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Enhancement of critical heat flux using nano-fluids for Invessel Retention–External Vessel Cooling
Applied Thermal Engineering 35 (2012) 157e165
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Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Enhancement of critical heat flux using nano-fluids for Invessel RetentioneExternal Vessel Cooling Q.T. Pham*, T.I. Kim, S.S. Lee, S.H. Chang Korea Advanced Institute of Science and Technology, 373-1, Guseong-dong, Yuseong-gu, Daejeon 305-701, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 April 2011 Accepted 6 October 2011 Available online 19 October 2011
This study investigated the pool boiling critical heat flux (CHF) of water-based nano-fluids under atmospheric pressure for Invessel Retention (IR)eExternal Vessel Cooling (EVC). The heated surface was a stainless steel foil inclined at different orientation angle from 0 (horizontal downward facing position) to 90 (vertical position). Three working nano-fluids with high suspension stability were selected by the zeta potential method to investigate the effect of each nano-fluid on CHF at the heated surface, which were 0.05% Alumine (Al2O3), 0.05% carbon nanotubes (CNT) þ 10% boric acid and 0.05% Al2O3 þ 0.05% CNT. It was observed that these nano-fluids enhanced CHF significantly (up to 220%) compared to deionized (DI) water. Furthermore, for all test fluids, CHF increased when the orientation angle increased. The surface characterization after boiling tests shows that the CHF enhancement with nano-fluids can be related to the increase of both surface roughness and wettability caused by nanoparticle deposition during the boiling processes. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Pool boiling CHF Nano-fluids IVR Contact angle Roughness
1. Introduction Severe accidents result from hypothetical Loss of Coolant Accidents (LOCA) coupled with a failure of Emergency Core Cooling System (ECCS) in a nuclear reactor. One of the management strategies for those accidents, which are currently being studied for advanced nuclear reactor, is Invessel Retention of corium debris (IVR). The IVR strategy consists of external cooling of the reactor vessel by flooding the reactor cavity so that decay heat from the molten core is removed through the lower head of the vessel. The decay heat is removed at low heat flux by natural convection. Pool boiling occurs as heat flux increases and heat removal is limited by the occurrence of critical heat flux (CHF) at the reactor vessel outer surface. CHF is the boiling crisis which refers to a condition characterized by a sharp reduction of local heat transfer coefficient resulting from the vapor blanket at the surface of the reactor vessel. This is accompanied by an inordinate increase in vessel outer surface temperature which damages the integrity of the vessel. The CHF should be investigated from the standpoint of IVR. The ex-vessel boiling process is described as boiling from a large downward facing curved surface. CHF at the surface of a downward
* Corresponding author. Tel.: þ82 42 350 3856; fax: þ82 42 869 3810. E-mail addresses: [email protected] (Q.T. Pham), skyimgf@ kaist.ac.kr (T.I. Kim), [email protected] (S.S. Lee), [email protected] (S. H. Chang). 1359-4311/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.applthermaleng.2011.10.017
facing configuration depends on surface inclination angle. The effect of heated surface orientation in a pool of isopropyl alcohol under atmospheric pressure was studied by Girhjji and Sabersky [1], with three different orientations (upward facing, vertical and downward facing). CHF of downward facing surface was found to be considerably smaller than other orientations. The effect of orientation in the nucleate boiling of R-11 was studied by Jung et al. [2]. The heated surface was changed from downward facing to upward facing. Increase of orientation from 0 to 165 caused the wall superheat to decrease by 15e25%. CHF decreased with orientation of heated surface from upward to downward facing [3]. CHF increased with increase in inclination angle when a 12.8 mm thick copper disk quenching in a pool of saturated water [4]. Nucleate boiling heat flux from a 0.3-m diameter curved surface exceeded 1 MW/m2 for highly subcooled water in a transient quenching integral experiments conducted [5]. CHF at the bottom center region of the IVR configuration under pool boiling conditions was approximately 300 kW/m2 [6]. Numerous attempts have been made to enhance the IVR capabilities of the reactor vessel to ensure that IVR can be achieved when a severe accident happens. The potential use of nano fluid to increase the CHF on downward facing surfaces is a possible IVR enhancing approach. Nano-fluids are engineered colloidal dispersions of nano-particles (of size less than 100 nm) in based fluids. Nano-fluids provide higher CHF than water at very low concentrations [7-9]. They are potential coolants for IVR to enhance the safety margin at the surface of reactor vessel.
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Nomenclature
q rl rv s a q00CHF g hfg Ra
contact angle, rad or liquid density, kg/m3 vapor density, kg/m3 liquidevapor surface tension force, N/m inclined angle (from horizontal downward to upward facing), rad or critical heat flux, kW/m2 gravity, m/s2 liquidevapor latent heat, J/kg surface roughness, mm
In this study, a series of experiments were performed to understand the pool boiling critical heat flux behavior on downward facing surfaces submerged in a pool of nano-fluids at very low concentration. Three types of nano-fluids were selected for study of fluid stability, which are alumina (Al2O3) 0.05% vol., Al2O3 0.05% vol. þ carbon nanotubes (CNT) 0.05% vol. and CNT 0.05% vol. þ boric acid (BA) 10% vol. The inclination angle was changed from horizontal to vertical. The influence of orientation on CHF enhancement due to the use of nano-fluids was investigated. Modifications of heated surface due to boiling in nano-fluids were characterized with an effort to find out the mechanisms underlying the CHF enhancement in nano-fluids. 2. Application of zeta potential method to select nano-fluids for use as IVReEVC coolants 2.1. Dispersion stability experiments with Al2O3 and CNT nanofluids Al2O3 and CNT nano-fluids were prepared by suspending Al2O3 and CNT nano-particles in DI water. The suspensions were transferred into an ultrasonic bath and sonicated for 1 h at room temperature. Boric acid was used as surfactant to change the pH of the fluids. Suspensions of nano-particles in DI water with and without boric acid and a combination of Al2O3 and CNT nano-particles in DI water were made in the same way for stability comparison. The zeta potentials of prepared nano-fluids were measured by ELSZ2 (Otsuka Electronics). The pH of fluids was measured by MIL-56-pH (Sechang) to verify the reliability of zeta potential results. Concentrations and dispersing time used in this study are shown in Table 1. 2.2. Zeta potential results and selection of nano-fluids for IVReEVC coolants
range of 35 w 45 mV within 3 days after sonication. Zeta potentials were significantly enhanced with the addition of boric acid. The Al2O3 nano-fluid with 10% volumetric concentration of boric acid was a highly stable suspension. CNT nano-fluid did not have good stability. The zeta potential of CNT-nano-fluid was very low (below 30 mV). The stability of CNT nano-fluid was increased by the addition of boric acid; the zeta potential reached the range of 35 w 45 mV, which is in the high stable dispersion range when the concentration of boric acid was 10% of volume. The zeta potential measurement of Al2O3 þ CNT combined nano-particles in water with low concentration (0.05% volume each) showed very good stability with increased zeta potentials up to 55 w 60 mV. Three different types of nano-fluids were selected for potential use as coolant in IVR (Al2O3 0.05%, CNT 0.05% þ Boric acid 10%, Al2O3 0.05% þ CNT 0.05%) considering stability and cost for an IVReEVC coolant.
3. Pool boiling experiment with an inclined downward facing surface 3.1. Experiment apparatus and procedure The test pool and test section were made of Type 316-stainless steel (cf. Fig. 2). The upper head of the pool was open for easy observation of phenomenon. The test section (1) was a plate 20-cm long, 5-cm wide and 1-mm thick, which was installed at the center of the test pool. A pulley (2) was installed at the side of the experiment loop which was used to control the orientation of the test section. The orientation of the test section was measured by an angler meter. The upper surface of the test section was isolated by high temperature resistant epoxy (3) for sealing the heated surface to diminish heat loss from the test section. Two copper electrodes (4) were welded to the ends of test section to transfer electric power from a 50 V, 4000A DC power supply system. The test section plate and copper electrodes were electrically insulated from the other parts of the loop by teflon rods. Five type-K thermocouples were installed along the test section to measure the temperature of the test section. Voltage signal from the test section were measured and processed by the system. The test pool was filled with DI water or one of the three selected nano-fluids. First, the test section was fixed at a desired orientation. The working fluid, preheated by a preheater, was pumped to the test pool by a centrifugal pump. The input power was slightly increased until CHF condition was detected. When CHF
The zeta potential of nano-fluids is shown in Fig. 1. Al2O3 nanofluid demonstrated good stability with the zeta potential in the Table 1 Zeta potential measurement test matrix. Base fluid
Nano-particles volume concentration
Boric acid volume concentration
Variation time
DI water
Al2O3 0.05% Al2O3 0.05% Al2O3 0.05% CNT 0.05% CNT 0.05% þ Al2O3 0.05% CNT 0.05% CNT 0.05%
0% 0.5% 10% 0% 0%
1 w 3 days
5% 10% Fig. 1. Zeta potential as a function of time.
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Fig. 2. Photos of (a) the test pool and b) the test section.
occurred, the power was immediately stopped. Before and after the experiment, samples of working fluids were extracted to measure zeta potential and pH to confirm the stability of fluids. The inclination angle was changed step by step from horizontal to vertical. CHF data with DI water were compared with nano-fluids. Experiments were conducted all in sub-cooled conditions with ΔTsub ¼ 7 w 14 C. Three selected nano-fluids were used as working fluids and the CHF data were compared with that of DI water. For each fluid, five different orientations of the test section were selected to perform the experiments, which are 0 (downward facing position), 30 , 45 , 60 , 90 (vertical position), corresponding to five positions at the outer surface of the reactor vessel (0 is equal to the bottom center of the vessel and 90 is the equator of the vessel). Each CHF test was repeated three times to ensure the repeatability of the data. 3.2. CHF data acquisition CHF data are shown in Fig. 3. The CHF data of DI water exhibits an increase of CHF with the increase of the inclined angle from 0 to 90 with respect to downward facing to vertical position of the test section. Similar dependence of CHF on orientation was also found by Githinji and Yang before [1,10]. For upward facing surfaces with
Fig. 4. Comparison of CHF between nano-fluids and DI water.
orientations of 90 (vertical) to 180 (horizontal upward facing), Kandlikar [11] developed a theoritical model based on force balance which also shows that the CHF increases with the inclined angle: 1= 1 þ cosq 4 q00CHF ¼ hlg r1=2 v ½sgðrl rv Þ 16 1= 2 2 p þ ð1 þ cosqÞcosð180 aÞ p 4
(1)
wherein q is the surface contact angle and a (in degree) is the inclined angle: for horizontal downward facing a ¼ 0 , for vertical facing a ¼ 90 and for horizontal upward facing a ¼ 180 .
Table 2 pH measurement before and after CHF experiment. Nano-fluids
pH
Fig. 3. Comparison of CHF between the present data and the data of Yang et al. [10].
Zeta potential (mV)
Before experiment After experiment Before experiment After experiment
0.05% Al2O3
0.05% Al2O3 þ 0.05% CNT
0.05% CNT þ 10% BA
4.68 4.85 39 41
4.80 4.84 53 51
3.27 3.05 35 38
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Fig. 5. 1a. SEM image of fresh surface. 2a. SEM image of Al2O3 nano-fluids heated surface. 3a. SEM image of CNT nano-fluids heated surface. 4a. SEM image of Al2O3þCNT nanofluids heated surface. 1b. EDS image of fresh surface. 2b. EDS image of Al2O3 nano-fluids heated surface. 3b. EDS image of CNT nano-fluids heated surface. 4b. EDS image of Al2O3þCNT nano-fluids heated surface.
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Fig. 6. a. Little CNT deposited on heated surface. b. Deposition layer of Al2O3 nano-particles. c. Deposition layer of Al2O3 þ CNT nano-particles.
However, as shown in Equation (1), the correlation of Kandlikar can only applied for upward facing conditions (a from 90 to 180 ). When the heated surface is positioned at downward facing (a from 0 to 90 ), cos(180 a) is negative and the CHF cannot be estimated by this equation. Therefore, in future works, it would be interesting to develop a more general theoretical model which enables to estimate the CHF from downward to upward facing (inclined angle from 0 to 180 ). For nano-fluids, significant CHF enhancements were obtained (cf. Fig. 4). The effect of orientation in nano-fluid tests demonstrates similar trend with DI water. CHF of nano-fluids also goes up when the inclined angle increases from downward facing to vertical. Enhancement of CHF due to nano-fluids was found to vary with orientation. Among the five tested orientations, maximum CHF enhancements occured at downward facing position. Enhancement of CHF by Al2O3 nano-fluids has been obtained in many other works previously [7,8,13,14] and this has been confirmed again in this study. Kashinath performed pool boiling experiment with a copper block using Al2O3 nano-fluids to investigate the effect of inclination to CHF. He also reported that CHF increased when the test specimen was inclined from downward facing up to upward facing. CNT nano-fluid showed larger CHF enhancement in comparison with Al2O3 nano-fluid. Interestingly, when Al2O3 and CNT nanofluids are dispersed together in DI water, achieved CHF was even higher than that of separate nano-fluid. 4. Nano-fluids stability verification One objective of this study is to select stable nano-fluids for the application to IVR system to enhance the thermal margin of the reactor vessel in accident condition. Therefore, the stability of nano-fluids is the key parameter that requires a significant attention. In this study, to confirm the dispersion stability of nano-fluids during the boiling experiment, zeta potential and pH of the selected nano-fluids were measured before and after each test. Results of these measurements are shown in Table 2. The zeta potentials and pHs of the three nano-fluids changed slightly during the boiling CHF experiments. These results guarantee that the selected nanofluids are stable enough to ensure a potential application of them as IVR coolants.
5. Investigation of CHF enhancement mechanisms in nanofluids The ability to enhance CHF is one of the most attractive features of nano-fluids, which has been investigated in many studies recently. However, a persuasive explanation of the mechanism of this phenomenon is yet to be found. During several decades of intense study a consensus explanation of the physical mechanism causing CHF, there have been many CHF models developed, most of which fall into one of the following four categories: hydrodynamic instability theory, macrolayer dryout theory, hot/dry spot theory, and bubble interaction theory [13]. Generally, existing CHF models suggests that a credible attempt at explaining the mechanism responsible for the large CHF enhancement observed in nano-fluids must include accurate knowledge of thermophysical property of the fluid and the heated surface conditions. Thermophysical properties of the fluid such as surface tension, thermal conductivity and viscosity can affect CHF according to the hydrodynamic instability model. However, at low concentration of nano-fluids, these properties of nano-fluids do not differ from those of water. Furthermore, the vapor density and enthalpy of evaporation are not expected to change due to low particle concentration [13,15]. Therefore, the influence of changes in thermophysical properties of nano-fluids compare to those of DI water is negligible and cannot be counted as the main reason for CHF enhancement. As shown in the macrolayer dryout model, hot/dry spot model as well as bubbles interaction model, CHF depends strongly on the surface condition of the heater. This has been confirmed experimentally by many works in literature [13]. In this study, the characterization of the heating surface modification occurring during the boiling experiments was investigated with an effort to find out the parameters governing the phenomena in which the unusual CHF enhancement of nano-fluids occurs. 5.1. SEM measurement The surfaces of the heater after boiling in nano-fluids were prepared for the Scanning Electron Microscope (SEM) and Energy Dispersive X-ray Spectroscopy (EDS) analysis to observe the effects of nano-particles on the morphology of the surface. This is of significance because both nucleate boiling heat transfer and CHF are
Table 3 Contact angle measurement data. Fluid
Fresh surface
Al2O3-nanofluid heated surface
CNT-nanofluid heated surface
Al2O3 þ CNT nano-fluid heated surface
DI water Al2O3 CNT Al2O3 þ CNT
68.213 66.961 64.947 69.201
16.734 1.253 14.658 2.492
42.794 2.089
22.308 0.762
1.436 1.681 2.367 2.643
39.443 2.097
17.617 1.116
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Fig. 7. Contact angle of DI water dropped on different surface.
highly affected by the surface configuration. According to the SEM and EDS analysis, some clear differences in the morphology of heated surfaces are evident among the three nano-fluids. The surface was clean during DI water boiling but nano-particles deposited on the heated surfaces during nano-fluids boiling (cf. Figs. 5.1e5.4). As stated in the literature, a porous layer of nano-particles deposited on the heater surface is responsible for the CHF enhancement in nano-fluids [7,8,12e14]. In this study, a micro-size layer of nano-particles on the heated surface was clearly observed by SEM after boiling in Al2O3 nano-fluid. The average thickness of this layer was 3.3 mm. A thicker layer of 7.58 mm was also seen on the heated surface after boiling in the Al2O3 þ CNT combined nanofluids. However, when the heater was boiled in CNT nano-fluid, the amount of CNT particle deposited was so little and was not enough to form a layer. EDS images show the compositions of the heated surfaces after boiling. It is clearly that the amount of CNT deposited is very minor compared to that of Al2O3 nano-particles (cf. Fig. 6). Although the deposition of Al2O3 nano-particles on the heated surface was much more than CNT, boiling CHF enhancement in CNT nano-fluids was remarkably higher than that of Al2O3 nano-fluids. These contradictory results opened a new insight into the CHF enhancement mechanism of nano-fluids. The nanoparticle deposited layer might have effects on CHF enhancement but is definitely not the only one governing factors contributing to CHF enhancement. It is important to stress that SEM images only allow a qualitative observation of the heated surface after nano-fluids boiling. For further quantification, the change in surface condition of the heater during boiling, wettability and roughness changes of the heater surface due to nano-fluids boiling were measured later.
5.2. Contact angle measurements Contact angle is a usual representative parameter of surface wettability. The retreating contact angle is often employed to correlate CHF because the heater surface becomes dry at the CHF conditions. Costello and Frea [16] reported the reduction of CHF on a poor wetting surface. As shown in the existing CHF models [11], a decrease in contact angle would lead to a significant increase in CHF. In this study, contact angles were measured for the sample surfaces after boiling test using a Phoenix 3000 Contact Angle Surface Analysis system. Drop of liquid (DI water or nano-fluids) was dropped on the surface of interest. Images of the liquid droplet were taken by a CCD camera. The software equipped with the system analyzes the contact angles on both sides of the droplet. All measurements were performed at room temperature. The general trend observed here is that contact angle of DI water on fresh surface has highest value compared to that on nano-fluid heated surface (Table 3). Al2O3 nano-fluid heated surface has the lowest contact angle overall. The contact angle of Al2O3 þ CNT combined nano-fluid heated surface is a little higher than the contact angle of Al2O3 nano-fluid heated surface but much smaller than that of CNT nano-fluid heated surface. The nano-fluids did not change the contact angle in comparison with DI water (cf. Figs. 7 and 8). This can demonstrate that the nanoparticles suspending on the working fluids did not take any role in surface wettability modification. In fact, the impact of the nanoparticles deposited on the heated surface is more important for CHF enhancement.
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Fig. 8. a. Contact angle of Al2O3 nano-fluids. b. Contact angle of CNT nano-fluids. c. Contact angle of Al2O3 þ CNT nano-fluids.
5.3. Surface roughness measurements
Another correlation of CHF as a function of surface roughness in water was introduced by Ferjancic and Golobic [21] as:
The effect of roughness on pool boiling CHF of water has been reported in several studies in literature. It was found that increases in surface roughness would result in CHF enhancement. Berenson (1960) [17] conducted boiling tests in n-pentane, showing that the roughest surface gives the highest CHF. Ramilison and Lienhard (1987) [18] also show that surface roughness enhances CHF significantly. Ramilson et al. (1992) [19] took into account the role of roughness and contact angle in CHF enhancement and a CHF correlation was developed. CHF was described as a function of the two mentioned factors, as shown in Equation (1):
q00CHF 00 qCHF;Zuber
p 3 0:125 q Ra ¼ 0:0336 p 180
(2)
where qCHF,Zuber is the CHF predicted by Zuber [20], q is the contact angle ( ) and Ra is the surface roughness of the heater surface (mm).
q00CHF ¼ 18:684lnðRa Þ þ 475:29
(3)
It was an agreement among the previous works that when roughness of the surface increases, CHF will increase as well. In this study, Atomic Force Microscope (AFM) was used to measure the surface roughness of the heater after boiling in nano-fluids. AFM allows obtaining 3D profile of the heated surfaces (cf. Fig. 9). It was found that the roughness of the fresh surface was 54 nm while that of the Al2O3 nano-fluids boiling surface was 55 nm. However, the roughness of the heater surface after boiling in CNT nano-fluid was increase up to 90 nm. The boiling of Al2O3 and CNT combined nanofluid brought a heated surface roughness of 92 nm, which is not much different from the one obtained by CNT nano-fluid boiling. It means Al2O3 nano-particles did not have any significant effect on roughness of the heated surface while CNT took a very important role in increasing the roughness of heated surface.
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Fig. 9. Surface roughness data.
5.4. Discussions on the CHF enhancement mechanisms As shown in the above sections, nano-fluids modified the characterization of the heated surface due to changes in contact angle and surface roughness. Contact angles of nano-fluids were lower than that of DI water. Especially, after boiling in Al2O3 nano-fluid, a super hydrophilic surface was obtained with a very low contact angle (16.7 ). According to the existing CHF models, when the contact is lower, CHF is higher. However, in this study, although the contact angle of CNT nano-fluid on the heater surface (39.4 ) is much higher than that of Al2O3 nanofluids, pool boiling CHF of CNT nano-fluid was remarkable higher than the one obtained by Al2O3 nano-fluids (Figs. 7 and 8). This result is contradictory with the previous works, which emphasizes the role of contact angle change to CHF enhancement in nano-fluids. This indicates that the pool boiling CHF enhancement mechanism in nano-fluids cannot be explained only from the decrease in contact angle. On the other hand, the roughness of the heater surface after boiling in nano-fluids was compared to that of the fresh surface before boiling. As shown in AFM results, while CNT nano-fluid made the heated surface become rougher (roughness was 90 nm), Al2O3 nano-particles almost had no impact on the surface roughness. The roughness of Table 4 Compositive effects of contact angle and roughness on CHF. Nano-fluid
Contact angle
Roughness
Fresh surface Al2O3 CNT Al2O3 þ CNT
68 YY (16.7 ) Y (39.4 ) YY (17.6 )
54 nm z(55 nm) [[ (90 nm) [[ (92 nm)
Dominant contributing factor to CHF enhancement Contact angle Roughness Contact angle and roughness
heated surface after boiling experiment of Al2O3 nano-fluids (55 nm) was considered to be the same with that of the fresh surface (54 nm). The roughness of heated surface after boiling Al2O3 þ CNT combined nano-fluid was 92 nm, which is frankly not different from that of CNT nano-fluid. Again, this reveals that only CNT nano-particles take the role of increasing roughness of heated surface. From the above discussion (cf. Table 4), it can be seen clearly that both changes in contact angle and surface roughness contribute to CHF enhancement of nano-fluids over that of DI water. Al2O3 nanofluid helped decrease contact angle compared to that of DI water, but did not show any increase in roughness of the heated surface. Therefore, CHF enhancement in Al2O3 nano-fluid could be explained by the effect of contact angle, as reported in literature. In spite of higher contact angle of CNT nano-fluid compared to Al2O3 nano-fluids, CNT nano-fluid still exhibited higher CHF because CNT nano-particles can increase the roughness of the heated surface significantly. In case of Al2O3þCNT combined nano-fluid, there was a synthesis of the two factors (decreased contact angle and increased roughness), resulting in the highest CHF among the three nano-fluids. The effects of contact angle and roughness on CHF can be explained basing on the theory of liquid rewetting as suggested by Phan [22]. Indeed, in his work, Phan [22] showed that a liquid movement occurs in the microlayer zone beneath the bubble during its growth. The greater the mass flow of this liquid movement, the greater CHF. Either the contact angle decreases or the surface roughness increases, the capillary forces contributing to move the liquid underneath the bubble increase, leading to a higher liquid mass flow in the microlayer. As a consequence, the CHF increases. Looking at the CHF data in this study, it was demonstrated that the higher the roughness, the greater the CHF, regardless of the contact angle. Therefore, even though CHF enhancement in nano-fluids was due to the contributions of both
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contact angle and surface roughness, effect of roughness is stronger and more dominant than that of contact angle.
6. Conclusions CHF enhancement of nano-fluids has been a topic of interest in recent years, bringing an opportunity to apply nano-fluids to heat transfer systems, including nuclear reactors, to enhance the thermal margin of the system, required for higher thermal power of the systems. One of the potential applications of nano-fluids is to be used as advanced coolant for Invessel Retention system to ensure the safety of the nuclear reactor during accident condition. This work provides a literature review of state-of-the-knowledge nanofluid research and experimental study of pool boiling CHF of nanofluids on an inclined heated surface. Some notable findings are summarized as follows: U Three low volume concentration of nano-particles suspended in DI water (0.05% Al2O3, 0.05% CNT þ 10% H3BO3, 0.05% Al2O3 þ0.05% CNT) were selected by zeta potential method to ensure the stability of working fluids during pool boiling CHF experiments, which is one of the key parameters that is needed for the application of nano-fluids to Invessel Retention system; U The three selected nano-fluids have been found to enhance CHF significantly over that of DI water. The highest CHF enhancement was obtained in Al2O3 þ CNT nano-fluid boiling experiment. Al2O3 nano-fluids showed the most modest CHF enhancement among the three fluids; U Inclination effect to CHF was investigated for each nano-fluid. The orientation angle was ranged from 0 (horizontal downward facing position) to 90 (vertical position). When inclined angle increased, CHF was increased. This trend was similar in the three nano-fluids as well as in DI water. For a given nanofluid, maximum CHF occurred at downward facing position (122% for Al2O3þCNT combined nano-fluid, 108% for CNT nanofluid þ H3BO3 and 33% for Al2O3 nano-fluid); U The thermal physical properties of nano-fluids are similar to those of DI water at low concentration was confirmed by thermal conductivity and surface tension measurements. It was concluded that CHF is not caused by the nano-particles in the fluids; U Nano-particles deposited layers were obtained after Al2O3 nano-fluids boiling experiment but deposition of CNT nanoparticles was not enough to form a layer. However, CHF of CNT is higher than Al2O3. Therefore, CHF enhancement cannot be explained by only the formation of nanoparticle layer on the heated surface; U The heated surface modification by nano-particles was considered as the main source of CHF enhancement. Two parameters contributing to this modification were contact angle and surface roughness;
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U Contact angle measurements showed that nano-fluids made the surface more hydrophilic by lowering the contact angle. However, it was proved that this factor is not representative enough to explain the CHF mechanism of nano-fluids; U Roughness was found to be the governing factor, contributing to CHF enhancement in nano-fluids. The larger the surface roughness, the higher the CHF
References [1] Girhjji, Sabersky, Some effects of the orientation of the heating surface in nucleate boiling, J. Heat Transfer 85 (1963) 379. [2] Jung, et al., Effects of enhanced surfaces and surface orientation on nucleate and film boiling heat transfer in R-11, Int. J. Heat Mass Transfer 30 (1987) 2627e2639. [3] Vishnev, et al., Study of heat transfer in boiling of helium on surfaces with various orientations, Heat Transfer Res.-Sov. Res. 24 (1976) 104e108. [4] Zhanxiong Guo, Mohamed S. El-Genk, An experimental study of saturated pool boiling from downward facing and inclined surfaces, Int.J. Heat Mass Transfer 35 (9) (1992) 2109e2117. [5] R.E. Henry, et al., External cooling of a reactor vessel under severe accident conditions, Nuclear Engineering and Design 139 (1993) 31e43. [6] T.G. Theofanous, et al., Critical heat flux through curved, downward facing, thick walls, Nuclear Engineering and Design 151 (1994) 247e258. [7] You, et al., Effect of nano-particles on critical heat flux of water in pool boiling heat transfer, Applied Physics Letter 83 (2003) 3373e3376. [8] I.C. Bang, S.H. Chang, Boiling heat transfer performance and phenomena of Al2O3ewater nanofluids from a plain surface in a pool, Internal Journal of Heat and Mass Transfer 48 (2005) 2407e2419. [9] P. Vassallo, R. Kumar, S. D’Amico, Pool Boiling Heat Transfer Experiments in Silica-Water Nano-Fluids, International Journal of Heat and Mass Transfer 47 47 (2004) 407e411. [10] Yang, et al., Pool boiling critical heat flux of water on small plates: effects of surface orientation and size, Int. Com. Heat Mass Transfer 24 (1997) 1093e1102. [11] S.G. Kandlikar, A theoretical model to predict pool boiling CHF incorporating effects of contact angle and orientation, J. Heat Transfer 123 (2001) 1071e1079. [12] B.H.Truong, Determination of pool boiling critical heat flux enhancement in nanofluids, Bachelor Thesis, Massachusetts Institute of Technology, 2007. [13] S.J. Kim, I.C. Bang, J. Buongiorno, L.W. Hu, Surface wettability change during pool boiling of nanofluids and its effect on critical heat flux, International Journal of Heat and Mass Transfer 50 (2007) 4105e4116. [14] H. Kim, J. Kim, M. Kim, Experimental studies on CHF characteristics of nanofluids at pool boiling, International Journal of Multiphase Flow 33 (2007) 691e706. [15] H. Kim, J. Kim, M. Kim, Effect of nanoparticles on CHF enhancement in pool boiling of nano-fluids, International Journal of Heat and Mass Transfer 49 (2006) 5070e5074. [16] C.P. Costello, J.W. Frea, A salient nonhydrodynamic effect on pool boiling burnout of small semicylindrical heaters, Chemical Engineering Progress Symposium Series 61 (57) (1965) 258e268. [17] P.J. Berenson, Experiments on pool-boiling heat transfer, Int. J. Heat Mass Transfer 5 (1962) 985e999. [18] J.M. Ramilison, J.H. Lienhard, Transition boiling heat transfer and the film transition regime, ASME J. Heat Transfer 102 (1987) 746e752. [19] J.M. Ramilison, P. Sadasivan, J.H. Lienhard, Surface factors influencing burnout on flat heaters, Journal of Heat Transfer 114 (1992) 287e290. [20] N. Zuber, Hydrodynamics aspects of boiling heat transfer, AEC Rep., AECU4439 (1959). [21] K. Ferjancic, I. Golobic, Surface effects on pool boiling CHF, Experimental Thermal and Fluid Science 25 (2002) 565e571. [22] H.T. Phan, Effects of nano- and micro-surface treatments on boiling heat transfer, Ph.D Thesis, Grenoble University, France, 2010.
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Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Enhancement of critical heat flux using nano-fluids for Invessel RetentioneExternal Vessel Cooling Q.T. Pham*, T.I. Kim, S.S. Lee, S.H. Chang Korea Advanced Institute of Science and Technology, 373-1, Guseong-dong, Yuseong-gu, Daejeon 305-701, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 April 2011 Accepted 6 October 2011 Available online 19 October 2011
This study investigated the pool boiling critical heat flux (CHF) of water-based nano-fluids under atmospheric pressure for Invessel Retention (IR)eExternal Vessel Cooling (EVC). The heated surface was a stainless steel foil inclined at different orientation angle from 0 (horizontal downward facing position) to 90 (vertical position). Three working nano-fluids with high suspension stability were selected by the zeta potential method to investigate the effect of each nano-fluid on CHF at the heated surface, which were 0.05% Alumine (Al2O3), 0.05% carbon nanotubes (CNT) þ 10% boric acid and 0.05% Al2O3 þ 0.05% CNT. It was observed that these nano-fluids enhanced CHF significantly (up to 220%) compared to deionized (DI) water. Furthermore, for all test fluids, CHF increased when the orientation angle increased. The surface characterization after boiling tests shows that the CHF enhancement with nano-fluids can be related to the increase of both surface roughness and wettability caused by nanoparticle deposition during the boiling processes. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Pool boiling CHF Nano-fluids IVR Contact angle Roughness
1. Introduction Severe accidents result from hypothetical Loss of Coolant Accidents (LOCA) coupled with a failure of Emergency Core Cooling System (ECCS) in a nuclear reactor. One of the management strategies for those accidents, which are currently being studied for advanced nuclear reactor, is Invessel Retention of corium debris (IVR). The IVR strategy consists of external cooling of the reactor vessel by flooding the reactor cavity so that decay heat from the molten core is removed through the lower head of the vessel. The decay heat is removed at low heat flux by natural convection. Pool boiling occurs as heat flux increases and heat removal is limited by the occurrence of critical heat flux (CHF) at the reactor vessel outer surface. CHF is the boiling crisis which refers to a condition characterized by a sharp reduction of local heat transfer coefficient resulting from the vapor blanket at the surface of the reactor vessel. This is accompanied by an inordinate increase in vessel outer surface temperature which damages the integrity of the vessel. The CHF should be investigated from the standpoint of IVR. The ex-vessel boiling process is described as boiling from a large downward facing curved surface. CHF at the surface of a downward
* Corresponding author. Tel.: þ82 42 350 3856; fax: þ82 42 869 3810. E-mail addresses: [email protected] (Q.T. Pham), skyimgf@ kaist.ac.kr (T.I. Kim), [email protected] (S.S. Lee), [email protected] (S. H. Chang). 1359-4311/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.applthermaleng.2011.10.017
facing configuration depends on surface inclination angle. The effect of heated surface orientation in a pool of isopropyl alcohol under atmospheric pressure was studied by Girhjji and Sabersky [1], with three different orientations (upward facing, vertical and downward facing). CHF of downward facing surface was found to be considerably smaller than other orientations. The effect of orientation in the nucleate boiling of R-11 was studied by Jung et al. [2]. The heated surface was changed from downward facing to upward facing. Increase of orientation from 0 to 165 caused the wall superheat to decrease by 15e25%. CHF decreased with orientation of heated surface from upward to downward facing [3]. CHF increased with increase in inclination angle when a 12.8 mm thick copper disk quenching in a pool of saturated water [4]. Nucleate boiling heat flux from a 0.3-m diameter curved surface exceeded 1 MW/m2 for highly subcooled water in a transient quenching integral experiments conducted [5]. CHF at the bottom center region of the IVR configuration under pool boiling conditions was approximately 300 kW/m2 [6]. Numerous attempts have been made to enhance the IVR capabilities of the reactor vessel to ensure that IVR can be achieved when a severe accident happens. The potential use of nano fluid to increase the CHF on downward facing surfaces is a possible IVR enhancing approach. Nano-fluids are engineered colloidal dispersions of nano-particles (of size less than 100 nm) in based fluids. Nano-fluids provide higher CHF than water at very low concentrations [7-9]. They are potential coolants for IVR to enhance the safety margin at the surface of reactor vessel.
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Nomenclature
q rl rv s a q00CHF g hfg Ra
contact angle, rad or liquid density, kg/m3 vapor density, kg/m3 liquidevapor surface tension force, N/m inclined angle (from horizontal downward to upward facing), rad or critical heat flux, kW/m2 gravity, m/s2 liquidevapor latent heat, J/kg surface roughness, mm
In this study, a series of experiments were performed to understand the pool boiling critical heat flux behavior on downward facing surfaces submerged in a pool of nano-fluids at very low concentration. Three types of nano-fluids were selected for study of fluid stability, which are alumina (Al2O3) 0.05% vol., Al2O3 0.05% vol. þ carbon nanotubes (CNT) 0.05% vol. and CNT 0.05% vol. þ boric acid (BA) 10% vol. The inclination angle was changed from horizontal to vertical. The influence of orientation on CHF enhancement due to the use of nano-fluids was investigated. Modifications of heated surface due to boiling in nano-fluids were characterized with an effort to find out the mechanisms underlying the CHF enhancement in nano-fluids. 2. Application of zeta potential method to select nano-fluids for use as IVReEVC coolants 2.1. Dispersion stability experiments with Al2O3 and CNT nanofluids Al2O3 and CNT nano-fluids were prepared by suspending Al2O3 and CNT nano-particles in DI water. The suspensions were transferred into an ultrasonic bath and sonicated for 1 h at room temperature. Boric acid was used as surfactant to change the pH of the fluids. Suspensions of nano-particles in DI water with and without boric acid and a combination of Al2O3 and CNT nano-particles in DI water were made in the same way for stability comparison. The zeta potentials of prepared nano-fluids were measured by ELSZ2 (Otsuka Electronics). The pH of fluids was measured by MIL-56-pH (Sechang) to verify the reliability of zeta potential results. Concentrations and dispersing time used in this study are shown in Table 1. 2.2. Zeta potential results and selection of nano-fluids for IVReEVC coolants
range of 35 w 45 mV within 3 days after sonication. Zeta potentials were significantly enhanced with the addition of boric acid. The Al2O3 nano-fluid with 10% volumetric concentration of boric acid was a highly stable suspension. CNT nano-fluid did not have good stability. The zeta potential of CNT-nano-fluid was very low (below 30 mV). The stability of CNT nano-fluid was increased by the addition of boric acid; the zeta potential reached the range of 35 w 45 mV, which is in the high stable dispersion range when the concentration of boric acid was 10% of volume. The zeta potential measurement of Al2O3 þ CNT combined nano-particles in water with low concentration (0.05% volume each) showed very good stability with increased zeta potentials up to 55 w 60 mV. Three different types of nano-fluids were selected for potential use as coolant in IVR (Al2O3 0.05%, CNT 0.05% þ Boric acid 10%, Al2O3 0.05% þ CNT 0.05%) considering stability and cost for an IVReEVC coolant.
3. Pool boiling experiment with an inclined downward facing surface 3.1. Experiment apparatus and procedure The test pool and test section were made of Type 316-stainless steel (cf. Fig. 2). The upper head of the pool was open for easy observation of phenomenon. The test section (1) was a plate 20-cm long, 5-cm wide and 1-mm thick, which was installed at the center of the test pool. A pulley (2) was installed at the side of the experiment loop which was used to control the orientation of the test section. The orientation of the test section was measured by an angler meter. The upper surface of the test section was isolated by high temperature resistant epoxy (3) for sealing the heated surface to diminish heat loss from the test section. Two copper electrodes (4) were welded to the ends of test section to transfer electric power from a 50 V, 4000A DC power supply system. The test section plate and copper electrodes were electrically insulated from the other parts of the loop by teflon rods. Five type-K thermocouples were installed along the test section to measure the temperature of the test section. Voltage signal from the test section were measured and processed by the system. The test pool was filled with DI water or one of the three selected nano-fluids. First, the test section was fixed at a desired orientation. The working fluid, preheated by a preheater, was pumped to the test pool by a centrifugal pump. The input power was slightly increased until CHF condition was detected. When CHF
The zeta potential of nano-fluids is shown in Fig. 1. Al2O3 nanofluid demonstrated good stability with the zeta potential in the Table 1 Zeta potential measurement test matrix. Base fluid
Nano-particles volume concentration
Boric acid volume concentration
Variation time
DI water
Al2O3 0.05% Al2O3 0.05% Al2O3 0.05% CNT 0.05% CNT 0.05% þ Al2O3 0.05% CNT 0.05% CNT 0.05%
0% 0.5% 10% 0% 0%
1 w 3 days
5% 10% Fig. 1. Zeta potential as a function of time.
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159
Fig. 2. Photos of (a) the test pool and b) the test section.
occurred, the power was immediately stopped. Before and after the experiment, samples of working fluids were extracted to measure zeta potential and pH to confirm the stability of fluids. The inclination angle was changed step by step from horizontal to vertical. CHF data with DI water were compared with nano-fluids. Experiments were conducted all in sub-cooled conditions with ΔTsub ¼ 7 w 14 C. Three selected nano-fluids were used as working fluids and the CHF data were compared with that of DI water. For each fluid, five different orientations of the test section were selected to perform the experiments, which are 0 (downward facing position), 30 , 45 , 60 , 90 (vertical position), corresponding to five positions at the outer surface of the reactor vessel (0 is equal to the bottom center of the vessel and 90 is the equator of the vessel). Each CHF test was repeated three times to ensure the repeatability of the data. 3.2. CHF data acquisition CHF data are shown in Fig. 3. The CHF data of DI water exhibits an increase of CHF with the increase of the inclined angle from 0 to 90 with respect to downward facing to vertical position of the test section. Similar dependence of CHF on orientation was also found by Githinji and Yang before [1,10]. For upward facing surfaces with
Fig. 4. Comparison of CHF between nano-fluids and DI water.
orientations of 90 (vertical) to 180 (horizontal upward facing), Kandlikar [11] developed a theoritical model based on force balance which also shows that the CHF increases with the inclined angle: 1= 1 þ cosq 4 q00CHF ¼ hlg r1=2 v ½sgðrl rv Þ 16 1= 2 2 p þ ð1 þ cosqÞcosð180 aÞ p 4
(1)
wherein q is the surface contact angle and a (in degree) is the inclined angle: for horizontal downward facing a ¼ 0 , for vertical facing a ¼ 90 and for horizontal upward facing a ¼ 180 .
Table 2 pH measurement before and after CHF experiment. Nano-fluids
pH
Fig. 3. Comparison of CHF between the present data and the data of Yang et al. [10].
Zeta potential (mV)
Before experiment After experiment Before experiment After experiment
0.05% Al2O3
0.05% Al2O3 þ 0.05% CNT
0.05% CNT þ 10% BA
4.68 4.85 39 41
4.80 4.84 53 51
3.27 3.05 35 38
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Fig. 5. 1a. SEM image of fresh surface. 2a. SEM image of Al2O3 nano-fluids heated surface. 3a. SEM image of CNT nano-fluids heated surface. 4a. SEM image of Al2O3þCNT nanofluids heated surface. 1b. EDS image of fresh surface. 2b. EDS image of Al2O3 nano-fluids heated surface. 3b. EDS image of CNT nano-fluids heated surface. 4b. EDS image of Al2O3þCNT nano-fluids heated surface.
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Fig. 6. a. Little CNT deposited on heated surface. b. Deposition layer of Al2O3 nano-particles. c. Deposition layer of Al2O3 þ CNT nano-particles.
However, as shown in Equation (1), the correlation of Kandlikar can only applied for upward facing conditions (a from 90 to 180 ). When the heated surface is positioned at downward facing (a from 0 to 90 ), cos(180 a) is negative and the CHF cannot be estimated by this equation. Therefore, in future works, it would be interesting to develop a more general theoretical model which enables to estimate the CHF from downward to upward facing (inclined angle from 0 to 180 ). For nano-fluids, significant CHF enhancements were obtained (cf. Fig. 4). The effect of orientation in nano-fluid tests demonstrates similar trend with DI water. CHF of nano-fluids also goes up when the inclined angle increases from downward facing to vertical. Enhancement of CHF due to nano-fluids was found to vary with orientation. Among the five tested orientations, maximum CHF enhancements occured at downward facing position. Enhancement of CHF by Al2O3 nano-fluids has been obtained in many other works previously [7,8,13,14] and this has been confirmed again in this study. Kashinath performed pool boiling experiment with a copper block using Al2O3 nano-fluids to investigate the effect of inclination to CHF. He also reported that CHF increased when the test specimen was inclined from downward facing up to upward facing. CNT nano-fluid showed larger CHF enhancement in comparison with Al2O3 nano-fluid. Interestingly, when Al2O3 and CNT nanofluids are dispersed together in DI water, achieved CHF was even higher than that of separate nano-fluid. 4. Nano-fluids stability verification One objective of this study is to select stable nano-fluids for the application to IVR system to enhance the thermal margin of the reactor vessel in accident condition. Therefore, the stability of nano-fluids is the key parameter that requires a significant attention. In this study, to confirm the dispersion stability of nano-fluids during the boiling experiment, zeta potential and pH of the selected nano-fluids were measured before and after each test. Results of these measurements are shown in Table 2. The zeta potentials and pHs of the three nano-fluids changed slightly during the boiling CHF experiments. These results guarantee that the selected nanofluids are stable enough to ensure a potential application of them as IVR coolants.
5. Investigation of CHF enhancement mechanisms in nanofluids The ability to enhance CHF is one of the most attractive features of nano-fluids, which has been investigated in many studies recently. However, a persuasive explanation of the mechanism of this phenomenon is yet to be found. During several decades of intense study a consensus explanation of the physical mechanism causing CHF, there have been many CHF models developed, most of which fall into one of the following four categories: hydrodynamic instability theory, macrolayer dryout theory, hot/dry spot theory, and bubble interaction theory [13]. Generally, existing CHF models suggests that a credible attempt at explaining the mechanism responsible for the large CHF enhancement observed in nano-fluids must include accurate knowledge of thermophysical property of the fluid and the heated surface conditions. Thermophysical properties of the fluid such as surface tension, thermal conductivity and viscosity can affect CHF according to the hydrodynamic instability model. However, at low concentration of nano-fluids, these properties of nano-fluids do not differ from those of water. Furthermore, the vapor density and enthalpy of evaporation are not expected to change due to low particle concentration [13,15]. Therefore, the influence of changes in thermophysical properties of nano-fluids compare to those of DI water is negligible and cannot be counted as the main reason for CHF enhancement. As shown in the macrolayer dryout model, hot/dry spot model as well as bubbles interaction model, CHF depends strongly on the surface condition of the heater. This has been confirmed experimentally by many works in literature [13]. In this study, the characterization of the heating surface modification occurring during the boiling experiments was investigated with an effort to find out the parameters governing the phenomena in which the unusual CHF enhancement of nano-fluids occurs. 5.1. SEM measurement The surfaces of the heater after boiling in nano-fluids were prepared for the Scanning Electron Microscope (SEM) and Energy Dispersive X-ray Spectroscopy (EDS) analysis to observe the effects of nano-particles on the morphology of the surface. This is of significance because both nucleate boiling heat transfer and CHF are
Table 3 Contact angle measurement data. Fluid
Fresh surface
Al2O3-nanofluid heated surface
CNT-nanofluid heated surface
Al2O3 þ CNT nano-fluid heated surface
DI water Al2O3 CNT Al2O3 þ CNT
68.213 66.961 64.947 69.201
16.734 1.253 14.658 2.492
42.794 2.089
22.308 0.762
1.436 1.681 2.367 2.643
39.443 2.097
17.617 1.116
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Fig. 7. Contact angle of DI water dropped on different surface.
highly affected by the surface configuration. According to the SEM and EDS analysis, some clear differences in the morphology of heated surfaces are evident among the three nano-fluids. The surface was clean during DI water boiling but nano-particles deposited on the heated surfaces during nano-fluids boiling (cf. Figs. 5.1e5.4). As stated in the literature, a porous layer of nano-particles deposited on the heater surface is responsible for the CHF enhancement in nano-fluids [7,8,12e14]. In this study, a micro-size layer of nano-particles on the heated surface was clearly observed by SEM after boiling in Al2O3 nano-fluid. The average thickness of this layer was 3.3 mm. A thicker layer of 7.58 mm was also seen on the heated surface after boiling in the Al2O3 þ CNT combined nanofluids. However, when the heater was boiled in CNT nano-fluid, the amount of CNT particle deposited was so little and was not enough to form a layer. EDS images show the compositions of the heated surfaces after boiling. It is clearly that the amount of CNT deposited is very minor compared to that of Al2O3 nano-particles (cf. Fig. 6). Although the deposition of Al2O3 nano-particles on the heated surface was much more than CNT, boiling CHF enhancement in CNT nano-fluids was remarkably higher than that of Al2O3 nano-fluids. These contradictory results opened a new insight into the CHF enhancement mechanism of nano-fluids. The nanoparticle deposited layer might have effects on CHF enhancement but is definitely not the only one governing factors contributing to CHF enhancement. It is important to stress that SEM images only allow a qualitative observation of the heated surface after nano-fluids boiling. For further quantification, the change in surface condition of the heater during boiling, wettability and roughness changes of the heater surface due to nano-fluids boiling were measured later.
5.2. Contact angle measurements Contact angle is a usual representative parameter of surface wettability. The retreating contact angle is often employed to correlate CHF because the heater surface becomes dry at the CHF conditions. Costello and Frea [16] reported the reduction of CHF on a poor wetting surface. As shown in the existing CHF models [11], a decrease in contact angle would lead to a significant increase in CHF. In this study, contact angles were measured for the sample surfaces after boiling test using a Phoenix 3000 Contact Angle Surface Analysis system. Drop of liquid (DI water or nano-fluids) was dropped on the surface of interest. Images of the liquid droplet were taken by a CCD camera. The software equipped with the system analyzes the contact angles on both sides of the droplet. All measurements were performed at room temperature. The general trend observed here is that contact angle of DI water on fresh surface has highest value compared to that on nano-fluid heated surface (Table 3). Al2O3 nano-fluid heated surface has the lowest contact angle overall. The contact angle of Al2O3 þ CNT combined nano-fluid heated surface is a little higher than the contact angle of Al2O3 nano-fluid heated surface but much smaller than that of CNT nano-fluid heated surface. The nano-fluids did not change the contact angle in comparison with DI water (cf. Figs. 7 and 8). This can demonstrate that the nanoparticles suspending on the working fluids did not take any role in surface wettability modification. In fact, the impact of the nanoparticles deposited on the heated surface is more important for CHF enhancement.
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Fig. 8. a. Contact angle of Al2O3 nano-fluids. b. Contact angle of CNT nano-fluids. c. Contact angle of Al2O3 þ CNT nano-fluids.
5.3. Surface roughness measurements
Another correlation of CHF as a function of surface roughness in water was introduced by Ferjancic and Golobic [21] as:
The effect of roughness on pool boiling CHF of water has been reported in several studies in literature. It was found that increases in surface roughness would result in CHF enhancement. Berenson (1960) [17] conducted boiling tests in n-pentane, showing that the roughest surface gives the highest CHF. Ramilison and Lienhard (1987) [18] also show that surface roughness enhances CHF significantly. Ramilson et al. (1992) [19] took into account the role of roughness and contact angle in CHF enhancement and a CHF correlation was developed. CHF was described as a function of the two mentioned factors, as shown in Equation (1):
q00CHF 00 qCHF;Zuber
p 3 0:125 q Ra ¼ 0:0336 p 180
(2)
where qCHF,Zuber is the CHF predicted by Zuber [20], q is the contact angle ( ) and Ra is the surface roughness of the heater surface (mm).
q00CHF ¼ 18:684lnðRa Þ þ 475:29
(3)
It was an agreement among the previous works that when roughness of the surface increases, CHF will increase as well. In this study, Atomic Force Microscope (AFM) was used to measure the surface roughness of the heater after boiling in nano-fluids. AFM allows obtaining 3D profile of the heated surfaces (cf. Fig. 9). It was found that the roughness of the fresh surface was 54 nm while that of the Al2O3 nano-fluids boiling surface was 55 nm. However, the roughness of the heater surface after boiling in CNT nano-fluid was increase up to 90 nm. The boiling of Al2O3 and CNT combined nanofluid brought a heated surface roughness of 92 nm, which is not much different from the one obtained by CNT nano-fluid boiling. It means Al2O3 nano-particles did not have any significant effect on roughness of the heated surface while CNT took a very important role in increasing the roughness of heated surface.
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Fig. 9. Surface roughness data.
5.4. Discussions on the CHF enhancement mechanisms As shown in the above sections, nano-fluids modified the characterization of the heated surface due to changes in contact angle and surface roughness. Contact angles of nano-fluids were lower than that of DI water. Especially, after boiling in Al2O3 nano-fluid, a super hydrophilic surface was obtained with a very low contact angle (16.7 ). According to the existing CHF models, when the contact is lower, CHF is higher. However, in this study, although the contact angle of CNT nano-fluid on the heater surface (39.4 ) is much higher than that of Al2O3 nanofluids, pool boiling CHF of CNT nano-fluid was remarkable higher than the one obtained by Al2O3 nano-fluids (Figs. 7 and 8). This result is contradictory with the previous works, which emphasizes the role of contact angle change to CHF enhancement in nano-fluids. This indicates that the pool boiling CHF enhancement mechanism in nano-fluids cannot be explained only from the decrease in contact angle. On the other hand, the roughness of the heater surface after boiling in nano-fluids was compared to that of the fresh surface before boiling. As shown in AFM results, while CNT nano-fluid made the heated surface become rougher (roughness was 90 nm), Al2O3 nano-particles almost had no impact on the surface roughness. The roughness of Table 4 Compositive effects of contact angle and roughness on CHF. Nano-fluid
Contact angle
Roughness
Fresh surface Al2O3 CNT Al2O3 þ CNT
68 YY (16.7 ) Y (39.4 ) YY (17.6 )
54 nm z(55 nm) [[ (90 nm) [[ (92 nm)
Dominant contributing factor to CHF enhancement Contact angle Roughness Contact angle and roughness
heated surface after boiling experiment of Al2O3 nano-fluids (55 nm) was considered to be the same with that of the fresh surface (54 nm). The roughness of heated surface after boiling Al2O3 þ CNT combined nano-fluid was 92 nm, which is frankly not different from that of CNT nano-fluid. Again, this reveals that only CNT nano-particles take the role of increasing roughness of heated surface. From the above discussion (cf. Table 4), it can be seen clearly that both changes in contact angle and surface roughness contribute to CHF enhancement of nano-fluids over that of DI water. Al2O3 nanofluid helped decrease contact angle compared to that of DI water, but did not show any increase in roughness of the heated surface. Therefore, CHF enhancement in Al2O3 nano-fluid could be explained by the effect of contact angle, as reported in literature. In spite of higher contact angle of CNT nano-fluid compared to Al2O3 nano-fluids, CNT nano-fluid still exhibited higher CHF because CNT nano-particles can increase the roughness of the heated surface significantly. In case of Al2O3þCNT combined nano-fluid, there was a synthesis of the two factors (decreased contact angle and increased roughness), resulting in the highest CHF among the three nano-fluids. The effects of contact angle and roughness on CHF can be explained basing on the theory of liquid rewetting as suggested by Phan [22]. Indeed, in his work, Phan [22] showed that a liquid movement occurs in the microlayer zone beneath the bubble during its growth. The greater the mass flow of this liquid movement, the greater CHF. Either the contact angle decreases or the surface roughness increases, the capillary forces contributing to move the liquid underneath the bubble increase, leading to a higher liquid mass flow in the microlayer. As a consequence, the CHF increases. Looking at the CHF data in this study, it was demonstrated that the higher the roughness, the greater the CHF, regardless of the contact angle. Therefore, even though CHF enhancement in nano-fluids was due to the contributions of both
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contact angle and surface roughness, effect of roughness is stronger and more dominant than that of contact angle.
6. Conclusions CHF enhancement of nano-fluids has been a topic of interest in recent years, bringing an opportunity to apply nano-fluids to heat transfer systems, including nuclear reactors, to enhance the thermal margin of the system, required for higher thermal power of the systems. One of the potential applications of nano-fluids is to be used as advanced coolant for Invessel Retention system to ensure the safety of the nuclear reactor during accident condition. This work provides a literature review of state-of-the-knowledge nanofluid research and experimental study of pool boiling CHF of nanofluids on an inclined heated surface. Some notable findings are summarized as follows: U Three low volume concentration of nano-particles suspended in DI water (0.05% Al2O3, 0.05% CNT þ 10% H3BO3, 0.05% Al2O3 þ0.05% CNT) were selected by zeta potential method to ensure the stability of working fluids during pool boiling CHF experiments, which is one of the key parameters that is needed for the application of nano-fluids to Invessel Retention system; U The three selected nano-fluids have been found to enhance CHF significantly over that of DI water. The highest CHF enhancement was obtained in Al2O3 þ CNT nano-fluid boiling experiment. Al2O3 nano-fluids showed the most modest CHF enhancement among the three fluids; U Inclination effect to CHF was investigated for each nano-fluid. The orientation angle was ranged from 0 (horizontal downward facing position) to 90 (vertical position). When inclined angle increased, CHF was increased. This trend was similar in the three nano-fluids as well as in DI water. For a given nanofluid, maximum CHF occurred at downward facing position (122% for Al2O3þCNT combined nano-fluid, 108% for CNT nanofluid þ H3BO3 and 33% for Al2O3 nano-fluid); U The thermal physical properties of nano-fluids are similar to those of DI water at low concentration was confirmed by thermal conductivity and surface tension measurements. It was concluded that CHF is not caused by the nano-particles in the fluids; U Nano-particles deposited layers were obtained after Al2O3 nano-fluids boiling experiment but deposition of CNT nanoparticles was not enough to form a layer. However, CHF of CNT is higher than Al2O3. Therefore, CHF enhancement cannot be explained by only the formation of nanoparticle layer on the heated surface; U The heated surface modification by nano-particles was considered as the main source of CHF enhancement. Two parameters contributing to this modification were contact angle and surface roughness;
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U Contact angle measurements showed that nano-fluids made the surface more hydrophilic by lowering the contact angle. However, it was proved that this factor is not representative enough to explain the CHF mechanism of nano-fluids; U Roughness was found to be the governing factor, contributing to CHF enhancement in nano-fluids. The larger the surface roughness, the higher the CHF
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