محمدمهدی دوستدار

In this research, the design of a can combustion chamber for a ramjet engine, the performance of this chamber and the role of flameholder have been studied. For this purpose, after talking about ramjet engine types, some general information about the combustion chamber has been explained. Then the process of chamber designing and determining its geometry has been discussed. The dimensions of the chamber were determined by using input conditions extracted from GasTurb software as well as applying a calculation code, and the geometry of the chamber have been determined by applying calculation codes. By evaluating the obtained geometry and ensuring the accuracy of the design, simulation of combustion with non-premixed liquid phase was carried out using Fluent software. While presenting the results, the effects of size, distance and number of flameholder have been investigated. It is shown that the use of flameholder in ramjet engines is essential but the use of large flameholder is not recommended.

In this research, the performance of ramjet engine combustion chamber has been evaluated by applying geometrical and physical changes such as changing the diameter of the chamber, changing the size and displacement of the secondary and dilution air holes and changing the mass flow passing through these holes. To do this, at first some typical ramjet combustion chambers were introduced and some of their features were discussed. Then using the information and guidance presented in scientific and technical references a suitable combustion chamber was designed for the defined inlet and outlet conditions. After the geometry has been determined, the simulation process has been carried out in Fluent software and while presenting the results, the performance of the chamber has been investigated. The results show that the values obtained for the geometry of the chamber and the position of the holes are reliable and changing the sizes has no global advantage and is not recommended. Furthermore, it is found that displacing air holes location could improve solution convergence.
 
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In this study, mixed convection inside a square cavity with a movable cap and baffle was simulated numerically using the finite volume method. The under-study cavity was two dimensional and affected by gravity and rotated perpendicular to the plane. Right and left walls of the cavity were adiabatic and the upper wall was warm source at a constant temperature. Lower surface was a movable cap that moved from the center to both sides and was assumed to be a cold source at constant temperature. The baffle was assumed to be at the same temperature as cold wall and had a height equal to two thirds of the side of the cavity. Experimental data was used for the thermal conductivity coefficient of the nanofluid. Simulations were performed at a constant Reynolds number to investigate the effects of three parameters of Richardson number, volume fraction of solid particles and cavity slope angle on isothermal lines, streamlines and mean Nusselt value, which created 36 different states. It was found that increasing of slop angle of cavity with respect to reference surface (0 to 90 deg), increasing Richardson number (0.01 to 100) and increasing the volume fraction (0 to 0.05), increase the mean Nusselt value, where the maximum value of which is equivalent to state , , . Increasing the volume fraction of the nanofluid causes an increment in average Nusselt number. It was also observed that at low Richardson values, cavity slope angle has no effect on the results.

In this research, the free vibration of rotating tapered Timoshenko beam with piezoelectric layer has been studied. It is assumed that the beam is made of Functionally Graded Materials (FGM) through the thickness direction and the boundary condition is a cantilever attached to the hub. The first-order shear deformation theory has been used to drive governing equations. At first, the total energy of the system such as potential and kinetic energies for the beam and piezoelectric layer has been derived, and then the natural frequencies of the beam have been determined by the Ritz approach based on minimizing the total system energy. After verifying the results by comparing them with other research, the effects of some parameters such as hub radius, rotational speed, taper ratios, rotary inertia, material gradient, piezoelectric voltage, and beam thickness on the natural frequencies of the tapered Timoshenko beam have been studied in detail. The results showed that with increasing the angular velocity of the beam, the natural frequency increases so that as the increasing velocity increases, the natural frequency increases with a steeper slope.

In this research, the natural convection heat transfer for a square enclosure containing two Nano-fluids, water-alumina and water-copper, which is affected by a diagonal magnetic field, is simulated numerically and the effects of some parameters such as Rayleigh number ( ), Hartmann number ( ), magnetic field angle ( ), nanoparticle volume fraction ( ), type of heat source (linear or sine), length of the heat source ( ) and the non-uniformity parameter of the source ( ) have been studied on the flow and temperature fields. The results show that with increasing Rayleigh  number and Hartmann number, the amount of heat transfer increases and decreases, respectively. Also, the thermal performance of the enclosure is improved by increasing the angle of the magnetic field (from 0 to 90 degrees) and by adding solid nanoparticles to the base fluid, it means a relative increment in enclosure heat transfer is observed. In both high and low Rayleigh numbers, the maximum heat transfer is related to the constant temperature source. After that, for high Rayleigh numbers a sinusoidal source (λ = 1) and for low Rayleigh numbers a linear source (λ =0.5), where the conduction heat transfer dominates on enclosure, have the highest average Nusselt number, respectively. The results also indicate that when the length of heat sources increases, the rate of heat transfer increases too.

In this investigation, grid generation difficulties in CFD analysis of internal combustion engines (ICEs) have been studied. Grid generation is primitive, extremely complicated and time consuming task in ICEs simulations. The KIVA open source code is one of the most popular CFD solvers in ICEs simulations. Grid generation for this solver is the most complex and difficult task. In the present work a methodology has been developed for rapid grid preparation in KIVA-3V code by using commercial grid generator, ANSYS ICEM CFD. In this methodology, all geometry details of ICEs, including valves and intake/exhaust ports, are taken into account. In this paper, the focus is not on modifying the KIVA code original mesh generator, however, the most popular grid generation and dynamic mesh management errors and some bugs in ICEM CFD and KIVA have been inspected and explained. By using the procedure described here, many of grid generation difficulties will be eliminated and grid generation time will be greatly reduced. It should be noted that many of features in this paper are general and can be used for geometries other than ICEs.

Turbocharging is a prevalent method for the promotion of an UAV flight level without having to make major changes in its engine. It depends on the correct selection and precise control of the turbocharger. Today, the mathematic simulation of the engine cyclic processes as a strong tool to estimate performance and reduce costs and testing time is taken into consideration. In this research multi-zones thermodynamic modeling according to the following steps is performed. At the first step, a geometrical model of the Wankel engine is developed and the geometrical characteristics of the equivalent reciprocating engine is achieved. At the second step, the equivalent reciprocating engine is simulated by GT-Power commercial software. Then, using the empirical results of dynamometer tests the developed model is vitrificated and by this means the effect of altitude conditions on the engine performance is studied. At the last step, according to the matching theories, a proper turbocharger is selected and by using appropriate control mechanisms, the performance of the turbocharged engine at the desired altitude is evaluated. Results indicate that for the operating engine speed and full load condition, turbocharging leads to 41% power increment and 5% specific fuel consumption reduction at the target altitude compared to the naturally aspirated engine at designed working altitude.

Using proper dimensionless coefficients that are insensitive to various operating conditions is a crucial issue during the utilization of a yawmeter probe. These dimensionless coefficients produce the deviation angle of flow, stagnation and static pressures. In the current study, these coefficients are analyzed using SPM analytical and experimental methods. A comparison of experimental and analytical results shows that SPM analytical method predicts the flow deviation coefficients satisfactorily at the operational angle range of three-hole probe. This method also calculates the stagnation pressure coefficient precisely at the deviation angle range of ±10 degrees. The experimental results show that due to the assumption of constant speed on the probe, the analytical method cannot calculate the static pressure accurately. Experimental observations also demonstrate that velocity is increased and pressure is decreased over the probe. This is due to the suction region at the downstream of probe. Unlike analytical results, experimental observations depict that at zero degrees, the flow static pressure is equal to the average of pressure at the left and the right side of probe. Due to sensitivity of dimensionless coefficients of flow static pressure to variation of Reynolds number, various values are reported at different kinds of literature for these coefficients. These coefficients change with Reynolds number variations and their accuracies are decreased. In the current study, a new proper dimensionless coefficient is introduced which represents minimum sensitivity to Reynolds number.

In this paper, design and simulation of fuel control unit (FCU) for a widespread turbojet engine is presented. For this purpose, a brief review on importance of control strategy and engine control modes is firstly presented. Next, the steady state and transient modeling flowchart for the engine is explained and a dynamic model for prediction of engine behavior is developed based on the described flowchart. Then, In order to confirm the ability and effectiveness of the used approaches, the engine simulation results are compared with the experimental results and the good agreement between them illustrates the effectiveness of the steady state and transient modeling. After that, the designed strategy for the engine fuel control unit is described in details and the mathematical equations of FCU parts are presented. Moreover, the mathematical modeling is used for development of a simulation model in SimHydraulic software and the FCU behavior is studied using the developed model. Finally, integration between engine and FCU simulation is done and the simulation results are presented in order to confirm the design and simulation process. The proposed design in this paper can be used for other similar case studies as a regular approach.

Exergy analysis is a method of evaluating the proportion of each process on the transmission of system availability and determining how the useful energy is loosed. In this research, a turboprop engine is simulated using GASTURB 10 commercial software which is developed based on zero-dimensional approach. The performance of turboprop engine is calculated at design point and some off-design points and thermodynamic properties of engine components at different flight altitudes and Mach numbers are calculated. After that, by defining exergy terms and using related exergy equilibrium equations, the necessary conceptual basis for exergy analysis is introduced. Then thermodynamic data obtained from GASTURB software are used to exergy analysis of engine components. Results show that combustion process has the dominant portion of the system irreversibility.

Using heat shield, especially in throat area has a significant effect on combustion chamber pressure and thermal efficiency of solid fuel engines, and so many studies have been made in this field. A precise prediction of regression in throat surface is essential to optimum design of high burning time engines. In this study, ablation of graphite nozzle in solid fuel engines is investigated numerically for a special ingredient composite fuel .Navier Stocks equations together with thermodynamic equations inside the engine as well as thermochemical and thermal conductivity equations on nozzle surface are derived and written in their suitable forms and solved to determine the regression rate of nozzle throat surface. Furthermore, a cartridge full size solid motor by polyester binder fuel was tested and the ablation rate was measured by using a 3D scanner. The experimental pressure-time and thrust-time curves were also derived and used as input data for numerical calculations. Comparison between numerical and experimental results shows a satisfactory agreement. The experimental results show that the ablation has maximum value in the inlet area, and in divergent section is approximately constant and its value is 0.2mm. Because of important effect of ablation rate on the geometry of nozzle throat and so on the performance of the engine, the results of this study may have applicable usage in analyzing and designing solid fuel engines.

Variable timing of valves is one of the effective factors on performance of a spark ignition (SI) engine. The time of opening, closing, duration of open state, and the amount of lift for inlet and exhaust valves are the parameters that must be considered in variable valve timing issue. These parameters influence thermodynamic performance factor and heat loss of an engine. In this paper at first, using K3PREP pre-processor, a structured moving mesh was generated for valves as well as for inner and outlet runners. Then, using KIVA-3V as the main solver, the variation effects of each valve timing parameters on the engine performance were studied. The simulation was done on a SI- engine. Comparison between simulation and experiment results indicates satisfactory agreement.

The purpose of this study is to investigate the effect of the simultaneous use of fuel injection injectors in an air cross flow. Nowadays, several methods are proposed for optimizing fuel injection in internal combustion engines. These optimizations are due to the high impact of this variable on engine performance and reduction of emissions. The method proposed in this study is to use two fuel injectors instead of a single injector in the air inlet manifold. The uses of two injectors in order to impingement two fuel sprays and increase the turbulent and collision of droplets, and so break them up faster. Also, the use of two injectors can provide more control over spatial and temporal distribution. Simulations are performed numerically using the generalized Kiva code. These simulations are similar to the fuel injection conditions in the manifold of the spark ignition internal combustion engine. The results indicate that the placement of two injectors in a longitudinal distance, the installation of two injectors at a 70 ° angle with the duct, placing two injectors in 180° or 90° relative angles and a 15° conical angle reduces the average diameter of the droplets. The results of this study can be used to design an internal combustion engine fuel injection system.

The basis of planing vessels motion is the use of low drag force to lift force. One of the main goals in the design of planing vessels is to reduce the hydrodynamic resistance and achieve higher speeds. For this purpose, using a hydrodynamic correct analysis is an important step.
In this study, a three-dimensional model of computational fluid dynamics is presented using a finite volume discretization on a structured network to study the flow pattern under the body of COUGAR model in different modes. Therefore, for the numerical simulation of COUGAR model, the commercial software of the ANSYS-CFX has been used. The VOF method was used to study the distribution of two fluid phases and free surface modeling. Thus, the fundamental equations governing the flow of fluid around the cougar model are solved and the distribution of velocity and pressure in the computational domain will be obtained.
In these simulations, the model is considered constant and the fluid moves. Simulations at trim angles of 1,2 and 3 degree and drafts of 10.7, 11,77 and 12/84 cm (equivalent to weights 9.5, 10.5 and 11.5 tons in the main vessel) and three speeds of 8.52, 11.5 and 14.95 meters per second (35, 50 and 65 knots for the main vessel) is done. The Studied geometry is two-steps cougar model. The length of the second step is three different distances from the aft and values 34, 38.5, and 43 cm. Thus, the exact results of the vessel hydrodynamic resistance, the pressure distribution on the body, the waveform and the lifting force will be obtained for all three bodies. For verification, the independence of the network as well as the validation of the results with the laboratory model is carried out. Comparison of the simulation results with the experimental results of the 38.5 cm model, which was tested in the Persian Gulf National Laboratory, is in good agreement. The maximum error rate for drag of two-step vessel at a speed of 11.5 m / s is 12.4% and satisfies the experimental results. Studies show that in a loading and constant velocity, the vessel drag increases with the increase of the second step length from of aft vessel.

Existence and growth of cracks in structures is an unavoidable matter, and presentation of economical repair methods is useful in this situation. Applications of repaired structures by composite patches are expanding. One of the most significant uses of this method is repairing cracked components and parts of aerospace industries. This type of patch includes many advantages such as high strength, resistance against corrosion and humidity, low weight and acceptable fatigue life. Patch repair causes the stress reduction around the crack and also the transition of stress from the cracked plate to the patch. Finally this type of repair decreases or totally stops the crack growth and increases the fatigue life of parts. In this study, the experimental investigation of crack growth and fatigue life in cracked Aluminum plates repaired by composite patches has been considered. Edge-cracked aluminum plates repaired with one-side composite patches are investigated. Due to avoidance of the disadvantages of mechanical joints such as; stress concentration, low strength against fatigue, the risk of failure in the main body, and so on, the adhesive has been used to join the parts. In addition, the effects of several parameters, such as the prime cracks angle in three levels $0^{0}$, $30^0$, and $45^0$, patch's width in three levels 25, 30, and 35mm and the layer's layout in three levels on fatigue life have been investigated. The results indicate that by the increase in crack angle from 0 to $45^0$, the fatigue life of parts under fatigue loads increases; and in several path layouts, there is not any difference in fatigue life. It is noticeable that increase in the patch's width, does not have any remarkable influence on the fatigue life and even may cause some decrease in lifetime.

Main difference between planning vessels, conventional displacement, and semi-displacement marine vessels is in their utilization hydrodynamic loads in addition to buoyancy force. The hydrodynamic pressure field on the bottom surface of planning vessels decreases the vessel draught and the thereafter their resistance. The resistance reduction introduces achieving higher speeds. One of the methods for improving hydrodynamic efficiency of planning vessels is utilization of transverse steps under their bottoms. Selecting their locations and heights has high sensitivity to loading conditions, speed, and center of gravity of the vessel. If proper design does not occur, it maybe followed by un-favorite effects. In this research, the flow field over the bottom of two stepped high speed planning vessels at different lading conditions were obtained, using CFD. VOF method was used for capturing the interface between water and air. Simulation was performed for one-step and two-step planning vessels of Cigar. For validating of the simulations, a grid-independency study and some comparisons with experiments are carried out. The emphasis of this study was on identification of the sensitivity of loading conditions on air-breathing of the transverse steps. The results show that the sensitivity is much higher than the traditional non-stepped planning vessels.

One of the basic parameter to design the hypersonic noses is the induced radiative heating to wall. During flight trajectory, the magnitude of aerodynamic heating changes. To accurate estimation of it, the different methods, is presented which, the numerical solution of navier stocks, chemical reactions, ablative modeling, species conservation, turbulence modeling, heat transfer equations with the finite volume algorithms is perfect method. Utilizing these solvers for flight trajectory require the high computational memory. Therefore, the finite difference method have been used, and the equations have been translated to curvature coordinate by mapping terms. The non propagation of data from flow downstream is the requirement to select the type of the space marching solver, and combination of viscous shock layer at body and similarity of viscous boundary layer at stagnation point methods are pass the mentioned requirement by using the lucidity assumption of the mixture elements. With utilizing this method, the radiative heating results of this research have been the excellence compliance with similar researchs. The results deviation started at Mach number greather than 40 but, in comparative to test results, the behavior of radiative heating variations in accordance with the curvature distance was more logical than the similar researchs. So, at Mach number smaller than 6, the radiative heating, in comparative to conduction and convection heating, is dispensable.

Because of irrational behavior of the load contours applied to aerospace structures, mapping the output contours of fluid software at text file as input file for structural software with respect to requirements of it is most accurate method to analysis and design of aerospace structures. The most applicable of design's software of those, are Fluent and Abacus, therefor in this research, the FACL algorithm, to create of the comprehensive input file of Abacus, is introduced by mapping of the output contours of fluent software. Induced aerodynamic -heating on the surface of the supersonic and hypersonic vehicles is one of the principal parameters of their design in aerodynamic, structural, and other terms, and Because of the deviation of it and surface temperature with empirical results due to steady state of solution and unspooling of shock layer chemical reactions at Fluent software, the temperature result files of CTCA software which written by these researchers is combined with the aerodynamic pressure results of Fluent during the flight trajectory, and the input file to structural automotive analysis by Abacus is created. Designing the attachment structure of ceramic ray dome of the vehicles which equipped to a radar seeker, using the FACL algorithm is essential. The flight test results of a typical vehicle equipped to ceramic ray dome and the structural analysis results of a controller wing of a typical vehicle show, to design and structural analysis of aerospace structures during flight trajectory, the FACL algorithm results is very accurate in comparison of other methods.

Derivation of temperature distribution, at the different sections of nose, to select the material, component, and sensitive system installation at inside of it, implicates to specifying the induced aeroheating to the nose surface. This parameter with surface temperature and recess due to surface ablation must be corrected at next time steps of flight trajectory. The different methods, to estimate or calculation of aeroheating, were created whereas the most accurate method for this purpose is numerical solution of fully navier stocks, chemical dissociation and ionization of air, mass conservation of species, turbulence modeling, combustion modeling due to surface ablation, nose heat transfer equations with time marching finite volume algorithms simultaneously. Utilizing these solvers for flight trajectory is snail, and it’s required the high computational memory. Therefore, the finite difference method is used, and the governing equations are translated to curvature coordinate by mapping terms. By using this translation, to solve the governing equations, the space marching solvers can be used. Therefore, in this research, the more accurate estimation of temperature distribution for 3-D nose of supersonic and hypersonic vehicles was presented by using the numerical space marching solvers such as viscous shock layers and viscous boundary layer methods. Therefore, the comprehensive code was created to this purpose. The results of this code were validated by using the temperature telemetry results of flight tests. The relative error of the results was less than 10 percent.

To investigate the effect of oxygen enrichment in internal combustion engines, we used GT-POWER code to simulate the XU7 engine. The result show an increase of %8 in power and torque, and a decrease of %7 in brake fuel consumption for %23 volumetric excess oxygen. However, the increase of NOX is the only undesired side effect. To reduce exhaust NOX a convenient time variable is selected which can modify this effect, while prevents too much reduction in excess power and torque. Spark timing is chosen as tha mentioned time variable. To optimize oxygen amount and spark timing, we link GT-POWER and SIMULINK-MATLAB as well as neural network toolbox of MATLAB. The target of optimization is %50 reduction in NOX with respect to %23 volumetric oxygen enrichment case and %5 increment in power with respect to natural aspiration case. The designed algorithm is capable of optimizing excess oxygen and spark timing

A finite volume method is used to numerically study the mixed convection of aluminum oxide-water Nano fluid inside a square cavity containing hot quadrilaterals obstacles on its bottom wall. The effects of Rayleigh number, volume fraction of Nano particles and the height of hot obstacles are investigated. The results are shown through streamlines and isotherm curves as well as Nusselt number. The average size of Nano particles and the temperature of Nano fluid are 40 nm and 300 K respectively. The applied rang of Rayleigh number is between 104 and 105, for volume fraction of Nano particles between 0 and 0.06, and for height of obstacles between 0.1 and 0.2 of the height of cavity. The results show that the increase of Rayleigh number as well as volume fraction of Nano particles will increase the heat transfer inside the cavity. Furthermore, the increase of the height of obstacles reduces the average Nusselt number. A comparison with experiments was made to validate the results.

The most accurate method for calculating the aeroheating is numerical solution method. Using finite volume method to solve the Navier-Stokes equations in time is very time consuming. So by using the finite difference algorithm, transfer equations to curvature coordinate by the mapping function, and combination of the viscous shock layer and similarity of boundary layer, the CTCA Code was developed. Determine the species mass concentration at the wall boundary due to chemical reactions of surface ablation and air dissociation/ionization, is one of the main inputs to solve the flow field around the hypersonic noses. Species mass concentration at the wall boundary due to chemical reactions is dependent on the catalytic wall type or time duration of flow stop at the specified point of wall. For fully catalytic walls, the time duration of flow stop is greater than the time required to equivalence chemical reactions and the wall boundary conditions are determined by using the chemical equilibrium assumption. Also in the non-catalytic walls, the time duration of flow stop is less than the time required to equivalence chemical reactions and they are determined by using the chemical frozen assumption. The severity of wall catalyst is dependent on the recombination average rate of species and this parameter is one of the inputs to solve the problem. Therefore in this study, the wall catalytic effects on the aeroheating of ablative noses were investigated by the space marching method. The result of current research and other similar researches showed that, the fully catalytic at stagnation points and the non-catalytic at body are reasonable assumptions.

Exergy analysis is a method of evaluating the contribution proportion of each process on transmission of initial availability of the system and determining the positions of useful energy losses. In this research exergic analysis of an internal combustion engine equipped with turbocharger and intercooler is attended. To do this, at first, the procedure of performance simulation of the system components i.e. engine, compressor, turbine and intercooler is briefly explained. Then by defining exergy terms and using related exergy equilibrium equations for open and closed systems necessary conceptual basis for exergy analysis, is introduced. Also exergoeconomic analysis, as a powerful tool for cost study and optimization of complex energy systems is developed by combining the second law of thermodynamics concept with economics point of view. The results show that combustion process produces the most proportion of the system irreversibility. Furthermore, regarding exergoeconomic parameters, engine has the most suitable performance among the related components of the system.

Nowadays, CAD/CAM simulation is considered as a strong tool to estimate performance of various systems and to reduce costs and testing time. In this study, we used GT-Power Software to model the performance of an SI engine. The results indicate that the engine power drops significantly as altitude increases. To compensate power loss, the engine is equiped with a turbocharger system. The proper turbocharger was selected with regard to the surge and choke limits of its compressor, as well as its compatibility with the engine (at mass flow rate point of view). To avoid engine knock, we used a special wastegate mechanism. Also, to reduce the temprature of the flow after compressor, we defined a proper type of intrcooler system. we used the experimental results derived in our engine lab. The results show that by using turbocharger and intercooler, the engine brake power improves. In addition, the specific fuel consumption decreases and the volumetric efficiency increases. Also, NOX emission reduction and safer cyclic performance of compressor are the effects of intercooling.