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Enhancement of critical heat flux using spherical porous bodies in saturated pool boiling of nanofluid
Applied Thermal Engineering 144 (2018) 219–230
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Research Paper
Enhancement of critical heat flux using spherical porous bodies in saturated pool boiling of nanofluid Shoji Moria, Fumihisa Yokomatsua, Yoshio Utakab,c,
T
⁎
a
Department of Chemical Engineering Science, Graduate School of Engineering, Yokohama National University, 79-5, Tokiwadai, Hodogaya-ku, Yokohama, Kanagawa 240-8501, Japan b School of Mechanical Engineering, Tianjin University, No. 135 Yaguan Road, Tianjin Haihe Education Park, Tianjin 300354, China c Key Laboratory of Efficient Utilization of Low and Medium Grade Energy (Tianjin University), Ministry of Education of China, Tianjin University
H I GH L IG H T S
is enhanced by a nanofluid and spherical porous bodies. • CHF proposed method can improve CHF of heated surface with curvature. • The was restricted by spherical porous body in a saturated boiling pool of nanofluid. • Dryout • Liquid was supplied by spherical porous bodies to the nanoparticle-deposited layer.
A R T I C LE I N FO
A B S T R A C T
Keywords: Critical heat flux Saturated pool boiling Porous beads Nanofluid Nanoparticle deposition
One strategy to address severe nuclear accidents is the in-vessel retention (IVR) of corium debris. IVR consists of the external cooling of the reactor vessel to remove the decay heat from the molten core through the lower head of the vessel. However, heat removal is limited by the occurrence of the critical heat flux (CHF) condition at the outer surface of the reactor vessel. Therefore, we propose a CHF enhancement technique in a saturated pool boiling by the attachment of a honeycomb porous plate (HPP) on the heated surface. However, the reactor vessel on which to install the HPP exhibits curvature, so the key to realizing IVR depends on the placement of the HPP on the curved surface of the reactor vessel. Accordingly, we propose an approach using porous cellulose beads and a nanofluid. Consequently, for the combination of the nanofluid (TiO2, 0.1 vol%) and spherical porous bodies, the CHF is demonstrated to be enhanced by up to a maximum factor of two compared to that of a plain surface of distilled water.
1. Introduction It is a great challenge to ensure the safety of nuclear power plants during severe accidents. One of the major concerns is the overheating of the reactor core, which may potentially create a nuclear crisis if the cooling of the core fails. In-vessel retention (IVR) is a method that is gaining attention for heat removal in core-melt accidents. It consists of the external water cooling of the reactor vessel to remove decay heat from the molten core through the lower head of the vessel. Unfortunately, heat removal by a boiling substance is limited by the critical heat flux (CHF) where the heat transfer rate drops drastically. The enhancement of the CHF is a great concern for increasing the capability of IVR, which will be implemented in many light water reactors [1–4].
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Numerous studies have proposed novel methods for improving the CHF in a pool boiling under saturated atmospheric conditions [5]. Table 1 summarizes studies on CHF enhancement of flat heaters in saturated pool boiling for water and water based nanofluid under atmospheric pressure condition. qCHF,max and qCHF,p indicate the critical heat flux of flat heater with and without surface modification, respectively. As shown in Table 1, there exist surface modifications such as the coating of a heat transfer surface by a porous layer, integrated surface structures such as channels, fins, and nanotubes, which have shown a significant enhancement in the CHF. Based on recent developments in nanotechnology, the use of a nanofluid instead of pure water has shown promising results in enhancing the CHF. The pioneer of this novel method was suggested by You et al. (2003) [6]. They reported that the CHF can be enhanced by a factor of
Corresponding author: School of Mechanical Engineering, Tianjin University, No. 135 Yaguan Road, Tianjin Haihe Education Park, Tianjin 300354, China. E-mail address: [email protected] (Y. Utaka).
https://doi.org/10.1016/j.applthermaleng.2018.08.047 Received 12 June 2018; Received in revised form 16 August 2018; Accepted 17 August 2018 Available online 18 August 2018 1359-4311/ © 2018 Elsevier Ltd. All rights reserved.
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Nomenclature A I λ h q“ qCHF S T Tsat V
Δx1 Δx2
area of heater surface [m2] current [A] thermal conductivity [W/(mK)] heat transfer coefficient [W/(m2K)] heat flux [W/m2] critical heat flux [W/m2] contact area ratio [-] temperature [K] saturated temperature of water [K] voltage [V]
length of one side of heater [m] length of other side of heater [m]
Greek symbols
ρ γ
density [kg/m3] thermal activity [m·
J ] m2K·S0.5
Subscripts sat
saturation
3.2. Copper block heater
about two compared to that of pure water with a low concentration of Al2O3/water nanofluid varying from 0 to 0.05 g/L. Kwark et al. (2012) [7] conducted an experiment using a low water/alumina nanoparticle concentration (≤1 g/L) and found that there was no further CHF enhancement with concentrations higher than 0.025 g/L. Nanoparticles deposited on the heated surface during vigorous boiling offer a good wettability and capillarity [8]. Therefore, it can retard the spreading of the dry region and, as a result, increase the CHF. In contrast, the CHF enhancement of a flat heater using a honeycomb porous plate (HPP) has also been experimentally shown to be more than approximately twice that of the plain surface under atmospheric and saturated pool boiling conditions [9–14]. This is attributed to the HPP, which provides an automatic liquid supply because of the capillary action and the reduction in the vapor escape flow resistance because of the separation of the liquid and vapor flow paths. By the enhancement of the CHF to improve the capability of the IVR by integrating these two elements, the HPP attachment and nanofluid as a working fluid have been proven. However, the reactor vessel on which to install the HPP has curvature, so the key to realizing IVR depends on the placement of HPPs on the curved surface of the reactor vessel.
The heat flux was supplied to the boiling surface through a copper cylinder using a cartridge electric heater, which was inserted into the bottom of the copper cylinder, and the cartridge heaters were controlled by an AC voltage regulator. The heat loss from the sides and bottom of the copper cylinder was reduced using a ceramic fiber insulation material. The top horizontal surface of the copper cylinder with a diameter of 30 mm was smooth and was used as the heat transfer surface in the experiment. Three sheathed thermocouples with an outer diameter of 1 mm were inserted horizontally into the centerline of the copper cylinder. The thermocouples (TC1, TC2, and TC3 shown in Fig. 2(a)) in the copper cylinder were set axially apart by 5 mm. The closest thermocouple was located 10-mm below the boiling surface. These thermocouples were calibrated using a platinum resistance thermometer. The wall temperature and wall heat flux were calculated by applying Fourier’s Law, where the thermal conductivity of the copper was evaluated at the arithmetic averaged temperature of TC1, TC2, and TC3, and the linearity of the data was confirmed.
2. Proposed method for improving chf of heated surface with curvature
3.3. ITO heater
In order to address the issue stated above, we propose a method using spherical porous bodies fixed by a stretchable metal net, which can guarantee contact with the curved heat transfer surface, as shown in Fig. 1. Fig. 1 shows a schematic of our idea for installing porous bodies on the curved heated surface. The spherical porous body can deform to keep contact with the heated surface having curvature. Note that the size and installation pitch of the spherical porous body are significantly different from those used in the present study. Therefore, the main objective of this study is to investigate the effect of spherical porous bodies on the CHF in a boiling pool of nanofluid.
Fig. 2(b) depicts the ITO heater used in the present study. This device was fabricated by vacuum-depositing a 250-nm-thick ITO film on a sapphire substrate (1 × 40 × 40 mm). The heating area of the ITO unit was 20 × 20 mm. The heater was installed with the ITO film facing upward and, in order to improve wettability, the film was coated with a 100-nm-thick layer of TiO2 because ITO is hydrophobic. Cr (30-nmthick) and Au (200-nm-thick) electrodes were deposited sequentially on the sapphire substrate and connected to an AC power supply to control the heat flux at the surface. An AC supply was used rather than DC because it was found that the ITO heater installed facing upward, to be in contact with the water and heated by a DC power supply, was often damaged as a result of electrochemical reactions. The heating cycle frequency of the AC power supply had to be as high as possible to not affect the primary bubble generation. Thus, in the present study, the heating frequency was set to 1000 Hz based on a consideration of the frequency of the primary bubble detachment. Taking into account the thermal activity value, γ = δ ρCp k [15], a thickness of 1 mm was selected for the sapphire glass (giving γ = 9.3) so that the heating conditions would be as close as possible to those associated with copper block heating. Moreover, the effect of the ITO heater and copper cylinder heater on CHF was considered experimentally. As a result, it is confirmed that the difference of the CHF in the two kinds of heaters was not so large. The temperature distribution on the heated ITO surface was
3. Experimental apparatus and procedure 3.1. Experimental apparatus Schematics of the pool boiling apparatus are illustrated in Fig. 2. Fig. 2(a) presents a conventional pool boiling experimental apparatus incorporating a copper block heater, while Fig. 2(b) presents the experimental setup in which an ITO heater is used to measure the temperature distribution of the heated surface with a high-speed IR camera. The main vessel, which is made of Pyrex glass, has an inner diameter of 87 mm and height of 500 mm. The pool container was filled with distilled water or a nanofluid to a height of approximately 60 mm above the heated surface.
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Table 1 Summary of studies on CHF enhancement of flat heaters in saturated pool boiling for water and water based nanofluid under atmospheric pressure condition. Water Reference
Information for surface modification
qCHF,max [MW/m2]
qCHF,max/qCHF,p [-]
Chen et al. (2009) [17]
Nanowires are fabricated on Si nanowires: diameter; 20–300 nm, length; 40–50 µm Cu nanowire: diameter; 200 nm, length; 40–50 µm Heater size:10 mm × 10 mm Sintered pure copper woven mesh screens (pore size: 119.2–232.2 μm, wire diameter: 56–191 μm) Heater size:8 mm × 8 mm(square copper block) Array of micro-scaled posts by the MEMS technique: (D: 21–26 μm, height: 6 μm, interval: 50 μm) ZnO nanorod: height was several hundred nanometers to over 1 μm, with a mode around 800–900 nm Heater size:15 mm × 10 mm (Ti thin film, 0.1 μm in thickness) Effective diameter: Silica nanofluid: 34 ± 10 nm Diamond nanofluid: 173 ± 10 nm No surfactant Concentration of silica and diamond nanoparticles: 0.1 vol% and 0.01 vol%, respectively nanofluid (Silica or diamond in water) Heater size:30 mm × 10 mm (ITO heater was deposited onto 0.4-mm-thick sapphire) Si nanowire array Height of SiNW: 16, 32, 59, and 122 μm Heater size:5 mm × 5 mm, 10 mm × 10 mm, 15 mm × 15 mm, 20 mm × 20 mm (silicon chip, 0.5 mm in thickness)
2.26
2.3
3.60
2.4
2.33
2.1
1.95
2.0
2.24 (5 mm × 5 mm heater)
2.8 (5 mm × 5 mm heater)
Li and Peterson (2010) [18]
Kim et al. (2010) [19]
Gerardi et al. (2011) [20]
Lu et al. (2011) [21]
1.51 (10 mm × 10 mm heater) 1.25 (15 mm × 15 mm heater) 1.26 (20 mm × 20 mm heater)
2.2 (10 × 10 mm –heater) 2.7 (15 mm × 15 mm heater) 2.8 (20 mm × 20 mm heater)
Yao et al. (2011) [22]
Li et al. (2011) [23] Zhang and Kim (2012) [24]
Chu et al. (2012) [25]
Cooke and Kandlikar (2012) [26]
Ahn et al. (2014) [27]
Kim et al. (2014) [28] Rahman et al. (2014) [29]
Xu et al. (2015) [30]
Cu and Si nanowire Height of CuNW: 2, 5, 10, and 20 μm Height of SiNW: 20 and 35 μm Heater size:10 mm × 10 mm multiscale modulated porous structures ∅8 mm(copper cylinder) Alumina nanoporous-structure (smaller pore diameter: approximately 100 nm, larger pore diameter: approximately 500 nm) Heater size:0.785 cm2(aluminum 6061 alloy heater) Micro-pillar arrays: Height: 10 or 20 μm Diameter: 5, 10 μm Spacing: 5, 10, 15 μm Heater size:20 mm × 20 mm Open microchannel Channel width: 197–400 μm, fin width: 200–300 μm, channel depth: 100–400 μm Heater size:10 mm × 10 mm Reduced grapheneoxide (RGO) nanoparticle Primary particle size: 47 nm Concentration of nanoparticles: 0.0001–0.001 wt% Nanofluid (reduced grapheneoxide in water) Heater size:15 mm × 10 mm(0.12-mm-thick-Pt heater deposited onto a silicon substrate) Si nano-wire Heater size:10 × 5mm Biotemplated nanofabrication using tobacco mosaic virus (TMV) TM (300-nm-long, 18-nm-diameter cylindrical structure) is formed on the microstructure Heater size:10 mm × 10 mm(silicon substrate) Composite porous surface electrochemical fabrication technique (macro pores above 200 μm in diameter, micro pores around 2 μm in diameter, and dendritic structure around 400 nm in diameter) Heater size: ∅12 mm(copper block)
1.34
2.0
4.35
3.1
2.55
2.0
2.08
2.7
2.50
2.0
1.54
2.0
2.03
2.3
2.57
3.4
2.42
2.1
(continued on next page)
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Table 1 (continued) Water Reference
Information for surface modification
qCHF,max [MW/m2]
qCHF,max/qCHF,p [-]
Mori et al. (2015) [12]
Honeycomb porous plate on a nanoparticle-deposited surface Vapor escape channel width: 1.4 mm Wall thickness of the grid: 0.45 mm Aperture ratio: 0.55 Height of the honeycomb porous plate: 1 mm Heater size: ∅10, 30, 50 mm(copper cylinder) Square micropillars (width a = 10 μm and height h = 12.75 μm) etched onto a 650-μmthick silicon substrate. Heater size:10 mm × 20 mm titanium thin-film heater was patterned on the electrically insulated backside of the 5 × 5 cm2 silicon samples The micro-pillar surface (diameter(μm): 5–20, pitch(μm): 5–40, height(μm): 10–40) Heater size: The platinum film heater (15 × 10 mm) Honeycomb porous plate with TiO2 nanofluid Vapor escape channel width: 1.4 mm Wall thickness of the grid: 0.45 mm Aperture ratio: 0.55 Height of the honeycomb porous plate: 1 mm Nanofluid (TiO2 in water) Heater size: ∅ 30 mm (copper cylinder) Microchannel with sintered surface (300,500,and 762 μm channel width) Heater size:10 mm × 10 mm micro/nano hierarchically structured surfaces The platinum film heater (Heater size:15 × 10 mm) graphene oxide colloids GO colloids The platinum film heater (Heater size:15 × 10 mm) porous artery structure Porous structure thickness (mm):2.0 Porous structure mesh count (μm):20–30, 40–60, 100–120 Artery width (mm): 1.0 Artery depth (mm): 1.0, 1.5, 2.0 Fin width (mm): 1.2 Heater size: ∅10 mm(copper cylinder) Honeycomb porous plate, metal solid structure, and nanofluid nanofluid Al2O3 in water or water Heater size: ∅50 mm(copper cylinder) nanoporous hydrophilic surface layers (particle diameter(nm): 20–100)
3.06 (at ∅10 mm) 2.08 (at ∅50 mm)
2.0
2.11
2.1
2.2
3.1
3.2
3.2
4.2
3.4
2.6
2.8
1.8
2.4
6.1
3.9
3.1
3.1
1.98
2.1
Dhillon el al. (2015) [31]
Kim et al.(2015) [32]
Mori et al.(2016) [11]
Jaikumar and Kandlikar (2016) [33,34] Moon et al.(2016) [35] Kim et al.(2016) [36]
Bai et al.(2016) [37,38]
Aznam et al.(2016) [39]
Tetreault-Friend et al. (2016) [40]
Fig. 1. Schematic diagram of the proposed method to install spherical porous bodies at the bottom of a nuclear reactor vessel.
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viewing the underside of the film accurately reflects the temperature on the top of the ITO heater surface, and the difference in the temperatures at the bottom of the ITO heater and the top surface temperature of the TiO2 was < 0.05 K even under high heat flux condition of 3 MW/m2. A sapphire plate is almost transparent for wave length of 2 to 4 μm. The transmittance is more than 80%. 3.4. Preparation of Water-based nanofluid TiO2 nanoparticles (Aeroxide TiO2 P 25, Aerosil) were selected as the test nanoparticles. The mean particle diameter supplied by the company was approximately 21 nm. For preparation, the TiO2 nanoparticles were dissolved in 300 ml of distilled water. Then, the nanofluid was dispersed in an ultrasonic bath for 2 h. In the present study, the nanoparticle concentration in the water-based nanofluid was prepared with 4.0 g/L (1.0 wt%, 0.26 vol%). The reason to choose TiO2 nanoparticle and the concentration of 4.0 g/L is that we have got significant CHF enhancement in our previous study [11], although these selection may not be the best for the CHF enhancement. Moreover, although no additives such as surfactants or dispersants were used to stabilize the nanoparticle suspension, it has proved to be stable. Analysis of particle diameter distribution before and after the experiment, using a Laser Diffraction Particle Size Analyser (SALD-7000) from Shimadzu. The mean particle diameter were approximately 200 nm before and after the experiment. Furthermore, sedimentation of nanoparticles was not detected.
(a) Copper block heater
3.5. Spherical porous body Fig. 3 presents the spherical porous body used in the present study, and a micrograph of its structure is shown on the right-hand side of the figure. The spherical porous body, which is commercially available, was used as a carrier for medicines such as fragrance and fungicide. The spherical porous body was made of cellulose, and its diameter was approximately 2.5 mm. The pore diameter distribution of the spherical porous body was measured using mercury penetration porosimetry, which exhibited a peak at approximately 80 μm, as shown in Fig. 4. The porosity of the spherical porous body was 93%. The spherical porous body was attached to the top of the boiling surface by pushing against the surface using a circular jig and stainlesssteel wire, 0.5 mm in diameter, as shown in Fig. 5. Note that cellulose porous bodies cannot be used for actual IVR because of thermal resistance and radiation resistance. The primary purpose of this paper is to present the idea of CHF improvement by the spherical porous body and nanofluid.
5
3.6. Temperature measurement using IR camera The output U of the infrared camera is given by the Eq. (1) when using optical setup with reference plane shown in Fig. 2 (b) [6].
U=εf (Tw ) + (1−ε ) f (Tamb)
(1)
where, f (T) is the output of the infrared camera in the case of temperature measurement for blackbody. Tw, Tamb, and ε are the temperature of the measurement surface, the ambient temperature, and the emissivity, respectively. In order to prevent the reflected image of the infrared camera itself from appearing, the infrared camera was installed with inclination of 5°from the measurement object as shown in Fig. 2 (b). Temperature of reference plane with black spray(ε = 0.94) was measured by a thermocouple. The procedure for correcting temperature at the bottom of ITO heater is as follows: The pool container was filled with saturated water and assumed the
(b) ITO heater Fig. 2. Schematic of experiment apparatus.
obtained using an IR high-speed camera with spatial and time resolutions of 130 μm and 2 ms, respectively. A similar technique was previously demonstrated by Nakamura [12]. The use of a relatively thin ITO film (250 nm) implies that data acquired from the IR camera when
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Fig. 3. Spherical porous bodies. (dp = 2.5 mm, d = 100 μm, ε = 93%).
Pore volume [ml/mg]
the measurements were performed at the steady state. A sheathed heater was installed near the heated surface in the vessel to confirm the saturation temperature (100 ˚C). The steady state was considered reached when the temperatures did not change by more than 0.25 K for at least 5 min. When burnout occurred, the heating was immediately stopped to prevent the heater and thermocouples from being damaged. The last quasi-steady-state heat flux was then measured before the transition to film boiling and was taken as the CHF. 4. Experimental results and discussion
Pore diameter [μm]
As we stated in the introduction, in order to apply the CHF improvement technique using an HPP to cool the reactor vessel with a curved surface in IVR, it is important to consider the effect of a gap between the HPP and heated surface on the CHF. Because a flat HPP cannot be placed completely in close contact with the heated surface with curvature. The HPP, which is commercially available, was used as a filter for purifying exhaust gases from combustion engines. The constituent ingredients are CaOAl2O3 (30–50 wt%), fused SiO2 (40–60 wt %), and TiO2 (5–20 wt%). The vapor escape channel width (cell width), the wall thickness of the grid, the aperture ratio (ratio of the open area to total area), the height of the honeycomb porous plate, and the diameter of the honeycomb porous plate are 1.3 mm, 0.4 mm, 0.55, 1.0 mm, and 30.0 mm, respectively. Fig. 6 illustrates the effect of a gap on the CHF. Two stainless-steel wires were placed between the heat transfer surface and an HPP (5-mm thickness), as shown in the images of Fig. 6, and the gap was controlled
Fig. 4. Pore radius distribution of test spherical porous bodies.
bottom temperature of the ITO film was 100° C. And the emissivity was adjusted on analysis software of the infrared camera so that the ITO film temperature obtained from the IR camera becomes 100° C. 3.7. Experimental procedure Pool boiling experiments were carried out using a water-based nanofluid or distilled water as working fluids under saturated conditions at atmospheric pressure. The heated surface was polished using waterproof sandpaper of 2000 grit. Distilled water of 700 ml was boiled using a heater installed in the test vessel for 30 min to remove as much dissolved gas as possible. At each run, the heat flux was increased in increments of approximately 0.1 MW/m2 until burnout occurred. All
Fig. 5. Attachment of spherical porous bodies on the heated surface.
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qCHF[MW/m2]
2
Bare Surface HPP(5mm) with gap Water P=0.1MPa
1
0 0
0.1
0.2
0.3
0.4
0.5
The gap between heated surface and HPP [mm] Fig. 6. The effect of the gap between the heated surface and an HPP on q”CHF.
by changing the diameter of the stainless-steel wire to 0.05, 0.1, and 0.3 mm. Repeatability was confirmed at least three times. As it can be seen from this figure, the CHF does not improve when the gap is 0.3 mm. The contact between the heated surface and HPP is an important factor for CHF enhancement, although it is very difficult to bring the HPP completely into close contact with the curved heat transfer surfaces. Therefore, this result signifies that the CHF of a curved surface cannot be enhanced by the flat HPP proposed in the previous study, because of the existence of the gap between the flat HPP and curved heated surface. Accordingly, we proposed an approach using the spherical porous body shown in Fig. 1. Fig. 7 depicts the experimental results by varying the number density of spherical porous bodies on the heated surface. A schematic of the heated surface with spherical porous bodies is also illustrated in the figure. Moreover, this figure includes the CHF in a saturated boiling pool of not only nanofluid but also distilled water. Comparing the case of distilled water and nanofluid with S = 0, the
2 q"CHF [MW/m ]
2
1 Nanofluid (Copper cylinder heater) Distilled water (Copper cylinder heater) Nanofluid (ITO heater)
0 0
10
20
30
40
50
60
Contact area ratio S [%] Fig. 7. Relation between q”CHF and contacted area ratio S [%] in saturated pool boiling of nanofluid and distilled water.
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occurs when the hovering time of large coalesced bubbles increases accidentally, resulting in burnout. In order to consider the effect of spherical porous bodies on the dryout and the CHF, the experiments have been performed at the same heat flux of Fig. 9. Fig. 10 presents IR images of the heated surface with spherical porous bodies in a saturated boiling pool of nanofluid, and the heat flux is the same as in Fig. 9. A spherical porous body is installed on the heat transfer surface in the areas indicated by the dashed line. As is clear from this figure, when the spherical porous body is installed on the heat transfer surface, dryout does not occur at all, although burnout does occur in the case without spherical porous bodies, as shown in Fig. 9. When a spherical porous body is installed, a temperature distribution is formed on the heat transfer surface. The high-temperature portion of the figure indicates that the spherical porous body is firmly in contact with the heat transfer surface. This is because the dryout may occur at the contact area between the spherical porous body and heated surface. However, based on temperature change with time, the temperature at the contact area did not change so much even large temperature increase at the non-contact area was observed as shown in Fig. 11. The reason for this is not clarified yet, but this may indicate the dryout at the contact area does not grow so much because porous body transports water to the dryout region. Fig. 11(a) and (b) depict IR images of the heated surface with the spherical porous body in a saturated boiling pool of nanofluid under the burnout occurrence condition (S = 20%, nanofluid, 1.78 MW/m2). As shown in Fig. 11, even under these conditions, a reversible dry spot is observed. This is because nanoparticles were deposited on the heated surface to improve the wickability. Nanoparticle deposition was
CHF of the nanofluid improves by a factor of about 1.5 compared to distilled water. The CHF enhancement for the case of the nanofluid is attributed to the surface wettability, surface roughness, and capillary wicking effect because of nanoparticle deposition on the heated surface, as reported in previous papers [7,11–13]. It is also interesting that the CHF with the spherical porous body using the nanofluid is enhanced up to an approximate factor of two compared to that of a plain surface of distilled water for the cases of S = 7.5% to 30%, although the CHF hardly improves in the case of distilled water. Because the capillary force of the spherical porous body is small because of large pore diameter (100 μm). Therefore CHF cannot be enhanced only by the spherical porous body in distilled water. While, for the case of the spherical porous body in the nanofluid, a thin porous layer caused by the nanoparticle deposition is formed between the spherical porous body and the heated surface. Therefore, the CHF may be enhanced by the similar effect of the two-layer-structured HPP proposed by Mori et al. [10]. According to the capillary limit model [9], the CHF is enhanced when the contacted area of the porous body with the heated surface increases. However, no increase in the CHF was observed in the range of 7.5% ≤ S < 30%. The reason why there is no optimal S to maximize the CHF has not yet been clarified. Nevertheless, for the range of 7.5% ≤ S < 30%, the CHF is enhanced roughly by a factor of two compared to that of the plain surface of distilled water. Therefore, the spherical porous bodies connected in a manner similar to beads can be applied to the CHF enhancement. The main improvement factor on the CHF is that liquid was supplied from the spherical porous body to the nanoparticle-deposited layer on the heated surface by the capillary force. Note that the capillary force used by the spherical porous body in the present study is small because the pore diameter of the spherical porous body itself is as large as 100 μm. The CHF enhancement mechanism may be a similar phenomenon of enhancement to the twolayer-structured HPP proposed by Mori et al. [10]. For S ≥ 30%, the CHF tends to decrease with S, as shown in Fig. 7. This is because the number density of spherical porous bodies is too large for the vapor to escape from the heat transfer surface, in other words, smaller vapor escape area causes a significant increase in the pressure drop for the vapor flow in the vapor escape area, resulting in CHF reduction. Fig. 8(a) and (b) present the boiling curves and HTCs in the nanofluid for the cases with and without surface modification by the spherical porous bodies. The arrows in Fig. 8 correspond to the CHF condition. As we pointed out in Fig. 7, the CHFs of S = 7.5% and 28% are higher than that of S = 0 (plain surfaces) of the nanofluid, while the CHF of S = 48% is lower than that of S = 0. Regarding the HTCs, the HTC does not change significantly from S = 7.5% to 28% except for S = 48%. Note that the HTC under burnout conditions for plain surfaces of distilled water is approximately 34 kW/(m2·K), which is roughly the same HTC from S = 7.5% to 28% of distilled water. In order to consider the thermo-fluid phenomena near the spherical porous body, an ITO film with a thickness of 250 nm was vapor-deposited on sapphire glass and used as a transparent heat transfer surface, and a temperature distribution was recorded from the lower side of the heat transfer surface using an IR camera, as shown in Fig. 2(b). Fig. 9(a) and (b) indicate the results of the IR data in the boiling pool of nanofluid under the burnout condition of S = 0% (q = 1.57 MW/m2). The large wall temperature changes frequently because of reversible dry spots within a wide region on the heated surface, as shown in Fig. 9(a). This is because a large coalesced bubble forms on the heated surface and departs periodically from the heated surface, and the wettability of the heated surface improves by the deposition of a nanoparticle. In addition, the frequency of temperature change corresponds to that of the departure frequency of large coalesced bubbles, whereas, as shown in Fig. 9(b), an irreversible dry spot
2
q" [MW/m2]
2
Nanofluid S=0% S = 7.5 % S = 28 % S = 48 %
1.5 MW/m 2 1.9 MW/m 2
1.9 MW/m
1 2
1.0 MW/m 0 0
20
40
60
T [K]
(a) Boiling curves 50
2
h [kW/(m K)]
40 30
Nanofluid S=0% S = 7.5 % S = 28 % S = 48 %
20 10 0 0
20
40
60
T [K]
(b) Heat transfer coefficients Fig. 8. Boiling curves and heat transfer coefficients with or without spherical porous bodies in saturated pool boiling of the nanofluid.
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(a) Reversible dry spot
(b) Irreversible dry spot
Fig. 9. IR image of bare surface in saturated pool boiling of nanofluid under the burnout condition, (S = 0%, nanofluid, 1.57 MW/m2).
spherical porous bodies. This is probably because the nanoparticle deposition layer supplies liquid to the heat transfer surface with an intense capillary force, and at the same time, the spherical porous body continues to supply liquid to the nanoparticle deposition layer around the porous body. The combination of the spherical porous body and nanoparticle-deposited layer is similar to the two-layered HHP proposed by Mori et al. [10]. The significance of making the HPP into a two-layer structure is as follows: (1) An HPP simply attached to a heated surface should have very fine pores to supply water to the heated surface because of the strong capillary action, and the HPP should be as thin as possible to decrease the frictional pressure drop caused by the internal water flow; (2) the other HPP, stacked on top of the thin HPP, must be structured to hold a sufficient amount of water over the plate. Therefore, principles (1) and (2) of the HHPs may correspond to the nanoparticle-deposited layer and spherical porous bodies, respectively, in the present study. Based on obtained results in the present study, cooling technology using spherical porous bodies in nanofluid has potential to be applied to IVR. However, further investigations of nanofluid with spherical porous bodies is needed in a wider variety of scenarios as different inclination angle of the heater surface, higher system pressure, concentrations of nanofluid, and flow boiling regime. 5. Conclusions Fig. 10. IR image using the spherical porous bodies on the heated surface in saturated pool boiling of nanofluid (S = 20%, nanofluid, 1.57 MW/m2).
In order to cool a heated curved surface, we proposed a method using a spherical porous body connected in a manner similar to beads. As a result, the following conclusions were obtained.
confirmed mainly on the location where spherical porous bodies were not installed after the experiments in nanofluid. However, burnout occurred once the irreversible dry spot formed. Even under the burnout condition, the size of the dryout is small compared to that without
1. For the combination of a nanofluid (TiO2, 0.1 vol%) and spherical porous bodies, the CHF is enhanced up to a factor of about 1.9 compared to the case of a plain surface in a boiling pool of distilled
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(a) Reversible dry spot
(b) Irreversible dry spot Fig. 11. IR image using the spherical porous bodies on the heated surface in saturated pool boiling of nanofluid under the burnout condition. (S = 20%, nanofluid, 1.78 MW/m2).
water in the range of 7.5% ≤ S < 30% 2. The area of temperature increase is small in the case of the spherical porous body in a saturated boiling pool of nanofluid. 3. It is considered that the CHF is enhanced by supplying the liquid retained by the spherical porous body to the nanoparticle-deposited layer.
characteristics of spherical porous bodies (size, pore diameter, and porosity), disposition of the spherical porous bodies, kind of nanofluid, and concentration of nanofluid on the CHF.
In order to enhance the CHF, we need to consider the effect of the
The present study was supported in part by the Futaba Foundation.
Acknowledgements
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Appendix Uncertainty analysis The individual standard uncertainties were combined to obtain the estimated standard deviation of the results, which is calculated using the law of the propagation of uncertainty [16]. For the case of the copper block heater, the uncertainties in the heat flux q, superheat ΔTsat, and heat transfer coefficient (HTC) h were evaluated using the following equations: 2
2
Δq =
2
⎜
⎟
⎜
⎟
2
Δ(ΔTsat ) =
⎜
⎟
2
(A.1) 2
2
⎛⎜ ∂ (ΔTsat ) Δq⎞⎟ + ⎛ ∂ (ΔTsat ) ΔT1 ⎞ + ⎛ ∂ (ΔTsat ) Δδ2 ⎞ + ⎛ ∂ (ΔTsat ) Δλ ⎞ ⎝ ∂λ ⎠ ⎝ ∂T1 ⎠ ⎝ ∂δ2 ⎠ ⎝ ∂q ⎠ ⎜
⎟
⎜
⎟
(A.2)
2
2
Δh =
2
⎛ ∂q Δλ ⎞ + ⎛ ∂q Δδ1 ⎞ + ⎛ ∂q ΔT1 ⎞ + ⎛ ∂q ΔT2 ⎞ ⎝ ∂λ ⎠ ⎝ ∂δ1 ⎠ ⎝ ∂T1 ⎠ ⎝ ∂T2 ⎠
⎛ ∂h Δ(ΔTsat ) ⎞ + ⎜⎛ ∂h Δq⎟⎞ ⎝ ∂ (ΔTsat ) ⎠ ⎝ ∂q ⎠ ⎜
⎟
(A.3)
where T1 and T2 are the temperatures at TC1 and TC2, respectively, λ is the thermal conductivity of copper evaluated at the arithmetic mean of T1 and T2, δ1 is the distance between TC1 and TC2, and δ2 is the distance between TC1 and the boiling surface. For the case of the ITO heater, the heat flux was determined from data measured by a voltmeter, ammeter, and the heated surface area, A. The uncertainties of the heat flux q and area were respectively obtained as follows:
Δq =
2 2 2 ⎛ ∂q ⎞ ΔV 2 + ⎛ ∂q ⎞ ΔI 2 + ⎛ ∂q ⎞ ΔA2 = ⎝ ∂V ⎠ ⎝ ∂I ⎠ ⎝ ∂A ⎠
ΔA =
⎛ ∂A ⎞ Δx12 + ⎛ ∂A ⎞ Δx22 ⎝ ∂x1 ⎠ ⎝ ∂x2 ⎠
2
⎜
⎟
I 2 V 2 VI 2 ⎛ ⎞ ΔV 2 + ⎛ ⎞ ΔI 2 + ⎛ ⎞ ΔA2 ⎝ A⎠ ⎝ A⎠ ⎝ A2 ⎠
(A.4)
2
⎜
⎟
(A.5)
where Δx1 and Δx2 are the lengths of one side of the heater. From the measurement, the length of the heated surface was Δx1 = Δx2 = 0.1mm . Table 1 and Table 2 list examples of the relative uncertainties calculated using Eqs. (A.1)–(A.5). As shown in the tables, the relative uncertainties depend on the experimental conditions and tend to decrease with the heat flux. (See Table 3) Table 2 Relative uncertainties of the measured quantities for the case of copper block heater. q [MW/m2]
ΔT sat [K]
h [kW/(m2·K)]
Δq/q [%]
Δ(ΔT [%]
1.2 1.5 1.8 2.0
16 18 20 21
77 83 91 95
2.7 2.3 1.9 1.8
2.7 2.3 2.0 1.8
Δh/h [%]
sat)/ΔT sat
3.8 3.2 2.8 2.5
Table 3 Relative uncertainties of the measured quantities for the case of ITO heater. V [V]
I [A]
q [MW/m2]
Δ q [MW/m2]
Δ q/q [%]
10.1 80.1
1.3 7.9
0.03 2.0
0.002 0.041
5.7 3.1
[6] S.M. You, J.H. Kim, K.H. Kim, Effect of nanoparticles on critical heat flux of water in pool boiling heat transfer, Appl. Phys. Lett 83 (16) (2003) 3374–3376. [7] S.M. Kwark, R. Kumar, G. Moreno, S.M. You, Transient characteristics of pool boiling heat transfer in nanofluids, J. Heat Transfer 134 (5) (2012) 051015. [8] S.M. Kwark, R. Kumar, G. Moreno, J. Yoo, S.M. You, Pool boiling characteristics of low concentration nanofluids, Int. J. Heat Mass Transf. 53 (5–6) (2010) 972–981. [9] S. Mori, K. Okuyama, Enhancement of the critical heat flux in saturated pool boiling using honeycomb porous media, Int. J. Multiph. Flow 35 (10) (2009) 946–951. [10] S. Mori, N. Maruoka, K. Okuyama, Critical heat flux enhancement by a two-layer structured honeycomb porous plate in a saturated pool boiling of water, Int. J. Heat Mass Transf. 118 (2018) 429–438 (Supplement C). [11] S. Mori, S. Mt Aznam, R. Yanagisawa, K. Okuyama, CHF enhancement by honeycomb porous plate in saturated pool boiling of nanofluid, J. Nucl. Sci. Technol. (2015) 1–8. [12] S. Mori, S. Mt Aznam, K. Okuyama, Enhancement of the critical heat flux in saturated pool boiling of water by nanoparticle-coating and a honeycomb porous plate,
References [1] J. Buongiorno, L.W. Hu, G. Apostolakis, R. Hannink, T. Lucas, A. Chupin, A feasibility assessment of the use of nanofluids to enhance the in-vessel retention capability in light-water reactors, Nuclear Eng. Des. 239 (5) (2009) 941–948. [2] S.D. Park, S.W. Lee, S. Kang, S.M. Kim, I.C. Bang, Pool boiling CHF enhancement by graphene-oxide nanofluid under nuclear coolant chemical environments, Nuclear Eng. Des. 252 (2012) 184–191. [3] Y.P. Zhang, S.Z. Qiu, G.H. Su, W.X. Tian, Analysis of safety margin of in-vessel retention for AP1000, Nuclear Eng. Des. 240 (8) (2010) 2023–2033. [4] Q.T. Pham, T.I. Kim, S.S. Lee, S.H. Chang, Enhancement of critical heat flux using nano-fluids for Invessel Retention-External Vessel Cooling, Appl. Thermal Eng. 35 (2012) 157–165. [5] S. Mori, Y. Utaka, Critical heat flux enhancement by surface modification in a saturated pool boiling: a review, Int. J. Heat Mass Transf. 108 (2017) 2534–2557.
229
Applied Thermal Engineering 144 (2018) 219–230
S. Mori et al.
Int. J. Heat Mass Transf. 80 (2015) 1–6. [13] S. Mori, L. Shen, K. Okuyama, Effect of cell size of a honeycomb porous plate attached to a heated surface on CHF in saturated pool boiling, 2010 14th International Heat Transfer Conference, American Society of Mechanical Engineers, 2010, pp. 255–265. [14] S.M. Aznam, S. Mori, F. Sakakibara, K. Okuyama, Effects of heater orientation on critical heat flux for nanoparticle-deposited surface with honeycomb porous plate attachment in saturated pool boiling of water, Int. J. Heat Mass Transf. 102 (2016) 1345–1355. [15] M. Arik, A. Bar-Cohen, Effusivity-based correlation of surface property effects in pool boiling CHF of dielectric liquids, Int. J. Heat Mass Transf. 46 (20) (2003) 3755–3764. [16] B.N. Taylor, C.E. Kuyatt, Guidelines for evaluation and expressing the uncertainty of NIST measurement results, NIST Tech. Note 1297 (1993) 1–20. [17] R. Chen, M.-C. Lu, V. Srinivasan, Z. Wang, H.H. Cho, A. Majumdar, Nanowires for enhanced boiling heat transfer, Nano Lett. 9 (2) (2009) 548–553. [18] C. Li, G.P. Peterson, Geometric effects on critical heat flux on horizontal microporous coatings, J. Thermophys. Heat Transf. 24 (3) (2010) 449–455. [19] S. Kim, H.D. Kim, H. Kim, H.S. Ahn, H. Jo, J. Kim, M.H. Kim, Effects of nano-fluid and surfaces with nano structure on the increase of CHF, Exp. Therm Fluid Sci. 34 (4) (2010) 487–495. [20] C. Gerardi, J. Buongiorno, L.W. Hu, T. McKrell, Infrared thermometry study of nanofluid pool boiling phenomena, Nanoscale Res. Lett. 6 (1) (2011). [21] M.-C. Lu, R. Chen, V. Srinivasan, V.P. Carey, A. Majumdar, Critical heat flux of pool boiling on Si nanowire array-coated surfaces, Int. J. Heat Mass Transf. 54 (25–26) (2011) 5359–5367. [22] Z. Yao, Y.W. Lu, S.G. Kandlikar, Effects of nanowire height on pool boiling performance of water on silicon chips, Int. J. Therm. Sci. 50 (11) (2011) 2084–2090. [23] C.H. Li, T. Li, P. Hodgins, C.N. Hunter, A.A. Voevodin, J.G. Jones, G.P. Peterson, Comparison study of liquid replenishing impacts on critical heat flux and heat transfer coefficient of nucleate pool boiling on multiscale modulated porous structures, Int. J. Heat Mass Transf. 54 (15–16) (2011) 3146–3155. [24] B. Zhang, K.J. Kim, Effect of liquid uptake on critical heat flux utilizing a three dimensional, interconnected alumina nano porous surfaces, Appl. Phys. Lett. 101 (5) (2012) 054104. [25] K.-H. Chu, R. Enright, E.N. Wang, Structured surfaces for enhanced pool boiling heat transfer, Appl. Phys. Lett. 100 (24) (2012) 241603. [26] D. Cooke, S.G. Kandlikar, Effect of open microchannel geometry on pool boiling enhancement, Int. J. Heat Mass Transf. 55 (4) (2012) 1004–1013. [27] H.S. Ahn, J.M. Kim, M. Kaviany, M.H. Kim, Pool boiling experiments in reduced
[28]
[29]
[30] [31] [32]
[33]
[34]
[35]
[36]
[37] [38]
[39]
[40]
230
graphene oxide colloids, Part I - Boiling Charact. Int. J. Heat Mass Transf. 74 (2014) 501–512. B.S. Kim, S. Shin, D. Lee, G. Choi, H. Lee, K.M. Kim, H.H. Cho, Stable and uniform heat dissipation by nucleate-catalytic nanowires for boiling heat transfer, Int. J. Heat Mass Transf. 70 (2014) 23–32. M.M. Rahman, E. Olceroglu, M. McCarthy, Role of wickability on the critical heat flux of structured superhydrophilic surfaces, Langmuir ACS J. of Surf. Coll. 30 (37) (2014) 11225–11234. P. Xu, Q. Li, Y. Xuan, Enhanced boiling heat transfer on composite porous surface, Int. J. Heat Mass Transf. 80 (2015) 107–114. N.S. Dhillon, J. Buongiorno, K.K. Varanasi, Critical heat flux maxima during boiling crisis on textured surfaces, Nat. Commun. 6 (2015). S.H. Kim, G.C. Lee, J.Y. Kang, K. Moriyama, M.H. Kim, H.S. Park, Boiling heat transfer and critical heat flux evaluation of the pool boiling on micro structured surface, Int. J. Heat Mass Transf. 91 (2015) 1140–1147. A. Jaikumar, S.G. Kandlikar, Ultra-high pool boiling performance and effect of channel width with selectively coated open microchannels, Int. J. Heat Mass Transf. 95 (2016) 795–805. A. Jaikumar, S. Kandlikar, Pool boiling enhancement through bubble induced convective liquid flow in feeder microchannels, Appl. Phys. Lett. 108 (4) (2016) 041604. H.W. Moon, Y.J. Yoon, J.H. Park, B.-S. Myung, D.E. Kim, Dynamic wetting and boiling characteristics on micro-structured and micro/nano hierarchically structured surfaces, Exp. Therm Fluid Sci. 74 (2016) 19–26. J.M. Kim, J.H. Kim, S.C. Park, M.H. Kim, H.S. Ahn, Nucleate boiling in graphene oxide colloids: morphological change and critical heat flux enhancement, Int. J. Multiph. Flow (2016). L. Bai, L. Zhang, G. Lin, G. Peterson, Pool boiling with high heat flux enabled by a porous artery structure, Appl. Phys. Lett. 108 (23) (2016) 233901. L. Bai, L. Zhang, J. Guo, G. Lin, X. Bu, D. Wen, Evaporation/boiling heat transfer characteristics in an artery porous structure, Appl. Therm. Eng. 104 (2016) 587–595. S.M. Aznam, S. Mori, K. Okuyama, Critical heat flux enhancement in water-based nanofluid with honeycomb porous plate on large heated surface, 2016 24th International Conference on Nuclear Engineering, American Society of Mechanical Engineers, 2016 V005T015A036-V005T015A036. M. Tetreault-Friend, R. Azizian, M. Bucci, T. McKrell, J. Buongiorno, M. Rubner, R. Cohen, Critical heat flux maxima resulting from the controlled morphology of nanoporous hydrophilic surface layers, Appl. Phys. Lett. 108 (24) (2016) 243102.
Contents lists available at ScienceDirect
Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Research Paper
Enhancement of critical heat flux using spherical porous bodies in saturated pool boiling of nanofluid Shoji Moria, Fumihisa Yokomatsua, Yoshio Utakab,c,
T
⁎
a
Department of Chemical Engineering Science, Graduate School of Engineering, Yokohama National University, 79-5, Tokiwadai, Hodogaya-ku, Yokohama, Kanagawa 240-8501, Japan b School of Mechanical Engineering, Tianjin University, No. 135 Yaguan Road, Tianjin Haihe Education Park, Tianjin 300354, China c Key Laboratory of Efficient Utilization of Low and Medium Grade Energy (Tianjin University), Ministry of Education of China, Tianjin University
H I GH L IG H T S
is enhanced by a nanofluid and spherical porous bodies. • CHF proposed method can improve CHF of heated surface with curvature. • The was restricted by spherical porous body in a saturated boiling pool of nanofluid. • Dryout • Liquid was supplied by spherical porous bodies to the nanoparticle-deposited layer.
A R T I C LE I N FO
A B S T R A C T
Keywords: Critical heat flux Saturated pool boiling Porous beads Nanofluid Nanoparticle deposition
One strategy to address severe nuclear accidents is the in-vessel retention (IVR) of corium debris. IVR consists of the external cooling of the reactor vessel to remove the decay heat from the molten core through the lower head of the vessel. However, heat removal is limited by the occurrence of the critical heat flux (CHF) condition at the outer surface of the reactor vessel. Therefore, we propose a CHF enhancement technique in a saturated pool boiling by the attachment of a honeycomb porous plate (HPP) on the heated surface. However, the reactor vessel on which to install the HPP exhibits curvature, so the key to realizing IVR depends on the placement of the HPP on the curved surface of the reactor vessel. Accordingly, we propose an approach using porous cellulose beads and a nanofluid. Consequently, for the combination of the nanofluid (TiO2, 0.1 vol%) and spherical porous bodies, the CHF is demonstrated to be enhanced by up to a maximum factor of two compared to that of a plain surface of distilled water.
1. Introduction It is a great challenge to ensure the safety of nuclear power plants during severe accidents. One of the major concerns is the overheating of the reactor core, which may potentially create a nuclear crisis if the cooling of the core fails. In-vessel retention (IVR) is a method that is gaining attention for heat removal in core-melt accidents. It consists of the external water cooling of the reactor vessel to remove decay heat from the molten core through the lower head of the vessel. Unfortunately, heat removal by a boiling substance is limited by the critical heat flux (CHF) where the heat transfer rate drops drastically. The enhancement of the CHF is a great concern for increasing the capability of IVR, which will be implemented in many light water reactors [1–4].
⁎
Numerous studies have proposed novel methods for improving the CHF in a pool boiling under saturated atmospheric conditions [5]. Table 1 summarizes studies on CHF enhancement of flat heaters in saturated pool boiling for water and water based nanofluid under atmospheric pressure condition. qCHF,max and qCHF,p indicate the critical heat flux of flat heater with and without surface modification, respectively. As shown in Table 1, there exist surface modifications such as the coating of a heat transfer surface by a porous layer, integrated surface structures such as channels, fins, and nanotubes, which have shown a significant enhancement in the CHF. Based on recent developments in nanotechnology, the use of a nanofluid instead of pure water has shown promising results in enhancing the CHF. The pioneer of this novel method was suggested by You et al. (2003) [6]. They reported that the CHF can be enhanced by a factor of
Corresponding author: School of Mechanical Engineering, Tianjin University, No. 135 Yaguan Road, Tianjin Haihe Education Park, Tianjin 300354, China. E-mail address: [email protected] (Y. Utaka).
https://doi.org/10.1016/j.applthermaleng.2018.08.047 Received 12 June 2018; Received in revised form 16 August 2018; Accepted 17 August 2018 Available online 18 August 2018 1359-4311/ © 2018 Elsevier Ltd. All rights reserved.
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Nomenclature A I λ h q“ qCHF S T Tsat V
Δx1 Δx2
area of heater surface [m2] current [A] thermal conductivity [W/(mK)] heat transfer coefficient [W/(m2K)] heat flux [W/m2] critical heat flux [W/m2] contact area ratio [-] temperature [K] saturated temperature of water [K] voltage [V]
length of one side of heater [m] length of other side of heater [m]
Greek symbols
ρ γ
density [kg/m3] thermal activity [m·
J ] m2K·S0.5
Subscripts sat
saturation
3.2. Copper block heater
about two compared to that of pure water with a low concentration of Al2O3/water nanofluid varying from 0 to 0.05 g/L. Kwark et al. (2012) [7] conducted an experiment using a low water/alumina nanoparticle concentration (≤1 g/L) and found that there was no further CHF enhancement with concentrations higher than 0.025 g/L. Nanoparticles deposited on the heated surface during vigorous boiling offer a good wettability and capillarity [8]. Therefore, it can retard the spreading of the dry region and, as a result, increase the CHF. In contrast, the CHF enhancement of a flat heater using a honeycomb porous plate (HPP) has also been experimentally shown to be more than approximately twice that of the plain surface under atmospheric and saturated pool boiling conditions [9–14]. This is attributed to the HPP, which provides an automatic liquid supply because of the capillary action and the reduction in the vapor escape flow resistance because of the separation of the liquid and vapor flow paths. By the enhancement of the CHF to improve the capability of the IVR by integrating these two elements, the HPP attachment and nanofluid as a working fluid have been proven. However, the reactor vessel on which to install the HPP has curvature, so the key to realizing IVR depends on the placement of HPPs on the curved surface of the reactor vessel.
The heat flux was supplied to the boiling surface through a copper cylinder using a cartridge electric heater, which was inserted into the bottom of the copper cylinder, and the cartridge heaters were controlled by an AC voltage regulator. The heat loss from the sides and bottom of the copper cylinder was reduced using a ceramic fiber insulation material. The top horizontal surface of the copper cylinder with a diameter of 30 mm was smooth and was used as the heat transfer surface in the experiment. Three sheathed thermocouples with an outer diameter of 1 mm were inserted horizontally into the centerline of the copper cylinder. The thermocouples (TC1, TC2, and TC3 shown in Fig. 2(a)) in the copper cylinder were set axially apart by 5 mm. The closest thermocouple was located 10-mm below the boiling surface. These thermocouples were calibrated using a platinum resistance thermometer. The wall temperature and wall heat flux were calculated by applying Fourier’s Law, where the thermal conductivity of the copper was evaluated at the arithmetic averaged temperature of TC1, TC2, and TC3, and the linearity of the data was confirmed.
2. Proposed method for improving chf of heated surface with curvature
3.3. ITO heater
In order to address the issue stated above, we propose a method using spherical porous bodies fixed by a stretchable metal net, which can guarantee contact with the curved heat transfer surface, as shown in Fig. 1. Fig. 1 shows a schematic of our idea for installing porous bodies on the curved heated surface. The spherical porous body can deform to keep contact with the heated surface having curvature. Note that the size and installation pitch of the spherical porous body are significantly different from those used in the present study. Therefore, the main objective of this study is to investigate the effect of spherical porous bodies on the CHF in a boiling pool of nanofluid.
Fig. 2(b) depicts the ITO heater used in the present study. This device was fabricated by vacuum-depositing a 250-nm-thick ITO film on a sapphire substrate (1 × 40 × 40 mm). The heating area of the ITO unit was 20 × 20 mm. The heater was installed with the ITO film facing upward and, in order to improve wettability, the film was coated with a 100-nm-thick layer of TiO2 because ITO is hydrophobic. Cr (30-nmthick) and Au (200-nm-thick) electrodes were deposited sequentially on the sapphire substrate and connected to an AC power supply to control the heat flux at the surface. An AC supply was used rather than DC because it was found that the ITO heater installed facing upward, to be in contact with the water and heated by a DC power supply, was often damaged as a result of electrochemical reactions. The heating cycle frequency of the AC power supply had to be as high as possible to not affect the primary bubble generation. Thus, in the present study, the heating frequency was set to 1000 Hz based on a consideration of the frequency of the primary bubble detachment. Taking into account the thermal activity value, γ = δ ρCp k [15], a thickness of 1 mm was selected for the sapphire glass (giving γ = 9.3) so that the heating conditions would be as close as possible to those associated with copper block heating. Moreover, the effect of the ITO heater and copper cylinder heater on CHF was considered experimentally. As a result, it is confirmed that the difference of the CHF in the two kinds of heaters was not so large. The temperature distribution on the heated ITO surface was
3. Experimental apparatus and procedure 3.1. Experimental apparatus Schematics of the pool boiling apparatus are illustrated in Fig. 2. Fig. 2(a) presents a conventional pool boiling experimental apparatus incorporating a copper block heater, while Fig. 2(b) presents the experimental setup in which an ITO heater is used to measure the temperature distribution of the heated surface with a high-speed IR camera. The main vessel, which is made of Pyrex glass, has an inner diameter of 87 mm and height of 500 mm. The pool container was filled with distilled water or a nanofluid to a height of approximately 60 mm above the heated surface.
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Table 1 Summary of studies on CHF enhancement of flat heaters in saturated pool boiling for water and water based nanofluid under atmospheric pressure condition. Water Reference
Information for surface modification
qCHF,max [MW/m2]
qCHF,max/qCHF,p [-]
Chen et al. (2009) [17]
Nanowires are fabricated on Si nanowires: diameter; 20–300 nm, length; 40–50 µm Cu nanowire: diameter; 200 nm, length; 40–50 µm Heater size:10 mm × 10 mm Sintered pure copper woven mesh screens (pore size: 119.2–232.2 μm, wire diameter: 56–191 μm) Heater size:8 mm × 8 mm(square copper block) Array of micro-scaled posts by the MEMS technique: (D: 21–26 μm, height: 6 μm, interval: 50 μm) ZnO nanorod: height was several hundred nanometers to over 1 μm, with a mode around 800–900 nm Heater size:15 mm × 10 mm (Ti thin film, 0.1 μm in thickness) Effective diameter: Silica nanofluid: 34 ± 10 nm Diamond nanofluid: 173 ± 10 nm No surfactant Concentration of silica and diamond nanoparticles: 0.1 vol% and 0.01 vol%, respectively nanofluid (Silica or diamond in water) Heater size:30 mm × 10 mm (ITO heater was deposited onto 0.4-mm-thick sapphire) Si nanowire array Height of SiNW: 16, 32, 59, and 122 μm Heater size:5 mm × 5 mm, 10 mm × 10 mm, 15 mm × 15 mm, 20 mm × 20 mm (silicon chip, 0.5 mm in thickness)
2.26
2.3
3.60
2.4
2.33
2.1
1.95
2.0
2.24 (5 mm × 5 mm heater)
2.8 (5 mm × 5 mm heater)
Li and Peterson (2010) [18]
Kim et al. (2010) [19]
Gerardi et al. (2011) [20]
Lu et al. (2011) [21]
1.51 (10 mm × 10 mm heater) 1.25 (15 mm × 15 mm heater) 1.26 (20 mm × 20 mm heater)
2.2 (10 × 10 mm –heater) 2.7 (15 mm × 15 mm heater) 2.8 (20 mm × 20 mm heater)
Yao et al. (2011) [22]
Li et al. (2011) [23] Zhang and Kim (2012) [24]
Chu et al. (2012) [25]
Cooke and Kandlikar (2012) [26]
Ahn et al. (2014) [27]
Kim et al. (2014) [28] Rahman et al. (2014) [29]
Xu et al. (2015) [30]
Cu and Si nanowire Height of CuNW: 2, 5, 10, and 20 μm Height of SiNW: 20 and 35 μm Heater size:10 mm × 10 mm multiscale modulated porous structures ∅8 mm(copper cylinder) Alumina nanoporous-structure (smaller pore diameter: approximately 100 nm, larger pore diameter: approximately 500 nm) Heater size:0.785 cm2(aluminum 6061 alloy heater) Micro-pillar arrays: Height: 10 or 20 μm Diameter: 5, 10 μm Spacing: 5, 10, 15 μm Heater size:20 mm × 20 mm Open microchannel Channel width: 197–400 μm, fin width: 200–300 μm, channel depth: 100–400 μm Heater size:10 mm × 10 mm Reduced grapheneoxide (RGO) nanoparticle Primary particle size: 47 nm Concentration of nanoparticles: 0.0001–0.001 wt% Nanofluid (reduced grapheneoxide in water) Heater size:15 mm × 10 mm(0.12-mm-thick-Pt heater deposited onto a silicon substrate) Si nano-wire Heater size:10 × 5mm Biotemplated nanofabrication using tobacco mosaic virus (TMV) TM (300-nm-long, 18-nm-diameter cylindrical structure) is formed on the microstructure Heater size:10 mm × 10 mm(silicon substrate) Composite porous surface electrochemical fabrication technique (macro pores above 200 μm in diameter, micro pores around 2 μm in diameter, and dendritic structure around 400 nm in diameter) Heater size: ∅12 mm(copper block)
1.34
2.0
4.35
3.1
2.55
2.0
2.08
2.7
2.50
2.0
1.54
2.0
2.03
2.3
2.57
3.4
2.42
2.1
(continued on next page)
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Table 1 (continued) Water Reference
Information for surface modification
qCHF,max [MW/m2]
qCHF,max/qCHF,p [-]
Mori et al. (2015) [12]
Honeycomb porous plate on a nanoparticle-deposited surface Vapor escape channel width: 1.4 mm Wall thickness of the grid: 0.45 mm Aperture ratio: 0.55 Height of the honeycomb porous plate: 1 mm Heater size: ∅10, 30, 50 mm(copper cylinder) Square micropillars (width a = 10 μm and height h = 12.75 μm) etched onto a 650-μmthick silicon substrate. Heater size:10 mm × 20 mm titanium thin-film heater was patterned on the electrically insulated backside of the 5 × 5 cm2 silicon samples The micro-pillar surface (diameter(μm): 5–20, pitch(μm): 5–40, height(μm): 10–40) Heater size: The platinum film heater (15 × 10 mm) Honeycomb porous plate with TiO2 nanofluid Vapor escape channel width: 1.4 mm Wall thickness of the grid: 0.45 mm Aperture ratio: 0.55 Height of the honeycomb porous plate: 1 mm Nanofluid (TiO2 in water) Heater size: ∅ 30 mm (copper cylinder) Microchannel with sintered surface (300,500,and 762 μm channel width) Heater size:10 mm × 10 mm micro/nano hierarchically structured surfaces The platinum film heater (Heater size:15 × 10 mm) graphene oxide colloids GO colloids The platinum film heater (Heater size:15 × 10 mm) porous artery structure Porous structure thickness (mm):2.0 Porous structure mesh count (μm):20–30, 40–60, 100–120 Artery width (mm): 1.0 Artery depth (mm): 1.0, 1.5, 2.0 Fin width (mm): 1.2 Heater size: ∅10 mm(copper cylinder) Honeycomb porous plate, metal solid structure, and nanofluid nanofluid Al2O3 in water or water Heater size: ∅50 mm(copper cylinder) nanoporous hydrophilic surface layers (particle diameter(nm): 20–100)
3.06 (at ∅10 mm) 2.08 (at ∅50 mm)
2.0
2.11
2.1
2.2
3.1
3.2
3.2
4.2
3.4
2.6
2.8
1.8
2.4
6.1
3.9
3.1
3.1
1.98
2.1
Dhillon el al. (2015) [31]
Kim et al.(2015) [32]
Mori et al.(2016) [11]
Jaikumar and Kandlikar (2016) [33,34] Moon et al.(2016) [35] Kim et al.(2016) [36]
Bai et al.(2016) [37,38]
Aznam et al.(2016) [39]
Tetreault-Friend et al. (2016) [40]
Fig. 1. Schematic diagram of the proposed method to install spherical porous bodies at the bottom of a nuclear reactor vessel.
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viewing the underside of the film accurately reflects the temperature on the top of the ITO heater surface, and the difference in the temperatures at the bottom of the ITO heater and the top surface temperature of the TiO2 was < 0.05 K even under high heat flux condition of 3 MW/m2. A sapphire plate is almost transparent for wave length of 2 to 4 μm. The transmittance is more than 80%. 3.4. Preparation of Water-based nanofluid TiO2 nanoparticles (Aeroxide TiO2 P 25, Aerosil) were selected as the test nanoparticles. The mean particle diameter supplied by the company was approximately 21 nm. For preparation, the TiO2 nanoparticles were dissolved in 300 ml of distilled water. Then, the nanofluid was dispersed in an ultrasonic bath for 2 h. In the present study, the nanoparticle concentration in the water-based nanofluid was prepared with 4.0 g/L (1.0 wt%, 0.26 vol%). The reason to choose TiO2 nanoparticle and the concentration of 4.0 g/L is that we have got significant CHF enhancement in our previous study [11], although these selection may not be the best for the CHF enhancement. Moreover, although no additives such as surfactants or dispersants were used to stabilize the nanoparticle suspension, it has proved to be stable. Analysis of particle diameter distribution before and after the experiment, using a Laser Diffraction Particle Size Analyser (SALD-7000) from Shimadzu. The mean particle diameter were approximately 200 nm before and after the experiment. Furthermore, sedimentation of nanoparticles was not detected.
(a) Copper block heater
3.5. Spherical porous body Fig. 3 presents the spherical porous body used in the present study, and a micrograph of its structure is shown on the right-hand side of the figure. The spherical porous body, which is commercially available, was used as a carrier for medicines such as fragrance and fungicide. The spherical porous body was made of cellulose, and its diameter was approximately 2.5 mm. The pore diameter distribution of the spherical porous body was measured using mercury penetration porosimetry, which exhibited a peak at approximately 80 μm, as shown in Fig. 4. The porosity of the spherical porous body was 93%. The spherical porous body was attached to the top of the boiling surface by pushing against the surface using a circular jig and stainlesssteel wire, 0.5 mm in diameter, as shown in Fig. 5. Note that cellulose porous bodies cannot be used for actual IVR because of thermal resistance and radiation resistance. The primary purpose of this paper is to present the idea of CHF improvement by the spherical porous body and nanofluid.
5
3.6. Temperature measurement using IR camera The output U of the infrared camera is given by the Eq. (1) when using optical setup with reference plane shown in Fig. 2 (b) [6].
U=εf (Tw ) + (1−ε ) f (Tamb)
(1)
where, f (T) is the output of the infrared camera in the case of temperature measurement for blackbody. Tw, Tamb, and ε are the temperature of the measurement surface, the ambient temperature, and the emissivity, respectively. In order to prevent the reflected image of the infrared camera itself from appearing, the infrared camera was installed with inclination of 5°from the measurement object as shown in Fig. 2 (b). Temperature of reference plane with black spray(ε = 0.94) was measured by a thermocouple. The procedure for correcting temperature at the bottom of ITO heater is as follows: The pool container was filled with saturated water and assumed the
(b) ITO heater Fig. 2. Schematic of experiment apparatus.
obtained using an IR high-speed camera with spatial and time resolutions of 130 μm and 2 ms, respectively. A similar technique was previously demonstrated by Nakamura [12]. The use of a relatively thin ITO film (250 nm) implies that data acquired from the IR camera when
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Fig. 3. Spherical porous bodies. (dp = 2.5 mm, d = 100 μm, ε = 93%).
Pore volume [ml/mg]
the measurements were performed at the steady state. A sheathed heater was installed near the heated surface in the vessel to confirm the saturation temperature (100 ˚C). The steady state was considered reached when the temperatures did not change by more than 0.25 K for at least 5 min. When burnout occurred, the heating was immediately stopped to prevent the heater and thermocouples from being damaged. The last quasi-steady-state heat flux was then measured before the transition to film boiling and was taken as the CHF. 4. Experimental results and discussion
Pore diameter [μm]
As we stated in the introduction, in order to apply the CHF improvement technique using an HPP to cool the reactor vessel with a curved surface in IVR, it is important to consider the effect of a gap between the HPP and heated surface on the CHF. Because a flat HPP cannot be placed completely in close contact with the heated surface with curvature. The HPP, which is commercially available, was used as a filter for purifying exhaust gases from combustion engines. The constituent ingredients are CaOAl2O3 (30–50 wt%), fused SiO2 (40–60 wt %), and TiO2 (5–20 wt%). The vapor escape channel width (cell width), the wall thickness of the grid, the aperture ratio (ratio of the open area to total area), the height of the honeycomb porous plate, and the diameter of the honeycomb porous plate are 1.3 mm, 0.4 mm, 0.55, 1.0 mm, and 30.0 mm, respectively. Fig. 6 illustrates the effect of a gap on the CHF. Two stainless-steel wires were placed between the heat transfer surface and an HPP (5-mm thickness), as shown in the images of Fig. 6, and the gap was controlled
Fig. 4. Pore radius distribution of test spherical porous bodies.
bottom temperature of the ITO film was 100° C. And the emissivity was adjusted on analysis software of the infrared camera so that the ITO film temperature obtained from the IR camera becomes 100° C. 3.7. Experimental procedure Pool boiling experiments were carried out using a water-based nanofluid or distilled water as working fluids under saturated conditions at atmospheric pressure. The heated surface was polished using waterproof sandpaper of 2000 grit. Distilled water of 700 ml was boiled using a heater installed in the test vessel for 30 min to remove as much dissolved gas as possible. At each run, the heat flux was increased in increments of approximately 0.1 MW/m2 until burnout occurred. All
Fig. 5. Attachment of spherical porous bodies on the heated surface.
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qCHF[MW/m2]
2
Bare Surface HPP(5mm) with gap Water P=0.1MPa
1
0 0
0.1
0.2
0.3
0.4
0.5
The gap between heated surface and HPP [mm] Fig. 6. The effect of the gap between the heated surface and an HPP on q”CHF.
by changing the diameter of the stainless-steel wire to 0.05, 0.1, and 0.3 mm. Repeatability was confirmed at least three times. As it can be seen from this figure, the CHF does not improve when the gap is 0.3 mm. The contact between the heated surface and HPP is an important factor for CHF enhancement, although it is very difficult to bring the HPP completely into close contact with the curved heat transfer surfaces. Therefore, this result signifies that the CHF of a curved surface cannot be enhanced by the flat HPP proposed in the previous study, because of the existence of the gap between the flat HPP and curved heated surface. Accordingly, we proposed an approach using the spherical porous body shown in Fig. 1. Fig. 7 depicts the experimental results by varying the number density of spherical porous bodies on the heated surface. A schematic of the heated surface with spherical porous bodies is also illustrated in the figure. Moreover, this figure includes the CHF in a saturated boiling pool of not only nanofluid but also distilled water. Comparing the case of distilled water and nanofluid with S = 0, the
2 q"CHF [MW/m ]
2
1 Nanofluid (Copper cylinder heater) Distilled water (Copper cylinder heater) Nanofluid (ITO heater)
0 0
10
20
30
40
50
60
Contact area ratio S [%] Fig. 7. Relation between q”CHF and contacted area ratio S [%] in saturated pool boiling of nanofluid and distilled water.
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occurs when the hovering time of large coalesced bubbles increases accidentally, resulting in burnout. In order to consider the effect of spherical porous bodies on the dryout and the CHF, the experiments have been performed at the same heat flux of Fig. 9. Fig. 10 presents IR images of the heated surface with spherical porous bodies in a saturated boiling pool of nanofluid, and the heat flux is the same as in Fig. 9. A spherical porous body is installed on the heat transfer surface in the areas indicated by the dashed line. As is clear from this figure, when the spherical porous body is installed on the heat transfer surface, dryout does not occur at all, although burnout does occur in the case without spherical porous bodies, as shown in Fig. 9. When a spherical porous body is installed, a temperature distribution is formed on the heat transfer surface. The high-temperature portion of the figure indicates that the spherical porous body is firmly in contact with the heat transfer surface. This is because the dryout may occur at the contact area between the spherical porous body and heated surface. However, based on temperature change with time, the temperature at the contact area did not change so much even large temperature increase at the non-contact area was observed as shown in Fig. 11. The reason for this is not clarified yet, but this may indicate the dryout at the contact area does not grow so much because porous body transports water to the dryout region. Fig. 11(a) and (b) depict IR images of the heated surface with the spherical porous body in a saturated boiling pool of nanofluid under the burnout occurrence condition (S = 20%, nanofluid, 1.78 MW/m2). As shown in Fig. 11, even under these conditions, a reversible dry spot is observed. This is because nanoparticles were deposited on the heated surface to improve the wickability. Nanoparticle deposition was
CHF of the nanofluid improves by a factor of about 1.5 compared to distilled water. The CHF enhancement for the case of the nanofluid is attributed to the surface wettability, surface roughness, and capillary wicking effect because of nanoparticle deposition on the heated surface, as reported in previous papers [7,11–13]. It is also interesting that the CHF with the spherical porous body using the nanofluid is enhanced up to an approximate factor of two compared to that of a plain surface of distilled water for the cases of S = 7.5% to 30%, although the CHF hardly improves in the case of distilled water. Because the capillary force of the spherical porous body is small because of large pore diameter (100 μm). Therefore CHF cannot be enhanced only by the spherical porous body in distilled water. While, for the case of the spherical porous body in the nanofluid, a thin porous layer caused by the nanoparticle deposition is formed between the spherical porous body and the heated surface. Therefore, the CHF may be enhanced by the similar effect of the two-layer-structured HPP proposed by Mori et al. [10]. According to the capillary limit model [9], the CHF is enhanced when the contacted area of the porous body with the heated surface increases. However, no increase in the CHF was observed in the range of 7.5% ≤ S < 30%. The reason why there is no optimal S to maximize the CHF has not yet been clarified. Nevertheless, for the range of 7.5% ≤ S < 30%, the CHF is enhanced roughly by a factor of two compared to that of the plain surface of distilled water. Therefore, the spherical porous bodies connected in a manner similar to beads can be applied to the CHF enhancement. The main improvement factor on the CHF is that liquid was supplied from the spherical porous body to the nanoparticle-deposited layer on the heated surface by the capillary force. Note that the capillary force used by the spherical porous body in the present study is small because the pore diameter of the spherical porous body itself is as large as 100 μm. The CHF enhancement mechanism may be a similar phenomenon of enhancement to the twolayer-structured HPP proposed by Mori et al. [10]. For S ≥ 30%, the CHF tends to decrease with S, as shown in Fig. 7. This is because the number density of spherical porous bodies is too large for the vapor to escape from the heat transfer surface, in other words, smaller vapor escape area causes a significant increase in the pressure drop for the vapor flow in the vapor escape area, resulting in CHF reduction. Fig. 8(a) and (b) present the boiling curves and HTCs in the nanofluid for the cases with and without surface modification by the spherical porous bodies. The arrows in Fig. 8 correspond to the CHF condition. As we pointed out in Fig. 7, the CHFs of S = 7.5% and 28% are higher than that of S = 0 (plain surfaces) of the nanofluid, while the CHF of S = 48% is lower than that of S = 0. Regarding the HTCs, the HTC does not change significantly from S = 7.5% to 28% except for S = 48%. Note that the HTC under burnout conditions for plain surfaces of distilled water is approximately 34 kW/(m2·K), which is roughly the same HTC from S = 7.5% to 28% of distilled water. In order to consider the thermo-fluid phenomena near the spherical porous body, an ITO film with a thickness of 250 nm was vapor-deposited on sapphire glass and used as a transparent heat transfer surface, and a temperature distribution was recorded from the lower side of the heat transfer surface using an IR camera, as shown in Fig. 2(b). Fig. 9(a) and (b) indicate the results of the IR data in the boiling pool of nanofluid under the burnout condition of S = 0% (q = 1.57 MW/m2). The large wall temperature changes frequently because of reversible dry spots within a wide region on the heated surface, as shown in Fig. 9(a). This is because a large coalesced bubble forms on the heated surface and departs periodically from the heated surface, and the wettability of the heated surface improves by the deposition of a nanoparticle. In addition, the frequency of temperature change corresponds to that of the departure frequency of large coalesced bubbles, whereas, as shown in Fig. 9(b), an irreversible dry spot
2
q" [MW/m2]
2
Nanofluid S=0% S = 7.5 % S = 28 % S = 48 %
1.5 MW/m 2 1.9 MW/m 2
1.9 MW/m
1 2
1.0 MW/m 0 0
20
40
60
T [K]
(a) Boiling curves 50
2
h [kW/(m K)]
40 30
Nanofluid S=0% S = 7.5 % S = 28 % S = 48 %
20 10 0 0
20
40
60
T [K]
(b) Heat transfer coefficients Fig. 8. Boiling curves and heat transfer coefficients with or without spherical porous bodies in saturated pool boiling of the nanofluid.
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(a) Reversible dry spot
(b) Irreversible dry spot
Fig. 9. IR image of bare surface in saturated pool boiling of nanofluid under the burnout condition, (S = 0%, nanofluid, 1.57 MW/m2).
spherical porous bodies. This is probably because the nanoparticle deposition layer supplies liquid to the heat transfer surface with an intense capillary force, and at the same time, the spherical porous body continues to supply liquid to the nanoparticle deposition layer around the porous body. The combination of the spherical porous body and nanoparticle-deposited layer is similar to the two-layered HHP proposed by Mori et al. [10]. The significance of making the HPP into a two-layer structure is as follows: (1) An HPP simply attached to a heated surface should have very fine pores to supply water to the heated surface because of the strong capillary action, and the HPP should be as thin as possible to decrease the frictional pressure drop caused by the internal water flow; (2) the other HPP, stacked on top of the thin HPP, must be structured to hold a sufficient amount of water over the plate. Therefore, principles (1) and (2) of the HHPs may correspond to the nanoparticle-deposited layer and spherical porous bodies, respectively, in the present study. Based on obtained results in the present study, cooling technology using spherical porous bodies in nanofluid has potential to be applied to IVR. However, further investigations of nanofluid with spherical porous bodies is needed in a wider variety of scenarios as different inclination angle of the heater surface, higher system pressure, concentrations of nanofluid, and flow boiling regime. 5. Conclusions Fig. 10. IR image using the spherical porous bodies on the heated surface in saturated pool boiling of nanofluid (S = 20%, nanofluid, 1.57 MW/m2).
In order to cool a heated curved surface, we proposed a method using a spherical porous body connected in a manner similar to beads. As a result, the following conclusions were obtained.
confirmed mainly on the location where spherical porous bodies were not installed after the experiments in nanofluid. However, burnout occurred once the irreversible dry spot formed. Even under the burnout condition, the size of the dryout is small compared to that without
1. For the combination of a nanofluid (TiO2, 0.1 vol%) and spherical porous bodies, the CHF is enhanced up to a factor of about 1.9 compared to the case of a plain surface in a boiling pool of distilled
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(a) Reversible dry spot
(b) Irreversible dry spot Fig. 11. IR image using the spherical porous bodies on the heated surface in saturated pool boiling of nanofluid under the burnout condition. (S = 20%, nanofluid, 1.78 MW/m2).
water in the range of 7.5% ≤ S < 30% 2. The area of temperature increase is small in the case of the spherical porous body in a saturated boiling pool of nanofluid. 3. It is considered that the CHF is enhanced by supplying the liquid retained by the spherical porous body to the nanoparticle-deposited layer.
characteristics of spherical porous bodies (size, pore diameter, and porosity), disposition of the spherical porous bodies, kind of nanofluid, and concentration of nanofluid on the CHF.
In order to enhance the CHF, we need to consider the effect of the
The present study was supported in part by the Futaba Foundation.
Acknowledgements
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Appendix Uncertainty analysis The individual standard uncertainties were combined to obtain the estimated standard deviation of the results, which is calculated using the law of the propagation of uncertainty [16]. For the case of the copper block heater, the uncertainties in the heat flux q, superheat ΔTsat, and heat transfer coefficient (HTC) h were evaluated using the following equations: 2
2
Δq =
2
⎜
⎟
⎜
⎟
2
Δ(ΔTsat ) =
⎜
⎟
2
(A.1) 2
2
⎛⎜ ∂ (ΔTsat ) Δq⎞⎟ + ⎛ ∂ (ΔTsat ) ΔT1 ⎞ + ⎛ ∂ (ΔTsat ) Δδ2 ⎞ + ⎛ ∂ (ΔTsat ) Δλ ⎞ ⎝ ∂λ ⎠ ⎝ ∂T1 ⎠ ⎝ ∂δ2 ⎠ ⎝ ∂q ⎠ ⎜
⎟
⎜
⎟
(A.2)
2
2
Δh =
2
⎛ ∂q Δλ ⎞ + ⎛ ∂q Δδ1 ⎞ + ⎛ ∂q ΔT1 ⎞ + ⎛ ∂q ΔT2 ⎞ ⎝ ∂λ ⎠ ⎝ ∂δ1 ⎠ ⎝ ∂T1 ⎠ ⎝ ∂T2 ⎠
⎛ ∂h Δ(ΔTsat ) ⎞ + ⎜⎛ ∂h Δq⎟⎞ ⎝ ∂ (ΔTsat ) ⎠ ⎝ ∂q ⎠ ⎜
⎟
(A.3)
where T1 and T2 are the temperatures at TC1 and TC2, respectively, λ is the thermal conductivity of copper evaluated at the arithmetic mean of T1 and T2, δ1 is the distance between TC1 and TC2, and δ2 is the distance between TC1 and the boiling surface. For the case of the ITO heater, the heat flux was determined from data measured by a voltmeter, ammeter, and the heated surface area, A. The uncertainties of the heat flux q and area were respectively obtained as follows:
Δq =
2 2 2 ⎛ ∂q ⎞ ΔV 2 + ⎛ ∂q ⎞ ΔI 2 + ⎛ ∂q ⎞ ΔA2 = ⎝ ∂V ⎠ ⎝ ∂I ⎠ ⎝ ∂A ⎠
ΔA =
⎛ ∂A ⎞ Δx12 + ⎛ ∂A ⎞ Δx22 ⎝ ∂x1 ⎠ ⎝ ∂x2 ⎠
2
⎜
⎟
I 2 V 2 VI 2 ⎛ ⎞ ΔV 2 + ⎛ ⎞ ΔI 2 + ⎛ ⎞ ΔA2 ⎝ A⎠ ⎝ A⎠ ⎝ A2 ⎠
(A.4)
2
⎜
⎟
(A.5)
where Δx1 and Δx2 are the lengths of one side of the heater. From the measurement, the length of the heated surface was Δx1 = Δx2 = 0.1mm . Table 1 and Table 2 list examples of the relative uncertainties calculated using Eqs. (A.1)–(A.5). As shown in the tables, the relative uncertainties depend on the experimental conditions and tend to decrease with the heat flux. (See Table 3) Table 2 Relative uncertainties of the measured quantities for the case of copper block heater. q [MW/m2]
ΔT sat [K]
h [kW/(m2·K)]
Δq/q [%]
Δ(ΔT [%]
1.2 1.5 1.8 2.0
16 18 20 21
77 83 91 95
2.7 2.3 1.9 1.8
2.7 2.3 2.0 1.8
Δh/h [%]
sat)/ΔT sat
3.8 3.2 2.8 2.5
Table 3 Relative uncertainties of the measured quantities for the case of ITO heater. V [V]
I [A]
q [MW/m2]
Δ q [MW/m2]
Δ q/q [%]
10.1 80.1
1.3 7.9
0.03 2.0
0.002 0.041
5.7 3.1
[6] S.M. You, J.H. Kim, K.H. Kim, Effect of nanoparticles on critical heat flux of water in pool boiling heat transfer, Appl. Phys. Lett 83 (16) (2003) 3374–3376. [7] S.M. Kwark, R. Kumar, G. Moreno, S.M. You, Transient characteristics of pool boiling heat transfer in nanofluids, J. Heat Transfer 134 (5) (2012) 051015. [8] S.M. Kwark, R. Kumar, G. Moreno, J. Yoo, S.M. You, Pool boiling characteristics of low concentration nanofluids, Int. J. Heat Mass Transf. 53 (5–6) (2010) 972–981. [9] S. Mori, K. Okuyama, Enhancement of the critical heat flux in saturated pool boiling using honeycomb porous media, Int. J. Multiph. Flow 35 (10) (2009) 946–951. [10] S. Mori, N. Maruoka, K. Okuyama, Critical heat flux enhancement by a two-layer structured honeycomb porous plate in a saturated pool boiling of water, Int. J. Heat Mass Transf. 118 (2018) 429–438 (Supplement C). [11] S. Mori, S. Mt Aznam, R. Yanagisawa, K. Okuyama, CHF enhancement by honeycomb porous plate in saturated pool boiling of nanofluid, J. Nucl. Sci. Technol. (2015) 1–8. [12] S. Mori, S. Mt Aznam, K. Okuyama, Enhancement of the critical heat flux in saturated pool boiling of water by nanoparticle-coating and a honeycomb porous plate,
References [1] J. Buongiorno, L.W. Hu, G. Apostolakis, R. Hannink, T. Lucas, A. Chupin, A feasibility assessment of the use of nanofluids to enhance the in-vessel retention capability in light-water reactors, Nuclear Eng. Des. 239 (5) (2009) 941–948. [2] S.D. Park, S.W. Lee, S. Kang, S.M. Kim, I.C. Bang, Pool boiling CHF enhancement by graphene-oxide nanofluid under nuclear coolant chemical environments, Nuclear Eng. Des. 252 (2012) 184–191. [3] Y.P. Zhang, S.Z. Qiu, G.H. Su, W.X. Tian, Analysis of safety margin of in-vessel retention for AP1000, Nuclear Eng. Des. 240 (8) (2010) 2023–2033. [4] Q.T. Pham, T.I. Kim, S.S. Lee, S.H. Chang, Enhancement of critical heat flux using nano-fluids for Invessel Retention-External Vessel Cooling, Appl. Thermal Eng. 35 (2012) 157–165. [5] S. Mori, Y. Utaka, Critical heat flux enhancement by surface modification in a saturated pool boiling: a review, Int. J. Heat Mass Transf. 108 (2017) 2534–2557.
229
Applied Thermal Engineering 144 (2018) 219–230
S. Mori et al.
Int. J. Heat Mass Transf. 80 (2015) 1–6. [13] S. Mori, L. Shen, K. Okuyama, Effect of cell size of a honeycomb porous plate attached to a heated surface on CHF in saturated pool boiling, 2010 14th International Heat Transfer Conference, American Society of Mechanical Engineers, 2010, pp. 255–265. [14] S.M. Aznam, S. Mori, F. Sakakibara, K. Okuyama, Effects of heater orientation on critical heat flux for nanoparticle-deposited surface with honeycomb porous plate attachment in saturated pool boiling of water, Int. J. Heat Mass Transf. 102 (2016) 1345–1355. [15] M. Arik, A. Bar-Cohen, Effusivity-based correlation of surface property effects in pool boiling CHF of dielectric liquids, Int. J. Heat Mass Transf. 46 (20) (2003) 3755–3764. [16] B.N. Taylor, C.E. Kuyatt, Guidelines for evaluation and expressing the uncertainty of NIST measurement results, NIST Tech. Note 1297 (1993) 1–20. [17] R. Chen, M.-C. Lu, V. Srinivasan, Z. Wang, H.H. Cho, A. Majumdar, Nanowires for enhanced boiling heat transfer, Nano Lett. 9 (2) (2009) 548–553. [18] C. Li, G.P. Peterson, Geometric effects on critical heat flux on horizontal microporous coatings, J. Thermophys. Heat Transf. 24 (3) (2010) 449–455. [19] S. Kim, H.D. Kim, H. Kim, H.S. Ahn, H. Jo, J. Kim, M.H. Kim, Effects of nano-fluid and surfaces with nano structure on the increase of CHF, Exp. Therm Fluid Sci. 34 (4) (2010) 487–495. [20] C. Gerardi, J. Buongiorno, L.W. Hu, T. McKrell, Infrared thermometry study of nanofluid pool boiling phenomena, Nanoscale Res. Lett. 6 (1) (2011). [21] M.-C. Lu, R. Chen, V. Srinivasan, V.P. Carey, A. Majumdar, Critical heat flux of pool boiling on Si nanowire array-coated surfaces, Int. J. Heat Mass Transf. 54 (25–26) (2011) 5359–5367. [22] Z. Yao, Y.W. Lu, S.G. Kandlikar, Effects of nanowire height on pool boiling performance of water on silicon chips, Int. J. Therm. Sci. 50 (11) (2011) 2084–2090. [23] C.H. Li, T. Li, P. Hodgins, C.N. Hunter, A.A. Voevodin, J.G. Jones, G.P. Peterson, Comparison study of liquid replenishing impacts on critical heat flux and heat transfer coefficient of nucleate pool boiling on multiscale modulated porous structures, Int. J. Heat Mass Transf. 54 (15–16) (2011) 3146–3155. [24] B. Zhang, K.J. Kim, Effect of liquid uptake on critical heat flux utilizing a three dimensional, interconnected alumina nano porous surfaces, Appl. Phys. Lett. 101 (5) (2012) 054104. [25] K.-H. Chu, R. Enright, E.N. Wang, Structured surfaces for enhanced pool boiling heat transfer, Appl. Phys. Lett. 100 (24) (2012) 241603. [26] D. Cooke, S.G. Kandlikar, Effect of open microchannel geometry on pool boiling enhancement, Int. J. Heat Mass Transf. 55 (4) (2012) 1004–1013. [27] H.S. Ahn, J.M. Kim, M. Kaviany, M.H. Kim, Pool boiling experiments in reduced
[28]
[29]
[30] [31] [32]
[33]
[34]
[35]
[36]
[37] [38]
[39]
[40]
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graphene oxide colloids, Part I - Boiling Charact. Int. J. Heat Mass Transf. 74 (2014) 501–512. B.S. Kim, S. Shin, D. Lee, G. Choi, H. Lee, K.M. Kim, H.H. Cho, Stable and uniform heat dissipation by nucleate-catalytic nanowires for boiling heat transfer, Int. J. Heat Mass Transf. 70 (2014) 23–32. M.M. Rahman, E. Olceroglu, M. McCarthy, Role of wickability on the critical heat flux of structured superhydrophilic surfaces, Langmuir ACS J. of Surf. Coll. 30 (37) (2014) 11225–11234. P. Xu, Q. Li, Y. Xuan, Enhanced boiling heat transfer on composite porous surface, Int. J. Heat Mass Transf. 80 (2015) 107–114. N.S. Dhillon, J. Buongiorno, K.K. Varanasi, Critical heat flux maxima during boiling crisis on textured surfaces, Nat. Commun. 6 (2015). S.H. Kim, G.C. Lee, J.Y. Kang, K. Moriyama, M.H. Kim, H.S. Park, Boiling heat transfer and critical heat flux evaluation of the pool boiling on micro structured surface, Int. J. Heat Mass Transf. 91 (2015) 1140–1147. A. Jaikumar, S.G. Kandlikar, Ultra-high pool boiling performance and effect of channel width with selectively coated open microchannels, Int. J. Heat Mass Transf. 95 (2016) 795–805. A. Jaikumar, S. Kandlikar, Pool boiling enhancement through bubble induced convective liquid flow in feeder microchannels, Appl. Phys. Lett. 108 (4) (2016) 041604. H.W. Moon, Y.J. Yoon, J.H. Park, B.-S. Myung, D.E. Kim, Dynamic wetting and boiling characteristics on micro-structured and micro/nano hierarchically structured surfaces, Exp. Therm Fluid Sci. 74 (2016) 19–26. J.M. Kim, J.H. Kim, S.C. Park, M.H. Kim, H.S. Ahn, Nucleate boiling in graphene oxide colloids: morphological change and critical heat flux enhancement, Int. J. Multiph. Flow (2016). L. Bai, L. Zhang, G. Lin, G. Peterson, Pool boiling with high heat flux enabled by a porous artery structure, Appl. Phys. Lett. 108 (23) (2016) 233901. L. Bai, L. Zhang, J. Guo, G. Lin, X. Bu, D. Wen, Evaporation/boiling heat transfer characteristics in an artery porous structure, Appl. Therm. Eng. 104 (2016) 587–595. S.M. Aznam, S. Mori, K. Okuyama, Critical heat flux enhancement in water-based nanofluid with honeycomb porous plate on large heated surface, 2016 24th International Conference on Nuclear Engineering, American Society of Mechanical Engineers, 2016 V005T015A036-V005T015A036. M. Tetreault-Friend, R. Azizian, M. Bucci, T. McKrell, J. Buongiorno, M. Rubner, R. Cohen, Critical heat flux maxima resulting from the controlled morphology of nanoporous hydrophilic surface layers, Appl. Phys. Lett. 108 (24) (2016) 243102.