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Interdisciplinary Transport Phenomena VI, Volterra, Italy, 2009 1 Paper No: ITP-09-09

Proceedings of ITP2009Interdisciplinary Transport Phenomena VI:

Fluid, Thermal, Biological, Materials and Space SciencesOctober 4-9, 2009, Volterra, Italy

ITP-09-09

NUCLEATE BOILING HEAT TRANSFERIN ALCOHOL AQUEOUS SOLUTIONS

Takashi SakaiDepartment of Aeronautics and Astronautics,

Kyushu University, Japan(JSPS Research Fellow DC)

Yuuki TsukinariDepartment of Aeronautics and Astronautics,

Kyushu University, Japan

Shinsaku YoshiiDepartment of Aeronautics and Astronautics,


Kazutoshi KajimotoDepartment of Aeronautics and Astronautics,


Yasuhisa ShinmotoDepartment of Aeronautics and Astronautics,


Haruhiko OhtaDepartment of Aeronautics and Astronautics,


ABSTRACTSaturated pool boiling experiments were conducted by using

alcohol aqueous solutions, i.e. 1-Propanol/Water, 2-

Propanol/Water and Water/Ethylene glycol mixtures at0.1MPa. The heating surface is a horizontal upward-facing

circular flat plate of 40mm in diameter. The increase in the

heat transfer coefficients was observed for 1-Propanol/Water

and 2-Propaonl/Water mixtures at very low alcohol

concentration range, where Marangoni convection towards the

three-phase interline is induced by the surface tension gradient

along the vapor-liquid interface due to preferential evaporation

of alcohol component. The heat transfer enhancement due to

Marangoni effect seems to overcome the heat transfer

deterioration due to mass transfer resistance. In the moderate

concentration range, however, the heat transfer enhancement

was turned to the heat transfer deterioration with increasing in

the alcohol concentration. For Water/Ethylene glycol mixture,on the contrary, no significant increase and decrease in the

heat transfer coefficients was observed in the entire

concentration range tested. On the other hand, the critical heat

flux was markedly decreased in the low alcohol concentration

range of 1-Propanol/Water and 2-Propanol/Water mixtures.

Marangoni effect is also suggested as a possible reason for the

peculiar trend of the critical heat flux. Marangoni effect seems

to promote the extension of drypatches underneath coalesced

bubbles and the critical heat flux becomes decreased.

NOMENCLATURED mass diffusion coefficient

g gravity accelerationLa Laplace constant

Ma Marangoni number

P pressure

q heat flux

T temperaturewx weight fraction of liquid

wy weight fraction of vapor

x mole fraction of liquid

y mole fraction of vapor

Greek symbols

heat transfer coefficient

viscosity density surface tension

Subscripts

1 low-boiling component or more volatile component

2 high-boiling component or less volatile component

CHF critical heat flux

g gas

l liquid

m mixture

sat saturation


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1. INTRODUCTIONNucleate boiling heat transfer is one of the most effective

means for the heat removal from thermal devices because of

its high performance of heat transfer and heat transportation.For this reason, boiling heat transfer and critical heat flux

(CHF) of several single components and mixtures have been

studied by many investigators.

In nucleate boiling of binary mixtures, mass transfer

resistance causes the variation of liquid concentration and itssaturation temperature underneath bubbles along a vapor-

liquid phase equilibrium diagram. Thus Marangoni force is

induced along the vapor-liquid interface by the surface tension

gradient due to the variation of concentration and temperature.

The Marangoni force induces the flow whose direction is

determined by the sign of surface tension gradient along the

interface.When the surface tension of mixture increases with

decreasing the concentration of more volatile component

(positive mixtures), Marangoni force is expected to induce the

flow which supplies liquid towards the three-phase interline

and prevents the extension of drypatch and the heat transfer

enhancement is expected. On the other hand, when the surface

tension of mixture decreases with decreasing the concentration

of more volatile component (negative mixtures), Marangoni

force is expected to induce the flow towards opposite direction

and promote the heat transfer deterioration. For alcohol

aqueous solutions, the value of surface tension changes

significantly in the low alcohol concentration range and the

existence of strong Marangoni force is expected.

There exist a number of experimental and theoretical

researches for the boiling heat transfer of binary mixtures. It is

a well known fact from numerous boiling experiments that thenucleate boiling heat transfer coefficients of mixtures areusually lower than those of ideal or, more precisely,

hypothetical single components with the same thermophysical

properties as the mixtures. Van Wijk et al.[1] explained for the

lower heat transfer coefficient of binary mixtures. They noted

that the reduction of local liquid concentration of more volatile

component increases the local saturation temperature

underneath bubbles because more volatile component

preferentially evaporates to maintain equilibrium between two

phases. Consequently, the wall temperature rises and the heat

transfer coefficient based on the saturation temperature of bulk

liquid is lowered. Stephan and Krner[2] proposed a simple

correlation based on the explanation given by Van Wijk etal.[1]. They defined the ideal heat transfer coefficients for

mixtures which are given by the interpolation of wall

superheats for both pure single components to represent the

mixture boiling heat transfer coefficients in the absence of

mixture effects. And they evaluated the heat transfer

deterioration rate from the ideal heat transfer coefficients by

using the difference between the molar fractions of both

phases on the vapor-liquid equilibrium diagram. Calus et

al.[3] considered the reduction of the bubble growth rate to

evaluate the deterioration rate by using Scriven[4] and Van

Stralen[5] analysis of the bubble growth. Thome[6] tried to

calculate the deterioration rate by only using phase

equilibrium diagram. He proposed a simple correlation which

uses only temperature difference between dew and bubbling

point temperatures at the bulk concentration. Kandlikar[7]

theoretically analyzed the mixture property effects on nucleate

boiling heat transfer. He introduced a new pseudo-singlecomponent heat transfer coefficient as the ideal heat transfer

coefficient to reflect mixture property effects more accurately.

And the mixture boiling heat transfer coefficient was derived

theoretically by estimating the effects of heat and mass

transfer at the vapor-liquid interface of a glowing bubble. Inthe above researches, only heat transfer deterioration of binary

mixtures was discussed and Marangoni effect on the heat

transfer coefficients of binary mixtures was ignored.

On the other hand, there also exist a lot of studies for critical

heat flux of mixtures. Several geometries of heating surface

were tested and the result of either increasing or decreasing in

the critical heat flux was observed. Hovestreijdt[8] first

speculated that the Marangoni force affects the critical heat

flux of binary mixtures. McGillis and Carey[9] suggested a

correlation based on the speculation given by Hovestreijdt.

They quantitatively estimated the Marangoni effect as an

additional liquid restoring force caused by the surface tension

gradient with a modified model derived from hydrodynamics.

Fujita et al.[10] also considered the Marangoni effect to

evaluate the increase in the critical heat flux of binary

mixtures and proposed a correlation using the Marangoni

number defined by using thermal diffusivity. McEligot[11]

attributed the increase in the critical heat flux of binary

mixtures to the increase in effective subcooling because the

interfacial temperature is increased due to the preferential

evaporation of more volatile component. Leddy and

Lienhard[12] quantitatively evaluated the increase in theeffective subcooling and suggested a correlation. The aboveresearches, however, considered no geometry effect on the

critical heat flux of mixtures.

3. EXPERIMENTAL APPARATUS AND PROCEDUREThe experimental setup shown in Fig.1 is composed of three

main parts, a boiling vessel, condensers and a heating section.

The boiling vessel is made from a stainless steel pipe with

inner diameter of 200mm and a volume of 0.023m3. Liquid

and vapor temperature are measured by four thermocouples

placed in the vessel. The condensers are installed in the upper

side of the boiling vessel and control the levels of system

pressure and saturation temperature. The detailed structure ofheating section is also illustrated in Fig.1. A heating surface,

which is the upper edge of the copper heating block, is heated

by the cartridge heaters inserted in the bottom of the heating

block. The horizontal upward-facing heating surface polished

by sandpaper (No. 600) has a dimension of 40mm in diameter

and is surrounded by a circular thin fin in order to preventpreferential bubble generation at the edge and minimize the

circumferential heating loss from the periphery. In the heating

block, eight thermocouples are inserted at the depth of 1, 7,

13, 19mm from the heating surface along two axes at the

center and 14mm from the center. A surface heat flux and

surface temperature are evaluated from the thermal conduction


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across the copper heating block by the indicated temperatures

of thermocouples.Pool boiling experiments were conducted under saturated

conditions. After the enough time is elapsed to confirm the

steady state of indicated temperatures of all thermocouples

and pressure, heat flux is increased. The condition of critical

heat flux is detected by the excursion of temperature in the

copper heating block. The heat flux one step before the value

resulting temperature excursion is defined as a critical heat

flux. The increment of input heat flux near the critical heat

flux is 1.0105 W/m2 and the value gives the resolution ofmeasured critical heat flux.

The experiments were performed at 0.1MPa on ground.

Three alcohol aqueous solutions, 1-Propanol/Water, 2-

Propanol/Water and Water/Ethylene glycol mixtures were

employed as test fluids. Concentration ranges tested are shown

in Table1 and the phase equilibrium diagrams of these

mixtures are shown in Fig.2. In Fig.3, surface tension behavior

of these mixtures is shown. 1-Propanol/Water and 2-

Propanol/Water mixtures have an azeotropic concentration, so

they act as the positive mixture at the lower concentration of

1-Propanol and 2-Propanol than the azeotrope while they act

as the negative mixture at the higher concentration of alcohol.

For Water/Ethylene glycol mixture, it acts as the negative

mixture in the entire concentration range.

In the present research, Marangoni effect was evaluated by

the Marangoni number using mass diffusion coefficient

defined by Eqs. (1) and (2).

Fig.1 Schematic diagram of experimental apparatus

Fig.2 Phase equilibrium diagrams at 0.1MPa

0 0.2 0.4 0.6 0.8 170

80

90

100

110

120

Weight fractionwx1

Tsat

C

Bubbling Line Dew Line

2-Propanol / Water

Positive system

Negative system

Azeotrope

(b) 2-Propanol/Water

0 0.2 0.4 0.6 0.8 170

80

90

100

110

120

Weight fractionwx1

Tsat

C

Bubbling Line

Dew Line

1-Propanol / Water

Positive system

Negative system

Azeotrope

(a) 1-Propanol/Water

0 0.2 0.4 0.6 0.8 1

100

200

Weight fractionwx1

Tsat

C

Bubbling Line Dew Line

Water / Ethylene glycol

Negative system

(c) Water/Ethylene glycol

Table1 Test concentration range

Test fluids Test bulk liquid

concentrations of alcohol

1-Propanol/Water 0 ~ 90wt%

2-Propanol/Water 0 ~ 89wt%

Water/Ethylene glycol 0 ~ 90wt%


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Fig.5 Heat transfer coefficients


0 0.2 0.4 0.6 0.8 10

1

2

3[104]

Ideal heat transfer coefficientsStephan and Krner correlationThome correlation

2-Propanol/Water

Weight fraction of alcoholwx

W/m2K

Azeotrope

q=1.0105W/m

2

q=2.0105W/m

2

q=4.0105W/m

2



0 0.2 0.4 0.6 0.8 10

1

2

3[104]

Ideal heat transfer coefficientsStephan and Krner correlationThome correlation

1-Propanol/Water


W/m2K

Azeotrope

q=1.0105W/m

2

q=2.0105W/m

2

q=4.0105W/m

2

0 0.2 0.4 0.6 0.8 10

1

2

3[104]

Ideal heat transfer coefficientsStephan and Krner correlation

Thome correlation

Water/Ethylene glycol


W/m2K

q=1.0105W/m

2

q=2.0105W/m

2

q=4.0105W/m

2

Fig.3 Surface tension under phase equilibriumcondition at 0.1MPa

0 0.2 0.4 0.6 0.8 10

0.02

0.04

0.06

0.08

Weight fraction of alcohol

N/m

1-Propanol/Water2-Propanol/Water


Fig.4 Marangoni number

0 0.2 0.4 0.6 0.8 1-1

0

1

2

3[108]

1-Propanol/Water2-Propanol/WaterWater/Ethylene glycol

Weight fraction of alcohol

Ma

( )

D

Laxyx

Mal

mm

11

1

= (1)

)( ,, mgml

mm

gLa

(2)

The calculated Marangoni number for the entire

concentration range is shown in Fig.4. The larger heat transfer

enhancement is expected for the larger Marangoni number.

4.EXPERIMENTAL RESULTS AND DISCUSSIONSFigure 5 shows the effect of concentration on the heat

transfer coefficients at selected heat fluxes (1.0105, 2.0105,4.0105 W/m2). For 1-Propanol/Water and 2-Propanol/Watermixtures, the increase in the heat transfer coefficients was


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Fig.6 Critical heat flux



0 0.2 0.4 0.6 0.8 1105

106

107



qCHFW/m

2

Experimental resultsZuber correlation

0 0.2 0.4 0.6 0.8 1105

106

10


1-Propanol/Water


qCHFW/m2

0 0.2 0.4 0.6 0.8 1105

106

107

2-Propanol/Water


qCHFW/m2


observed at the very low concentration of alcohol. In the

moderate concentration range, however, the heat transfer

enhancement was turned to the heat transfer deterioration with

increasing in the alcohol concentration. And at the azeotropicconcentration, the heat transfer coefficient has a local

maximum. For Water/Ethylene glycol mixture, on the other

hand, no marked heat transfer enhancement or deterioration

was observed and the heat transfer coefficients gradually

decreased with increasing in the alcohol concentration. In thefigure, predicted heat transfer coefficients by the existing

correlations are also shown. Stephan and Krner[2] correlation

and Thome[6] correlation are used, and the heat transfer

coefficients of each pure alcohol are predicted by the

Kutateladze[13] correlation because the experiments for the

pure alcohol were not conducted due to the safety problem

inherent in the experimental apparatus. Here, multiplying

factors are introduced to adjust the predicted values to

coincide with the experimental data for pure water and at the

azeotropic concentration. The multiplying factors in the

intermediate concentration range between zero (pure water)

and the azeotropic concentration and between the azeotropic

concentration and unity (pure alcohol) was evaluated by the

linear interpolation of these multipliers, where a multiplier of

unity was introduced for pure alcohols. As can be seen from

Fig.5, both Stephan and Krner[2] correlation and Thome[6]

correlation can reproduce the trends of measured heat transfer

coefficients in the moderate concentration range for 1-

Propanol/Water and 2-Propanol/Water mixtures. For

Water/Ethylene glycol mixture, however, the prediction of the

heat transfer coefficients by Thome correlation is

overestimated because its temperature difference between thedew and bubbling temperature at the bulk liquid concentrationis too large. And the heat transfer enhancement at the very low

concentration range can be reproduced by neither of these

correlations. The heat transfer enhancement seems to be

caused by the Marangoni effect, and the heat transfer

enhancement overcomes the heat transfer deterioration due to

mass transfer resistance because the Marangoni number has a

peak value in the low concentration range and is markedly

reduced with further increasing in the alcohol concentration

as shown in Fig.4.

Figure 6 shows the effect of concentration on the critical heat

flux. The critical heat flux predicted by the Zuber

correlation[14] for hypothetical single component liquids withthe same properties as mixtures is also shown in Fig.6. In the

moderate concentration range, the measured critical heat flux

value was gradually decreased with increasing in the alcohol

concentration. Zuber correlation can reproduce the critical

heat flux trend. In the low alcohol concentration range,

however, the critical heat flux was significantly decreased.

The peculiar trend of critical heat flux was also reported by

using a flat plate heating surface[15] while the markedly

increase in the entire concentration range was observed by

using a wire heating surface[10,12]. Thus the trend of the

critical heat flux depends on the geometry of heating surfaces,

but the logical explanation or correlations which consider the

geometry effect has not be provided. The low concentration


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range, where the peculiar decrease in the critical heat flux was

observed, corresponds to the peak Marangoni number.

Consequently, Marangoni effect is suggested again as a

possible cause for the peculiar trend of the critical heat flux.How Marangoni effect influences to the critical heat flux

mechanism is deduced as follows. Marangoni force is directed

towards the three-phase interline underneath primary bubbles

because of the preferential evaporation of alcohol for positive

mixtures. The Marangoni effect supplies liquid spontaneouslyto the microlayer of the primary bubbles. As a result,

evaporation of microlayer is enhanced and heat transfer

coefficient is increased. On the other hand, the enhanced

evaporation causes finally the shortage of liquid stored in

macrolayer and drypatches are extended from some locations

where primary bubbles were existed. The extension of

drypaches is continued under a coalesced bubble, and the

excursion of surface temperature occurs before entire part of

liquid in macrolayer is consumed. Thus the critical heat flux

condition seems to be realized independent of the period for

the detachment of a coalesced bubble and it varies with the

intensity of Marangoni force induced around primary bubbles.

This mechanism might be true for the flat plate heating surface

where liquid supply to primary bubbles is indirectly via the

formation of macrolayer during the detachment of coalesced

bubbles and not directly by bulk liquid.

5. CONCLUTIONSThe boiling heat transfer and the critical heat flux of alcohol

aqueous solutions were investigated through the saturated pool

boiling experiments using 1-Propanol/Water, 2-

Propanol/Water and Water/Ethylene glycol mixtures at0.1MPa. The following conclusions were derived.

(1) The increase in the heat transfer coefficients was observed

at the very low alcohol concentration of 1-Propanol/Water

and 2-Propanol/Water mixtures while the critical heat flux

was markedly deceased in the same alcohol concentration

range.

(2) The above trends of the heat transfer and the critical heat

flux are deduced to be caused by the Marangoni effect,

which accelerates the evaporation of the primary bubbles

in macrolayer underneath coalesced bubbles.

ACKNOWLEDGMENTS

This work was supported by Grant-in-Aid for JSPS Fellows(211862) from Japan Society for the Promotion of Science

(JSPS). The authors express the appreciation for the support.

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transfer to boiling binary liquid mixtures. Chem. Engng. Sci.,

5: 68-80 (1956).

[2] K. Stephan & M. Krner, Calculation of heat transfer in

evaporating binary liquid mixtures. Chem. Ing. Tech., 41:

409-417 (1969).

[3] W.F. Calus & D.J. Leonidopoulos, Pool boiling - Binary

liquid mixtures.Int. J. Heat Mass Transfer, 17: 249-256

(1974).

[4] L.E. Scriven, On the dynamics of phase growth. Chem.

Eng. Sci., 10: 1-13 (1959).

[5] S.J.D. Van Stralen, Bubble growth rates in boiling binary

mixtures.Br. Chem. Eng., 12: 390-394 (1967).

[6] J.R. Thome, Prediction of binary mixture boiling heat

transfer coefficients using only phase equilibrium data.Int. J.

Heat Mass Transfer, 26: 965-974 (1983).

[7] S.G. Kandlikar, Boiling heat transfer with binary

mixtures : Part I - A theoretical model for pool boiling. Trans.

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[8] J. Hovestreijdt, The influence of the surface tension

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[9] W.R. McGillis & V.P. Carey, On the role of marangoni

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mixtures. Trans. ASME, 118: 103-109 (1996).

[10] Y. Fujita & Q. Bai, Critical heat flux of binary mixturesin pool boiling and its correlation in terms of Marangoni

number.Int. J. Refrig., 20: 616-622 (1997).

[11] D.M. McEligot, Generalized peak heat flux for dilute

binary mixtures.AIChE J., 10: 130-131 (1964).

[12] R.P. Reddy & J.H. Lienhard, The peak boiling heat flux

in saturated ethanol-water mixtures. Trans. ASME, 111: 480-

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[13] S.S. Kutateladze, Heat transfer in condensation and

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[14] N. Zuber, Hydrodynamic aspects of boiling heat

transfer. USAEC Report, AECU-4439 (1959).

[15] G.I. Bobrovich, I.I. Gogonin, S.S. Kutateladze, & V.N.

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