Sharif Digital Repository

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... 

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.... 

This article deals with the design and optimization of a cascade thermoacoustic Stirling engine. Different configurations for cascade thermoacoustic Stirling heat engine are investigated. A simplified model is proposed to enable initial design guidelines. The most important characteristics of the cascade system are examined. The results indicate that the thermoacoustic engines in cascade configuration may achieve very low onset temperatures. A suitable configuration is selected and the locations and dimensions of different thermoacoustic units are optimized to achieve low onset temperatures as well as high values for the acoustic power output and efficiency. The results support the... 

Residual stresses resulted from localized non-uniform heating and subsequent cooling during welding processes enact an important role in the formation of cracks and welding distortions and have severe effect on performance of welded joints. The present research performs a three dimensional transient thermo Elasto-plastic analysis using finite element technique to simulate welding process. Welding simulation procedure is developed using the parametric design language of commercial code ANSYS for single pass T and butt welded joints. The procedure verified with predicted residual stress field found in literature to confirm the accuracy of the method. The material of the weld metal, HAZ and the... 

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  

In the present work the production of biodiesel using bitter almond oil (BAO) in a potassium hydroxide catalyzed transesterification reaction was investigated. The BAO was obtained from resources available in Iran and its physical and chemical properties including iodine value, acid value, density, kinematic viscosity, fatty acid composition and mean molecular weight were specified. The low acid value of BAO (0.24. mg. KOH/g) indicated that the pretreatment of raw oil with acid was not required. The fatty acid content analysis confirmed that the contribution of unsaturated fatty acids in the BAO is high (84.7. wt.%). Effect of different parameters including methanol to oil molar ratio (3-11.... 

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  

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... 

Dispersion of heavy gases is considered to be more hazardous than the passive ones. This is because it takes place more slowly. In this paper, based on the extensive experimental work of Hanna and Chang, the CFD model (Ansys-CFX) was tested compared with Kit Fox experiments. In order to accomplish this validation, the multiphase approach was employed as a new method in this area. In addition, the temperature gradient effects were investigated. The survey of wind speed was done taking factors such as time, height and direction into the consideration. To reduce the number of elements in computational domain, a combination of 2D and 3D geometries were utilized. Results showed that the wind... 

Dispersion of heavy gases is considered to be more hazardous than the passive ones. This is because it takes place more slowly. In this paper, based on the extensive experimental work of Hanna and Chang, the CFD model (Ansys-CFX) was tested compared with Kit Fox experiments. In order to accomplish this validation, the multiphase approach was employed as a new method in this area. In addition, the temperature gradient effects were investigated. The survey of wind speed was done taking factors such as time, height and direction into the consideration. To reduce the number of elements in computational domain, a combination of 2D and 3D geometries were utilized. Results showed that the wind... 

We study the natural convection heat transfer in a tilted square cavity with different tilt angles. The cavity is subject to a high gradient temperature resulting in high Rayleigh number flows. The fluid is air and is treated as an ideal gas. The flow is laminar. The fluid properties change with temperature variation using Sutherland's law. Because of imposing large temperature gradients to the two cavity opposite walls, there is substantial density variation in the domain. We use a novel non-Boussinesq algorithm to model the density variation fully. Therefore, the current results are considerably different from those obtained using the classical Boussinesq-based methods, which replace the... 

Here we have optimized shape and location of cooling passages of a C3X turbine blade using a multi-objective strategy. The objective functions is selected to be the maximum temperature gradient and the maximum temperature through the three dimensional blade. Shape of cooling channels is modeled using a new method based on the Bezier curves and using forty design variables. The optimized channel shapes are found to be smooth and without corners. To reduce the computational time, parallel processing and the reduced conjugate heat transfer methodology RCHT is used. Using RCHT, the heat transfer between channels and blade are coupled, while the experimental data is used for heat transfer... 

Installation of a significant number of distributed generators (DGs), besides the application of non-sinusoidal loads such as; Plug-in Hybrid Electric Vehicles (PHEVs), in the emerging smart distribution networks and the industrial plants have posed a major challenge to the existing methods of optimal transformer sizing (OTS). The harmonic currents generated in these new environments not only substantially increase the transformer load losses, but also cause abnormal winding temperature rise and hence transformer excessive loss of life. Therefore, the harmonic contents of the loads currents should be accounted in choosing the appropriate size of distribution transformers. To address this... 

The radiative heat transfer should be calculated very accurately in many combustion domains. This work uses the spectral-line based weighted-sum-of-gray-gases (SLW) model to calculate the radiation heat transfer in a model furnace representing a combustion field with high temperature gradient variations. The past researchers have commonly implemented the reference approach in the SLW model. However, the reference approach produces error because of enforcing a reference temperature to perform the non-gray radiation heat transfer calculations in non-isothermal domains. In this work, we implement a modified reference approach, which reduces such errors in non-gray gas calculations. We show that... 

Temperature gradient on the thick Reactor Pressure Vessel (RPV), caused by sudden overcooling events, especially in the downcomer, would intensify the propagation of structural defects. This situation known as Pressurized Thermal Shock (PTS) could be created in case of Emergency Core Cooling System (ECCS) actuation which leads to injection of cold water into the cold leg of the primary loop in some accidents, e.g. Loss Of Coolant Accident (LOCA). Prediction of Plant response to LOCA and water temperature gradient in the downcomer are performed in thermal-hydraulic section of PTS analysis. Employment of system codes is one of the proposed procedures in literature to obtain plant response and...