase in db,lift asan overall trend with small number of exceptions. In the case ofinitially spheroidal bubble oscillating in an infinite inviscid liquid,the frequency of the shape oscillation caused by the surface ten-sion force is inversely proportional to the sphere-equivalent bubblediameter db to the three halves power (Lamb, 1932). When a bubbleis present in close proximity to the wall, surface tension-inducedshape oscillation should cause the displacement of bubble centroidas illustrated in Fig. 8. Based on this concept, Vb,lift may be estimatedby

Vb,lift = 2�x

T(3)

where T is the half cycle of the bubble shape oscillation and �x isthe lateral displacement of bubble centroid (see Fig. 8). Applyingthe inviscid results (Lamb, 1932), these values are evaluated by√

Fig. 8. Definitions of the lateral displacement of bubble centroid �x and the halfcycle of the bubble shape oscillation T.

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T. Harada et al. / Nuclear Enginee

here RA0 is the initial bubble aspect ratio. The bubble lift-off veloc-ties calculated from Eqs. (3)–(5) are also indicated by solid andashed lines in Fig. 7 for comparison; here, RA0 was set to 0.7 and.8 as the typical values in the present experiments. It can be seenhat the experimental values of Vb,lift are in fairly good agreementsith the above theory, indicating that the change in bubble shape

nduced by surface tension force would be one of the primal causesf the bubble lift-off from a hydrophilic surface observed in thisork. Since several plots are deviated significantly from the theory

n Fig. 7, the main reason of the deviation was also investigated.hese data were measured for the surface of � = 56◦ (Run No. 9);his value belonged to the largest contact angles at which the bubbleift-off was observed. It is hence expected that the surface tensionffect to hold the bubble to the wall was comparable with the effecto detach it in these cases.

The present experimental results indicate that surface tensionould have the effects to hold the bubble to the wall as supposedy Levy (1967) and at the same time to remove the bubble from theall through the change in bubble shape as illustrated in Fig. 8. In

he present experiments, bubbles were usually flattened along theeated surface in the initial stage regardless of � (see Figs. 3 and 4).herefore, assuming the presence of three-phase contact line on theubble base, it is considered that the removing effect competedith the holding effect in the present experiments. The holding

ffect is expected to diminish with a decrease in the contact angleFritz, 1935). Therefore, although other surface properties such ashe thermal resistance and roughness might also be influential inhe bubble dynamics, the present experimental results would benterpreted that the bubble lift-off occurred when the contact angle

as small enough for the removing effect to overcome the holdingffect.

. Conclusions

Photographic studies were carried out to investigate the depen-ences of the bubble dynamics in water pool boiling on the staticontact angle of the heated surface and the liquid subcooling. Tonable clear visualization of bubble behavior, experiments wereonducted in the isolated bubble regime near the onset of nucle-te boiling. In the present pool boiling experiments, surface tensionas considered of particular importance in determining the bubble

ehavior. The surface tension first held the bubble at the nucleationite for a certain period of time after the nucleation as hypoth-sized in many available models. When the bubble size becamearge enough for buoyancy to overcome the surface tension effect,ubbles started to slide up the vertical heated surface. Then, dis-inct difference was found in the bubble behavior depending onhe heated surface. Although bubbles kept sliding up the surfaceor a long distance in the case of hydrophobic surface, they wereifted-off the vertical heated surface within a short period of time

hen a hydrophilic surface was used. Since the bubble was usuallyropelled into a bulk liquid after the detachment from the wall,ubble life-time was significantly shortened when the bubble lift-ff took place in subcooled liquid. In the present experiments, theeated surface was oxidized or nano-particles were deposited onhe surface to change surface properties. The bubble behavior mightence be influenced not only by the contact angle but also by otherurface properties such as thermal resistance and roughness. How-ver, reduction of the contact angle was supposed as a main cause

f bubble lift-off since the surface tension effect to hold bubbles onhe wall was considered to diminish with a decrease in the contactngle and consistent with the present observations.

In the present experiments, no bubble could adhere to theeated surface even at the onset of nucleate boiling when a

d Design 240 (2010) 3949–3955 3955

hydrophilic surface was used as the heated wall. This would indi-cate the possibility that the bubble detachment concept widelyused in available subcooled flow boiling models does not apply topredict the point of net vapor generation if the contact angle of theheated surface is not large enough. Further studies are considerednecessary to elucidate the role of surface wettability in determiningthe void fraction distribution in subcooled flow boiling.

References

Al-Hayes, R.A., Winterton, R.H.S., 1981. Bubble diameter on detachment in flowingliquids. International Journal of Heat and Mass Transfer 24, 223–230.

Bibeau, E.L., Salcudean, M., 1994. A study of bubble ebullition in forced-convectivesubcooled nucleate boiling at low pressures. International Journal of Heat andMass Transfer 37, 2245–2259.

Bowring, R.W., 1962. Physical model based on bubble detachment and calculationof steam voidage in the subcooled region of a heated channel. OECD HaldenReactor Project Report, HPR-10.

Collier, J.G., Thome, J.R., 1994. Convective Boiling and Condensation, 3rd ed. OxfordUniversity Press, Oxford.

Fritz, W., 1935. Maximum volume of vapor bubbles. Physik Zeitschr 36, 379–384.Gunther, F.C., 1951. Photographic study of surface-boiling heat transfer to water

with forced convection. Transactions of the ASME 73, 115–123.Kandlikar, S.G., Stumm, B.J., 1995. A control volume approach for investigating forces

on a departing bubble under subcooled flow boiling. Transactions of the ASME:Journal of Heat Transfer 117, 990–997.

Kandlikar, S.G., Nariai, H., 1999. Flow boiling in circular tubes. In: Kandlikar, S.G.,Shoji, M., Dhir, V.K. (Eds.), Handbook of Phase Change: Boiling and Condensation.Taylor & Francis, Philadelphia, pp. 367–402.

Kim, H.D., Kim, J., Kim, M.H., 2007. Experimental studies on CHF characteristicsof nano-fluids at pool boiling. International Journal of Multiphase Flow 33,691–706.

Kim, S.J., Bang, I.C., Buongiorno, J., Hu, L.W., 2007. Surface wettability change duringpool boiling of nanofluids and its effect on critical heat flux. International Journalof Heat and Mass Transfer 50, 4105–4116.

Kudritskiy, G.R., 1995. Effect of heating-surface roughness on the mecha-nism of growth of a vapor bubble in boiling. Heat Transfer Research 24,445–454.

Lamb, H., 1932. Hydrodynamics, 6th ed. Cambridge University Press (Chapter 9).Levy, S., 1967. Forced convection subcooled boiling prediction of vapor volumetric

fraction. International Journal of Heat and Mass Transfer 10, 951–965.Liaw, S.P., Dhir, V.K., 1989. Void fraction measurements during saturated pool boiling

of water on partially wetted vertical surface. Transactions of the ASME: Journalof Heat Transfer 111, 731–738.

Oka, T., Abe, Y., Mori, Y.H., Nagashima, A., 1996. Pool boiling heat transfer inmicrogravity (experiments with CFC-113 and water utilizing a drop shaft facil-ity). JSME International Journal Series B: Fluids and Thermal Engineering 39,798–807.

Okawa, T., Ishida, T., Kataoka, I., Mori, M., 2005. Bubble rise characteristics after thedeparture from nucleation site in vertical upflow boiling of subcooled water.Nuclear Engineering and Design 235, 1149–1161.

Okawa, T., Kataoka, I., Mori, M., 2006. Numerical simulation of two-dimensionalbubbles initially flattened along a flat plate. Numerical Heat Transfer Part A:Applications 49, 393–409.

Okawa, T., Hayashi, K., Tomiyama, A., 2008. A numerical study on the mechanism ofbubble lift-off from a flat plate. In: Proceedings of 6th Japan–Korea Symposiumon Nuclear Thermal Hydraulics and Safety, Okinawa, Japan, Paper No. N6P1024.

Saha, P., Zuber, N., 1974. Point of net vapor generation and vapor void fraction in sub-cooling. In: Proceedings of 5th International Heat Transfer Conference, Tokyo,Japan, pp. 175–179.

Shoji, M., Takagi, Y., 2001. Bubbling features from single artificial cavity. Interna-tional Journal of Heat and Mass Transfer 44, 2763–2776.

Situ, R., Hideki, T., Ishii, M., Mori, M., 2005. Bubble lift-off size in forced convec-tive subcooled boiling flow. International Journal of Heat and Mass Transfer 48,5536–5548.

Staub, F.W., 1968. The void fraction in subcooled boiling: prediction of the initialpoint of net vapor generation. Transactions of the ASME: Journal oh heat transfer90, 151–157.

Takata, Y., Hidaka, S., Cao, J.M., Nakamura, T., Yamamoto, H., Masuda, M., Ito,T., 2005. Effect of surface wettability on boiling and evaporation. Energy 30,209–220.

Thorncroft, G.E., Klausner, J.F., Mei, R., 1998. An experimental investigation of bubblegrowth and detachment in vertical upflow and downflow boiling. InternationalJournal of Heat and Mass Transfer 41, 3857–3871.

Thorncroft, G.E., Klausner, J.F., Mei, R., 2001. Bubble forces and detachment models.

Multiphase Science and Technology 13, 35–76.

Unal, H.C., 1975. Determination of the initial point of net vapor generation in flowboiling systems. International Journal of Heat and Mass Transfer 18, 1095–1099.

Yang, S.R., Kim, R.H., 1988. A mathematical model of the pool boiling nucleation sitedensity in terms of the surface characteristics. International Journal of Heat andMass Transfer 31, 1127–1135.