Journal of Aerospace Mechanics
Year:2019 | Volume:15 | Issue:2 (56)|Papers Count:13

Correct heat management is one of the major problems in fuel cells that should be somehow solved. The performance of the cell at high temperatures causes the membranes to dry, increasing ohmic resistance of cell, shrinking and rupture of membranes and at low temperatures decrease the reaction rate, voltage, efficiency, output power, as well as condensation of water and occurrence of flooding at the cathode side. Increasing power in fuel cells associated with increasing the number of cells in a fuel cell stack. By increasing power, high flow rate of the cooling fluid is required to dissipate more generated heat. Increasing cooling flow rate increases the volume of the cooling system, parasitic power and reduces the efficiency of the stack. In this paper, using of nanofluids as an approach to solving this problem is presented and its effect on reducing the parasitic power is investigated. The results showed with using mixture of water and 2% volume fraction of Al2O3, at Re=6000 the temperature difference of all parts of flow field compare to inlet is smaller than 5℃ . At the case that base fluid is used, this goal achieved at Re=9000.

In this paper, a Min-Max controller design is presented for a double shaft unmixed turbofan engine’ s thermodynamic model at design point. For this objective, first, the safe operation’ s limitations for turbofan engine are determined from the related catalogues and references and used in controller design process. Thermodynamic Modelling and controller design are conducted in Simulink environment of MATLAB software. Simulation results show that the controller effectively provides the thrust command and the limitations of high pressure spool speed and acceleration, output pressure of high pressure compressor and output temperature of combustion chamber are satisfied and surge, stall and flameout events are prevented.

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.

In recent centuries energy has become of great importance due to the shortage of energy resources and energy losses. Efficient use of energy and minimizing energy waste has become one of the largest research purposes and achieving this goal has provided the field of production of the devices. In this research, the construction of a high-temperature heat pipe with mercury as the working fluid as well as CFD simulation of the heat pipe is investigated and the obtained results are presented. The factors affecting the heat transfer of the heat pipe, including the evaporator and condenser internal heat transfer coefficients and the thermal conductivity coefficient are empirically and numerically investigated and the results are reported as the diagram. The results show that the heat pipe in the heat flux of 278 watts has an efficiency of 0. 89 and has the lowest thermal resistance (R=1. 3 ° C/W) against the heat transfer, which has a better performance than the lower input heat fluxes. The heat pipe built in temperature range close to the design conditions delivers more acceptable performance.

The flow affected by a magnetic field is applied in cooling electronic devices and voltage transformers and in physical phenomenon such as geology. In this study, the effects of magnetic field on the flow field and heat transfer of Cu-water nanofluid natural convection and by considering the Brownian motion of nanoparticles have been studied in a trapezoidal enclosure. The study has been done for Rayleigh numbers 103 to 1010, Hartmann numbers 0 to 100 and the nanoparticles volume fraction of 0 to 0. 04. The governing equations have been solved numerically by use of finite volume method and SIMPLER algorithm. The results showed that by applying the magnetic field and increasing it, the nanofluid convection and the strength of flow decrease and the flow tends toward natural convection and finally toward pure conduction. For all of the Reynolds numbers and volume fractions which are considered, by increasing the Hartmann number, the average Nusselt number decreases.

Electromagnetic launcher uses electrical energy to launch an armature. In order to simulate the movement of this type of launchers, it is necessary to solve the Maxwell equations in rails and armature. Due to the high acceleration and low speed of the armature at the start of the movement, it is necessary to apply the non-uniform meshes at the beginning of the rails. Changes in the physical properties of the rail and armature must be considered versus the temperature. The calculated results are in the agreement with the experimental velocity results reported in the previous study. The magnetic contours indicate that the high share of electrical current passes through the interior of the rails and the armature. The results of the temperature distribution show that the highest temperature is produced in the armature wings and the inner edge of the rails. The effect of temperature and magnetic force distribution on the thermal stress in rails and armature has been investigated and it has been shown that the moment of occurrence of the highest thermal stress in rails and armature is different.

In this research, the governing differential equation of heat conduction problem in a non-homogenous, functionally graded material (FGM) is solved using the boundary elements method (BEM). Except for some limited cases, there is not known Green function or fundamental singular solution for this kind of problems, which is necessary to have the boundary elements analysis. In this paper, the thermal conductivity of the functionally graded plate is considered as a quadratic function of one direction and then an auxiliary variable is introduced into the governing differential equation in order to simplify the problem to a kind with known fundamental singular solution and therefore, heat transfer is solved in a 2D functionally graded material by simple boundary elements method which is not possible by common methods. Based on the proposed approach, a computer code is developed using the MATLAB. The validity of this developed code is verified by solving and analyzing a number of heat transfer problems.

In this paper, a novel high speed forming process called warm electrohydraulic forming (WEHF) process is introduced. In this new forming process, the blank is heated before the forming process and then formed using the Electrohydraulic forming process. In order to investigate the formability improvement in this hybrid process, first the electrohydraulic forming of aluminum alloy AA5182-O at room temperature is simulated using the commercial software LS-DYNA and the results of simulation are verified with the experimental results available in the literature. After verification, the warm electrohydraulic forming process in different temperatures is simulated and the Forming Limit Diagrams (FLD) are obtained and compared to each other. The results indicate that considerable improvement in formability of the aluminum alloys in warm electrohydraulic forming process in different forming paths. Due to the ability of the WEHF in formability improvement, a novel WEHF is introduced and its formability improvement is investigated. In the experimental tests 23. 6% increasing in failure strain of aluminum alloy is observed at WEHF in comparison with EHF. Therefore the formability improvement in WEHF process can be concluded.

Currently, most of the available tool wear condition monitoring systems are based on the signal features quantities that are correlated with tool wear. The evaluation of tool wear based on one sensor is not reliable because the measured features depends not only on tool wear but also on other process parameters and random disturbances. For solving this problem, multi sensor data fusion is used to combine disparate sensory data. The obtained information are more accurate and reliable. In this research, combination of vision, current, strain and vibration sensors for predicting flank wear land is proposed. An optimized adaptive neuro-fuzzy inference system (ANFIS) model is developed to fuse the surface image, motor current, strain and vibration signal features. The structure of proposed ANFIS model has four inputs and one output. The inputs of the model are entropy of surface image (which is filtered by undecimated wavelet transform), time-frequency marginal integral of the motor current, Shannon entropy of strain and Shannon entropy of vibration signals, while output of the model is the flank wear. The results showed that the optimized ANFIS model can be used to fuse the signal features and predict tool flank wear with high accuracy.

Ultrasonic assisted surface rolling (UASR) process is one of the most prominent methods to create severe plastic deformation and fine-grained structures. The aim of this paper is to study the ultrasonic vibration effects in surface rolling process, experimentally. In this regard, the desired horn was designed and manufactured in order to vibrate at longitudinal mode, which the proposed horn is able to change the mechanical properties of the metal surface by applying static pressure and dynamic impacts. Experimental tests were performed for three samples: CK15, CK45 and CK60 (Low, medium and high carbon) with and without applying ultrasonic vibrations. Micro surface hardness tests of samples were compared before and after UASR. The results showed that the higher static pressure leads to the higher micro surface hardness of samples. In addition, the use of ultrasonic vibrations in UASR can cause considerable rise in micro surface hardness. Based on the experimental results, it was found that, applying ultrasonic vibrations in surface rolling process are more effectiveness specifically in high carbon samples. Fatigue life investigation in various amplitudes showed the relative fatigue life improved in UASR process, especially in high vibrations amplitudes.

Productions of metallic ultra-fine and Nano microstructure have been addressed by researchers and engineers in recent years. The purpose of the production of these materials with specific procedures is to achieve lightweight parts with high strength and capabilities. Various methods for manufacturing solid round or square sections using equal channel angular pressing have been offered by researchers. However, few researches have been reported the production of pipes with high strength-to-weight property. A direct extrusion method is proposed for the production of high-strength tubes. Aluminum alloy 3003 that is widely used in industries because of the structural characteristics of a cold working is used in this study. Previous studies have been focused on channel angle of 90o, but in this study, angles above 90o are investigated. Strain and stress distribution results in finite element simulation have been used to obtain the optimal internal and external channel angles. The optimal channel angles are first investigated by simulation results. The required die with optimal channel angles was then manufactured and the samples were experimentally tested. The results showed that the use of these angles with optimal channel angels in equal channel angular presses for aluminum 3003 tube increases significantly the tensile strength and hardness.

Semi-solid metal (SSM) processing has been used to produce of the engineering components with the suitable proper-ties and at the closet form of the final part. Thixoforging is one of SSM processes that in this process the forming opera-tion is performed on materials with the solid fraction of less than 55%. The purpose of this paper is to discuss on the temperature (mold and specimen temperature and holding time) for the producing of a relatively complex piece. The characteristics of the microstructures and mechanical properties of thixoforged Al-A356 alloy were studied. For this aim, some experimental tests were done. Also the results of these tests are compared with the results of simulations. The results showed that by increasing mold temperature, causes more inhomogeneous microstructure and therefore the hardness and forming force decreased %12. 5 and %20. 6 respectively. Also the results showed that increasing the spec-imen temperature and holding time, the grain size of primary α – al phase increased. For example in holding time of 5 min, by increasing the temperature from 570° C to 600° C, the grain size of primary α – al increased 3. 5 percent. Also in the temperature from 570° C, by increasing holding time of 5 min to 30 min, the grain size of primary α – al increased 73. 9 percent. In this work, the T6 heat treatment process was used for improving the hardness. The hardness was in-creased %17. 3, because of changing in primary α – al phase and produced the homogeneous microstructure.