The Heat Transfer Performance of MWCNT, CuO, and Al2O3 Nanofluids in an Automotive Engine Radiator

The effect of enhancing heat transfer using three nanofluids, Multi-Walled Carbon Nanotubes MWCNT, Copper oxide CuO, and aluminum oxide Al2O3, have been experimentally studied on the automotive radiator with a concentration of 1% vol and different flow rates (4-8) l/min, air velocity of 3 m/s and inlet temperatures range (60-80) °C. The results showed that the use of nanofluids improved the thermal performance compared to the base fluid. Using the (MWCNT-Water) achieved 41.7% of Nusselt number where as, copper oxide (CuO) and aluminum oxide (Al2O3) have improved the Nussult number by 31.7 % and 12.3 % respectively. The Nusselt number showed an increase with the increase in the flow rate and the inlet temperature.


Introduction
The Environmental Protection Agency EPA imposes strict standards on automobile manufacturers to protect the environment from the influence of emissions of combustion engine exhausts. One of the essential things in reducing pollution and increasing engine performance is to improve the automotive cooling system. Nanofluids are new technology which consists of tiny particles (less than 100 nm) suspended in conventional cooling fluids (water and/or ethylene glycol) [1]. Many experiments on nanofluids performance as a coolant in automotive radiators in order to improve the performance and reduce the radiator's size have been carried out in the last decades. Chaurasia et al. [2] conducted an experimental study on a mixture of nanofluid (alumina with distilled water) at a concentration of 0.1%, 0.15%, and 0.2% with different flow rates and different temperatures. They found that a concentration of 0.2% of nanofluid has increased the heat transfer rate by 44.29%. Topuz et al. [3] carried out an experimental investigation on nanofluid containing aluminum oxide Al2O3 nano-particles mixed with water and ethylene glycol (50-50%), 0.5% concentration with different flow rates and fixed temperature 95 °C.
Results showed an increase in cooling performance by 15% compared to the base fluid. Ravisankar et al. [4] applied an experimental and numerical study to reveal the performance of nanofluid (CuO with water) on a tractor radiator with a concentration of (0.025, 0.05%) and turbulent flow (8000 ≤ Re ≤ 25000). They found that the overall heat transfer coefficient has increased by 4% at a concentration of 0.05%. Ali et al. [5] studied experimentally the improvement in the heat transfer of automobile radiators using (MgO + water) nanofluid at different flow rates (8-16) l/min and different volume fraction. The study showed a 31% increase in the heat transfer rate at a concentration of 0.12%. M'hamed et al. [6] applied a nano-coolant fluid mix of MWCNT with water and EG at a 50-50% ratio on the Proton Kelisa 1000cc automobile engine. A volumetric concentration of 0.1 to 0.5% and a different flow rate with the laminar flow were suggested. Results showed an increase in the average heat transfer coefficient by 196.3% at a concentration of 0.5%. In the current study, nanofluids and their effect on automotive coolants are studied. The main aim is to enhance the average Nusselt number of the nanofluid as a coolant at the laminar flow conditions. Also, to overcome emissions of harmful gases and pumping power, to improve fuel consumption, and to reduce automotive radiators' size.

Preparation of Nanofluid
The nanoparticles used in the current study are multi-walled carbon nanotubes MWCNT, Copper Oxide CuO and Aluminum Oxide Al 2 O 3 , the characteristics of nano-particles are shown in Table 1. The nanoparticles were suspended at a volume fraction of 1% with pure water (pH 6.6). An ultrasonic for 3 hours with a frequency of 37 kHz and 100 Watts was used to produce the nanofluid. The particles' stability was monitored for 15 days and no sedimentation of nanoparticles was found during this period when a surfactant (gum Arabic GA) was added to increase the nanofluid stability [7]. The prepared samples are shown in

2 Experimental Work
Fig . 2 shows the rig test carried out to study the three nanofluids' thermal performance. It contains a coolant tank with 30 liters with a heater capacity of 3000 watts. The liquid's temperature is controlled by a thermostat placed on the heater with a range (40 -80) °C. The centrifugal pump speed and the amount of flow are controlled by a frequency converter (AC driver), flow meter instrument, a pressure gauge to measure the inlet and outlet coolant pressure of radiators, insulated pipelines with diameter 0.5", two thermocouples (k-type) to measure the inlet and outlet temperature of the fluid, six thermocouples (k-type) attach to the surface of the tube are distributed as shown in Fig 3 to record the temperature of the tube surface. Automotive engine radiators with 60 vertical finned flat tubes, their specifications and dimensions are listed in Table 2. An axial fan with airflow speed 3 m/s was fixed to the radiator to simulate the radiator cooling engine. The study was conducted at different temperatures range 60, 70, 80 °C for three types of nanofluids at a concentration of 1%. The ambient temperature was 25 °C, and the flow rate of (4-8) liters per minute.

Nanofluid Physical Properties
The following equation was used to evaluate the volumetric concentration of the nanoparticles and the amount of liquid. [8]. Nanofluid concentration is inlet temperature range of the coolants is 60-80 °C The following equations evaluate the effective thermal properties such as density, specific heat, viscosity, and thermal conductivity at various temperatures. Where (r) Density, kg/m 3 , (Cp) Specific heat J/kg., (bf) base fluid, (nf) nanofluid, (p) particles.
By (Maxwell) model of the effective thermal conductivity for (Al 2 O 3 , CuO) nanofluids have approximate.
The (Hamilton and crosser) model of the effective thermal conductivity for (MWCNT ) nanofluids have approximate. [9] where the (k) Thermal Conductivity, W/m. K.

Data Extraction
According to Newton's cooling law Eq (7), the heat transfer rate can be measured by. [10] = ℎ ( � − � ) =̇ Where ( � ) is hydraulic diameter, (k) the fluid thermal conductivity. ( � ) the flat tube surface area, (T b ) is the bulk temperature which is supposed to be the average temperature of the inlet, and (T w ) is the wall temperature.

Base Fluid (water)
Before starting experimental work on nanofluids in the radiator, it is necessary to conduct experiments on the same base fluid (pure water) and verify that the receiver operates under the same operating conditions of temperature and flow rate to compare results. It's also necessary to compare the Nusselt number measured from the experimental data with the predictions and other correlations for accuracy and reliability. There are three correlations; the first is suggested by (Dittus-Boelter) Eq.13 [11], the second developed by the researcher (Dehghandokht et al.) Eq.14 [12] and the last one was presented by (Sieder -Tate) Eq.15 [13].  shows the experimental result for the base fluid (distilled water), which showed a good agreement with the (Sider-Tate) correlation at the fluid inlet temperature of 80 ° C. However, increasing the Reynolds number has led to an increase in the Nusselt number, where the error rate according to equation (15) was 6.7 %, while the error rate for the other correlation was 35% for Eq.13 and 39.8% for Eq.15.    [10] found that Nusselt number increased by 12% when using a nanofluid at 80 ° C inlet temperature by increasing the flow rate from 7.63 to 8.82 l/min, this is the result of increasing the convection heat transfer rate, this also occur when increasing the nanofluid inlet temperature as a result of the decrease of specific heat of nanofluid which in turn lead to increase the rate of heat transfer [14]. The main cause of enhancement of heat transfer rate can be attributed to the improve in the nanofluid's thermal properties (especially the thermal conductivity and specific heat) compared to the base fluid heat as showen in table 3. As well as, many research studies attributed this thermal improvement to Brownian motion, which causes random movement of particles in the fluid. Heris et al. [15] found that the thermal improvement rate was increased by 40% when using water-Al 2 O 3 with a turbulent flow rather than laminar flow.

Conclusions
In this research, the effect of using MWCNT, CuO, and Al 2 O 3 nanofluids has been investigated in automotive radiators. A significant improvement in Nusselt number compared to the base fluid was achieved when using the mentioned nanofluids. The highest improvement was 41.8% for MWCNT mixed with distilled water at a concentration of 1%. The experimental results showed that the Nusselt number for base fluid and nanofluid has increased with the increase in the flow rate due to the increases in fluid turbulence inside the tube, resulting in more heat to transfer by convection. The increase in the inlet temperature has also resulted in the Nusselt number increase due to the change in the thermal properties. This is attributed to the increases in thermal conductivity and the the decrease in viscosity and specific heat of the fluids. Nanofluid can be a promising for coolants' radiators' future; this provides profitable methods for engineers to promote effective and compacted radiators to reduce the size and weight, thus; reducing the fuel consumption.