Sharif Digital Repository

Nonequilibrium molecular dynamics simulations are used to study the motion of a C60 molecule on a graphene sheet subjected to a temperature gradient. The C60 molecule is actuated and moves along the system while it just randomly dances along the perpendicular direction. Increasing the temperature gradient increases the directed velocity of C60. It is found that the free energy decreases as the C60 molecule moves toward the cold end. The driving mechanism based on the temperature gradient suggests the construction of nanoscale graphene-based motors  

This article applies a novel non-Boussinesq numerical algorithm to solve the free-convection problem in a wide range of thin to thick vertical cavities subject to different side-wall temperatures. In this regard, the compressible flow equations are solved using a primitive incompressible method. No Boussinesq approximation and low Mach number consideration are included in the formulation. To implement the compressibility effect, the density field is calculated via the equation of state for gas. The temperature gradient is suitably varied to generate different low to high thermobuoyant fields, where the Boussinesq approximation may or may not be valid. Contrary to published works on the thin... 

For the production of synthesis gas utilised in Midrex direct reduction plants, catalytic steam/CO2 hydrocarbon reforming in tubular reformer is the major process. Owing to the high heat input through the Midrex reformer tube wall, the endothermic nature of reforming reactions, low mass velocity of feed gas and large tube diameter, the catalyst bed is exposed to considerable axial and radial temperature gradients. These radial concentration and temperature gradients may create local areas with potential for carbon formation. To investigate this phenomenon, a rigorous two-dimensional model is developed for simulating the operation of a Midrex reformer which applies a dual catalyst loading... 

The effect of temperature, pressure and bonding time on microstructure of temperature gradient transient liquid phase (TGTLP) diffusion bonded Al7075 alloy using liquid gallium interlayer was investigated. The selected bonding method relies on using the minimum amount of liquid gallium on faying surfaces, using a very fast heating rate to reach the joining temperature and imposing a temperature gradient across the bond region. The microstructure of the diffusion bonded joints was evaluated by light optical microscopy, scanning electron microscopy and energy dispersive spectroscopy (EDS). Results show that by increasing the temperature, pressure and time of joining, a more uniform... 

In this work, we study the gas mixing behavior in a micro T-mixer using the direct simulation Monte Carlo (DSMC) method. The gas mixing process is monitored through a T-mixer, which is fed by two different CO and N2 gases; flowing into the T-mixer through the upper and lower inlets. We investigate the effects of axial and lateral wall temperature gradients on the mixing evolution at different rarefaction levels. The achieved results show that any temperature difference between the channel walls would result in an increase in mixing length for the chosen wall temperature gradient ranges and the studies pressure cases. Our observations show that a positive temperature gradient toward the... 

We study numerically the heat transfer of steady laminar flow in a square cavity filled with electrically conducting fluids, in the presence of an external uniform magnetic field. Imposing a large temperature gradient between two opposite vertical walls, there are substantial temperature and density variations in the domain. The fluid is treated as an ideal gas. Indeed, high temperature gradient thermo-buoyant cavity flows result in natural convection flow domains with high Rayleigh number. To implement the temperature variation effect, the fluid properties, including the conductivity and viscosity coefficients, are considered to vary with temperature in accordance to the Sutherland's law.... 

We use three-dimensional molecular dynamics simulation to investigate the driven flow between two parallel plates separated by argon atoms. Our simulations show that fluids in such channels can be continuously driven. Difference in surface wettability can cause a difference in fluid density along the nano channel. To control the nanochannel temperature walls, we use the thermal wall idea, which models the walls using atoms connected to their original positions by enforcing linear spring forces. In this study, we propose a nanochannel system in which, half of the channel has a low surface wettability, while the other half has a higher surface wettability and that the middle part of channel... 

In this article, an analytical elastic-plastic solution for thick-walled cylinders made of Functionally Graded Materials (FGMs) subjected to internal pressure and thermal loading is presented. Based on the experimental results, a mathematical model to predict the yielding through the thickness of FG AlA359/SiCp cylinder is developed. It is shown that under the temperature gradient loading, there is a point in the cylinder where the circumferential stress changes from compressive to tensile. The position of this point depends on the geometry and material properties of the FG cylinder and is independent of the temperature gradient  

Transient liquid phase (TLP) bonding of Al2024-T6 alloy, using gallium (Ga) interlayer, has been investigated. Bonding process was carried out at 470°C for 6 min, and homogenising temperature and time were 495°C and 2 h respectively. Conventional TLP bonding using Ga interlayer was not an appropriate method for joining of Al2024. In this method, the boundary between two Al2024 specimens was not fully eliminated during bonding because of solidification with planar front. In addition, bonding zone was depleted of copper, and as a result, tensile and shear strength of joint decreased to 200 and 110 MPa respectively. TLP bonding under temperature gradient offered very good results in bonding of... 

Abstract Fatigue properties of Al7075-T6 alloy joined by temperature gradient transient liquid phase (TGTLP) diffusion bonding using liquid gallium interlayer was investigated. The fatigue specimen was jointed at 460 °C under 10 MPa pressure for 10 min. The TLP bonded samples were homogenized at 465 °C for 2 h and then T6-heat treated. The fatigue life of Al7075-T6 alloy was determined as 107 cycles under 90 MPa while the fatigue life of TLP bonded sample under this stress amplitude was 1.2×106 cycles, which is about 10% of the total Al7075 alloy fatigue life. The fatigue fracture surfaces of Al7075 sample and TGTLP bonded sample were studied using scanning electron microscope to... 

In this article, experimental and numerical investigations are performed to study a thermogravitational column (TGC) for the separation of toluene/n-heptane mixture. This research has tried to determine the main significant parameters and their effects on the performance of the process. In experimental examinations, the influence of the main parameters such as feed flow rate, cut and temperature gradient on the performance of the TGC efficiency is studied. In addition, computational fluid dynamics is used to simulate the separation process in this review. The response surface methodology (RSM) was also applied to minimize the number of runs and investigate the optimum operating conditions.... 

In the selective laser sintering (SLS) method, layers of powder are scanned by a laser beam and sintered. The thermal gradients created by laser heating and the subsequent cooling of the sintered sections results in thermal stresses and part warping in the final part. Thermal gradients are dependent on the scanning algorithm, in particular, the scan vector length. In this work, an efficient scanning algorithm for the SLS process is presented with the aim to minimise the part warping in the final part due to thermally induced residual stresses, while maintaining the production time at a minimum. The proposed algorithm is implemented in a finite element simulation and scanning parameters... 

This is the first research on the nonlinear frequency analysis of a multi-scale hybrid nanocomposite (MHC) disk (MHCD) resting on an elastic foundation subjected to nonlinear temperature gradient and mechanical loading is investigated. The matrix material is reinforced with carbon nanotubes (CNTs) or carbon fibers (CF) at the nano- or macroscale, respectively. We present a modified Halpin–Tsai model to predict the effective properties of the MHCD. The displacement–strain of nonlinear vibration of multi-scale laminated disk via third-order shear deformation theory (TSDT) and using Von Karman nonlinear shell theory is obtained. Hamilton’s principle is employed to establish the governing... 

One important heat transfer application in engineering is to predict the flow behavior and heat transfer rate in thin vertical air layers. There are numerous applications in engi- neering where high temperature gradients exist between the slot walls. In such cases, the methods based on simple Boussinesq approximations do not provide reliable predictions. Unfortunately, the compressibility effect in heat transfer rate through thin vertical slots has not been much investigated by the past investigators. In this work, a compressible algorithm is properly developed and utilized to solve compressible natural convection in vertical air layers. The current technique employs discretization equations... 

A numerical analysis has been performed to investigate the ground effects on the main parameters of a two-phase unconfined cloud of fuel and air to study its detonability. Equivalence ratio, turbulence intensity, cloud shape and volume, uniformity, temperature gradient and delay time distribution are the most important factors that affect the detonability of a vapor cloud. The effects of the altitude of the dispersing device from the ground on these significant factors have been demonstrated. A modified KIVA-based program has been employed to carry out the computations. A finite volume method is used to solve the equations describing the gas phase. A discrete particle technique is applied to... 

The application of the multi-component Fe-based filler metals (FMs) for transient liquid phase (TLP) bonding of AISI 304 austenitic stainless steel has been overshadowed by dissimilar interlayers merely due to their shorter isothermal solidification time. However, the latter usually suffers from low efficiency in terms of mechanical properties even after homogenization of heat treatment. This study shows that by imposing a temperature gradient across the bond line during the TLP bonding process (TG-TLP), it is possible to reduce the isothermal solidification time significantly. This renews the interest in utilizing multi-component Fe-based FMs. In this regard, the TG-TLP bonding process was... 

In this work, we aim to study the effect of temperature gradients on the combined natural convection-radiation heat transfer problem in participating media. To impose this combined effect, we first solve the radiative transfer equation in an absorbing and emitting media. Then, we suitably add the radiation source terms to the energy equation and solve the fluid flow equations. Literature shows that many incompressible algorithms use the Boussinesq assumption to model the thermobuoyant force; however, the validity of this assumption is limited to cases with low temperature gradient distributions. Evidently, Boussinesq assumption would result in considerable errors in high temperature gradient... 

Field evidence indicates that the thermal and chemical regimes in wellbore considerably affect the wellbore stability. This study presents a general coupled model for transport of solute, solvent and heat including their combined effects on the wellbore stability. Optimization of drilling fluid parameters is crucial for wellbore stability analysis particularly in high pressure-high temperature environments. The coupled effects of chemical potential and temperature gradients on fluid flow significantly change the pore pressure and stress around a borehole. The effects of various parameters such as mud weight, solute concentration gradient, shale properties, and temperature gradient on... 

Governing equilibrium equations of thick-walled spherical vessels made of material following linear strain hardening and subjected to a steady-state radial temperature gradient using elasto-plastic analysis are derived. By considering a maximum plastic radius and using the concept of thermal autofrettage for the strengthening mechanism, the optimum wall thickness of the vessel for a given temperature gradient across the wall thickness is obtained. Finally, in the case of thermal loading on a vessel, the effect of convective heat transfer on the optimum thickness is studied and a general formula for the optimum wall thickness and design graphs for several different cases are presented. © 2008... 

In this work, the rising of a single bubble in a quiescent liquid under microgravity condition was simulated. In addition to general studies of microgravity effects, the initiation of hydrodynamic convection, solely due to the variations of interface curvature (surface tension force) and thus the generation of shearing forces at the interfaces, was also studied. Then, the variation of surface tension due to the temperature gradient (Marangoni convection), which can initiate the onset of convection even in the absence of buoyancy, was studied. The related unsteady incompressible full Navier-Stokes equations were solved using a finite difference method with a structured staggered grid. The...