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Enhancement of the critical heat flux in saturated pool boiling of water by nanoparticle-coating and a honeycomb porous plate
International Journal of Heat and Mass Transfer 80 (2015) 1–6
Contents lists available at ScienceDirect
International Journal of Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ijhmt
Technical Note
Enhancement of the critical heat flux in saturated pool boiling of water by nanoparticle-coating and a honeycomb porous plate Shoji Mori ⇑, Suazlan Mt Aznam, Kunito Okuyama Department of Chemical Engineering Science, Graduate School of Engineering, Yokohama National University, 79-5, Tokiwadai, Hodogaya-ku, Yokohama, Kanagawa 240-8501, Japan
a r t i c l e
i n f o
Article history: Received 17 February 2014 Received in revised form 8 August 2014 Accepted 21 August 2014 Available online 21 September 2014 Keywords: Pool boiling Enhancement of critical heat flux Porous media Nanofluid
a b s t r a c t Various surface modifications of the boiling surface, e.g., integrated surface structures, such as channels and micro-pin fins, and the coating of a micro-porous layer using sintered metal powders and nanoparticle deposition onto the heat transfer surface, have been proven to effectively enhance the critical heat flux (CHF) in saturated pool boiling. In particular, novel methods involving nanofluids have gained a great deal of attention because the CHF for the use of nano-fluids is increased drastically, by up to approximately three times compared to that of pure water. CHF enhancement using nanofluids is related to surface wettability, surface roughness, and capillary wicking performance due to nanoparticle deposition on the heated surface. Several studies have proposed the use of nanofluids to enhance the in-vessel retention (IVR) capability in the severe accident management strategy implemented at certain light-water reactors. Systems using nanofluids for IVR must be applicable to large-scale systems, i.e., sufficiently large heated surfaces compared to the characteristic length of boiling (capillary length). However, as for the effect of the size of heater with nanoparticle deposition, it was revealed that the CHF tends to be decreased with the increased heater size. On the other hand, the CHF in saturated pool boiling of water using a honeycomb porous plate was shown experimentally to become approximately twice that of a plain surface with a heated surface diameter of 30 mm, which is comparatively large. The enhancement is considered to result from the capillary supply of liquid onto the heated surface through the microstructure and the release of vapor generated through the channels. In the present paper, in order to enhance the CHF on a large heated surface, the effects of a honeycomb porous plate and/or nanoparticle deposited heat transfer surface on the CHF were investigated experimentally. As a result, the CHF was enhanced greatly by the attachment of a honeycomb porous plate to the modified heated surface by nanoparticle deposition, even in the case of a large heated surface. Under the best performing surface modifications, the CHF for 10-mm-, 30-mm- and 50-mm-diameter surfaces was enhanced up to 3.1, 2.3, and 2.2 MW/m2, respectively. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction One strategy for severe accidents, such as loss of coolant accidents (LOCA), is in-vessel retention (IVR) of corium debris [1]. The IVR consists of external cooling of the reactor vessel in order to remove decay heat from the molten core through the lower head of the vessel. However, the heat removal is limited by the occurrence of critical heat flux (CHF) at the outer surface of the reactor vessel. Therefore, in order to enhance the capability of the IVR in the severe accident of the light-water reactors, methods to increase the CHF should be considered.
⇑ Corresponding author. Tel./fax: +81 45 339 4010. E-mail address: [email protected] (S. Mori). http://dx.doi.org/10.1016/j.ijheatmasstransfer.2014.08.046 0017-9310/Ó 2014 Elsevier Ltd. All rights reserved.
Various modifications of the boiling surface, e.g., integrated surface structures, such as channels and micro-pin fins, and the coating of a micro-porous layer using sintered metal powders and nanoparticle deposition onto the heat transfer surface, have been proven to effectively enhance the CHF in saturated pool boiling [2–6]. Several researchers have offered novel structures of the porous media in which the liquid and vapor flow paths will be separated [7–12]. The CHF has been enhanced significantly up to approximately twice that of a plain surface with a heated surface diameter of 30 mm using a honeycomb porous plate [11]. Moreover, the effect of the channel width on the CHF in the saturated pool boiling has been investigated experimentally [12]. You et al. [6] introduced a novel method to enhance the CHF of pool boiling using nanofluids. The CHF was increased drastically up
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to approximately three times (1.68 MW/m2) compared to that of pure water. The drastic enhancement in the CHF obtained by using nanofluids is of particular interest [13,14]. The CHF enhancement obtained by using nanofluids is related to the surface wettability, surface roughness, and capillary wicking performance due to nanoparticle deposition on the heated surface [15]. Based on the studies mentioned above, the mechanism of the CHF enhancement can be explained by the capillary suction effect, the extended surface area effect, the liquid supply caused by the hydrodynamic effect, and the wettability of the heated surface. There are several ways to enhance the CHF. However, approaches for increasing the IVR capability must be simple and installable at low cost. The use of nanofluids has been proposed in order to enhance the IVR capability in severe accident management strategies for advanced light-water reactors [16,17]. The strategy of using nanofluid for IVR should be applicable to a large heated surface compared to a characteristic length of boiling (capillary length). However, for Al2O3 nanoparticle-coated flat heaters, the CHF tends to be decrease with an increase in heater size [18]. On the other hand, as stated above, the CHF using a honeycomb porous plate was shown experimentally to be approximately twice that of a plain surface having a heated surface diameter of 30 mm [11,12], which is comparatively large. The enhancement is considered to result from the capillary supply of liquid onto the heated surface and the release of generated vapor through the channels. In the present paper, in order to succeed in heat removal from a large heated surface with high heat flux, we focused on the use of a honeycomb porous plate and nanofluid. Therefore, the CHF in a
saturated pool boiling of water was investigated experimentally using a honeycomb porous plate and/or nanoparticle deposited heat transfer surface. As a result, the CHF was greatly enhanced under the attachment of a honeycomb porous plate on the modified heated surface by nanoparticle deposition. 2. Experimental apparatus and procedure 2.1. Experimental apparatus A schematic diagram of the pool boiling test facility is shown in Fig. 1. The main vessel is made of Pyrex glass and has an inner diameter of 87 mm and a height of 500 mm. The pool container was filled with distilled water to a height of approximately 60 mm above the heated surface. The heat flux was supplied to the boiling surface through a copper cylinder using a cartridge electric heater, which was inserted into the copper cylinder and was 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 a copper cylinder with diameters of 10 mm, 30 mm, and 50 mm is smooth and is used as the heat transfer surface in the experiments. Three sheathed thermocouples with an outer diameter of 0.5 mm were inserted horizontally to the centerline of the copper cylinder. The thermocouples (TC1, TC2 and TC3 shown in Fig. 1) in the copper cylinder were set apart axially by 5.0 mm. The closest thermocouple was located 10.0 mm below the boiling surface. These thermocouples were calibrated using a
Fig. 1. Schematic diagram of the experimental apparatus.
S. Mori et al. / International Journal of Heat and Mass Transfer 80 (2015) 1–6
platinum resistance thermometer. The wall temperature and the 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. The linearity of the temperature distribution was confirmed.
3
depositing on the heated surface. Finally, the vessel was cleaned and refilled with distilled water. 2.4. Honeycomb porous plates
Experiments were carried out using distilled water as a working fluid under saturated conditions at atmospheric pressure. A sheathed heater was installed above the heated surface in the liquid bath in order to maintain the liquid temperature at the saturation temperature. At each run, the heated surface was polished using waterproof 2000-grit sand paper and then cleaned using acetone in order to minimize the effect of oxidation of heat transfer surface. The heat flux was increased in increments of approximately 0.1 MW/m2 until burnout occurred. All of the measurements were performed at the steady state. The steady state was regarded as being reached when the temperatures did not change more than 0.25 K for at least 10 min. When burnout occurred, the heating was immediately stopped in order to prevent the heater and the 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.
Fig. 2 shows the honeycomb porous plate used in the present study [11], and a micrograph of its structure is shown on the right-hand side of the figure. The honeycomb porous plate, which is commercially available, was used as a filter for purifying exhaust gases from combustion engines. The constitutive ingredients are CaOAl2O3 (30 to 50 wt%), fused SiO2 (40 to 60 wt%), and TiO2 (5 to 20 wt%). The vapor escape channel width (cell width) dV‘, the wall thickness dS of the grid, the aperture ratio, and the height of the honeycomb porous plate dh are 1.4 mm, 0.45 mm, 0.55, and 1 mm, respectively, as shown in Fig. 2. The honeycomb porous plate was pressed against the top of the boiling surface by stainless steel wire of 0.3 mm in diameter. Any thermally conductive grease was not used between honeycomb porous plate and heated surface. The pore radius distribution of the honeycomb porous plates was measured by mercury penetration porosimetry, which peaked at approximately 0.17 lm. The average pore radius, the median pore radius, and the porosity of the honeycomb porous plates based on porosimetry are 0.037 lm, 0.13 lm, and 24.8%, respectively.
2.3. Preparation of the nanoparticle-deposited heat transfer surface
2.5. Uncertainty analysis
The preparation of the nanoparticle-deposited surface is similar to that described in previous studies [19,20]. The TiO2 nanoparticles (Aero-sil Corporation, Aeroxide TiO2 P 25) were selected as the test nanoparticles and were dispersed in distilled water, i.e. nanofluid. The mean particle diameter of the particle supplied by the company was approximately 21 nm. The heated surface was polished using waterproof 2000-grit sand paper and then cleaned using acetone. For preparation, 800 ml of distilled water was kept boiling in a test vessel at 500 kW/m2 for at least 30 min. A volume of 200 ml of nanofluid (C = 0.0201 wt%), which was stirred for at least four hours using an ultrasonic bath, was then added to the boiling distilled water in the test vessel. Therefore, the final nanoparticle concentration was 0.0040 wt% (volume concentration: 0.0011 vol%). In order to form the nanoparticle-deposited surface, the time from the addition of nanofluid was set to 20 min. The reason for these conditions was that the CHF was not changed greatly under a higher concentration and a longer deposition time compared to the present condition (C = 0.0040 wt%, deposition time = 20 min) [20]. Heating was then turned off, and the liquid containing nanoparticles was removed from the test vessel to prevent nanoparticles from continuously
The individual standard uncertainties are combined to obtain the estimated standard deviation of the results, which is calculated using the law of propagation of uncertainty [21]. The uncertainties of the heat flux q, the superheat DTsat, and the heat transfer coefficient h, respectively, are evaluated in the following equations:
1 mm
2.2. Experimental procedure
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 2 2 @q @q @q @q Dq ¼ Dk þ Dd 1 þ DT 1 þ DT 2 @k @d @T 1 @T 2
DðDT sat Þ ¼
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 2 2 @ðDT sat Þ @ðDT sat Þ @ðDT sat Þ @ðDT sat Þ Dq þ DT 1 þ Dd2 þ Dk @q @T 1 @d2 @k ð2Þ
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 @h @h Dh ¼ DðDT sat Þ þ Dq @ðDT sat Þ @q
ð3Þ
where T1 and T2 are the temperatures at TC1 and TC2, respectively, k is the thermal conductivity of copper evaluated at the arithmetic
1.4 mm
φ10, 30, and 50 mm
ð1Þ
0.45 mm
Aperture ratio: 0.55 Fig. 2. Dimensions of the honeycomb porous plate.
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S. Mori et al. / International Journal of Heat and Mass Transfer 80 (2015) 1–6
Table 1 Relative uncertainties of the measured quantities.
DTsat (K)
h (kW/ (m2 K))
Dq/q (%)
D(DTsat)/DTsat (%)
Dh/h (%)
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
3.8 3.2 2.8 2.5
q [MW/m2 ]
q (MW/ m2)
0 0
1.5 MW/m2 2.0 MW/m2
10
20
30
q [MW/m2 ]
Tsat [K]
Water 3 P = 0.1 MPa TSUB = 0 K (b) 30mm 2
2.2 MW/m2 2.2 MW/m2 1.1 MW/m2
1 1.0 MW/m2
0 0
10
20
30
Tsat [K]
Water P = 0.1 MPa TSUB = 0 K
3 q [MW/m2]
Fig. 3(a)–(c) show the boiling curves for the cases with and without surface modifications, i.e., a nanoparticle-deposited surface (D), a honeycomb porous plate installed on a plain surface (h), a honeycomb porous plate installed on a nanoparticle-deposited surface (.), and a plain surface (s) for surfaces having diameters of 10, 30, and 50 mm, respectively. The arrows in Fig. 3 correspond to the CHF condition. As shown in the figure, the CHFs of the nanoparticle-deposited surfaces are higher than those of plain surfaces. The CHF enhancement for the case of the nanocoated surface is attributed to the surface wettability, surface roughness, and the capillary wicking effect due to nanoparticle deposition on the heated surface, as reported in previous studies [6,13–20]. As shown in the figure, the CHF for the case of a honeycomb porous plate, as indicated by the h and . symbols, is enhanced compared to that for the cases of plain and nano-coated surfaces (s, D). The CHF enhancements in the case of a honeycomb porous plate are due to the automatic liquid supply to the heated surface due to capillary action and the reduction of the vapor escape flow resistance due to the separation of liquid and vapor flow by the honeycomb porous structure [11,12]. Moreover, it is interesting that the CHF in the case of a honeycomb porous plate installed on a nanoparticle-deposited surface is higher than that for the case of a honeycomb porous plate alone on a plain surface for diameters of 10, 30, and 50 mm. The mechanisms of liquid supply to the heated surface due to a honeycomb porous plate, which are believed to be responsible for this phenomenon, are explained in the following. Fig. 4 shows a schematic diagram of the liquid supply mechanism in a honeycomb porous plate. In Fig. 4, (1) and (2) depict the liquid flow caused by the capillary force and the inflow of liquid through the vapor escape channels from the top surface by gravity, respectively [12]. Liquid passing through the vapor escape channels from the top surface due to gravity, as indicated by (2) in Fig. 4, spreads quickly at the bottom of the vapor escape channel due to the nanoparticle-deposited surface, which prevents dryout from growing on the heated surface because of good wettability and the capillary wicking effect. Consequently, the case of a honeycomb porous plate placed on a nanoparticle-deposited surface has the best performance with respect to CHF enhancement. Fig. 5(a)–(c) indicates the heat transfer coefficient for the cases shown in Fig. 3. As shown in the figure, the boiling heat transfer coefficients of plain surfaces are higher than that of nanoparticledeposited surfaces in all cases. This tendency is caused by the thermal resistance due to nanoparticle-deposition. This similar tendency is observed even in the case with a honeycomb porous plate (h and .). Moreover, irrespective of nanoparticle deposition
3.1 MW/m2
1
mean of T1 and T2, d1 is the distance between TC1 and TC2, and d2 is the distance between TC1 and the boiling surface. Table 1 shows an example of the relative uncertainties calculated using Eqs. (1)–(3). As shown in the table, the relative uncertainties depend on the experimental conditions and tend to become smaller with increasing heat flux.
3. Experimental results and discussion
2.8 MW/m2
Water 3 P = 0.1 MPa TSUB = 0 K (a) 10mm 2
(c)
2
1.9 MW/m2
50mm
2
2.1 MW/m 1.0 MW/m2
1
1.3 MW/m2
0
0
10
20 Tsat [K]
30
Plain surface Nanoparticle deposited surface Honeycomb porous plate installed on a plain surface Honeycomb porous plate installed on a nano-particle deposited surface Fig. 3. Boiling curves for different surface modifications 10, 30, and 50 mm in diameter.
occurred on the heated surface, the boiling heat transfer coefficients in the case with a honeycomb porous plate (h and .) were clearly higher than those without a honeycomb porous plate (D, s). As shown in Fig. 5, this enhancement was caused by the honeycomb porous plate. As stated in the introduction, nanofluids have received a great deal of attention because of the drastic CHF enhancement they provide. As such, a number of studies have proposed enhancing the IVR capability for severe accident management strategies using nanofluid [16,17,22]. In order to establish the cooling system for IVR, the CHF must be enhanced even for a sufficiently large heated surface. Therefore, the effect of heater size on CHF enhancement is very important for IVR. Fig. 6 depicts the relationship between the CHF and representative length of the heater size Lh corresponding to diameters of heated surface with or without surface modifications. The values of the CHF reported by Kwark et al. [18] are included in the figure
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S. Mori et al. / International Journal of Heat and Mass Transfer 80 (2015) 1–6
(2)
(1)
Honeycomb porous plate (1) Water flow Steam flow
Heat flux
Fig. 4. Schematic diagram of the liquid supply mechanisms to the heated surface generated by a honeycomb porous plate.
q [MW/m2]
3 2 1 0
0
10
20
30 40 Lh [mm]
50
60
Plain surface Nanoparticle deposited surface
h [W/m2 K]
[
2 10 ] 5
(a)
Water P = 0.1 MPa TSUB = 0 K
10mm
Honeycomb porous plate installed on a plain surface Honeycomb porous plate installed on a nano-particle deposited surface Kwark et al.(2010) ,(Nano-particle deposited surface)
1 Fig. 6. Relationship between the CHF and the size of heated surfaces with various surface modifications.
0 0
10
20
30
Tsat [K]
2
h [W/(m K)]
[×10 5] 2
(b)
30mm
Water P = 0.1 MPa TSUB = 0 K
1
0
0
10
20 Tsat [K]
30
[×10 5] 2
h [W/(m2K)]
(c)
50mm
4. Conclusions
Water P = 0.1 MPa TSUB = 0 K
1
0
0
10
20 Tsat [K]
nanoparticle-deposited surface of 30 and 50 mm in diameter was not enhanced greatly compared to a plain surface. Therefore it is possible that a nanoparticle-deposited surface alone cannot enhance the CHF when applied to a large heated surface. This is because the liquid may be pumped two-dimensionally toward the center of the heated surface due to capillary suction and wettability for smaller heated surfaces. On the other hand, when using a honeycomb porous plate, the CHF was enhanced compared with a plain surface (s) even for a diameter of 50 mm. Furthermore, as stated above, the best performing surface modification for CHF enhancement was confirmed to be a honeycomb porous plate installed on a nanoparticle-deposited surface. Therefore, the strategy of combining a nanofluid and a honeycomb porous plate is suitable for high heat flux removal from a large heated surface.
30
Plain surface Nanoparticle deposited surface Honeycomb porous plate installed on a plain surface Honeycomb porous plate installed on a nano-particle deposited surface Fig. 5. Heat transfer coefficients for different surface modifications 10, 30, 50 mm in diameter.
for comparison. The CHF obtained in the present study for a diameter of 10 mm is in good agreement with the data obtained by Kwark et al. [18]. As shown in the figure, the CHF tends to decrease with increasing heater size for all cases. In particular, the CHF for a
In the present study, the CHF in a saturated pool boiling of water was investigated experimentally using a honeycomb porous plate and nanofluid. After preparation of the nanoparticle-deposited heat transfer surface, liquid containing nanoparticles was removed from the test vessel in order to prevent nanoparticles from continuously depositing on the heated surface. The vessel was then cleaned and refilled with distilled water. The CHF with a nanoparticle-deposited surface was verified to decrease with the increase of heater size, as shown in a previous study [18]. The CHF in the case of a nanoparticle-deposited surface was significantly influenced by the heater size. The best performing surface modification for CHF enhancement was confirmed to be a honeycomb porous plate installed on a nanoparticle-deposited surface. Under the best performing surface modifications, the CHF for 10mm-, 30-mm- and 50-mm-diameter surfaces was enhanced up to 3.1, 2.3, and 2.2 MW/m2, respectively. Therefore, high heat flux removal of a large heated surface can be achieved by the combined use of a honeycomb porous plate and nanofluid.
Conflict of interest None declared.
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S. Mori et al. / International Journal of Heat and Mass Transfer 80 (2015) 1–6
Acknowledgments The present study was supported in part by the Research Foundation for Electro technology of Chubu and TEPCO Memorial Foundation.
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[11] S. Mori, K. Okuyama, Enhancement of the critical heat flux in saturated pool boiling using honeycomb porous media, Int. J. Multiphase Flow 35 (10) (2009) 946–951. [12] 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, in: The 14th International Heat Transfer Conference, ASME, Washington DC, 2010. [13] S.M.S. Murshed, C.A. Nieto de Castro, M.J.V. Lourenço, M.L.M. Lopes, F.J.V. Santos, A review of boiling and convective heat transfer with nanofluids, Renew. Sustain. Energy Rev. 15 (5) (2011) 2342–2354. [14] H. Seon Ahn, M. Hwan Kim, A review on critical heat flux enhancement with nanofluids and surface modification, J. Heat Transfer 134 (2) (2012) 024001. [15] H. Kim, M. Kim, Experimental study of the characteristics and mechanism of pool boiling CHF enhancement using nanofluids, Heat Mass Transfer 45 (7) (2007) 991–998. [16] 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. Therm. Eng. 35 (2012) 157–165. [17] 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, Nucl. Eng. Des. 239 (5) (2009) 941–948. [18] S.M. Kwark, M. Amaya, R. Kumar, G. Moreno, S.M. You, Effects of pressure, orientation, and heater size on pool boiling of water with nanocoated heaters, Int. J. Heat Mass Transfer 53 (23–24) (2010) 5199–5208. [19] H.D. Kim, M.H. Kim, Effect of nanoparticle deposition on capillary wicking that influences the critical heat flux in nanofluids, Appl. Phys. Lett. 91 (1) (2007) 014104. [20] T. Okawa, M. Takamura, T. Kamiya, Boiling time effect on CHF enhancement in pool boiling of nanofluids, Int. J. Heat Mass Transfer 55 (9–10) (2012) 2719– 2725. [21] B.N. Taylor, C.E. Kuyatt, Guidelines for evaluation and expressing the uncertainty of NIST measurement results, NIST Tech. Note 1297 (1993) 1–20. [22] 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, Nucl. Eng. Des. 252 (2012) 184–191.
Contents lists available at ScienceDirect
International Journal of Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ijhmt
Technical Note
Enhancement of the critical heat flux in saturated pool boiling of water by nanoparticle-coating and a honeycomb porous plate Shoji Mori ⇑, Suazlan Mt Aznam, Kunito Okuyama Department of Chemical Engineering Science, Graduate School of Engineering, Yokohama National University, 79-5, Tokiwadai, Hodogaya-ku, Yokohama, Kanagawa 240-8501, Japan
a r t i c l e
i n f o
Article history: Received 17 February 2014 Received in revised form 8 August 2014 Accepted 21 August 2014 Available online 21 September 2014 Keywords: Pool boiling Enhancement of critical heat flux Porous media Nanofluid
a b s t r a c t Various surface modifications of the boiling surface, e.g., integrated surface structures, such as channels and micro-pin fins, and the coating of a micro-porous layer using sintered metal powders and nanoparticle deposition onto the heat transfer surface, have been proven to effectively enhance the critical heat flux (CHF) in saturated pool boiling. In particular, novel methods involving nanofluids have gained a great deal of attention because the CHF for the use of nano-fluids is increased drastically, by up to approximately three times compared to that of pure water. CHF enhancement using nanofluids is related to surface wettability, surface roughness, and capillary wicking performance due to nanoparticle deposition on the heated surface. Several studies have proposed the use of nanofluids to enhance the in-vessel retention (IVR) capability in the severe accident management strategy implemented at certain light-water reactors. Systems using nanofluids for IVR must be applicable to large-scale systems, i.e., sufficiently large heated surfaces compared to the characteristic length of boiling (capillary length). However, as for the effect of the size of heater with nanoparticle deposition, it was revealed that the CHF tends to be decreased with the increased heater size. On the other hand, the CHF in saturated pool boiling of water using a honeycomb porous plate was shown experimentally to become approximately twice that of a plain surface with a heated surface diameter of 30 mm, which is comparatively large. The enhancement is considered to result from the capillary supply of liquid onto the heated surface through the microstructure and the release of vapor generated through the channels. In the present paper, in order to enhance the CHF on a large heated surface, the effects of a honeycomb porous plate and/or nanoparticle deposited heat transfer surface on the CHF were investigated experimentally. As a result, the CHF was enhanced greatly by the attachment of a honeycomb porous plate to the modified heated surface by nanoparticle deposition, even in the case of a large heated surface. Under the best performing surface modifications, the CHF for 10-mm-, 30-mm- and 50-mm-diameter surfaces was enhanced up to 3.1, 2.3, and 2.2 MW/m2, respectively. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction One strategy for severe accidents, such as loss of coolant accidents (LOCA), is in-vessel retention (IVR) of corium debris [1]. The IVR consists of external cooling of the reactor vessel in order to remove decay heat from the molten core through the lower head of the vessel. However, the heat removal is limited by the occurrence of critical heat flux (CHF) at the outer surface of the reactor vessel. Therefore, in order to enhance the capability of the IVR in the severe accident of the light-water reactors, methods to increase the CHF should be considered.
⇑ Corresponding author. Tel./fax: +81 45 339 4010. E-mail address: [email protected] (S. Mori). http://dx.doi.org/10.1016/j.ijheatmasstransfer.2014.08.046 0017-9310/Ó 2014 Elsevier Ltd. All rights reserved.
Various modifications of the boiling surface, e.g., integrated surface structures, such as channels and micro-pin fins, and the coating of a micro-porous layer using sintered metal powders and nanoparticle deposition onto the heat transfer surface, have been proven to effectively enhance the CHF in saturated pool boiling [2–6]. Several researchers have offered novel structures of the porous media in which the liquid and vapor flow paths will be separated [7–12]. The CHF has been enhanced significantly up to approximately twice that of a plain surface with a heated surface diameter of 30 mm using a honeycomb porous plate [11]. Moreover, the effect of the channel width on the CHF in the saturated pool boiling has been investigated experimentally [12]. You et al. [6] introduced a novel method to enhance the CHF of pool boiling using nanofluids. The CHF was increased drastically up
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to approximately three times (1.68 MW/m2) compared to that of pure water. The drastic enhancement in the CHF obtained by using nanofluids is of particular interest [13,14]. The CHF enhancement obtained by using nanofluids is related to the surface wettability, surface roughness, and capillary wicking performance due to nanoparticle deposition on the heated surface [15]. Based on the studies mentioned above, the mechanism of the CHF enhancement can be explained by the capillary suction effect, the extended surface area effect, the liquid supply caused by the hydrodynamic effect, and the wettability of the heated surface. There are several ways to enhance the CHF. However, approaches for increasing the IVR capability must be simple and installable at low cost. The use of nanofluids has been proposed in order to enhance the IVR capability in severe accident management strategies for advanced light-water reactors [16,17]. The strategy of using nanofluid for IVR should be applicable to a large heated surface compared to a characteristic length of boiling (capillary length). However, for Al2O3 nanoparticle-coated flat heaters, the CHF tends to be decrease with an increase in heater size [18]. On the other hand, as stated above, the CHF using a honeycomb porous plate was shown experimentally to be approximately twice that of a plain surface having a heated surface diameter of 30 mm [11,12], which is comparatively large. The enhancement is considered to result from the capillary supply of liquid onto the heated surface and the release of generated vapor through the channels. In the present paper, in order to succeed in heat removal from a large heated surface with high heat flux, we focused on the use of a honeycomb porous plate and nanofluid. Therefore, the CHF in a
saturated pool boiling of water was investigated experimentally using a honeycomb porous plate and/or nanoparticle deposited heat transfer surface. As a result, the CHF was greatly enhanced under the attachment of a honeycomb porous plate on the modified heated surface by nanoparticle deposition. 2. Experimental apparatus and procedure 2.1. Experimental apparatus A schematic diagram of the pool boiling test facility is shown in Fig. 1. The main vessel is made of Pyrex glass and has an inner diameter of 87 mm and a height of 500 mm. The pool container was filled with distilled water to a height of approximately 60 mm above the heated surface. The heat flux was supplied to the boiling surface through a copper cylinder using a cartridge electric heater, which was inserted into the copper cylinder and was 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 a copper cylinder with diameters of 10 mm, 30 mm, and 50 mm is smooth and is used as the heat transfer surface in the experiments. Three sheathed thermocouples with an outer diameter of 0.5 mm were inserted horizontally to the centerline of the copper cylinder. The thermocouples (TC1, TC2 and TC3 shown in Fig. 1) in the copper cylinder were set apart axially by 5.0 mm. The closest thermocouple was located 10.0 mm below the boiling surface. These thermocouples were calibrated using a
Fig. 1. Schematic diagram of the experimental apparatus.
S. Mori et al. / International Journal of Heat and Mass Transfer 80 (2015) 1–6
platinum resistance thermometer. The wall temperature and the 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. The linearity of the temperature distribution was confirmed.
3
depositing on the heated surface. Finally, the vessel was cleaned and refilled with distilled water. 2.4. Honeycomb porous plates
Experiments were carried out using distilled water as a working fluid under saturated conditions at atmospheric pressure. A sheathed heater was installed above the heated surface in the liquid bath in order to maintain the liquid temperature at the saturation temperature. At each run, the heated surface was polished using waterproof 2000-grit sand paper and then cleaned using acetone in order to minimize the effect of oxidation of heat transfer surface. The heat flux was increased in increments of approximately 0.1 MW/m2 until burnout occurred. All of the measurements were performed at the steady state. The steady state was regarded as being reached when the temperatures did not change more than 0.25 K for at least 10 min. When burnout occurred, the heating was immediately stopped in order to prevent the heater and the 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.
Fig. 2 shows the honeycomb porous plate used in the present study [11], and a micrograph of its structure is shown on the right-hand side of the figure. The honeycomb porous plate, which is commercially available, was used as a filter for purifying exhaust gases from combustion engines. The constitutive ingredients are CaOAl2O3 (30 to 50 wt%), fused SiO2 (40 to 60 wt%), and TiO2 (5 to 20 wt%). The vapor escape channel width (cell width) dV‘, the wall thickness dS of the grid, the aperture ratio, and the height of the honeycomb porous plate dh are 1.4 mm, 0.45 mm, 0.55, and 1 mm, respectively, as shown in Fig. 2. The honeycomb porous plate was pressed against the top of the boiling surface by stainless steel wire of 0.3 mm in diameter. Any thermally conductive grease was not used between honeycomb porous plate and heated surface. The pore radius distribution of the honeycomb porous plates was measured by mercury penetration porosimetry, which peaked at approximately 0.17 lm. The average pore radius, the median pore radius, and the porosity of the honeycomb porous plates based on porosimetry are 0.037 lm, 0.13 lm, and 24.8%, respectively.
2.3. Preparation of the nanoparticle-deposited heat transfer surface
2.5. Uncertainty analysis
The preparation of the nanoparticle-deposited surface is similar to that described in previous studies [19,20]. The TiO2 nanoparticles (Aero-sil Corporation, Aeroxide TiO2 P 25) were selected as the test nanoparticles and were dispersed in distilled water, i.e. nanofluid. The mean particle diameter of the particle supplied by the company was approximately 21 nm. The heated surface was polished using waterproof 2000-grit sand paper and then cleaned using acetone. For preparation, 800 ml of distilled water was kept boiling in a test vessel at 500 kW/m2 for at least 30 min. A volume of 200 ml of nanofluid (C = 0.0201 wt%), which was stirred for at least four hours using an ultrasonic bath, was then added to the boiling distilled water in the test vessel. Therefore, the final nanoparticle concentration was 0.0040 wt% (volume concentration: 0.0011 vol%). In order to form the nanoparticle-deposited surface, the time from the addition of nanofluid was set to 20 min. The reason for these conditions was that the CHF was not changed greatly under a higher concentration and a longer deposition time compared to the present condition (C = 0.0040 wt%, deposition time = 20 min) [20]. Heating was then turned off, and the liquid containing nanoparticles was removed from the test vessel to prevent nanoparticles from continuously
The individual standard uncertainties are combined to obtain the estimated standard deviation of the results, which is calculated using the law of propagation of uncertainty [21]. The uncertainties of the heat flux q, the superheat DTsat, and the heat transfer coefficient h, respectively, are evaluated in the following equations:
1 mm
2.2. Experimental procedure
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 2 2 @q @q @q @q Dq ¼ Dk þ Dd 1 þ DT 1 þ DT 2 @k @d @T 1 @T 2
DðDT sat Þ ¼
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 2 2 @ðDT sat Þ @ðDT sat Þ @ðDT sat Þ @ðDT sat Þ Dq þ DT 1 þ Dd2 þ Dk @q @T 1 @d2 @k ð2Þ
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 @h @h Dh ¼ DðDT sat Þ þ Dq @ðDT sat Þ @q
ð3Þ
where T1 and T2 are the temperatures at TC1 and TC2, respectively, k is the thermal conductivity of copper evaluated at the arithmetic
1.4 mm
φ10, 30, and 50 mm
ð1Þ
0.45 mm
Aperture ratio: 0.55 Fig. 2. Dimensions of the honeycomb porous plate.
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Table 1 Relative uncertainties of the measured quantities.
DTsat (K)
h (kW/ (m2 K))
Dq/q (%)
D(DTsat)/DTsat (%)
Dh/h (%)
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
3.8 3.2 2.8 2.5
q [MW/m2 ]
q (MW/ m2)
0 0
1.5 MW/m2 2.0 MW/m2
10
20
30
q [MW/m2 ]
Tsat [K]
Water 3 P = 0.1 MPa TSUB = 0 K (b) 30mm 2
2.2 MW/m2 2.2 MW/m2 1.1 MW/m2
1 1.0 MW/m2
0 0
10
20
30
Tsat [K]
Water P = 0.1 MPa TSUB = 0 K
3 q [MW/m2]
Fig. 3(a)–(c) show the boiling curves for the cases with and without surface modifications, i.e., a nanoparticle-deposited surface (D), a honeycomb porous plate installed on a plain surface (h), a honeycomb porous plate installed on a nanoparticle-deposited surface (.), and a plain surface (s) for surfaces having diameters of 10, 30, and 50 mm, respectively. The arrows in Fig. 3 correspond to the CHF condition. As shown in the figure, the CHFs of the nanoparticle-deposited surfaces are higher than those of plain surfaces. The CHF enhancement for the case of the nanocoated surface is attributed to the surface wettability, surface roughness, and the capillary wicking effect due to nanoparticle deposition on the heated surface, as reported in previous studies [6,13–20]. As shown in the figure, the CHF for the case of a honeycomb porous plate, as indicated by the h and . symbols, is enhanced compared to that for the cases of plain and nano-coated surfaces (s, D). The CHF enhancements in the case of a honeycomb porous plate are due to the automatic liquid supply to the heated surface due to capillary action and the reduction of the vapor escape flow resistance due to the separation of liquid and vapor flow by the honeycomb porous structure [11,12]. Moreover, it is interesting that the CHF in the case of a honeycomb porous plate installed on a nanoparticle-deposited surface is higher than that for the case of a honeycomb porous plate alone on a plain surface for diameters of 10, 30, and 50 mm. The mechanisms of liquid supply to the heated surface due to a honeycomb porous plate, which are believed to be responsible for this phenomenon, are explained in the following. Fig. 4 shows a schematic diagram of the liquid supply mechanism in a honeycomb porous plate. In Fig. 4, (1) and (2) depict the liquid flow caused by the capillary force and the inflow of liquid through the vapor escape channels from the top surface by gravity, respectively [12]. Liquid passing through the vapor escape channels from the top surface due to gravity, as indicated by (2) in Fig. 4, spreads quickly at the bottom of the vapor escape channel due to the nanoparticle-deposited surface, which prevents dryout from growing on the heated surface because of good wettability and the capillary wicking effect. Consequently, the case of a honeycomb porous plate placed on a nanoparticle-deposited surface has the best performance with respect to CHF enhancement. Fig. 5(a)–(c) indicates the heat transfer coefficient for the cases shown in Fig. 3. As shown in the figure, the boiling heat transfer coefficients of plain surfaces are higher than that of nanoparticledeposited surfaces in all cases. This tendency is caused by the thermal resistance due to nanoparticle-deposition. This similar tendency is observed even in the case with a honeycomb porous plate (h and .). Moreover, irrespective of nanoparticle deposition
3.1 MW/m2
1
mean of T1 and T2, d1 is the distance between TC1 and TC2, and d2 is the distance between TC1 and the boiling surface. Table 1 shows an example of the relative uncertainties calculated using Eqs. (1)–(3). As shown in the table, the relative uncertainties depend on the experimental conditions and tend to become smaller with increasing heat flux.
3. Experimental results and discussion
2.8 MW/m2
Water 3 P = 0.1 MPa TSUB = 0 K (a) 10mm 2
(c)
2
1.9 MW/m2
50mm
2
2.1 MW/m 1.0 MW/m2
1
1.3 MW/m2
0
0
10
20 Tsat [K]
30
Plain surface Nanoparticle deposited surface Honeycomb porous plate installed on a plain surface Honeycomb porous plate installed on a nano-particle deposited surface Fig. 3. Boiling curves for different surface modifications 10, 30, and 50 mm in diameter.
occurred on the heated surface, the boiling heat transfer coefficients in the case with a honeycomb porous plate (h and .) were clearly higher than those without a honeycomb porous plate (D, s). As shown in Fig. 5, this enhancement was caused by the honeycomb porous plate. As stated in the introduction, nanofluids have received a great deal of attention because of the drastic CHF enhancement they provide. As such, a number of studies have proposed enhancing the IVR capability for severe accident management strategies using nanofluid [16,17,22]. In order to establish the cooling system for IVR, the CHF must be enhanced even for a sufficiently large heated surface. Therefore, the effect of heater size on CHF enhancement is very important for IVR. Fig. 6 depicts the relationship between the CHF and representative length of the heater size Lh corresponding to diameters of heated surface with or without surface modifications. The values of the CHF reported by Kwark et al. [18] are included in the figure
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(2)
(1)
Honeycomb porous plate (1) Water flow Steam flow
Heat flux
Fig. 4. Schematic diagram of the liquid supply mechanisms to the heated surface generated by a honeycomb porous plate.
q [MW/m2]
3 2 1 0
0
10
20
30 40 Lh [mm]
50
60
Plain surface Nanoparticle deposited surface
h [W/m2 K]
[
2 10 ] 5
(a)
Water P = 0.1 MPa TSUB = 0 K
10mm
Honeycomb porous plate installed on a plain surface Honeycomb porous plate installed on a nano-particle deposited surface Kwark et al.(2010) ,(Nano-particle deposited surface)
1 Fig. 6. Relationship between the CHF and the size of heated surfaces with various surface modifications.
0 0
10
20
30
Tsat [K]
2
h [W/(m K)]
[×10 5] 2
(b)
30mm
Water P = 0.1 MPa TSUB = 0 K
1
0
0
10
20 Tsat [K]
30
[×10 5] 2
h [W/(m2K)]
(c)
50mm
4. Conclusions
Water P = 0.1 MPa TSUB = 0 K
1
0
0
10
20 Tsat [K]
nanoparticle-deposited surface of 30 and 50 mm in diameter was not enhanced greatly compared to a plain surface. Therefore it is possible that a nanoparticle-deposited surface alone cannot enhance the CHF when applied to a large heated surface. This is because the liquid may be pumped two-dimensionally toward the center of the heated surface due to capillary suction and wettability for smaller heated surfaces. On the other hand, when using a honeycomb porous plate, the CHF was enhanced compared with a plain surface (s) even for a diameter of 50 mm. Furthermore, as stated above, the best performing surface modification for CHF enhancement was confirmed to be a honeycomb porous plate installed on a nanoparticle-deposited surface. Therefore, the strategy of combining a nanofluid and a honeycomb porous plate is suitable for high heat flux removal from a large heated surface.
30
Plain surface Nanoparticle deposited surface Honeycomb porous plate installed on a plain surface Honeycomb porous plate installed on a nano-particle deposited surface Fig. 5. Heat transfer coefficients for different surface modifications 10, 30, 50 mm in diameter.
for comparison. The CHF obtained in the present study for a diameter of 10 mm is in good agreement with the data obtained by Kwark et al. [18]. As shown in the figure, the CHF tends to decrease with increasing heater size for all cases. In particular, the CHF for a
In the present study, the CHF in a saturated pool boiling of water was investigated experimentally using a honeycomb porous plate and nanofluid. After preparation of the nanoparticle-deposited heat transfer surface, liquid containing nanoparticles was removed from the test vessel in order to prevent nanoparticles from continuously depositing on the heated surface. The vessel was then cleaned and refilled with distilled water. The CHF with a nanoparticle-deposited surface was verified to decrease with the increase of heater size, as shown in a previous study [18]. The CHF in the case of a nanoparticle-deposited surface was significantly influenced by the heater size. The best performing surface modification for CHF enhancement was confirmed to be a honeycomb porous plate installed on a nanoparticle-deposited surface. Under the best performing surface modifications, the CHF for 10mm-, 30-mm- and 50-mm-diameter surfaces was enhanced up to 3.1, 2.3, and 2.2 MW/m2, respectively. Therefore, high heat flux removal of a large heated surface can be achieved by the combined use of a honeycomb porous plate and nanofluid.
Conflict of interest None declared.
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Acknowledgments The present study was supported in part by the Research Foundation for Electro technology of Chubu and TEPCO Memorial Foundation.
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