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The effect of nanofluid stability on critical heat flux using magnetite-water nanofluids
Nuclear Engineering and Design 292 (2015) 187–192
Contents lists available at ScienceDirect
Nuclear Engineering and Design journal homepage: www.elsevier.com/locate/nucengdes
The effect of nanofluid stability on critical heat flux using magnetite-water nanofluids Jong Hyuk Lee a,b , Dong Hoon Kam b , Yong Hoon Jeong b,∗ a Thermal-Hydraulic Safety Research Division, Korea Atomic Energy Research Institute, 989-111 Daedeok-daero, Yuseong-gu, Daejeon 305-353, Republic of Korea b Department of Nuclear and Quantum Engineering, Korea Advanced Institute of Science and Technology, 373-1, Guseong-dong, Yuseong-gu, Daejeon 305-701, Republic of Korea
h i g h l i g h t s • • • •
We conduct the CHF experiment with magnetite-water nanofluid. Nanofluids should be initially sonicated to enhance the CHF. Nanofluids have the limited concentration to guarantee CHF enhancement. CHF enhancement using nanofluids can be guaranteed for a year at least.
a r t i c l e
i n f o
Article history: Received 18 December 2014 Received in revised form 11 May 2015 Accepted 22 May 2015
a b s t r a c t In nuclear safety, the critical heat flux (CHF) is a very important value that can determine the limit of the safety design. To prevent a minor nuclear accident from mitigating into a severe accident, the coolants should have a higher CHF value. Among the strategies to improve the CHF, many feasibility studies using several kinds of nanofluids were performed as an alternative coolant because of the abnormally CHF enhancement with a very small volume fraction of nanoparticles. Although many researches have tried to apply nanofluids into a real system as a coolant, there are some problems left to solve. In particular, it is necessary to understand the characteristics of nanoparticle stability in the base fluid in order to guarantee the thermal performance of the nanofluids. In this study, three kinds of effects were considered to clarify the relation between nanofluid stability and CHF improvement: the effect of sonication, dilution, and storage time. The effect of sonication in the manufacturing process of the nanofluids should be considered. A two-step method is the most popular for manufacturing nanofluids. Based on the results of the present study, nanofluids made through a two-step method should be initially sonicated to guarantee their CHF enhancement. In the process of dilution, the limitation of nanoparticle concentration exists in the process of dilution to guarantee CHF enhancement. Finally, CHF enhancement using a nanofluid made by a two-step method can be guaranteed for at least a year. In addition, we also proved that CHF using nanofluids can be decreased according to the preservation time. © 2015 Elsevier B.V. All rights reserved.
1. Introduction A primary interest with regard to nanofluids is on heat transfer without phase changes. Thus, many works have been focused on a single phase heat transfer such as in thermal conductivity (Choi et al., 2001; Das et al., 2003; Eastman et al., 2001; Eastman et al., 1996; Hong et al., 2005; Jang and Choi, 2004). However, the
∗ Corresponding author. Tel.: +82 42 350 3826; fax: +82 42 350 3801. E-mail address: [email protected] (Y.H. Jeong). http://dx.doi.org/10.1016/j.nucengdes.2015.05.026 0029-5493/© 2015 Elsevier B.V. All rights reserved.
application of nanofluids is extended to a boiling heat transfer because of the excellent enhanced thermal performance with a very small volume fraction of nanoparticles. In the process of removing the high heat flux, boiling is the most effective method of heat transfer mechanisms. However, a limit of effective heat transfer exists, i.e., the so-called critical heat flux (CHF), which can be generated from nucleate boiling to film boiling. CHF is the main constraint on the design process. Many researches have been tried to evaluate the CHF with several kinds of nanofluids (Bang and Chang, 2005; Jeong et al., 2008; Kim et al., 2006; Kim and Kim, 2009; Kwark et al., 2010; Lee et al., 2012, 2013a,b; Park et al., 2010).
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All experiments, which dealt with an improvement of the heat performance using several kinds of nanofluids, were performed under stable conditions of nanoparticle dispersion. In other words, they did not consider the condition of nanoparticle dispersion in the base-fluid. All researches set a precondition that nanoparticles are well-dispersed in the base-fluid used in their experiments. However, Timofeeva et al. (2007) addressed the idea that nanoparticles are agglomerated according to the passage of time. As shown by the dynamic light scattering (DLS) results, there is a significant nanoparticle agglomeration in the alumina nanofluids. Shima et al. (2010) studied the effect of nanoparticle agglomeration on the thermal conductivity, viscosity, and size distribution of iron oxide and copper oxide. They concluded the prominent role of agglomeration on thermal properties of nanofluid. With the elapsed time, the thermal conductivity of nanofluids was decreased and finally reached an equilibrium value after a time interval. Therefore, it is important to evaluate the nanofluid stability because the thermal performance of the nanofluids can be changed according to their stability characteristics. To guarantee the performance of the nanofluids, it is necessary to clarify between the nanofluid stability and improved thermal performance. This study is aimed at clarifying the CHF characteristics, which is one of the important thermal performances in nuclear power safety, considering the nanofluid stability, and assessing the feasibility of nanofluid applications in nuclear power plants as a coolant. Pool boiling CHF experiments using a Ni–Cr wire were performed to assess the thermal performance of the nanofluids. Among several kinds of nanofluids, we chose a magnetite-water nanofluid, which can improve the CHF with a small fraction of nanoparticles and may also have many advantages for overcoming the shortcomings of the conventional nanofluids in our previous study (Lee et al., 2012, 2013a,b). In addition, DLS data of magnetite-water nanofluids were measured to evaluate the degree of agglomeration qualitatively. Based on these data, we focused on the effects of several characteristics of nanoparticle stability such as the sonication, dilution, and storage time on the CHF enhancement to estimate the possibility in the uses of nanofluids for nuclear applications considering realistic phenomena and problems. These results will be discussed in detail.
2. Assessment of nanofluid stability and thermal performance 2.1. Stability of magnetite-water nanofluids Stable dispersions of nanoparticles are produced by two methods: a one-step method and a two-step method. The two-step method first manufactures nanoparticles using physical or chemical process and then disperses them into base fluids. The one-step method simultaneously manufactures and disperses the nanoparticles into base fluids. Although it can produce well-dispersed nanofluids with monosized nanoparticles, the manufacturing process is very complicated and it is necessary to conduct more studies to overcome the limitations of the manufacturing techniques. Therefore, most nanofluids reported in the literature are made using a two-step method. In most nanofluids prepared by the two-step method, the agglomeration of nanoparticles, which occurs due to the van der Waals force between nanoparticles, is not fully separated. As I mentioned before, the thermal characteristics of nanofluids can be changed according to the condition of the nanoparticles dispersion. However, it is difficult to clarify the relation between nanoparticle dispersion and the excellent performance of nanofluids. In this study, we focused on the effect of nanoparticle dispersion on the thermal performance of nanofluids, especially CHF enhancement. It is necessary to measure the degree of nanoparticle dispersion to clarify the relation. There are several
methods used to evaluate the nanoparticle dispersion stability as indicated below: A sedimentation analysis is the simplest method. The sedimentation method depends on the sedimentation rate of the nanoparticles in the fluid to measure the particle size distribution. By measuring the time required for the particles to settle down, the spherical particle sizes can be determined. Due to dipolar characteristics and ionic attributes, the colloidal particles suspended in fluids are charged electrically. The Zeta potential is an electric potential in the interfacial double layer at the location of the slipping plane and exists around each particle. The magnitude of the zeta potential gives an indication of the potential stability of the colloidal system. In other words, colloids with high zeta potential (negative or positive) will tend to repel each other and there is no tendency to agglomerate. However, if the colloids have a low zeta potential then there is no force to prevent the colloids from coming together and flocculating. The general dividing line between stable and unstable dispersions is generally taken at ±30 mV. A spectroscopy analysis is another efficient way to evaluate the stability of nanofluids. Among the various types of such analysis, Ultraviolet–visible (UV–vis) or photon correlation spectroscopy is useful to evaluate the stability of nanofluids by measuring the optical properties. If the nanoparticles dispersed in fluids have characteristic absorption bands in a wavelength of 190–1100 nm, it is a reliable method to evaluate the stability of nanofluids using UV–vis spectral analysis. Photon correlation spectroscopy, also known as Dynamic light scattering (DLS), can be used to determine the size distribution profile of the colloidal in a suspension. In a previous study (Lee et al., 2012), the zeta potential and pH were measured to guarantee the stability of manufactured magnetite-water nanofluids. The results show that the zeta potential for 100 ppm vol. of magnetite-water nanofluid is measured to be over +30 mV. In addition, the measured pH (4.1–4.5) of magnetitewater nanofluid at 20 ◦ C is far from the iso-electric point (IEP, IEP of magnetite ∼7), which is given as an equal number of positively and negatively charged particles in the colloid. In other words, it can be verified that the magnetite-water nanofluids will be preserved in a stable condition within a short elapsed time (∼1 day). However, the measured data of the zeta potential has more scattered values without a certain tendency according to more elapsed time. In addition, it is difficult to quantify the comprehensive stability of nanoparticle dispersion using the zeta potential and pH. The UV–vis were also measured, but cannot be used to evaluate the stability of a magnetite-water nanofluid. There is no characteristic peak within the valid absorption band from the results of the UV–vis. The DLS was measured in this study to evaluate that of the magnetite-water nanofluid. However, the DLS measurements were performed within a limited range of nanoparticle concentration. A magnetite-water nanofluid is originally opaque, and it is hard to penetrate into the nanofluids with more concentration by light because a magnetite-water nanofluid is more turbid as the nanoparticle concentration is increased. 2.2. CHF for magnetite-water nanofluid In nuclear safety, CHF is a very important value that can determine the limit of the safety design. When CHF occurred, the temperature suddenly soars up to hundreds or thousands of degrees Celsius. To prevent a minor nuclear accident from mitigating into a severe accident, the coolant should have a higher CHF value. Therefore, many researchers have been making an effort to enhance the CHF. In this study, the thermal performance of a magnetite-water nanofluid was estimated based on the CHF values to assess the relation between nanofluid stability and thermal performance. The pool boiling CHF experiment facility in Fig. 1 is
J.H. Lee et al. / Nuclear Engineering and Design 292 (2015) 187–192
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Fig. 3. Comparison of CHF data of magnetite-water nanofluids considering the effect of sonication with various nanoparticle concentrations. Fig. 1. Schematic of pool boiling CHF experiment.
3. Results
CHF / CHFwater
3.1. The effect of sonication
2.0
Magnetite-water nanofluid Alumina-water nanofluid Titania-water nanofluid 1.0 1
10
100
Nanoparticle concentration (ppm Vol.) Fig. 2. Comparison of CHF ratio between magnetite-water nanofluids and other nanofluids with increasing nanoparticle concentrations from 1 ppm Vol. to 100 ppm Vol.
comprised of a rectangular main vessel, a pre-heater, copper electrodes, a thermocouple, visualization windows, and a condenser. A Ni–Cr wire (Ni: 80%, Cr: 20%) with a 0.4 mm diameter was used as a heated surface and was heated by a DC power supply of OPE-3050DI with a maximum capacity of 1.5 kW (50 V, 30 A), The voltage, current and temperature signals were measured using a National Instrument data acquisition system. The detailed experiment apparatus and procedure were described in our previous studies (Lee et al., 2012). The previous CHF results can be summarized by Fig. 2. The CHF of magnetite-water nanofluids can be improved by 60–140% according to the nanoparticle concentration. As the nanoparticle volume concentration is increased, the CHF of the magnetite-water nanofluids is enhanced. A methodology on the assessment of nanofluid stability and thermal performance is introduced and prepared. Clarifying the relation between nanofluid stability and CHF through these measurements, the characteristics of magnetite-water nanofluid will be studied and supported to design uses of the nanofluids in nuclear power plants.
An advantage of a two-step method for manufacturing the nanofluids is that nanoparticles produced in bulk at low prices can be used. The manufacturing process of nanoparticle powders is very simple. However, it is inevitable that individual particles quickly agglomerate before dispersion. Therefore, it is expected that the initial sonication should be necessary to disperse the nanoparticles in a fluid. To understand the effect of sonication for the manufacturing of the nanofluid on the CHF, two kinds of nanofluids were prepared. For the first, magnetite nanoparticles are well dispersed ultrasonically in pure water, and for the second, magnetite nanoparticles were simply mixed and stirred in pure water. Outwardly, the former has a light red color but the latter is visibly separated between the nanoparticles and pure water. Pool boiling CHF experiments were performed using these nanofluids. Fig. 3 shows the results through a comparison of CHF data between magnetite-water nanofluids according to the effect of sonication when increasing the nanoparticle concentration. The CHF data without sonication show that the CHF values are similar to the pure water case and that the CHF is not improved with an increase in the nanoparticle concentration. In other words, the results show that nanofluids should be initially sonicated to guarantee the CHF enhancement of a nanofluid manufactured using the two-step method.
3.2. The effect of dilution It seems like nanofluids have become a potential future application in nuclear power plants as a coolant in an emergency cooling system. In particular, it is expected that a nanofluid can be used to enhance the in-vessel retention capability in a severe accident mitigation strategy owing to the high CHF margin of the nanofluids. However, it is not possible to prepare a nanofluid with a target concentration in the present system. Because the volume of the flooded cavity is too big and a large amount of nanofluid is needed, this does not agree with the economics and space utilization in a real system. Accordingly, it is necessary to prepare a concentrated nanofluid to reduce its volume, and a dilution process should be required to apply the nanofluid into the cavity. However, we do not know how the CHF can be affected by the dilution process of the nanofluids. The effect of dilution on the CHF
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Fig. 4. Schematic of dilution process of nanofluids. Table 1 Relationship of nanoparticle concentrations between concentrated nanoflud and diluted nanofluid. VNF /VTotal
Effective diameter (nm)
Concentrated Magnetite-water nanofluids 100 ppm
5000
Nanoparticle concentration Concentrated nanofluid
Diluted nanofluid 0
1 10 100
1 10 100
0.01
100 1000 10,000
1 10 100
0.001
1000 10,000
1 10
1
0
2
Critical heat flux (kW/m )
VNF / VTotal = 0.01 VNF / VTotal = 0.001
2x10
3
Pure water 1x10
3
1
10
300
400
Fig. 6. Time-dependent DLS data according to the setting time.
VNF / VTotal = 1 3
200
Setting time (day)
is still unknown, and there have been no researches about the effect of dilution on the CHF enhancement using nanofluids. To assess the effect of dilution on the CHF, various kinds of concentrated nanofluids were prepared. In addition, pool boiling CHF experiments were conducted using a diluted nanofluid in a range of 1–100 ppm Vol. concentration. VNF is the volume of concentrated magnetite-water nanofluid with a range of 100–10,000 ppm Vol. of concentration. VWater is the volume of pure water to dilute the concentrated nanofluid. In addition, VTotal is total volume of diluted nanofluid. Through the dilution process shown in Fig. 4 and Table 1, the diluted magnetite-water nanofluids were prepared, and CHF experiments were then carried out using them. Fig. 5 shows the CHF results according to the effect of dilution compared to the CHF data without the dilution. For all cases using 1000 ppm Vol. of concentrated nanofluids, there is no CHF difference between the dilution and non-dilution cases, despite the changes in nanoparticle volume concentrations. However, the CHF tends to decrease the improvement using 10,000 ppm Vol. of concentrated nanofluids compared to the non-dilution cases. As a result, we proved that a limitation of nanoparticle concentration exists in the process of dilution. A CHF enhancement can be
3x10
100
100
Diluted nanoparticle concentration (ppm Vol.) Fig. 5. CHF results according to the effect of dilution compared to the CHF data without the dilution.
guaranteed until 1000 ppm Vol. of concentrated nanofluid without consideration of the effect its dilution. It is expected that the limitation of the concentrated nanoparticle concentration will be used as a main criterion in the design process.
3.3. The effect of storage time In this study, a two-step method was used for manufacturing the water-based nanofluids. An advantage of this two-step method is that the manufacturing process is simple, cheap, and producible in bulk. However, the agglomeration of nanoparticles is not fully separated in nanofluids prepared by a two-step method. From a practical perspective, manufactured nanofluids should be conserved while keeping the excellent heat transfer. However, nanoparticles will become agglomerated after manufacturing using a two-step method. In the field of heat transfer using nanofluids, the poor dispersion quality generally causes a lower performance of the thermal heat transfer. Considering their dispersion stability, the thermal conductivity of nanofluids, which is the mechanism of a single-phase heat transfer and is related to Brownian motion, was investigated experimentally and theoretically in many research groups. However, it is not easy to find a study on the relationship between dispersion stability and CHF, which is the mechanism of two-phase heat transfer and is related to the changes in the surface characteristics. To evaluate the effect of storage time on the CHF, 100 and 1000 ppm Vol. of concentrated magnetite-water nanofluid were prepared using the two-step method. To assess the dispersion stability quantitatively, the sizes of the nanoparticles were measured by using dynamic light scattering (DLS). Fig. 6 shows the DLS data including the effective diameter of the magnetite nanoparticles according to the setting time in years. The results of 100 ppm Vol. of magnetite-water nanofluid representing the agglomeration of nanoparticles was initially undergoing rapid progress, and after 1 month, the progress of agglomeration was saturated with an approximately 2000 nm nanoparticle size. Unfortunately, it is difficult to measure the DLS data for 1000 ppm Vol. of a magnetite-water nanofluid because of its opacity. Pool boiling CHF experiments were also performed using 1 and 10 ppm Vol. of diluted nanofluids, which are diluted with 100 and 1000 ppm Vol. of concentrated nanofluids, respectively. Time-dependent CHF data for a magnetite-water nanofluid were plotted in Fig. 7 according to the nanoparticle concentration. From this, there is no decrease in CHF values for 1 and 10 ppm Vol. of nanofluids when increasing the preserved time compared with the original CHF values.
J.H. Lee et al. / Nuclear Engineering and Design 292 (2015) 187–192
a nanofluid injection system, repairing the period time to guarantee the thermal performance of the nanofluids, their nanoparticle concentration, and so on. For example, the volume of the APR 1400 flooded reactor cavity is approximately 760 m3 . If a concentration of 1 ppm Vol. of nanofluid is determined as the target, the volume of the stored nanofluid is almost 0.76 m3 because the concentrated nanoparticle concentration should be limited to below 1000 ppm Vol.
2 Critical heat flux (kW/m )
3000
2000
4. Conclusion
Magnetite-water nanofluids 1 ppm 1 ppm (dillution) 10 ppm 10 ppm (dillution)
1000
0
100
200
300
Setting time (day) Fig. 7. Time-dependent CHF data of diluted magnetite-water nanofluids with 1 ppm vol. and 10 ppm vol. nanoparticle concentration.
3000
Critical heat flux (kW/m2)
191
Magnetite-water nanofluids 1 ppm (dillution)
2000
Based on this study, we concluded that design criteria are required to guarantee the excellent thermal performance of nanofluids considering their stability. First, nanofluids, which are made by a two-step method, should be initially sonicated to guarantee the CHF enhancement of the nanofluids. Second, a limitation of nanoparticle concentration existed in the process of dilution to keep the CHF enhancement. The CHF enhancement can be guaranteed until 1000 ppm Vol. of concentrated magnetite-water nanofluid without considering the effect of nanofluid dilution. It is expected that the existence of nanoparticle concentration limitation should be considered as an important design criteria in the design process. The CHF enhancement using a nanofluid made using a two-step method can be guaranteed for a year at least. In addition, we also proved that the CHF using nanofluids can be decreased according to the preservation time. Therefore, the maintenance of nanofluid stability should be required to guarantee the CHF enhancement. Based on these results, we expect that this study can be utilized as a basic research for moving forward to step to apply nanofluids into a real system of nuclear power plants. Acknowledgment This work was supported by the Nuclear Safety Research Center Program of the KORSAFe (Grant Code 1305011) grant funded by Nuclear Safety and Security Commission of the Korean Government.
1000
0
100
200
300
400
500
600
700
Setting time (day) Fig. 8. CHF data of diluted magnetite-water nanofluids with 1 ppm vol. of nanoparticle concentration according to preservation time.
Additionally, CHF experiments with 1 ppm Vol. of diluted magnetite-water nanofluid, which have been preserved for 2 years, were conducted, and the results are shown in Fig. 8. The results show that the value of the CHF for 1 ppm of magnetite-water nanofluid preserved during 2 years decreased, which is similar with that of the water case. Although we cannot compare the CHF performance with a varying nanoparticle concentration for 2-year reserved cases, we proved that CHF performance can be decreased according to preservation time. Based on this result, we can conclude that the CHF enhancement using a nanofluid made by a two-step method can be guaranteed for a year at least. In addition, we also proved that the CHF using nanofluids can be decreased according to preservation time. In other words, it is necessary to maintain the nanofluid stability after 1 year to guarantee the CHF performance. Based on this study, we can determine the design criteria for the safety system of a nuclear power plant using nanofluids. In other words, if magnetite-water nanofluid is used in external reactor vessel cooling (ERVC) for in-vessel retention (IVR) as a coolant, we can easily determine the required capacity of the storage tank of
References Bang, I.C., Chang, S.H., 2005. Boiling heat transfer performance and phenomena of Al2 O3 –water nano-fluids from a plain surface in a pool. Int. J. Heat Mass Transf. 48, 2407–2419. Choi, S., Zhang, Z., Yu, W., Lockwood, F., Grulke, E., 2001. Anomalous thermal conductivity enhancement in nanotube suspensions. Appl. Phys. Lett. 79, 2252–2254. Das, S.K., Putra, N., Thiesen, P., Roetzel, W., 2003. Temperature dependence of thermal conductivity enhancement for nanofluids. J. Heat Transf. 125, 567–574. Eastman, J., Choi, S., Li, S., Yu, W., Thompson, L., 2001. Anomalously increased effective thermal conductivities of ethylene glycol-based nanofluids containing copper nanoparticles. Appl. Phys. Lett. 78, 718–720. Eastman, J., Choi, U., Li, S., Thompson, L., Lee, S.,1996. Enhanced thermal conductivity through the development of nanofluids. In: MRS Proceedings. Cambridge Univ Press, p. 3. Hong, T.-K., Yang, H.-S., Choi, C., 2005. Study of the enhanced thermal conductivity of Fe nanofluids. J. Appl. Phys. 97, 064311. Jang, S.P., Choi, S.U., 2004. Role of Brownian motion in the enhanced thermal conductivity of nanofluids. Appl. Phys. Lett. 84, 4316–4318. Jeong, Y.H., Chang, W.J., Chang, S.H., 2008. Wettability of heated surfaces under pool boiling using surfactant solutions and nano-fluids. Int. J. Heat Mass Transf. 51, 3025–3031. Kim, H., Kim, J., Kim, M., 2006. Experimental study on CHF characteristics of waterTiO2 nano-fluids. Nucl. Eng. Technol. 38, 61. Kim, H., Kim, M., 2009. Experimental study of the characteristics and mechanism of pool boiling CHF enhancement using nanofluids. Heat Mass Transf. 45, 991–998. Kwark, S.M., Kumar, R., Moreno, G., Yoo, J., You, S.M., 2010. Pool boiling characteristics of low concentration nanofluids. Int. J. Heat Mass Transf. 53, 972–981. Lee, J.H., Lee, T., Jeong, Y.H., 2012. Experimental study on the pool boiling CHF enhancement using magnetite-water nanofluids. Int. J. Heat Mass Transf. 55, 2656–2663. Lee, J.H., Lee, T., Jeong, Y.H., 2013a. The effect of pressure on the critical heat flux in water-based nanofluids containing Al2 O3 and Fe3 O4 nanoparticles. Int. J. Heat Mass Transf. 61, 432–438.
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Lee, T., Lee, J.H., Jeong, Y.H., 2013b. Flow boiling critical heat flux characteristics of magnetic nanofluid at atmospheric pressure and low mass flux conditions. Int. J. Heat Mass Transf. 56, 101–106. Park, S.D., Lee, S.W., Kang, S., Bang, I.C., Kim, J.H., Shin, H.S., Lee, D.W., Lee, D.W., 2010. Effects of nanofluids containing graphene/graphene-oxide nanosheets on critical heat flux. Appl. Phys. Lett. 97, 023103.
Shima, P.D., Philip, J., Raj, B., 2010. Influence of aggregation on thermal conductivity in stable and unstable nanofluids. Appl. Phys. Lett. 97, 153113. Timofeeva, E.V., Gavrilov, A.N., McCloskey, J.M., Tolmachev, Y.V., Sprunt, S., Lopatina, L.M., Selinger, J.V., 2007. Thermal conductivity and particle agglomeration in alumina nanofluids: experiment and theory. Phys. Rev. E 76, 061203.
Contents lists available at ScienceDirect
Nuclear Engineering and Design journal homepage: www.elsevier.com/locate/nucengdes
The effect of nanofluid stability on critical heat flux using magnetite-water nanofluids Jong Hyuk Lee a,b , Dong Hoon Kam b , Yong Hoon Jeong b,∗ a Thermal-Hydraulic Safety Research Division, Korea Atomic Energy Research Institute, 989-111 Daedeok-daero, Yuseong-gu, Daejeon 305-353, Republic of Korea b Department of Nuclear and Quantum Engineering, Korea Advanced Institute of Science and Technology, 373-1, Guseong-dong, Yuseong-gu, Daejeon 305-701, Republic of Korea
h i g h l i g h t s • • • •
We conduct the CHF experiment with magnetite-water nanofluid. Nanofluids should be initially sonicated to enhance the CHF. Nanofluids have the limited concentration to guarantee CHF enhancement. CHF enhancement using nanofluids can be guaranteed for a year at least.
a r t i c l e
i n f o
Article history: Received 18 December 2014 Received in revised form 11 May 2015 Accepted 22 May 2015
a b s t r a c t In nuclear safety, the critical heat flux (CHF) is a very important value that can determine the limit of the safety design. To prevent a minor nuclear accident from mitigating into a severe accident, the coolants should have a higher CHF value. Among the strategies to improve the CHF, many feasibility studies using several kinds of nanofluids were performed as an alternative coolant because of the abnormally CHF enhancement with a very small volume fraction of nanoparticles. Although many researches have tried to apply nanofluids into a real system as a coolant, there are some problems left to solve. In particular, it is necessary to understand the characteristics of nanoparticle stability in the base fluid in order to guarantee the thermal performance of the nanofluids. In this study, three kinds of effects were considered to clarify the relation between nanofluid stability and CHF improvement: the effect of sonication, dilution, and storage time. The effect of sonication in the manufacturing process of the nanofluids should be considered. A two-step method is the most popular for manufacturing nanofluids. Based on the results of the present study, nanofluids made through a two-step method should be initially sonicated to guarantee their CHF enhancement. In the process of dilution, the limitation of nanoparticle concentration exists in the process of dilution to guarantee CHF enhancement. Finally, CHF enhancement using a nanofluid made by a two-step method can be guaranteed for at least a year. In addition, we also proved that CHF using nanofluids can be decreased according to the preservation time. © 2015 Elsevier B.V. All rights reserved.
1. Introduction A primary interest with regard to nanofluids is on heat transfer without phase changes. Thus, many works have been focused on a single phase heat transfer such as in thermal conductivity (Choi et al., 2001; Das et al., 2003; Eastman et al., 2001; Eastman et al., 1996; Hong et al., 2005; Jang and Choi, 2004). However, the
∗ Corresponding author. Tel.: +82 42 350 3826; fax: +82 42 350 3801. E-mail address: [email protected] (Y.H. Jeong). http://dx.doi.org/10.1016/j.nucengdes.2015.05.026 0029-5493/© 2015 Elsevier B.V. All rights reserved.
application of nanofluids is extended to a boiling heat transfer because of the excellent enhanced thermal performance with a very small volume fraction of nanoparticles. In the process of removing the high heat flux, boiling is the most effective method of heat transfer mechanisms. However, a limit of effective heat transfer exists, i.e., the so-called critical heat flux (CHF), which can be generated from nucleate boiling to film boiling. CHF is the main constraint on the design process. Many researches have been tried to evaluate the CHF with several kinds of nanofluids (Bang and Chang, 2005; Jeong et al., 2008; Kim et al., 2006; Kim and Kim, 2009; Kwark et al., 2010; Lee et al., 2012, 2013a,b; Park et al., 2010).
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All experiments, which dealt with an improvement of the heat performance using several kinds of nanofluids, were performed under stable conditions of nanoparticle dispersion. In other words, they did not consider the condition of nanoparticle dispersion in the base-fluid. All researches set a precondition that nanoparticles are well-dispersed in the base-fluid used in their experiments. However, Timofeeva et al. (2007) addressed the idea that nanoparticles are agglomerated according to the passage of time. As shown by the dynamic light scattering (DLS) results, there is a significant nanoparticle agglomeration in the alumina nanofluids. Shima et al. (2010) studied the effect of nanoparticle agglomeration on the thermal conductivity, viscosity, and size distribution of iron oxide and copper oxide. They concluded the prominent role of agglomeration on thermal properties of nanofluid. With the elapsed time, the thermal conductivity of nanofluids was decreased and finally reached an equilibrium value after a time interval. Therefore, it is important to evaluate the nanofluid stability because the thermal performance of the nanofluids can be changed according to their stability characteristics. To guarantee the performance of the nanofluids, it is necessary to clarify between the nanofluid stability and improved thermal performance. This study is aimed at clarifying the CHF characteristics, which is one of the important thermal performances in nuclear power safety, considering the nanofluid stability, and assessing the feasibility of nanofluid applications in nuclear power plants as a coolant. Pool boiling CHF experiments using a Ni–Cr wire were performed to assess the thermal performance of the nanofluids. Among several kinds of nanofluids, we chose a magnetite-water nanofluid, which can improve the CHF with a small fraction of nanoparticles and may also have many advantages for overcoming the shortcomings of the conventional nanofluids in our previous study (Lee et al., 2012, 2013a,b). In addition, DLS data of magnetite-water nanofluids were measured to evaluate the degree of agglomeration qualitatively. Based on these data, we focused on the effects of several characteristics of nanoparticle stability such as the sonication, dilution, and storage time on the CHF enhancement to estimate the possibility in the uses of nanofluids for nuclear applications considering realistic phenomena and problems. These results will be discussed in detail.
2. Assessment of nanofluid stability and thermal performance 2.1. Stability of magnetite-water nanofluids Stable dispersions of nanoparticles are produced by two methods: a one-step method and a two-step method. The two-step method first manufactures nanoparticles using physical or chemical process and then disperses them into base fluids. The one-step method simultaneously manufactures and disperses the nanoparticles into base fluids. Although it can produce well-dispersed nanofluids with monosized nanoparticles, the manufacturing process is very complicated and it is necessary to conduct more studies to overcome the limitations of the manufacturing techniques. Therefore, most nanofluids reported in the literature are made using a two-step method. In most nanofluids prepared by the two-step method, the agglomeration of nanoparticles, which occurs due to the van der Waals force between nanoparticles, is not fully separated. As I mentioned before, the thermal characteristics of nanofluids can be changed according to the condition of the nanoparticles dispersion. However, it is difficult to clarify the relation between nanoparticle dispersion and the excellent performance of nanofluids. In this study, we focused on the effect of nanoparticle dispersion on the thermal performance of nanofluids, especially CHF enhancement. It is necessary to measure the degree of nanoparticle dispersion to clarify the relation. There are several
methods used to evaluate the nanoparticle dispersion stability as indicated below: A sedimentation analysis is the simplest method. The sedimentation method depends on the sedimentation rate of the nanoparticles in the fluid to measure the particle size distribution. By measuring the time required for the particles to settle down, the spherical particle sizes can be determined. Due to dipolar characteristics and ionic attributes, the colloidal particles suspended in fluids are charged electrically. The Zeta potential is an electric potential in the interfacial double layer at the location of the slipping plane and exists around each particle. The magnitude of the zeta potential gives an indication of the potential stability of the colloidal system. In other words, colloids with high zeta potential (negative or positive) will tend to repel each other and there is no tendency to agglomerate. However, if the colloids have a low zeta potential then there is no force to prevent the colloids from coming together and flocculating. The general dividing line between stable and unstable dispersions is generally taken at ±30 mV. A spectroscopy analysis is another efficient way to evaluate the stability of nanofluids. Among the various types of such analysis, Ultraviolet–visible (UV–vis) or photon correlation spectroscopy is useful to evaluate the stability of nanofluids by measuring the optical properties. If the nanoparticles dispersed in fluids have characteristic absorption bands in a wavelength of 190–1100 nm, it is a reliable method to evaluate the stability of nanofluids using UV–vis spectral analysis. Photon correlation spectroscopy, also known as Dynamic light scattering (DLS), can be used to determine the size distribution profile of the colloidal in a suspension. In a previous study (Lee et al., 2012), the zeta potential and pH were measured to guarantee the stability of manufactured magnetite-water nanofluids. The results show that the zeta potential for 100 ppm vol. of magnetite-water nanofluid is measured to be over +30 mV. In addition, the measured pH (4.1–4.5) of magnetitewater nanofluid at 20 ◦ C is far from the iso-electric point (IEP, IEP of magnetite ∼7), which is given as an equal number of positively and negatively charged particles in the colloid. In other words, it can be verified that the magnetite-water nanofluids will be preserved in a stable condition within a short elapsed time (∼1 day). However, the measured data of the zeta potential has more scattered values without a certain tendency according to more elapsed time. In addition, it is difficult to quantify the comprehensive stability of nanoparticle dispersion using the zeta potential and pH. The UV–vis were also measured, but cannot be used to evaluate the stability of a magnetite-water nanofluid. There is no characteristic peak within the valid absorption band from the results of the UV–vis. The DLS was measured in this study to evaluate that of the magnetite-water nanofluid. However, the DLS measurements were performed within a limited range of nanoparticle concentration. A magnetite-water nanofluid is originally opaque, and it is hard to penetrate into the nanofluids with more concentration by light because a magnetite-water nanofluid is more turbid as the nanoparticle concentration is increased. 2.2. CHF for magnetite-water nanofluid In nuclear safety, CHF is a very important value that can determine the limit of the safety design. When CHF occurred, the temperature suddenly soars up to hundreds or thousands of degrees Celsius. To prevent a minor nuclear accident from mitigating into a severe accident, the coolant should have a higher CHF value. Therefore, many researchers have been making an effort to enhance the CHF. In this study, the thermal performance of a magnetite-water nanofluid was estimated based on the CHF values to assess the relation between nanofluid stability and thermal performance. The pool boiling CHF experiment facility in Fig. 1 is
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Fig. 3. Comparison of CHF data of magnetite-water nanofluids considering the effect of sonication with various nanoparticle concentrations. Fig. 1. Schematic of pool boiling CHF experiment.
3. Results
CHF / CHFwater
3.1. The effect of sonication
2.0
Magnetite-water nanofluid Alumina-water nanofluid Titania-water nanofluid 1.0 1
10
100
Nanoparticle concentration (ppm Vol.) Fig. 2. Comparison of CHF ratio between magnetite-water nanofluids and other nanofluids with increasing nanoparticle concentrations from 1 ppm Vol. to 100 ppm Vol.
comprised of a rectangular main vessel, a pre-heater, copper electrodes, a thermocouple, visualization windows, and a condenser. A Ni–Cr wire (Ni: 80%, Cr: 20%) with a 0.4 mm diameter was used as a heated surface and was heated by a DC power supply of OPE-3050DI with a maximum capacity of 1.5 kW (50 V, 30 A), The voltage, current and temperature signals were measured using a National Instrument data acquisition system. The detailed experiment apparatus and procedure were described in our previous studies (Lee et al., 2012). The previous CHF results can be summarized by Fig. 2. The CHF of magnetite-water nanofluids can be improved by 60–140% according to the nanoparticle concentration. As the nanoparticle volume concentration is increased, the CHF of the magnetite-water nanofluids is enhanced. A methodology on the assessment of nanofluid stability and thermal performance is introduced and prepared. Clarifying the relation between nanofluid stability and CHF through these measurements, the characteristics of magnetite-water nanofluid will be studied and supported to design uses of the nanofluids in nuclear power plants.
An advantage of a two-step method for manufacturing the nanofluids is that nanoparticles produced in bulk at low prices can be used. The manufacturing process of nanoparticle powders is very simple. However, it is inevitable that individual particles quickly agglomerate before dispersion. Therefore, it is expected that the initial sonication should be necessary to disperse the nanoparticles in a fluid. To understand the effect of sonication for the manufacturing of the nanofluid on the CHF, two kinds of nanofluids were prepared. For the first, magnetite nanoparticles are well dispersed ultrasonically in pure water, and for the second, magnetite nanoparticles were simply mixed and stirred in pure water. Outwardly, the former has a light red color but the latter is visibly separated between the nanoparticles and pure water. Pool boiling CHF experiments were performed using these nanofluids. Fig. 3 shows the results through a comparison of CHF data between magnetite-water nanofluids according to the effect of sonication when increasing the nanoparticle concentration. The CHF data without sonication show that the CHF values are similar to the pure water case and that the CHF is not improved with an increase in the nanoparticle concentration. In other words, the results show that nanofluids should be initially sonicated to guarantee the CHF enhancement of a nanofluid manufactured using the two-step method.
3.2. The effect of dilution It seems like nanofluids have become a potential future application in nuclear power plants as a coolant in an emergency cooling system. In particular, it is expected that a nanofluid can be used to enhance the in-vessel retention capability in a severe accident mitigation strategy owing to the high CHF margin of the nanofluids. However, it is not possible to prepare a nanofluid with a target concentration in the present system. Because the volume of the flooded cavity is too big and a large amount of nanofluid is needed, this does not agree with the economics and space utilization in a real system. Accordingly, it is necessary to prepare a concentrated nanofluid to reduce its volume, and a dilution process should be required to apply the nanofluid into the cavity. However, we do not know how the CHF can be affected by the dilution process of the nanofluids. The effect of dilution on the CHF
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Fig. 4. Schematic of dilution process of nanofluids. Table 1 Relationship of nanoparticle concentrations between concentrated nanoflud and diluted nanofluid. VNF /VTotal
Effective diameter (nm)
Concentrated Magnetite-water nanofluids 100 ppm
5000
Nanoparticle concentration Concentrated nanofluid
Diluted nanofluid 0
1 10 100
1 10 100
0.01
100 1000 10,000
1 10 100
0.001
1000 10,000
1 10
1
0
2
Critical heat flux (kW/m )
VNF / VTotal = 0.01 VNF / VTotal = 0.001
2x10
3
Pure water 1x10
3
1
10
300
400
Fig. 6. Time-dependent DLS data according to the setting time.
VNF / VTotal = 1 3
200
Setting time (day)
is still unknown, and there have been no researches about the effect of dilution on the CHF enhancement using nanofluids. To assess the effect of dilution on the CHF, various kinds of concentrated nanofluids were prepared. In addition, pool boiling CHF experiments were conducted using a diluted nanofluid in a range of 1–100 ppm Vol. concentration. VNF is the volume of concentrated magnetite-water nanofluid with a range of 100–10,000 ppm Vol. of concentration. VWater is the volume of pure water to dilute the concentrated nanofluid. In addition, VTotal is total volume of diluted nanofluid. Through the dilution process shown in Fig. 4 and Table 1, the diluted magnetite-water nanofluids were prepared, and CHF experiments were then carried out using them. Fig. 5 shows the CHF results according to the effect of dilution compared to the CHF data without the dilution. For all cases using 1000 ppm Vol. of concentrated nanofluids, there is no CHF difference between the dilution and non-dilution cases, despite the changes in nanoparticle volume concentrations. However, the CHF tends to decrease the improvement using 10,000 ppm Vol. of concentrated nanofluids compared to the non-dilution cases. As a result, we proved that a limitation of nanoparticle concentration exists in the process of dilution. A CHF enhancement can be
3x10
100
100
Diluted nanoparticle concentration (ppm Vol.) Fig. 5. CHF results according to the effect of dilution compared to the CHF data without the dilution.
guaranteed until 1000 ppm Vol. of concentrated nanofluid without consideration of the effect its dilution. It is expected that the limitation of the concentrated nanoparticle concentration will be used as a main criterion in the design process.
3.3. The effect of storage time In this study, a two-step method was used for manufacturing the water-based nanofluids. An advantage of this two-step method is that the manufacturing process is simple, cheap, and producible in bulk. However, the agglomeration of nanoparticles is not fully separated in nanofluids prepared by a two-step method. From a practical perspective, manufactured nanofluids should be conserved while keeping the excellent heat transfer. However, nanoparticles will become agglomerated after manufacturing using a two-step method. In the field of heat transfer using nanofluids, the poor dispersion quality generally causes a lower performance of the thermal heat transfer. Considering their dispersion stability, the thermal conductivity of nanofluids, which is the mechanism of a single-phase heat transfer and is related to Brownian motion, was investigated experimentally and theoretically in many research groups. However, it is not easy to find a study on the relationship between dispersion stability and CHF, which is the mechanism of two-phase heat transfer and is related to the changes in the surface characteristics. To evaluate the effect of storage time on the CHF, 100 and 1000 ppm Vol. of concentrated magnetite-water nanofluid were prepared using the two-step method. To assess the dispersion stability quantitatively, the sizes of the nanoparticles were measured by using dynamic light scattering (DLS). Fig. 6 shows the DLS data including the effective diameter of the magnetite nanoparticles according to the setting time in years. The results of 100 ppm Vol. of magnetite-water nanofluid representing the agglomeration of nanoparticles was initially undergoing rapid progress, and after 1 month, the progress of agglomeration was saturated with an approximately 2000 nm nanoparticle size. Unfortunately, it is difficult to measure the DLS data for 1000 ppm Vol. of a magnetite-water nanofluid because of its opacity. Pool boiling CHF experiments were also performed using 1 and 10 ppm Vol. of diluted nanofluids, which are diluted with 100 and 1000 ppm Vol. of concentrated nanofluids, respectively. Time-dependent CHF data for a magnetite-water nanofluid were plotted in Fig. 7 according to the nanoparticle concentration. From this, there is no decrease in CHF values for 1 and 10 ppm Vol. of nanofluids when increasing the preserved time compared with the original CHF values.
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a nanofluid injection system, repairing the period time to guarantee the thermal performance of the nanofluids, their nanoparticle concentration, and so on. For example, the volume of the APR 1400 flooded reactor cavity is approximately 760 m3 . If a concentration of 1 ppm Vol. of nanofluid is determined as the target, the volume of the stored nanofluid is almost 0.76 m3 because the concentrated nanoparticle concentration should be limited to below 1000 ppm Vol.
2 Critical heat flux (kW/m )
3000
2000
4. Conclusion
Magnetite-water nanofluids 1 ppm 1 ppm (dillution) 10 ppm 10 ppm (dillution)
1000
0
100
200
300
Setting time (day) Fig. 7. Time-dependent CHF data of diluted magnetite-water nanofluids with 1 ppm vol. and 10 ppm vol. nanoparticle concentration.
3000
Critical heat flux (kW/m2)
191
Magnetite-water nanofluids 1 ppm (dillution)
2000
Based on this study, we concluded that design criteria are required to guarantee the excellent thermal performance of nanofluids considering their stability. First, nanofluids, which are made by a two-step method, should be initially sonicated to guarantee the CHF enhancement of the nanofluids. Second, a limitation of nanoparticle concentration existed in the process of dilution to keep the CHF enhancement. The CHF enhancement can be guaranteed until 1000 ppm Vol. of concentrated magnetite-water nanofluid without considering the effect of nanofluid dilution. It is expected that the existence of nanoparticle concentration limitation should be considered as an important design criteria in the design process. The CHF enhancement using a nanofluid made using a two-step method can be guaranteed for a year at least. In addition, we also proved that the CHF using nanofluids can be decreased according to the preservation time. Therefore, the maintenance of nanofluid stability should be required to guarantee the CHF enhancement. Based on these results, we expect that this study can be utilized as a basic research for moving forward to step to apply nanofluids into a real system of nuclear power plants. Acknowledgment This work was supported by the Nuclear Safety Research Center Program of the KORSAFe (Grant Code 1305011) grant funded by Nuclear Safety and Security Commission of the Korean Government.
1000
0
100
200
300
400
500
600
700
Setting time (day) Fig. 8. CHF data of diluted magnetite-water nanofluids with 1 ppm vol. of nanoparticle concentration according to preservation time.
Additionally, CHF experiments with 1 ppm Vol. of diluted magnetite-water nanofluid, which have been preserved for 2 years, were conducted, and the results are shown in Fig. 8. The results show that the value of the CHF for 1 ppm of magnetite-water nanofluid preserved during 2 years decreased, which is similar with that of the water case. Although we cannot compare the CHF performance with a varying nanoparticle concentration for 2-year reserved cases, we proved that CHF performance can be decreased according to preservation time. Based on this result, we can conclude that the CHF enhancement using a nanofluid made by a two-step method can be guaranteed for a year at least. In addition, we also proved that the CHF using nanofluids can be decreased according to preservation time. In other words, it is necessary to maintain the nanofluid stability after 1 year to guarantee the CHF performance. Based on this study, we can determine the design criteria for the safety system of a nuclear power plant using nanofluids. In other words, if magnetite-water nanofluid is used in external reactor vessel cooling (ERVC) for in-vessel retention (IVR) as a coolant, we can easily determine the required capacity of the storage tank of
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