Investigation on the Use of Graphene Oxide as Novel Surfactant for Stabilizing Carbon Based Materials

The present work reported on the use of graphene oxide (GO) as effective dispersant to isolate different carbon allotropes. The nature of its chemical structure which consists of hydrophobic and hydrophilic components enables GO to behave as surfactant, paving routes for dissolution of graphitic materials and achieving surfactant free all-carbon solutions. Two additional carboneous materials under the family of fullerene (carbon nanofiber—CNF) and graphite (graphene nanoplatelets—GnP) were introduced within the present study to form a new GO based hybrid complexes on top of the commonly investigated carbon nanotube (CNT) based GO hybrid. Investigation on GO stability with respect to particle size and zeta potential measurements showed that the strength of its dispersibility was highly dependent on its morphological size and less affected by the pH. Rheological study revealed that GO shear–strain relationship is highly sensitive to the particle size. The GO viscosity experienced dramatic changes from Newtonian toward shear thinning behaviors as the particle size increases. Thermal conductivity measurement highlighted as high as 8% increase in magnitude with the addition of CNT, CNF, and GnP carbon constituents, indicating that the enhancement may be attributed to the much efficient thermal transport along the conducting path of pristine carbon allotropes.     

Similarity solutions to viscous flow and heat transfer of nanofluid over nonlinearly stretching sheet

The boundary-layer flow and heat transfer in a viscous fluid containing metallic nanoparticles over a nonlinear stretching sheet are analyzed. The stretching velocity is assumed to vary as a power function of the distance from the origin. The governing partial differential equation and auxiliary conditions are reduced to coupled nonlinear ordinary differential equations with the appropriate corresponding auxiliary conditions. The resulting nonlinear ordinary differential equations (ODEs) are solved numerically. The effects of various relevant parameters, namely, the Eckert number Ec, the solid volume fraction of the nanoparticles φ, and the nonlinear stretching parameter n are discussed. The comparison with published results is also presented. Different types of nanoparticles are studied. It is shown that the behavior of the fluid flow changes with the change of the nanoparticles type.     

Lie symmetry group transformation for MHD natural convection flow of nanofluid over linearly porous stretching sheet in presence of thermal stratification

The magnetohydrodynamics(MHD) convection flow and heat transfer of an incompressible viscous nanofluid past a semi-infinite vertical stretching sheet in the presence of thermal stratification are examined.The partial differential equations governing the problem under consideration are transformed by a special form of the Lie symmetry group transformations,i.e.,a one-parameter group of transformations into a system of ordinary differential equations which are numerically solved using the Runge-Kutta-Gillbased shooting method.It is concluded that the flow field,temperature,and nanoparticle volume fraction profiles are significantly influenced by the thermal stratification and the magnetic field.     

Influence of thermophoretic diffusion of nanoparticles with Joule heating in flow of Maxwell nanofluid

The aspects of activation energy in magnetized Maxwell nanofluid flow with Brownian movement and thermophoretic diffusion have been elaborated here. Furthermore, Joule heating, variable conductivity and chemical reaction are scrutinized. The Buongiorno nanofluid thought is ratify to incorporate the importance of thermophoretic and Brownian diffusion. The attained ODEs have been solved via homotopic algorithm. The performance of operational variables is inspected. The Maxwell temperature field for Eckert number and variable conductivity factor have similar trend. The fluid concentration exaggerates for activation energy and decelerates for Brownian motion parameter. Furthermore, the brilliant outcomes attained and associated with possible existing prose accurately.     

饱和纳米流体多孔介质中竖直嵌入板上边界层的非Newton流

在层流条件下,对饱和多孔介质中的竖直板,研究幂指数型非Newton流的自由对流热交换.非Newton纳米流体服从幂指数型的数学模型,模型综合考虑了Brown运动和热泳的影响.通过相似变换,将问题的偏微分控制方程组,转化为常微分方程组,得到了常微分方程组的数值解.数值解依赖于幂指数n,Lewis数Le,浮力比Nr,Brown运动参数Nb,以及热泳参数Nt.在n和Le的不同取值下,研究并讨论了对相关流体性质参数的影响和简化的Nusselt数.     

Influence of various shapes of nanoparticles on unsteady stagnation-point flow of Cu-H2O nanofluid on a flat surface in a porous medium: A stability analysis

The nanofluid and porous medium together are able to fulfill the requirement of high cooling rate in many engineering problems. So, here the impact of various shapes of nanoparticles on unsteady stagnation-point flow of Cu-H₂O nanofluid on a flat surface in a porous medium is examined. Moreover, the thermal radiation and viscous dissipation effects are considered. The problem governing partial differential equations are converted into self-similar coupled ordinary differential equations and those are numerically solved by the shooting method. The computed results can reveal many vital findings of practical importance. Firstly, dual solutions exist for decelerating unsteady flow and for accelerating unsteady and steady flows, the solution is unique. The presence of nanoparticles affects the existence of dual solution in decelerating unsteady flow only when the medium of the flow is a porous medium. But different shapes of nanoparticles are not disturbing the dual solution existence range, though it has a considerable impact on thermal conductivity of the mixture. Different shapes of nanoparticles act differently to enhance the heat transfer characteristics of the base fluid, i.e., the water here. On the other hand, the existence range of dual solutions becomes wider for a larger permeability parameter related to the porous medium. Regarding the cooling rate of the heated surface, it rises with the permeability parameter, shape factor (related to various shapes of Cu-nanoparticles), and radiation parameter. The surface drag force becomes stronger with the permeability parameter. Also, with growing values of nanoparticle volume fraction, the boundary layer thickness (BLT) increases and the thermal BLT becomes thicker with larger values of shape factor. For decelerating unsteady flow, the nanofluid velocity rises with permeability parameter in the case of upper branch solution and an opposite trend for the lower branch is witnessed. The thermal BLT is thicker with radiation parameter. Due to the existence of dual solutions, a linear stability analysis is made and it is concluded that the upper branch and unique solutions are stable solutions.     

Analysis of periodic and aperiodic convective stability of double diffusive nanofluid convection in rotating porous layer

The onset of periodic and aperiodic convection in a binary nanofluid saturated rotating porous layer is studied considering constant flux boundary conditions. The porous medium obeys Darcy’s law, while the nanofluid envisages the effects of the Brownian motion and thermophoresis. The Rayleigh numbers for stationary and oscillatory convection are obtained in terms of various non-dimensional parameters. The effect of the involved physical parameters on the aperiodic convection is studied graphically. The results are validated in comparison with the published literature in limiting cases of the present study.     

Heat transfer enhancement by combination of serpentine curves and nanofluid flow in microtube

Heat transfer and flow characteristics of Cu/water nanofluids' flow in the serpentine microtubes are investigated experimentally. The serpentine microtubes are fabricated by bending a straight copper microtube with an inner diameter of 787 μm. Also, the Cu/water nanofluids are prepared using a novel one-step technique, namely electro-exploded wire. The effects of serpentine microtubes' geometrical parameters (pitch spacing, p, and straight section, l) and nanofluid concentration (weight fraction, φ) are examined. It is found that the heat transfer enhances by decreasing both the pitch spacing and the straight section of the serpentine microtube as well as increasing the weight fraction of the nanofluid. Also, the results show that the friction factor tends to increase in the same manner. A noticeable average enhancement in the thermal performance factor of 21.8% is obtained for a specific operating condition, i.e., the nanofluid at φ = 0.3% through the serpentine microtube with p = 9.6 mm and l = 10 mm. Finally, two correlations of Nusselt number and friction factor for the Cu/water nanofluids across the serpentine microtubes are proposed.     

Influence of container material on the heat transfer characteristics of nanofluids

The average and local thermal conductivity measurements of water-based Ag-nanofluid held in polypropylene and metallic containers using transient hot-wire method revealed a new phenomenon. The local thermal conductivity of water-based Ag nanofluid measured at different locations of containers was found to depend strongly upon the metallic container, but not on the polypropylene container. Similar observations have been found in water-based NiAl nanofluid, but not in water-based Al₂O₃ nanofluid. In contrast, this phenomenon was not observed for ethylene glycol-based Ag nanofluid, possibly due to the insignificant charge on the container wall, which partly explains the diversity in thermal conductivity by different researchers.