Sharif Digital Repository

Detecting of coherent structures in bidirectional vortex combustion chamber is the original aim of this project. For this aim a numerical simulation is performed by using OpenFOAM, open-source CFD software. Also influence of fuel nozzle places in formation of coherent structures is analyzed. Effects of combustion in these structures and differences in cold and hot flows are analyzed too. There are papers that present influence of coherent structures in vortex burners and other applications, but based on reviews that are performed in this work, it seems that influence of coherent structures have not seen in bidirectional vortex chambers. So it can be one of innovations of this work. Swirl... 

There are numerous ways available to achieve low temperatures that cryocoolers might be among the most important of them. In the meantime, the pulse tube refrigerator has a special place due to its simple geometry and not be using any moving parts. This simple property make the pulse tube refrigerator one of the most popular methods to achieve low temperatures. Nowadays, a pulse tube refrigerator is used for cooling in the range of 4 to 123 Kelvin in two form of one-stage and two-stage and up to an approximate power of 1 KW. Today, due to the temperature and the amount of power required for cooling, as well as the time it takes to reach that temperature, this chiller is used in medical,... 

Lab-on-a-chip devices have gained a lot of attention in chemical and biomedical analyses during the past two decades. These devices employ microfluidics basics and fundamentals to combine multifold laboratory processes in one single portable chip. The electric field has been often used in most microfluidics applications for the ease of sample control as well as become easily integrated to other chip components. Instabilities in microflows would occur when two fluids of different electric properties are exposed to an adequately strong electric field. Studying these electrokinetic instabilities is not only important for the fundamental studies but also for practical applications in micromixers... 

Recent developments in MEMS related areas have increased the demand for practical and novel pumping methods. Utilizing Electroosmotic force for flow generation in microchannels has become really popular recently, because of its reliable operation and control. One of potential applications of MEMS devices is biological and medical analysis which most samples are considered to be non-Newtonian; consequently, thermal transport characteristics of non-Newtonian electroosmotic flow of power-law fluids is investigated in this paper. In this study, thermal and hydrodynamic behavior of non-Newtonian electroosmotic flow of power-law model in a slit microchannel is analyzed. It is assumed that the flow... 

In recent two decades, with developments in micro and nano scale fabricating techniques, use of microfluidic systems is increasing in different fields. How to drive fluid is a hard question because of high pressure drop in micro scale. Different methods has been suggested to solve this problem, among which, electroosmotic pump is preferred because of its advantages. One of the main applications of microsystems is in biomechanical devices like microreactors which are usually working with non-Newtonian fluids. The cross section of these devices are most often close to rectangular shape.Therefor, in this study,Hydrodynamic charactristics and mass transport of non-Newtonian fluids under... 

In the study of heat transfer in micro-channels, free and force convections are two limit cases and these two methods of heat transfer are combined together generally, so to achieve the most accurate informations about the flow field they should be considered in combination. In the first part of the thesis the fully developed slip flow mixed convection in vertical micro-ducts of arbitrary shapes is investigated.Uniform axial heat flux and uniform peripheral wall temperature (H1) is considered. The method considered is analytical-numerical in which the governing equations and three of the boundary conditions are exactly satisfied but the remaining slip boundary condition on the duct wall... 

One strategy for increasing efficiency and decreasing brake specific fuel consumption in spark ignition engines is using variable valve timing mechanism which is proportional to engine speed and load. Variable timing refers to the opening and closing time of inlet and outlet valves and their duration and overlap. In conventional SI engines the performance of valves is designed on a fixed point at special engine load and speed. While investigations show that if performance time of valves is changed according to engine load and speed, performance parameters of engine will improve.
In reviewing articles related to variable valve timing, their effect was considered on volumetric efficiency.... 

Electronic engine control unit (ECU) operates the different actuators using information signals received from the different sensors. Also in the case of any fault in the sensors and actuators operation ECU is responsible to detect and alert the accruing faults. Most of the common faults in the engine such as leakage in intake system and ignition system are not detectable by ECU, on the other hand ECU fault codes some times are hard to interpret or misleading. So fault diagnosis is usually based on experience of repair staff, and might be time consuming and inaccurate. In this study, using the knowledge of premixed fuel injection SI engines and emission generation and statistical data,... 

Nowadays, cooling towers are widely used in power plants, refineries, petrochemicals and ventilation industries, but the water consumption of these towers is high and because of water crisis in Middle East and Iran, it is necessary to devise in order to reduce the water consumption of these towers. In this study separated hybrid cooling tower are used to investigate the water consumption at power plants. The rigorous Poppe method are used in order to estimate the exact amount of water loss due to evaporation in wet cooling towers. First, the effect of ambient condition, wet cooling tower cells and dry cooling tower size on the performance of hybrid cooling tower are investigated, then the... 

Many of industrial processes include suspension in their middle section of production, final product or residue. Majority of these two-phase mixtures are unstable due to their large particle size (larger than one micrometer). Therefore, Study of particle distribution in suspensions for generation of stabilized suspension becomes significant. In this study a new mechanism in stabilization of microparticle suspension known as nanoparticle halo mechanism has been assessed. Experiments have been conducted in dilute suspension and two-way coupling range. The effect of the presence of silica nanoparticle on zirconia microparticle suspension has been investigated. The effect of zeta potential of... 

First studies in internal combustion engines have focused on increasing engine power and efficiency, reducing engine volume and improving fuel quality. However, nowadays most studies are concentrated on decreasing exhaust emissions and usage of regenerative fuels while maintaining engine size, efficiency and power. Surveying fuel type in direct injection engines, requires investigation of microscopic and macroscopic spray characteristics including spray tip penetration, spray cone angle, spray area and sauter mean diameter, since these parameters determine air-fuel mixture quality. In this study, spray of ethanol as a regenerative fuel that reduces emissions such as NOx and CO is... 

Helium gas is the working fluid in common Pulse Tube Refrigerators (PTRs) due to its brilliant properties in reaching cryogenics temperature as low as 4 K. Generally ideal gas equation of state has been used to evaluate thermodynamic performance of He. However using ideal gas equation of state for He in 10 bar and 5 K causes considerable error on estimation of density. The objective is to use real gas equation of state for temperatures near zero Kelvin. This approach makes an accurate prediction of PT cooling capacity and COP. In this thesis the objective is to realize the most relevant equations of state describing the proper behavior of He in the regions of operating point of PTR. Later on... 

Electroosmosis is the predominant mechanism for flow generation in lab-on-a-chip devices. These microfluidic devices are microscale laboratories on a microchip that can perform clinical diagnoses. Since most biofluids encountered in these devices are considered to be non-Newtonian and the cross section of microchannels in these devices is close to a rectangular shape, In this study, the hydrodynamically and thermally fully developed combined electroosmotically and pressure driven flow of power-law fluids in rectangular microchannels is analyzed. The governing equations are first made dimensionless and then transformed into new ones based on the computational parameters which provide mesh... 

Investigation of flow field and vortex structures in swirl flows and their velocity components, have received widespread interest among the researchers in the field of fluid mechanics. Due to the importance of vortex structures and their applications in combustion chambers, a special type of combustion chamber which is called “bidirectional swirl combustor” has been investigated in this work. Accurate identification of vortex phenomenon and flow structure in vortex engine leads to better design and control of the engine in the field of stability, geometry, cooling and combustion efficiency. Eperimnetal methods in this area are difficult but very valuable and efficient. Numerical methods are... 

In order to promote the efficiency of fossil fuel power stations, rotary air preheaters are used to warm the air entering the boiler. So, knowing the essential parameters of such exchangers and their effect on the efficiency is the primary objective and choosing the appropriate parameters and the dimensions reducing the liquid pressure, weight of exchangers and subsequently the maintenance expenses would be the secondary objective.
In this research we tried to know the principles of regenerative convertors introducing the tentative air preheaters and their functional characteristics .We also assessed the factors in designing the ideal tentative air preheaters including the type of... 

The aim of this research has been to modeling and optimization of steam injected gas turbine with inlet air cooling using absorption chiller. The whole system consists of compressor, combustion chamber, turbine, heat recovery steam generator, absorption chiller and cooling coil. The tasks carried out in this research include four points of view. The thermodynamic, exergetic, thermo-economic and environmental points of view are considered along modeling and optimization process. Design variables including temperature of combustion chamber exhausted gas, pinch point temperature of heat recovery steam generator and steam injection flow rate. Finally this modeling and optimization process is... 

Vortex engine has shown to be a credible substitute in thermal power and aerospace industries mainly due to their advantages such as better mixing, improved combustion, and considerable cooled walls of combustion chamber. The engine geometry and tangential inlet oxidizer create inner and outer vortices rotating in the same angular direction. The inner vortex moves from head to the end of engine in the axial direction and outer vortex moves in the opposite direction. In the vortex engine, vortices affect spray droplets and force them to have a spiral path and apply a centrifugal force on the droplets to push them towards sidewall. Also axial relative velocity of droplets and flow filed act as... 

Improving efficiency of heat exchangers has always been an important concern for the industry. Pulsating heat pipes are novel and efficient technology in the field of heat transfer. Pulsating heat pipes are widely used in solar water heaters, solar desalination systems, air conditioning systems, cooling of electronical boards, etc. these applications explain the necessity of performing this study in our country. In this research, thermal performance of open loop pulsating heat pipes using two operating fluids (ferrofluid with surfactant and aluminum microfluid) is experimentally investigated. Start-up and steady thermal performance of pulsating heat pipes charged with these two operating... 

A new experimental setup was fabricated; equipped with an adjustable chamber’s length, with air tangentially flowing through the wall of vortex engine. In a vortex combustion chamber, the tangential inlet fuel or oxidizer creates both inner and outer vortices which are rotating in the same angular direction. Indeed, inner vortex moves from head to the end of engine in the axial direction; however, outer vortex moves in the opposite direction. This setup was fabricated to see the effect of fuel nozzle characteristics on the wall temperature of the chamber. In a vortex engine, vortices affect spray droplets and force them to have a spiral path. Moreover, relative velocity of droplets and flow... 

Vortex flow is one of the most sophisticated fluid movements in nature which has great industrial and scientific importance. In this dissertation, in order to study the combustion process due to a vortex flow, a new experimental model of a cold wall vortex engine, was designed, fabricated and tested with oxidizer issuing tangentially toward the end of the chamber. The cold wall effect in these chambers is a result of the cool layer of oxidizer which significantly reduces the amount of heat transfer to the chamber’s wall. This experimental model was designed to withstand the massive temperature rise resulting from the usage of Polyethylene as fuel and Nitrous oxide as oxidizer. In a series of...