- Email: [email protected]
SAE50 nanolubricant
Journal Pre-proof Viscosity, cloud point, freezing point and flash point of zinc oxide/ SAE50 nanolubricant
Jilin Ma, Amin Shahsavar, Abdullah A.A.A. Al-Rashed, Arash Karimipour, Hooman Yarmand, Sara Rostami PII:
S0167-7322(19)35245-6
DOI:
https://doi.org/10.1016/j.molliq.2019.112045
Reference:
MOLLIQ 112045
To appear in:
Journal of Molecular Liquids
Received date:
19 September 2019
Revised date:
27 October 2019
Accepted date:
31 October 2019
Please cite this article as: J. Ma, A. Shahsavar, A.A.A.A. Al-Rashed, et al., Viscosity, cloud point, freezing point and flash point of zinc oxide/SAE50 nanolubricant, Journal of Molecular Liquids(2018), https://doi.org/10.1016/j.molliq.2019.112045
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© 2018 Published by Elsevier.
Journal Pre-proof
Viscosity, Cloud Point, Freezing Point and Flash Point of Zinc oxide/SAE50 Nanolubricant Jilin Ma1, Amin Shahsavar2,*, Abdullah A.A.A. Al-Rashed3, Arash Karimipour4, Hooman Yarmand5, Sara Rostami6,7,* 1
2
3
China Classification Society, Beijing 100007, China
Department of Mechanical Engineering, Kermanshah University of Technology, Kermanshah, Iran
Department of Automotive and Marine Engineering Technology, College of Technological Studies, The Public
5
Department of Mechanical Engineering, Najafabad Branch, Islamic Azad University, Najafabad, Iran
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4
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Authority for Applied Education and Training, Kuwait
Centre for Energy Sciences, Department of Mechanical Engineering, University of Malaya, 50603 Kuala Lumpur,
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Malaysia
Laboratory of Magnetism and Magnetic Materials, Advanced Institute of Materials Science, Ton Duc Thang
Faculty of Applied Sciences, Ton Duc Thang University, Ho Chi Minh City, Vietnam
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7
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University, Ho Chi Minh City, Vietnam
*Corresponding authors
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Emails: [email protected] (A. Shahsavar)
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[email protected] (H. Yarmand)
Abstract
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[email protected] (S. Rostami)
This study aimed at improving the performance of engine oil (SAE50) by adding zinc oxide nanoparticles. First, the microstructure of nanoparticles was studied by scanning electron microscopy (SEM) and X-ray diffraction (XRD). After confirming the microstructure of nanoparticles, a two-step method was used to prepare the engine oil/zinc oxide nanolubricant. The nanolubricant remained stable without any sedimentation for more than 5 weeks in static mode. Different volume fractions of the nanolubricant were prepared and the rheological behavior and nanofluid viscosity were studied in the range of 25 to 65 °C and, additionally, the cloud, freezing 1
Journal Pre-proof and flash points were measured. According to the results of dynamic light scattering (DLS) test, the nanoparticles in the base nanofluid had an average diameter of 55 nm. The viscosity increased with increasing concentration up to 25.3% relative to the base fluid. The viscosity of the nanolubricant decreased with increasing temperature. An accurate relation was proposed based on different temperatures and nanofluid volume fractions to calculate the nanofluid viscosity. The cloud, freezing and flash points were improved respectively by 22.2, 19.4 and 7.2% at the highest
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concentration, indicating the reasonable performance of nanoparticles.
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Keywords: Experimental study; Nanolubricant; Rheological behavior; Cloud point; Freezing point;
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Flash point.
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1. Introduction
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Optimizing and improving the performance of energy and heat transfer systems have recently received considerable attention due to the population growth and ever-increasing need for energy
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throughout the world. Therefore, the lubrication and cooling of lubricants in advanced systems
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should be highly efficient. Improved thermal properties of nanoparticles, in particular high thermal
systems [1-10].
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conductivity of the nanostructure, may lead to improved lubrication and heat transfer in such
As a recently emerging and advancing technology, nanofluids have received considerable attention in energy management and heat transfer systems given their unique thermophysical properties. Nanolubricants are a new class of nanofluids. Stability, rheological behavior, and oil performance at very high and very low temperatures are the most important issues in obtaining a practical nanolubricant with improved performance in lubrication and thermal systems [11, 12]. In addition to lubrication and heat transfer, mass transfer and nanofluid flow are of great importance in such systems as the pumping power and electricity costs become a matter of concern. Thus, it is necessary to obtain a nanofluid with proper viscosity and high thermal performance as an alternative 2
Journal Pre-proof to the base fluid to achieve a higher efficiency. The changes in the base fluid by adding nanoparticles can significantly affect the improvements in heat transfer, design and investment on thermal systems [13-20]. Today, various lubricants with different properties are produced and used in various mechanical systems such as internal combustion engines. Friction is the main cause of energy loss in mechanical systems [21], where lubricants can be used as an effective method to reduce friction and excessive heat. Thus, the use of lubricants leads to energy saving and prevention of energy loss in
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mechanical systems [22, 23].
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Nanolubricants have received considerable attention due to their ability in improving surface properties and engine efficiency while reducing fuel consumption and maintenance costs [24, 25].
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Moreover, the anti-wear and anti-coating properties of nanolubricants containing various
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nanoparticles such as fluorine, titanium oxide, copper oxide, nano-diamond or spherical structures
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have been studied by many researchers [26-29]. Ku et al. [30] experimentally studied the lubrication properties of mineral oil in the presence of different volume fractions of fluorine nanoparticles.
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Experiments were carried out at different volume fractions of nanoparticles and normal loads using
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a rotating disc. They experimentally investigated the friction surface temperature, friction coefficient and lubrication properties of nanoparticles. According to their results, the friction and
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wear coefficients decreased at higher volume fractions indicating an improvement in the lubrication properties of mineral oil by adding fluorine (C60) nanoparticles [30]. Saeidinia et al. [31] studied a nanofluid containing CuO nanoparticles in an oil base fluid under laminar flow and constant thermal flux conditions in a smooth tube. The nanoparticles were dispersed in the base fluid by a sonicator. The volume fraction of nanoparticles ranged from 0.07 to 0.3%. The effect of several factors including Reynolds number, volume fraction of nanoparticles, and thermal flux on heat transfer was investigated. The results showed an 45% increase in the heat transfer coefficient. Timofeeva et al. [32] studied thermophysical properties of SiO2/Thermal Oil66 nanofluid including thermal conductivity, density, specific heat capacity and viscosity at different temperatures and volume 3
Journal Pre-proof fractions. A cationic surfactant was used to disperse nanoparticles in the base fluid. The results showed an improvement in the thermal conductivity of the oil by adding silica nanoparticles and cationic surfactant. The nanofluid viscosity increased with increasing the surfactant level and volume fraction of nanoparticles. Liu et al. [33] investigated the effect of a nanofluid containing carbon nanotubes (CNTs) in an engine oil base fluid. Their results showed an 30% increase in the thermal conductivity of the nanolubricant containing 2 vol.% of CNTs compared to the pure base fluid. Esfe et al. [34] experimentally studied the rheological behavior of a nanolubricant containing
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hybrid ZrO2-MWCNT nanoparticles in 10W40 base fluid at different temperatures, volume
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fractions and shear rates. The operating temperature and volume fraction of nanoparticles respectively ranged from 5-55 °C and 0.05-1 Vol%. The viscosity of the nanolubricant was
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measured at different shear rates in the aforementioned temperature range. According to the results,
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despite the non-Newtonian behavior of the pure oil, the nanolubricant showed a pseudo-plastic
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behavior. Moreover, the non-Newtonian behavior was intensified as the temperature increased. The nanolubricant showed the highest viscosity at 55 °C in the presence of 1 vol.% of nanoparticles.
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Esfe et al. [35] investigated the rheological behavior of TiO2-MWCNT/10W40 hybrid nanolubricant
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in a temperature range of 5 to 55 °C. Their results indicated the non-Newtonian behavior of both base oil and nano-oil. Finally, a relation was proposed to explain the rheological behavior of
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nanolubricant with temperature. Ilyas et al. [36] studied the effect of 0.5 to 3 wt.% of alumina nanoparticles on thermal properties of a thermal oil base fluid. The stability of the nanofluid was evaluated under different conditions, and its rheological behavior was investigated at high shear rates. The thermal conductivity and specific heat capacity of the nanolubricant were also measured. The results showed a significant improvement in the thermal properties of the oil containing alumina nanoparticles. In another study, Ettefaghi et al. [37] investigated the effect of different volume fractions of multi-walled carbon nanotubes (MWCNT) on the properties of engine oil. Viscosity, thermal conductivity, flash point and pour point were measured as four qualitative parameters affecting the engine. According to their results, the thermal conductivity and flash point 4
Journal Pre-proof of the nanolubricant respectively increased by 13.2% and 6.7% compared with the base oil by adding 0.1 wt.% of MWCNTs. The pour point was also improved by 3.3% by adding 0.2 wt.% of nanoparticles. A review of the numerous studies conducted on various nanolubricants in the literature shows that practical parameters of engine oil have not been fully studied. To achieve an optimal nano-oil for energy systems, parameters such as viscosity, rheological behavior, freezing point, cloud point and flash point should be thoroughly investigated. In other words, it is not sufficient to study one or two
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of the aforementioned properties. To this end, the stability of the nanolubricant was first studied as a
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key parameter, and then the nanoparticles were characterized. The viscosity and rheological behavior were studied at different volume fractions, temperatures and shear rates. The freezing, pour
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and flash points of the engine oil were experimentally measured at different volume fractions of
2. Experimental
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2.1 Materials and methods
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nanoparticles.
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The present study aimed at improving the performance of SAE50 engine oil manufactured by Behran CO, Iran. Various parameters were investigated at different volume fractions of
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nanoparticles. Table 1 lists the specifications and properties of the oil used in this study. ZnO nanoparticles manufactured by TECNOLOGIA NAVARRA DE NANOPRODUCTOS, S. L. (TECNAN) were used in this study. Based on the company report, the nanoparticles are almost spherical with an average diameter of 30 nm. The use of spherical nanoparticles in lubricants is of great importance due to reduced friction and increased wear resistance caused by contact between the surfaces.
2.2. Nanofluid Preparation
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Journal Pre-proof A two-step process was used to prepare nanofluid samples. The weight of ZnO nanoparticles was first determined at different concentrations from Eq. (1). The nanoparticles and base fluid were weighed by a digital scale (A&D GR200) with an accuracy of 0.001 g.
m ( ) ZnO 100 m m ( ) ZnO ( )OilEngine
(1)
where ρ, m and respectively represent density, mass and volume fraction of the nanofluid.
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The main challenge in the use of nanofluids is their stability [38-40]. Mechanical and ultrasound
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processes were used to prepare a stable nanofluid. A magnetic stirrer (IKA- RET BASIC, Germany)
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was used to mix the nanoparticles and base fluid for 45 min. The oil temperature was increased to reduce viscosity while mixing the nanoparticles with the base fluid by the magnetic stirrer. During
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mixing, the nanoparticles were gradually added to the oil. The mixture was then exposed to
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ultrasound waves for homogenizing and breaking agglomerates to obtain a stable nanofluid, for
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which a probe sonication (UP400St, 400W, 24 kHz, Hielscher, Germany) was used.
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2.3. Viscosity Measurements
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The viscometer parameters like measurement range and accessories should be selected considering the base fluid properties [41]. The viscosity of nanolubricant was measured according to ASTM D78 by Brookfield Rotational Viscometer (DV2EXTRA-Pro) and small sample adapter measurement system. A permissible torque range of 10 to 90 N.m was taken into consideration while setting the spindle speed in the Brookfield RheocalcT Software. Considering the direct effect of temperature on the viscosity of fluids, a circulating temperature control system with accuracy of 0.1 °C was used to achieve a constant temperature during viscosity measurements. The LAUDA Alpha RA 8 bath was used for this purpose. For ensuring the truth of the measurements, the viscosity of pure water is verified at temperatures of 20, 30, 40, 50 and 60 °C earlier the experiments. The findings are compared with the data reported by International Association for the 6
Journal Pre-proof Properties of Water and Steam (IAPWS) [42]. The comparison is presented in Table 2. As seen, there is a good agreement between these values.
2.4. Freezing Point and Pour Point Measurements Freezing point is defined as a temperature at which hydrocarbon crystals are formed as the oil is cooled and a rigid opaque oil is obtained. This nanolubricant property is of great importance at low temperatures in cold regions. The freezing point was measured by a graduated cylinder, lid, 5 °C
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thermometer and cooling bath according to ASTM D97. Petrotest cooling system with a power of
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1300 W (Fig. 1) was used for this purpose. The thermal stability of samples based on the above standard is of great importance to increase the accuracy of measurements.
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The engine oil used in this system should have a low cloud point so that the engine can be easily
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started at low temperatures. The experimental setup shown in Fig. 1 was used for cloud point
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measurements according to ASTM D97. The freezing point of the samples should be determined to increase the accuracy of cloud point measurements. The cloud point was then measured by lowering
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the temperature of the frozen oil. The distance between the thermometer and graduated cylinder
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should be always 3 mm during cloud point measurements.
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2.5. Flash Point Measurements
The flash point was measured by close cup method using Pensky-Martens apparatus according to ASTM D93. Flash point measurements are of great importance to prevent explosion of storage tanks. To measure the flash point, 250 mL of nano-oil was used as shown in Fig. 2. The Protest apparatus consists of a mixer, thermometer, gas cylinder, a brass cup, heater and temperature setting system.
3. Results 3.1. Stability of Nanofluids 7
Journal Pre-proof The method used for preparation of nanofluid significantly affects the quality of nanoparticles dispersed in the base fluid. Agglomeration of nanoparticles negatively affects preparation of nanofluids, because agglomeration caused by attractive intermolecular forces will change thermophysical properties of the resulting nanofluid [43, 44]. According to the analytical relation for settling velocity of spherical particles in a stagnant fluid (Stoke’s law) and the sample imaging results, the nanolubricant shows a reasonable long-term stability which is considered a useful practical property for nanolubricants. DLS test was used to investigate the stability and diameter of
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nanoparticles in the base fluid. The results are shown in Fig. 3.
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A fixed-wavelength laser beam was used in the DLS test. The DLS mechanism calculates the size of dispersed nanoparticles by colliding the laser beam with the nanoparticles in the oil. The particle
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size distribution is obtained based on Mie scattering theory [45]. According to the DLS results, the
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maximum intensity of ZnO nanoparticles was observed at 42 nm and size of all particles was less
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than 100 nm. The nanosized particles improve long-term stability and properties of the
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nanolubricant as compared with micron-sized particles.
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3.2. Evaluating dynamic viscosity
Given the importance of the rheological behavior of fluids, the rheological behavior of the
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nanolubricant is discussed in this section. The effect of various parameters on the dynamic viscosity of the nanofluid was studied and the results were compared with those of the base fluid. The behavior of a fluid is said to be Newtonian if its shear rate is linearly related to the applied shear stress. The equation governing the behavior of Newtonian fluids is as follows:
*
(2)
du dx
Where ??, µ and respectively show the shear stress (dyne/cm2), dynamic viscosity (mPa.s) and shear rate (1/s).
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Journal Pre-proof To evaluate the rheological behavior of the nanolubricant, the changes in the shear stress with respect to shear rate should be first determined. To this end, the effect of shear rate on the shear stress was investigated at different volume fractions and the results are shown in Fig. 4. The experiments were carried out within a temperature range of 25 to 65 °C. The results indicated the oil base fluid to exhibit a Newtonian behavior, since a direct linear relationship is observed between the shear rate and shear stress of various nanolubricant samples. The nanolubricant also shows a Newtonian behavior like the oil base fluid.
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The right-hand side diagrams show the viscosity of the nanolubricant as a function of shear rate at
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25°C. The results show a small deviation from the ideal linear behavior, neglecting which we can conclude that the nanolubricant exhibits a Newtonian behavior at this temperature. The
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nanolubricant shows a constant linear behavior with increasing the shear rate. Fluctuations are
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usually observed at high volume fractions due to the hybrid structure of the nanolubricant. The
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results also indicate the independence of viscosity of the shear rate at different temperatures. At
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as the shear rate increased.
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different volume fractions, the viscosity did not change significantly and remained almost constant
3.3. The Effect of Temperature and Volume Fraction on Viscosity
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Fig. 5 shows the viscosity of the nanolubricant at different volume fractions in a temperature range of 25 to 65 °C. As can be seen, the dynamic viscosity of the nanolubricant significantly decreases with increasing temperature at a certain volume fraction. This is one of the major drawbacks in the use of engine oils in the industry. The maximum viscosity was observed at 1.5 Vol% of nanoparticles at 25 °C. The resistance against flow increases with increasing the volume fraction of ZnO nanoparticles in the base fluid, which means an increase in the number of nanoparticle collisions due to their random motions. With increasing the volume fraction of nanoparticles at a constant oil volume, nanoparticle
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Journal Pre-proof aggregates are formed in the base fluid due to van der Waals forces, which prevent the movement of the base fluid particles on each other, consequently leading to an increase in the viscosity. The relative viscosity of the nanofluid is calculated from Eq. (3) as the ratio of changes in the viscosity of the nanolubricant to the base oil. As shown in Fig. 5, the relative viscosity increases with increasing temperature at a constant volume fraction of nanoparticles. The viscosity is changing more significantly at lower temperatures. The viscosity changes, however, become insignificant with increasing temperature. This is a key
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parameter in heat and mass transfer applications where pumping power and viscosity-related issues
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are of great importance. The changes in thermal conductivity significantly increases with increasing temperature, indicating the sensitivity of viscosity to temperature and volume fraction of
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nanoparticles.
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In liquid flow, especially oils, molecules with higher energy at higher temperatures overcome inter-
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molecular adhesive forces. As a result, the nanofluid viscosity decreases with increasing temperature. The results clearly indicate a significant reduction in the viscosity of the base oil with
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increasing temperature. As temperature rises, the inter-molecular distance between the nanoparticles
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and base fluid increases. Consequently, the resistance against the flow of fluid layers decreases and nanolubricant viscosity is reduced.
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Fig. 6 shows the dynamic viscosity and relative viscosity as a function of temperature at different volume fractions of ZnO nanoparticles. The viscosity of the nanofluid decreases with increasing temperature at a constant volume fraction. The viscosity decreases more significantly at higher volume fractions, such that the greatest slope occurs at the highest volume fraction. This can be partly attributed to adhesion forces between the fluid molecules which are intrinsically decreased with temperature. As can be seen, with increasing the volume fraction of nanoparticles in the base fluid, the dynamic viscosity increases due to resistance of flow against shear rate at different concentrations. At higher temperatures, the viscosity of nanolubricant is not significantly affected
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Journal Pre-proof by increasing the volume fraction of nanoparticles, which is considered as an advantage due to high operating temperatures of engine oils. The changes in the viscosity of the nanolubricant relative to the base oil were measured at different temperatures. Due to low volume fraction of nanoparticles in the base oil at low concentrations, the viscosity does not change significantly. However, the slope of changes in relative viscosity increases significantly as the volume fraction of nanoparticles increases. As the temperature rises, the relative viscosity of the nanolubricant decreases. As a result, the relative viscosity further
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increases relative to the base fluid at lower temperatures.
3.4. Providing a New Relation
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According to the literature, analytical relations are not usually able to predict the changes in the
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properties of nanofluids, because of influence of multiple parameters on the nanofluid properties
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[46, 47]. The complex structure and properties of nanofluids are influenced by various parameters such as temperature, volume fraction, particle size, surface, chemical and atomic structure of
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nanoparticles and nanofluid preparation method, consequently introducing to a significant
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difference between experimental and analytical results. Therefore, an exact experimental relation (Eq. 4) with a coefficient of determination of 0.998 was proposed to calculate the dynamic viscosity
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of the nanofluid as a function of temperature and volume fraction of nanoparticles. The relation was obtained by curve-fitting on experimental data. nf 1 0.986 0.937 ( 0.47 ) 0.703 bf T
)4(
(25o C T 65o C & 0.125% 1.5%) where T, φ respectively represent temperature and solid volume fraction. μnf and μbf are respectively the viscosity of nanofluid and base fluid. Fig. 7 compares the dynamic viscosity ratio calculated from the above experimental relation with experimental results. The dynamic viscosity ratio is shown at a temperature range of 25 to 65 °C. 11
Journal Pre-proof The proposed relation can be used to calculate the changes in the dynamic viscosity within the specified range. These charts clearly show the difference between the results at different temperatures. The deviation margin of the calculated and experimental results is expressed by Eq. (5): nf )exp -( nf ) pred bf Deviation margin(%) bf *100 nf ( )exp bf (
nf )exp and ( nf ) pred respectively show the experimental and calculated (Eq. 4) viscosity bf bf
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where (
)5(
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ratios. Given the wide range of experiments in terms of temperature and volume fraction of
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nanoparticles, the maximum deviation margin equals 1.16%. Fig. 8 shows the deviation margin
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calculated at different temperatures. The results indicate the reasonable accuracy of the proposed
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correlation.
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3.5. Freezing Point and Cloud Point
The use of oils is of great importance in cold weather at the beginning of engine or lubrication
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system operation. Maximum wear occurs when the system fails to operate properly. Thus, engine
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start in cold weather is very critical. Fig. 10 shows the cloud point for the nanolubricant as an important property of oil at low temperatures. The changes in the cloud point of the nanolubricant relative to the base fluid are also shown. According to the results, the performance of the nanolubricant is improved by lowering the oil temperature and formation of a cloud layer or a halo of paraffinic crystals. The nanolubricant samples are still in the fluid phase at this temperature and can thus be used. The best operating conditions for the nanolubricant occurs at the highest concentration, where the performance increases by 22.2%. Fig. 8 shows the freezing point of the nanolubricant. As shown, in the absence of ZnO nanoparticles, the oil is frozen at 17 °C, while the nanoparticles improve the performance of oil. The performance of the engine oil significantly increases with increasing the volume fraction of 12
Journal Pre-proof nanoparticles. The lowest freezing temperature of -20.3 °C was observed at a volume fraction of 1.5 %. The changes in the nanolubricant relative to the base oil indicate significant variations with increasing the volume fraction of nanoparticles. An improvement in the freezing point is useful in cold regions. In addition to other advantages, addition of nanoparticles delay oil freezing by -3.3 °C.
3.6. Flash Point Flash point is the lowest temperature at which the oil is sufficiently evaporated and forms a
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flammable mixture with air so that the oil is ignited for a brief moment in the presence of flame.
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Therefore, oil melting point is a key parameter in determining the operating range of oils. This physical property is a criterion for measuring the flammability and volatility of oil. Addition of
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different volume fractions of ZnO nanoparticles improves the performance of oil at high
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temperatures, leading to an increase in the operating range of oil. Fig. 11 shows the flash point with
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respect to the volume fraction. The largest improvement in the flash point of the nanolubricant relative to the base oil was 7.2%. The flash point, in fact, increases from 235 to 252 °C. Even low
4. Conclusion
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volume fractions of nanoparticles may have a positive impact on the performance of engine oil.
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A nanolubricant containing different volume fractions of ZnO nanoparticles was studied from various aspects. The two-step process is considered a suitable method for preparation of nanolubricants, since the nanoparticles can be perfectly mixed with the base fluid by increasing the base oil temperature and the subsequent reduction in the viscosity. The resulting nanofluid was then homogenized for 60 min with the help of a probe sonication. The stability of the nanolubricant was assessed as acceptable using the DLS test. The results showed the nanofluid to exhibit a Newtonian behavior based on the rheological behavior of the base fluid, which is considered a very important feature. The viscosity of the nanofluid experienced a maximum increase of up to 1.25 times that of the base fluid at 1.5 Vol% of ZnO nanoparticles. On the other hand, the viscosity of both base oil 13
Journal Pre-proof and nano-oil decreased with increasing temperature. The results on cloud point, freezing point and flash point indicated an improvement in the performance of oil at low and high temperatures. In addition to improved oil properties, the operating range of the oil also increased so that the cloud point, freezing point and flash point were respectively improved by 22.2, 19.4 and 7.2% under optimal conditions. According to the results of this study, one can conclude that the use of this nanofluid may improve the performance of engine oils.
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[20] A.A.A.A. Al-Rashed, A. Shahsavar, S. Entezari, M.A. Moghimi, S.A. Adio, T.K. Nguyen, Numerical investigation of non-Newtonian water-CMC/CuO nanofluid flow in an offset strip-fin
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microchannel heat sink: Thermal performance and thermodynamic considerations, Applied Thermal Engineering 155 (2019) 247-258. [21] C.R. Ferguson, A.T. Kirkpatrick, Internal combustion engines: applied thermosciences. 2015: John Wiley & Sons. [22] L. Yang, K. Du, X.S. Zhang. A theoretical investigation of thermal conductivity of nanofluids with particles in cylindrical shape by anisotropy analysis. Powder Technology, 314 (2017) 328-338 . [23] L. Yang, K. Du, X. Niu, Y. Zhang. Numerical Investigation of ammonia falling film absorption outside vertical tube with nanofluids. Int. J.Heat Mass Transfer. 79 (2014) 241-250
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Journal Pre-proof [24] Vakili-Nezhaad, G. and A. Dorany, Investigation of the effect of multiwalled carbon nanotubes on the viscosity index of lube oil cuts. Chemical Engineering Communications, 2009. 196(9): p. 997-1007. [25] Esfe, M.H., et al., Effects of temperature and concentration on rheological behavior of MWCNTs/SiO2 (20–80)-SAE40 hybrid nano-lubricant. International Communications in Heat and Mass Transfer, 2016. 7(6): p. 133-138. [26] Wu, Y., W. Tsui, and T. Liu, Experimental analysis of tribological properties of lubricating oils
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with nanoparticle additives. Wear, 2007. 262(7-8): p. 819-825.
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[27] Liu, L., et al., Lubrication effect of the paraffin oil filled with functionalized multiwalled carbon nanotubes for bismaleimide resin. Tribology Letters, 2011. 42(1): p. 59-65.
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[28] Hwang, Y., et al., Effect of the size and morphology of particles dispersed in nano-oil on
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friction performance between rotating discs. Journal of Mechanical Science and Technology, 2011.
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[29] Ali, M.K.A., et al., Improving the tribological characteristics of piston ring assembly in
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automotive engines using Al2O3 and TiO2 nanomaterials as nano-lubricant additives. Tribology
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International, 2016. 103: p. 540-554.
[30] Ku, B.-C., et al., Tribological effects of fullerene (C 60) nanoparticles added in mineral
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lubricants according to its viscosity. International Journal of Precision Engineering and Manufacturing, 2010. 11(4): p. 60611-7 [31] Saeedinia, M., M. Akhavan-Behabadi, and M. Nasr, Experimental study on heat transfer and pressure drop of nanofluid flow in a horizontal coiled wire inserted tube under constant heat flux. Experimental Thermal and Fluid Science, 2012. 36: p. 15. 168-8 [32] Timofeeva, E.V., M.R. Moravek, and D. Singh, Improving the heat transfer efficiency of synthetic oil with silica nanoparticles. Journal of colloid and interface science, 2011. 364(1): p. 7179.
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Journal Pre-proof [33] Liu, M.-S., et al., Enhancement of thermal conductivity with carbon nanotube for nanofluids. International communications in heat and mass transfer, 2005. 32(9): p. 1202-1210. [34] Esfe, M.H., et al., Rheological behavior characteristics of ZrO2-MWCNT/10w40 hybrid nanolubricant affected by temperature, concentration, and shear rate: An experimental study and a neural network simulating. Physica E: Low-dimensional Systems and Nanostructures, 2018. 102: p. 160170. [35] Esfe, M.H., et al., Rheological behavior characteristics of TiO2-MWCNT/10w40 hybrid nano-
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oil affected by temperature, concentration and shear rate: An experimental study and a neural
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network simulating. Physica E: Low-dimensional Systems and Nanostructures, 2017. 94: p. 231240.
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[36] Ilyas, S.U., et al., Stability, rheology and thermal analysis of functionalized alumina-thermal
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oil-based nanofluids for advanced cooling systems. Energy conversion and management, 2017. 142:
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p. 215-229.
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[37] E. Ettefaghi, H. Ahmadi, A. Rashidi, A. Nouralishahi, S.S. Mohtasebi, Preparation and thermal properties of oil-based nanofluid from multi-walled carbon nanotubes and engine oil as nano-
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lubricant, Int. Commun. Heat Mass Transfer, 46 (2013), pp. 142-147
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[38] A. Ahmadi Nadooshan, M. Hemmat Esfe, M. Afrand, Evaluation of rheological behavior of 10W40 lubricant containing hybrid nano-material by measuring dynamic viscosity, Physica E: Lowdimensional Systems and Nanostructures, 92 (2017) 47-54. [39] A. Hajatzadeh Pordanjani, S. Aghakhani, M. Afrand, B. Mahmoudi, O. Mahian, S. Wongwises, An updated review on application of nanofluids in heat exchangers for saving energy, Energy Conversion and Management, 198 (2019) 111886. [40] Ranjbarzadeh, R., et al., An experimental study on stability and thermal conductivity of water/silica nanofluid: Eco-friendly production of nanoparticles. Journal of Cleaner Production, 2019. 206: p. 1089-1100.
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Journal Pre-proof [41] Ranjbarzadeh, R., et al., Effects of graphene oxide‑ silicon oxide hybrid nanomaterials on rheological behavior of water at various time durations and temperatures: Synthesis, preparation and stability. Powder Technology, 2018. 335: p. 375-387. [42] M.K. Daucik, R. Dooley, The international association for the properties of water and steam, Revised supplementary release on properties of liquid water at 0.1 MPa, (2011) 1-12. [43] Wang, X.-Q. and A.S. Mujumdar, A review on nanofluids-part II: experiments and applications. Brazilian Journal of Chemical Engineering, 2008. 25(4 :)p. 631-648.
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[44] J.A. Eastman, Anomalously increased effective thermal conductivities of ethylene glycol-based
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[46] M. Afrand, K.N. Najafabadi, M. Akbari, Effects of temperature and solid volume fraction on
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viscosity of SiO 2-MWCNTs/SAE40 hybrid nanofluid as a coolant and lubricant in heat engines. Applied Thermal Engineering, 102 (2016) 45-54.
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[47] M. Soltanimehr, M. Afrand, Thermal conductivity enhancement of COOH-functionalized
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MWCNTs/ethylene glycol–water nanofluid for application in heating and cooling systems. Applied
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Thermal Engineering, 105 (2016) 716-723.
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Journal Pre-proof Table 1. Characteristics of SAE50 engine oil. Characteristic
Value
Kinematic viscosity @ 100 °C
1.8×10-5 (m2/s)
Viscosity Index (VI)
90
Flash point
246 (°C)
Pour point
-9 (°C)
Total base number (TBN)
4.1 (mg KOH/g) 0.906 (g/cm-3)
Density @ 15 °C
1900 (J/kg.°C)
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specific heat capacity
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Table 2. Validation of measurements
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20 30 40 50 60
Viscosity (cP) Measurement IAPWS [42] 1.059 1.0016 0.846 0.7972 0.685 0.6527 0.581 0.5465 0.491 0.466
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Temperature (oC)
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Fig. 1. The experimental setup used for measuring the freezing point and pour point of the nanolubricant.
Fig. 2. The experimental setup used to measure freezing and pour points of the nano-oil.
Fig. 3. The average diameter of ZnO nanoparticles in the oil base fluid.
Fig. 4. Results of the behavior of nanolubricant viscosity in terms of shear rate at different temperatures.
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Fig. 5. The effect of volume fraction on viscosity and relative viscosity of nanolubricant.
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Fig. 6. Effect of temperature on viscosity and relative viscosity of Nanolubricants in different volume fractions.
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Fig. 7. Comparison of correlation and experimental results of viscosity for base and nanolubricant fluid.
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Fig. 8. Deviation margin of calculated and experimental viscosities.
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Fig. 9. The cloud point of the nanolubricant and cloud point ratio at different volume fractions.
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Fig. 10. Freezing point of the nanolubricant and freezing point ratio at different volume fractions.
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Fig. 11. The flash point of the nanolubricant and flash point ratio at different volume fractions of ZnO nanoparticles.
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Journal Pre-proof Declaration of interests
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The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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Highlights Improving the performance of engine oil by adding zinc oxide nanoparticles Measuring the rheological behavior, cloud, freezing and flash points The viscosity increased with increasing concentration up to 25.3% relative to the base fluid. The cloud, freezing and flash points were improved respectively by 22.2, 19.4 and 7.2%.
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The nanofluid is a good alternative to the engine oil base fluid in practical systems.
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Figure 1
Figure 2
Figure 3
Figure 4
Figure 5
Figure 6
Figure 7
Figure 8
Figure 9
Figure 10
Figure 11
Jilin Ma, Amin Shahsavar, Abdullah A.A.A. Al-Rashed, Arash Karimipour, Hooman Yarmand, Sara Rostami PII:
S0167-7322(19)35245-6
DOI:
https://doi.org/10.1016/j.molliq.2019.112045
Reference:
MOLLIQ 112045
To appear in:
Journal of Molecular Liquids
Received date:
19 September 2019
Revised date:
27 October 2019
Accepted date:
31 October 2019
Please cite this article as: J. Ma, A. Shahsavar, A.A.A.A. Al-Rashed, et al., Viscosity, cloud point, freezing point and flash point of zinc oxide/SAE50 nanolubricant, Journal of Molecular Liquids(2018), https://doi.org/10.1016/j.molliq.2019.112045
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© 2018 Published by Elsevier.
Journal Pre-proof
Viscosity, Cloud Point, Freezing Point and Flash Point of Zinc oxide/SAE50 Nanolubricant Jilin Ma1, Amin Shahsavar2,*, Abdullah A.A.A. Al-Rashed3, Arash Karimipour4, Hooman Yarmand5, Sara Rostami6,7,* 1
2
3
China Classification Society, Beijing 100007, China
Department of Mechanical Engineering, Kermanshah University of Technology, Kermanshah, Iran
Department of Automotive and Marine Engineering Technology, College of Technological Studies, The Public
5
Department of Mechanical Engineering, Najafabad Branch, Islamic Azad University, Najafabad, Iran
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4
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Authority for Applied Education and Training, Kuwait
Centre for Energy Sciences, Department of Mechanical Engineering, University of Malaya, 50603 Kuala Lumpur,
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Malaysia
Laboratory of Magnetism and Magnetic Materials, Advanced Institute of Materials Science, Ton Duc Thang
Faculty of Applied Sciences, Ton Duc Thang University, Ho Chi Minh City, Vietnam
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7
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University, Ho Chi Minh City, Vietnam
*Corresponding authors
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Emails: [email protected] (A. Shahsavar)
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[email protected] (H. Yarmand)
Abstract
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[email protected] (S. Rostami)
This study aimed at improving the performance of engine oil (SAE50) by adding zinc oxide nanoparticles. First, the microstructure of nanoparticles was studied by scanning electron microscopy (SEM) and X-ray diffraction (XRD). After confirming the microstructure of nanoparticles, a two-step method was used to prepare the engine oil/zinc oxide nanolubricant. The nanolubricant remained stable without any sedimentation for more than 5 weeks in static mode. Different volume fractions of the nanolubricant were prepared and the rheological behavior and nanofluid viscosity were studied in the range of 25 to 65 °C and, additionally, the cloud, freezing 1
Journal Pre-proof and flash points were measured. According to the results of dynamic light scattering (DLS) test, the nanoparticles in the base nanofluid had an average diameter of 55 nm. The viscosity increased with increasing concentration up to 25.3% relative to the base fluid. The viscosity of the nanolubricant decreased with increasing temperature. An accurate relation was proposed based on different temperatures and nanofluid volume fractions to calculate the nanofluid viscosity. The cloud, freezing and flash points were improved respectively by 22.2, 19.4 and 7.2% at the highest
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concentration, indicating the reasonable performance of nanoparticles.
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Keywords: Experimental study; Nanolubricant; Rheological behavior; Cloud point; Freezing point;
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Flash point.
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1. Introduction
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Optimizing and improving the performance of energy and heat transfer systems have recently received considerable attention due to the population growth and ever-increasing need for energy
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throughout the world. Therefore, the lubrication and cooling of lubricants in advanced systems
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should be highly efficient. Improved thermal properties of nanoparticles, in particular high thermal
systems [1-10].
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conductivity of the nanostructure, may lead to improved lubrication and heat transfer in such
As a recently emerging and advancing technology, nanofluids have received considerable attention in energy management and heat transfer systems given their unique thermophysical properties. Nanolubricants are a new class of nanofluids. Stability, rheological behavior, and oil performance at very high and very low temperatures are the most important issues in obtaining a practical nanolubricant with improved performance in lubrication and thermal systems [11, 12]. In addition to lubrication and heat transfer, mass transfer and nanofluid flow are of great importance in such systems as the pumping power and electricity costs become a matter of concern. Thus, it is necessary to obtain a nanofluid with proper viscosity and high thermal performance as an alternative 2
Journal Pre-proof to the base fluid to achieve a higher efficiency. The changes in the base fluid by adding nanoparticles can significantly affect the improvements in heat transfer, design and investment on thermal systems [13-20]. Today, various lubricants with different properties are produced and used in various mechanical systems such as internal combustion engines. Friction is the main cause of energy loss in mechanical systems [21], where lubricants can be used as an effective method to reduce friction and excessive heat. Thus, the use of lubricants leads to energy saving and prevention of energy loss in
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mechanical systems [22, 23].
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Nanolubricants have received considerable attention due to their ability in improving surface properties and engine efficiency while reducing fuel consumption and maintenance costs [24, 25].
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Moreover, the anti-wear and anti-coating properties of nanolubricants containing various
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nanoparticles such as fluorine, titanium oxide, copper oxide, nano-diamond or spherical structures
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have been studied by many researchers [26-29]. Ku et al. [30] experimentally studied the lubrication properties of mineral oil in the presence of different volume fractions of fluorine nanoparticles.
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Experiments were carried out at different volume fractions of nanoparticles and normal loads using
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a rotating disc. They experimentally investigated the friction surface temperature, friction coefficient and lubrication properties of nanoparticles. According to their results, the friction and
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wear coefficients decreased at higher volume fractions indicating an improvement in the lubrication properties of mineral oil by adding fluorine (C60) nanoparticles [30]. Saeidinia et al. [31] studied a nanofluid containing CuO nanoparticles in an oil base fluid under laminar flow and constant thermal flux conditions in a smooth tube. The nanoparticles were dispersed in the base fluid by a sonicator. The volume fraction of nanoparticles ranged from 0.07 to 0.3%. The effect of several factors including Reynolds number, volume fraction of nanoparticles, and thermal flux on heat transfer was investigated. The results showed an 45% increase in the heat transfer coefficient. Timofeeva et al. [32] studied thermophysical properties of SiO2/Thermal Oil66 nanofluid including thermal conductivity, density, specific heat capacity and viscosity at different temperatures and volume 3
Journal Pre-proof fractions. A cationic surfactant was used to disperse nanoparticles in the base fluid. The results showed an improvement in the thermal conductivity of the oil by adding silica nanoparticles and cationic surfactant. The nanofluid viscosity increased with increasing the surfactant level and volume fraction of nanoparticles. Liu et al. [33] investigated the effect of a nanofluid containing carbon nanotubes (CNTs) in an engine oil base fluid. Their results showed an 30% increase in the thermal conductivity of the nanolubricant containing 2 vol.% of CNTs compared to the pure base fluid. Esfe et al. [34] experimentally studied the rheological behavior of a nanolubricant containing
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hybrid ZrO2-MWCNT nanoparticles in 10W40 base fluid at different temperatures, volume
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fractions and shear rates. The operating temperature and volume fraction of nanoparticles respectively ranged from 5-55 °C and 0.05-1 Vol%. The viscosity of the nanolubricant was
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measured at different shear rates in the aforementioned temperature range. According to the results,
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despite the non-Newtonian behavior of the pure oil, the nanolubricant showed a pseudo-plastic
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behavior. Moreover, the non-Newtonian behavior was intensified as the temperature increased. The nanolubricant showed the highest viscosity at 55 °C in the presence of 1 vol.% of nanoparticles.
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Esfe et al. [35] investigated the rheological behavior of TiO2-MWCNT/10W40 hybrid nanolubricant
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in a temperature range of 5 to 55 °C. Their results indicated the non-Newtonian behavior of both base oil and nano-oil. Finally, a relation was proposed to explain the rheological behavior of
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nanolubricant with temperature. Ilyas et al. [36] studied the effect of 0.5 to 3 wt.% of alumina nanoparticles on thermal properties of a thermal oil base fluid. The stability of the nanofluid was evaluated under different conditions, and its rheological behavior was investigated at high shear rates. The thermal conductivity and specific heat capacity of the nanolubricant were also measured. The results showed a significant improvement in the thermal properties of the oil containing alumina nanoparticles. In another study, Ettefaghi et al. [37] investigated the effect of different volume fractions of multi-walled carbon nanotubes (MWCNT) on the properties of engine oil. Viscosity, thermal conductivity, flash point and pour point were measured as four qualitative parameters affecting the engine. According to their results, the thermal conductivity and flash point 4
Journal Pre-proof of the nanolubricant respectively increased by 13.2% and 6.7% compared with the base oil by adding 0.1 wt.% of MWCNTs. The pour point was also improved by 3.3% by adding 0.2 wt.% of nanoparticles. A review of the numerous studies conducted on various nanolubricants in the literature shows that practical parameters of engine oil have not been fully studied. To achieve an optimal nano-oil for energy systems, parameters such as viscosity, rheological behavior, freezing point, cloud point and flash point should be thoroughly investigated. In other words, it is not sufficient to study one or two
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of the aforementioned properties. To this end, the stability of the nanolubricant was first studied as a
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key parameter, and then the nanoparticles were characterized. The viscosity and rheological behavior were studied at different volume fractions, temperatures and shear rates. The freezing, pour
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and flash points of the engine oil were experimentally measured at different volume fractions of
2. Experimental
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2.1 Materials and methods
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nanoparticles.
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The present study aimed at improving the performance of SAE50 engine oil manufactured by Behran CO, Iran. Various parameters were investigated at different volume fractions of
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nanoparticles. Table 1 lists the specifications and properties of the oil used in this study. ZnO nanoparticles manufactured by TECNOLOGIA NAVARRA DE NANOPRODUCTOS, S. L. (TECNAN) were used in this study. Based on the company report, the nanoparticles are almost spherical with an average diameter of 30 nm. The use of spherical nanoparticles in lubricants is of great importance due to reduced friction and increased wear resistance caused by contact between the surfaces.
2.2. Nanofluid Preparation
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Journal Pre-proof A two-step process was used to prepare nanofluid samples. The weight of ZnO nanoparticles was first determined at different concentrations from Eq. (1). The nanoparticles and base fluid were weighed by a digital scale (A&D GR200) with an accuracy of 0.001 g.
m ( ) ZnO 100 m m ( ) ZnO ( )OilEngine
(1)
where ρ, m and respectively represent density, mass and volume fraction of the nanofluid.
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The main challenge in the use of nanofluids is their stability [38-40]. Mechanical and ultrasound
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processes were used to prepare a stable nanofluid. A magnetic stirrer (IKA- RET BASIC, Germany)
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was used to mix the nanoparticles and base fluid for 45 min. The oil temperature was increased to reduce viscosity while mixing the nanoparticles with the base fluid by the magnetic stirrer. During
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mixing, the nanoparticles were gradually added to the oil. The mixture was then exposed to
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ultrasound waves for homogenizing and breaking agglomerates to obtain a stable nanofluid, for
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which a probe sonication (UP400St, 400W, 24 kHz, Hielscher, Germany) was used.
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2.3. Viscosity Measurements
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The viscometer parameters like measurement range and accessories should be selected considering the base fluid properties [41]. The viscosity of nanolubricant was measured according to ASTM D78 by Brookfield Rotational Viscometer (DV2EXTRA-Pro) and small sample adapter measurement system. A permissible torque range of 10 to 90 N.m was taken into consideration while setting the spindle speed in the Brookfield RheocalcT Software. Considering the direct effect of temperature on the viscosity of fluids, a circulating temperature control system with accuracy of 0.1 °C was used to achieve a constant temperature during viscosity measurements. The LAUDA Alpha RA 8 bath was used for this purpose. For ensuring the truth of the measurements, the viscosity of pure water is verified at temperatures of 20, 30, 40, 50 and 60 °C earlier the experiments. The findings are compared with the data reported by International Association for the 6
Journal Pre-proof Properties of Water and Steam (IAPWS) [42]. The comparison is presented in Table 2. As seen, there is a good agreement between these values.
2.4. Freezing Point and Pour Point Measurements Freezing point is defined as a temperature at which hydrocarbon crystals are formed as the oil is cooled and a rigid opaque oil is obtained. This nanolubricant property is of great importance at low temperatures in cold regions. The freezing point was measured by a graduated cylinder, lid, 5 °C
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thermometer and cooling bath according to ASTM D97. Petrotest cooling system with a power of
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1300 W (Fig. 1) was used for this purpose. The thermal stability of samples based on the above standard is of great importance to increase the accuracy of measurements.
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The engine oil used in this system should have a low cloud point so that the engine can be easily
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started at low temperatures. The experimental setup shown in Fig. 1 was used for cloud point
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measurements according to ASTM D97. The freezing point of the samples should be determined to increase the accuracy of cloud point measurements. The cloud point was then measured by lowering
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the temperature of the frozen oil. The distance between the thermometer and graduated cylinder
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should be always 3 mm during cloud point measurements.
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2.5. Flash Point Measurements
The flash point was measured by close cup method using Pensky-Martens apparatus according to ASTM D93. Flash point measurements are of great importance to prevent explosion of storage tanks. To measure the flash point, 250 mL of nano-oil was used as shown in Fig. 2. The Protest apparatus consists of a mixer, thermometer, gas cylinder, a brass cup, heater and temperature setting system.
3. Results 3.1. Stability of Nanofluids 7
Journal Pre-proof The method used for preparation of nanofluid significantly affects the quality of nanoparticles dispersed in the base fluid. Agglomeration of nanoparticles negatively affects preparation of nanofluids, because agglomeration caused by attractive intermolecular forces will change thermophysical properties of the resulting nanofluid [43, 44]. According to the analytical relation for settling velocity of spherical particles in a stagnant fluid (Stoke’s law) and the sample imaging results, the nanolubricant shows a reasonable long-term stability which is considered a useful practical property for nanolubricants. DLS test was used to investigate the stability and diameter of
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nanoparticles in the base fluid. The results are shown in Fig. 3.
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A fixed-wavelength laser beam was used in the DLS test. The DLS mechanism calculates the size of dispersed nanoparticles by colliding the laser beam with the nanoparticles in the oil. The particle
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size distribution is obtained based on Mie scattering theory [45]. According to the DLS results, the
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maximum intensity of ZnO nanoparticles was observed at 42 nm and size of all particles was less
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than 100 nm. The nanosized particles improve long-term stability and properties of the
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nanolubricant as compared with micron-sized particles.
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3.2. Evaluating dynamic viscosity
Given the importance of the rheological behavior of fluids, the rheological behavior of the
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nanolubricant is discussed in this section. The effect of various parameters on the dynamic viscosity of the nanofluid was studied and the results were compared with those of the base fluid. The behavior of a fluid is said to be Newtonian if its shear rate is linearly related to the applied shear stress. The equation governing the behavior of Newtonian fluids is as follows:
*
(2)
du dx
Where ??, µ and respectively show the shear stress (dyne/cm2), dynamic viscosity (mPa.s) and shear rate (1/s).
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Journal Pre-proof To evaluate the rheological behavior of the nanolubricant, the changes in the shear stress with respect to shear rate should be first determined. To this end, the effect of shear rate on the shear stress was investigated at different volume fractions and the results are shown in Fig. 4. The experiments were carried out within a temperature range of 25 to 65 °C. The results indicated the oil base fluid to exhibit a Newtonian behavior, since a direct linear relationship is observed between the shear rate and shear stress of various nanolubricant samples. The nanolubricant also shows a Newtonian behavior like the oil base fluid.
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The right-hand side diagrams show the viscosity of the nanolubricant as a function of shear rate at
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25°C. The results show a small deviation from the ideal linear behavior, neglecting which we can conclude that the nanolubricant exhibits a Newtonian behavior at this temperature. The
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nanolubricant shows a constant linear behavior with increasing the shear rate. Fluctuations are
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usually observed at high volume fractions due to the hybrid structure of the nanolubricant. The
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results also indicate the independence of viscosity of the shear rate at different temperatures. At
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as the shear rate increased.
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different volume fractions, the viscosity did not change significantly and remained almost constant
3.3. The Effect of Temperature and Volume Fraction on Viscosity
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Fig. 5 shows the viscosity of the nanolubricant at different volume fractions in a temperature range of 25 to 65 °C. As can be seen, the dynamic viscosity of the nanolubricant significantly decreases with increasing temperature at a certain volume fraction. This is one of the major drawbacks in the use of engine oils in the industry. The maximum viscosity was observed at 1.5 Vol% of nanoparticles at 25 °C. The resistance against flow increases with increasing the volume fraction of ZnO nanoparticles in the base fluid, which means an increase in the number of nanoparticle collisions due to their random motions. With increasing the volume fraction of nanoparticles at a constant oil volume, nanoparticle
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Journal Pre-proof aggregates are formed in the base fluid due to van der Waals forces, which prevent the movement of the base fluid particles on each other, consequently leading to an increase in the viscosity. The relative viscosity of the nanofluid is calculated from Eq. (3) as the ratio of changes in the viscosity of the nanolubricant to the base oil. As shown in Fig. 5, the relative viscosity increases with increasing temperature at a constant volume fraction of nanoparticles. The viscosity is changing more significantly at lower temperatures. The viscosity changes, however, become insignificant with increasing temperature. This is a key
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parameter in heat and mass transfer applications where pumping power and viscosity-related issues
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are of great importance. The changes in thermal conductivity significantly increases with increasing temperature, indicating the sensitivity of viscosity to temperature and volume fraction of
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nanoparticles.
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In liquid flow, especially oils, molecules with higher energy at higher temperatures overcome inter-
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molecular adhesive forces. As a result, the nanofluid viscosity decreases with increasing temperature. The results clearly indicate a significant reduction in the viscosity of the base oil with
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increasing temperature. As temperature rises, the inter-molecular distance between the nanoparticles
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and base fluid increases. Consequently, the resistance against the flow of fluid layers decreases and nanolubricant viscosity is reduced.
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Fig. 6 shows the dynamic viscosity and relative viscosity as a function of temperature at different volume fractions of ZnO nanoparticles. The viscosity of the nanofluid decreases with increasing temperature at a constant volume fraction. The viscosity decreases more significantly at higher volume fractions, such that the greatest slope occurs at the highest volume fraction. This can be partly attributed to adhesion forces between the fluid molecules which are intrinsically decreased with temperature. As can be seen, with increasing the volume fraction of nanoparticles in the base fluid, the dynamic viscosity increases due to resistance of flow against shear rate at different concentrations. At higher temperatures, the viscosity of nanolubricant is not significantly affected
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Journal Pre-proof by increasing the volume fraction of nanoparticles, which is considered as an advantage due to high operating temperatures of engine oils. The changes in the viscosity of the nanolubricant relative to the base oil were measured at different temperatures. Due to low volume fraction of nanoparticles in the base oil at low concentrations, the viscosity does not change significantly. However, the slope of changes in relative viscosity increases significantly as the volume fraction of nanoparticles increases. As the temperature rises, the relative viscosity of the nanolubricant decreases. As a result, the relative viscosity further
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increases relative to the base fluid at lower temperatures.
3.4. Providing a New Relation
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According to the literature, analytical relations are not usually able to predict the changes in the
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properties of nanofluids, because of influence of multiple parameters on the nanofluid properties
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[46, 47]. The complex structure and properties of nanofluids are influenced by various parameters such as temperature, volume fraction, particle size, surface, chemical and atomic structure of
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nanoparticles and nanofluid preparation method, consequently introducing to a significant
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difference between experimental and analytical results. Therefore, an exact experimental relation (Eq. 4) with a coefficient of determination of 0.998 was proposed to calculate the dynamic viscosity
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of the nanofluid as a function of temperature and volume fraction of nanoparticles. The relation was obtained by curve-fitting on experimental data. nf 1 0.986 0.937 ( 0.47 ) 0.703 bf T
)4(
(25o C T 65o C & 0.125% 1.5%) where T, φ respectively represent temperature and solid volume fraction. μnf and μbf are respectively the viscosity of nanofluid and base fluid. Fig. 7 compares the dynamic viscosity ratio calculated from the above experimental relation with experimental results. The dynamic viscosity ratio is shown at a temperature range of 25 to 65 °C. 11
Journal Pre-proof The proposed relation can be used to calculate the changes in the dynamic viscosity within the specified range. These charts clearly show the difference between the results at different temperatures. The deviation margin of the calculated and experimental results is expressed by Eq. (5): nf )exp -( nf ) pred bf Deviation margin(%) bf *100 nf ( )exp bf (
nf )exp and ( nf ) pred respectively show the experimental and calculated (Eq. 4) viscosity bf bf
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where (
)5(
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ratios. Given the wide range of experiments in terms of temperature and volume fraction of
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nanoparticles, the maximum deviation margin equals 1.16%. Fig. 8 shows the deviation margin
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calculated at different temperatures. The results indicate the reasonable accuracy of the proposed
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correlation.
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3.5. Freezing Point and Cloud Point
The use of oils is of great importance in cold weather at the beginning of engine or lubrication
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system operation. Maximum wear occurs when the system fails to operate properly. Thus, engine
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start in cold weather is very critical. Fig. 10 shows the cloud point for the nanolubricant as an important property of oil at low temperatures. The changes in the cloud point of the nanolubricant relative to the base fluid are also shown. According to the results, the performance of the nanolubricant is improved by lowering the oil temperature and formation of a cloud layer or a halo of paraffinic crystals. The nanolubricant samples are still in the fluid phase at this temperature and can thus be used. The best operating conditions for the nanolubricant occurs at the highest concentration, where the performance increases by 22.2%. Fig. 8 shows the freezing point of the nanolubricant. As shown, in the absence of ZnO nanoparticles, the oil is frozen at 17 °C, while the nanoparticles improve the performance of oil. The performance of the engine oil significantly increases with increasing the volume fraction of 12
Journal Pre-proof nanoparticles. The lowest freezing temperature of -20.3 °C was observed at a volume fraction of 1.5 %. The changes in the nanolubricant relative to the base oil indicate significant variations with increasing the volume fraction of nanoparticles. An improvement in the freezing point is useful in cold regions. In addition to other advantages, addition of nanoparticles delay oil freezing by -3.3 °C.
3.6. Flash Point Flash point is the lowest temperature at which the oil is sufficiently evaporated and forms a
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flammable mixture with air so that the oil is ignited for a brief moment in the presence of flame.
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Therefore, oil melting point is a key parameter in determining the operating range of oils. This physical property is a criterion for measuring the flammability and volatility of oil. Addition of
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different volume fractions of ZnO nanoparticles improves the performance of oil at high
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temperatures, leading to an increase in the operating range of oil. Fig. 11 shows the flash point with
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respect to the volume fraction. The largest improvement in the flash point of the nanolubricant relative to the base oil was 7.2%. The flash point, in fact, increases from 235 to 252 °C. Even low
4. Conclusion
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volume fractions of nanoparticles may have a positive impact on the performance of engine oil.
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A nanolubricant containing different volume fractions of ZnO nanoparticles was studied from various aspects. The two-step process is considered a suitable method for preparation of nanolubricants, since the nanoparticles can be perfectly mixed with the base fluid by increasing the base oil temperature and the subsequent reduction in the viscosity. The resulting nanofluid was then homogenized for 60 min with the help of a probe sonication. The stability of the nanolubricant was assessed as acceptable using the DLS test. The results showed the nanofluid to exhibit a Newtonian behavior based on the rheological behavior of the base fluid, which is considered a very important feature. The viscosity of the nanofluid experienced a maximum increase of up to 1.25 times that of the base fluid at 1.5 Vol% of ZnO nanoparticles. On the other hand, the viscosity of both base oil 13
Journal Pre-proof and nano-oil decreased with increasing temperature. The results on cloud point, freezing point and flash point indicated an improvement in the performance of oil at low and high temperatures. In addition to improved oil properties, the operating range of the oil also increased so that the cloud point, freezing point and flash point were respectively improved by 22.2, 19.4 and 7.2% under optimal conditions. According to the results of this study, one can conclude that the use of this nanofluid may improve the performance of engine oils.
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Journal Pre-proof Table 1. Characteristics of SAE50 engine oil. Characteristic
Value
Kinematic viscosity @ 100 °C
1.8×10-5 (m2/s)
Viscosity Index (VI)
90
Flash point
246 (°C)
Pour point
-9 (°C)
Total base number (TBN)
4.1 (mg KOH/g) 0.906 (g/cm-3)
Density @ 15 °C
1900 (J/kg.°C)
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specific heat capacity
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Table 2. Validation of measurements
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20 30 40 50 60
Viscosity (cP) Measurement IAPWS [42] 1.059 1.0016 0.846 0.7972 0.685 0.6527 0.581 0.5465 0.491 0.466
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Temperature (oC)
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Fig. 1. The experimental setup used for measuring the freezing point and pour point of the nanolubricant.
Fig. 2. The experimental setup used to measure freezing and pour points of the nano-oil.
Fig. 3. The average diameter of ZnO nanoparticles in the oil base fluid.
Fig. 4. Results of the behavior of nanolubricant viscosity in terms of shear rate at different temperatures.
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Fig. 5. The effect of volume fraction on viscosity and relative viscosity of nanolubricant.
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Fig. 6. Effect of temperature on viscosity and relative viscosity of Nanolubricants in different volume fractions.
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Fig. 7. Comparison of correlation and experimental results of viscosity for base and nanolubricant fluid.
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Fig. 8. Deviation margin of calculated and experimental viscosities.
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Fig. 9. The cloud point of the nanolubricant and cloud point ratio at different volume fractions.
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Fig. 10. Freezing point of the nanolubricant and freezing point ratio at different volume fractions.
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Fig. 11. The flash point of the nanolubricant and flash point ratio at different volume fractions of ZnO nanoparticles.
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Journal Pre-proof Declaration of interests
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The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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Journal Pre-proof
Highlights Improving the performance of engine oil by adding zinc oxide nanoparticles Measuring the rheological behavior, cloud, freezing and flash points The viscosity increased with increasing concentration up to 25.3% relative to the base fluid. The cloud, freezing and flash points were improved respectively by 22.2, 19.4 and 7.2%.
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The nanofluid is a good alternative to the engine oil base fluid in practical systems.
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Figure 1
Figure 2
Figure 3
Figure 4
Figure 5
Figure 6
Figure 7
Figure 8
Figure 9
Figure 10
Figure 11