Sharif Digital Repository

In this thesis the steady state convective heat transfer for turbulent, two-dimensional, incompressible gas flow in a circular microchannel under slip flow and temperature jump conditions is numerically investigated by means of finite volume scheme. The low Reynolds number k-ε turbulence model is employed using a new boundary condition for turbulent kinetic energy at solid surface. To calculate variables at control volume surfaces in the axial direction upwind differencing scheme and in the radial direction central differencing scheme are used. Rhie-Chow interpolation technique is used to prevent pressure field oscillations. The set of discrete equations are solved using SIMPLE Algorithm. In... 

In this thesis, the quasi-two-dimensional modeling of polymer electrolyte membrane fuel cell cathode side is presented as a single phase. Polymer membrane fuel cell modeling equations are solved in the equations of continuity, momentum, and chemical species are potential equation. In this report, for modeling the electrochemical reaction in the catalyst layer of the relationship Baltr-Vollmer is used. The present analytical solution based on two fixed pressure gradient and the velocity dependent pressure gradient is developed. Momentum and continuity equations in the model with separation, velocity and pressure field is achieved. By comparing the results of the modeling analysis with... 

In this thesis, two-phase flow modeling in two-dimensional micro channel with liquid injection from wall is studied. Two models as "separated flow model" and "two-phase mixture flow" is developed. The separated flow model considers gas and liquid flow in separated films of flow with an interface between the phases. The two-phase mixture flow considers equivalent fluid mixture from gas and liquid with local modification in density and viscosity. Governing equations include continuity and momentum solved with control volume method. SIMPLE algorithm's been used to pressure-velocity linked equations. Velocity and pressure profiles plotted for mentioned models at a given situations. By comparing... 

Numerical simulation of air-water annular two-phase flow in a tube is carried out by using level set method for determining position of interface between two phases. Governing equations include continuity, momentum and level set equations that considered for each phase. The finite difference scheme is used to discretize the differential equations. The SIMPLE algorithm is adopted to solve the discretized equations for linking continuity and momentum equation. An unequally spaced staggered grid size for flow field is adopted. The Level Set method is used to capture interface position between two phases. To increase the accuracy of solution at level set equation, 5th order finite difference and... 

This thesis is about modeling of liquid-vapor two phase flows through nozzle. Liquid-vapor two phase flow is very applicable in industries such as boilers, expansion valves, refrigeration and sudden failures in pipelines. Two models are used for modeling of fluid flow through nozzle, transient and steady state. In Both model the solution field is considered as saturated liquid and vapor and the mass, momentum and energy equations as well as equation of state is used for describing of fluid flow properties. Homogeneous equilibrium model is used and for complete modeling of fluid flow heat transfer and friction force is also considered.
Numerical solutions are used for solving of the... 

The purpose of this study is to investigate the distribution of air flow and temperature in electric machines. The fluid flow equations are solved by using the characteristic Galerkin. The unsteady equations of the air flow and heat transfer between the rotor, stator and radial cooling channels of the generator are solved by finite element method. Tetrahedral elements are used, but hexahedral elements can also be used. The upwind method is used to discretize the convection terms, but the central difference for the diffusion parts, that is equal to Petrov-Galerkin method in steady state situation. Due to the large number of channels and specific geometry, to reduce the computation costs, the... 

This paper numerically studies the convection heat transfer enhancement of a developing two dimensional laminar flow in a pipe partially filled with porous materials. One of the most important effects of the systems with porous materials in them, is that they can improve some heat transfer components if they be used in a proper way. This study has been performed under both local thermal equilibrium (LTE) and local thermal non-equilibrium (LTNE) conditions. Two energy equations are used in non-thermal equilibrium condition between fluid and porous material. Darcy-Brinkman-Forchheimer model is used to model the flow inside the porous medium. The effects of different parameters such as, Darcy... 

In this study we solve the porous and semi-porous flow in the entrance region in a channel filled with porous material or partially filled with it. We present a semi-analytical method for solving the momentum equation in porous and semi-porous channel and pipes at the development and fully developed regions in order to acquire the entrance length and velocity profile and its dependence to the interface boundary conditions between the porous and fluid flow. For solving the equations of entrance region we utilize the definition of boundary layer and use integral solution. Subliminally we find the thickness of boundary layer at the walls and interface of regions in the porous layer. For fully... 

This work numerically simulates pressure drop and heat transfer for flow in a pipe with porous wall. Incompressible two dimensional, ax symmetric laminar or turbulence flow with constant mass flow rate is considered. It is assumed that there is no thermal equilibrium in the porous media region between solid and fluid phase. Effect of porous wall thickness and Darcy number on the rate of heat transfer and pressure drop were investigated. The Darcy number varies from to and porous wall thickness can fill entire pipe. the turbulent model is used to calculate the fluid flow and heat transfer characteristics in turbulent flow. Nusselt number can enhance about ... 

The silencer is used to reduce sound in the vehicle, air conditioning channels, power plants and guns. Functional parameters of the sound amplifier are the amount of noise reduction and pressure drop. There are few studies on the flow and pressure distribution in the silencer, and there is no article about the effect of the size of the internal passages on the pressure drop and acoustical performance of the silencer. Few studies have been done on the effect of the absorbent fiber diameter on the acoustic performance of silencer. In this study, the effect of different shapes of internal channels (extended tubes, baffles and number of chambers) on the pressure drop and acoustic performance of... 

The increasing demand of energy and energy consumption, urged researchers of different fields of science to investigate different aspects of energy efficiency methods. One way of optimization of energy consumption is to reduce drag force in moving objects in fluids. Changing the geometry of objects, bubble injection, air suction and blowing on surface, using nano fluid and superhydrophobic surface are just a number of drag reduction methods. Among the mentioned methods, using superhydrophobic surface due to its simplicity, low cost and no need to change in geometry of vehicles attracted attentions of researchers of all over the world. These surfaces with micro/nano roughness combined with... 

The present study's main aim is to investigate the drag-reduction ability of liquid-infused surfaces inspired by the pitcher plant. The present study can be divided into experimental and numerical sections. In the numerical section, the drag reduction ability of these surfaces was investigated in the internal flows. For this purpose, the effect of different parameters, such as Reynolds number, viscosity and density ratio of working fluid and lubricant, overlayer film thickness, and Ohnesorge number representing the interfacial tension, on the drag reduction was investigated. The results revealed that the variation in density ratio and Ohnesorge number had insignificant impact on drag... 

Two-phase flows are widely used in petrochemical industry, chemical processes, boilers, nuclear reactors, cooling systems and air conditioning equipment. Since the problem of earth’s ozone layer and global warming, the analysis of flow characteristics in evaporators are necessary to improve performance of refrigerating systems and saving the environment. Most of the researchers have been studied the evaporation of two phase flow refrigerant through experimental studies and numerical analysis has less attention in the literature. In this Thesis, a mathematical model based on the two-phase separated flow model is developed to simulate the evaporation of pure refrigerant under constant heat... 

The aim of this research is to consider the effect of geometric changes on pumping performance of two phase gas-fluid flow in a vertical pipe. The idea is to inject a light gas to the bottom of a tube full of fluid so we will obtain an upward flow due to density differences. Potential benefits of this device are numerous such as dredging of river estuaries and harbors, sludge extraction in sewage treatment plant, water treatment and many others. There are almost four distinct flow patterns which occurs in the rise pipe, bubbly flow, slug flow, churn flow and annular flow that each of them require special conditions. heat transfer between two phases, phase changes, mass and volume rate... 

In this thesis, gas-liquid two phase flow through channels of spatially varying cross-sections are investigated numerically. These flows have several industrial applications such as nozzles, diffusers, pipeline blowdown, valves and channels with sudden expansions or contractions. Finite volume method in conjunction with a second order Advection Upstream Splitting Method (AUSM+_up) is employed to discretize the governing equations. At low Mach numbers, a pressure-based algorithm with a pressure correction equation is derived. Using the pressure-based algorithm accelerates the convergence rate of the numerical solution of the flows through sudden expansions. The pressure adjacent to the... 

Water treatment from industrial and domestic pollutants is one of the most important issues in recent decades. In the present research, simulation of water purification using single-layer graphene membrane were studied. The effects of nanopores diameter, applied pressure, concentration of contaminant ions, and functional groups on the amount of filtered water and ion rejection through the membrane were quantified. Contaminant ions including Na+, K+, Mg2+, Ca2+, and Cl- were dissolved in water. Based on the results, by increasing the pore diameter and applied pressure, more water would pass through the membrane. Also it was concluded that pores with functional groups have better performance... 

Superhydrophobic surfaces have attracted so mucha attenetions in recent years. High contact angle, Low adhesion and low friction drag are the most important features of these surfaces. Two main parameters of these surfaces are hydrophobicity and micro-( or nano) scale roughness. In this Project a 2-D simulation of flow on superhydrophobic surfaces have been considered. Shan-Chen multiphase Lattice Boltzmann Model has been used for the simulation and carnahan-starling equation of state have been used or a higher density ratio. In the first step flow in a channel with smooth walls with different hydrophobicity have been simulated and effects of hydrophobicity on pressure drop have been... 

Supersonic separators with neither rotating parts nor chemical process, physically separate particles in a multi-phase flow. In comparison with general separation techniques, which are utilized in industries, specifically this equipment is cheaper, smaller and lighter. Condensation and separation at supersonic velocity is the key to achieve a significant reduction in both capital and operating costs. In this study, a two dimensional de Laval nozzle and few three-dimensional supersonic separator have been modeled. The 3D models modified in shape and dimensions. Impellers were created to form swirl flow. The models were simulated by the use of CFD software ANSYS FLUENT. Diverse inlet pressures... 

Capturing the atmospheric CO2 and turning it into value added products is a topic that has been extensively studied in recent years. One of the methods through which CO2 is valorized is via converting it into carbon nanofiber, CNFs. CNFs can be an important material in future human activities due to its valuable properties such as wide range of electrical conductivity, their superior flexibility and strength, But it is currently obstructed in use due to the high cost of manufacturing and the complexity of the synthesis. Furthermore, in today's world, human life is heavily influenced by greenhouse gases, and avoiding emitting gases like CO2, is almost infeasible. This thesis provides an... 

Osmotically assisted reverse osmosis (OARO) assimilates high efficiency due to non-thermal processes and accessing high salinity. This makes it suitable for high water recovery for the purpose of zero- and minimal liquid discharge desalination. Moreover, forward osmosis (FO) has low fouling efficiency which makes the operating costs of cleaning and replacement lower. This study aims at incorporating OARO and FO in a novel configuration with multiple inlet of the diluted draw solution to the regeneration section. Pinch analysis with simplifying assumptions was carried out for FO and OARO standalone stages. The results showed that pinch point in FO resides either on the beginning or at the end...