By Dr. Reza Azizian
Research Scientist
Advanced Thermal Solutions, Inc.
1. Introduction
Power requirements for electronic devices have risen steadily in recent years, with the rate of increase sloping upwards, and that has necessitated enhanced thermal management solutions to preserve performance and maintain the mean time between failures (MTBF) of these devices. There are a variety of solutions that can be implemented for cooling high power electronic devices from air to liquid cooling. While air cooling is the default, liquid cooling is necessary when high-power electronic devices dissipate more than of 300-520 W/cm2. [1,2]
The addition of nanoparticles to a coolant (i.e. Nanofluid) is another alternative that can provide further improvement within a liquid cooled system.
Engineered suspensions of nanoparticles in liquids have become known recently as nanofluids. The nanoparticles dispersed in a base fluid are typically metal or metal oxide particles with a size range of 1-100 nanometers. When suspended in the base fluid, nanoparticles create a colloidal solution that has been shown to eliminate the issues of erosion, sedimentation and clogging that plagued earlier solid-liquid mixtures that used larger particles.
Dispersion of nanoparticles in a base fluid alters the fluid’s overall thermo-physical properties (such as enhancing the thermal conductivity). Researchers were able to demonstrate as much as 20% enhancement in heat transfer performance of the single phase, liquid cooled system when nanoparticles were introduced. [3]
This white paper tries to offer a basic overview of nanoparticles and nanofluids for engineers unfamiliar with this topic, including a basic introduction of what nanofluids are, terminology and selecting criteria for engineers, price points, market trends, basic applications, challenges and conclusions. The end of this paper contains both a definition of terms for this topic and a list of further references.
2. Nanofluid Composition
Nanofluids are made up of two basic components: nanoparticles and a base fluid. Surfactant is also used in some cases to increase colloidal stability.
The nanoparticles used in nanofluids are typically metals, metal oxides, carbides, or carbon nanotubes. Common base fluids include water, ethylene glycol and oil. [4] The mixture of nanoparticles into a base fluid creates a colloid, as shown in Fig. 1.
Figure 1. Colloidal dispersion of nanoparticles in a base fluid (Nanofluids). [5]
The addition of nanoparticles to a base fluid enhances thermal conductivity and viscosity of the resultant nanofluids. The degree of enhancement depends on various factors such as volume fraction of the particles being added, particle shape, etc. The reasons for the enhancement are attributed to several microscopic phenomena including particle dynamic effect, liquid layering on the surface of the nanoparticles, and particle clustering. [6]
3. Markets and Applications
Several key markets and applications for nanofluids include:
- Heat Transfer Enhancements
- Power Systems (nuclear and conventional); heat exchangers (chemical industry); heat pipes; thermal management of power electronics and other semiconductors; air conditioning; and refrigeration systems
- Mass Transfer Enhancements
- Fermentations; protein/cell separation; drug delivery; and catalysts
- Rheology
- Lubricants (deep hole drilling)
- Magnetic Nanofluids (Ferrofluids)
- Rotary seals; cancer therapy (hyperthermia); NMR imaging; and magnetic cell sorting
4. Selecting the right nanofluid from the thermal standpoint
In the case of single-phase heat transfer, it has been shown that the heat transfer coefficient and the amount of pressure loss can be predicted by means of traditional correlations and models (Eq. 1 and 2 for heat transfer coefficient of laminar and turbulent flow respectively), as long as the measured temperature and loading dependent nanofluid properties are used. [7, 8]
Where hnf is the convective heat transfer coefficient of nanofluids; Knf, µnf, ρnf, cp,nf, are the thermal conductivity, viscosity, density and specific heat of nanofluid respectively; V is the fluid velocity, x is the distance from the entrance of the tube and D is the tube diameter.
It is clear that if accurate values of the thermo-physical properties for a specific nanofluid are available, the thermal behavior can be predicted. Apart from the specific heat and density, which can be identified by mixture rules, thermal conductivity and viscosity are the two most important parameters for overall thermal evaluation of a specific nanofluid. It is expected that by dispersing nanoparticles with higher thermal conductivity in comparison to the base fluid overall thermal conductivity of the resultant nanofluid will be higher than that of the original base fluid.
This is beneficial because having a working fluid with higher thermal conductivity would result in a reduction of pumping power while achieving the same amount of heat removal. It can also be shown that if one can increase the thermal conductivity of the working fluid by a factor of three (enhancing overall thermal conductivity is basically what nanofluid does), the heat transfer in the same apparatus doubles. [3] To give readers an idea, to increase the heat transfer of conventional fluid by a factor of two, pumping power must usually be increased by a factor of eight. Therefore, using nanofluid with high value of thermal conductivity can provide large pumping power reduction in the system.
Effective medium theory (i.e. Eq. 3) with Ck = 3 (for a dilute system of spherical particles) can be used as a rule of thumb criteria of the thermal conductivity estimation for the specific nanofluid. Obviously, measuring thermal conductivity and identifying Ck provides much more accurate value.
where knf and kbf are the thermal conductivity of nanofluid and base fluid respectively; Ck is the constant that should be found by matching Eq. 3 to the experimental data; and ∅ is the volume fraction of the particles.
However, this is just half of the story, in addition to thermal conductivity, it was also evident that viscosity has a significant impact on the overall performance of a heat transfer fluid. Adding nanoparticles to the base fluid results in a higher viscosity of the resultant nanofluid. Clearly, pumping a fluid with increased viscosity through a heat exchanger requires an increase in pumping energy, thereby reducing the overall benefit of a fluid with higher thermal conductivity. [9] Unfortunately, most of the equations for predicting viscosity of nanofluids under-predicts the experimental data by a huge margin. It was shown that for the first order estimation value of Cµ = 10 can be used in Eq. 4.
where µnf and µbf are the viscosity of nanofluid and base fluid respectively and Cµ is the constant that should be found by matching Eq. 4 with the experimental data.
This shows that generally the enhancement in viscosity is higher than enhancement in thermal conductivity (Cµ = 10 compared to Ck = 3). Then the key question to be answered is: “Is there any benefit of using nanofluids as a heat transfer fluid?” [10]
Criteria for the overall effectiveness of nanofluids as heat transfer fluids have been proposed, [9,11] which suggest that the increase in the viscosity has to be more than four times larger than the comparable increase in thermal conductivity for a nanofluid to be not beneficial at all.
Therefore, the overall quick guide line for selecting a nanofluid for a specific application would be as follows:
A. Use Eq. 3 and 4 to estimate the thermal conductivity and viscosity of the nanofluid (measured values of the thermal conductivity and viscosity are obviously preferred).
B. If the measured thermal conductivity and viscosity of the nanofluid of interest meets the requirements of Cµ <4 Ck move to the next step. If not, the choice of nanofluid should be changed.
C. Calculated/measured thermal conductivity and viscosity values should be implemented in Eq. 1 or Eq. 2 (depending on the flow) for the first order estimation of heat transfer coefficient enhancement.
In applications that involve boiling, it was shown that the boiling of nanofluids result in a deposition of nanoparticles on the heated surface (Fig. 2). [12,13] Deposition of nanoparticles on the surface provides a porous, hydrophilic layer that alters the wettability of the surface. Due to the nano-scale porous nature of this layer, capillary wicking also plays an important role. Lateral flow of the liquid due to the capillary wicking supplies a fresh liquid to the dry region beneath the bubble and delays the irreversible growth of the hot spot, which results in critical heat flux (CHF). [13] The exact mechanism behind the observed enhancement in the CHF value is still subject of debate in heat transfer community. [14]
Figure 2. Scanning electron microscopy of the heated surface after boiling with (a) DI-water and (b) nanofluid. [12]
5. Cost estimation of Nanofluids
The overall cost of a nanofluid is determined by the price of the chosen base fluid and the price of the nanoparticles themselves. Prices are higher when the nanoparticles are more difficult to synthesize or the process for creating the stable colloidal suspension is more challenging. Table 1 presents an estimation of the price for some common nanoparticles with water as the base fluid. The thermal conductivity of the resultant nanofluid is a function of the nanoparticle’s thermal conductivity. Clearly, nanoparticles with higher value of thermal conductivity provide higher enhancement.
6. Geographic Considerations for Nanofluid R & D
Nanotechnology is a field that has worldwide investment in R&D both from governments and private enterprises. The Industrial Research Institute’s “2016 Global R&D Funding Forecast” [15] notes that the most important technologies from a world-wide Industrial R&D investment standpoint include nanotechnology for both biological and industrial applications. Indeed, nanotechnology research for industrial applications (including thermal management of electronics) is forecast to have a 30% increase in R&D investment over the next two years. Only Information Technologies is forecast to have a larger increase at 31%.
The United States nanotechnology industry is led by the National Nanotechnology Initiative, which (in 2017 alone) has budgeted $1.4 billion for R&D. The budget supports nanoscale science, engineering, and technology R&D at 11 agencies, including the National Science Foundation, National Institute of Health, Department of Energy and the Department of Defense. [16] In addition, the United States has published 6,926 scientific papers on nanotechnology as of 2015. [17]
Other countries that are also heavily investing in nanotechnology include:
– China is second to the United States in investment for nanotechnology research having invested $1.3 billion in R&D and having published 2,898 scientific papers.
– Japan has $850 million dollars invested in nanotechnology research and has published 2,626 papers.
– Germany also has $850 million dollars invested with 1,767 published scientific papers.
– South Korea has $400 million dollars invested and 1,461 published scientific papers.
There has been a major effort in the European Union (EU) to make nanotechnology available for commercial purposes. In the European Commission’s “Horizon 2020” framework for research and innovation, nanofluids were labeled a “Key Enabling Technology,” although it also noted that many nanofluids are at Technical Readiness Level (TLR) 1-3, making them several years away from full commercial viability. [18] The European Commission is devoting €80 billion to spearhead research into various technologies, in addition to private investments.
While the amount of investment shows the overall commitment of a country to this technology, another metric is the concentration of nanotechnology startup companies by geography. For example, in the United States, the highest concentrations of nanotechnology startup companies are in California, Massachusetts, New York and Texas. The densest locations (with 30 or more companies) are Boston, Mass.; San Francisco, Calif.; San Jose, Calif.; Raleigh, N.C.; Middlesex-Essex, Mass.; and Oakland, Calif. [19]
7. Regulatory Considerations
Many nanoscale materials are regarded as “chemical substances” under the U.S. Environmental Protection Agency (EPA) Toxic Substance Control Act (TSCA). [20] TSCA requires manufacturers of new chemical substances to provide specific information to the Agency for review prior to manufacturing chemicals or introducing them into commerce. The EPA can take action to ensure that chemicals that may pose an unreasonable risk to human health or the environment are effectively controlled. The EPA’s bulletin, “Control of Nanoscale Materials under the Toxic Substances Control Act” outlines the regulatory framework for nanoscale materials. Most engineers implementing nanofluids for thermal management would be acquiring these materials from another manufacturer.
8. Key Market Drivers
Market drivers are forces that compel products and services to change in order to meet consumer or business demand. A useful construct for those drivers is a STEEPLE analysis, which considers seven broad factors of market drivers: Socio-Cultural, Technological, Economic, Environmental, Political, Legal and Ethical. This section deals with two to three key technological drivers for the most relevant heat transfer enhancement applications such as: power plants, power electronics and other semiconductors, and heat pipes.
8.1. Power Systems (nuclear and conventional)
Virtually all developed countries rely on a national power grid for their nations to grow and prosper. Nanofluids can play an important role in this space by improving the outcome and cost-efficiency of existing power plants. The use of nanofluids to cool a power plant can significantly reduce the resources consumed and the cost in electricity to run it. These cost savings and the greater efficiency that nanofluids provide can also eliminate the need for building additional power plants, which is a massive financial boost for a country, state, or municipality.
In power systems, the key technological drivers to consider for the implementation of nanofluids include:
o Nuclear [21]
– Improving the energy efficiency of a nuclear power plant by improving the functionality of existing power systems.
– Cost and regulatory avoidance by not having to build additional infrastructure since the efficiency of existing infrastructure can be improved to the degree that more building is unnecessary.
– Safety of nuclear power plants by improving the cooling of nuclear rods (in a pressurized water reactor, Fig. 3 [22]) by pushing the point at which a rod might overheat (CHF) to a higher value.
Figure 3. Pressurized water reactor (PWR). [22]
o Solar [23]
– Improvement to heat transfer by increasing the heat transfer performance of the fluid in the solar collectors for hot water supply (Fig. 4).
Figure 4. Solar hot water system. [24]
o Conventional [24]
– Improvement in power plant efficiency through various methods including higher heat capacity of the heat transfer fluid.
– Reduction in the size of heat exchangers.
– Reducing vapor pressure to reduce plant risk.
– Improvement in the stability of heat transfer fluids with a goal of keeping the consistency of energy conversion stable.
8.2. Power electronics and semiconductors [25, 26, 27]
With the advent of cloud computing, electric and hybrid vehicles, and the Internet of Things (IoT), electronics have become increasingly integrated into our lives. This integration of and the growth in applications drives an increasing need for more performance but at lower power and in smaller packages.
In power electronics and semiconductors the key technology drivers are:
• Increasing speeds of semiconductors with simultaneous increase in component densities.
• Increasing the performance of fluids for thermal management of electronics and migration from air to liquid cooling systems.
• Improvement in energy efficiency of electronic systems.
• Improving rack density for computing systems through reducing systems to sub-1U platforms.
• Improvement in power module life (MTBF).
• Thermal management of ever-increasing power densities.
8.3. Heat Pipes
Heat pipes are increasingly used as a key component in thermal management solutions. Their ability to transfer large amounts of heat from one section of a system to another makes them invaluable for keeping semiconductor junction temperatures within their proper range.
Key technology drivers for heat pipes and nanofluids are:
• Improving the heat transfer performance and temperature uniformity in heat pipes.
• Reduction of the thermal resistance (because of higher thermal conductivity of nanofluids).
• Increasing maximum allowable heat load of heat pipes (Qmax) by improving the wettability and providing larger surface area for thin-film evaporation in an evaporator (because of particle deposition as a result of evaporation).
9. Applications and Specifications
Nanofluids are a broad category of engineered fluids. The composition and selection of a particular colloid is generally specific to a given application category. The following are some examples of applications that could use nanofluids and some general specifications as to why an engineer may want to choose them.
9.1. Computer servers for cloud computing
Cloud computing, with its large scale of servers, continues to grow and with this trend so too does the amount of energy data centers consume. Moving from air cooling to liquid cooling can create a more energy- and computationally-efficient data center. When using water as the base fluid with nanoscale particles of zinc oxide, copper oxide or carbon nanotubes, for example, the colloid’s capability to conduct heat is improved 10-40% over water alone. The lower range of improvement is with a conventional heat exchanger, while the higher range requires a specially designed heat exchanger. [29]
9.2. Power Electronics
Insulated-gate bipolar transistor (IGBT) modules, power brick and power semiconductors have ever-increasing heat dissipation requirements reaching 300 W/cm2 and higher. At those heat dissipation levels the constraints of cabinet size, system density, noise levels and fan size make using liquid cooling with nanofluids a good thermal management strategy. As in other applications, an improvement of 10-40% may be possible by using nanofluids rather than water alone.
The implementations of liquid cooling can vary, but commonly involve heat pipes, vapor chambers or cold plates (either single or two-phase) with a pump. Because of the unique thermal capabilities of nanofluids, all three implementations can be smaller, lighter, quieter and thinner than when deployed without nanofluids. [30]
10. Trends
The idea of using liquid cooling for power electronics applications is no longer confined to theoretical observations or laboratory experiments. There is widespread use of heat pipes, for example, and personal computers frequently incorporate elaborate liquid cooling systems. Nanofluids represent an enhancement to the technologies that are increasingly being used for cooling electronics.
Much of the ongoing research is focused on immersion cooling and boiling. In tests, boiling with nanofluids has been shown to improve the value of critical heat flux (CHF) by as much as 200% as a result of nanoparticle deposition on the surface of component. This has led to other research on, potentially, engineering the surface of high-powered electronic devices with nanoparticles to improve heat dissipation without the need for an engineered liquid.
Researchers are also continually testing new materials to disperse in different fluids. For instance, aluminum oxide, iron oxide, zinc oxide, cerium oxide, and bismuth oxide are just some of the options that Nanophase Technologies Corporation offers to customers. In addition, there is work being done with carbon nanotubes that have shown promising results in heat transfer. [31] New materials, such as graphene, are also being developed and tested to determine their potential for application in thermal management.
Other trends in the technology include the use of different materials such as copper, aluminum, and newly developed polymers for the loop of a liquid cooling system or different wicking materials in the production of heat pipes and vapor chambers. In addition, improvements are being made in heat exchangers, pumps, and other components of liquid cooling systems.
11. Challenges
While nanofluids have gained momentum and notoriety in the past two decades, there are still several challenges to their widespread commercialization. [32] One of those challenges is that the concept of nanofluids is still relatively young. The term was only coined in the early 1990s and, while the number of studies have increased in recent years, there are no long-term assessments of how the addition of nanoparticles could affect a cooling system over time. Nanoparticles could collect and cause degradation in a pump or heat exchanger, for example. There is a widespread belief in the stability and reliability of nanofluid solutions, but it has not been possible to document that over the course of a long period of time.
Also, there are questions that have been raised about the environmental impacts of disposing nanofluids from a system, whether as vapor in the case of a nuclear power reactor or as liquid when replacing the fluid in a data center system. While it is possible that nanoparticles in high quantities could be harmful, there are few studies that have focused on the environmental impacts [33] or the impact on humans. [34]
12. Conclusions
This white paper summarized the potential applications of nanofluid in electronic cooling. It was shown that the thermal performance of the liquid cooled system can be further improved by dispersing nanoparticles in a base fluid (i.e. nanofluid). The guide for selection of the right nanofluid for specific application is provided. It was shown that the heat transfer coefficient enhancement in nanofluid can be predicted simply by implementing thermo-physical properties of the nanofluid of interest in the traditional models/correlation for single phase convective heat transfer. It was also shown that the large enhancement in CHF value can be achieved by boiling nanofluid on a heated surface. Based on these basic principles employing nanofluid in different applications, their opportunities and challenges were discussed.
Acknowledgment
The author would like to thank Ms. Rebecca O’Day and Mr. Joshua Perry for their help in preparing this manuscript.
Definition of Terms
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There are several terms that are used in discussion nanoparticles and nanofluids in electronics cooling applications. These terms are listed below and defined for reference.
Base Fluid – Fluids generally used to contain nano particles to create a colloidal suspension. Base fluids typically but not exclusively used include water, ethylene glycol, engine oil, Tolouene, refrigerants.
Colloid – Nanoscale or microscale particles suspended in another medium.
Nanoparticle – Particles with size on the order of 1-100 nm. [35]
Nanoparticle Types: Nanoparticles can be made of materials with good thermal conductivity such as metallic oxides (Al2O3, CuO, SiO2, Fe3O4), nitride ceramics (AlN, SiN), Carbide ceramics (SiC, TiC), metals (Cu, Ag, Au), semiconductors (TiO2, SiC), carbon nanotubes (SWCNT, DWCNT, MWCNT).
Nanofluid – Fluid containing nanometer-sized particles, called nanoparticles. These fluids are engineered colloids of nanoparticles in a base fluid.
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