Enhanced thermophysical properties via PAO superstructure

Abstract

For the last few years, molten salt nanomaterials have attracted many scientists for their enhanced specific heat by doping a minute concentration of nanoparticles (up to 1% by weight). Likewise, enhancing the specific heat of liquid media is important in many aspects of engineering such as engine oil, coolant, and lubricant. However, such enhancement in specific heat was only observed for molten salts, yet other engineering fluids such as water, ethylene glycol, and oil have shown a decrease of specific heat with doped nanoparticles. Recent studies have shown that the observed specific heat enhancement resulted from unique nanostructures that were formed by molten salt molecules when interacting with nanoparticles. Thus, such enhancement in specific heat is only possible for molten salts because other fluids may not naturally form such nanostructures. In this study, we hypothesized such nanostructures can be mimicked through in situ formation of fabricated nano-additives, which are putative nanoparticles coated with useful organic materials (e.g., polar-group-ended organic molecules) leading to superstructures, and thus can be directly used for other engineering fluids. We first applied this approach to polyalphaolefin (PAO). A differential scanning calorimeter (DSC), a rheometer, and a customized setup were employed to characterize the heat capacity, viscosity, and thermal conductivity of PAO and PAO with fabricated nano-additives. Results showed 44.5% enhanced heat capacity and 19.8 and 22.98% enhancement for thermal conductivity and viscosity, respectively, by an addition of only 2% of fabricated nanostructures in comparison with pure PAO. Moreover, a partial melting of the polar-group-ended organic molecules was observed in the first thermal cycle and the peak disappeared in the following cycles. This indicates that the in situ formation of fabricated nano-additives spontaneously occurs in the thermal cycle to form nanostructures. Figure of merit analyses have been performed for the PAO superstructure to evaluate its performance for heat storage and transfer media.

Understanding the Specific Heat Enhancement in Metal-Containing Nanofluids for Thermal Energy Storage: Experimental and Ab Initio Evidence for a Strong Interfacial Layering Effect

“…Specific heat represents the capacity of the fluid to exchange sensible heat given a temperature change, and it is a desirable property for HTF to be used in CSP. , A higher specific heat implies that a more thermal energy per unit volume can be directly stored by the fluid, without implicating a secondary medium for thermal energy storage, and higher rates for convection heat transfer from the collector, which improves its thermal and hydraulic performance. , Previous research papers have reported either an increase − or decrease − in isobaric specific heat by dispersing a small load of nanoparticles within the fluid, thereby generating a nanofluid . The ability to tune the specific heat of the fluids has suggested potential applications in the field of CSP. , Current numerical models do not provide an adequate description of the variation of isobaric specific heat in nanofluids with respect to the base fluid, as they always predict increases or decreases, regardless.…”

An overall review on the tribological, thermal and rheological properties of nanolubricants

“…The percentage enhancement, i.e. increase in thermal conductivity is depicted in Figure 3 [16,21,[156][157][158].…”

“…Specific heat of oil increases from 1.331 at 0 wt.% to 1.424 at 0.5 wt.% concentration, afterwards a decrease in specific heat is observed. Pournorouz et al [158] mixed silica nanoparticles (10 nm), polyethylene-block-poly (PBP) with PAO-8 and studied the thermo-physical properties of synthetic oil. It is observed that the heat capacity of the oil with PBP and silica remarkably increases by 44.5%.…”

“…The variation of viscosity with shear rate, shear stress and temperature determines the flow behaviour of fluids. The percentage increase in viscosity is depicted in Figure 4 [13,158,[176][177][178][179][180][181][182]. A fluid is said to be Newtonian if the viscosity is constant with varying shear rate where fluid is non-Newtonian, if viscosity increases or decreases with shear rate.…”

“…The percentage enhancement, i.e. increase in thermal conductivity is depicted in Figure 3 [16,21,156–158].

Figure 3 Increase in thermal conductivity with the addition of nanoparticles.

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