Saturation temperature of each component and equilibrium temperature of immiscible mixtures at 0.1 MPa
To satisfy the requirements for the cooling of small and large semiconductors operated at high heat flux density, an innovative cooling method using boiling heat transfer to immiscible liquid mixtures is proposed. Immiscible liquid mixtures discussed here are composed of more-volatile liquid with higher density and less-volatile liquid with lower density, and appropriate volumetric ratios become a key to realize high-performance cooling. The chapter reviews the experimental results obtained by the present authors, where critical heat flux accompanied by the catastrophic surface temperature excursion is increased up to 300 W/cm2 for FC72/water by using a flat heating surface of 40 mm in diameter facing upwards under the pressure 0.1 MPa.
- cooling of electronics
- immiscible mixture
- insoluble mixture
- pool boiling
- flow boiling
1.1. Possibility of non-azeotropic mixtures
For the systems of air conditioning and refrigeration, non-azeotropic miscible mixtures are often used as the working fluids alternative to the discontinued ones. However, these fluids have a well-known unavoidable disadvantage of heat transfer deterioration resulting from the increased interfacial temperature due to the existence of mass diffusion resistance. At the same time, for the non-azeotropic miscible mixtures, there is an unknown effect of Marangoni force exerted mainly by the concentration difference along the liquid-vapor interface as a result of the preferential evaporation of more-volatile component. In aqueous solutions of alcohol with a large carbon number, the surface tension is increased with increasing temperature depending on the range of concentration and the level of temperature. In such a condition, Marangoni force due to the concentration gradient is enhanced by also the temperature gradient along the interface, especially near the three-phase interline extended at the base of bubbles. The enhancement of critical heat flux (CHF) was shown by Vochten-Petre  and Van Stralen  based on the experiments using a wire heater. Abe  verified the drastic increase of maximum heat transportation for heat pipes using "self-rewetting mixtures". Sakai et al.  confirmed the small enhancement of heat transfer in the ranges of very low alcohol concentration in water, while no appreciable increase in CHF for a flat heating surface facing upwards. As a consequence, non-azeotropic mixtures have no advantage from the view point of the improvement of heat transfer.
1.2. Expected performance of immiscible mixtures
Innovative cooling systems which meet the requirement for the increased heat generation density from electronic devices are urgently required. To enhance the values of CHF for the cooling of a large area at high heat flux larger than 200 W/cm2 as shown in Figure 1, the present authors confirmed the validity of the devised structure which reduces the effective heating length by the liquid supply directly to the downstream of the heating duct from the transverse direction. An example of the structure is illustrated in Figure 2 [5,6]. However, such structure is rather complicated. On the other hand, to ensure the high reliability for a long-term operation, microstructures on the "enhanced surface" cannot be accepted depending on their application. The present authors noticed the superior heat transfer characteristics in nucleate boiling of immiscible mixtures even on a smooth surface (Kobayashi et al. , Ohnishi et al. , Kita et al. ), which are summarized as follows.
The value of critical heat flux is increased by the self-sustained subcooling of less-volatile liquid as a result of the excessive compression by the high partial vapor pressure of more-volatile component.
The operation at a pressure higher than the atmospheric is possible keeping low liquid temperature to prevent the mixing of incondensable air which seriously deteriorates the heat transfer in the condensation process, i.e., a final heat dissipation process, due to the existence of mass diffusion resistance.
The surface temperature is decreased during the free convection or nucleate boiling of less-volatile liquid, which is caused by the substantial heat transfer enhancement by the existence of vapor bubbles generated form the more-volatile component.
The excessive overshooting of the surface temperature at the initiation of boiling can be reduced by the selection of more-volatile component and the optimization of its distribution on the heating surface, which is required, e.g., for the cooling of automobile power controllers with a large fluctuation of thermal load.
1.3. Existing researches on boiling of immiscible mixtures
A large number of reports on nucleate boiling of oil mixtures exist. Filipczak et al.  used emulsions of oil and water, where the distribution of two liquids and vapor was investigated at different levels of heat flux. The heat transfer coefficients for high oil concentration were quite smaller than those for pure water, because the free convection of oil dominates the heat transfer to water-oil mixture. At the initial stage of nucleate boiling, foaming was observed before the formation of emulsion. Roesle and Kulacki  studied nucleate boiling of FC72/water and pentane/water on a horizontal wire. The discontinuous phase of more-volatile components FC72 and pentane were dispersed in a continuous phase of water, where the concentrations of more-volatile component were varied as 0.2–1.0% and 0.5–2.0%, respectively. Nucleate boiling of dispersed component or of dispersed and continuous components was observed depending on the level of heat flux. The heat transfer was enhanced by nucleate boiling of dispersed liquid if its volume fraction was larger than 1%. Bulanov and Gasanov  studied the heat transfer to four emulsions, n-pentane/glycerin, diethyl ether/water, R113/water and water/oil, where more-volatile liquids were dispersed in the continuous phase of less-volatile liquids. The reduction of surface superheat at the boiling initiation was observed compared to that for pure less-volatile liquids.
On the other hand, the number of investigations on immiscible mixtures which form stratified layers of component liquids before the heating is quite limited. There are old studies by Bonilla and Eisenbuerg , Bragg and Westwater , Sump and Westwater . Bragg and Westwater  classified heat transfer modes for individual layers of liquids. The interpretation of data, however, was not described in detail. Gorenflo et al.  studied boiling of water/1-butanol on a horizontal tube, where the liquid mixture becomes soluble or partially soluble depending on its concentration, and levels of temperature and pressure. From the experiments performed under various combinations of concentration and pressure, they reported that the nucleate boiling heat transfer is not largely depending on the solubility.
2. Immiscible mixtures
2.1. Phase equilibrium
Immiscible mixtures employed here consist of insoluble component liquids and their phenomena during nucleate boiling have a unique feature characterized by self-sustaining subcooling of liquids. An example of phase equilibrium diagrams for FC72/water at the total pressure of 0.1 MPa is shown in Figure 3. The concentration where the two curves merge, which is corresponding to the azeotropic point frequently observed in miscible mixtures, is the concentration of vapor phase independent of the liquid composition for immiscible mixtures. The concentrations
2.2. Conditions of component liquids
The conditions of liquid phase are represented in Figure 4, where two saturated vapor pressure curves with a red line for a more-volatile component and a blue line for a less-volatile one are drawn. Since immiscible mixtures have the total pressure
|More-volatile component (||Less-volatile component (||Mixtures|
|FC72 (55.9°C)||Water (100°C)||51.6°C|
|Novec7200 (78.4°C)||Water (100°C)||66.4°C|
|More-volatile component ||Less-volatile component |
|FC72/Water||4.3 K||48.4 K|
|Novec7200/Water||12.0 K||33.6 K|
3. Pool Boiling
3.1. Experimental apparatus for pool boiling
The outline of the apparatus is shown in Figure 5. A flat heating surface of 40 mm in diameter made of copper is located horizontally facing upwards. The upper surface of cylindrical copper heating block is operated as the heating surface surrounded by a thin fin cut out in one unit body to prevent the preferential nucleation at the periphery. Nineteen cartridge heaters are inserted in the heating block and the maximum amount of heat generation is 5700 W. Eight thermocouples are inserted in the center and side of copper heating block at four different depths of 1, 7, 13 and 19 mm below the heating surface to estimate the heat flux and heating surface temperature. Fluid temperatures are measured by three thermocouples at 2, 80 and 160 mm above the heating surface in the boiling vessel, where the liquid level is located between the second and third thermocouples.
The experiments are performed at 0.1 MPa changing volume ratio of the components. Immiscible mixtures of FC72/water and Novec7200/water are used as test fluids, where FC72 and Novec7200 are more-volatile components with higher density and water is less-volatile one with lower density. The conditions for the volume ratio of component liquids are represented by Figure 6 , where
|1. More-volatile c. / 2. Less-volatile c.||Height of liquid|
|FC72/Water||[0/100], [5/95], [10/90], [50/50]|
|Novec7200/Water||[0/100], [5/95], [10/90]|
3.2. Experimental results for pool boiling
Experimental results are shown in Figures 7 and 8 for FC72/water and Novec7200/water, respectively. Figure 7 represents the relation between heat flux q and heating surface temperature
3.3. Consideration on the mechanisms of intermediate heat flux burnout
The phenomena of limited jump of heating surface temperature referred here to as the intermediate heat flux burnout occurs if the thickness of the more-volatile liquid with higher density attached to the horizontal heating surface is small as observed for FC72/water [10 mm/90 mm] at
After the jump of the surface temperature, the heat transfer mode is changed from the nucleate boiling of more-volatile liquid to the natural convection or nucleate boiling of less-volatile liquid at higher heat fluxes. It is very important that the generation of bubbles or vapor slugs of more-volatile component continues also in this region enhancing the heat transfer to the less-volatile liquid. The intermediate heat flux burnout is observed when Taylor instability occurs by the growth of a coalesced bubble with more-volatile component after its lateral coalescence below the bulk of less-volatile liquid with lower density.
The value of minimum bubble diameter
|Most dangerous wavelength ||28.9||28.4|
|Minimum penetration diameter ||4.2||3.9|
|Bubble departure diameter ||0.36||0.43|
Bubbles of more-volatile component grow by the evaporation or lateral coalescence in the vicinity of liquid-liquid interface and become sizes beyond
There is another type of intermediate heat flux burnout confirmed by the present authors, where the heating surface temperature gradually deviates from that for nucleate boiling of pure more-volatile liquid . In such a case, bubbles can coalesce easily below the liquid-liquid interface because of thicker thickness of its layer and exceed the value of most dangerous wave length at lower heat flux. As a consequence, the less-volatile liquid starts to contact partially the heating surface, and the contribution of the heat transfer to the less-volatile liquid gradually increased as the increase of heat flux keeping the steady-state conditions at each heat flux level. Similar phenomenon occurs in the cases of smaller wavelength of Taylor instability or of partially soluble combination of liquids depending on the selection of component liquids. It is clear that the physical and/or chemical mixing of component liquids is a key factor to determine the heat transfer characteristics at low heat flux.
4. Flow boiling
4.1. Experimental apparatus for flow boiling
Figures 11 and 12 show the outline of test section and test loop . Test loop is composed of test section, condenser, liquid-vapor separation tank, circulating pump, pre-heater. Immiscible liquids are stratified in liquid-vapor separation tank, and both flow rates are controlled by valves. The test section is composed of a heated section of stainless tube spirally coiled by sheath heaters on the outer surface and a transparent unheated section of Pyrex glass for the observation of liquid-liquid and liquid-vapor interfaces. The inner diameter of both tubes is 7 mm and heated length is 310 mm. Six thermocouples are inserted in the top and bottom tube walls at the upstream, midstream and downstream locations. The experiments are conducted for the combination of FC72 and water, i.e. more-volatile component with higher density and less-volatile one with lower density whose saturation temperatures as pure components are 55.7 and 100°C, respectively, at 0.1 MPa.
4.2. Experimental results for flow boiling
Various flow patterns such as stratified flow, FC72 slug flow, emulsion-like flow, "wavy stratified + FC72 droplet flow", "FC72 churn + FC72 droplet flow" and "FC72 slug + FC72 droplet flow" are observed depending on the combinations of flow rates for both components under unheated conditions, and they are summarized as a liquid-liquid flow pattern map in Figures 13 and 14 .It is known by the preliminary experiments that the heat transfer characteristics at low heat fluxes are strongly influenced by the liquid-liquid behaviors under unheated conditions.
To evaluate the local heat transfer coefficients, the distribution of mixture temperature along the tube axis is needed. Outlet mixture temperature can be reproduced by the heat balance equations which introduce a parameter
Figures 16 and 17 show heat transfer coefficient and wall temperature versus heat flux at midstream averaged by the top and the bottom values. Heat transfer coefficient is higher and wall temperature is lower than pure water for immiscible liquid because the convection of water is enhanced by the generation of FC72 bubbles.
To clarify the boiling heat transfer characteristics of immiscible mixtures, experiments of pool boiling and flow boiling in a tube were conducted, and the following results were obtained.
By optimizing the volume ratio of immiscible mixtures in pool boiling using a flat heating surface facing upwards, the drastic increase of CHF and/or the decrease of heating surface temperature from those of pure less-volatile component become possible.
The above characteristics resulted from the self-sustaining high subcooling of less-volatile liquid and the substantial heat transfer enhancement caused by the preferential evaporation of more-volatile liquid.
In flow boiling, outlet fluid temperature is well reproduced by heat balance equations, which introduced a parameter representing the ratio of heat supplied to more-volatile component to the total.
In flow boiling, at low and moderate heat fluxes, the convection of less-volatile liquid is enhanced by boiling of more-volatile liquid, and the enhancement is largest corresponding to the flow pattern of emulsion-like flow at low flow rate of more-volatile component classified under the unheated conditions.
For the cooling systems in an enclosure, the distribution of liquid layers for both immiscible components becomes a key to determine the heat transfer characteristics due to nucleate boiling. For a vertical heating surface or a heating surface operated under the reduced gravity conditions, some methods to transfer the heat to the more-volatile liquid with larger density are needed to obtain the superior cooling characteristics.
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