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.  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  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. , 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 . 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.  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.  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.  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. , 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.  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.