The Effecting of Various Parameters on the Thermophysical and Rheological Properties of SiO 2 Nano-lubricating Oil in Petroleum Refineries

The improvement of the performance of automotive engines necessitated the use of effective lubricating oil. Experimentally explored are the thermophysical characteristics and rheological behavior of Nano-lubricating oil made by mixing SiO 2 nanoparticles with stock-60. Nano-lubricant has been prepared with a two-step method using a magnetic stirrer as a first step mixing and an ultrasonic homogenizer. The rheological properties were investigated at various shear rates, temperature, and solid volume percent. This research was carried out on concentrations ranging from 0.1 % to 1.0 %. The results showed elevation in temperature, the dynamic viscosity dropped in all shear rates due to lower SiO 2 NP concentrations and that all samples exhibited Newtonian behavior at all temperatures fixed. The thermal conductivity of nano-lubricating oil was tested at various temperatures ranging from 20°C to 50°C. The different solid volume percent of SiO 2 NPs were in the range of 0 to 1% maximum enhancement value was achieved at higher temperature with increased volume percent of SiO 2 NPs. Other important thermophysical properties of lube oil including flashpoint and pour point were also measured. The results revealed that adding SiO 2 NPs to stock-60 at a solid volume percent of 1% will result in a maximum improvement of 12 % in flashpoint over the base oil, while the improvement of pour point was achieved at nano-addition of 0.75% and 1% compared with the base oil was increased from -3°C to -6°C.


Introduction
Nanofluids are currently being employed in engine lubrication, rolling processes, ball bearings in engines, and also gearbox lubrication. Nano-lubricants have an influence not only on engine performance and thermal efficiency but also on the engine part's durability and wear. As a result, research into nanofluids utilized as lubricants and coolants under a variety of engine loads and speeds is crucial for enhancing engine performance [1][2][3][4]. Nano-lubricants are used in internal combustion engines. Baskar et al. [5] found that enough nano-lubricating oil reduced journal bearing system friction and wear. According to their ideal results, the coefficient of friction (COF) and specific wear rate was reduced utilizing a load of 100 N, a sliding speed of 2 m/s, and bronze bearing material/SN500 base oil with 0.5wt. % CuO NPs.
Aberoumand and Jafarimoghaddam [6] discovered that adding 1wt. % of Cu NPs to motor oil increased thermal conductivity and viscosity by 49% and 37%, respectively.
Understanding the impact of nano-lubricants on shear rate and shear stress has been aided by the rheological behavior of nano-lubricating oil [7]. Moreover, the viscosity of nanolubricating oil is important for understanding their tribological characteristics. The addition of nanoparticles to engine oil has been shown to minimize friction and wear of the engine parts that come into contact during movement. At temperatures ranging from 20 o C to 60 o C, Aghahadi et al. [8] investigated the rheological behavior of WO 3 -MWCNTs/engine oil Newtonian fluid with varied volume fractions ranging from 0 % to 0.6 %. they noted that the base fluid and nano-lubricating oil are both Newtonian fluids, according to the authors. In addition, when the temperature rises, the viscosity of nano-lubricant reduces, whereas the viscosity rises as the volume fraction of nano-additions rises. Various factors affecting the rheological behavior of nanofluids were examined by Nadooshan et al. [9]. They discovered that size of nanoparticles, volume percent, and shear rate all influence rheological properties.
Kharabati et al. [10] studied the impacts of adding SiO2 NPs on the rheological and thermal characteristics of lubricant at various temperatures in a low range of gasoline and diesel engines with solid fractions varying between 0.05 % and 1%. They found that the viscosity drops in all shear rates when the temperature increases. At 30°C and 1% volume percent, the greatest thermal conductivity value recorded was 0.169 (W/m. K). At various temperatures and solid percentages, Kashefi et al. [11] investigated the rheological behavior and tribological characteristics of a nano-lubricant comprising SiO 2 NPs in engine oil. The authors demonstrated that nano-lubricating oil had a non-Newtonian performance. Additional key variables such as pour point and flash point were also measured, revealing that adding SiO 2 NPs to the lubricating oils at a volume percent of 0.1 % improves the flashpoint by 3.8 % when compared to the initial oil. According to Mousavi et al. [12], adding MoS 2 NPs to diesel oil boosts nano-lubricant viscosity index and viscosity, with the maximum viscosity index and viscosity obtained at a mass fraction of 0.7 weight percent of NPs The results revealed that the additives had no appreciable anti-friction and anti-wear capabilities when compared to the basic oil at various fractions. In addition, as compared to base oil, the MoS 2 NPs in base oil did not affect the friction factor readings. Shafi and Charoo [13] investigated the rheological properties of oil combined with various fractions of ZrO 2 nano-additives and the influence of shear rate, solid portions, and temperatures on oil rheological behavior. The tests are carried out at different temperatures and with different shear rates. At a temperature of 40 °C, the authors reported that a rise in the viscosity of produced nano-lubricating oil with maximal improvement at mass fraction1.5 weight fractions is equivalent to 5.8%. The pure oil and nanolubricant are representations of Newtonian behavior as measured by the power law.
Hu et al. [14] investigated graphite/engine oil viscosity, solid fractions, and shear rates at various temperatures. The findings demonstrated that adding graphite NPS does not influence the viscosity of base oil, indicating that graphite is a suitable NPs addition. When the temperature was decreased, the viscosity increased by 593%. Nano-lubricant behaved as Newtonian fluids in the lower shear rate limit, but as non-Newtonian fluids in the higher shear rate range. The findings showed that solid fraction and temperature have no influence on the power-law index, but that increasing the temperature resulted in a clear decrease in the consistency index. Based on temperature, the volume percentage of NPs, and shear rate, Esfe et al. [15] investigated the viscosity of SAE40 engine oil using MWCNTs and SiO 2 NPs. The nano-lubricant flow regime was discovered to be a non-Newtonian fluid, according to the authors. The nano-lubricant developed the largest increase of 42.53 % at a volume percentage of 1%, a temperature of 45 °C, and a shear rate of 6665/s. The temperature has a greater impact on nano-lubricant viscosity than other parameters. It was also discovered that the viscositytemperature, solid-fraction-temperature relationships are direct, inverse, and direct, respectively. Del et al. [16] used two separate pristine graphene nano-platelets to study the tribological characteristics of nano-lubricating oil. The lateral diameters of these GNPs are 7 and 40 µm, and the thickness is 3 and 10 nm. Various amounts of nano-additives were utilized.
The visual control and the refractive index evolution over time were used to study the stability of nano-lubricants, demonstrating slightly superior stability for nano-lubricants formed with the biggest lateral size graphene nanoplatelets (GNP40). At a temperature range of 0-60°C, Heris et al. [17] evaluated the rheological characteristics of nano-lubricating oil with a dispersion of zinc oxide NPs with varied mass fractions ranging from 0.1 % to 4 %. They demonstrated that nano-lubricating oil and base oil act similarly to Bingham fluid. The influence of particle concentration on the viscosity of nano-lubricants. Rheological tests demonstrated that nanoparticles agglomerate in the lubricating oils in this way. Furthermore, the influence of temperature on the synthesized nano-lubricating oil's viscosity and yield stress.
Esfe et al. [18] investigated the viscosity of an alumina oxide nano-lubricating oil using different solid percentages and temperatures. The nano-lubricants were made in various solid percent at varied working temperatures, and novel correlations for estimating the viscosity of aluminum oxide/ engine oil were provided. They demonstrated that all nano-lubricant samples behave in the same way as a Newtonian fluid. The results indicated that when the solid percentage grows, the viscosity of the nano-lubricant increases. Furthermore, it was discovered that the viscosity of nano-lubricants reduces with rising temperature, and it was more noticeable at lower temperatures. The greatest viscosity improvement of nano-lubricating oil was 132% when compared to base oil, according to the findings. The purpose of this study is to explore the impacts of SiO 2 NPs additions on the rheological behavior, thermal conductivity, flash point, and pour point of base stock-60 oil in an experimental setting.

Materials
In this study, the base stock-60 lube oil was received as products for the vacuum distillation unit from Iraqi refineries (Middle Oil Refining Company, Baghdad, Iraq). It is employed in the experiments as a base fluid. The impacts of SiO 2 nano-additives were compared to the effects of pure stock-60) oil to study the SiO 2 NPs as lubricant additives. The parameters of the employed Iraqi base stock, as well as the test method, were listed in Table (1 Table (2). X-ray (XRD) diffractometer (Schimadzu 7000, Japan) was used to determine the crystalline structure of the samples, scanning electron microscopy was used to examine the morphology (SEM, Quanta FEG 250, FEI, Republic of Czech) and Fourier transform infrared spectroscopy (FTIR) spectroscopic study of the sample was performed by (Tensor 27, Bruker, Germany), using KBr as a reference. The following equation was used to calculate the amounts of SiO 2 NPs required for specific solid volume fractions: The terms (ƿ), (Ք), and (w) density of the nano-lubricant, the particle volume percent of the SiO 2 NPs, and needed mass for the target particle volume percent, respectively, are used in the above equation.
Volume percent of SiO 2 NPs were dispersed in 300 ml of base oil (Ք= 0.1%, 0.25%, 0.5%, 0.75%, and 1%). To reduce the early aggregation of the nano-additions in the base oil, each sample was agitated for 2 hours at room temperature with a continuous stirring at 1600 rpm to guarantee adequate dispersion of the nanoparticles in the base oil. After that, all samples were subjected to 3 hours of temperate sonication [ultrasonic bath, oscillating frequency 20 kHz, power 100 W]. This technique was utilized to break down agglomeration between the particles, resulting in a stable suspension and uniform dispersion.
The samples were placed in closed vials and placed in a 45°C water bath. During the sonication process, there was no substantial change in the temperature of the samples.
Following that, all suspensions were kept in hermetic containers at room temperature. There was no discernible phase separation before and after the rheological measurements. The creation of suspensions with a solid Before and after the rheological tests, there was no obvious phase separation. In all of the conditions tested, creating suspensions with a solid volume percentage of 2% resulted in samples that were no longer liquid but rather pasty and behaved like gels (no flow under the effect of gravity, by turning the vial upside down). As a result, present work is restricted to a solid concentration of less than 1.5 weight percent. Figure (2) shows a picture of SiO 2 NPs, as well as base (stock-60) oil and nano-lubricant samples.

Rheological measurement
The viscosity and shear stress of prepared nano-lubricating oil with nanoparticles concentration ranging from 0 vol.% to 1 vol.% were evaluated using a [Brookfield Viscometer DV3TLV, USA] analysis software device at four temperature range: 40°C, 60°C, 80°C, and 100°C, with shear rates ranging from 6.115/s to 12.236/s. The adaptor for the ULA-49EAY water jacket, a cylindrical experiment container, and the ULA spindle will be utilized as a part of the setup.
The test sample container has a capacity of 16 ml, and the spindle is put within the sample and rotated swiftly to calculate the torque while maintaining a consistent temperature throughout the sample. A temperature sensor will be immersed into the sample to measure the temperature. The Viscometer system has a precision of 1% and repeatability of 0.2%. Because temperature has a direct influence on fluid viscosity, a circulating temperature control system with a 0.1 o C precision was utilized to maintain the temperature of the samples throughout the experiment. For this part, the temperature bath model LAUDA Alpha RA 8 was used. Table   (3) lists some of the data collected.

Thermal conductivity coefficient of SiO 2 nano-lubricant
In the present experimental part, the thermal conductivity of nano-lubricating oils was measured to study their capability to deliver heat transfer. Thermal characteristics based on hotwire technology were characterized using the KD2 Pro, a transportable instrument. This device was located at the Polymer Engineering Department, University of Babylon. This instrument has a probe of 1.3 mm diameter, cable length 50 cm, and 60 mm long. A high accuracy water bath was used for temperature control during each measurement. Figure

Characterization of SiO 2 nanoparticles
FE-SEM analysis was used to examine the morphological properties of SiO 2 nanoparticles, such as particle diameter and morphological parameters. Figure ( stable suspension, these agglomerates must be broken down during the nano-lubricant preparation process. Magnetic and ultrasonic agitation can be used to achieve this. The average particle volume diameter of the SiO 2 NPs was 20-30 nm, according to the morphological examination.

Fig. (4): SEM morphological images of SiO 2 NPs
EDX testing is used to determine the elemental composition of SiO 2 NPs, as illustrated in Figure 5.

Rheological Behavior of Base Oil and Nano-lubricating Oil
The viscosity of base oil (stock-60) and SiO 2 /stock-60 nano-lubricant at five NPs volume   and thermal physical characteristics of lubricating oils since they are standard temperatures.
With rising shear rate and lower operating temperature, the shear tension of base stock-60 increases. this result agrees with much previous work [7,12,13,17], and that it is similar to the base oil, which is consistent with the results of Ranjbarzadeh [22], except for the behavior of nano-lubricant at two concentrations of 0.75% and 1% at temperature 40 o C, which showed non-Newtonian fluid of the Pseudo-plastic fluids. The dynamic viscosity of these fluids will decrease as the shear rate increases.  The non-Newtonian (shear-thinning) behavior may be seen since viscosity decreases as the shear rate increases. Because stress rates impact viscosity, the connection will be non-linear, as seen by power-law index values less than 1 for these nano-lubricant samples. Shear tension will arise in these liquids if they are sheared at low speeds. Shear stress causes the molecules to rearrange to lower overall stress, which is consistent with Kashefi's findings [23].
The results in Figure ( Rising temperature aids particles in overcoming Van der Waals attraction contacts, which may dissolve clusters of nanoparticles floating in the base fluid, reducing viscosity by reducing intermolecular connections between molecules [23][24][25][26]. The Brownian motion will grow as the temperature increases, generating chaos and lowering the viscosity of the nano-lubricating oil [23,25]. suggested that the nano-lubricants with a 0.5% concentration of SiO 2 NPs a suitable for engine lubricity due to enhanced heat transfer properties and thermal conductivity.
One of the variables determining enhanced service life is lubricant oil's heat transferability.
The thermal conductivity of the lube oil is critical for sustaining the lube oil's characteristics and service life [24]. Temperature and thermal conductivity are found to be strongly connected in many studies, with most liquids seeing a drop in thermal conductivity as temperatures rise [25,28,29]. The thermal conductivity of nanofluids increases as the temperature rises, according to previous research [24,[30][31][32][33].

Flash Point and Pour Point of SiO 2 Nano-lubricant
The major distinctive identification for this oil functioning in internal combustion engines under various operating situations is its lubricating oil parameters. When an ignition fire is conducted over this sample, the flashpoint as property usually denotes the minimal temperature at which the vapor above the lubricating oil will quickly ignite [2,3,7].
The connection between the flashpoint of lubricating oil and the concentration of SiO 2 nanoadditions is noted in Figure (12). The mixing of NPs with lube stock-60 oil has resulted in a change in the flashpoint, which causes it to rise until it reaches a substantial level [29,31,33].
With a nano-additive increase, the flashpoint was raised [4,11,12].  In general, managing the quality of nano-lubricating oil is a critical component of improving lubricant specifications for increased energy conservation in the mechanical system inside the engine [3,4,22,23]. After then, the addition of NPs to the base lubricating oil will focus on high-performance operation for a long-lasting engine [4]. The peculiar features of SiO 2 NPs additions are to blame for this. Then, with the addition of SiO 2 NPs nanoparticles, the flashpoint improves, indicating that high-performance operation is possible [4,12,22,28]. The pour point in lubricating oil systems is defined as the temperature limit beyond which liquid cannot travel [22,23]. The largest value of wear on the engine occurs when the internal combustion engine first starts up. When lubricating oil is pumped, it does not reach all areas of the engine quickly, and to avoid and prevent this problem, the lubricating oil must be sufficiently viscous stirred to flow smoothly and rapidly to all sections of the engine [19,22,24,28]. The pour point of lubricating lubricants was next investigated using nano-additions of  interpretations [22][23][24]28]. The pour point values of lubricants are typically in the range of (-2 to -30)°C. The temperature range is determined by the kind of lubricating oil used and the additives used. As a result, when the lubricating oils pour point is reached, the paraffin's compounds solidify at the freezing point, forming a cross-link between them. The lubricating oil seems to be milky in color and cloudy in this situation [25,[31][32][33].
The lubricating oil's viscosity rises until it becomes a semi-solid substance. As a result, when nano-additions of SiO 2 NPs were added to the base stock-60 lubricating oil, the pour point parameters were improved in the current study [4,13,17,22,31].

Conclusions
The additions of SiO 2 NPs to base stock-60 lubricating oil enhanced the rheological behavior and thermophysical properties. The experimental results indicated that the stock-60 lube oil and all nano-lubricating oil samples are to be Newtonian fluids. Also, it was noted that the viscosity was unaffected by the shear rate or the type of nano-lubricant utilized. Moreover, the NPs concentration at a fixed temperature rising the viscosity, and rising the temperature in a