ISSN 0018-151X, High Temperature, 2017, Vol. 55, No. 4, pp. 524–534. © Pleiades Publishing, Ltd., 2017. Original Russian Text © I.A. Popov, A.V. Shchelchkov, Yu.F. Gortyshov, N.N. Zubkov, 2017, published in Teplofizika Vysokikh Temperatur, 2017, Vol. 55, No. 4, pp. 537–548.
HEAT AND MASS TRANSFER AND PHYSICAL GASDYNAMICS
Heat Transfer Enhancement and Critical Heat Fluxes in Boiling of Microfinned Surfaces I. A. Popova, A. V. Shchelchkova, *, Yu. F. Gortyshova, and N. N. Zubkovb aTupolev
Kazan National Research Technical University (KAI), Kazan, 420111 Russia Moscow State Technical University, Moscow, 105005 Russia *e-mail:
[email protected]
bBauman
Received April 2, 2015
Abstract—A brief review is presented of experimental investigations of heat transfer enhancement during boiling on surfaces with nano- or microstructure. Distilled water boiling on microfinned surfaces made by deforming cutting was investigated. It was found that, for distilled water boiling on these surfaces with 3D microfinning (having an interfin gap of u = 120–180 μm, a microfin height of h = 340–570 μm, and a microfin pitch of w = 240–400 μm), heat transfer increased by a maximum factor of 4 to 5 as compared with that on smooth surfaces. Critical heat fluxes increase by a factor of up to 6 as compared with boiling on smooth surfaces. DOI: 10.1134/S0018151X17030208
INTRODUCTION At present heat-releasing components of power electrical equipment and microelectronic hardware are cooled with a liquid cooling system. In these systems, liquid boiling occurs right on electronic components for which purpose these components are immersed directly into the liquid coolant. Implementation of similar cooling arrangements requires suitable dielectric liquids such as Freons. Cooling can be effected by boiling in an open circuit with condensation of vapors in an external air-cooled condenser or in an enclosed volume with condensation of vapors inside the volume on the surface of a liquid-cooled condenser. To reduce the boiling onset point, enhance heat transfer, and raise critical heat fluxes, surfaces of cooled electronic components are provided with metal films or metal plates with microdeformations, microroughness, or porous coating [1–3]. A BRIEF REVIEW OF AVAILABLE PUBLICATIONS AND FORMULATION OF THE PROBLEM Many publications on heat transfer enhancement in boiling on surfaces with nano- or microroughness appeared in the 2000s. Findings from certain investigations are presented in Table 1. An analysis of data from Table 1 has revealed that the use of surfaces with nano- or microroughness enhances heat transfer by a factor of 1.1 to 10, increases critical heat fluxes by as much as four times, and reduces the boiling onset point. This is important for development of effective cooling systems. Each struc-
ture of nano- or microroughened surface and used working fluid is characterized by its own best sizes of roughness elements. The factors affecting heat transfer enhancement and critical heat fluxes depend on the surface wettability and properties of the liquid. The surface operating conditions also have an effect. If, for example, considerable heat transfer enhancement is observed during pool boiling on surfaces with gridtype or columnar microstructure [6, 13, 14, 25], no heat enhancement can occur when a coolant flows along these surfaces [36, 37]. To corroborate processes responsible for heat transfer enhancement, investigations using various boiling visualization techniques involving measurement of bubble departure diameter, bubble release frequency, floatation velocity, etc. were carried out [6, 8, 14, 19, 31, 35, 38]. It has been found that heat transfer enhancement is accompanied with a decrease in the bubble departure diameter and an increase in the bubble release frequency. An increase in critical fluxes is attributed to surface wettability and depends on a ratio of capillary constant to dimensions of roughness elements. Reviews of available models of boiling on surfaces with porous coatings or various nano- and microroughness structures (Fig. 1) can be found in [35, 39–44]. Technologies that apply porous nanocoatings, sintered microgrids and microfins based on metal or silicon microelements, are intricate and expensive. The above-presented analysis and results [45] demonstrate that surfaces with metal or silicon micro or nanocoatings having various structures affecting wettability of heat transfer surfaces rank below microstructured sur-
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Table 1. Characteristics and results of investigation of heat transfer in boiling on microstructured surfaces Reference [4]
Working surface
Working fluid
Pipe, copper, horizontal R-11, R-123, R-134a
Intensifier type Solid fins made of porous materials
Working surface characteristics
Results
dpor = (200–270) × 10–6 m; α/α0 = 1–10 h = 540 × 10–6 m; δ = 605 × 10–6 m; s = 855 × 10–6 m
[5]
Plate, silicon, horizontal Distilled Columnar structure d = (5–10) × 10–6 m; degassed water h = (10–20) × 10–6 m; s = (5–15) × 10–6 m
(q/q0)cr = 2–3
[6]
Plate, silicon, horizontal Water
Columnar structure h = 500 × 10–6 m; s = (200–1000) × 10–6 m
(q/q0)cr = 1.5
[7]
Plate, copper, horizontal Water
TiO2 nanotubes, vertical bundle
h < 200 × 10–9 m
α/α0 = 1.3–2.1
[8]
Plate, aluminum, horizontal
SiO2 coating, PTFE, columnar structures
h < (1–50) × 10–6 m
α/α0 = 1.8–2.1
GewaTM-T, GewaTM-K, Turbo BIITM-LP, High FluxTM
s = (1.36–1.5) × 10–3 m; δ = 236 × 10–6 m , δ = 700 × 10–6 m; h = 10–6 m; h = 1.53 × 10–6 m
α/α0 = 1.5–5.35
Water
[9, 10] Pipe, copper, horizontal R-123, R-123 + n-hexane
Columnar h = (79.6–5863) × 10–9 m α/α0 = 5.35 nanostructure, (q/q0)cr = 3.8 Al–ZnO, Cu–ZnO
Water
[11]
Plate, aluminum, copper, horizontal
[12]
Pipe, copper, horizontal Water
[13–17] Plate, steel, titanium, horizontal
Distilled degassed water, ethanol, 60% glycerine water solution, S11 antifreeze
Roughness after PO = 40–65%; sand blasting, grids, dpor = (5–30) × 10–6 m; sintered particles h = (0.1–0.4) × 10–3 m
α/α0 = 2
Columnar h = (90–420) × 10–6 m; structures, δ = (25–180) × 10–6 m; microfinning made s = (40–350) × 10–6 m by DC method
α/α0 = 9 (q/q0)cr = 6
α/α0 = 3
[18]
Pipe, horizontal, vertical R-113
Microfinning, made by DC method
[19]
Plate, silicon, horizontal,
Water
Columnar PO = 50%; nanostructure made dpor = 200 × 10–6 m; of Si and Cu wires h = (40–60) × 10–6 m
(q/q0)cr = 1.2–4
[20]
Plate, horizontal
Water
Inclined columnar structures, Cu
α/α0 = 1.4–4.5
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h = 310 × 10–6 m; δ = 182 × 10–6 m; s = 215 × 10–6 m
PO = (40–65) %; s = 50 × 10–9 m; h = 450 × 10–9 m
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Table 1. (Contd.) Reference
Working surface
Working fluid
Intensifier type
Working surface characteristics
Results
[21]
Plate, horizontal, copper Water
Rectangular δ = 1.8 × 10–6 m; columnar structures δ = 3.6 × 10–9 m; s = (2.1–6.98) × 10–9 m; h < 10.2 × 10–9 m
[22]
Pipe, copper, brass, horizontal
Mechanical roughness
h = (0.07–10.5) × 10–6 m; α/α0 = 2
[23, 24] Plate, copper, horizontal n-pentane
Porous fins and studs
dpor = 100 × 10–6 m; PO = (15–41)%, h = 1.3 × 10–3 m; δ = 500 × 10–6 m; s = 10–3 m
[25, 26] Plate, copper, horizontal Water
Inline and staggered dpor = (119–232) × 10–6 m; α/α0 = 4 grid arrangements h = (210–2300) × 10–6 m; (q/q0)cr = 1.6–2 δ = 0.56 × 10–6 m; s = 120 × 10–6 m
R-134a, R-123
(q/q0)cr = 2.5
(q/q0)cr = 2–3.3
α/α0 = 1.5–4.8 (q/q0)cr = 1.6–2.25
[27]
Plate, copper, horizontal HFO-1234yf, HFC-134a
Microroughness
PO = (40–50)%; δpor = 150 × 10–6 m
[28]
Plate, horizontal
PF-5060,
Microporous surface h = 9 × 10–6 m; made of nanotubes h = 25 × 10–6 m; δ = (2–15) × 10–6 m
[29]
Plate, horizontal
FC-72
Microdimples
α/α0 = 2.75 (q/q0)cr = 2.4
dpor = (100–400) × 10–6 m; α/α0 = 2.5 h = (110–220) × 10–6 m; s = (230–620) × 10–6 m δ = (248–1148) × 10–6 m; α/α0 = 1–5 (q/q0)cr = 3 h = (20–30) × 10–6 m; –6 m s = (304–1200) × 10
[30, 31] Plate, silicon, horizontal Ethanol, water Microdimples HFE-7000, FC-72 [32]
Plate, horizontal
FC-72
Chrome nitride porous coating
δ = (0.12–0.45) × 10–6 m; α/α0 = 1–5 (q/q0)cr = 1.3–2.5 h = (35–45) × 10–6 m
[33]
Plate, horizontal
Water
Si/SiO2 coating
h = 30 × 10–9–5 × 10–6
(q/q0)cr = 2.35
[34]
Plate, silicon, horizontal Water
Columnar fibers, silicom
h = (900–3600) × 10–9
α/α0 = 2.2–3.54
[35]
Plate, horizontal
Microfinning
δ = (193–402) × 10–6; h = (245–402) × 10–6; s = (387–808) × 10–6
α/α0 = 1–5
Ethanol, FC-87
d—diameter, h—fin height, s—knurling pitch, α—heat transfer coefficient, δ—fin thickness, ϕ—fin inclination angle, PO—porosity, subscripts: por—pore, cr—critical, 0—smooth, DC—deforming cutting. HIGH TEMPERATURE
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(a) (а)
(g)
10 μm
(b)
1 μm
10 μm (h)
100 μm (i)
500 μm
(c)
1 μm (j)
310 ± 59 μm
wall tube
(d)
t
215 μm
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(e)
b 33 ± 7 μm
grooves
(f)
(k)
Fig. 1. Nano- and microroughened surfaces for boiling: (a) silicon microstuds [5], (b) nanostructured surface [11], (c) porous microfins [23], (d) sintered microgrids [25], (e), (f) microfinning made by deformed cutting (DC) [10, 18], (g) deformed microfins [9], (h) copper and silicon microwires [19], (i) microporous structure [35], (j) silicon microstuds [24, 34], (k) 2D and 3D microfining by DC [13, 14].
faces in heat transfer enhancement and heat fluxes. The deformed cutting method [46–48] is among effective techniques for pool boiling heat transfer enhancement. This technology is simple, waste free, and consists in metal machining in standard machine tools. For these surfaces with 2D and 3D structure (with a fin height of h = 90–420 μm, fin thickness of δ = 25–180 μm, and fin pitch w = 40–350 μm), the pool boiling heat transfer coefficient increases by as much as a factor of 4.1 [13, 14]. Heat transfer enhancement was maximized on 3D-structured surfaces (with microstuds). Results of the investigation into heat transfer enhancement for these surfaces during the boiling of distilled water, 98% ethanol, 60% glycerin water solution, and other liquids are presented in [16, 17]. This paper presents the results of additional investigations focused on extensions of the range of investigated geometric parameters of a microroughened surface made by the deforming cutting technique and of finding the best structures for heat transfer enhancement. EXPERIMENTAL CONDITIONS The experiments were performed in an experimental set-up, a schematic diagram of which is shown in Fig. 2. The set-up comprises a heat-insulated vessel used as boiling chamber 1 with sizes 150 × 250 × 200 mm filled with degassed distilled water (bidistilled). The boiling chamber had double walls. The gap between the walls is filled with asbestos heat-insulating packing. Plate 2, on which boiling enhancement was studied, was installed in the chamber. The test plate was heated by a passing electric current (experiments were carried twice: with direct current and alternating current). Voltage was fed to specimen 2 from terminals 3 via 4.5-mm thick and 30-mm wide flat copper current leads 4. The current leads were secured to electrically insulating textolite cover 6 by means of screwed joint 5. The cover had sealing collar 7. The experimental plate was placed respective to cover 6 and vessel 1 to achieve the best visibility of its working surface. Experimental plate 2 was attached to a 30–50-mm wide and 6-mm HIGH TEMPERATURE
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thick supporting textolite plate 15 and pressed to the current leads with threaded joint 16. Thermowell 8 of thermometer 9 was attached to the cover. During the experiments, thermometer 9 was installed into thermowell 8 so that its mercury bulb was at the same level with experimental plate 2. Thus, the temperature of water in the immediate vicinity of plate 2 location zone was measured. To observe boiling, two windows 10 were provided. One window made of matted glass was used for illumination. The other window made of transparent glass was used for observing the boiling process. The water was heated to the boiling temperature using tubular electric heater 11, the power of which was controlled during the experiment. Steam generated during boiling was cooled on the wall of a condenser connected to the cavity of chamber 1 through nozzle 12 located at the top of the vessel. The produced condensate flowed back into the chamber. The condenser was also used for maintaining saturation conditions in the working chamber. The chamber was filled with water and emptied via drain nozzle 13 at the vessel bottom with valve 14 in open position. When the set-up was in operation, valve 14 was closed. The layer of liquid above the experimental plate had a height of 60–80 mm. To control water heating with a guard heater and an experimental heater, a power unit and a control unit were used. The power unit is a welded frame with an autotransformer in the front panel for controlling the supply voltage of the guard heater and the experimental heater. It also has two switches for turning on and off the power supply to the guard and experimental heaters, ammeters for monitoring current through a test specimen, a voltmeter for monitoring voltage across the specimen, a millivoltmeter for measurement of emf of chromel-copel thermocouples on the specimen surface, and indicators as applicable. In the experiment, water temperature in the plate vicinity, tf, was measured using a mercury thermometer with a scale from 50 to 100°С and a scale division of 0.1°С.
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220 V
5
9
3
12
6
A
16
10
2
7 1
4
8
220 V
15 14 13
mV V
11
Fig. 2. Schematic diagram and appearance of the test set-up.
The heat transfer coefficient was calculated as formula
were routed via a hole in the chamber cover, which was sealed during the experiments.
α = Q (F Δt ) .
To prevent water boiling on the smooth (bottom plane) surface of the specimen and to eliminate temperature measurement errors caused by steam bubbles periodically generated at the thermocouple welding points, the surface was coated with a layer of epoxy glue and a layer of silicon sealant, and glued to a textolite substrate. The layers of glue and sealant also made the connection of thermocouples with the specimen stronger. The 6-mm thick textolite substrate protected the plates from being damaged and deformed during the experiment and decreased heat losses from the opposite nonfinned side of the plate.
Here, F is the plate surface area (i.e., the product of plate width by the plate length between the current leads without accounting for an increase in the surface area due to finning); Q = IΔU is the heat flux released in the plate; I is the current through the plate; ΔU is the voltage drop across the plate; Δt = tw – tf is the difference between the average temperature of sample surface and the water temperature, К. There was a four to nine times increase in the heat transfer area due to finning; however, the surface extension was not considered in processing of experimental data. The established heat transfer enhancement was correlated with a change in number of nucleation sites and boiling processes as demonstrated in [13], rather than with the surface extension. For the shape and sizes of the specimens, see Table 2 and Fig. 3. Investigations were carried out with specimens of various materials having a thickness from 0.2 to 0.5 mm, a working (finned) section length of 87 to 115 mm, and a width of 5 to 30 mm. To make reliable electrical contact of the specimen with current-carrying busbars and minimize heat removal to the busbars at contact points, the area of the corresponding edge of a specimen was increased. The temperature of the specimen surface was measured with three chromelcopel thermocouples (made of thermocouple wire 0.2 mm in diameter). The hot junction of one thermocouple was near the transverse symmetry axis of specimen. The hot junctions of the other two thermocouples were at a distance of 40 mm of the specimen edges. The hot junction of each thermocouple was welded to the specimen surface. Thermocouple leads
The experimental boiling surfaces were heated by passing electric current through them. The configuration of finned elements has certain specific features with respect to the flow of current and heat propagation. The boiling surface has the shape of a rectangle in its plan view, and fins are arranged along its long side. Electric current passed through the plate in the longitudinal direction causes release of the Joule heat only in the base of the fins (studded structures) or in both fin bases and fins proper (longitudinal structures). Thus, the fins can either dissipate or release heat. This investigation is focused basically on heat-releasing microfins, i.e., 2D microfins having the generatrix coinciding with the direction of electric current flow. The heating of these microfins along the height can be considered uniform. Galvanic and electrical isolations were used in measuring circuits to reduce uncertainties in temperature measurements with thermocouples. Results of the investigation from the heating of the boiling surface using alternating current agreed with the results obtained from DC heating. HIGH TEMPERATURE
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Table 2. Parameters of studied boiling surfaces made by the deforming cutting method Specimen h × 106, w × 106, δ × 106, Material no. * m m m 17 18 19 20 21 22 23 24 25 26 27 28 29 30
AISI1020 316L ВТ1-0 AISI1020 AISI1020 ВТ1-0 ВТ1-0 ВТ1-0 ВТ1-0 316L 316L 316L 316L ВТ1-0
420 300 550 570 480 300 360 360 200 500 400 400 400 500
350 70 250 350 240 250 200 200 100 400 400 400 400 400
145 26 150 140 105 140 115 115 65 200 200 200 200 200
ϕ, deg 20 7 10 10 20 0 25 25 7 10 10 10 7 10
u × 106, s × 106, m × 10 6, z × 106, n × 106, k × 106, m m m m m m 120 0 0 180 120 0 0 0 0 0 0 300 300 0
320 0 0 320 320 0 0 0 0 0 0 600 600 0
0 0 0 0 50 0 0 15 0 0 0 0 20 0
0 0 0 0 100 0 0 95 0 0 0 0 50 0
0 0 0 20 50 0 0 30 0 0 0 0 10 0
205 10 100 210 135 110 85 85 10 200 100 100 100 200
u—longitudinal gap width, m—interfin gap, z—microfin pitch, n—knurling dept, k—gap width. * Numbering continues designation of the specimen surfaces investigated in [13–17].
RESULTS OF HEAT TRANSFER ENHANCEMENT INVESTIGATION The experimental data were obtained for distilled water at atmospheric pressure. Heat flux density varied from 10 to 3500 kW/m2. Single-phase liquid convection, surface boiling, fully developed nucleate boiling, and burnout were observed in the studied range of heat flux densities. The boiling investigations were performed with saturated liquid. Prior to taking measurements, a so-called “burn-in” of the surface was performed, during which the liquid was brought to boiling on the surface many times. Many experiments were carried out on all the plates, the duration of which, including intervals, was as long as two or three days. The experiments demonstrate that the use of surfaces made by the deformed cutting technique, enhances boiling heat transfer as compared with that on smooth specimens. Experimental results for nucleate water boiling (Fig. 4) differ by 10% from predictions by the Mikheev correlation α = 3q0.7p 0.15 [49], which is satisfactory. An analysis of the experimental results for heat transfer on microstructured surfaces shown in Fig. 4 suggests that the highest heat transfer coefficients are observed on surfaces nos. 17, 20, and 21, having 3D microfins. There was three to five times heat transfer enhancement during developed nucleate boiling. It should be considered that 3D fin structure with the selected heating arrangement makes fin heating uniformity along the height poorer due to shallow transverse grooves in the fin. It was demonstrated in [13] on HIGH TEMPERATURE
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the basis of comparison of measured heat transfer on the surfaces with longitudinal and transverse 2D fins (surface nos. 14 and 16, respectively) that poorer heating of the fins decreased heat transfer by 7.5%. However, boiling visualization [14] shows that these surfaces have smaller bubble departure diameters, a greater number of released bubbles, and a higher bubble release frequency, especially at low temperature drops. This is explained by a higher density of nucleation sites and a limited increase in the bubble volume on the discontinuous surface of a 3D fin. Boiling on these surfaces occurs at considerably lower (by a maximum factor of 5) wall overheating (Fig. 5). The level of heat transfer enhancement attained on surfaces nos. 17, 20, and 21 having 3D microfins agrees well with the results of [13, 15–17] obtained for surfaces nos. 10 and 15. As in [13], it is noted that, on surfaces nos. 18, 19, 22–27, 30 with 2D fins, irrespective of the tip microstructure of fin (straight, bent, or with microstuds), heat transfer enhancement is by a factor of 1.2 to 1.4. With fins heavily bent using a roller to form open microchannels, heat transfer enhancement increases by as much as a factor of 2 to 3, as demonstrated in Fig. 4 for surface no. 25. An analysis of the experimental data confirms the experimental results of [13]. Surfaces with 3D microfin structure made by the deformed cutting method are most suitable for heat transfer enhancement during water boiling at atmospheric pressure. For microfin heights from 340 to 570 μm, microfin longitudinal pitches from 240 to 400 μm, and a knurling pitch of s = 320 μm, we could not find adequate dimensions max-
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(b)
(a)
(c)
(a)
Surface no. 17
(a)
no. 19
(b)
(c)
(b)
(c)
no. 18
(b)
(c)
(a)
no. 20
s
(a)
no. 21
(b)
(c)
(a)
(a)
no. 23
(b)
(c)
(a)
(a)
no. 25
(b)
(c)
(a)
(b)
(c)
(a)
(b)
(c)
(a)
(a)
(a)
no. 27
no. 29
(b)
(c)
no. 24
(b)
(c)
no. 26
(b)
(c)
(b)
(c)
(b)
(c)
no. 22
no. 28
no. 30
Fig. 3. Metallographic sections (a), appearance (b), and models (c) of microfinned surfaces nos. 17–30 (Table 2) made by the deforming cutting method.
imizing heat transfer enhancement. It should be noted that, with these dimensions of microfins, the interfin gap u should not be greater than 120–180 μm. Its increase, for example, to 300 μm considerably decreases heat transfer enhancement (to the level of 2D finning).
EXPERIMENTAL RESULTS FOR CRITICAL HEAT FLUXES In the investigated range of experimental conditions, the highest critical heat fluxes were measured on surfaces nos. 20 and 21 (Fig. 6) with 3D structure. The HIGH TEMPERATURE
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α, W/m2 K 3 × 105
q, W/m2 3 × 106
5
106
21
17 18 20 21 22 23 24 25
20 17
24
30 28
27 29
25
26 23
Burnout
10
531
22 18
104 Smooth plate Predictions by Mikheev’s formula
17 22 26 30
103 5 × 102 103
104
18 23 27
20 24 28
21 25 29
Smooth surface
105
106 4 × 106 q, W/m2
105
104 Fig. 4. Water boiling heat transfer as a function of the heat flux for surfaces nos. 17–30 (see Table 2). Smooth surface
26 29
α, W/m K 3 × 105 2
10
103 1
5
27 30
28
10
30 ΔT, °C
Fig. 6. Water boiling curves for various structures of microroughened surfaces nos. 17–30 (Table 2): arrows indicate critical heat flux levels.
104 Smooth surface
103 1
17 21 24 27 30
10
18 22 25 28
20 23 26 29
40 ΔT, °C
Fig. 5. Water boiling heat transfer as a function of the overheating of surfaces nos. 17–30 (see Table 2).
critical heat fluxes increased by a factor of 6 as compared to a smooth surface. Unfortunately, geometric characteristics of the investigated 3D surface are nearly the same (Table 2) that prevented the best dimensions from being found. However, comparing the results of this investigation and of [14], we can conclude that, for water boiling at atmospheric pressure, structures with 250–340 μm-high 3D fins would be the best. An increase in the critical heat flux for these surfaces is explained by capillary inleakage of liquid via additional gaps 120–180 μm wide under the formed steam films (unlike 2D surfaces). An increase in the gap width to 300 μm reduces heat flux as shown for surface no. 29. It should be noted that boiling-up occurs on 3D surfaces at lower, by a factor of 5, temperature differences between the wall and liquid as compared with a 3D or smooth surface. It is shown in [14] that boiling HIGH TEMPERATURE
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surface no. 25 with open microchannels formed by heavily bent microfins also features high heat fluxes at low temperature differences. Surfaces nos. 18, 19, 22–27, and 30 having 2D microfins with different tip structure (straight, bent, or with microstuds) feature an increase in the critical heat fluxes by a maximum factor of 4 that agrees with data obtained in [14]. At high temperature differences, heat fluxes on these surfaces are close to heat fluxes on a smooth surface due to relatively low heat transfer coefficients. THE EFFECT OF ORIENTATION OF THE BOILING SURFACE ON HEAT TRANSFER AND CRITICAL HEAT FLUXES The above-presented results are for horizontal plates with boiling surfaces facing upwards. In practice, cooling systems can have boiling surface of various orientation. Investigation results on the effect the boiling surface orientation has on the heat transfer rate are presented in [50–52]. For boiling on a pipe surface, it is demonstrated in [50, 51] that the lower pipe surface has the highest heat transfer coefficient while the upper surface features the lowest. This is caused by the fact that, on departure of a vapor bubble from a horizontal plate facing upwards, the microlayer of overheated liquid under the bubble is entrained and replaced with colder liquid consuming additional heat
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α, W/m2 K 7 × 105 Surface no. 13
Side q
Smooth surface Top Side Bottom
Bottom q
106
Surface no. 10 Top Side Bottom Burnout
105
q
Top
Top Side Bottom Surface no. 14 Top Side Bottom
q, W/m2 4 × 106
104 Smooth surface no. 1 Top Side Bottom
103 3.5 × 103 104
105 q
Top
105
106
Side
4 × 106 q, W/m2
q
q
Fig. 7. Water boiling heat transfer as a function of the heat flux for different orientations of the boiling surface.
for heating. If a horizontal plate has a boiling surface facing downwards, on departure of a vapor bubble, the microlayer of superheated liquid is held at the surface under action of gravitational forces, thereby contributing to quicker nucleation and growth of a new vapor bubble. At the same level of q = idem, the difference between the heat transfer coefficients on horizontal plates with the boiling surface facing downwards or upwards is as great as 30%. The side surface has a heat transfer coefficient of intermediate value falling between the two above-mentioned heat transfer coefficients. This fact is examined in [52]. A horizontal plate with the boiling surface facing downwards has higher heat transfer coefficients as compared with horizontal plates with boiling surface facing upwards or vertical plates. Nevertheless, at high heat fluxes or in developed nucleate boiling, no such difference in the heat transfer rate can occur. The effect of microstructure and surface orientation on the heat transfer and critical heat fluxes during boiling was investigated for specimens nos. 1, 10, 13– 15 (for specimen characteristics, see [13]). It has been found that the orientation of smooth boiling surfaces has a more pronounced effect than that of microroughened surfaces. On a smooth plate with the boiling surface facing upwards, heat transfer is 20 to 50% lower as compared with that on a plate with the boiling surface facing downwards (Fig. 7). For roughened surfaces, this difference hardly manifests itself at all. It should be noted that orientation of the boiling surface has a considerable effect on critical heat fluxes including those on microroughened surfaces (Fig. 8). On the plates with the boiling surface facing upwards, burnout occurred at heat fluxes that were up to 2.2 times lower than those on plates with the boiling surface facing downwards. This is explained by a rapid growth of vapor bubbles and their merging into films in the near
Bottom
104 3.5 × 103 0.1
1
10
30 ΔT, °C
Fig. 8. Water boiling curves for different orientations of the boiling surface: arrows indicate critical heat flux levels.
wall region of plates with the boiling surface facing downwards due to a problem with bubble removal from the surface and the presence of an overheated liquid microlayer on the surface. CONCLUSIONS The results of the experimental investigation confirm that microstructured surfaces made by the deforming cutting method can be used for heat transfer enhancement during the boiling of heat carriers and the increasing of critical heat fluxes. The technology offers high output, enables us to produce a wide range of sizes of microstructured surface, can be implemented using standard metal-cutting equipment, and has a number of other advantages over intricate and expensive technologies for the manufacture of porous, nano-, or microroughened materials. It was found that, for distilled water boiling at atmospheric pressure on these surfaces with 3D microfinning (having a interfin gap of 120–180 μm, a microfin height of 340–570 μm, and a longitudinal pitch of 240–400 μm), heat transfer increased by a maximum factor of 4 to 5 as compared with that on smooth surfaces. Critical heat fluxes increase by a factor of up to 6 as compared with boiling on smooth surfaces. HIGH TEMPERATURE
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Translated by T. Krasnoshchekova
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