Effect of Nano-fluids on Heat Transfer Enhancement in Automotive Cooling Circuits

Unceasing technological development has compelled the automobile industry to manufacture engines with higher efficiencies. To meet the weight requirements of a vehicle, a radiator with optimum size and design is required. Fins are generally used in radiators to increase the rate of heat transfer. Moreover, conventional approach of improving the heat transfer by using micro-channels and fins have some limitations. Additionally, Ethylene Glycol and water have very low thermal conductivities, therefore there is a dire need of introducing new fluids to increase the heat transfer in automobile cooling circuits. Recent researches have shown that Nano-fluids perform better than conventional fluids in heat transfer applications. Leong et al. [33] studied the heat transfer performance of Copper and Ethylene Glycol Nano-fluid and found a better rate of heat transfer than the base fluid alone. The Reynolds number for air and coolant was kept 5000 and 6000 respectively and a 3.8% increase in the rate of heat transfer was narrated for 2% volume fraction. The study also suggested a decrease in the dimensions of frontal surface. Hussein et al. [34] carried out a study on the heat transfer performance of SiO2 and TiO2 Nano-fluid with water. Volume concentration, volume flow rate and inlet temperature of Nano-fluid was kept 1-2%, 2-8 LPM, 60°-80°C respectively and flow was kept laminar. The results indicated that Nusselt number increased slightly with the volume concentration and inlet temperature of the Nano-particles and showed a significant rise with the increase in volume flow rate. 

Subhedar [35] carried out an experimental study to observe the heat transfer performance of water monoethylene glycol (MEG) and Nano-particles of Al2O3 with a volumetric ratio of 50-50%. MEG is usually used as a coolant in radiators. The flow was kept laminar and an automobile cooling system was replicated in the experimental system. The Nano-fluid preparation consisted of two-step method. Ultrasound technique was used to disperse the Nano-particles (20nm diameter) of Al2O3 in a mixture of MEG. The experimental study showed an increase in the heat transfer relative to the base fluid. With the increase in volume concentration from 0-8%, an increase in the overall heat transfer coefficient was noticed. The increase of the inlet temperature from 65 °C to 85 °C. Lastly, the study also established that when the volume concentration of Nano-particles of Al2O3 is 2%, it permits a reduction of 36.9% in the heat exchanger surface. Bozorgan et al. [36] performed numerical analysis on the radiator of a diesel engine. He circulated the Nano-fluid of CuO-water through the flat tubes. The study concluded that an increase of 23.8% and 10% compared to base fluid was attained in pumping power and overall heat transfer coefficient respectively when the speed of the vehicle was 70km/h and the volume concentration of Nano-fluid was 2%. 

Hwa Ming Nieh [7] carried out an experimental study in which he used Nano-coolant (NC), Titania (TiO2) and Alumina Al2O3 to increase the heat transfer of an air-cooled radiator. The study concluded that Nano-fluids perform better than Ethylene Glycol in the terms of heat dissipation and efficiency. Moreover, experimental data revealed that Nano-fluid of TiO2 is efficient than the Nano-fluid of Al2O3. Gulhane and Chincholkar [37] performed experimental analysis on the radiator of a car to determine the heat transfer rate. The Nano-fluid of Al2O3 with water was taken in lower concentration in this study. The result revealed that with the increase in inlet temperature of the coolant, coolant flow rate and particle concentration, heat transfer coefficient of the coolant also rises. They noticed a maximum increase of 45.87% in the heat transfer coefficient. Ray and Das [39] used three different types of Nano-particles i.e. silicon dioxide, copper oxide and aluminum oxide in a car radiator. The base fluid for all three Nano-particles was a mixture of water and Ethylene Glycol in a ratio of 40:60. It was observed that a mixture with lower concentrations i.e. 1% performed better than the mixture that had higher concentration of Nano-particles. A reduction of 35.3% in the pumping power and 7.3% in the heat transfer surface was achieved for 1% volume concentration of Al2O3 Nano-fluid. 

In the case of CuO Nano-fluid, pumping power reduction was comparatively lesser i.e. 33.1% and heat transfer surface reduction i.e.7.2% was almost similar to Al2O3 Nano-fluid. SiO Nano-fluid performed lowest but still a reduction of 26.2% and 5.2% was attained in the pumping power and heat transfer surface respectively. Ali et al. [40] studied the heat transfer performance of ZnO Nano-particles at different volume concentrations i.e. 1%, 2%, 3%, and 8%. The flow rate and Reynolds number was kept in the range of 7-11 LPM and 17,500-27,600 respectively. All the concentrations of Nano-fluid showed a better heat transfer performance compared to the base fluid. The maximum increase of 46% was achieved for 2% volume concentration of the Nano-particles. When the concentration was increased up to 3%, a decrease in the heat transfer was noticed. In addition to that change in inlet temperature from 45°C to 55°C, an increase of 4% was observed in heat transfer velocity. Moghaieb et al. [41] performed the experimental analysis to study the convective heat transfer of a car engine for water and Al2O3 Nano-fluid. He used different diameters [21-37 nm] of Nano-particles in his study. He found that with the increase in flow velocity, the heat transfer coefficient increase whereas it decreases with the increase in the temperature. For a volumetric concentration of 1%, 78.76 % increase in the heat transfer was attained as compared to the traditional fluid. 

Elbadawy [42] performed a numerical analysis to study the flow characteristics and thermal performance of water based Nano-fluid of Al2O3 and CuO. He observed that Nano-fluids can increase the rate of heat transfer and obtained a major volume reduction in the radiator. He found that pumping power is depends upon the concentration of Nano-particles and Reynolds number of the flow. Moreover, 38% and 45% increase in the rate of heat transfer was obtained for CuO and Al2O3 Nano-fluid respectively. Optimum value of volume concentration of both Nano-fluid was found to be 4.5%. At this value, a reasonable increase and decrease in the heat transfer and pumping power was achieved respectively. Harsh et al. [43] carried out an experimental analysis on the heat transfer performance of Nano-fluids in an engine. A solution of Ethylene Glycol (EG) and water (80:20) was used as a base fluid for the dispersion of Al2O3 Nano-particle. Three different values of flow rate were chosen to be 4, 6 and 7 LPM whereas the speed of air flow inside the duct was 4.9 m/sec. The study concluded that with the use of Nano-particle, 24.5% and 13.9% increase in the overall heat transfer coefficient and rate of heat transfer can be obtained. In this research, the diameter of Nano-particles of Al2O3 was 45 nm and its volumetric concentrations was kept between 2% and 6%. The analysis was carried out on three different operating ranges of car (High, Medium and Low) and study was focused on determining the rate of heat transfer, pumping power and outlet temperature of radiator. The obtained results were then compared with the results obtained using base fluid. 

Thermal Conductivity Enhancement in Nanofluids

Thermal conductivity of conventional fluids can be improved using two approaches i.e. usual suspensions and Nanofluids. Nanofluids generally have higher thermal conductivity than usual micro-sized suspensions. 

However, when toluene, transformer oil, ethylene glycol and water are used as base fluids, the improvement in thermal conductivity achieved by using Nanofluids is better than that could have been achieved through suspensions. 

Nano-particles can be categorized into three different types i.e. Carbon nanotubes (CNTs), ceramic particles and pure metallic particles. These particles can be used in different combinations to form different Nanofluids.

Ceramic Nanofluids

ANL group investigated the ceramic Nanofluids for the first time. The pioneer work in this field was carried out to determine the thermal conductivity of Cu-O and Al2O3 Nano-particles in the base fluid of ethylene glycol and water.

Conventional transient hot-wire (THW) method was used to measure thermal conductivity. Different volumetric fractions i.e. 1-5% were used to analyze thermal conductivity enhancement. 

It was concluded that thermal conductivity was greatly enhanced by using Al2O3 and Cu-O Nano-particles. For the 4% volumetric fraction of CuO, the increase in thermal conductivity was 20%. 

Though the increase in thermal conductivity in the case of water as a base fluid was substantial but the rise in thermal conductivity was comparatively higher in the case of ethylene glycol. 

The increase of 12% and 10% for 3.5 vol. % of Cu-O and 4 vol. % of Al2O3 was achieved in the case of water as a base fluid. These values of thermal conductivity were higher than the values obtained for suspensions in the study carried out by Maxwell in 1962 [28]. 

The model he used employ point source method to predict the values of effective thermal conductivity as a weighted average of liquid and solid conductivity. 

Previous work also predicted the thermal conductivity of Al2O3 and Cu-O water Nanofluids. They used smaller Nano-particles of size 23 nm and 28 nm for Cu-O and Al2O3 respectively. 

They used engine oil (Pennzoil 10W-30) and ethylene glycol as the base fluids and results showed that thermal conductivity is strongly dependent on the size of particle and method of dispersion. 

Xie et al. [29] used particles of Al2O3 of size 1.2-3.02 nm to measure the thermal conductivity with water as a base fluid. They studied the effect of base solution as well as the size of particle on thermal conductivity. 

Though the ceramic particles are not known for their high thermal conductivity enhanced the thermal conductivity of Nano-fluid. Many researches have been conducted on oxide particles-based Nanofluids due to their easy availability.

Murshed et al. [30] predicted the thermal conductivity of TiO2-Water Nano-fluid. He used cylindrical and spherical shaped particles of size 10 × 40 nm and 15 nm respectively. 

They measured an increase of 33% in thermal conductivity for a volumetric fraction of 5%. The obtained values of thermal conductivity were significantly higher than the values predicted by Hamilton-Crosser model. 

They also found that, at lower volume fraction, particle concentration and thermal conductivity are non-linearly related to each other.

Metallic Nanofluids 

Though the importance of Nano-fluid was obvious from ceramic-based Nanofluids but the arrival of metallic based Nanofluids was a one-step forward. 

A study was done to first measure the thermal conductivity of copper-transformer oil based Nano-fluid. Large sized particles of size approximately equal to 100 nm were used in this study and 55% increase in thermal conductivity was obtained at a volumetric fraction of 5%. 

ANL group used copper Nano-particles of 10 nm in size in ethylene glycol and narrated an increase of 40% with the volumetric fraction of only 0.3 % which was a major breakthrough. 

This study proved that thermal conductivity is greatly affected by the size of Nano-particle. They used thioglycolic acid to stabilize the Nano-fluid. Predicted values of thermal conductivity are shown in Fig. 3. Patel et al. [31] used silver and gold particles for the first time in preparing Nanofluids and employed transient hot wire method to measure the thermal conductivity. 

They reported a noticeable increase in the thermal conductivity for very small concentrations of Nano-particles. The increase in the thermal conductivity for toluene-gold Nano-fluid was 3-7% for a very low volume fraction of only 0.005–0.011% and the increase was around 3.2-5% for 0.0013–0.0026% volumetric fraction in the case of water-gold Nano-fluid. 

The key reason of increase was the particle size which was around 10-20 nm. Water based Nano-fluid showed more improvement due to use of bare particles whereas thiolate coated Nano-particles were used in toluene-gold Nano-fluid to avoid agglomeration. 

Though the thermal conductivity of silver is higher but the silver-water Nano-fluid showed relatively lower thermal conductivity due to the large sized Nano-particles (∼60–80 nm). 

This is important to note that size of particle can dominate the concentration effects or thermal conductivity. Xie et al. [32] studied the effect of base fluid on thermal conductivity. Glycerol-water mixture, glycol-water mixture, glycerol, pump oil, ethylene glycol, and deionized with α-Al2O3 were used in the study. 

Base fluids with higher thermal conductivity showed a decrease in thermal conductivity. Study Iron-Ethylene Glycol Nano-fluid and obtained an increase of 18% in the thermal conductivity for the volumetric fraction of only 0.55%. 

It was observed that thermal conductivity is greatly affected by the sonication of Nano-fluid which indirectly proves the dependence of thermal conductivity on particle size.

Factor affecting the nanofluid performance in heat transfer

A Nano fluid can of the size of 100 nm particles of non-metallic and metallic substance or it can also be a Nano fiber particles. When these particles are mixed in any fluid then they are called Nano fluids. Adding Nano fluids in any regular fluid like water will enhance the thermal conductivity of that fluid as they provide better flow of mixing. Masuda work on Al2O3 Nano fluid and mixed about 4.3 percent by volume of these Nano fluids in water. This increases the thermal conductivity of water mixed Nano fluid by 30 percent as compared to pure water. Similarly Eastman performs an experiment with Nano crystalline copper oxide and water. Nano crystalline copper oxide particle with 5 percent by volume were mixed in water which result an increase in thermal conductivity of water by almost 60 percent as compare to simple water.

An experimental work was done by Tzeng to investigate the thermal performance of Nano fluids on a real vehicle cooling system. Al2O3, antifoam and CuO Nano particles were used this this practical work. These Nano particles were added in the transmission oil of the vehicle engine. Four different engine speed (400, 800 1200 and 1600 revolution per minute) were analyzed in this test. Test result concluded that CuO perform the best among the Nano particles and it provides the best heat transfer from engine to fluid Test result concluded that CuO perform the best among the Nano particles and it provides the best heat transfer from engine to fluid Test result concluded that CuO perform the best among the Nano particles and it provides the best heat transfer from engine to fluid as it has the best effect in terms of heat transfer and heat transfer distribution for all four different speeds.

Vajjha performed numerical study using CFD and ansys fluent to check and analyze the thermal performance of the CuO and Al2O3 Nano fluid. A flat tube radiator was used for this numerical study and result show that at the Reynolds number of 2000 and 10 percent by volume of Al2O3 improve the heat transfer coefficient up to 94 % as compare to simple fluid. For CuO case a 6 percent by volume increase the heat transfer coefficient of heat exchanger to about 82 percent as compared to pure water.

Factor affecting the nanofluid performance

Coolant efficiency and effective tube geometry of a radiator are the two most important factors that affect the pressure drop and heat transfer taking place in the cooling system of a vehicle engine. Flat tubes and Nano-fluids are used in lieu of circular tubes and conventional fluids (e.g. EG, water) respectively to enhance the cooling system efficiency.  Factors that affect the performance of the cooling system are highlighted in Fig. 3.

 

Figure 1 factor effecting nano fluid working

Tube geometry used for nanofluids heat exchanger 

Flat tubes usually pronounced as flattening of tube profile is an important parameter. Rate of heat transfer is largely dependent on the flattening of tube profile. When the tubes are flattened, the upper and lower walls come more close to each other which escalates the boundary layer phenomenon and an increase in the heat transfer is attained. It can also be said that when the tubes are flattened the hydraulic diameter decreases which increases the heat transfer coefficient. Safikhani et al. [21] performed CFD analysis to analyze the heat transfer coefficient of five flat tubes of different cross-section. Al2O3 Nano-fluid was used in this analysis and flow was kept laminar. With the help of Multi-objective or vector optimization techniques, they noticed an improvement in HTC with the flattening of tubes. Abbassi and Safikhani [22] studied the thermal-hydraulic performance of Al2O3 Nano-fluid with relation to the tube flattening. In this case, they modelled two-phase mixture using FORTRAN programming language and the results were found similar to the previous study. The experimental study on tubes of different internal heights i.e. 8.6 mm, 10.6mm, 11.7mm and 13.4 mm was performed. It concluded that hydraulic diameter reduces with the tube flattening and hence an increase in HTC is noticed at the same flow rate. In the experimental studies performed by Razi et al. [23], he used four different flat tube geometries and one circular tube to study the pressure drop and heat transfer characteristics of flat tubes and found that when the tube is flattened pressure drop becomes high. To find a trade-off between the minimum value of pressure drop and maximum value heat transfer, they calculated a performance index. A tube with internal height of 7.5 mm gave the maximum value of factor as compared to the tube with the internal height of 6.3mm.

Nanofluids (Nanoparticle) concentration 

Weight concentration or volume fraction of Nano-particles in a Nano-fluid is a very important parameter that affects the performance of any thermal system or heat exchanger devices like radiator. When the concentration of Nano-particles is increased, the Brownian motion increases and hence it escalates the micro-convection. The phenomenon of micro-convection and Brownian motion delay the formation of thermal boundary layer. This delay causes the HTC at the entrance region to increase which helps in the rise of overall value of HTC. Weight concentration or volume fraction of nano-particles in the base fluid largely affects its the viscosity and thermal conductivity [24]. Increase in the concentration of Nano-particles increases the thermal conductivity of Nano-fluid but it has adverse effects on its viscosity. Heat transfer is inversely proportional to the viscosity and directly proportional to the thermal conductivity of the Nano-Fluid. Thermal conductivity is an important parameter for effective heat transfer at lower concentrations but at higher concentrations viscosity also becomes very significant. So the HTC increase with the increase in particles’ concentration up to a certain limit and beyond that it goes on decreasing due to the adverse effects of viscosity. Ali et al. [25] studied the thermal behavior of Al2O3 and water Nano-fluid and narrated an increase of 14.97% for 1 vol. %. He found that with increase in the vol. % to 1.5 % and 2 %, HTC started to decrease. Vajjha studied the Nano-fluids of CuO and Al2O3 to predict their thermal behavior by changing their concentration. He noticed that, at lower concentration HTC remained unchanged but its value increases when the vol. % was increased beyond 1 %. Ali et al. [25] used the Nano-fluid of ZnO and water at different volume concentrations i.e. 0.3%, 0.2%, 0.08% and 0.01%. He noticed an increase of 46% in the value of HTC at 0.2 vol. % and the value of HTC started to decrease beyond the vol. % of 0.3. Author used MWCNT/water Nano-fluid and narrated a decrease in the value of HTC with the increase in the concentration of Nano-particles beyond 0.16 wt. %. Ahmed et al. [26] also narrated the similar trend for TiO2 Nano-fluid. He noticed a rise in the value of HTC at 0.2vol. % and it started to decrease beyond the vol. % of 0.3.

Flow rate/Reynolds number of coolant (nanofluid) 

Flow rate or the Reynolds number of the Nano-fluid is also an important parameter that decides the amount of heat transfer taking place. At higher values of Reynolds number, the flow becomes turbulent which increases the molecular collisions and better mixing of the Nano-particles takes place. So high Reynolds number increases the value of HTC. It is evident from the literature that Reynolds number of the flow and concentration of Nano-particles affect the heat transfer. It was noticed an increase of 1.4 % in the rate of heat transfer with the increase in Reynolds number from 4000 to 7000. Abdolbaqi et al. [26], believed that increase of flow rate enhances the rate of heat transfer. They mentioned that increasing the Reynolds number increases the pressure drop and decreases the friction factor.  Nano-fluid temperature also plays an important role in deciding the thermal performance of radiator devices. When the temperature of the fluid is high, kinetic energy of the particles also becomes high and their collisions increases. A rise in heat exchange is noticed with the increase in number of collisions. Initial temperature of Nano-fluids also affects their viscosity and thermal conductivity. With the rise in temperature, viscosity decreases while thermal conductivity increases. A slight increase in the thermal performance of a radiator with the increase in the inlet temperature of Nano-fluid was observed. Selvam et al. [27] increased the inlet temperature of Nano-fluid up to 35°C and noticed an increase of 10% in the HTC. The HTC kept rising with the increase in inlet temperature and at 45°C, it showed an increase of 21 %. According to literature, the temperature of Nano-fluid has a very nominal effect on the thermal performance of a radiator.  The changed the temperature of Nano-fluid from 56°C to 64° and narrated an increase of only 6.7% in the value of HTC. In the same study, only 4% rise in the value of HTC was observed when the temperature was varied form 45 ◦C to 55 ◦C. A study related to reduction in the friction factor when the temperature of Nano-fluid was increased.  Because at high temperature, the viscosity of the fluid decreases and hence shear stress are also less. With the change in temperature from 35°C to 45°C, a significant decrease in the friction factor was noticed. It was also noticed that a decrease in the value of pressure with the increase in temperature from 35 ◦C to 45 ◦C.

Advantages of Using Nanofluids in Engine Cooling System

Lesser particle momentum, high mobility and larger surface area are the features that made nanoparticles a prospect in the field of suspended particles. 

In the terms of thermal conductivity, use of multi-walled carbon nanotubes (MWCNTs) as a suspended particle can increase the thermal conductivity by 20,000 times as compared to engine oil at room temperature. 

If the proper dispersion of nano-fluid is achieved, they are expected to provide the following advantages over a simple liquid coolant.

Higher heat conduction of nanofluid

Nano-particles have larger surface area which increases the rate of heat transfer. Particles whose size is less than 20nm have 20% of their atoms on their surface which are readily available for heat transfer purposes. 

Nano-particles due to their tiny size have high mobility and can bring about micro-convection which can increase the rate of heat transfer. The increased rate of heat transfer and micro-convection can also increase the rate of heat dissipation in fluid. 

This is already known that increase of thermal conductivity in Nano-fluid rises significantly with the rise in temperature which can be related to the facts discussed above.

Stability of nanofluid

Nano-particles have small size and hence have less weight due to which the susceptibility of sedimentation is also less. Sedimentation, which is a major issue in suspended particles can be curbed using Nano-fluids and hence it makes Nano-fluids relatively more stable.

Micro-channel cooling without clogging using Nanofluids

Nano-fluids are not only a better option for heat transfer purposes in general, but they are also very ideal in the application of micro-channels where the heat load is very high. 

A combination of Nano-fluids and micro-channels can provide large surface area and high thermal conductivity which cannot achieved with micro or meso-particles because they can clog in the micro-channels. 

Nano-particles that have very few number of atoms are very small in size as compared to the size of micro-channels.

Reduced chances of erosion using Nanofluids

Nano-particles have less momentum due to their smaller size and lesser weight and hence they are not able to transfer high amount of momentum to the channel walls. 

So, when the momentum is lower, the probability of erosion of pipelines, pumps and heat exchanger becomes less.

Reduction in pumping power using Nanofluids

To double the amount of heat transfer in the case of conventional fluid, pumping power should be increased by ten times. 

It can also be shown that the rate of heat transfer is doubled in the same equipment when the thermal conductivity is tripled [15]. But, in the case of Nano-fluids the increase in pumping power is very nominal unless the fluid viscosity becomes very high with the introduction of Nano-particles. 

Thus, large amount of pumping power can be saved if the volume fraction of Nano-particles is small. Nature, often, proves itself to be more thrilling than fascination. 

According to Henry Becquerel, uranium ore absorbed sunlight and radiated it back until the concept of radioactivity was discovered and it was revealed that uranium emits radiations without absorbing sunlight. 

In the similar fashion, Nano-fluids showed attributes that were beyond the expectations [16]. Four unique attributes of Nano-particles are given below.

a. Abnormal enhancement of thermal conductivity. Nano-particles show an abnormal increase in thermal conductivity. This abnormal increase in the thermal conductivity was the most significant feature and it was way beyond the expectations. None of theory was able to predict the increase that they had shown.

b. Nano-fluids have proved their self to be more stable as compared to the use of stabilizing agent .

c. Small concentration and Newtonian behavior. As discussed earlier, Nano-particles depicted a sharp increase in thermal conductivity even with the small concentrations of Nano-particles. This helped fluid to maintain its Newtonian behavior because the increase in viscosity was very less and increase in pressure drop was also very nominal.

d. Particles size dependence. In the case of micro-slurries, the thermal conductivity varied with size of particles. But in the case of Nano-particles, the thermal conductivity remained the function of concentration only. At large, increase in thermal conductivity was noticed with the decrease in particle size.

Use of Nanofluids in Car Engine Cooling System

INTRODUCTION To Nano Fluid  in Car Engine Cooling System

In the recent few decades, researchers have paid attention to enhance the performance of automobile cooling systems. Radiator keeps engine from overheating due to friction [1]. Different techniques are used to enhance the heat transfer performance of automobile radiators. 

These techniques include improving the coolant properties, increasing the coolant flow rate, manipulating geometry properties and increasing the surface area of radiator fins etc. Using micro-channels and fins is a way of increasing the heat transfer performance of the radiator. 

Micro-channels and fins extend the area and thus increase the heat transfer [2]. Due to the confined space available, different shapes of fins i.e. serrated fin, wavy fin and louvered fins are utilized. Louvered fins are the most commonly used type of fins [3]. 

Optimizing the geometric parameters and size of fins to increase the heat transfer is a challenging task [4]. Use of flat tubes increases the heat transfer due to their efficient geometry. 

Flat tubes cover lesser space and have high surface-to-cross-section flow area ratio which increases their heat transfer capacity [5]. Moreover, pressure drop is lesser in these type of heat exchangers due to their aerodynamic shape [6]. 

Enhancement of coolant properties is another approach to increase the rate of heat transfer in radiator. In the past, water has been used as a coolant which absorbed heat by spreading into the different parts of the engine. 

But, thermal properties of water and its non-resistive behavior to corrosion is a limitation in using water as a coolant [7]. A coolant with comparatively higher boiling point, lower freezing point and high thermal conductivity is required to overcome this limitation. 

Ethylene Glycol (EG) have wide range of working temperature and is used as an anti-freeze agent. Keeping in view the weather conditions, Ethylene Glycol (EG) can be mixed with water to enhance the properties of water as a coolant [8]. 

Due to the low thermal conductivity of Ethylene Glycol (EG), the mixture of water and Ethylene Glycol (EG) have weak thermal properties. 

This drawback can be overcome by using different additives in the coolant to increase the heat transfer performance. A fluid that contains nano-sized particles are called nanofluids and introduction of nanoparticles in coolant increases the rate of heat transfer which subsequently enhances the thermal performance of the system [9]. 

Nanofluids in lieu of conventional fluids were first used by Eastman and Choi [10]. Nano-fluids are developed by mixing metal oxide or metals in the base fluid [11]. 

Nanoparticles have attracted the attention of modern day researchers in numerous heat transfer applications such as automotive industry, dairy, heat exchangers, heat pipes, boiling, micro-power applications, manufacturing and electronic cooling.

THE RATIONALE BEHIND NANOFLUIDS in Car Engine Cooling

It is evident from a review that fluids which are used as coolants in different heat transfer applications have low thermal conductivity (only liquid metals show good thermal conductivity but these are not suitable to be used as a coolant due to their working temperature range). 

For example, thermal conductivity of copper is three times higher than the thermal conductivity of water which is same for almost all the lubricants, organic coolant and engine coolant. 

Due to this limitation of low thermal conductivity, the efforts to increase the thermal performance by increasing surface area and turbulence go in vain. 

Thus, this becomes very necessary to increase the thermal conductivity of coolants. Introduction of nanoparticles in the base fluid to increase the heat transfer is one century old concept. 

Maxwell [12] was the trailblazer who introduced a theoretical method to calculate the effective thermal conductivity of suspension. He was followed by different researchers such as Wasp [13] who carried out experimental and theoretical studies. 

These models performs very well in forecasting the thermal conductivity of suspended particles. However, these studies are limited in the terms of size of the suspended particle. Study of suspended particles have following disadvantages.

1- Suspended particles settle quickly and form a layer on the surface of heat exchanger which results in the reduction of heat transfer.

2- The settlement of the particles can be reduced by increasing the circulation rate, but this can cause erosion of the heat exchanging devices.

3- In the case of flow through narrow channels, the particles can obstruct the flow.

4- Suspended particles can increase the pressure drop and hence more power is required to pump the coolant.

5- Finally, increase in thermal conductivity in relationship with particle concentration is achieved (i.e. when the volume fraction of nano-particles is high, increase in thermal conductivity is also high and more will the problems as shown in 1-4 above). 

Thus the behavior of suspended particles became well known but it was considered to be a bad option for heat transfer purposes. But the arrival of nano-particles gave a new life to this option. 

Modern technology in material science made it possible to manufacture nano-sized particles which have different thermal, optical, mechanical and electrical properties. 

This has given researchers a new option to re-study the behavior suspended particles or nanofluids in heat transfer applications [14]. 

Through the pivotal efforts of nano-suspension in fluid were given the name of nanofluids in Argonne National Laboratory (ANL).  

First proposal in the field of nano-particles came from the pivotal effort of and it was named as nanofluid in Argonne National Laboratory (ANL). 

Looking at this name with an eye of a purist, the name may seem unacceptable, because motion in every fluid is at nano level, but the name became popular was accepted by the scientific community. 

One should note the fact that the name ‘Nano-fluid’ have been used by biologist for different particles like RNA, DNA, proteins and fluid in Nano-pores. 

Lesser particle momentum, high mobility and larger surface area are the features that made nanoparticles a prospect in the field of suspended particles. 

In the terms of thermal conductivity, use of multi-walled carbon nanotubes (MWCNTs) as a suspended particle can increase the thermal conductivity by 20,000 times as compared to engine oil at room temperature.