DOI 10.1007/s10891-014-1146-6
Journal of Engineering Physics and Thermophysics, Vol. 87, No. 6, November, 2014
BOILING OF VARIOUS LIQUIDS ON MICROSTRUCTURIZED SURFACES I. A. Popov and A. V. Shchelchkov
UDC 536.24
This paper presents the results of experimental studies of the heat transfer of microstructurized surfaces of various configurations and sizes obtained by the method of deforming cutting. It has been shown that the intensity of heat transfer on such surfaces with three-dimensional columnar and channel structures increases by 20 times, and on microfinned surfaces by 2.5 times, compared to the smooth boiling surface. The critical heat flow density increases 4.1–6 times thereby. The obtained results on the heat transfer on the above surfaces and the critical flow densities on them can be used for calculating the heat transfer coefficients and the heat loads in boiling of various saturated liquids on such surfaces with sizes of fin elements from 50 to 420 μm at a pressure of 0.1 MPa under free convection conditions. Keywords: microstructurized surface, boiling crisis, heat transfer enhancement. Introduction. The reliability and safe operation of aviation radio-electronic equipment and measuring and computing complexes call for effective cooling systems of microelectronic devices. Today the trend in microelectronics is towards enhancement of the specific heat flows in chip cooling systems. The first step in choosing the type of the cooling system is determining the level of its heat transfer necessary for removal of the given flow of heat released by the electronic element. Another problem is meeting the requirements on weight-size characteristics of the cooling system. Such a system must fit into the electronic equipment and not lead to a sharp increase in its weight. It is always necessary to maintain the optimum between the weight-size parameters of the cooling system and its design simplicity. There exist various cooling systems providing heat removal from heated elements of electronic devices in the course of their operation: passive (natural convection) and active (providing forced convection) radiator systems, flow water cooling systems (operating on the basis of an intermediate coolant), and open evaporation systems (based on the phase transition of the coolant). It is most expedient to use natural-convection cooling systems since they do not require additional power for circulating the coolant, have no moving parts, and are reliable and noiseless. Cooling systems of electronic equipment for aircrafts, satellites, space ships, and rockets should be developed with account for the complex forms of attaching points of such equipment and the narrowness of the space of its location. Cooling systems of the above equipment must be of small size and provide reliable heat removal from its elements for a long time. The choice of the method for cooling the electronic device and the provision of its reliable operation are determined by the maximum permissible temperature of this device and the heat transfer coefficient of the cooling system. At the present time, the following methods of cooling electronic devices are widely used: 1) air free-convection cooling and radiative cooling; 2) forced-air cooling; 3) immersion or free-convection cooling with immersion of the object to be cooled into dielectric liquids including cryogenic liquids or freons; 4) cooling in boiling in a dielectric liquid, including jet cooling; 5) forced-water cooling; 6) cooling due to open evaporation; 7) cooling in water chillers; and 8) cooling with the use of Peltier cells. To realize methods 1–4 and 6, it is necessary to develop effective, reliable, and compact high heat transfer cooling systems providing large critical heat flows in the process of boiling of various liquids. At present, air cooling systems (methods 1 and 2) contain heat pipes that permit removing considerable heat flows from small surfaces and transporting them into zones where there are no limitations for the development of a heat transfer surface when heat is released into the atmosphere. Liquid and evaporative cooling systems (methods 3, 4, and 6), as well as cooling systems with heat pipes, require also a significant increase in their heat transfer coefficients for removing large heat flows from relatively small chip areas. This problem can be solved by using boiling of liquids in cooling systems. In spite of the high heat transfer coefficients of liquids, methods for enhancing the heat transfer of the boiling surface are needed because of the intense phase transformation in boiling. The boiling heat transfer from the wall to the liquid can be enhanced by: A. N. Tupolev Kazan′ National Research Technical University — KAI, 10 K. Marx Str., Kazan′, 420111, Tatarstan, Russia; email:
[email protected]. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 87, No. 6, pp. 1362–1374, November– December, 2014. Original article submitted April 8, 2014. 1420
0062-0125/14/8706-1420 ©2014 Springer Science+Business Media New York
1) choosing a proper working liquid; 2) using boiling during motion of the coolant; 3) forming a microstructure on the heat transfer surface in order to increase the rate of formation and disengagement of bubbles. Microstructurized surfaces are heat exchange surfaces with small-scale deformations comparable in geometric parameters to roughness obtained by refining these surfaces and/or applying coatings on them. In this case, the roughness is small for changing the intensity of single-phase heat transfer. Such surfaces are used primarily for boiling processes. The development of structurized surfaces for intensifying the boiling process is based on the creation of a large number of vaporization centers or traps for vapor bubbles on the boiling surface, which expedites the initiation of boiling or brings the liquid to a boil at lower temperature heads. Significant heat transfer enhancement was reported in the works of A. E. Bergles [1], J. R. Thome [2], R. L. Webb et al. [3], S. Yilmaz et al. [4], and many other foreign authors. Among the domestic works on the boil of structurized surfaces are the investigations carried out by M. A. Styrikovich [5], Yu. A. Kuzma-Kichta [6–8], S. A. Kovalev [6, 9], M. D. Diev [10], L. L. Vasil′ev [11], V. M. Polyaev [12], I. Z. Kopp [13], L. I. Roizen [14], and A. V. Borishanskaya [15]. Modern heat transfer surfaces for boiling used in developing cooling systems must meet the following requirements: a) nucleate boiling should be initiated at a small temperature difference between the hot wall and the liquid, i.e., the boundaries between the free convection and the nucleate boiling should be narrowed; b) the heat transfer of the heat transfer surface should be high enough at a given temperature difference between the wall and the liquid; c) the critical heat flow corresponding to the beginning of the boiling crisis should be large. Present-day technologies permit obtaining boiling surfaces with disordered and ordered structures. Disordered structures can be obtained on the boiling surface by sputtering powder on it, sintering powders or fibers, applying composite coatings (including glue-based ones) to the surface, abrasive treatment and chemical etching of the surface, and applying to it electrolytic coatings of various types. Ordered structures are created on the boiling surface by cutting or under the action of pressure, as well as by a combined method including the above two kinds of treatment. Composite structures with superimposed elements are also used. To enhance boiling processes, one can use, for microstructurized surfaces, surfaces obtained by the method of deforming cutting representing a combination of undercutting and folding of the surface layers of the heat transfer surface [16, 17]. Experimental Facility and Experimental Conditions. Experimental studies were made on the facility schematically represented in Fig. 1a. The facility represents a heat-insulated vessel in the form of a boiling chamber 1 of size 150 × 250 × 200 mm filled with the working liquid. The boiling chamber has double walls, between which an asbestos heat-insulating packing is located. Inside the chamber 1, a plate 2 intended for investigating the boiling intensification is situated. The experimental plates were heated by passing electric current directly through them. Voltage is fed from terminals 3 through copper flat current leads 4 of thickness 3 mm and width 20 mm to the specimen 2. The leads are fastened by a threaded connection 5 to the electroinsulating fabric-based laminate cover 6 with a clamp 7 gasketing it. The position of the experimental plate relative to the cover 6 and vessel 1 is determined by the condition of the best view of its working surface. The experimental plate 2 is fastened on a supporting fabric-based laminate plate 15 of width 30 mm and thickness 6 mm and is retained against the current leads by a threaded connection 16. Fastened to the cover is a shell 8, in which a thermometer 9 is set so that the mercury reservoir is at the level of the experimental plate 2, which permits measuring the water temperature in the zone where the plate 2 is located. The facility has two windows 10 intended for lighting and observing the boiling process. The window through which lighting is realized is made of ground glass, and the window for observing the boiling process is made of transparent glass. The working liquid was heated to the boiling temperature by means of a thermoelectrical heater (TEH) 11 whose power is regulated during the experiment. Liquid vapors formed by boiling cool down on the wall of the condenser connected to the chamber 1 through a pipe union 12 located in the upper part of the vessel, and the condensate flows back into the chamber. The condenser maintains also the saturation conditions in the working chamber. The chamber is filled with the working liquid and is drained through a discharge pipe 13 located in the lower part of the vessel with an open valve 14. In the course of operation of the facility the valve 14 is shut off. The thickness of the liquid layer over the experimental plate is 60–80 nm. To control the parameters of liquid heating by the protective and experimental heaters, a power unit and a control unit were used. The power unit incorporated a welded frame with voltage-control autotransformers located on its front panel for
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Fig. 1. Scheme (a) and external view (b) of the experimental facility: 1) heat-insulated case; 2) investigated specimen; 3) terminals; 4) leads; 5, 16) threaded connections; 6) cover; 7) clamp; 8) shell; 9) thermometer; 10) viewing window; 11) TEH; 12) condenser union; 13) outlet pipe; 14) valve; 15) fabric-based laminate base for the specimen. the experimental heater and the protective TEH, two switches for turning on and off the heating voltage of the experimental heater and the protective TEH, ammeters for measuring the current intensity in heating the specimen, a voltmeter for determining the voltage drop on the specimen, a millivoltmeter for measuring the thermoelectromotive force of the chromelcopel thermocouples on the surface of the specimen, and signal fittings. Visualization of the boiling process was realized with the help of a Photron Fastcam SA4-500K-C1 high-speed video camera with Nikon Nikkor AF (60 mm, F/2.8, D Micro and Navitar DO-2595 (25 mm, F/0.95) optical systems that permitted video recording in the course of the experiment with a frequency of 3000–20,000 frames/s. Lighting was carried out by an ARRI ST1 lighting plant of power 1 kW with a Fresnel lens of diameter 175 mm, and a Schott DCR III system of local fiber optic illumination with an EKE lamp of power 150 W. The temperature of water tf was measured in the course of the experiment by means of a mercury thermometer with a scale from 50 to 100oC and a value of the scale division of 0.1oC. The heat flow density and the heat transfer coefficient were calculated by the formulas q = Q /F = I ΔU /F , α = Q /( F Δt ) . Investigations were carried out on specimens from various materials of thickness from 0.2 to 0.5 mm with a length of the working (finned) part of 115 mm and a width of 5–7 mm. The length of the specimen was chosen so that a clear view of the boiling surface through the viewing window was provided. The width and thickness of the working part of the specimen were chosen from the condition of obtaining the necessary heat flows on the heat transfer surface. At the points of contact of the specimen with the current-carrying buses, the area of its corresponding face was increased in order to provide a reliable electrical contact and the minimum heat removal at the points of contact to the current-carrying buses. A microrelief was applied to the straight narrow part of the specimen. We used, as the blank for making finned plates, sheets of BTI-00 titanium of thickness 0.5 mm, stainless steels 12X18H9T and 12X18H10T of thickness 0.2 mm, and carbon steel AISI 1020 of thickness 0.3–0.35 mm (Table 1, Fig. 2). Such a choice was motivated, firstly, by the high quality of the form of fins that can be obtained on the surface of titanium plates and, secondly, by the comparatively high specific resistance of the material, which is of no small importance since in the experiment the specimen was heated by passing current directly through it. The temperature of the specimen surface was measured by means of three chromel-copel thermocouples. The junction of one thermocouple was near the transverse symmetry axis, and the junctions of the other two extreme thermocouples were at a distance of 40 mm from the edges of the specimen. Each junction of the thermocouples was welded to the surface of 1422
Fig. 2. Top external view and metallographic specimens of the investigated boiling surfaces with three-dimensional roughness in the form of spherical recesses (a), with two-dimensional roughness in the form of conducting fins (a–f, h, i, k), with threedimensional roughness in the form of pin dissipating fins (g, l), and with two-dimensional roughness in the form of a microchannel structure formed by bent conducting fins (j). The characteristics of surfaces a–l are given in Table 1. TABLE 1. Parameters of the Plates Investigated
Material
Height of ribs, μm
Fin pitch, μm
Interfin gap, μm
Inclination of fins, deg
Depth of grooves, mm
Smooth
12Х18Н9Т
–
–
–
–
–
–
–
–
a
12Х18Н9Т
–
–
–
–
1
2
–
–
4
a
12Х18Н9Т
–
–
–
–
0.5
1
–
–
5
b
ВТ1–00
95
40
15
87
–
–
–
–
Designation of surface
Type of surface
1–2 3
Diameter of Knurling the recesses, pitch, mm μm
Width of the groove, μm
6
c
ВТ1–00
310
160
63
87
–
–
–
–
7
d
ВТ1–00
200
120
46
87
–
–
–
–
8
e
ВТ1–00
230
90
35
87
–
–
–
–
9
f
ВТ1–00
220
60
22
87
–
–
–
–
10
g
AISI 1020
420
350
–
90
–
–
318
140
11
h
12Х18Н10Т
150
160
50
90
–
–
–
–
12
i
12Х18Н10Т
90
160
50
90
–
–
–
–
13
j
ВТ1–00
200
200
30–40
60
–
–
–
–
14
k
12Х18Н10Т
200
160
50
90
–
–
–
–
15
l
AISI 1020
340
240
–
75
–
–
318
140
16
k
12Х18Н10Т
200
160
50
90
–
–
–
–
1423
the specimen, and the free ends of the thermocouples were brought out through the hole in the vessel cover, which was sealed during the experiment. All wires of the thermocouples were isolated from one another and from the liquid with the help of polychlorovinyl tubes. The thermocouples were located on the side of the plate reverse to the boiling surface and, therefore, we recalculated their readings with account for the heat conductivity of the plate material and the heat transfer conditions on its sides. In the course of estimating the measurement error and calibrating the readings of the thermocouples, it was established that the boiling surface temperature was measured with an error of ±3oC. In the course of boiling experiments, the boiling surface temperature in the zone of the thermocouples varied drastically over the range of ±0.1–1oC depending on the heat load due to the formation and disengagement of vapor bubbles. In processing the experimental data, the average temperature was used. In this case, the experimental error was ±0.25–0.9% depending on the level of heat loads. To prevent boiling of the liquid on the smooth (lower unfinned) surface of the specimen and rectify the temperature measurement errors that are due to the periodic formation of vapor bubbles at welding points of the thermocouples, this surface was covered with layers of epoxy adhesive and silicon sealant and bonded to the fabric-based laminate substrate. The adhesive and sealant layers also provided additional strength of the thermocouple–specimen joint. The 6-mm thick fabricbased laminate substrate prevented damage and deformation of the plates during the experiment and decreased the heat loss on their opposite unfinned side. Heating of the experimental boiling surfaces was realized by passing electric current through them. It should be noted that finned surfaces have a specific feature as to the current flow and heat propagation on them. Such a boiling surface in plane can have the form of a rectangle, and fins can be located along both the long side of the rectangle and along its short side. Therefore, the electric current passed in the longitudinal direction of the plate can cause Joule heat release only at the base of the (transverse) fins, as well as simultaneously at the base of the fins and in the fins proper (longitudinal fins), i.e., fins can be heat-dissipating and heat-releasing. Practically all investigations of the boil on finned surfaces were carried out for the first (traditional) case. In the present work, we investigated mainly heat-dissipating microfins, i.e., twodimensional microfins whose generatrices coincided with the direction of the electric current flow and whose heating with height can be considered to be uniform. Here the heat-dissipating microfins are three-dimensional fins, i.e., fins that are intermittent in the direction of the electric current flow through them and in which temperature distribution is characteristic of fins. The surface of specimen Nos. 5–12 and 14–16 was increased by a factor of 2.1–8.3 by applying a microstructure to it. The results obtained in the present work pertained mainly to the change in the hydrodynamical boiling pattern on a microcapillary surface rather than to the development of the heat transfer surface. Analysis of the Experimental Data. The boiling heat transfer of distilled water (bidistillate) on the above-described surfaces was investigated in [18–20]. Below we present the results obtained in these works for 96% ethanol, 60% aqueous solution of D-98 glycerol, S11 antifreeze, and 0.05% aqueous solution of Al2O3 at atmospheric pressure. The heat flow was varied over the range of 10–1200 kW/m2. In the investigated range of heat flow densities, regimes of convection, surface and developed nucleate boiling, and boiling crisis were observed. Investigations were conducted in the process of boiling of saturated liquids. In the course of experiments, it was noted that in the case of long-term operation of the experimental plates and periodic boiling of the liquid on their surface, the heat transfer intensity of these plates remains unchanged, and the so-called "self-accommodation" of their surfaces occurs. The experiments including seven boils on the surface of the above plates have shown that after four boils the level of heat transfer of their surfaces remains practically unchanged. All results were obtained on "stuck" plates. The experiments performed have shown that the application of surfaces with a relief obtained by the method of deforming cutting permits enhancing the boiling heat transfer compared to smooth surfaces. The levels of heat transfer enhancement were determined at q = idem. Boiling Heat Transfer of Various Liquids. Figure 3 presents the results of the test experiments on the boil of various liquids on a smooth surface. Experiments were performed in the process of boiling of water, 96% ethanol, 60% aqueous solution of D-98 glycerol, S11 antifreeze, and 0.05% aqueous solution of Al2O3. The experimental data on the nucleate boiling of water and the results of calculations by the Mikheev dependence α = 3q0.7p0.15 [21] differed by 10–30%, which is satisfactory. From Fig. 3 it is seen that the addition of 0.05% of Al2O3 powder to the distilled water enhanced the heat transfer of the investigated surface by 30–40%. As was noted in the works of Yu. A. Kuzma-Kichta [22, 23], in boiling of nanoliquids, including H2O with Al2O3 additives, the heat transfer enhancement is due to the increase in the number of vaporization centers owing to the release and growth of submicron microstructures on the boiling surface.
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Fig. 3. Boiling heat transfer of the smooth surface for various liquids: 1) distilled water; 2) 96% ethanol; 3) 60% aqueous solution of D-98 glycerol; 4) S11 antifreeze; 5) 0.05% Al2O3 + distilled water; A, calculation by the Mikheev dependence α = 3q0.7p0.15 [21].
Fig. 4. Heat transfer of surfaces of various geometries in boiling on them of large volumes of distilled water (a), 96% ethanol (b), 60% aqueous solution of D-98 glycerol (c), and S11 antifreeze (d): A, calculation by the Labuntsov dependence α = 3.4q2/3p0.18/(1 – 0.0045p) [22]; numbers designate the surfaces whose characteristics are given in Table 1. 1425
Fig. 5. Heat transfer intensity of surfaces of various geometries in boiling on them of large volumes of distilled water (a), 96% ethanol (b), 60% aqueous solution of D-98 glycerol (c), and S11 antifreeze (d) depending on the temperature difference between the wall and the liquid. Numerical designations are same as in Fig. 4.
Figure 4 compares the experimental data on the boiling of large volumes of the above liquids on surfaces of various geometries with the data calculated by the Labuntsov relation α = 3.4q2/3p0.18/(1 – 0.0045p) [24]. Figure 5 shows the dependence of the heat transfer of these surfaces on the temperature difference between the wall and the liquid. From the figures it is seen that the heat transfer of the horizontal surfaces of the plates under consideration is influenced by the geometry of their roughness. The boiling curves were plotted according to the data obtained by increasing or decreasing the heat load (there is practically no hysteresis of the boiling curve of water). According to Fig. 5, the presence of microroughness on the plate surface permits increasing its heat transfer coefficient and expedite the initiation of nucleate boiling of the liquid at a given temperature difference between the wall and the liquid. Figure 6 presents the results on the enhancement of the heat transfer of microstructured surfaces. The highest enhancement of the boiling heat transfer was observed in water and 96% ethanol on surface Nos. 10 and 15 with threedimensional columnar microroughness. Here the boiling heat transfer depending on the heat flow density was enhanced 5–20 times for water and 1.3–23 times for 96% ethanol. The presence of small interfin spaces allowed the liquid to flow to the vaporization centers, and the large sizes of lateral recesses provided vapor outflow. In the case of boiling of a more viscous 60% aqueous solution of D-98 glycerol, the heat transfer was enhanced by a factor of more than 1.1–3. Surface No. 13 with
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Fig. 6. Heat transfer enhancement in boiling of various liquids on microstructured surface Nos. 15 (a), 13 (b), and 10 (c). Numerical designations are same as in Fig. 4.
solid fins, whose ends were bent horizontally and formed microchannels, also showed a high level of boiling heat transfer enhancement for the above liquids. Such surfaces are close in enhancement mechanisms to porous coatings. The heat transfer of the above surface in boiling of water was enhanced 2.5–3 times, in boiling of 96% ethanol it was enhanced 10 times, and in the case of boiling of 60% aqueous solution of D-98 glycerol its heat transfer almost doubled. Analysis of the experimental data obtained for surface Nos. 5–9, 11, 12, 14, and 16 with two-dimensional microfins has shown that the level of heat transfer enhancement on them due to the boiling of water depends on the height of microfins, the width of gaps between microfins, the position of fins, and the angle of inclination of fins to the vertical. The boiling heat transfer of water on microfinned surface Nos. 5–9 was enhanced by a factor of 1.2–2.5 throughout the range of heat flow densities. In the case of boiling of 96% ethanol, 60% D-98 aqueous solution of glycerol, and antifreeze, practically no enhancement of the heat transfer of the above surfaces was observed. On the basis of the results obtained on specimen Nos. 5–9 we proposed that with increasing gap between fins (fin pitch) the level of heat transfer enhancement drops. On surface Nos. 3 and 4 with microroughness from spherical recesses the heat transfer enhancement in boiling of water was minimal — it was enhanced by a factor of no more than 1.2 compared to the smooth surface. As is seen from Figs. 4–6, the best data on the heat transfer enhancement were obtained for surfaces with a columnar structure Nos. 10 and 15. For surface No. 13, we observed "steaming" of the pores under bent fins in boiling on the outer sides of the fins and the typical distribution of the heat transfer coefficient depending on the surface superheating and the heat flow density. This was especially pronounced in boiling of 96% ethanol (Figs. 4b and 5b). The data presented in Fig. 6 1427
show that the greatest heat transfer enhancement of the investigated surfaces takes place in the case of boiling of water on them, which makes the application of these surfaces most effective in water-cooling systems of microprocessor devices, in water heat pipes, and in water thermosyphons. For other liquids, it is necessary to choose the geometric parameters of surface microstructuring depending on the capillary constant of a particular liquid. To substantiate the mechanisms of heat transfer enhancement of the investigated surfaces, in the course of experiments, video filming of the process of boiling of liquids on them was carried out. The process of boiling of water on surface Nos. 1, 3, and 9 was visualized in [18, 20]. Filming was carried out mainly in the regime of 3000–4500 frames with a resolution of 1024 × 800. There was no considerable difference in the number of vaporization centers in the case of boiling of water on surface Nos. 5–16. The number of vaporization centers was 0.05–0.1 1/m2 at q = 7.7–30 kW/m2 and increased from the above value to 0.25 1/mm2 at q = 80–140 kW/m2. However, as the visualization results show, there was a change in the wetting angle — it decreased sharply because of the finned surface, which just led to an increase in the frequency of disengagement of vapor bubbles and a certain decrease in their diameter at the moment of disengagement. At large heat loads the vaporization centers are difficult to determine because of the large number of these centers and the coalescence of bubbles before they are disengaged from the surface. Comparative analysis of the diameter distribution of vapor bubbles in the volume of water boiling on various surfaces at a height of 20 mm at q > 220–313 kW/m2 has shown that surface Nos. 10, 13, and 14 contain smaller vapor bubbles (the content of bubbles of diameter from 1 to 6 mm in the volume of water was 8–25%, and that of bubbles of diameter more than 7 mm was about 35–40%). It should be noted that in the case of boiling on a smooth surface the diameters of the majority of bubbles (66–100%) did not exceed 7 mm and only 0–25% of bubbles had a diameter of 1–2 mm, and they were also formed by the collapse of large bubbles when they are disengaged from the boiling surface (film). Large diameters of emerging bubbles cause their coalescence before disengagement because of the high density of vaporization centers. Critical Heat Flows. We have determined the critical heat flows depending on the geometry of the enhanced surfaces (Fig. 7). The minimum increase in such flows in the case of boiling of water (up to their doubling) was observed on surface Nos. 3 and 4 with microroughness in the form of systems of spherical recesses. The critical heat flows on surface Nos. 11– 14 and 16 with microroughness in the form of two-dimensional microfins increased 3.3–4.1 times. We could not determine the critical heat flows for the perspective surface No. 15 in the investigated range of heat flow densities. The critical heat flows on surface No. 10 with three-dimensional roughness increased by a factor of 6, which is due to the presence on them of zones where the water is "drawn" to the evaporation centers through the microchannels between the fins and the large channels for vapor outlet due to the preliminary knurling and the relatively large height of the fins. In the case of boiling of ethanol and an aqueous solution of glycerol, the increase in the critical heat flows on the investigated surfaces was minimal or not observed. Boiling of water on the smooth surface No. 1 was initiated upon its superheating Δt ≈ 4oC. For surface Nos. 10, 13, and 15 distinguished by a significant heat transfer enhancement, the initiation of boiling corresponded to Δt ≈ 0.3–0.5oC. Boiling on surface Nos. 4–9 was also initiated sooner than on the smooth surface and corresponded to Δt ≈ 0.3–1oC. Boiling on surface Nos. 11, 12, 14, and 16 was also initiated at Δt ≈ 3–4oC, as on smooth surfaces. Principles of the Physical Model. Boling heat transfer enhancement on microstructured surface is of great practical importance, since it permits decreasing considerably the sizes of cooling systems and heat transfer equipment. Enhanced surfaces designed for nucleate boiling have many clear advantages. For example, the heat transfer of the surface of low-finned tubes is enhanced, compared to a smooth tube, by a factor of 2–4, and that of the surface of tubes with a porous coating or deformed fins of small height is enhanced by a factor of 10 or more. The greatest increase in the heat transfer coefficient of the surfaces under consideration is observed at moderate heat flow densities. Practically important parameters of the heat transfer enhancement of surfaces are the intensification of the critical heat flows on them and the lowering of their superheating level necessary for initiating nucleate boiling. The obtained positive results cannot be explained by the development of the heat transfer surface alone. Vaporization occurs both on the outside of the enhanced surface and in its pores or half-closed channels. Consequently, there are four possible ways of heat removal from the enhanced heating surface (Fig. 8) [25]: 1) in the form of latent heat of vaporization in vapor bubbles formed in the half-closed channels of the porous coating or in the deformed projections; 2) in the form of latent heat of vaporization in vapor bubbles displaced from the pores and half-closed channels and "growing" on the outer side of the coating and projections; 3) in the form of heat contained in the liquid passing through the pores and half-closed channels; 4) in the form of heat contained in the liquid heated on the outer surface of projections and porous coats. The efficiency of the above factors depends on the type of geometry of the enhanced surface and its determining sizes.
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Fig. 7. Heat flows in boiling on surfaces of various configurations of distilled water (a), 96% ethanol (b), 60% aqueous solution of D-98 glycerol (c), nanoliquid H2O + 0.05% Al2O3 (d), and S11 antifreeze (e). Numerical designations are same as in Fig. 4. 1429
Fig. 8. Principles of the physical model of boiling on microfinned surfaces.
In the case of propagation of small heat flows on the heat transfer surface, the liquid fills all interfin spaces, some of which become subsequently active vaporization centers. When heat flows increase, vapor bubbles are formed in the interfin space in active vaporization centers. The liquid contained between vaporization centers soaks into the interfin space due to the capillary effect and spreads over the interfin spaces, arriving at the active vaporization centers. With further enhancement of the heat flows, vapor fills the whole of the interfin space and vapor bubbles grow in the upper parts of fins, i.e., fins pierce the vapor layer. There exist many physical and mathematical models of boiling on microfinned and porous surfaces, for example, the models proposed by P. S. O′ Neill, S. A. Kovalev, U. Nakayama, D. H. Chen, R. L. Webb, and K. Ramaswami. Most of them were considered in detail in [2, 3] and fit the above physical model. The aim of the present work was to verify the existing mathematical models. In the absence of mathematical models, one can use empirical criterial equations to calculate the heat transfer and the critical heat flows on heat transfer surfaces. To this end, systematization and processing of experimental data are needed. An attempt to generalize the obtained experimental data on the heat transfer of the surfaces under consideration was made in [19], and in [20] we attempted to generalize the experimental data on the critical heat flows on these surfaces. In [19], we analyzed the influence of the height of microfins, the width of the gap between them, the position of fins, and the angle of inclination of fins to the vertical on the heat transfer coefficient of surface Nos. 5–9, 11, 12, 14, and 16. However, we did not manage to establish the influence of the relative width of the interfin gap, the relative width of the fin, and the relative increase in the heat transfer area, on the level of heat transfer of the above surfaces, which necessitates further experimental investigations with the use of the literature data. Conclusions. We have made an experimental investigation and formed a data array on the boiling heat transfer for various saturated liquids (distilled water, 96% ethanol, 60% aqueous solution of D-98 glycerol, S11 antifreeze, a mixture of distilled water with 0.05% of Al2O3) at a pressure of 0.1 MPa in the regime of free convection on microstructured plates with sizes of fin elements from 50 to 420 μm obtained by the method of deforming cutting, as well as an array of experimental data on the critical heat loads, the above plates lying horizontally with the boiling surface upward under uniform heating. On the basis of the analysis of the obtained data array it has been established that the most suitable substance for investigating the microgeometry of boiling surfaces is water. The highest heat transfer enhancement in the case of boiling of water is attained on surfaces with three-dimensional columnar and channel structures, whose heat transfer is 3–20 times that 1430
of a smooth surface. The formation of two-dimensional microfins on the boiling surface permits enhancing its heat transfer by a factor of no more than 2.5, and a decrease in the distance between fins and in their thickness permits increasing the heat transfer intensity. The critical heat flow density increases by a factor of 4.1–6 in the case of boiling of water on surfaces with three-dimensional columnar and channel structures as compared to its boiling on the smooth surface. It should be noted that nucleate boiling on the above microstructured surfaces is initiated at much lower temperature differences between the hot wall and the liquid, for example, at Δt ≈ 0.3–0.5oC as compared to Δt ≈ 4oC on the smooth surface. Thus, problems for future investigations and selection of effective microstructures on the boiling surface have been formulated. This work is a part of agreement No. 14.Z50.31.0003 of 0.4.03.2014 on supporting scientific investigations of leading scientists in Russian universities (leading scientist S. A. Isaev).
NOTATION I, current intensity of the plate, A; F, surface area of the plate between leads, m2; Q = IΔU, heat flow generated on the plate, W; q, heat flow density, W/m2; Δt = tw − tf , difference between the average temperature of the specimen surface and water, K; ΔU, voltage drop on the plate, V; α and αsm, heat transfer of the structurized and smooth surfaces, W/(m2·K). Subscripts: sm, smooth; f, fluid; w, wall.
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