فتح الله امی

In this paper, the starting and steady-periodic characteristics of a multi-stage looped-branched thermoacoustic cryocooler and its uncertainty with respect to effective parameters are calculated and discussed. In this regard, the start-up and nominal performance of the system are simulated using the numerical solution of Rott's one-dimensional equations. Numerical and experimental results of a similar system confirm the validity of the results of the present article. Simulation outcomes show that the number of stages, mean pressure and loop perimeter can significantly change the system performance. In contrast to previous findings, the results in this paper show that increasing the number of stages or increasing the mean pressure does not necessarily improve thermodynamic performance. Besides, Morris method is implemented to quantify uncertainty of the operating parameters of a four-stage cryocooler in relation to the geometric, modeling and material characteristics. The results reveal that the gas characteristics, solid conductivity and mesh geometry of the engine and refrigerator regenerator have a dominant nonlinear effect on the required hot temperature, exergy efficiency, relative Carnot efficiency, cooling load and temperature. Moreover, by adding additional thermodynamic cores in each stage, the amount of acoustic work and total enthalpy and its uncertainty increase and the effect of solid thermal conductivity decreases. With the developed model and main findings of this paper, one can optimize a reliable thermoacoustic cryocooler based on its onset and steady characteristics.

To improve the mixing of fuel and air in supersonic flows, various methods are used, including the implementation of a ramp ahead of the injection port. In the present work, the effect of ramp height on the area of double hydrogen jet cross-jet flow in supersonic airflow has been investigated numerically and the effect of ramp height on parameters such as mixing efficiency, effective mixing area ratio and stagnation pressure losses has been investigated. Numerical simulations have been performed using the solution of the three-dimensional Reynolds-Averaged Navier-Stokes equations with the two-equation sst k-ω turbulent model. Initially, the results of the numerical solution are validated with the experimental data. It was shown that the numerical solution results have good agreement with experimental data. Then, the effect of the presence of a ramp upstream of the injection port is numerically investigated for several ramps with different heights. The results show that by increasing the height of the ramp from zero to 6 mm, the mixing efficiency at the exit plane increases from 0.37 to 0.52 and the effective mixing area ratio at the exit plane increases from 0.059 to 0.1. Stagnation pressure losses also increase from 8% to 9%.

In this research, the internal flow of open-end pressure-swirl atomizer, which has many applications in gas turbine engines and space propulsion, is investigated numerically. A multi-phase fluid volume model is used to simulate the internal flow of the atomizer. Also, due to the nature of the rotational flow inside the atomizer, the RNG k-ε turbulence model was used. Structured mesh has been used to achieve better results. In this research, spray parameters such as liquid film thickness, spray cone angle, discharge coefficient and velocity and pressure distribution contours have been studied. Simulation of an open-end atomizer with an L⁄D = 5 ratio, as well as the use of a structured mesh and the inclusion of kerosene fuel to create conditions close to the operation of a gas turbine, are among the things that differentiate the present study from other studies. The results also show that at a pressure of 0.5 MPa, the time required to reach the fuel in the atomizer used is less than 1.5 milliseconds and the failure of the to spray cone and turn into larger droplets can be observed. The spray angle and discharge coefficient for the studied atomizer were 74.8 and 0.17, respectively.

Due to the fact that the velocity of air flow in the combustion chamber of scramjets is supersonic, so there is not enough time for proper mixing of fuel and air at these high speeds, and studying the process of mixing of fuel and air in this situation is one of the important issues for scramjet engine design. One of the most widely used methods of fuel injection in supersonic flows of scramjet engine combustion chambers is the method of transverse injection into supersonic air crossflow. Since the velocity of the passing air is very high in such streams, it is challenging to achieve a suitable mixture for combustion. In the present work, by solving the three-dimensional Reynolds-Averaged-Navier-Stokes equations with the k-ω sst turbulence model and the ideal gas state equation, the transverse injection domain in the supersonic flow is numerically simulated and the effect of injection diameter is investigated based on the mixing properties such as fuel penetration depth, mixing efficiency, effective mixing area ratio and stagnation pressure losses. After that, results of reactive flow are presented.

This paper presents a numerical method to evaluate the system conditions in transient mode for a standing wave thermo-acoustic engine (STAE). This method focuses on increasing the accuracy of predictions and creating a better eyesight of the operating conditions of these devices. Combining the numerical solution of transient mode with the electrical circuit analogy of standing wave TAE, is the main idea of this work. Electrical circuit analogy of STAE is done by assuming each component of a STAE as a lumped element. The result of Combining the time-dependent numerical solution and the electrical circuit analogy is a code, which determines the condition of startup, steady periodic, and damping regime of the spontaneous oscillations in the STAE. Accordingly, a nonlinear temperature distribution is obtained along the hot core. The developed numerical approach is in line with the experimental results of a STAE. Based on this method, the temperature variations along the stack and the impact of stack material on startup time and onset temperature are numerically investigated. Additionally, the onset temperature profiles are calculated and presented comparatively for the numerical and experimental results. The startup time of spontaneous oscillations is calculated for the standing wave system. Consequently, the best geometry that quickly reaches the sustained oscillations can be selected using this model. Examining the temperature profile during the startup and damping process highlights a temperature difference between these two processes. Observations show that using a material with lower thermal conductivity in the stack section can reduce the onset-damping temperature difference. Also, with the help of this method the startup moment of spontaneous oscillations was calculated as second in the system. The data obtained from the experimental tests and the temperature profiles resulting from the numerical solution method, showed a good agreement with each other for the onset and damping process in the system. The novel approach described here shows potential in capturing the onset-damping behavior of thermoacoustic systems efficiently.

In this study, the effects of adding methylated homopolymer to gasoline on the performance of an internal combustion engine have been investigated. This additive increases the number of octane gasoline. In this research, these fuels which include ordinary gasoline and the other gasoline with the methylated homopolymer, were subjected to performance tests on the internal combustion engine. Also, the amount of exhaust emissions of the engine was measured. The results of experimental tests show that with the addition of additive to gasoline, the brake torque and braking power were increased by 4.28 % and 2.31 %, respectively, in the mean of average values (different engine operating speed). By combining gasoline and additive, carbon dioxide and nitrogen oxide contaminants increase by 7.41 % and 6.21 %, respectively. Also, contaminants of uncured hydrocarbons and carbon monoxide emissions from the engine are reduced by 18.17 % and 9.05 %, respectively, in the mean of average values.

The challenge of the relationship between academia and industry is an issue that experts and academics and industry officials have been addressing more and more in recent years and are looking for safe and planned solutions and ways to strengthen this relationship. The relationship between industry and academia has been discussed in the country for several decades, and the first statements of the Supreme Leader in this regard date back to 1990, but what is certain is that there has been no effective relationship between science, academia and industry; Today, more than ever, the structure of science, technology and industry in the country needs to be reviewed and interacted as much as possible, and industry stakeholders must accept the fact that, along with the university, they will be able to fully absorb technology.This is part of His Holiness's statement in a group meeting of university professors, elites and researchers: "We have talked a lot about 'beneficial science'; "That is, the science that solves the problems of the country; the solution of the problems of the country is a beneficial science, that is, it is a scientific confrontation with the various problems that exist in the country." At first glance, perhaps the biggest problem in the communication space between industry, society and academia is the technical issue, but upon entering this space, it is seen that this is not the case in practice, and it seems that lack of information, lack of trust and lack of motivation are the three main pillars. ; Too often, our industry and society are unaware of the capacity and potential of knowledge-based companies, academic companies, and faculty members.In the current era, universities and entrepreneurs have a greater mission and social responsibility in the face of economic development, productivity in production and the creation of sustainable income, and a connection with industry and production. These basic problems led to the presentation of research in this area to be able to be effective in removing existing barriers. One of the most important challenges in the relationship between industry and the university is the academic level of Iran's first-ranked universities, which is at the level of international knowledge, while Iran's industry is at the level of developing countries. Be

Thermoacoustic engine is an energy conversion device that uses the energy carrying capacity of sound waves to generate sound power from thermal energy. Although it is not difficult to build thermoacoustic engines due to having no moving parts, many researchers have always tried to reduce the temperature difference required to run thermoacoustic engines, so that these devices can be used in most industries. To investigate the onset conditions of the system, temperature changes in the stack section of a standing wave Thermoacoustic engine were investigated. Numerical analysis of temperature changes along the stack, was performed using the rotts thermoacoustic equations. The temperature was calculated instantaneously along the stack, and this process continued until the thermal equilibrium was established in the system. A standing wave with an open end was designed and built to validate the temperature curves obtained at different moments. This thermoacoustic engine was able to display the temperature instantaneously along the stack with parallel plates structure. The data obtained from the experimental tests and the temperature changes diagram resulting from the numerical solution method, showed a good agreement with each other for the onset process in the system.

In this paper, the heat transfer and ablation thermal insulators in solid rocket motor are investigated. Therefore, by collecting and solving the thermal ablation equations, a computer program, using MATLAB software, is developed which can predict the thermal response of insulators in different operating conditions and compare the performance of these insulators. The heat and mass transfer equations are considered in two dimensions in a solid body. We used the equations, finite volume method with implicit formulation for time dependency to solve equations. The reaction equation which written in the form of Arrhenius, is solved using Runge-Kutta method, and the density and the flux of the gas produced at each step are obtained. Also we represent a model for the rate of recession.

Today's Human Life is Undeniably Dependent on Satellites that have being sent to Earth's Orbit by Launch vehicles. Delivery of any Kind of Cargo to Space must be carried out by Launchers, which are Designed and Optimized for the Best Energy Consumption Mood. Use of Porous Materials in the Engine Injector Plate culd be One of the Optimization Steps in this vehicles Engine. In this Paper, Applications that Utilize Porous Materials to Improve Combustion Efficiency have been Reviewed. In this Regard, the Results of Experiments Performed by Four Different Porous Injector Designs, were Studied. The First Design was a Porous Injector with Five Liquid Oxygen Injection Posts, which has been Undergone Cold and Hot Tests for Optical Researches.The Test Results of the Porous Injectors API68, API80-168 and the Improved API68 Injector, Using Two Subscale Combustion Chambers Designed and Built at the German Space Research Center, were Reviewed. Combustion Chamber Used for the Optical Tests, was Cooled by Water and Made Possibility of Taking Picture From Combustion Process. One of the Important Points, was the Reduction in Combustion Efficiency of the API80-168 Injector Head, due to Injector Head Correction, to Prevent Excess Oxygen Accumulation near the Combustion Chamber Wall. Another Important Point, was the Acoustic Damping Characteristic of the Porous Injector Plate, which Results in More Combustion Stability, Compared to the Coaxial Injector Head. However, During Some Reviewed Test Runs of API80- 168 Injector Head, Strong Oscillatory Instabilities were Observed which caused the Engine to Automatically Shut Down. Finally the Experimental Results of Four Different Porous Injector Heads with Oxidizer Transfer Tubes, Under Different Fluid Flow Regimes and Deep Throttling was Analyzed

The two-dimensional numerical simulation of the spray flame formed in a laminar counterflow configuration has been performed to investigate flame regimes and its effect on changes in important species and radicals. Ethanol is used as a liquid spray fuel and the skeletal mechanism of ethanol/air with 40 species and 180 preliminary reactions is used for combustion reactions. The Sauter mean diameter of fuel droplets and randomly injected into the air inlet are considered using a UDF code, and the motion of the droplets is calculated using the Lagrangian approach. The results show that the predominant flame regime is Non-Premixed in the high equivalences ratio, high strain rates and large droplet diameters. Also, the combustion reactions are suppressed in the center areas of the flame, due to the reduction of the oxidant fraction and the OH mass fraction, and the flame index is zero, which indicates the internal group of droplets in this area.

This paper presents numerical study on spray characteristics and droplet distribution by using Lagrangian method in the discrete phase model of CFD. A two-fluid Eulerian method and Lagrangian approach is selected for modeling two phases turbulence flow in mixing chamber and atomization at outlet of nozzle while turbulence has been modeled by K-ɛ. In this study, water has been used instead of fuel and Nitrogen instead of atomization gas or oxidizer, while their ratio has been considered 0.32 to provide 26 degrees cone angle and this way, droplet‘s characteristic has been studied and compared with maximum entropy methods. Then droplet‘s diameter has been investigated by changing liquid and gas phase flow rateand based on that, we can optimize atomizer ‘s working condition with maximum efficiency with respect to its cone angle, droplet ‘s diameter and velocity and level of penetration by minimum need of experimental tests.

A two-dimensional numerical simulation of the spray flame has been carried out in a smooth counterflow configuration, and the region of formation of droplets group combustion by the flame index parameter for strain rate, equivalence ratio and the diameter of different droplets. n-decane (C10H22) is used as a liquid spray fuel, and a one-step global reaction is employed for the combustion reaction model. Fuel droplets are randomly injected using an UDF code in the air inlet and the droplet motion is calculated by Lagrange method. According to the results, in the ratio of high ratios, high strain rates and large droplet diameters, The temperature of the flame center decreases due to the suppression of the chemical reactions resulting from the reduction of oxidizing fractionation, and the dominant flame regime in these states is diffusion. The external group combustion of droplets occurs in the upper and lower layers of flame, and combustion of the internal group occurs at the center of the flame.

Studying the combustor flow field helps identify effective fuel and air mixing areas. The purpose of this paper is to investigate the structure of cold flow in a single-element Lean Direct Injection combustor. In studying and modeling of a turbine engine combustor, it is important to identify the areas of swirling, reversal and symmetry of the flow inside the chamber due to its influence on the rate of fuel and air mixing and also to study the reaction flow accurately. In this paper, the effect of operating pressure on the turbulent flow is studied. Since computational fluid dynamics are very time-consuming to solve the reacting flow problems, it is necessary to select the appropriate turbulence model that saves time with good accuracy. Comparing the computational results of the k-ɛ Realizable and Reynolds stress turbulence models (RSTM) and computational errors analysis, the maximum error was obtained for the Reynolds stress model of 31% and for the k-ɛ Realizable model of 37% compared to the experimental data. The difference between numerical results of turbulence models showed that the low-cost k-ɛ Realizable model is acceptable for the studied studies and can be used to model the reacting flow of an LDI combustor with highly swirl flow.

In this paper, the heat transfer and ablation thermal insulators in solid rocket motor are investigated. Therefore, by collecting and solving the thermal ablation equations, a computer program, using MATLAB software, is developed which can predict the thermal response of insulators in different operating conditions and compare the performance of these insulators. The heat and mass transfer equations are considered in two dimensions in a solid body. We used the equations, finite volume method with implicit formulation for time dependency to solve equations. The reaction equation which written in the form of Arrhenius, is solved using Runge-Kutta method, and the density and the flux of the gas produced at each step are obtained. Also we represent a model for the rate of recession.

In this study, the effects of using multi-electrode spark plugs on the performance parameters of an internal combustion engine were investigated using experimental tests. In this regard, experimental tests have been carried out using two types of lead-free gasoline, as well as a combination of gasoline with commercial additive (Keropur-G). The results of experimental tests show that with the use of multi-electrode spark plugs in conventional fuel and fuel with additive, the braking torque and braking power are increased and in comparison the specific fuel consumption (SFC) is decreased. Also the use of multi-electrode spark plugs increases the emission of nitrogen oxides and reduces the emissions of unburnt hydrocarbons, carbon monoxide and carbon dioxide from the engine.

The purpose of this paper is to investigate the magnetohydrodynamics technology in creating artificial earthquakes. For this purpose, the possibility of expressing the electronegative mechanism for artificial earthquakes is first discussed. Then, the mechanism of such earthquakes and their impact, using this technology has been investigated. Also, application of these artificial earthquakes is described. As follows, using field and laboratory experiments, the results of tectonic tensions with high power pulses to reduce earthquake hazards have been addressed. Subsequently, the description of this technology and its mechanism and further the measurement of the tectonic stresses of the earth's crust and its evacuation, using a magnetohydrodynamic high power generator, have been made. Finally, an example of the creation of an artificial earthquake is mentioned and examined.

Now, the required samples to achieve the specific precision of sensitivity analysis in design are performed based on trial and error methods. The purpose of this paper is to develop for determining the number of the required sample to achieve the specific precision of sensitivity analysis. Thus, in this paper, a new sensitivity analysis method is proposed based on the Progressive Latin hypercube Sampling (PLHS) and the convergence of the analysis results. For this purpose, a PLHS method has been developed. This cystic approach has led to a sensitivity analysis of accuracy, efficiency and speed in a variety of models with a large number of large parameters and large changes. Sensitivity analysis has been performed on the design of a hydrazine monopropellant thruster catalyst bed model as a case study. The results of this study indicate that in the sensitivity analysis based on the PLHS, the minimum population required for sensitivity analysis with specified accuracy can be determined. This leads to lower processing costs in the sensitivity analysis process, especially in complex models.

Transverse fuel injection in supersonic crossflow of scramjet combustors is one of the common methods for fuel-air mixing. Air jet injection before fuel jet can improve fuel penetration but with the excess stagnation pressure loss. In this paper, the position of air jet is changed to find the position that maximum fuel jet penetration height and minimum stagnation pressure loss is achieved. For numerical simulations, Two-dimensional Navier-Stokes equations and k-ω sst turbulence model and the perfect gas equation are solved. Then the results of the numerical solution are compared and validated with experimental data. Numerical results showed good agreement with the experimental values. In the experimental test, Helium is injected into supersonic crossflow and so numerical simulation is started with Helium injection. In scramjet usually, hydrogen was used as fuel. Therefore, hydrogen fuel injection is used for the parametric study. In the case of air injection near the fuel injection, maximum fuel penetration with minimum stagnation pressure loss is achieved.

One-dimensional modeling of aircraft piston engines is one of the methods for studying their behavior and performance in different environmental conditions. In this research, the Rotax 914, an aerial Spark Ignition (SI) turbocharged engine, was modeled one-dimensionally to predict its behavior at different altitudes. To address this issue, the one-dimensional model of all engine components including air filter, carburetor, air manifold, inlet and outlet valves, cylinder and piston along with crankshaft, exhaust manifold and turbocharger were created in GT-Power software.Then by validating the model with the data provided by the manufacturer and ensuring the accuracy, the significant performance parameters of engine and turbocharger performance curves were evaluated for different flight conditions up to 30,000 feet. According to the results, the turbocharger prevented a dramatic decline in engine performance parameters up to 18,000 feet. However above that altitude, due to the compressor reaching the choking region, it is not possible to charge the required air and avoid severe engine performance loss. It was also observed, by studying the effect of ambient temperature changes on engine performance, an increase of 40 °C in the ambient temperature caused a 10% reduction in the braking horse power, and consequently, performance loss during climb period. The effect of changes in ambient temperature and altitude on engine performance varies so that, until the engine reaches the compressor's choking region, the impact of the ambient temperature variations is more perceptible, after that, the height factor will have a greater influence on the engine performance.

The idea of designing new geometries for catalytic bed in the decomposition chamber of monopropellant thrusters is introduced with numerical simulations of pore-scale turbulent flows. The LES numerical technique is used for simulation of turbulent structures in the flow-field. The efficiency and reliability of the results obtained from numerical simulation have been determined by solving a benchmark problem of turbulent flow over the pack of cubes. The results show very good agreement with the experimental data, indicating the accuracy of the used model and numerical solution process. The characteristics of turbulent flow over two different geometries have been investigated using the numerical method. The results have been analyzed to evaluate the effectiveness of geometrical changes on the parameters associated with the catalytic reaction. All simulations have been conducted for cold flow, and the exact effects of the geometrical design of porous bed on reactive flow have not been quantified. The eddy dissipation and length scales of turbulence have been considered as the main parameters, because of their effect on rates of turbulent mixing and rate of reaction. The difference between the turbulent dissipation and length scales in the investigated flows in two different geometries indicates the effectiveness of the geometrical changes of the porous bed on the flow characteristics. Coherent structures are seen in the new geometry and the wall shear stress is reduced significantly, which increases the life of the catalytic coating.

In this research, the possibility of measuring total pressure, mass flow, and velocity of a high energy flow of air at 0.6-0.7 Mach is investigated experimentally, using 6 different types of commercial sensors. For this end, a fixed area annular nozzle, mounted at the exit of micro-turbojet engine, was used. Also, a test bench with the capability of measuring total pressure, static pressure, total temperature, mass flow, RPM, and the thrust force was used. The results of the L-type sensors calibrated for such velocities indicate that the total pressure and velocity are similar, near to each other, and among the engineering precision. The largest difference between the measured and calculated mass flow was 9.1% and related to L-type probes with the length of 68mm and the outside diameter of 3mm. This difference for all other probes was less than 8%. Also, the calculated mass flow based on Rod-type probe data shows a difference of only 4.4% with the measured mass flow; so, there is a distinct difference between these two kinds of probes. Also, the measurements include useful information of the variations of main flow characteristics along the length of annular nozzle, among which the most important are an intense drop of about 29% in total pressure and about 48°C drop in total temperature.