Comparison of effects of nanofluid utilization (Al2O3, SiO2, TiO2) with reference water in automotive radiators on exergetic properties of diesel engines

In this study, nanofluids formed by using ethylene glycol and three kinds of nanoparticles such as Al2O3, SiO2, and TiO2 were added to the four-stroke internal combustion engine radiator and compared with the conventional coolant (pure water). This comparison is based on the exergy performances which are the main theme of the second law of thermodynamics. The tests were carried out at a fixed engine speed of 1800 rpm using diesel fuel, and the outputs were obtained from the test setup experimentally. A total of six nanofluid tests were performed on two different dispersions (0.2% and 0.4%). As a result of this study, the best exergy efficiency was obtained by using TiO2 particles with a 35.67% value. Increasing the percentage of nanoparticles in the fluid from 0.2 to 0.4 positively affected efficiency. Pure water generally lagged behind nanofluid performance in experimental parameters. Compared to conventional coolant (pure water), the lowest exhaust temperature value was measured by using an Al2O3/Ethylene Glycol mixture with a difference of 59 K. Also, by using Al2O3 nanoparticles as a coolant, 8.858 kW of exergy exhaust value was obtained. This is the best emission value measured in the experimental study. While calculating values close to each other in the use of other nanoparticles, the worst exergy exhaust results were obtained by using the conventional refrigerant. Consequently, in this paper, exergetic outputs such as exergetic efficiency, exergy destruction, exergy heat, exergy work, exergy total exhaust, and entropy production rate were calculated for pure water and each nanofluid.


Introduction
The cooling system has vital importance in regulating the engine temperature. The resulting temperature in the combustion chamber of the engine cylinder reaches 1500-2000 °C. These temperature values are above the melting point of the materials from which the cylinder block and head of the engine are produced. Most of the engine blocks are produced from cast iron and aluminum alloys that have lower melting points compared to burning temperature. Therefore, if the high temperatures generated by combustion gases cannot be removed, serious deformations will occur on the cylinders.
The radiator is the main part of an engine cooling system. It consists of three basic parts; inlet tank, outlet tank, and core. Two types of working fluids are generally in the engine cooling system. They are air and coolant. The main purpose of the air is to remove the heat from the hot coolant and ensure circulation that it is cold in the engine block [1]. Trivedi and Vasava modeled the mass flow rate of the air passing through the automotive radiators on the CFD [2]. The current automotive radiators work with circulating fluid which is regulated by water and a mixture of water and coolant materials like ethylene glycol (EG). Under normal weather conditions, the mixing ratio of water and EG is generally applied as 50:50 in the radiators. However, in colder weather conditions, when the percentage of EG in the mixture is increased, the circulating fluid becomes more resistant to freezing. The conventional heat transfer fluids such as water and ethylene glycol are often limited due to their low thermal conductivities [3]. The enhancement of heat transfer in an automotive radiator may create a saving in energy, reduce process time, raise the thermal rating, and lengthen the working life of the equipment. The current system is no longer enough and so, a new heat transfer technology requirement rises day by day.
Yu et al. explained applications area of nanofluids such as transportation, electronics cooling, defense, space, nuclear systems cooling, and biomedicine, etc. in their study [4]. Nanoparticles have also the potential to be used in the engine cooling system since they increase the rate of heat transfer of the fluids. Hussein et al. examined the variation of Nusselt numbers at different Reynold numbers using TiO 2 -Water and SiO 2 -Water nanofluids as coolants in automotive radiators. They obtained maximum Nusselt number enhancements of up to 11% for TiO 2 and 22.5% for SiO 2 in water [5]. Vajjha et al. used Al 2 O 3 /Ethylene glycol and CuO/Ethylene glycol nanofluids as a coolant to flat tube in the automotive radiator and studied laminar heat transfer numerically [6]. In this experimental study, the effects of nanoparticle utilization in diesel engine radiator will be investigated. Nanoparticles, water, and ethylene glycol mixture will be used as a coolant fluid to enhance heat transfer. Exergy analysis will be done on the system. The control volume of the system is composed of an engine and heat exchanger (engine radiator) together. In terms of understanding energy efficiency, the first law of thermodynamics, that is, the quality of energy is as important as the quantity. Considering this quality, not enough information has been given in the studies in the literature so far. Therefore, it is very important to make the second law analysis of thermodynamics in terms of understanding the nature of heat transfer in automotive radiators. This study was made to fill this gap in the literature. Exergy is briefly defined as the maximum useful work of a system as it comes to equilibrium with a reference environment [7]. The second law of thermodynamics forms the basis of the exergy analysis which has recently become popular [8]. Çalışkan et al. studied the effect of exergetic output of different dead state temperatures using HOME fuel on the John Deere 45 T diesel engine [9]. Tosun has made exergy analyses of diesel No.2, canola biodiesel, and peanut biodiesel on ICE [10]. Çalışkan used diesel No.2, SME, and HOME fuels on the John Deere 45 T diesel engine to calculate exergy analyses in his study [7]. Ma et al. conducted energy and exergy studies using different inlet airflow rates on the natural convection energy recovery loop analysis [11]. Heydari et al. conducted exergy analyses to optimize the hydrothermal performance of a specific heat exchanger through Taguchi empirical method [12]. Yan et al. worked on the experimental studies for energy economy management on a new generation air conditioning system [13]. Ahmadi et al. examined thermodynamically the effects of regeneration of the combined heat and power (CHP) system existing in a petrochemical plant and calculated the exergy efficiency [14]. In a similar study, Ahmadi et al. conducted energy, exergy, and environment analysis on the CHP system, known as 3E [15]. Dabiri et al. calculated a parametric analysis of the thermal characteristics in a special receiver for a special 3-collector concentrator [16]. Jozaalizadeh et al. used MILD (moderate or intense low oxygen dilution) technology in their study to increase thermal efficiency [17]. Toghraie et al. examined the effect of the solar chimney on thermal properties and some other results by changing the geometric parameters [18].
Nanoparticle effects on heat transfer can easily be observed when the literature is reviewed. Yu et al. used various types of nanofluids in their research and they reported that heat transfer enhancement can be achieved about 15-40% [4]. Particle volume fraction, particle material, particle size, particle shape, base fluid (water + ethylene glycol), temperature, and preparation method could be the effect on the heat transfer [19]. The thermal conductivity of water, ethylene glycol, and nanofluid mixture increase almost linearly with temperature, and the heat transfer rate is increased with an increase in volume concentration of nanoparticles (0.1%, 0.5%, 1%) reported by Bhanu et al. [20]. According to Eastman et al. research, the thermal conductivity of ethylene glycol nanofluids containing 0.3% volume fraction of copper particles can be enhanced up to 40% compared to that of ethylene glycol base fluid [21]. Rostami et al. tried to obtain the best neuron number by using the SiO 2 /water-ethylene glycol mixture in different volume fractions and temperatures with artificial neural network (ANN) and fitting methods, and it was investigated which method would be a more accurate estimation of nanofluidic thermal conductivity [22]. Rostami et al. compared ANN, the experimental and fitting method to estimate the thermal conductivity of multi-walled carbon nanotubes (MWCNTs)-CuO / water mixture. As a result of the study, it was found that the margin of error is less about the estimated data of the ANN method. In addition, it has been stated that the increase in the volume fraction of the nanoparticle at constant temperature causes an increase in its thermal conductivity [23]. Another MWCNT experiment was conducted by Afshari et al. In this study, MWCNT-Alumina/water (80%)ethylene glycol (20%) mixture was tested in different solid volume fractions and temperatures, and the effects of hybrid nanofluid on viscosity were investigated [24]. Aghahadi et al. obtained hybrid nanofluid by mixing Tungsten oxide (WO 3 )-MWCNTs with engine oil and observed that the increase in the volume fraction of this nanofluid increases viscosity. In addition, it has been observed that the increase in temperature has an increasing effect on the nanofluid viscosity [25]. In the experimental study, Ruhani et al. developed a new model to investigate the rheological behavior of Silica-ethylene glycol/water (30:70 vol %) hybrid Newtonian nanofluid. As a result of the study, a linear relationship was found between shear stress and shear rate. Also, while the temperature increased from minimum to maximum in the maximum volume fraction, the viscosity value lost 89% [26]. In a similar study, Ruhani et al. used ZnO-Ag (50% -50%) / water hybrid nanofluid and concluded that viscosity decreases when temperature increases and increases when volume fractions increase [27]. Arasteh et al. observed the effects of different volume fractions on PEC number, average Nusselt number, and friction coefficients using water-graphene nanoplate/ platinum hybrid nanofluid. As a result of the observation, all values increased due to the increase in the volume fraction [28]. In another nanofluid study, researchers tried liquid paraffin containing oleic acid and Al 2 O 3 nanofluid at different temperatures and concentrations. Studies have shown that nanofluid act like a shear-thinning fluid. Besides, boosting nanoparticle concentration caused an increase in thermal conductivity. Also, while boosting the temperature decreased the viscosity, thermal conductivity increased [29]. Samani et al. worked on the removal of hexavalent chromium from water using chemically synthesized Polyanilin/Sawdust/Poly Ethylene Glycol (PANI/SD/ PEG) composite as absorbent. In this study, PANI/SD/PEG performed well in removing hexavalent chromium from aqueous media. Also, the presence of PEG in the mixture provided a homogeneous mixture and more absorption [30]. Esfe et al. aimed to reduce the cost and increase the heat transfer coefficient by optimizing the MgO-water mixture. Accordingly, the researchers used different volume fractions, diameters of nanoparticles, and Reynolds numbers. As a result of the study, it was stated that the cost was reduced by 38% [31]. Golestaneh et al. examined the natural gas flow in cyclotubes by injecting black powder particles of different diameters and velocities into cyclotubes. As a result, boosting velocity increased the pressure drop [32].
In this study, different nanoparticles (Al 2 O 3 , SiO 2 , TiO 2 ) and ethylene glycol were mixed in two different dispersions (0.2 and 0.4%) to form nanofluids. These coolants have been experimentally tested on the diesel-fuelled internal combustion engine and various empirical results have been obtained. The results were evaluated with the mathematical calculations given under the methodology title, their effects on the exergy parameters were examined and all coolants were compared with each other. As far as our research, although nanoparticles have been tested separately and examined different thermal properties in the literature, it has been observed that the number of sources from which the exergy properties of nanofluids are made is limited. This point is the most interesting part of the article. Also, experimental studies have been carried out with these fluids in automotive radiators and compared with conventional coolants. There are no similar examples in this direction in the literature.
This study consist of two main stages. In the first stage, a comprehensive literature review was made, then the nanoparticle properties were specified and the mixing method of nanofluids was explained. The second stage mainly includes the analysis method and the conclusion parts. From this stage, a mathematical method was defined and experimental tests were carried out to make the thermodynamic analysis. As a result of these tests, the thermodynamic effects of nanoparticles on the diesel engine radiator were observed and future studies were mentioned in the conclusion part.

Exergy analysis
The basic exergy formula is shown below.
Ėx heat (kW) means that the exergy transfer rate is related to heat loss from control volume to the environment, Ėx work is called exergy coming from engine work; Ėx dest (kW) represents exergy destruction due to irreversibilities. Table 1 shows that some values used in the experimental test to make an exergy analysis.
In Eq. (2), ṁ fuel (kg/s) expresses the mass flow rate of fuel consumption (measured values are shown in Table 2) while where H u (kJ/kg) means lower heating value (LHV) and φ means that chemical exergy factor. φ can be defined as follows (4) [33]: where h, c, o, ⍺ indicate the mass fraction of hydrogen, carbon, oxygen, and sulfur contents in the fuel.
where ṁ air (kg/s) is the mass flow rate of air and Ɛ air (kJ/ kg) is the specific exergy of the air which can be explained as in Eq. 6.
h and h 0 are enthalpies of the working and reference temperatures, respectively. s and s 0 is the entropy of the working and reference temperatures, respectively in Eq. 6. Exhaust measurements obtained from the experiments are shown in Table 3.
where ṁ i (kg/s) is mass flow rates of the combustion gases, Ɛ tm (kJ/kg) and Ɛ chem (kJ/kg) are the thermomechanical and chemical exergies of these gases. Thermomechanical and chemical exergies are described as follows: (Eqs. 8-9) [34]. Based on exhaust temperatures which are shown in Table 4, enthalpy and entropy values of the generated emission gases are found [35].
where − R is the general gas constant (8,314 J/mol.K), y i is the mole fraction of the component and y e is the mole fraction of the component given under the definition of the environment in Table 5.
Ėx heat is the exergy of the heat transfer rate from the automotive radiator. It has been formulated as follows: (Eq. 10) [10].
where measured cooling water temperatures from the radiator are shown in Table 6. Ȯ (kW) is the heat rate from the automotive radiator to the environment through the coolants. Ȯ is explained in Eq. 11. where Ė fuel (kJ/s) is the energy input rate to the control volume (Eq. 13). Ėx work (kW) is the net exergy work rate and as formulated in Eq. 14.
In Eq. (12), the mass flow rate balance is described. In other words, it could be said that input mass flow rates are equal to output mass flow rates.
where N and T (Nm) are obtained revolution per minute and torque of the engine (Table 7), respectively.
Exergetic efficiency can be found by the ratio of the net work exergy to input exergy. (Eq. 15) Ṡ (kW/K) is the entropy production rate. It can be found as follows: (Eq. 16)

System description
The experimental setup is shown in Fig. 1. Experimental setup composed of a radiator, radiator fan, test engine, dynamometer, emission analyzer, and computer-controlled engine tester. K-type thermocouples were used to measure radiator inlet and outlet temperatures The experimental setup is also schematically in Fig. 2. Pressure,   Tables 8 and 9.
All experiments in this study were carried out on a fourstroke, four-cylinder Mitsubishi Canter diesel engine. Test engine specifications are presented in Table 10.

Nanofluids properties and preparation
Al 2 O 3 , SiO 2 , TiO 2 nanoparticles were used as an additive to water and ethylene glycol mixture. To provide a more homogeneous mixture of nanoparticles and the base fluid, SONICS vibra cell magnetic stirrer was used at 20 min., pulse 10 03 and Amp1 60%. All nanoparticles were purchased from Sigma-Aldrich company are shown in Table 11. Nanofluids were tested shortly after they were prepared homogeneously with a magnetic stirrer and were continuously circulated in the cooling system throughout the experiment. For this reason, no precipitation has occurred in nanofluids and stability has been neglected.
For all nanoparticles, the first mixture consists of 20 gr nanoparticles, 1 kg EG, and 9 kg water (10% EG-%90 water). As a result, the mass percentage of nanoparticles in the first mixture was obtained as 0.2%. The second mixture consists of 40 gr nanoparticles, 1 kg EG and 9 kg water. The mass percentage of nanoparticles in the second mixture was obtained as 0.4%. Thus, the percentage of nanoparticles in the mixture was increased and its effects on the exergetic properties of the system were examined. The coolant with based nanoparticle in the cooling system is circulated with the help of the pump and mixed. Experiments were performed three times for each nanoparticle and their averaged results were evaluated.

Results and Discussion
In this paper, different types of nanofluids were added to the automotive radiator, and comparisons were made with the conventional coolant used in the present according to the exergy parameters. By mixing the nanofluids with    ethylene glycol already used in radiators, the accuracy of the experiments has been increased. These aspects of the work reveal their originality. Besides, the results of the study are more suitable for practice, since the tests are carried out completely experimentally in the laboratory environment. In this section, each exergy parameter has been examined separately, its graphics are drawn and interpreted.

Effect of different types of coolants on exergetic efficiency
The exergy method quantifies the locations, types, and sizes of wastes and losses and provides meaningful efficiency, which is always a measure of the approach to the ideal. For this reason, exergy analysis accurately represents the margin available to design more efficient energy systems by reducing inefficiencies [38]. As shown in Fig. 3, the best result with 35.67% was obtained when the TiO 2 nanoparticle was used with 0.4% dispersion. This result shows that TiO 2 is 62.14% more efficient than the pure water currently used. In addition, SiO 2 and Al 2 O 3 nanoparticles obtained similar results at 0.4% dispersion (28.99% and 26.32%, respectively). Generally, the exergy efficiency tended to increase as the nanoparticle concentration ratio increased for all nanofluids. The thermal performance of the nanofluid consisting of titanium dioxide and water on the flat plate solar collector was investigated. The researchers observed that the TiO 2 /water mixture used at 0.02% volume concentration increased the efficiency by 34% [39].

Effect of different types of coolants on exergy destruction
Exergy loss caused by irreversibility in the system is called exergy destruction. Therefore, systems with high exergy destruction values are expected to have a low exergetic efficiency. While the TiO 2 nanoparticle with the best exergy efficiency showed the lowest result with 18.516 kW for this parameter, the pure water coolant with the worst exergy efficiency showed the highest exergy destruction result with 23.584 kW as seen in Fig. 4

Effect of different types of coolants on exergy heat
As indicated by Eqs. 10 and 11, the heat of exergy value varies depending on many factors. The heat rate (Ȯ) value will change depending on the work, exhaust exergy, and energy input rate (Ė fuel ). When the exergy heat results are compared, the best result was obtained with the use of 0.4% dispersed TiO 2 based nanofluid with a value of CuO/water, in heat exchanger and obtained the worst heat transfer coefficient from CuO/water [41]. In these results, unlike the other results, 0.4% Al 2 O 3 nanofluid gave better results than 0.4% SiO 2 nanofluid. The effects of Al 2 O 3 -ZnO/ water nanofluids on thermal performances were observed by using 1: 1 (50% Al 2 O 3 and 50% ZnO), 1: 2 (33.34% Al 2 O 3 and 66.66% ZnO), and 2: 1 (66.64% Al 2 O 3 and 33.34% ZnO) mixture ratios on the flat plate collectors. The average exergy efficiency enhancements for the Al 2 O 3 -ZnO/ water nanofluid at 1:1, 1:2, and 2:1 mixture ratios were 6%, 7.5%, and 9%, respectively. These results carried out that the increase in Al 2 O 3 in the mixture positively affected the exergy efficiency [40]. When 0.4% Al 2 O 3 was used, 1.547 kW value was obtained, and 1.586 kW was obtained when 0.4% SiO 2 was used. Boosting nanoparticle concentration decreases the exergy heat values. Heat transfer through heat exchanger was examined using Al 2 O 3 nanofluid at different concentrations (0.3-2%). Researchers obtained the best heat transfer characteristic at the highest volume concentration [42].

Effect of different types of coolants on exergy work
As shown in Eq. (15), the increase in exergetic efficiency is due to the increase in exergy work. For the increase of the exergy work, the motor gets more torque, which makes the system more efficient according to exergy. As can be seen in Fig. 6, the Ėx work value increased when the nanoparticle was used, unlike the conventional coolants. The highest increase was obtained from the use of TiO 2 nanoparticles with 16.014 kW. Experimental results show that adding nanoparticles into the coolant increases the exergy work value. The highest increase was obtained when TiO 2 nanoparticle-based nanofluid was added to the radiator. The highest increase value was obtained when TiO 2 nanoparticle-based nanofluid was added (16.014 kW). This result is 58.88% more than pure water. The use of SiO 2 at 0.4% dispersion achieved a 15.62% better result than Al 2 O 3 use at 13.942 kW. Increasing nanoparticle concentrations from 0.2 to 0.4 in the nanofluid caused an increase of approximately 1.5 kW. Among TiO 2 /water, Al 2 O 3 /water, and CuO/water nanofluids, the worst pumping power value were obtained with the use of CuO / water. The pumping power value increased with the volume concentration value [41].

Effect of different types of coolants on exergy total exhaust
Total exergy produced by components such as O 2 , CO 2 , CO, NO, NO 2 , N 2, and H 2 O, which is caused by combustion reaction, is called total exhaust exergy. Looking at Fig. 7

Effect of different types of coolants on entropy production rate
Entropy production, which is caused by irreversibilities in a process, is proportional to the exergy consumption of that process. Exergy and entropy analyses are known as interrelated terms because they are based on the second law of fundamental thermodynamics [44]. As shown in Eq. (16), the entropy production rate (Ṡ) varies depending on exergy destruction at constant T 0 (298 K). The low entropy production rate for a system means that the system has fewer irregularities, that is, the system is more efficient. Even when looking at the results of other parameters, the entropy production rate can be sorted. When pure water is used as a refrigerant, an entropy production rate of 0.0791 kW/K is obtained, as seen in Fig. 8. This value is the worst result compared to others. TiO 2 nanoparticles showed the best performance in this parameter (0.0621 kW/K). Better results were obtained in the use of SiO 2 with a slight difference compared to the use of Al 2 O 3 .
In this part, it has been observed that the increase in dispersion has a positive effect on the system by decreasing the value of the entropy production rate. Kumar et al. examined the exergy and energy performances by using different shapes of Al 2 O 3 nanoparticles on a wavy fin radiator. In the study, it was stated that based fluid water has a high irreversibility value and that nanoparticles added with different shapes increase entropy generation efficiency. However, it has been reported that hybrid nanofluids have less irreversibility compared to base fluids [45].

Distributions of exergy for each test
According to the equation of exergy balance, the exergy coming from the system should be equal to the exergy entering the system. Figures 9, 10

Conclusion
After the experimental results were evaluated mathematically, the coolant with the best exergetic efficiency was TiO 2 based nanofluid. When using the 0.2% and 0.4% concentrations of TiO 2 nanoparticles, efficiency parameters of 31.76% and 35.67% were obtained, respectively. As can be seen from these results, boosting nanoparticle concentration in the nanofluid has a positive effect on the exergetic efficiency. The value of exergy destruction refers to the irregularities in the system. According to this definition, the nanofluid with the lowest exergy destruction value is expected to have a high exergetic efficiency value. While the highest exergy destruction value was obtained with pure water (23.584 kW), 0.4% TiO 2 /EG nanofluid (18.516 kW) with the lowest exergy destruction value was also specified as the most efficient nanofluid. Exergy work value is generally higher inefficient systems where the motor produces more torque. Calculations were made according to the measured torque values in the experiments and the best exergy work value with 16.014 kW was obtained when 0.4% TiO 2 /EG nanofluid was used. While SiO 2 (13.942 kW) and Al 2 O 3 (12.058 kW) nanoparticles were carried out close values, the worst result was obtained using conventional coolant (10.079 kW). Generally, the most desirable emission values obtained with Al 2 O 3 / Ethylene glycol nanofluids use as coolants according to other nanofluids. And exergy total exhaust (8.858 kW) values were obtained used 0.4% Al 2 O 3 based nanofluids. The emission values can be further improved by increasing the weight percentage of the nanoparticle in the coolants. Exergy heat value varies depending on many different parameters (such as Tcw in,out , T exh , Ė fuel , Ėx work , ṁ exh ). From the results, the highest Ėx heat value was obtained from pure water with 1.715 kW, however, the results for other nanofluids are close to each other.
Generally desired results were achieved in the system, but the cost of nanoparticles is the biggest obstacle to increasing the concentration of nanoparticles in the nanofluid. In addition, a circulation pump can be installed in the cooling system of the engine to prevent the precipitation of these nanoparticles in the mixture and to obtain a more homogeneous mixture. As a result, better thermal results can be obtained. Different cogeneration systems can be tried to reduce the exergy destruction value of the system.