Proceedings of the AIAA/ASME Thermophysics and Heat Transfer Conference, St. Louise, MO, June 2002

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Heat Transfer from a Single Nucleation SiteDuring Saturated Pool Boiling of FC-72 Using an Array of 100 Micron Heaters

Fatih DemirayUniversity of Maryland

Department of Mechanical EngineeringCollege Park, MD 20742 USA

Email: fatih@eng.umd.edu

Jungho Kim*University of Maryland

Department of Mechanical EngineeringCollege Park, MD 20742 USAEmail: kimjh@eng.umd.edu

ABSTRACTBoiling heat transfer has been used in the thermal

control of compact devices since the discovery of highheat transfer coefficients associated with boilingphenomena. The objective of the current work is toinvestigate and determine boiling heat transfermechanisms in single and multiple bubble modes with aconstant surface temperature boundary condition. Theheat transfer data contained in this paper is uniquebecause it is acquired at many locations simultaneouslyusing a microscale heater array instead of at a singlepoint underneath the bubbles, which yields a moredetailed picture of the heat transfer process. Images ofthe growing bubbles were captured from below andfrom the side using two high-speed digital videocameras, allowing bubble behavior to be correlated withthe heat transfer measurements. The data indicated thatthe area influenced by the bubble departure wasapproximately half the departure diameter. Microlayerevaporation was observed to contribute a significant,but not dominant, fraction of the wall heat transfer inthe single bubble mode. Microlayer evaporation wasinsignificant in the multiple bubble mode, and heattransfer occurred mainly through transientconduction/microconvection during liquid rewetting asthe bubble departs the surface.*

INTRODUCTIONDespite many decades of study, there are still many

conflicting models regarding the mechanisms by whichheat is transferred during boiling. Many of the earlymodels were based on bubble agitation/microconvection being the primary mechanism. Thesemodels did not include phase change, but relied on ananalogy with forced convection, i.e., the role of thebubble is to change the length and velocity scales usedto correlate data.

The vapor-liquid exchange model proposed byForster and Grief [1] assumes that bubbles act asmicropumps that remove a quantity of hot liquid fromthe wall equal to a hemisphere at the maximum bubble * All correspondence should be addressed to this author.

radius, replacing it with cold liquid from the bulk. Theheat transfer from a single site is the energy required toheat this volume of liquid from the bulk temperature tothe average of the wall and bulk temperatures.

Building on the work of Han and Griffith [2],Mikic and Rohsenow [3] developed a model of bubbleheat transfer that assumes the bubble scavenges awaythe superheated layer surrounding the bubble over somearea of influence as it departs, allowing colder liquidfrom the bulk to contact the surface. The superheatedlayer is then renewed during the waiting time (timeafter bubble departure and before nucleation of a newbubble) by transient conduction into a semi-infiniteliquid. Transient conduction into this bulk liquid afterbubble departure was the assumed mode of heattransfer.

Cooper and Lloyd [4] and van Stralen et al. [5]proposed a model based on the evaporation of a thinmicrolayer that forms between a hemisphericallygrowing bubble and the wall during inertially controlledgrowth. Heat transfer through this microlayer wasproposed to be the primary mechanism for bubblegrowth.

The proliferation of models and continuingcontroversy surrounding single bubble heat transfer is adirect result of the lack of reliable data regarding localheat transfer information in the vicinity of the bubble.The vast majority of experimental work performed todate regarding boiling has utilized single heaters thatwere large compared to individual bubbles, making itdifficult to look at details of the boiling process. Theseexperiments usually used a single heating elementoperated at constant power, making it difficult to obtaininformation about local temperature variationsunderneath bubbles. Other experiments have utilizedsurfaces held at constant temperature averaged over theentire heater, but the local heat flux and temperaturewere not measurable and could vary significantly acrossthe heater.

Yaddanapuddi and Kim [6] and Kim et al. [7] useda 96-element microheater array with individual heatersnominally 270 mm on a side to obtain wall heat transferinformation under single bubbles at two wall

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superheats. In both cases they found that bubble heattransfer mechanisms were different from the widelyaccepted view that microlayer evaporation is thedominant heat transfer mechanism in saturated poolboiling. The results also indicated that transientconduction and/or microconvection was the primaryheat transfer mechanism, consistent with the model ofMikic and Rohsenow [3]. However, the area ofinfluence of the departing bubble was much smallerthan the model suggested, and the temperature of theliquid coming into contact with the wall after bubbledeparture was much higher than the bulk temperature.Because the departing bubble was about 380 mm indiameter, the spatial resolution used in this study wasinsufficient to determine the area of influence of thebubble. In this paper, we examine the results obtainedusing a 96 element heater array similar to the one usedearlier, but with individual heaters nominally 100 mm x100 mm. Wall heat transfer due to single bubblesseparated by a distinct waiting time as well as multiplebubble nucleating one after another from a single siteare examined.

EXPERIMENTAL APPARATUS

Heater array An array of 96 platinum resistance heater elements

deposited on a quartz wafer provided local surface heatflux and temperature measurements. A photograph ofthe heater array is shown in Figure 1. Each element inthe array was square in shape, nominally 0.01 mm2 inarea, and consisted of 2 mm wide Pt lines spaced 2 mmapart. Each heater had a nominal resistance of 8 kWwith a temperature coefficient of resistance of0.00125 °C-1. The lines that supply power to theheaters are routed between the heaters and the PGAboard. The details of the construction of a similar heaterarray are given in Rule and Kim [8].

Feedback control circuitEach heater in the array was kept at constant

temperature by individual feedback circuits similar tothose used in hotwire anemometry–see Figure 2 for aschematic of the circuit. Any imbalance in theWheatstone bridge was sensed by an amplifier, whichprovided enough power to the heater to bring the bridgeback into balance. The output of the circuit was thevoltage across the heater. The heat dissipated by a givenheater could be directly calculated from this voltage andthe heater resistance. The heater temperature wascontrolled by a digital potentiometer placed in a leg ofthe Wheatstone bridge.

Sixteen of these circuits were constructed on asingle card. Six of these cards plugged into amotherboard that routed the signals from the hostcomputer to the individual feedback circuits. The reader

is referred to Bae et al. [9] for additional detailsregarding the electronics of the circuits.

Heater calibrationThe heater array was calibrated in an oven held

within 0.1 °C of the set temperature. Calibrationconsisted of finding the digital potentiometer wiperposition that caused the feedback loop to just beginregulating for that given chamber temperature. Eachheater in the array could be varied over a 20 °C range in4 °C increments. The uncertainty in heater temperatureis less than 1 °C.

Data acquisition systemThe two data acquisition cards (PCI-DAS6402/16),

each capable of scanning 64 analog input channels at amaximum speed of 200 kHz, were installed inside aDell OptiPlex GX110 computer. Each card sampledthe outputs of 48 heaters. The system was used toobtain time-resolved data at 3704 Hz from each heaterfor a period of four seconds. Both data acquisitioncards were triggered by the same rising edge of a TTLsignal from the computer.

Boiling rigThe boiling rig shown in Fig. 3 was provided by

NASA and used in the experiments. The bellows andthe surrounding housing allowed the test sectionpressure to be controlled. A stirrer was used to breakup any stratification within the test chamber, while aseries of Kapton heaters attached to the outside of thechamber were used to control the bulk liquidtemperature.

High speed videoThe semi-transparent nature of the heater array

enabled images to be taken from below with a high-speed digital video camera (Vision Research PhantomIV) set to acquire 256x256 resolution images at 3704fps. A group of high performance white LEDs wasmounted over the heater array within the chamber inorder to provide a bright, diffuse background forbottom-view pictures of the bubble. A second high-speed digital video camera (Vision Research PhantomIV) was used to record side-view images at the samespeed and resolution. A halogen lamp next to a glasswindow in the boiling chamber provided enough lightfor side view images. Recording of both cameras wasinitiated using the same signal used to trigger the dataacquisition system, enabling heat transfermeasurements and video records to be madesimultaneously.

Data reductionBecause each heater had its own feedback control

circuit, we were able to measure the instantaneouspower required to maintain each heater at a constant

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temperature. Some of the power supplied to the heaters,however, is conducted from the heater elements to thesurrounding substrate and can eventually be lost bynatural convection to the bulk liquid. In this study, weare interested in the heat transfer induced only by thebubble action. The heat transfer excursions around aslowly varying baseline were considered and assumedto be a consequence of bubble formation and departure.The baseline of the heat transfer curve exhibited a lowfrequency oscillation, which is likely due to a naturalconvection flow over the heater driven by thetemperature difference between the bulk liquid and theheater array. To obtain the effect of the bubble only, asixth degree polynomial was fitted to selected points onthe baseline and subtracted from the total time-resolvedheat trace for each heater in the array. The resultingheat transfer curve could exhibit both positive andnegative values. Negative values of heat transfer couldresult if liquid dryout during bubble growth above aheater occurred, resulting in a lower heat transfer thanwould have occurred in the case of natural convectionin the absence of a bubble.

An example of the data reduction is shown on Fig.4, in which the total heat transferred from the array isobtained by summing the heat transferred from eachheater together and plotted on the upper curve.Excursions in heat transfer above a slowly varyingbaseline are observed. These excursions correspond toa single or multiple bubble growth sequence from asingle nucleation site on the surface, and each wasassigned a number as indicated in Fig. 4. The baselineobtained by a curve fit is overlaid on this curve. Thelower curve was obtained by subtracting the baselinefrom the total heat transfer curve and is the net changein heat transfer due to the presence of the bubble on thesurface.

RESULTSData were obtained under slightly subcooled pool

boiling conditions using FC-72 at 1 atm (Tsat=56.7 °C).The surface temperature of the heater array and the bulkfluid temperature during the experiment were 76 °C and52 °C, respectively. It was observed that the bubblesnucleated persistently from a single site on the array.

Bubbles that nucleated at this site alternatedbetween two modes: single bubble mode and multiplebubble mode. In the single bubble mode, discretebubbles departed from the heater array with a waitingtime between the departure of one bubble andnucleation of the following bubble. In the multiplebubble mode, bubble nucleation was observedimmediately after the previous bubble departed. Thedeparting bubble pulled the growing bubble off thesurface prematurely and the bubbles merged verticallyforming small vapor columns. Events 10, 12, and 13on Figure 4 were multiple bubble modes, with the

remainder being single bubble modes. Events 10, 12,and 13 were composed to two, three, and threenucleation events, respectively.

Single bubble ModeTime resolved images of a single bubble event 1

taken from below are shown on Fig. 5. Each heater inthe array has been colored according to the heattransfer. Side view images are shown on Fig. 6.Nucleation occurred between 0 ms – 0.27 ms. Based onthe bottom view images, the bubble grew to nearly fullsize by 1.89 ms after nucleation. The bubble shapeseemed to be approximately hemispherical. A largeincrease in the heat transfer under almost the entirebubble was observed during this time, consistent withevaporation from a microlayer between the bubble andthe wall. Starting from 2.16 ms, the development of alow heat transfer region at the center of the bubble isobserved, indicating progressive dryout of themicrolayer. The dry spot size, as evidenced by theinner circle, reaches a maximum around 3.51 ms. Thebubble began to depart the surface at this time, and thedry spot shrinks as the bubble necks down. Higher heattransfer is observed on the center heaters as they arerewetted by the bulk liquid. Bubble departure occurredat 5.13 ms, and is associated with a spike in heattransfer at the center heaters that decays with time.

Evaluation of superheated liquid modelA plot of the time varying physical bubble

diameter was obtained by fitting a circular template tothe bubbles shown on Fig. 5. The wall heat transferdata shown in Fig. 4. can be used to compute anequivalent bubble diameter (deq) by assuming that allthe heat transferred from all of the heaters goes intolatent heat:

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Assuming the time t = 0 to be the start ofnucleation for a single bubble, the equivalent diameteris plotted along with the physical diameter on Fig. 7.The slight decrease in measured physical diameter afterabout 0.8 ms is due to the distortion of the bubblegeometry during the growth time. It is seen that deq issignificantly lower than the measured bubble diameterduring the bubble growth time, implying that the heattransferred from the wall cannot account for the bubblegrowth alone. This indicates that the bubble must havegained the great majority of its energy from thesuperheated liquid layer surrounding the bubble. Thisconclusion is consistent with the results of the studyperformed by Kim et al. [7].

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Evaluation of transient conduction modelMikic and Rohsenow [3] assumed that the bubble

scavenged away the superheated layer over an areacovering twice the bubble departure diameter (area ratioAR=4.0). They also assumed that the temperature ofthe liquid coming into contact with the wall was thebulk liquid temperature. As observed in the colorizedbottom-view images (Fig. 5), the area of influence afterbubble departure is contained within approximately halfof the bubble departure diameter (Area RatioAR=0.25). Plots of the transient conduction curveassuming various wall-to-liquid temperature differences(DT) and AR are compared to the actual heat transfervariation in Fig. 8. Assuming the rewetting liquid is atthe bulk liquid temperature, the transient conductioncurve is best fit to the measured heat trace whenAR=0.1 as seen in Fig. 8. However, the rewettingliquid temperature is unknown. From the present data,one can clearly state that DT should be smaller sincethe heaters within the area of influence would besubjected to rewetting of colder liquid from the bulkduring shrinkage of base diameter until departure. Thisallows the liquid surrounding the bubble base to bepreheated gradually in advance

In a study conducted by Kim et al. [7], it waspredicted that AR=0.4, which is consistent with thepresent data and images. We can conclude from thehigher spatial resolution underneath the bubble andbetter time-resolved data that the temperature of therewetting liquid is higher than that of the bulk.However, it still needs to be studied numerically tomake a quantitative conclusion.

Multiple bubble modeMultiple bubble mode is defined as an event that

consists of at least two bubbles that nucleate and departwithout any waiting time in between. In this paper, wewill examine only the data that were recorded duringmultiple bubble event 13. Side view images of thisprocess are shown on Fig. 9. The multiple bubble modestarts with the first bubble nucleating between 0 - 0.27ms. This bubble departs from the surface at 5.4 ms. Asecond bubble is observed to immediately nucleate inthe wake of the first bubble. After a growth period, thesecond bubble is pulled off the surface prematurely bythe influence of the first bubble rising in the liquid. Theinitial spherical bubble shape is distorted under theinfluence of the first-departed bubble and the verticalmerging with it. Departure of the second bubble occursat 10.26 ms and is immediately followed by thenucleation of a third bubble. This bubble grows on thesurface and departure occurs at 17.82 ms -no immediatenucleation is observed after the third bubble departs.

The heat transfer distribution at the wallsuperimposed on the high-speed images are shown onFig. 10. The heat transfer distribution for the first

bubble is seen to be very similar to that observed for thesingle bubble event. A large heat transfer is observedjust after bubble nucleation due to microlayerevaporation. The dry spot at the center of the bubblegrows as the microlayer dries out, then shrinks as thebubble departs at 5.4 ms. The large heat transfer at thecenter of the array at bubble departure suggests that theentire heater surface is wetted with liquid, i.e., thedeparting bubble does not leave a vapor layer coveringthe heaters. The heat transfer after departure of the firstbubble decays with time even as the second bubblegrows (5.40 ms to 7.83 ms), and may be associated withgrowth of another dry spot on the surface. Bubblegrowth during this time may be due to evaporation atthe three-phase contact line, or from the superheatedliquid layer. Microlayer evaporation is not observed,indicating that much of the energy contained within thesuperheated liquid layer was depleted during the growthof the first bubble. The side view images (Fig. 9)indicate that merging of the second bubble with the firstbubble occurs between 7.83 ms and 10.26 ms. The heattransfer distribution during this time indicates high heattransfer along the circumference of the bubble as it ispulled off the surface, which is consistent withrewetting of the surface with liquid. The majority ofheat transfer during growth and departure of the secondbubble seems to occur during the rewetting processbefore bubble departure by transient conduction and/ormicroconvection. Heat transfer distributions for thethird bubble as it grows and departs the surface are verysimilar to those for the second bubble, but the thirdbubble is not prematurely pulled off the surface bymerger with the previous bubbles.

Time-varying physical and equivalent diametersare plotted along with the total heat transfer for event13 on Fig. 11. The first nucleation is followed by rapidbubble growth. The corresponding rapid increase inheat transfer is likely due to microlayer evaporation.As in the single nucleation event, however, theequivalent diameter indicates that the wall heat transfercannot account for the large bubble size. Thenucleation of the second bubble is not accompanied byan increase in heat transfer, and the bubble growth rateis seen to be much slower than for the first bubble. Theequivalent diameter again indicates that the bubblegains energy from the superheated liquid layer duringbubble growth. The heat transfer continues to decreaseafter nucleation until the second bubble begins to mergewith the first bubble at 7.83 ms. The heat transferincreases during this merging process and peaks whenthe second bubble departs the surface. Nucleation of thethird bubble is accompanied by a decrease in heattransfer as well. The heat transfer rises again as thebubble begins to pull off the surface at 15 ms. The peakin heat transfer again corresponds to bubble departure.Similar observations were made for events 10 and 12.

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CONCLUSIONSSpace and time resolved heat transfer variations

due to bubble activity on a 96 element heater arrayconsisting of 100 mm heaters were visualized by using ahigh speed digital video camera to obtain images ofbubble growth and departure, then colorizing eachheater in the array according the instantaneous heattransfer. The current study indicates that the area ofinfluence of the bubble is much smaller than theprojected bubble area at departure, which validates thepredicted area ratio in the study conducted by Kim et al.[7]. The examination of a calculated equivalentdiameter with the measured physical diameter duringthe single bubble mode supports the idea that thebubble gains energy from the wall as well as from asuperheated layer that develops during the waiting time.Microlayer evaporation was observed to contribute asignificant, but not dominant, fraction of the wall heattransfer in this mode. Examination of wall heat transferduring multiple bubble modes indicated that microlayerevaporation is insignificant after the first bubble, andthat heat transfer occurs mainly through transientconduction and/or microconvection during liquidrewetting as the bubble departs the surface.

ACKNOWLEDGEMENTSThe funding for this work was provided by NASA

MSAD under Grant No. NCC3-783, and is gratefullyacknowledged. The grant monitor is John McQuillen.

NOMENCLATUREA Aread DiameterDT Wall-to-liquid temperature differencehfg Latent heatq EnergyT Temperaturet Time

Greekr Density

Subscriptseq Equivalenth Heatersat Saturationv Vapor

Superscripts. Per unit time'' Per unit area

REFERENCES[1] Forster, K.E., and Greif, R., 1959, “Heat Transfer toa Boiling Liquid-Mechanism and Correlations,”Journal of Heat Transfer, 81, pp. 43-54.[2] Han, C.Y., and Griffith, P.,1965, “The Mechanismof Heat Transfer in Nucleate Pool Boiling - Part II,”International Journal of Heat and Mass Transfer, 8, pp.905-913.[3] Mikic, B.B., and Rohsenow, W.M., 1969, “A NewCorrelation of Pool-Boiling Data Including the Effectof Heating Surface Characteristics,” Journal of HeatTransfer, 91, pp. 245-250.[4] Cooper, M.G., and Lloyd, A.J.P., 1969, “Themicrolayer in nucleate pool boiling”, InternationalJournal of Heat and Mass Transfer, 12, pp. 895-913.[5] Van Stralen, S.J.D., Sohal, M.S., Cole, R., andSluyter, W.M., 1975, “Bubble growth rates in pure andbinary systems: Combined effect of relaxation andevaporation microlayers”, International Journal of Heatand Mass Transfer, 18, pp. 453-467.[6] Yaddanapudi, N., and Kim, J., 2000, “Single BubbleHeat Transfer of Saturated Pool Boiling of FC-72,”Proceedings of the Boiling 2000 EngineeringFoundation Conference, Anchorage, AK.[7] Kim, J., Demiray, F., and Yaddanapudi, N.,“Saturated Pool Boiling Mechanisms During SingleBubble Heat Transfer: Comparison at Two WallSuperheats,” Proceeding of 2000 IMECE Conference,Orlando, FL.[8] Rule, T.D., and Kim, J., 1999, “Heat TransferBehavior on Small Horizontal Heaters During PoolBoiling of FC-72,” Journal of Heat Transfer, 121, No.2, pp. 386-393.[9] Bae, S.W., Kim, J., Mullen, J.D., and Kim, M.H.,1999, “Preliminary Wall Heat Transfer Measurementsand Visualization of bubble Growth and Departure:Saturated Nucleate Boiling of FC-72,” Proceedings ofthe 5th ASME/JSME Joint Thermal EngineeringConference, San Diego, CA.

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Figure 1: Photograph of heater array with a single bubble nucleating on the surface.

Figure 2: A schematic diagram of the feedback control circuit.

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.

Figure 3: Schematic of experimental apparatus.

Figure 4: Total heat transfer vs. time.

0

0.01

0.02

0.03

0.04

0.05

0 0.25 0.5 0.75 1 1.25 1.5 1.75

baseline baseline reduced total heat transfertotal heat transfer

Total heat transfer [W]

time [s]

12

34

56 7 8 9

10 11 12

13 14

P

P

FilterP

T

Compressed air

Fill port/vacuum

Not to scale

Microscale heater array

Stirrer

Window

Viewports

Stainless steel bellows

FC-72Window

Pressure regulator

Light

Mirror High-speed digital video

LED

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Figure 5: Colorized bottom-view images. The non-functional heaters are noted in gray.

Figure 6: Side-view images of bubble growth.The numbers printed below the images indicate the time in milliseconds.

Departure

Nucleation

0 0.02 W

6942

2246

76

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Figure 7: Physical and equivalent bubble diameter vs. time.

Figure 8: Wall heat transfer compared with transient conduction model.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 1 2 3 4 5

Bubble 1 - EquivalentBubble 2Bubble 3Bubble 4Bubble 5Bubble 6Bubble 7Bubble 8

Bubble 1 - Physical Bubble 2Bubble 3Bubble 4Bubble 5Bubble 6Bubble 7Bubble 8

Bubble diameter [mm]

time [ms]

0

0.005

0.01

0.015

0 5 10 15 20

DT = 24 K and AR = 0.1DT = 24 K and AR = 0.2DT = 24 K and AR = 0.25DT = 24 K and AR = 0.3DT = 24 K and AR = 0.4

DT = 5 K and AR = 0.4DT = 5 K and AR = 0.25Bubble 3, measured heat transfer

Total heat transfer [W]

time [ms]

bubbledeparture

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Figure 9: Side-view images of multiple bubble event 13.The numbers printed below the images indicate the time in milliseconds. The time interval

between succeeding frames is 0.27 ms. The black-underlined images in the timeline show bubblenucleation and the last red-underlined image indicates the final bubble departure.

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Figure 10: Colorized bottom-view images of multiple bubble event 13.Non-functional heaters are noted in gray. The black-underlined images in the timeline show

bubble nucleation and the red-underlined image indicates the final bubble departure.

0 0.02 W

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Figure 11: Physical and equivalent diameters of multiple bubble event 13along with total heat transfer trace.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0

0.005

0.01

0.015

0.02

0.025

0 5 10 15 20

Bubble 13 - physical

Bubble 13 - equivalent

total heat transfer

Bubble diameter [mm]

total heat transfer [W]

time [ms]

departure

departure

departure