Abstract
Since its discovery more than 40 years ago, thermal-hydraulic technology has been used widely to enhance the performance, design, and safety of nuclear power plants. Over the years, money and time have been invested in this field to come up with improved technologies to efficiently and fully develop thermal hydraulic systems. In this paper, the author provided a critical review of promising nanotechnology innovations that can be used to exploit the potential of these applications.
Introduction
Nanotechnology is an emerging field in modern science. It can be defined as the manipulation of matter at the atomic and molecular levels. The concept was first introduced by physicist Richard Feynman in 1959. However, widespread application of the technology started in the early 1970s. The term ‘nanotechnology’ was first used by Norio Taniguchi. It was coined in 1974. A critical analysis of this field reveals the endless possibilities associated with the application of nanotechnology. Such applications include those in thermal-hydraulic systems used in nuclear reactors (Rocha et al. 26). Modern thermal-hydraulic systems rely on a wide range of materials to cool the reactors. The materials used depend on the intensity of the operations undertaken.
In this paper, the author provides a critical literature review of the probable applications of nanotechnology in thermal-hydraulic systems. The review is based on an article published by Bang and Jeong (218).
Using Nanotechnology in Thermal-Hydraulic Systems
Thermal-Hydraulic Systems and Reactors
There are different types of reactors. The first category includes light water reactors. In this cluster, there is PWR and BWR (Rocha et al. 25). The second class includes heavy water reactors. The cluster is made up of HWRs. Finally, there are gas-cooled reactors. The category includes LFR and LMFR. The fourth-generation reactors require more efficient cooling materials compared to the earlier technology. The reason is due to an increase in the power density of the reactors (Rocha et al. 24).
Nanotechnological Advancements with Practical Applications in Thermal-Hydraulic Systems
Since the 1970s, significant advancements have been made in nanotechnology. Some of the new technologies introduced have potential practical applications in thermal-hydraulic systems. They include the following:
Nanofluids
In their article, Bang and Jeong provide a working definition of a nanofluid (218). The component comprises a base fluid, which may be water or any other aqueous solution. The fluid has chemically stable nanoparticles suspended in it. The volume of the nanoparticles suspended in the base fluid is around 50% of the total concentration (Rocha et al. 26). The nanoparticles used are usually derived from metals, metal oxides, and carbon compounds. The latter includes carbon nanotubes, graphite, and diamond (Saidur, Leong, and Mohammad 1648). There are different types of these fluids. Most nuclear reactors in operation today rely on conventional fluids to cool down their cores. The fluids are associated with various limitations. One of these weaknesses touches on their thermal conductivity. As a result, there is poor heat transfer. Compared to conventional fluids, the thermal conductivity of solids is generally higher. As a result, they are ideal for heat transfer. For practical applications, these solids are broken down into their respective nanoparticles. The size of these particles is usually below 100nm. The small size makes it possible to suspend the nanoparticles in the base fluid without apparent damage to the structure of heat transfer systems (Bang and Jeong 223).
The performance of nanofluids is relatively high. It is noted that the use of these particles increases thermal conductivity by 150% (Saidur, Leong, and Mohammad 1660). In addition, the coefficient of single-phase heat transfer rises by 60% (Bang and Jeong 223). Quenching efficiency is also significantly improved. According to Li et al., the boiling Critical Heat Flux (CHF) also increases by up to 200% at low concentrations of nanoparticles (1). According to Bang and Jeong, CHF is considered as the upper limit of phase change nucleate boiling heat transfer (225). As such, it is the most efficient method of heat transfer (Bang & Jeong 225).
Despite the practical applications of this technology, nanotechnology has several weaknesses. For example, when nanoparticles are suspended in the base fluid, the resulting nanofluid has a higher viscosity than the base fluid used. High viscosity means that more pumping power is required to produce a thermal performance that is similar to that of conventional fluids. In addition, nanofluids are associated with constant drops in pressure. It is also noted that the thermal performance of these fluids in turbulent flow lowers the specific heat (Saidur, Leong, and Mohammad 1648).
Surface modification technology
The technology involves modification of the heating surface related to boiling mechanisms (Bang and Jeong 221). The alteration is aimed at, among others, improving the texture of the surface and increasing the number of cavities. The roughness of the heating plane is achieved through the use of appropriate microstructures to coat the surface (Bang and Jeong 221). The modification is usually at the molecular level. The intervention is a form of nanotechnology. It results in an improved boiling heat transfer (BHT).
Surface modification is also practical when enhanced critical heat flux is required. The elevation at which a fluid establishes contact with a particular plane can be controlled through the process. As a result, it changes the wetting structure of a surface. Due to the porosity of the modified plane, the boiling regime is extended. On its part, CHF is delayed given that the liquid is spreading over the heated area.
The use of nanotechnology in surface modification can be broadly categorized into five classes:
Nanorod deposition
In an experiment conducted by Li et al., an electron beam evaporator was used to deposit copper nanorods on a copper substrate (4). It was observed that the density of active bubble nucleation increased by 30 folds. The move led to an increase in the boiling heat transfer coefficient. The surface’s wettability also improved. The wettability was measured using the contact angle with the surface. It was concluded that effective wetting of a surface suppresses the bubble ebullition process to higher superheat (Bang and Jeong 222). The observation is attributed to the fact that the enhanced wettability of the nanorods could not support the increase in nucleation sites and the reduced superheat.
Nanowires
Compared to plain surfaces made of the same material, nanowires contain almost twice the amount of critical heat flux and heat transfer enhancement. The wires possess unique properties that include high nucleation site density, enhanced capillary pumping effect, and super hydrophilicity (Bang and Jeong 225). Research has revealed that higher levels of critical heat flux and heat transfer can be attained in these components by designing and synthesizing nanowire arrays rationally.
Nanomaterial deposition
In this class, surface modification is achieved through a process called Microreactor Assisted Nanomaterial Deposition (MAND). Surfaces modified by nanomaterial deposition exhibit a boiling heat transfer that is 10-fold higher than that of conventional materials. Their CHF pool boiling is also 4-fold higher.
Coarse micro-structures
Micro or nanostructures are surfaces with mountain-like structures. When passed through an anodic oxidation process, they display higher CHF values and ultimately lower boiling heat transfer compared to bare substrates with the microstructures. The varying readings are attributed to the increased wettability and super hydrophilicity of the surface (Bang and Jeong 222).
Nanoparticle thin-film coatings
Forrest et al. experimented using nanoparticle thin-film coating (59). The coating was made of PAH/SiO2. It was used to prepare heat transfer surfaces. The planes were used to study BHT and CHF in pool boiling. Compared to a bare surface of the same material, the coating gave a higher BHT and a higher CHF. The results were attributed to changes in advancing contact angle (Forrest et al. 59). The alterations affected the BHT and the receding contact angle which determines the CHF.
Conclusion
Thermal hydraulic systems play an important role in the safety and performance of nuclear power plants. However, due to an increase in the power density of nuclear reactors, highly advanced and reliable technology is needed to sustain the growth. Technological advancements have revealed new ways through which the potential of nanotechnology in thermal-hydraulic systems can be exploited. The future of this field looks bright. Engineers hope that the new ideas will be adopted at the industrial level.
Works Cited
Bang, In, and Ji Jeong. “Nanotechnology for Advanced Nuclear Thermal-Hydraulics and Safety: Boiling and Condensation.” Nuclear Engineering and Technology 43.3 (2011): 217-242. Print.
Forrest, Eric, Williamson Erik, Buongiorno Jacopo, Hu Li-Wen, Rubner Michael, and Cohen Robert. “Augmentation of Nucleate Boiling Heat Transfer and Critical Heat Flux Using Nanoparticle Thin-Film Coating.” International Journal of Heat and Mass Transfer 53 (2010): 58-67. Print.
Li, Chen, Wang Zuankai, Wang Pei-I, Peles Yoav, Koratkar Nikhil and Peterson P. “Nanostructured Copper Interfaces for Enhanced Boiling.” Small 10.10 (2008): 1-5. Print.
Rocha, Marcelo, Cabral Eduardo, Sabundjian Gaiane, Yoriyaz Helio, Ana Lima, Junior Antonio, Prado Adelk, Filho Tufic, Andrade Delvonei, Shorto Julian, Mesquita Roberto, Souza Francisco, Otubo Larissa and Filho Benedicto. Perspectives of Heat Transfer Enhancement in Nuclear Reactors Toward Nanofluids Applications. 2013. Web.
Saidur, Rahman, Leong Kin, and Mohammad Hussein. “A Review on Applications and Challenges of Nanofluids.” Renewable and Sustainable Energy Reviews 15.3 (2011): 1646-1668. Print.