ISTP-16, 2005, PRAGUE

16TH INTERNATIONAL SYMPOSIUM ON TRANSPORT PHENOMENA

BUBBLE GENERATION AND HEAT TRANSFER IN DIRECT-CONTACT EVAPORATION PROCESS WITH A CONTINUOUS LIQUID-LIQUID INTERFACE Katsuhiko KADOGUCHI* * National Institute of Advanced Industrial Science and Technology [email protected], Phone: +81-29-861-8077, FAX: +81-29-861-8208 Keywords: Experimental Study, Direct-Contact Evaporation, Non-Dispersed Liquid-Liquid Interface, Interfacial Curvature, Heat Transfer Characteristics.

Abstract Heat transfer characteristics were experimentally examined in the field where a volatile liquid pool of a per-fluorocarbon PF5050 (boiling point 306 K) was directly in contact with an immiscible water layer. Heat was supplied to the horizontal continuous liquidliquid (LL) interface by downward hot water jet flow. Bubble incipience occurred with a thin PF5050 liquid film atop the growing bubble. The characteristics of bubble departure from the droplets were discussed based on the comparison with Mersmann’s formula. On the heat transfer, the curvature of the liquid-liquid interface controlling the bubble departure size had a predominant effect in both droplet and LL cases. The heat transfer characteristics could be explained based on this result, by using Weber number and buoyancy-interfacial tension ratio. 1 Introduction Low-grade waste heat with temperature close to the ambient has a possibility to cause the warming up of air especially in the urban area, if it is emitted to the environment. It should therefore be recovered and supplied to the appropriate thermal energy demand not only for avoiding this problem but also for energy conservation and for reducing CO2 emission. When the heat source exists in the form of liquid such as the industrial waste water, the sewage, sea or river water, etc., the performance of the heat exchangers required for the purpose

described above often deteriorates due to the contamination of the heat transfer surface. If the direct-contact evaporation could be applied efficiently to these heat exchangers, it should become one of the options to avoid this problem. Many kinds of works have already been carried out on the direct-contact heat transfer, a part of which before 1988 was summarized in [1]. Some of the works after that on the liquidliquid systems with phase change (evaporation) are surveyed as follows. Tadrist, et al. [2] investigated the heat transfer characteristics on n-pentane/water counter-current system and proposed a model taking the coalescence of particles of n-pentane into account. Shimaoka and Mori [3] experimentally investigated the evaporating heat transfer of n-pentane/n-hexane mixture heated by water. Celata, et al. [4] carried out the experiment with flow visualization on the evaporation of liquid CFC114 jet in water flowing concurrently. Song, et al. [5, 6] carried out an analytical work for the prediction of the heat transfer coefficient in a bubble column. Attentions of these works were paid to the evaporation of the volatile liquid droplets (dispersed phase) in another liquid flow (continuous phase). In the practical application of the liquid-liquid direct-contact evaporation, however, there is the possibility that the droplets coalesce into large liquid lumps or a liquid pool, especially when the flow rate and/or the temperature of the continuous liquid phase changes significantly with time. For designing the direct-contact heat exchanger, therefore, it is 1

Katsuhiko Kadoguchi

required to make clear the characteristics of the heat transfer under the condition of the very small liquid-liquid interfacial area concentration, i.e. through the large interface. The author already investigated experimentally the boiling heat transfer phenomena in the fields where a volatile liquid layer was directly in contact with an immiscible non-volatile hot liquid layer [7-9]. In these cases, a continuous liquid-liquid interface with large area appeared between these two layers. In the previous work, the overall heat transfer characteristics were investigated experimentally with the visual observation of the direct-contact boiling pattern near the continuous liquid-liquid interface [8]. Based on the results of this work, the mechanism of the departure of the generated bubbles, both from the continuous liquid-liquid interface and from the liquid droplets floating on this large liquid-liquid interface was examined experimentally, and a physical model was proposed for the evaluation of the condition at the bubble departure [9]. In the present paper, the bubble departure mechanism was further examined taking into account the correlation for the condition of the bubble-droplet separation [5]. In addition, the direct-contact evaporation heat transfer characteristics on the individual vapor bubble generation were experimentally examined with attention focused on the configuration of the liquid-liquid interface.

5 mm in diameter on its side wall facing downward. The water flowed out of this hole as a round jet and impinged on the surface of the liquid PF5050 pool. Because water and PF5050 were immiscible with each other and the liquid density of water was smaller than that of PF5050, the supplied water accumulated on the PF5050 pool. As a result, the hot water pool of 200 mm in depth was formed on the PF5050 pool. The water flowed out of the box through the tube (2) with an exit hole of 10 mm in diameter. The following temperatures were measured by thermocouples shown by “TC” in Fig.1. : temperatures of the cooling water at the inlet and the outlet of the tube (3), hot water temperatures at the inlet of the tube (1) Ti and at the exit of the tube (2) To, liquid and/or vapor temperatures in the water and PF5050 pools. Cooling and heating water flow rates were measured by electromagnetic flow-meters. Pressure in the box was measured by pressure transducer shown by “p” in Fig.1. In all the experiment the behaviors of the liquid and bubbles were recorded on a digital video camera. After conversion of the recorded movie into the electric jpg files of the continuous still frames, the sizes of the bubbles and droplets were measured by using Image-Pro Plus ver.4.5.1J, Media Cybernetics. Cooling water (in)

2 Experiment Figure 1 shows the test section of the experimental setup. It was a closed box of 500 mm in height, 200 mm in width and 100 mm in extent, which had two glass walls of 500 mm x 200 mm for visual observation. The other walls were made of stainless steel. The working liquid (per-fluorocarbon PF5050, 3M) was charged in the box at first. It formed a liquid pool of 90 mm in depth on the bottom wall. Cooling water began to be circulated in a coiled condenser tube [(3) in Fig.1]. Hot water was supplied simultaneously into the box through the closed-end tube (1), 10 mm I.D., inserted horizontally at the point 100 mm higher than the bottom wall. It had a hole of

TC

Cooling water (out)

p

TC

To Falling condensate 500

Hot water flow (in)

Ti

(3)

TC

(2) Hot water pool (1)

TC

Td

TC

PF5050 pool

Liquid-liquid interface

(out)

90

200

2

BUBBLE GENERATION AND HEAT TRANSFER IN DIRECT-CONTACT EVAPORATION PROCESS WITH A CONTINUOUS LIQUID-LIQUID INTERFACE

Fig.1 Experimental setup Table 1 Experimental conditions

3 Results and Discussions 3.1 Boiling Patterns

Heating fluid : Water Evaporating fluid : PF5050 - Boiling point : 29 oC - Liquid density : 1638 kg/m3 - Vapor density : 12.35 kg/m3 - Liquid viscosity : 0.43 mPas - Latent heat of vaporization : 100.1 kJ/kg System pressure : 0.1 MPa Rew (=vcdc/νw)

Ti oC

3000, 6000, 8000, 10000

32, 34, 36 (ΔT=3, 5, 7)

Experimental conditions are summarized in Table 1. The experiment was carried out under the condition of the atmospheric pressure. In the experiment the degree of superheat at the inlet ∆T and Reynolds number Rew of the water jet were controlled as the experimental parameters.

(a) ΔT=3 K, ReW=3000

Figures 2-5 show the examples of the boiling pattern appearing in the region near the continuous liquid-liquid interface (abbreviated to “LL” in the present paper) between the water and PF5050 pools. In all cases the LL interface was depressed at the point of the impingement of the water jet, which could be clearly seen in the upper photos in Figs.2 and 3. The hot water jet rebounded at this depressed point on the LL interface. Because the jet did not impinge on the interface vertically but it tilted, a large-amplitude wave of PF5050 liquid was generated only in the right-hand side region of the depressed LL interface (cf. slant views in Fig.3, for example). Because of heating, PF5050 vapor bubbles were generated mainly in the region around the depressed point on the LL interface [cf. slant views in Fig.2]. They rose up in the water pool and were released into the space above the pool. The vapor was then condensed on the outer surface of the condenser tube [(3) in Fig.1]. The

(b) ΔT=5 K, ReW=3000

(c) ΔT=7 K, ReW=3000

Fig.2 Boiling pattern on the liquid-liquid interface in case of Rew= 3000 3

Katsuhiko Kadoguchi

(upper photo: slant view, lower photo: elevation view)

(a) ΔT=3 K, ReW=6000

(b) ΔT=5 K, ReW=6000

(c) ΔT=7 K, ReW=6000

Fig.3 Boiling pattern on the liquid-liquid interface in case of Rew= 6000 (upper photo: slant view, lower photo: elevation view)

(a) ΔT=3 K, ReW=8000

(b) ΔT=5 K, ReW=8000

(c) ΔT=7 K, ReW=8000

Fig.4 Boiling pattern on the liquid-liquid interface in case of Rew=6000 (upper photo: slant view, lower photo: elevation view) 4

BUBBLE GENERATION AND HEAT TRANSFER IN DIRECT-CONTACT EVAPORATION PROCESS WITH A CONTINUOUS LIQUID-LIQUID INTERFACE

(a) ΔT=3 K, ReW=10000

(b) ΔT=5 K, ReW=10000

(c) ΔT=7 K, ReW=10000

Fig.5 Boiling pattern on the liquid-liquid interface in case of Rew=10000 (upper photo: slant view, lower photo: elevation view)

interface for several seconds before they sunk into, and coalesced with, PF5050 liquid pool. During this period, the smaller vapor bubbles were also generated from these droplets. Figure 6 shows the summary of the observed results of the bubble generation with superimposed explanations. Two sources of vapor generation existed in the present heat exchange field, i.e. the LL interface and the droplets, as shown in Fig.6. Fig.6 Summary of the boiling pattern (close-up taken at ΔT=3 K and ReW=3000) condensate fell down into the water pool in the form of liquid droplets. Some minute droplets evaporated again quickly in the upper portion of the water pool, because the degree of water superheat was still positive even in this portion. The other larger droplets sunk down on the LL interface. These droplets were floating on the LL

3.2 Mechanism of Bubble Departure Figures 7 (a)-(f) show the example of a vapor bubble departure from a droplet floating on the LL interface. The lapse of time from the bubble incipience was also shown in Fig.7. The vapor bubble of about 2 mm in diameter was observed in Fig.7 (a) at the top of the droplet. This bubble grew up mainly in the vertical upward direction by evaporation of the liquid PF5050 droplet [(b)-(d)]. By this growing up, the droplet was deformed significantly longer in the vertical 5

Katsuhiko Kadoguchi

direction (e). Finally, the bubble departed from the droplet (f).

(a) 0.50 sec.

(b) 1.30 sec.

film was broken down and the bubble was released into the water pool. Regarding the departure of a vapor bubble

(c) 1.54 sec.

(a) 0 ms

(d) 1.70 sec.

(e) 1.74 sec.

(b) 134 ms

(c) 334 ms

(f) 1.77 sec.

Fig.7 Bubble growth and departure from a droplet [ΔT=3 K, Rew=3000] (d) 467 ms Figures 8(a)-(h) show the process of a bubble growth and departure from the LL interface. As shown by an arrow in Fig.8 (a), the incipience of the bubble generation did not occur on the LL interface but beneath it. The vapor bubble grew rather horizontally than vertically (a)-(c), then rose up (d)-(f) and finally broke the LL interface (g). The bubble then departed from the LL interface and lifted up in the water pool. The above-mentioned behavior can be summarized by using Fig.9. In case of the bubble departure from the LL interface, the bubble was generated at first in the bulk liquid close to the interface, not on the interface itself. The same phenomenon should occur in case of droplets. The bubble grew up with time by evaporation with water pool as the heat source. In case of droplets, the liquid-liquid interfaces had very large curvatures. As a result, the growth of the bubble in the horizontal direction was restricted, compared with the case of the LL interface. As the bubble grew and rose up, the liquid film between the liquid-liquid interface and the top of the bubble became thinner. Finally this liquid

(e) 601 ms

(g) 835 ms

(f) 801 ms

(h) 868 ms

Fig.8 Bubble growth and departure from the LL interface [ΔT=5 K, Rew=3000]

from a droplet, Mersmann’s equation has already been proposed for the maximum diameter of stable dispersed droplets as follows [5, 10]: d max =

kσ (ρ w − ρV )g (1)

6

BUBBLE GENERATION AND HEAT TRANSFER IN DIRECT-CONTACT EVAPORATION PROCESS WITH A CONTINUOUS LIQUID-LIQUID INTERFACE

Here the value of k was about 9. Following Song, et al. [5], the vapor bubble could separate from the liquid droplet if the diameter of the whole non-separated bubble and droplet was Thin PF5050 liquid film

Small vapor bubble

Film breakdown

equal to 0.055, which was the interfacial tension in the present liquid-liquid system measured a priori by using the liquid pendant method. This value was smaller than the gas-liquid interfacial

Lift off

8

Water pool

∆T [K]

3

5

Rew

PF5050 pool

Bubble departure from a droplet Small vapor bubble

Growing horizontally

Thin PF5050 liquid film

Film breakdown

dh [mm]

PF5050 droplet

Occurrence of bubble departure following Mersmann's Eq.

3000 6000 8000 10000

6

7

4

PF5050 pool Bubble departure from the LL interface

2

2

4

6

8

dLd [mm] Fig.9 Schematic drawing of the bubble growth and departure

larger than this value. However, the existence of the thin liquid film over the bubble, which was explained by using Fig.9, was not taken into consideration in the reference [5]. Namely, the upper surface of the vapor bubble before separation from the droplet was directly in contact with the surrounding continuous liquid. Eq.(1) was obtained simply by taking into account the balance of forces acting on a bubble without the liquid film over it, i.e. the buoyancy force and the interfacial tension at the boundary between the bubble, the droplet and the surrounding continuous liquid [10]. Fig. 10 shows the comparison of dmax calculated by using Eq.(1) with the data on dLd. In the present experiment, the characteristic diameter of the bubble plus droplet was dLd, shown in Fig.10, at the moment just before departure of bubbles from the droplets. The ordinate in Fig 10 is the horizontal size of a bubble just before departure dh, which was slightly smaller than the characteristic length of the departed bubble. The calculated value of dmax was 7.2 mm in the present case assuming σ

Fig.10 Comparison of calculated and experimental bubble departure diameters

(i.e. surface) tension of water and larger than that of PF5050. Assuming no liquid film over the non-departed bubble on the droplet, the real value of the interfacial tension might be larger than this value at the boundary between PF5050 vapor, PF5050 liquid and the liquid water. As shown in Fig.10, however, almost all the present data showed that vapor bubbles could depart from the liquid droplet under the condition of smaller bubble-droplet diameters than dmax. In addition, it was often observed in the present experiment that the bubbles did not depart from the droplets even if the values of dLd were within the range shown in Fig.10. In such a case, the vapor bubble and droplet floated on the LL interface and rose up together without separation. Namely, the condition at the occurrence of the bubble departure could not be determined simply by the exceeding of buoyancy to the interfacial tension at the bubble-droplet boundary. According to the results of the present observation, the mechanism of the bubble departure should be 7

Katsuhiko Kadoguchi

discussed by taking the breakdown of the liquid film atop the bubble into account. The vapor bubble cannot depart from the droplet even at the moment of the liquid film

Figure 12 shows the relationship between dh and the time needed for a vapor bubble to grow to the size of departure tD. Hollow symbols show the data in case of bubbles from the LL interface, 12 ∆T [K] 3

0.05

dh [mm]

3

Vd [cm ]

10

∆T [K]

3

5

7

5

7

LL D

8 6

Rew 3000 6000 8000 10000

0.01

4 2 0.0

0.05

0.01

0.5

1.0

3

tD

Vdmin [cm ] Fig. 11 Volume of the departed bubbles Vd

1.5

2.0

[sec.]

Fig.12 Growing time of bubbles tD 12 ∆T [K]

breakdown under the following condition: (2)

It should be noticed in Eq.(2) that σP is the surface tension of pure PF5050, since a bubble is contained as a whole inside a droplet just before the liquid film breakdown. From Eq.(2), the minimum volume of a vapor bubble required for the departure after the liquid film breakdown Vdmin was obtained as follows.

Vd min

σ Pπd h = (3) (ρ w − ρV )g

Figure 11 shows the comparison between Vdmin calculated by Eq.(3) and the experimentally obtained volumes of the departed vapor bubbles Vd. The values of Vd were obtained by assuming the spheroidal shape of the departed bubbles. All the present data showed that the vapor bubbles grew enough for their departure in the droplets. 3.3 Horizontal Bubble Size before Departure

10

dh [mm]

(ρ w − ρV )Vd g < σ Pπd h

3

5

7

LL D

8 6 4 2 2000

4000

6000

8000 10000 12000

Rew Fig. 13 Effect of the water jet flow rate

and the solid ones are in case of bubbles from the droplets. There was no remarkable difference of tD between both cases. However, the values of the size dh were larger in case of LL than in the case of droplets under almost all experimental conditions. 8

BUBBLE GENERATION AND HEAT TRANSFER IN DIRECT-CONTACT EVAPORATION PROCESS WITH A CONTINUOUS LIQUID-LIQUID INTERFACE

equal to dh [9]. Therefore, the velocity was defined as (gdh)0.5 in Eq.(4). Conclusively, Fig.15 became the non-dimensional expression of Fig.13. The decrease of WeLL meant that the waves with shorter wavelength appeared on the

Rew

1

0.1

0.01

3.4 Evaporating Characteristics (LL)

WeLL

ρ gd ≡ L h σ

0

1

2

3

4

5

6

7

8

∆Td , ∆T

9

10

[K]

Fig.14 Evaporation rate of individual bubbles 40 ∆T [K] 3

5

7

Rew 3000 6000 8000 10000

WeLL

Figure 14 shows the evaporation rate of the individual vapor bubbles QB obtained by using the measured values of the volume of the departed bubbles. QB increased with the degree of water superheat ∆T in case of the bubble generation from the LL interface. This meant that larger vapor bubbles generated in case of large ∆T. On the other hand, QB did not change with the increase of ∆Ti, the superheat of the water pool near the LL interface. According to the above result, the decision of the bubble size was the most important point for evaluating the heat transfer characteristics for the individual bubbles in case of the bubble generation from the LL interface. In addition, this bubble size was affected by the wavy structure of the LL interface. Fig.15 shows the change of this structure with Rew by using the Weber number, which was defined as follows.

3000 6000 8000 10000

LL D

QB [W]

Figure 13 shows the effect of the flow rate of water jet impinging on the LL interface on dh. The data in case of the LL interface had the tendency to decrease as Rew, i.e. the water flow rate, increased. On the other hand, dh was not affected by Rew in case of the droplet. Namely, the horizontal size of bubbles before departure from the LL interface became close to that in case of the departure from the droplet with the increase of the water jet flow rate. In case of the large water flow rate, the impingement of the water jet made the LL interface disturbed significantly and the interface became wavy. As a result, the curvature of the interface became close to that of the droplets. The horizontal growth of the bubble beneath the LL interface was restricted by this effect. Then the bubble departure size became smaller.

20

0 2000

4000

6000

8000

10000 12000

Rew Fig.15 Weber number on the LL interface

2

(4)

As a characteristic velocity, the wave velocity was selected in Eq.(4). This should be defined by using the wave height. In the present experiment, however, this height was almost

LL interface. By using this non-dimensional parameter, the data shown in Fig.14 were correlated. The result is shown in Fig.16. The ordinate is the 9

Katsuhiko Kadoguchi

Nusselt number defined by the following equation. Nu LL ≡

∆T [K] 3

5

QB k L ∆Td h

(5)

should be different from that in the previous section. Fig. 17 shows the data on the heat flux representing the vapor bubble generation from the droplets. Here qd was obtained by dividing QB by the surface area of the combined bubble and droplet. The abscissa r-1 in Fig.17 represents

7

10000

Rew

2 3 4 5

2

qd [W/m ]

3000 6000 8000 10000

NuLL

1000

Droplet

∆Td [K]

1000

100

5

10

15

20 25 30

100 200

300

400

r

WeLL

-1

500 600 700 -1

[m ]

Fig.17 Heat flux on the droplet surface Fig.16 Evaporation heat transfer on individual bubbles in case of the LL interface

Although the value of NuLL varied widely, the increasing tendency of NuLL with WeLL could be recognized. Namely, In case of the large WeLL the waviness of the LL interface was not significant. As a result, the vapor bubble could grow easily in the horizontal direction beneath the LL interface, leading to the generation of large bubbles. The heat transfer coefficient increased due to this effect. The reason for the scattering of the data might be the existence of the non-uniform temperature distribution on the LL interface, which could not be measured in the present experiment. 3.5 Evaporating Characteristics (Droplet) In case of the vapor bubble generation from the droplets, the bubble size did not change significantly compared with the case of the LL interface, as shown in Figs. 13 and 14. Therefore, the way of approach for the data

the curvature of the surface of the combined bubble and droplet, which was evaluated as 2/dLd. As the size of the combined bubble and droplet decreased, the heat flux increased. This result was consistent with the general relationship between the heat transfer area and the volume of the heat sink, that is, the heat transfer augmentation due to the increase of the interfacial area concentration. In order to correlate those data shown in Fig.17, the following non-dimensional parameter was introduced. Fd ≡

(ρ L − ρV )gd h σ / dh

(6)

This parameter meant the ratio of pressures concerning the thin liquid film over the bubble. The numerator of the right-hand side of Eq.(6) represents the upward pressure caused by the buoyancy force acting on the bubble. On the other hand, the denominator represents the 10

BUBBLE GENERATION AND HEAT TRANSFER IN DIRECT-CONTACT EVAPORATION PROCESS WITH A CONTINUOUS LIQUID-LIQUID INTERFACE

critical value for the liquid film breakdown, which was investigated in detail by using the proposed physical model in the previous work [9]. The smaller value of Fd meant that the bubble could not depart from the droplet easily, i.e. the smaller droplet.

addition, the difference of the values of ρL and ρL-ρV was negligible. However, the heat transfer deteriorated with the increase of Fd in case of the droplet, and it was augmented with the increase of WeLL in case of the LL interface. Regarding this result, the relationship 1

Droplet

Nud

2

4

6

8

10

400

r

Fig.18 Evaporation heat transfer on individual bubbles in case of the droplet

Figure 18 shows the experimental result of the relationship between the non-dimensional heat transfer coefficient Nud and Fd. Here, qd d h k L ∆Td

0.1

0.01 200

Fd

Nu d ≡

LL D

3

Vd , VLL [cm ]

2 3 4 5

100

10

Rew 3000 6000 8000 10000

∆Td [K]

.

(7)

Nud decreased with the increase of Fd. Namely, the departed bubble size increased with Fd, which meant the large amount of vapor generation, i.e. the increase of the heat transfer rate. However, this effect was exceeded by the effect of the increase of the surface area of the droplet, leading to the heat transfer deterioration. 3.6 Effect of Curvature of the Interface It was clearly noticed that the definitions of WeLL in Eq.(4) and Fd in Eq.(6) were similar, with only one difference of the density part. In

-1

600

800

-1

[m ]

Fig.19 Effect of curvature of the liquid-liquid interface on the size of departed bubbles

between the curvature of the liquid-liquid interface and the volume of the departed vapor bubble was examined experimentally. Fig.19 shows the result. Here r-1 was evaluated as 2/dh in case of the LL interface (hollow symbols). In case of the bubble departure from the droplets (solid symbols), the volume of the bubble Vd increased with the increase of the droplet diameter, i.e. with the decrease of r-1. However, this increasing tendency almost disappeared in the range of r-1 less than 400. Namely, there was the limitation on the growth of the vapor bubble in the droplet due to the existence of the curved liquid-liquid interface, which became the resistance for the bubble growth. Since the LL interface could be deformed more easily and freely in case of the small r-1 compared with the surface of the droplet, however, this limitation did not exist in the LL interface case within the present experimental range. Based on the above discussion, the conflicting tendencies of the heat transfer 11

Katsuhiko Kadoguchi

performance between the LL interface and the droplet cases with respect to WeLL or Fd could be explained as follows. When the curvature of the liquid-liquid interface was large, like smaller droplets, the heat transfer performance was excellent due to the large interfacial area concentration. As the curvature decreased, the heat transfer deteriorated under the condition that the liquid-liquid interface could not deform easily, i.e. in case of the droplet. Under the condition of such a large droplet, the merit of the large interfacial area concentration disappeared, and the liquid-liquid interface worked as the resistance for the bubble growth. In the case that the liquid-liquid interface had the large area with almost no curvature like the present LL interface, however, there were the advantages that the liquid-liquid interface could deform easily and that there was no resistance for bubble growth in the direction parallel to the interface. As a result, the vapor bubble could grow to the large size in both directions parallel and normal to the liquid-liquid interface. Although the increase of the heat transfer area for each vapor bubble was accompanied by the bubble growth, the value of the heat flux increased with the bubble size conclusively. The heat transfer performance was improved in this case due to the above effect. 4 Conclusion In the present experimental work, the heat transfer characteristics were examined on the direct-contact evaporation in the liquid-liquid system with a continuous interface of large area and the droplets floating on it. Attention was paid on the generation and departure of the individual vapor bubbles. The results are summarized as follows. (1) Departure of the vapor bubbles from the liquid-liquid interface was triggered by the breakdown of the thin film of the working volatile liquid covering the upper surface of the bubble. (2) The existence of the liquid film over the growing bubble must be taken into account to evaluate the characteristics of bubble

departure, especially the departing bubble size. (3) Heat transfer deteriorated as the diameter of the droplet increased due to the decrease of the interfacial area concentration. In such a case, the liquid-liquid interface worked as the resistance for the bubble growth. (4) In case of a continuous liquid-liquid interface of large area, the increase of the departing bubble size caused the heat transfer augmentation. In this case there were the advantages that the liquid-liquid interface could deform easily and that there was no resistance for bubble growth, compared with the case of the droplet. Nomenclature dh

horizontal diameter of a bubble just before its departure, m dLd diameter of a combined bubble and droplet, m dmax maximum diameter of stable bubble and droplet defined by Eq.(1), m ds inner diameter of the tube (1) in Fig.1, m Fd ratio of buoyancy to the interfacial tension defined by Eq.(6) g acceleration due to gravity, m/s2 kL thermal conductivity of liquid PF5050, W/mK Nu Nusselt number QB evaporation heat transfer rate for each bubble, W qd heat flux on the surface of a combined bubble and droplet, W/m2 Rew Reynolds number of water jet, =vsds/νw r radius of curvature of the liquid-liquid interface, m T temperature, K tD time from the incipience to the departure of a bubble, second V volume, cm3 vs water velocity in the tube (1) in Fig.1, m/s WeLL Weber number defined by Eq.(4) Greek letters ∆T degree of superheat of the supplied hot water, K 12

BUBBLE GENERATION AND HEAT TRANSFER IN DIRECT-CONTACT EVAPORATION PROCESS WITH A CONTINUOUS LIQUID-LIQUID INTERFACE

∆Td degree of superheat of the water pool near the drifting droplets, K νw dynamic viscosity of water, m2/s σP surface tension of PF5050, N/m Subscript d bubble departing from the PF5050 droplet dmin minimum volume of the departed bubble defined by Eq.(3) i inlet L liquid LL bubble departing from the continuous liquidliquid interface V vapor w water References [1] Jacobs, H.R., Direct-Contact Heat Transfer for Process Technologies, Trans. ASME J. Heat Transfer, vol. 110, pp. 1259-1270, 1988. [2] Tadrist, L., et al. Heat Transfer with Vaporization of a Liquid by Direct Contact in Another Immiscible Liquid: Experimental and Numerical Study, Trans. ASME, Ser. (C), vol. 113, pp. 705-713, 1991. [3] Shimaoka, H. and Mori, Y.H., Evaporation of Single Drops of n-Pentane/n-Hexane Mixtures in Water,

ρ σ

density, kg/m3 liquid-liquid interfacial tension, N/m

Trans. ASME, J. Heat Transfer, Ser. (C), vol. 114, pp. 965-971, 1992. [4] Celata, G.P., et al., Direct Contact Boiling of Immiscible Liquids, Proc. 10th Int. Heat Transfer Conf., vol. 5, pp. 37-43, 1994. [5] Song, M., Steiff, A. and Weinspach, P.-M., Parametric Analysis of Direct Contact Evaporation Process in a Bubble Column, Int. J. Heat Mass Transfer, vol. 41, pp. 1749-1758, 1998. [6] Song, M., Steiff, A. and Weinspach, P.-M., DirectContact Heat Transfer with Change of Phase: a Population Balance Model, Chem. Eng. Sci., vol. 54, pp. 3861-3871, 1999. [7] Kadoguchi, K., Direct-Contact Boiling Phenomena in a Field with the Continuous Liquid-Liquid Interface, Progress in Transport Phenomena, Elsevier, pp.285289, 2002. [8] Kadoguchi, K., Heat Recovery from a Waste Liquid by Direct-Contact Boiling Heat Exchanger with Liquid Jet Heating, Proc. 1st Int. Exergy, Energy and Environment Symp., pp.695-700, 2003. [9] Kadoguchi, K., Behavior of Bubble Departure in the Direct-Contact Boiling Field with a Continuous Liquid-Liquid Interface, Proc. 3rd International Symposium on Two-Phase Flow Modeling and Experimentation (CD-ROM), Pisa, jp16, 2004. [10] Blass, E., Formation and Coalescence of Bubbles and Droplets, Int. Chem. Eng., vol.30, pp. 206-221, 1990.

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