مجتبی قطعی

Nowadays, the utilization of smart materials is experiencing rapid growth across a wide range of industrial systems. Magnetorheological elastomers are a class of smart materials and possess two unique features, i.e. adjustable hardness and damping capabilities. These characteristics make them widely used in various industrial applications. Hence, understanding their behavior in different systems is necessary. The focus of this work is to study the dynamic behavior of magnetorheological elastomers under different static pre-strains in tension-compression mode. In this study, three isotropic samples were fabricated and the force-deflection features of them were acquired under harmonic excitation with various strain amplitudes, static pre-strains, frequencies, and magnetic flux densities. Assess the effect of static pre-strain on the dynamic response of the magnetorheological elastomers, studying the effects of other parameters like strains, frequencies, and magnetic flux densities on the dynamic modulus of magnetorheological elastomers, and proposing a novel phenomenological-based model to predict the viscoelastic behavior of magnetorheological elastomers are the innovative aspects of this study. The results showed that the dynamic modulus of magnetorheological elastomers will increase by superimposing the static pre-strain. Furthermore, the relative MR effect decreases when the static pre-strain is applied. The maximum relative MR effect of 288.3158% have been achieved at a strain of 4%, a frequency of 7 Hz, and a static pre-strain of 0%.

The present study aims to synthesis and characterization of spinel coating for spark plug aero-engine application. At first spinel coatings were synthesized by mechanical alloying. The different spinel coatings with spinel different composition and modulation were applied on alumina substrate. The phase content of the samples was characterized by X-ray diffraction (XRD) method. The microstructure and thickness of the coatings were analyzed by scanning electron microscopy. Nano-hardness and Young’s modulus of the coating were measured by nano-indentation method. The electrical resistance of the coating was studied at different temperatures. The electrical wear resistance of the coating was evaluated by spark method. The XRD method confirmed the synthesis of copper based spinels. The coatings had a thickness of 19-22 μm with proper density. The average nano-hardness of the coatings was 10 GPa. The wear rate after 140000 of spark was less than 1% of coating weight. It was also found that the optimum modulation of the multilayer coating is 4 successive layers composed of CuFe2O4+CuCr2O4, followed by two layer of CuCr2O4 and again CuFe2O4+CuCr2O4 layer. The ratio of the components in the composite layers were 50:50 wt.%

Sedimentations and hard cakes formation of magnetic particles restrict magnetorheological fluid response to magnetic field and can cause the MR fluid containing device to collapse. Therefore, researches on MR fluids sedimentation reduction procedures and its effective factors are in great deal of interest to improve magnetorheological applications. In this study, the effects of some parameters on typical MRF stability were investigated. For this purpose, at first, MRF samples were constructed and the effects of various factors including carrier fluid type, particles concentrations and MRF mixing methods on its stability were investigated and the importance of each factor were determined by Taguchi algorithm and the stable MRF sample for application of magnetorheological dampers was chosen. Next, by investigation of the most stable MRF sample, based on the combination of stability and off- state viscosity factors, the relation for yield stress in various magnetic fields was presented. This relation was derived based on fitting the Herschel- Bulkley model with experimental data in conjunction with the existing relations of yield stress. As the results shown, after 168 hours, sedimentation for the most stable sample is 7%. This sample consists of silicon oil and 70%wt iron powder which prepared with mechanical stirrer. Adding 3%wt stearic acid to carrier fluid for increasing the stability, results to increasing the viscosity of carrier fluid up to 39 times. In spite of this, an acceptable MR effect is presented so that, in magnetic field of 146KA/m the sample yield stress is 15KPa.

With respect to special conditions apply to the gas turbine, its blades are affected by many different factors such as, hot corrosion, oxidation, wear, impact of external particles, and etc. and are destroyed. Due to the reduction of their working life time, the turbine efficiency reduces and ultimately the heavy costs of periodic repairs are needed, and also new replacements of their blades are unavoidable. The aim of this study is investigation of the effects of corrosion and blade damage on flow field and gas turbine performance, by numerical simulation. In this research, a two stage turbine is modeled in the form of three dimensional and the results are validated with experimental data. To analyze of the behavior of entire flow, conservation of mass, momentum, and energy equations are solved. The numerical simulation of the turbine is done with ANSYS CFX software. Then the increased rotors tip clearance effects with decreasing thickness due to corrosion in both nozzles and blade leading edge and trailing edge were separately studied on turbine flow field and its performance in five actual different pressure ratios. The results showed that the most important factor in reducing the efficiency of gas turbine is due to rotor tip clearance increasing. Also corrosion of the blade edge respect to the trailing edge damage is a little more affected on reducing efficiency and increasing loss coefficients.

High temperatures and different properties of entering gas into the turbine of a gas turbine cycle can decrease its performance. Considering the complexity of the flow distribution inside the turbine، three-dimensional analysis to find out the flow and temperature field in the turbine stages is very important. As time passing the increasing of the roughness of blades is unavoidable. The aim of this paper is investigation of the blades roughness effects on flow field and efficiency of gas turbine with numerical calculations. In this research، a two-stage turbine is modeled in the form of three-dimensional and the results are validated with experimental data. Then the effects of blades roughness on flow field and performance of turbine in five pressure ratios is investigated. Also، in order to determine the role of stators and rotors in decreasing the turbine efficiency، in a special roughness، the first and second stators and then corresponding rotors have separately been examined and then this phenomenon affected on blades simultaneously. Results showed that the efficiency drop by applying all together on the turbine stage is approximately equal to summation of efficiency drops by applying separately.