ISSN 0018�151X, High Temperature, 2012, Vol. 50, No. 3, pp. 407–411. © Pleiades Publishing, Ltd., 2012.Original Russian Text © Yu.A. Zeigarnik, K.A. Khodakov, Yu.L. Shekhter, 2012, published in Teplofizika Vysokikh Temperatur, 2012, Vol. 50, No. 3, pp. 436–441.

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INTRODUCTION

In [1] it was shown that many external effects thataccompany subcooled water boiling, in particular, themicrobubble ejection phenomenon, are connectedwith the presence of a certain amount of dissolved airin water. This fact forced us to return to this problemand to analyze to what extent the air dissolved in wateraffects the visual picture of subcooled water boiling.

A specific feature of boiling is the formation of bub�bles of the substance (coolant) being boiled during thephase transition process. If we speak about a liquidsubcooled relative to the saturation temperature (theso�called “surface” boiling), the existence of vaporbubbles, which have sizes of several hundredmicrometers and short lifetimes (hundreds µs) and donot depart from the heating surface, provides intensiveheat transfer from the heating wall to the single�phaseflow core or liquid pool, whose temperatures are lessthan the saturation temperature. This process incor�porates liquid evaporation to and condensation fromvapor bubbles, which act as miniature heat pipes, andturbulent convective heat exchange between the coldsingle�phase liquid core and the heated surface in thespace between the bubbles. Periodically originatingand collapsing vapor bubbles stimulate this convec�tion. At high heat flow rates (1 MW/m2 and more), thefirst of the above heat transfer mechanisms prevails.

The statistical characteristics of vapor bubbles(their amount, total surface area, duration of the bub�ble growth and collapse stages, etc.) form the data�base, which is necessary to construct physical models.However, water contains dissolved gas that is liberatedin the course of liquid heating as air bubbles, which donot participate in heat transfer (in any case, their con�tribution to this process is small). This factor must beaccounted for when the obtained bubble structures arestatistically processed.

Air bubble identification contains some uncertain�ties and ambiguities and suffers from subjectivity. In amethodological sense, the acoustic effects thataccompany the boiling process can help to a certain

extent when identifying vapor bubbles. Below, we willconsider this effect (dependence between vapor bub�ble liberation and acoustic�signal characteristics) inmore detail.

As was stated above, the water used in engineeringpractice and thermophysical experiments containsdissolved air. To exclude this air, which in technologi�cal processes leads to equipment corrosion and in thecourse of boiling visualization distorts the actual pic�ture of the process, water is commonly exposed tothermal deaeration. In industrial technologies, specialdeaerators are applied for this purpose, in which waterfilms, jets, or droplets are heated up to the saturationtemperature and the liberated air is removed in a mix�ture with steam to the atmosphere (a so�called dumpsteam). In experimental installations (if deaeration isapplied, although it is not always done) long�termwater boiling (for several hours) is used with the airremoval to a free volume of the installation or to theatmosphere.

The above deaeration methods considerably reducethe amount of dissolved air, especially when accom�plished at rarefaction; however, they do not allowcomplete removal of air. Some amount of air alwaysremains in the water.

The problem of monitoring small concentrationsof air is not simple. The existing instruments for mea�suring the oxygen content require continuous circuitwater flow through them, and it must be provided in anamount that is too large for an experimental installa�tion. Therefore, as the proved achievable oxygen con�centration, we can assume its standardized concentra�tion in a feed water of high�pressure boilers, i.e.,10 µg/kg of water. With account for the share of nitro�gen in the air and its solubility in water, this correspondsto a dissolved air content in water of about 16 µg/kg.

The oxygen solubility coefficient at 100°С (at long�term boiling under atmospheric conditions and achiev�ing the equilibrium state) is about 26 mg/(kg MPa).This gives for air (the mixture of oxygen and nitrogen)a value of about 16 mg/(kg MPa). If long�term deaer�ation is accompanied by rarefaction of the installation

HEAT AND MASS TRANSFERAND PHYSICAL GASDYNAMICS

Behavior of Air Bubbles during Subcooled Water BoilingYu. A. Zeigarnik, K. A. Khodakov, and Yu. L. Shekhter

Joint Institute of High Temperatures, Russian Academy of Sciences, ul. Izhorskaya 13, Moscow, 125412 RussiaReceived December 27, 2011

Abstract—The data of video filming of the behavior of air bubbles near the heating surface during boiling ofsubcooled water are presented. The effects the regime parameters (heat flow rate and water subcooling rela�tive to the saturation temperature) have on the characteristics of the vapor–air–water mixture are described.

DOI: 10.1134/S0018151X12030224

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free volume to 1 Tor or the dump steam is dischargedfor a long time, it is theoretically possible to achievethe air content of 2–3 µg/kg in the deaerated water. Inpractice, the effectiveness of deaeration is limited bygas diffusion in the liquid volume of a tank in whichthis liquid is boiled, and with the absence of liquidfragmentation (as it is done in a deaerator column), itis hardly possible to obtain an air content in water con�siderably less than that stated in the boiler standards.

Therefore, in order to correctly interpret the resultsof photo and video filming, including those obtainedusing high�speed video cameras, which form the data�base for statistical processing, it is necessary to have acomprehensive model regarding the behavior of the airbubbles that are formed by residual air in the course ofboiling of subcooled water.

RESULTS

To solve the above problem, in addition to theexperiments conducted earlier on flat surfaces [1], thetests on subcooled�water pool boiling on a capillary of1 mm diameter and 24 mm length with a wall thicknessof 0.1 mm were carried out under atmospheric condi�tions. A capillary of such a diameter was chosen toexclude considerably shading of objects in the photo�graphs to be done with great enlargement by neighbor�ing bubbles, which is the case at extended heating sur�faces when filming from the side. The regularitiesrevealed in these experiments were compared andqualitatively confirmed by the data of filming the boil�ing process on the flat surfaces of greater dimensions(5 × 20 mm) with electrical heating and on hot spots of3–5 mm diameter with laser heating [1].

Air bubbles originally appear at the heating surface,where the cavities (sites) filled with air exist. Thesebubbles leave the heating surface, form a cloud ofsmall bubbles, and float up to the free liquid surfacewith a velocity of several mm/s practically withoutchanging their diameter. Under the conditions whenthe bulk�water is subcooled relative to the saturationtemperature, the above fact attests that we observe themotion of noncondensing air bubbles. The dimensions

of these bubbles are several tens of µm. At small liquidsubcoolings and high boiling intensity, air bubbles ofup to 100 µm dimension exist. This fact must beaccounted for when analyzing boiling�process photosand movies, and these bubbles must be excluded fromthe data mass to be statistically processed. This is evenmore the case, because vapor bubbles are short�livedmembers, while any air bubble after its incipienceexists several orders of magnitude longer than a vaporbubble and these air bubbles can be seen in photos in agreat amount.

Visual observations show that the amount of airbubbles and their diameters sharply increase when weswitch from the water deaerated under rarefaction tothe water deaerated by boiling under atmospheric con�ditions and especially further to nondeaerated distilledwater (Fig. 1). These qualitative observations are con�firmed by the measurements of the amount of air thatentered into a glass of 15 mm diameter with its bottominstalled above the working capillary at a distance of5 mm. During nondeaerated water boiling at a heatflow rate of 2.9 MW/m2, 306 mm3/min of air entered theglass. For water deaerated by long�term boiling underatmospheric pressure, this figure was 55 mm3/min. Forwater deaerated by boiling under rarefaction, theamount of air captured during the same time at thesame heat flow rate was hardly distinguished. Theabove rough estimates reasonably correlate with thedata on the air content in water at the different degreesof water deaeration presented earlier.

Simultaneously, during nondeaerated water boil�ing, coalescence of small air bubbles into large bubblesof more than 1 mm diameter takes place on the heat�ing surface (Fig. 2). An increase in the heat flow ratedue to natural convection enhancement (during poolboiling), as well as an increase in the velocity of theexternal flow (during boiling in a channel), both con�tribute to the departure of these bubbles and a decreasein a number of large air bubbles that simultaneouslyexist on the heating surface. At heat flow rates morethan 1.5–2 MW/m2, there are no large air bubbles onthe heating surface.

(a) (b) (c)

Fig. 1. Effect of the degree of water deaeration on theintensity of the emission of air bubbles. (a) Nondeaerated distilled water;(b) deaeration by boiling; (c) deaeration by boiling under rarefaction. Capillary 1 × 0.1 × 25 mm; q = 2.3 MW/m2.

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BEHAVIOR OF AIR BUBBLES DURING SUBCOOLED WATER BOILING 409

In the experiments with nondeaerated water, aninteresting phenomenon was recognized. As was statedabove, at heat flow rates less than 1.5 MW/m2 large airbubbles of about 1 mm exist on the heating surface. Inthe space between these air bubbles, no vapor bubbles,which usually provide heat transfer from the heatingsurface through the “evaporation–condensation”process, are observed. We can suppose that, in this sit�uation, long�living air bubbles on the heating surfacetake the role of miniature heat pipes, which is usuallyperformed by vapor bubbles. We should note thatclouds of small air bubbles above the heating surfaceare absent in this case.

At somewhat higher heat flow rates, large air bub�bles coexist with small vapor bubbles on the heatingsurface. When this happens, zones (clouds) of fine airbubbles are located above vapor bubbles, meanwhilesuch clouds are absent above large air bubbles (Fig. 3).Thus, collapsing vapor bubbles leave a certain amountof small air bubbles, which float up in the liquid, aftertheir “death.”

The described picture of the behavior of air bubbleswas never seen in the experiments with deaeratedwater, where vapor bubbles always existed on the heat�ing surface, as well as the clouds of the air bubblesabove them. It is most reasonable to suppose that to aconsiderable extent the mass of the air bubbles isformed from the air that enters into the vapor bubblewith water in the course of its evaporation and isretained after vapor condensation and bubble collapse.

Note, that heat transfer by large air bubbles was notaccompanied by acoustic effects, which are typical ofthe boiling process with formation of vapor bubbles(Fig. 4). Further, we will consider this fact in moredetail.

It is interesting to point out that after switching offthe heat load the air bubbles existed on the heating sur�face remain there unchanged for a very long time (abouta day). Thus, reverse solution of the air liberated fromthe bubbles in the cold water does not occur (Fig. 2).

To analyze vapor bubble evolution, special testswith boiling at small water subcoolings were con�ducted, because the bubble lifetime and dimension arehigher under these conditions. As we can see from Fig. 5,the vapor bubbles rather quickly degrade in the coldliquid to some stable dimensions, which are consider�ably smaller than original ones, and form the vapor�bubble region, similar to that observe earlier at highersubcooling values, but with noticeably larger bubbles.

We repeat that, in subcooled liquid, the long�termbubble existence without collapsing undoubtedlyproves that the bubble observed is an air bubble (in Fig. 5such a bubble is marked with an arrow).

The phenomenon observed in our studies to a con�siderable extent resembles the processes attributed inseveral papers [2–4] to microbubble�emission boiling.We suppose that the microbubble emission observedduring subcooled water boiling is the emission of airbubbles. When this happens, the microbubble emis�sion is more intense at a low degree of water deaerationin the experiment.

With an increase in the water temperature to 85–95°С (correspondingly, water subcooling relative tothe saturation temperature decreases to 15–5°С) thenumber of vapor bubbles detached from the heatingsurface gradually increases. Initially, these bubblesrather quickly condense in the liquid after moving sev�eral millimeters. As the subcooling value approacheszero, the amount of vapor bubbles that reach the freeliquid surface increases and the amount of small airbubbles decreases because a considerable amount ofair comes to the surface with vapor bubbles.

It is known that boiling is accompanied by acousticeffects [5, 6]. Intensive studies of this problem have beenconducted for a long time. For example, the list of ref�erences in [6] comprises 424 items. Unfortunately, thefinal result of this intense activity is rather poor. There isno mutually consistent description of the process. Wecan practically always find experimental and, which iseven more amazing, theoretical substantiation of anyproposed version of the sound source (it may be bubble

(a) (b)

Fig. 2. Stability of air bubbles after switching off the heat supply. (a) At the instant of switching off; (b) after 5 h.

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Fig. 3. Destruction of the air�bubble structure after an increase in the heat flow rate.

–30

–60

–90

–1201086420

dB

1, 2

3

kHz

Fig. 4. Spectral intensity of the acoustic noise during distilled nondearated water boiling on the capillary. (1) Background; (2) q =0.9 MW/m2; (3) q = 2.8 MW/m2.

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BEHAVIOR OF AIR BUBBLES DURING SUBCOOLED WATER BOILING 411

collapse or bubble incipience, phase interface oscilla�tions, etc.), any shape of the pressure pulse or the effectof regime parameters on the signal amplitude–fre�quency characteristics, as well as the absence of such aneffect, etc. Therefore, in our studies, records of theacoustic signals emitted during boiling were used asadditional indicator, which made it possible to identifythe phenomena observed more efficiently. In so doing,we did not attempt to quantitatively analyze these dataand the microphone of the NV�MX500 movie cameracalibrated with the use of a sound generator was appliedto record sound signals.

The tests showed that the sound appears in thepresence of vapor bubbles, and, most likely, it is aresult of the collapse of vapor bubbles. It is possiblethat the collapse of air bubbles would give the sameresult. However, the amount of air participating in theprocess under consideration is significantly less, and,according to our observations, the air bubbles do notcollapse.

According to our experiments, nothing changed inthe acoustic characteristics while switching fromboiled to nonboiled water, i.e., with an increase in theamount of the air bubbles in the liquid volume. Thesound appeared at heat flow rates close to the limitingones for the case of heat removal by natural convection(0.25–0.30 MW/m2), i.e., with the incipience ofintense boiling, and resembles white noise in the rangeof 0–10 kHz.

At high subcoolings (cold water) and one and thesame heat flow rate, the number of vapor bubbles andtheir dimensions become smaller. At small subcoolingsthe vapor bubbles are rather large, they reach free liquidsurface, the sound becomes weaker and then ceasesaltogether. When this happens, water turbidity alsodecreases, because, as was pointed out earlier, small airbubbles appear from collapsing vapor bubbles.

It was stated above that, when boiling nondeaeratedwater, there exists a heat flow rate range (up to about

1.5 MW/m2) within which large air bubbles on the heat�ing surface act as miniature heat pipes that transfer heatfrom the heating surface to the cold liquid. It is impor�tant that in this case there is no acoustic signal (Fig. 4).The appearance of this signal occurs simultaneouslywith the incipience of short�lived vapor bubbles andclouds of small air bubbles above them.

CONCLUSIONS

The air dissolved in water and liberated in thecourse of water boiling, which does not affect integralthermohydraulic characteristics of boiling subcooledwater, distorts the picture of the process observed visu�ally. This fact should not be ignored, because videopictures play significant role when constructing physi�cal models of the processes.

REFERENCES

1. Zeigarnik, Yu.A., Platonov, D.N., Khodakov, K.A., andShekhter, Yu.L., High Temp., 2011, vol. 49, no. 4,p. 566.

2. Kumagai, S., Kawata, K., Katagiri, T., and Shimada, R.,Proceedings of the 11th International Heat Transfer Con�ference (IHTC�1998), Kyongju, Korea, August 23–28,1998, Seoul: The Korean Society of Mechanical Engi�neers, 1998, vol. 2, p. 279.

3. Suzuki, K., ECI International Conference on BoilingHeat Transfer, Spoleto, Italy, May 7–12, 2008.

4. Inada, S. and Yang, W.�J., Proceedings of the 11th Inter�national Heat Transfer Conference (IHTC�1998),Kyongju, Korea, August 23–28, 1998, Seoul: TheKorean Society of Mechanical Engineers, 1998, vol. 2,p. 509.

5. Nesis, E.I., Kipenie zhidkostei (Boiling of Liquids),Moscow: Nauka, 1973.

6. Dorofeev, B.N. and Volkova, V.I., Akustika kipeniya(Boiling Acoustics), Stavropol: Stavropol State Univer�sity, 2007.

1 mm

0 ms 1 ms 2 ms 3 ms

6 ms 9 ms 14 ms 19 ms

Fig. 5. Vapor–air bubble evolution. q = 1.3 MW/m2; Ts – Tl = 10°C; exposure time is 10 µs.