Sharif Digital Repository

A very simple and computationally low cost numerical algorithm is developed to generate a quasi-structured data structure for an unstructured grid. To achieve this purpose, the data structure in the matrices of an unstructured grid is classified to address the element layers and node lines in the computational domain. In this regard, elements and nodes of the unstructured grid are renumbered in a directional ordering-based strategy. The elements and nodes arrangement in each layer and line is accomplished in a unique direction around an interior object of the grid either clockwise or counterclockwise. Furthermore, a new searching scheme is introduced which guarantees a quick search inside... 

In this article, NO production mechanisms for CH 4-H 2 combustion under MILD (moderate or intense low-oxygen dilution) conditions are studied using CDF and also zero-dimensional well stirred reactor (WSR) analysis. A H 2/CH 4 jet into a heated and diluted coflow is modeled in CFD analysis. The RANS equations with modified k equations are solved in an axisymmetric 2D computational domain. The GRI2.11 full mechanism is considered to represent the chemical reactions. The effects of oxidizer oxygen concentration, fuel hydrogen content, and fuel jet Reynolds number are studied on NO formation reactions. Results show that the measurements are predicted with an acceptable accuracy. The NNH and N 2O... 

This research investigated the forced convection heat transfer by using the swirling impinging jets. This study focused on nozzles, which equipped with twisted tapes via a numerical approach. The computational domain created by utilizing the fully structured meshes, which had very high quality from the viewpoint of aspect ratio and skewness. The numerical simulations were performed at four different jet-to-plate distances (L/D) of 2, 4, 6 and 8, four Reynolds numbers of 4000, 8000, 12,000 and 16,000, and also four different twist ratios (y/w) of 3, 4, 5 and 6. The mesh-independent tests were conducted based upon the average Nusselt number. The obtained results revealed good agreement with... 

This paper demonstrates a numerical study on the combination of Oxy-Fuel and MILD (moderate or intense low-oxygen dilution combustion) combustions, i.e. OXY-MILD. The N2 of a hot oxidizer was replaced with CO2 and H2O in a MILD combustion test case. The study was conducted using a CFD analysis, a zero-dimensional well-stirred reactor analysis, and a reactors network analysis. In the CFD analysis, RANS equations with modified k-ε equations were solved for a 2D-axisymmetric computational domain. Results showed a decrease in temperature gradient, reaction rate, and Damköhler number under the OXY-MILD condition in comparison with the MILD one. It seems the higher the oxygen level in the... 

A brass vortextube is used to carry out a series of experiments. The main objective of the present research is to investigate the separation performances of a vortex tube (VT) for a hydrocarbon mixture. Examination is also applied to study the effects of nozzle intakes number and cold fraction on the gas species separation at specific inlet pressure 236.37 kPa in a VT with two gas mixtures (LPG as a hydrocarbon mixture and LPG-N2). A two-dimensional computational fluid dynamic (CFD) model simulation of a VT is presented. CFD code after validation is also applied to investigate the role of cold fraction and nozzle intakes number on the gas species separation. The highly rotating flow field... 

Performance of polymer electrolyte membrane fuel cells (PEMFC) at high current densities is limited to transport reactants and products. Furthermore, large amounts of water are generated and may be condensed due to the low temperature of the PEMFC. Development of a two-phase flow model is necessary in order to predict water flooding and its effects on the PEMFC performance. In this paper, a multiphase mixture model (M2) is used, accurately, to model two-phase transport in porous media of a PEMFC. The cathode side, which includes channel, gas diffusion layer (GDL), microporous layer (MPL), and catalyst layer (CL), is considered as the computational domain. A multidomain approach has been used... 

Reduction of air pollutants formation from hydrocarbon combustion process requires improvements in combustion systems. The moderate and intense low oxygen dilution (MILD) combustion technique is an opportunity to achieve such a goal. MILD combustion is a combustion regime which can be attained by high temperature preheating and high level dilution. In this paper, the mechanism of CO and CO2 formation for a CH4/H2 fuel mixture is studied under MILD combustion condition of a jet in hot coflow (JHC) burner. This investigation is done using the computational fluid dynamics (CFD) and also zero dimensional well-stirred reactor (WSR) analysis. The RANS equations with modified k-ε equations are... 

A general method for deriving electromagnetic particle in cell (EMPIC) algorithms has been given by Eastwood [Comput. Phys. Commun. 64, 252 (1991)]. This method devises variation of the action-integral to find discrete governing equations. The most important advantage of this method is automatic inclusion of the time coordinate via the action integral into the computational domain. This inclusion is inevitable because electromagnetic algorithms are based on time evolution of the system from its initial state. The drawback of this method is that it is rather abstract. This causes obscurity of particle-mesh interactions and makes it hard to analyze physical treatments of the computational... 

This paper presents an advanced grid adaptation strategy to be used by unstructured grid users. The idea behind this strategy originates from the need for automatic control of computational grids during iterative procedures utilized by fluid flow solvers. This strategy eliminates unnecessary grid computations by dividing the unstructured grid into active and inactive zones automatically. The active zones are extended automatically in order to capture the propagation of disturbances in solution domain. In this work, we focus to solve the grid deformations which are imposed in some portions of the main body and are propagated into computational domain during the iterative solutions. To achieve... 

Moderate or Intense Low-oxygen Dilution (MILD) combustion which known as a combustion under highly preheated and diluted condition is a relatively new combustion regime including many differences in comparison with the traditional one. Among many distinguished disparities between combustion modeling of conventional and MILD ones, turbulent-chemistry interaction modeling is an open field of study. Current knowledge on modeling this combustion regime shows that the eddy dissipation concept (EDC) model could be successfully used for modeling of this combustion regime. In this paper, it has been tried to propose some guidelines to modify the semi-empirical constants of the original EDC model for... 

One promising solution to achieve moderate or intense low-oxygen dilution (MILD) combustion under lower temperatures may be using non-equilibrium plasma discharge. Plasma assisted combustion can extend the flammability limits by reducing the auto-ignition temperature of the reactants and can be a great approach to achieve steady MILD combustion under extreme conditions through decreasing ignition delay time. In particular, the influence of the outlet flows from a co-flowing cylindrical dielectric barrier discharge (DBD) on the CH4/N2 combustion under MILD combustion has been investigated numerically through some simplifications on plasma discharge. A three streams coaxial burner are modeled... 

Numerical simulation of dam-foundation-fault system, considering the earthquake source, propagation path, and local site effects, was carried out for realistic and reasonable seismic safety analysis of concrete dams. The Domain Reduction Method (DRM) was used for seismic analysis of Dam-Foundation-Fault (DFF) system, in which a modular two-step methodology for reducing the computational costs in large domain analysis was introduced. In this method, seismic excitation is directly applied to the computational domain such that assigning artificial boundary to the finite element models is more comfortable. In order to verify the implementation of the DRM in Finite Element Method (FEM), a simple... 

Numerical simulation of dam-foundation-fault system, considering the earthquake source, propagation path, and local site effects, was carried out for realistic and reasonable seismic safety analysis of concrete dams. The Domain Reduction Method (DRM) was used for seismic analysis of Dam-Foundation-Fault (DFF) system, in which a modular two-step methodology for reducing the computational costs in large domain analysis was introduced. In this method, seismic excitation is directly applied to the computational domain such that assigning artificial boundary to the finite element models is more comfortable. In order to verify the implementation of the DRM in Finite Element Method (FEM), a simple... 

In this study, the effects of geometry and operating conditions upon the thermal and hydraulic performance of Finned Microchannel Heat Sink (FMCHS) were investigated. Water and aluminum were considered as fluid and solid for the computational domain (30 mm × 0.8 mm × 0.8 mm). The Microchannel (MC) was supposed to have 0.65 mm height with an aspect ratio of 0.5. CFD analysis was applied for the assessments of four-types of micro-fins (i.e., conical, pyramidal, cylindrical and cubical). In order to evaluate the effects of height, diameter, the spacing of fins and Reynolds number on the overall performance of FMCHS, central composite design at five levels was used to generate design points.... 

Stability has been one of the major issues of time-domain numerical methods used for solving Maxwell equations. This problem takes a more severe form when additional algorithms are introduced to the computation domain (e.g. Absorbing Boundary Conditions-ABCs). There are a number of methods for investigating the stability of a simulation. In this article the problem of stability of ABCs, has been tackled through a control system's state-space point of view. Thus, occurrence of instability in a simulation can be predicted. © 2007 IEEE  

In this research, it is investigated to numerically evaluate the performance of a polymer electrolyte membrane fuel cell (PEMFC). The performance is investigated through the nonuniformity gradient loading at the catalyst layer (CL) of the considered PEMFC. Computational fluid dynamics is used to simulate a 2D domain in which a steady-state laminar compressible flow in two-phase for the PEMFC has been considered. In this case, a particular nonuniform variation inside the CL along the channel is assumed. The nonuniform gradient is created using a nonisothermal domain to predict the flooding effects on the performance of the PEMFC. The computational domain is considered as the cathode of PEMFC,... 

In this paper, a two-dimensional numerical approach is used to study the effect of micro combustor height, mass flow rate and external convection heat transfer coefficient on the temperature and species mass fraction profiles. A premixed mixture of H2-Air with a multi-step chemistry is used. The transient gas phase energy and species conservation equations result in an Advection-Diffusion-Reaction system that leads to two stiff systems of PDEs. In the present work, the computational domain is solved through the Strang splitting method, which is suitable for a nonlinear stiff system of PDEs. A revised boundary condition for the velocity equation is applied and its effect on the flow... 

A numerical model is developed to simulate the borehole heat exchanger both in the short and long time. In this regard, the computational domain is divided into the inside and outside borehole regions. A two-dimensional finite volume method is implemented in a cylindrical coordinate system for modeling of the outside borehole. Also, a thermal resistance-capacity model is presented for the borehole cross section. This model is extended to take into account the fluid transport through the U-tube and the temperature variation of the borehole components with depth. The governing equations of the two regions are solved iteratively in each time step. The proposed model is verified with the... 

Overall turbine analysis requires large CPU time and computer memory, even in the present days. As a result, choosing an appropriate computational domain accompanied by a suitable boundary condition can dramatically reduce the time cost of computations. This work compares different geometries for numerical investigation of the 3D flow in the runner of a Francis turbine, and presents an optimum geometry with least computational effort and desirable numerical accuracy. The numerical results are validated with a GAMM Francis Turbine runner, which was used as a test case (GAMM workshop on 3D computation of incompressible internal flows, 1989) in which the geometry and detailed best efficiency...