Heat transfer in nanofluids

Chapter One

1-1-Introduction:

Common fluids such as water, oils and ethylene glycol, which are usually used as heat transfer media, have a limited ability in terms of thermal properties, which is the first obstacle to compress and increase the efficiency of heat exchangers. One of the ways to improve heat transfer is adding particles to the fluid. They are used with millimeter or micrometer sizes. Due to low stability, rapid sedimentation, obstruction and clogging of the flow path, rapid wear and erosion of the pipe and equipment walls, and a sharp increase in pressure drop in the fluid flow, it was not welcomed. In 1995 AD, Choi introduced a new type of fluid called nano fluid, which consisted of a base fluid that contained nano-sized solid particles with a high conductivity coefficient and low volume ratio. Research showed that nano fluids behave completely differently. Compared to common fluid mixtures with millimeter and micrometer particles, compared to mixtures containing millimeter and micrometer suspended particles, the mixture containing nanometer particles showed better stability and flow properties. Also, they show a much higher conductive heat transfer coefficient and at the same time no specific pressure drop is seen. In this way, a new branch in thermal engineering science was created and it seems that nano fluids are a good alternative to common heat transfer medium fluids due to their ability to increase heat transfer and can be used as the next generation of heat transfer fluids. Therefore, many experimental and numerical studies have been conducted to investigate the thermal and hydrodynamic behavior of these fluids in different thermal systems. These numerical studies with two single-phase and two-phase approaches have attracted the attention of many researchers. In the single-phase method, the mixture of nanoparticles and the homogeneous base fluid are considered, and this mixture is considered as a fluid with effective thermal and hydrodynamic properties for study, while in the two-phase method, the behavior of the fluid and the particle are considered separately, which increases the calculation time. However, the accuracy of the calculations based on the single-phase approach directly depends on the accuracy of the effective physical properties of the fluid. Among these characteristics is viscosity, which is mostly obtained from experimental values ??and analytical relationships with high accuracy are not available to predict it. Lubricating oils have also been noted for improving surface properties, increasing engine efficiency, reducing fuel and engine oil consumption, and reducing the cost of engine repairs. 1-2-History of Nano Technology: Nano is a Greek word that means small. Around 400 years before Christ, the Greek philosopher Democritus first used the word atom, which in Greek means indivisible, to describe the particles that make up materials. worked Therefore, he may be considered the father of nanotechnology and science.

The first spark of nanotechnology (although it was not yet known by this name at that time) was struck in 1959. In this year, Richard Feynman introduced the idea of ??nanotechnology during a speech entitled "There is a lot of space at low levels". He presented the theory that in the near future we will be able to directly manipulate molecules and atoms.

The term nanotechnology was first used by Noriotainguchi, a professor at Tokyo University of Science in 1974. He used this term to describe the construction of precise materials (devices) whose dimensional tolerance is on the order of nanometers. In 1986, this term was recreated and redefined by K. Eric Drexler in a book titled: "The Engine of Creation: The Beginning of the Era of Nanotechnology". He investigated this term in a deeper way in his doctoral dissertation and later developed it in a book titled "Nanosystems, molecular machines, how to make and calculate them". The nano century is called the century of health, economy and peace. Nano is neither a material nor an object, it is just a scale prefix like kilo or milli or giga. Nano means the reduction of a scale, nano is one billionth of a meter so small that it cannot be seen. For example, we have nanometers in the unit of length, which is a billionth of a meter, or nanoseconds in the time scale. A nanometer is as long as 5 to 10 atoms placed next to each other.. If we want to make a cube with dimensions of 2.5 nanometers, approximately 1000 atoms are used. But what separates nanotechnology from other branches of science and engineering is that in the world of nanometers, the physical, chemical and biological properties of individual atoms and molecules are different from the properties of the mass of matter. In our normal world, which is called (classical), the mass of matter obeys Newton's laws. But in the nano world, quantum laws rule. For this reason, making a nanometer piece like a nanotube is different from making a tube with normal dimensions from the ground to the sky.

1-4- Heat transfer by nanofluids:

A new group of fluids that are able to transfer heat are called nanofluids. Nanofluids, which are actually a stable suspension of nanofibers and solid nanoparticles, are made by spreading nanometer-sized particles in common heat transfer fluids, in order to increase thermal conductivity and improve heat transfer performance. Nanofluids, which are obtained from the distribution of nano-sized particles in normal fluids, are a new generation of fluids with great potential in industrial applications. The size of the particles used in nanofluids is from 1 nm to 100 nm. These particles are made of metal particles such as copper, silver, etc. Or metal oxide such as aluminum oxide, copper oxide, etc. Common fluids that are used in the field of heat transfer have a low thermal conductivity coefficient. Nano particles, due to their high conductivity coefficient, when distributed in the base fluid, increase the thermal conductivity coefficient of the fluid, which is one of the basic parameters of heat transfer.

Cooling systems are one of the most important concerns of factories and industries and any place that is faced with heat transfer in some way. With the advancement of technology in industries such as microelectronics, fast and massive operations at very high speeds (several gigahertz) occur at scales below one hundred nanometers. Using engines with power and heat load becomes very important, using advanced and optimal cooling systems is inevitable. The optimization of the existing heat transfer systems is usually done by increasing their surface, which always increases the volume and size of these devices; Therefore, to overcome this problem, new and effective coolants are needed, and nanofluids have been proposed as a new solution in this field. Nanofluids have attracted the attention of many scientists in recent years due to the significant increase in thermal properties, for example, a small amount (about 1% by volume) of copper nanoparticles or carbon nanotubes in ethylene glycol or oil causes a 40% and 150% increase in the thermal conductivity of these fluids, respectively; While in order to achieve such an increase in normal suspensions, concentrations higher than ten percent of particles are needed. This is while the rheological and stability problems of these suspensions in high concentrations prevent their widespread use in heat transfer. In some researches, the thermal conductivity of nanofluids is several times higher than predicted by theories. Among the other very interesting results, the extreme subordination of the heat conduction of nanofluids and their effect is almost three times the critical heat flux compared to normal fluids. These changes in the heat properties of nanofluids have not only attracted the attention of academics. Of course, the suspension of metallic nanoparticles has also been used in other fields, including the pharmaceutical industry and cancer treatment.

(Images can be seen in the main file)

Images of copper nanofluid (left), copper oxide nanoparticles (middle) and gold-lead colloidal particles (right) that have been used in interface resistance studies.

1-5-Preparation of nanofluids:

Improving the thermal properties of nanofluids requires choosing the appropriate preparation method of these suspensions to prevent their sedimentation and instability.