Deep Understanding of the Mechanism and Thermophysical Properties of Prepared Nanofluids Lube Oil Stock-60 with Al 2 O 3 NPs

Iraqi petroleum refineries produce large quantities of base lubricating oils (lube oils). Managing the influence of nano-additives on the lube oil nanofluids is required deep understanding to explain the resulting new specifications of produced nano-lubricants. The present study investigated the effect of Al 2 O 3 NPs addition on the thermal properties of lube oil stock-60. Different mass additions of 0.25, 0.65, 1.05, 1.45, and 1.85 wt.% of Al 2 O 3 NPs at operating temperatures of 20-50°C were evaluated. Also, the thermal conductivity coefficient of the prepared nanofluid was studied at the full range of the experimental temperatures (20-50°C). It was noted that the addition of Al 2 O 3 NPs improved the thermal properties of the prepared nano-lubricant due to the high thermal conductivity of the added Al 2 O 3 NPs. Moreover, the greatest improvement in the thermal conductivity of modified nano-lubricating oil was 13.02% at added Al 2 O 3 mass fraction of 1.85%. The results indicated that the viscosity index of the prepared nano-lubricant was improved dramatically with Al 2 O 3 NPs addition increase at measured standard temperatures of 40 and 100°C. The viscosity index of lubricant nanofluid is increased up to 2.46% at a weight fraction of 1.85%. The flashpoint increased by 1.33, 3.54, 5.75, 7.52, and 9.73% for mass fraction of 0.25, 0.65, 1.05, 1.45, and 1.85 wt.%, respectively. o of nanofluid lube wt.% Al O NPs. Finally, nano-lubricating oil has high feasibility. accurate for predicting the viscosity of both types of nano-lubricants was NPs the results of the XRD Al 2 O 3 the same key characteristics peaks of Al 2 O 3 NPs of PDF-Card-00-048-1548. in at The experimental data then using the curve approach. To forecast the relative viscosity of nano-lubricant, new correlations were developed. Relationship of µ nf in the other direction with temperature. A novel experimental correlation for estimating the relative viscosities of nano-lubricants of each kind of nanomaterial was presented. With acceptable accuracy, the suggested correlation could anticipate nano-lubricant dynamic viscosity.


Experimental Procedure 3.1. Materials
The lube oil base stock-60 was used as the base fluid in the present investigation and obtained from the vacuum distillation unit, Al-Daura Refinery, Baghdad, Iraq. Table 1 summarizes the general specifications of the stock-60 lubricating oil. Also, Al2O3 NPs (with a purity of 99.6 %, density of 2.33 g/cm 3 , a particle diameter of 28 nm, and surface area of 144 m 2 /g) were imported from C.S.M. Tech. Company.

Synthesis of nano-lube oil
The Al2O3 NPs were dispersed in the base lubricating oil (stock-60) to obtain high desperation nano-lubricants. Accordingly, Al2O3 NPs at different nano-additions of 0.25, 0.65, 1.05, 1.45, and 1.85 wt.% were mixed with the base lubricating oil. Then, the required amount of the Al2O3 NPs was carefully weighed using an accurate electronic digital balance and then mixed with the base lubricating oil in two steps. The first one is achieved by combining the produced nanofluid with a mechanical mixer for 30 minutes. In the second step, the nanofluid is exposed to an ultrasonic bath (P120-Elmasonic) for 1 hr. The sonication method was achieved at 40 o C and 20 kHz [16,18,22,31]. Figure 2 summarizes the schematic diagram of the nanofluid (nano-lubricating oil) preparation stages.

Measuring of Viscosity and Viscosity Index
The viscosity index of parent oil and nano-lubricants at different concentrations of NPs was measured using Stabinger viscometer (SVM 3000 where, k is the kinematic viscosity at 100°C of the engine lubricating oil whose viscosity index is to be evaluated.

Thermal Conductivity of Prepared Nano-lube Oil
In the present experimental part, the thermal conductivity of nano-lubricating oils was measured to study their capability to deliver heat transfer. The KD2 Pro, a portable device, was used to characterize thermal properties based on the hot wire technology. 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. Table 2 illustrates the main specifications of the KD2 Pro analyzer, and Figure 3 shows the schematic representation of the thermal conductivity measuring system. The thermal conductivity was tested at different concentrations of Al2O3 NPs (0.25, 0.65, 1.05, 1.45, and 1.85 wt.%) and various operating temperatures of (20,30,40, and 50°C).

Measuring of Flashpoint and Pour Point
The lube oil and nano-lube oil flashpoints were measured using the standard test method of Cleveland open cup for flash and fire points (model: FP92 5G2, USA) according to ASTM D-92 with a maximum error of ±2%. On the other hand, the pour point of nano-lubricating oils was tested using an automatic cloud and pour point device (CPP 5GS, USA) depending on ASTM D-97. Also, the XRD apparatus (Shimadzu 6000) and FE-SEM (JEOL-7610F) are used to identify the structural and morphological specifications of Al2O3 NPs. Figure 4 shows the XRD measurements of the crystalline structures of the Al2O3 NPs. Then, from this figure, it can be noted that the Al2O3 NPs have a high crystalline structure with high purity due to present the general standard peaks in the structure.  Also, the morphological properties specifications of the Al2O3 NPs were analyzed using FE-SEM images. Figure  5 reveals the morphological feature of the Al2O3 NPs. The results indicated that the nanoparticles are spherical. This shape provided excellent rolling specifications for the lubricity of moving parts required for internal combustion engines. Furthermore, the morphological results pointed to that the primary average particles size of Al2O3 NPs was 28 nm. Additionally, it can be observed that most Al2O3 NPs are agglomerated before disperse in the base stock lubricating oil [10,14,27].

Viscosity-Index of Nano-lube Oil
Evaluating the viscosity index (VI) is an essential step to understanding the influence of nanomaterial addition on the rheological behaviour of lubricating oil. This index can be obtained from the results of kinematic viscosity at standard operating temperatures of 40 °C and 100 °C according to equation (1). Table 3 illustrates the changes in the viscosity index with the concentration of added Al2O3 NPs. Then, the results in this table indicated that the viscosity index increased with nano-additions. Accordingly, for both standard temperatures of 40 and 100 °C, it was noted that as the kinematic viscosity of the prepared nano-lubricant increased, the viscosity index of these lubricants was undergone an apparent increase. The maximum viscosity index was at 2.46% at nano-addition of Al2O3 NPs of 1.85 wt.%. Additionally, Figure 6 illustrates the changes in the viscosity index with a concentration of added Al2O3 NPs. The results show that the viscosity of nano-lubricating oil increases with increasing the concentration of added Al2O3 NPs. This behaviour can be attributed to the Brownian motion of nanoparticles that allows collision of nanoparticles and then make the particles narrow to each other. Accordingly, in this case, the system tends to form clusters of Al2O3 NPs attracted to each other due to strong van der Waals forces between them. Then, as the concentrations of nanoparticles recorded higher numbers, more collisions between nanoparticles are produced, and big clusters are formed. Furthermore, an increase of the cluster size leads to a clear increase in the shear stress of the nanofluid and then increases the viscosity of lubricating oil previous investigations by researchers Dalkılıç et al. [25] and Sepyani et al. [28] have validated these findings.

Thermal Conductivity of Nano-lube Oil
The thermal conductivity measurements were achieved to evaluate the ability of Al2O3 NPs to improve the thermal specification of base stock lubricating oil. The results of the measured thermal conductivity of base lubricant and nano-lubricants are shown in Figure 7. The results indicated that the nano-lubricants have higher thermal conductivity values than the base stocks lubricating oil. Then, it can be seen that, with increased Al2O3 NPs additions, the thermal conductivity of the prepared nano-lubricants increased. These results are related to the high thermal conductivity of the Al2O3 NPs compared to that of the parent engine lubricating oil. According to the results of many authors, the properties of the base fluid and the added nanoparticles are the two key factors in determining the final specifications of nano-lubricant [16,22,29]. Also, the results in Figure 7 indicated that the values of thermal conductivity increase with operating temperature increase. For example, the highest thermal conductivity value was recorded at a temperature of 50 o C and nano-addition of 1.85% of Al2O3 NPs. It is essential to mention here that one duty of engine oil is to cool the engine down, and then it is suggested that the nanolubricants with 0.25 wt.% concentration of Al2O3 NPs a suitable for engine lubricity due to enhanced heat transfer properties and thermal conductivity. Dey et al. [14], Ali et al. [17], and Dambatta et al. [20] indicated that the thermal conductivity of a nanofluid is a key factor that determines the lubricity performance of engine lubricating oil. Higher thermal conductivity of a lubricant means the heat transfer process occurs at a greater rate, and better cooling is achieved. The performance of nano-lubricant is related to the concentration, size, shape, and thermophysical properties of added nanomaterials [3,8,30]. The mechanisms of the effect of thermal conductivity on the thermal specification of nanofluids can be related to the Brownian motion of nanoparticles in the system [33][34][35][36][37]. Moreover, according to the collision of theory, the Brownian motion usually improves with thermal conductivity, and a solid-solid conduction heat transfer can be achieved. Also, as the thermal conductivity of nanolubricants increases with motion, the convective heat transfer mechanism will be highly effective [11,20,22]. Finally, the stability of the prepared nano-lubricating oils (nanofluids) was evaluated for 60 days.  In other words, the high stability of nanomaterials within the bulk lubricants is attributed to the high dispersion provided by the ultrasonic mixing step. Accordingly, utilizing the ultrasonic dispersion technique enhanced Al2O3 NPs in the base stock lubricating oil with a more uniform distribution with time. Many publications pointed to the high influence of the sonication method on nanofluid stability [5,8,12,[25][26][27]. Finally, it is essential to mention here that the thermal conductivity of lubricants plays a dominating role in determining their heat transfer and cooling behaviour. However, the low thermal conductivity of conventional base lubricant oils limits their performance. Recent researches show the application of nanoparticles as thermal conductivity improvers. Thus, nanoparticles have played an important role in improving the efficiency of internal combustion engines, reducing maintenance requirements and more extended service periods for lubricants, and improving fuel economy in automotive [21,30,42].

Evaluation of Flashpoint and Pour Point
Flashpoint and pour point determine the high and low-temperature abilities of lubricants. Flashpoint is the temperature at which lubricant vapour starts burning and then gets extinguished, thus limiting its high-temperature abilities [1][2][3][4][5][6]. From an operating point of view, the flashpoint is a major property for lubricating oil inside the internal combustion engine due to its direct interaction with operating and environmental temperatures. Table 4 exhibits the values of flashpoints at five various mass fractions. This table shows that the flashpoints of nanolubricants were enhanced dramatically with the Al2O3 NPs additions. Then, the flashpoint increased by 1.33, 3.54, 5.75, 7.52, and 9.73% for nano-additions of 0.25, 0.65, 1.05, 1.45, and 1.85 wt.%, respectively. Also, it was seen that the highest flashpoint value was 248 o C at Al2O3 NPs addition of 1.85 wt.%. Accordingly, in this case, the ability of lube oil toward ignition increased due to increasing thermal conductivity with a concentration of nanoadditions. Consequently, the improved flashpoint can be considered an advantage in improving the lubricity of base lube oil. This is in good agreement with previous research [7,8,18,[33][34][35][36][37][38]. Additionally, the pour point refers to the lowest temperature at which oil stops flowing and thus becomes important for low-temperature surroundings. Table 5  Al2O3 NPs inside the engine. Therefore, temperature reduction decreases the convenient movement of nanoparticles; furthermore, the effectiveness of the nanoparticles decreases due to the agglomeration of the nanoparticles at higher concentrations. Moreover, nano-addition of more than 1.05 wt.% of Al2O3 NPs is not favourable due to no higher change in the pour point value have been observed. It is essential to mention that as the operating temperature was reduced, the lubricating oil motion will be restricted with the increasing nanoaddition due to agglomerated NPs.

Effect of Temperature and Volume Concentration on Relative Viscosity of Nano-lubricant
The relative viscosity ( μ nf μ Bf ) of nanofluids may be used to indicate the fluctuation of dynamic viscosity of nanolubricants at different temperatures for both nano-additives. For example, Figure 9 shows the relative viscosity of Al2O3 nano-lubricants at different temperatures (40°C, 60°C, 80°C, and 100°C), as well as different Al2O3 solid fractions. As seen in Figure 9, the relative viscosities vary significantly as temperature changes. Also, Figure 9 demonstrates that when the temperature rises from 40 o C to 60 o C, the relative viscosity of Al2O3 nano-lubricant with various five mass fractions decreases significantly, but relative viscosity values soon begin to fall increase as the temperature rises to 80 o C and 100 o C. When the temperature rises above 80 o C, the agglomeration of spherical nanoparticles of alumina oxide, which are perpendicular to the direction of the fluid flow during the viscosity measurement process and thus work to increase the relative viscosity compared to the base fluid, rather than decrease, where the nanospheres work on sliding the fluid layers over each other, causes the relative viscosity to increase. The aggregation disintegrates due to weak intermolecular adhesion forces [48][49][50]. Furthermore, Minitab optimization and statistical program version 19 was used to fit the curves of nano-lubricant experimental data. Then, the following predicted mathematical correlation equations (number 4) were achieved to represent the present experimental results. This equation is dependent on the temperature and volume fraction, and the viscosity of the Al2O3 nano-lubricant. µr = 13.321-10.11 T 0.042 + 0.251 ϕ 1.47 (4) Where µr, ϕ, and T are the relative viscosity of Al2O3 nano-lubricant, mass concentration, and temperature, respectively. These correlations may be used at temperatures ranging from 40 to 100 °C, with mass fractions ranging from 0.25 to 1.85 %. To test the correctness of the correlation above, Figure 10 compares experimental data with data obtained by the proposed correlation at various temperatures. The absence of variation between these two data groups implies that the recommended correlation for predicting these nano-viscosity lubricants is pretty accurate.

Conclusions
The rheological and thermal characteristics of base stock-60 lubricating were improved successfully by adding Al2O3 NPs. The kinematic viscosity of nano-lubricating oil showed an apparent increase with increasing mass fraction of Al2O3 NPs additions. Also, the viscosity index rises dramatically with the rise in the concentration of Al2O3 NPs as measured at standard temperatures of 40 and 100 o C. The highest value of viscosity index was 2.46% at Al2O3 NPs addition of 1.85wt.%. Moreover, the experimental results indicated that the thermal conductivity of nano-lubricating oil increased as the operating temperature increased with nano-addition. Furthermore, it is suggested that the nano-lubricants with nano-addition of 0.25 wt.% are more suitable for engine lubricity due to enhanced heat transfer properties and thermal conductivity. The flashpoint indicates the quality and operating of the lubricant oil in hot seasons and especially at the highest temperatures. By adding nanoparticles, the flashpoint increment from 228 o C to 248 o C. This advantage enhances the nano-lubricant oil performance at high temperatures. The results of the pour point show that the lube oil performance has improved in lower temperatures. The best mass fraction of nano additives was 1.05 wt.%, giving a pour point of -5.4 o C with an enhancement of 80%. This fraction represents a good nano-addition with no agglomeration of alumina oxide inside the engine.
Under these conditions, this nano-lubricant can be used in state base oil at lower temperatures. The experimental data were then fitted using the curve fitting approach. To forecast the relative viscosity of nano-lubricant, new correlations were developed. Relationship of µnf in the other direction with temperature. A novel experimental correlation for estimating the relative viscosities of nano-lubricants of each kind of nanomaterial was presented. With acceptable accuracy, the suggested correlation could anticipate nano-lubricant dynamic viscosity. The correctness of the recommended models was determined by comparing the data to the correlation outputs.