Nano-oils comprising stable and dilute dispersions of synthesized Graphene (Gr) nanoflakes and carbon nanotubes (CNT) have been experimentally observed for the first time to exhibit augmented dielectric breakdown strengths compared to the base transformer oils. Variant nano-oils comprising different Gr and CNT samples suspended in two different grades of transformer oils have yielded consistent and high degrees of enhancement in the breakdown strength. The apparent counter-intuitive phenomenon of enhancing insulating caliber of fluids utilizing nanostructures of high electronic conductance has been shown to be physically consistent thorough theoretical analysis. The crux mechanism has been pin pointed as efficient charge scavenging leading to hampered streamer growth and development, thereby delaying probability of complete ionization. The mathematical analysis presented provides a comprehensive picture of the mechanisms and physics of the electrohydrodynamics involved in the phenomena of enhanced breakdown strengths. Furthermore, the analysis is able to physically explain the various breakdown characteristics observed as functions of system parameters, viz. nanostructure type, size distribution, relative permittivity, base fluid dielectric properties, nanomaterial concentration and nano-oil temperature. The mathematical analyses have been extended to propose a physically and dimensionally consistent analytical model to predict the enhanced breakdown strengths of such nanooils from involved constituent material properties and characteristics. The model has been observed to accurately predict the augmented insulating property, thereby rendering it as an extremely useful tool for efficient design and prediction of breakdown characteristics of nanostructure infused insulating fluids. The present study, involving experimental investigations backed by theoretical analyses and models for an important dielectric phenomenon such as electrical breakdown can find utility in design of safer and more efficient high operating voltage electrical drives, transformers and machines.
We report a systematic study on the role of Marangoni convection on the evaporation kinetics of pure water drops, considering the influence of heating regime and surface wettability. The Marangoni flows were induced via heating under constant wall temperature (uniform heating) and constant heat flux (local heating) regimes below the drops. To visualize the thermal patterns/flows emerging within the water drops we employed infrared (IR) thermography and we captured the evolution of the drop profile with a CCD camera to follow the evaporation kinetics of each drop. We observed a strong correlation between the temperature difference within the drop and the evolution of drop shape during different modes of evaporation (i.e. constant radius, angle or stick-slip) resulting in different Marangoni flow patterns. Under uniform heating, stable recirculatory vortices due to Marangoni convection emerged at high temperature which faded at later stages of the evaporation process. On the other hand, in the localized heating case, the constant heat flux resulted in a rapid increase of the temperature difference within the drop capable of sustaining Marangoni flows throughout the evaporation. Surface wettability was found to also play a role in both the emergence of the Marangoni flows and the evaporation kinetics. In particular, recirculatory flows on hydrophobic surfaces were stronger when compared to hydrophilic for both uniform and local heating. To quantify the effect of heating mode and the importance of Marangoni flows, we calculated the evaporative flux for each case and found to it to be much higher in the localized heating case. Evaporative flux depends on both diffusion and natural convection of the vapor phase to the ambient. Hence, we estimated the Grashof number for each case and found a strong relation between natural convection in the vapor phase and heating regime or Marangoni convection in the liquid phase. Subsequently, we demonstrate the limitation of current diffusion-only models describing the evaporation of heated drops. 32 90°, the CCA mode of evaporation is reported and the 33 decrease in weight/volume is observed to be non-linear. 34 Further, the decrease in volume according to a power law 35 is reported for drops evaporating on hydrophobic and su-36 perhydrophobic surfaces [8, 9]. Apart from the extreme 37 modes of evaporation (CCR and CCA), a stick-slip mode 38 of evaporation with repetitive cycles of stick and slip of 39 the contact line is observed for pure fluids [10] and also 40 for colloidal suspensions [11]. The strong influence of 41 substrate wettability [8, 12], shape of the sessile drop 42 [13], ambient conditions [14-16] and substrate proper-43 ties [17, 18] on the evaporation process are extensively 44 reported. 45 For a sessile drop in contact with a solid substrate, 46 the evaporative flux at the liquid-vapor interface is non-47 uniform and depends on the drop shape [19]. The evap-48 orative flux is higher near the contact line for drops with 49 contact angles less than 90°, whereas for d...
A thermal transport mechanism leading to the enhanced thermal conductivity of Graphene nanofluids has been proposed. The Graphene sheet size is postulated to be the key to the underlying mechanism. Based on a critical sheet size derived from Stokes-Einstein equation for the poly-dispersed nanofluid, sheet percolation and Brownian motion assisted sheet collisions are used to explain the heat conduction. A collision dependant dynamic conductivity considering Debye approximated volumetric specific heat due to phonon transport in Graphene has been incorporated. The model has been found to be in good agreement with experimental data.
The viscosity of Graphene nano-sheet suspensions (GNS)and its behavior with temperature and concentration have been experimentally determined. A physical mechanism for the enhanced viscosity over the base fluids has been proposed for the poly-dispersed GNSs. Experimental data reveals that enhancement of viscosity for GNSs lie in between that of Carbon Nanotube Suspensions (CNTSs) and nano-Alumina suspensions (nAS) , indicating the hybrid mechanism of percolation (like CNTs) and Brownian motion assisted sheet dynamics (like Alumina particles). Sheet dynamics and percolation, along with a proposed percolation Network Dynamicity Factor; have been used to determine a dimensionally consistent analytical model to accurately determine and explain the viscosity of poly-dispersed GNSs. It has been hypothesized that the dynamic sheets behave qualitatively analogous to gas molecules. The model alsoprovides insight into the mechanisms of viscous behavior of different dilute nanoparticle suspensions. The modelhas been found to be in agreement with the GNS experimental data, and even for CNT and nano-Alumina suspensions.
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