Sharif Digital Repository

In the present thesis, the extensive study of reactive and non-reactive supercritical and transcritical flows has been discussed in shear-axial and swirling forms. Considering the need for combustion models with the application of detailed chemistry for accurate simulation of the field, the Laminar Flamelet Models (LFM) are suitable in order to reduce computational expensive. For this purpose, it is necessary to produce thermodynamic tables specific to real gas conditions for the flamelet model. The tables related to the flamelet model in real gas conditions are extracted from the open source software Cantra. The difference point in the Flamelet tables of real gas and ideal gas is for... 

Low emissions combustion V-flame combustors are exceptionally prone to combustion instabilities. These systems for land based gas turbines rely on a premixed or partially premixed combustion process. As V-flames do not anchored on the lip of combustor, an experimental study of the transfer function of these flames shows that they are more sensitive to fuel oscillations and in the range of frequencies, they behave like an amplifier. This paper presents an experimental study of the transfer function of a ducted, laminar, premixed V and M-flames. V-flame, laminar, propane/air premixed flame stabilised at the rim of the burner. The limit of M and V-flame are identified without excitation, then... 

In this work, the unsteady response of a laminar diffusion flame to harmonic mass fraction oscillations and also to flow velocity fluctuations is investigated. Flame-sheet assumption is utilized to model the laminar unsteady two-dimensional co-flow diffusion flame mathematically. The initial combustion of most combustion chambers is through diffusion mechanism. Therefore, developing analytical model of diffusion combustion is of great importance. The flow is assumed subsonic (incompressible), inviscid, and uniform. The convection-diffusion equation for conserved scalar variable with appropriate boundary conditions is solved. Considering stoichiometric mass fraction surface to be the flame... 

In the present study, the axisymmetric liquid gas/air diffusion flame in the presence of fuel concentration oscillations and velocity perturbations has been investigated experimentally. Several industrial combustions take a place by diffusion mechanism which necessitate scientists to make far better understanding of diffusion flames. In order to study diffusion flames, the instrument with two separated lines of fuel and air has been developed. Nitrogen is used for fuel dilution oscillatory with pulse of 1, 2, and 4 Hz frequencies. The pulse duration effects on flame structure are also studied. The flame shape has been captured by high speed camera and the results are processed by image... 

This thesis is a complementary experimental and theoretical investigation on stability of premixed flames and a study of combustion instability and combustion dynamics in a swirl-stabilized combustor, aiming to understand the fundamental mechanisms responsible for combustion oscillations in gas turbine combustors. Theoretical investigations on acoustic modeling of a simple combustor and a study on kinematic response of premixed flames to flow perturbations have been discussed in the first part of this work. In another part of thesis, experimental studies on the response of premixed flames to acoustic perturbations have been performed. In addition, experimental investigations on combustion... 

Swarm robotics is a new approach to the coordination of large numbers of relatively simple robots. The approach takes its inspiration from the system level functioning of social insects which demonstrate three desired characteristics for multi-robot systems: robustness, flexibility and scalability. This research describes development of an autonomous miniature mobile robot for swarm robotics research and education. Several common platforms were studied and this prototype was designed. Wireless communication was selected to transmit robot’s messages. The design of this testbed aims for enhancing the sensing capability of the robotic agents, overcoming the constraints led by lack of... 

One of the problems associated with lean premixed combustion is their stability. Low swirl flame is a modern technology for reducing NOx emissions and increasing stability.In this thesis, first, the effect of geometric parameters on the flame stability was investigated. This study was carried out in high fuel and oxidizer mixture flow rate. Having find, the most stable burner, two other burners with the same geometrical parameters were built. Stability of these three burners were analyzed. Second, they were located in a ring together in three different relative positions. In each configuration stability and NOx emissions of the burners were determined.The study shows that NOx and CO... 

Based on the results of a cryogenic swirl flow numerical simulation under supercritical conditions, a global injection tester has been designed. For numerical simulation of the injection process by using ANSYS FLUENT as the CFD solver, the full 3D flowfield has been meshed by a fully structure manner. k-ω(SST) has been used as the turbulence model which has been captured many of the swirl cryogenic flow structures in the both steady and unsteady conditions. Also, the modified Soave-Redlich-Kwong EOS has been accommodated in a broad range of supercritical pressures for estimating state functions, thermodynamic properties and transport properties. The results show that increasing ambient... 

Burner scaling criteria have been studied and the most appropriate one has been selected to scale down a large industrial burner. The sub-scaled non-premixed swirl-stabilized burner is manufactured to ensure that the available experimental setup and equipment are sufficient and practical for characterizing the burner. The aim of the research is to assist the future design optimization with the identifications which have been done. The constant residence time method is selected in this project and it is shown that this is the proper criteria for non-premixed and swirl-stabilized burners. For better identification, stability maps of the flame are extracted with the experimental methodology for... 

Combustion in supercritical conditions (thermodynamic) is one of the most important applications of today's systems. One of the reasons for the increased use of combustion in supercritical conditions is the need to develop high pressure combustion systems, such as liquid fuel engines, gas turbines and internal combustion engines. Combustion is a technique in which the fuel is injected into the combustion chamber with its supercritical state (Any substance at a temperature and pressure above its critical point). Fuels in the supercritical conditions exhibit new features such as low viscosity, high diffusivity and low surface tension, which lead to higher efficiency, high power generation and... 

Experimental studies of regular cell structure of a weak detonation wave in a gaseous mixture of hydrogen and oxygen diluted with argon have shown that for a constant initial thermodynamic condition, the final cell size differs by repeating the tests. The question is that if these different final cell sizes for a constant initial condition are because of test errors, or the experiments would lead to a domain of solution in the form of different final cell sizes? Therefore, the aim of this study is to investigate, numerically, the evolution process and domain of variation of cellular structure of a weak detonation, and their dependency to unstable wavelengths obtained from linear instability... 

The aim of the present research is to investigate the combustion chamber cooling with vortex method in presence of kerosene and liquid oxygen as propellants. For this purpose, a part of oxidizer is entered into the cylindrical combustion chamber tangentially (from bottem of the chamber), and creates a vortex flow (outer vortex). With motion of created outer vortex along the wall and eventually hit its top wall (the head of the chamber), secondary vortex (inner vortex) occurs along the center line, with a downward direction (opposite to the primary vortex). The fuel and other part of the oxidizer injection is also carried in the head of chamber, so by mixing the fuel and oxidizer that happens... 

Turbine engines performance is notably dependent on environmental conditions, i.e. pressure and temperature and also on flight speed. Every engine has a specific flight ceiling, i.e. altitude above which engine shuts down. Engine shut down during altitude increase, is because of combustor performance loss and its efficiency and stability decrease. Combustor stability limit can be extended by performing modifications such as replacing fuel injectors and/or improving fuel characteristics, and the combustor can be used for higher altitude whitout any required change in design. In this thesis, influence of combustor various parameters like air flow rate, fuel equivalence ratio, inlet pressure... 

Excited jet flow has many physical and industrial applications, e.g. in aeroacoustics and the combustion instability. Analysis of this type of flow needs an accurate simulation of flow dynamics. This work presents the large eddy simulation of this type of flow. The numerical method used in the large eddy simulation must have low numerical dissipation and high order of accuracy. Compact methods which satisfy these requirements and have high resolution of frequency, are favorable ones for the large eddy simulation. A fourth-order compact finite volume method which had been developed in the MSc thesis of the author is extended and completed in the present work. This extension includes the... 

Turbulent combustion at supercritical condition have a considerable effect in modern high performance rocket propulsion systems, gas turbines and diesel engines. In such a reaction conditions, an abrupt changes occur in thermodynamics and transport properties of fluid. So numerically modeling of a real fluid behavior of the cryogenic propellants and the turbulent trans-critical mixing and combustion processes faces serious challenges.In this study, in order to realistically represent turbulence–chemistry interactions, detailed chemical kinetics and real-fluid thermodynamic behaviors related to the gaseous hydrogen and cryogenic liquid oxygen combustion under supercritical pressures, the... 

The liquid breakup analysis and spray characteristics measurement of atomizers are very essential for the numerical modeling of the primary atomization in combustion chambers. The rotary atomizers are used in many applications especially in small gas turbine engines as a fuel injection system. An experimental investigation of primary breakup dynamics and spray characteristics of rotary atomizers with high aspect ratio discharge channels are described. Four different rotary atomizers with round radial, round radial-axial, round radial tangential, and square radial discharge channels are studied. The purposes of this study are to identify the effect of channel orientation and its geometry on... 

During the last six decades, there were remarkable developments in the field of gas turbines. In the early 1950’s, the effciency of gas turbines was 20% and it is improved up to more than 40% for modern gas turbines. In addition, due to the implementation of the combined power cycles, the thermal effciency have been reached to 60%. According to the recent developments, further improvements in this field seem to be more costly and diffcult to obtain. Rotating detonation combustor (RDC) is a new generation of detonation combustors which has simpler design and several advantages compared to jet engine combustors, but it has its intrinsic problems which seem resolvable. Given that the RDC... 

Lean premixed combustion is highly susceptible to thermoacoustic instabilities. Such instabilities are compounded in low swirl flames by the fact that low swirl flames are stabilized aerodynamically at a relatively considerable distance downstream of the burner rim, such stabilization mechanism makes the flame more prone to both vertical and acoustic perturbations. The main objective of the present thesis is to evaluate effects of acoustic and hydrodynamic perturbations on low swirl flame features. To such aim, first, low swirl flame characteristics and stabilization mechanism are studied using large eddy simulations along with thickened flame combustion model. The obtained results show that... 

The liquid breakup analysis and spray characteristics measurement of atomizer are very essential for the numerical modeling of the primary atomization in combustion chambers. The atomization process is irregular and random nature and is a chaotic phenomenon. This study was motivated by the lack of temporal analysis of the breakup process especially for the first and second wind-induced breakup regimes and the atomization regime. The aim of this study was to determine the location, velocity and structures of breakup liquid jet at different outlet pressures from 0.3 bar to 50 bar. The results indicate that the breakup length, ligament size, and the breakup time period are randomly varied in... 

Flame Response to acoustic excitation which plays an important role in combustion chamber stability state may be found through numerical simulation. A finite volume code of higher order accuracy developed for compressible flows was adopted. Species equations along with a single-step irreversible reaction model were added to this non-reacting program. Changes were also made to the energy equation. Premixed methane/air flame in tubes was studied and the same problem was solved with FLUENT software. It was found that whether the Lewis number is considered constant or temperature dependent has a strong effect on flame structure and shape and that using mean values for gas properties yields out...