نشریه مهندسی مکانیک مدرس
سال بیستم شماره 8 (امرداد 1399)

Research on DNA is particularly important in the diagnosis, control, and treatment of many diseases, including cancer. Today, the use of digital droplet polymerase chain reaction (ddPCR) for different DNA tests has attracted much researchers’ attention. A large number of micron-sized droplets are required to perform ddPCR. In the present study, a ddPCR system was designed, fabricated, and evaluated using a microfluidic chip. The system comprises a microfluidic chip for droplet generation and a thermal cycling device needed for PCR. The droplet generation in the microchip was simulated in 3D. The simulation results were validated. The average error is about 5% in the radius of the droplets. The constructed thermal cycling device controls the chip temperature with a precision of ±1.5°C. The in-chip PCR process was successfully performed by applying 25 heat cycles. The fluorescent property was observed in most droplets that prove the thermal cycling device can provide the conditions for DNA proliferation in the laboratory. The images were processed, and different levels of fluorescent light were identified in the droplets. The coefficient of variation of the selected droplets is 2.5%, which gives a good accuracy compared to the acceptable amount for these types of systems (less than 8%). The results obtained from this fully native device can be used in many fields, including cancer detection, examination of malignant tissue, and evaluation of the success in tissue surgery.

The study of turbulent boundary layer trailing edge noise as one of the most important aerodynamic sound generation mechanisms is a fundamental issue in design and production of equipment with minimum noise. In the present study, the utilization of finlets as a turbulent boundary layer trailing edge noise control technique is investigated. For this purpose, a flat-plate model, equipped with surface pressure transducers has been designed and built and the main parameters of trailing edge noise including the surface pressure spectra, the spanwise length scale, and eddy convection velocity in the trailing edge region have been measured. Moreover, in order to study the structure of the boundary layer flow downstream of the surface treatments, a single hot-wire anemometer has been used. The results showed that the presence of finlets leads to a significant reduction in the surface pressure spectra at all frequency ranges except for frequencies close to the maximum surface pressure spectrum. Furthermore, passing the flow structures through the finlets, although did not create significant changes to the spanwise length scale at high frequencies, however, they have led to an increase at low to mid frequencies. Finally, the Amiet-Roger model has been used to evaluate the changes in far field trailing edge noise due to the presence of the finlets and the results show the effectiveness of finlets in reducing trailing edge noise over a wide range of frequencies.

Smart polymers as a subset of smart materials have the ability to memorize their original form and return after reforming by inducing some stimulus. In this study, shape-memory polymers were manufactured in layers by 3D printing methods. Using this method, by controlling the percentage of each material in the sample and layer design the shape memory properties are investigated. The advantages of this method compared to other methods such as blending are the control simplicity of the impacting factors on the shape memory property, construction of complex parts, and improved shape memory property. TPU with elastic property and semi-crystalline PLA materials were used to achieve shape memory property and the samples printed out in TPU-UP and PLA -UP states to investigate the layer design effect. The results of shape memory tests showed that the number of layers, their arrangement, and shape memory properties can be easily controlled and designed. The results of DMTA test indicated that the recovery temperature in layered samples is lower than the other methods and the percentage of PLA and TPU can be controlled the recovery temperature. The recovery speed of layered samples in this study is very higher than previous studies, due to the amount of saved energy in TPU and the multilayered construction. Shape memory tests depicted that TPU increases the recovery ratio and the PLA increases the fixity ratio. The reason lay in the increase of the switching point percentage including crystallization, Tg, and reduction of cross-links which play the role of network cross.

Residual stress is one of the most substantial defects of welded parts caused by intensive thermal gradient. In this study, different mechanical and thermal techniques for reducing residual stresses have been investigated and the effectiveness of contributing parameters has been discussed afterwards. Subsequently, some equations have been proposed for welding energy and exergy efficiency and the effects of parallel flame heating, vibration method, and hammer working method on reducing welding residual stresses are expressed. The results show that by using parallel heating technique, the enhancement of flame power would result in reducing both energy and exergy efficiencies. However, the decremental rate of the two efficiencies would slow down and they approach to an asymptotic value. Increasing the speed of welding improves two efficiencies more than 2 times. On the other hand, the normalized entropy is reduced by increasing the heat input of the flames. This fact is an indicator of a reduction in welding residual stress. This reduction is more at high speeds. Eventually, the ratio of the two efficiencies shows that in this study, economical power was about 1800j/s. The reduction of normalized entropy for the vibration, hammering, and parallel flame methods are 0.001, 0.1, and 10, respectively. Overall, it is expected that thermal methods are more efficient than mechanical methods in reducing residual stresses.

In this paper, experimental defect diagnosis and the classification of its size in the outer race of angular contact ball bearing with acoustic emission method and artificial neural network are presented. In an experimental system, bearings are loaded at four speeds of 600, 900, 1200, and 1500rpm with four loads from low to high. Loads are applied to the outer race with the help of four bolts with equal and specific torques. Since the bearing is angular type, this type of loading is converted to radial and axial combined loading simultaneously and differs from conventional loads in deep groove bearings. Acoustic emission waves are recorded using broadband sensors in two states, healthy and defective. Therefore, to diagnose the defect, different states can be compared with the healthy. The spark method was used to create an artificially seeded defect. In analyzing the results, a new parameter called “the total time above threshold” was introduced to increase the efficiency of defect diagnosis and classification of its size. With the help of the introduced parameter and 4 conventional acoustic emission parameters and using an artificial neural network, the performance of the artificial intelligence system was 95.1% in defect diagnosis and 94.4% in defect size classification.

Blow molding is one of the most widely used processes for producing hollow plastic parts. In this process, the wall thickness uniformity of blow molded part is a prime concern. Processing parameters such as blowing pressure, melting temperature, and parison thickness affect the uniformity. In this paper, extrusion blow molding process for Peugeot 405 and Peugeot Pars water tanks has been studied by simulations and experiments. The effects of parison thickness in three levels and blowing pressure in two levels were investigated on the wall thickness of blow molded part. Parison thickness was varied by manipulating air gap between mandrel and die. The results indicated that the increase of blowing pressure had no effect on the part thickness. However, the parison thickness significantly influenced the thickness of molded part. Parison thickness was optimized by considering the weight and required strength of the part, so that, the material consumed was decreased. Also, Polyflow software was used to simulate the blow molding process. For this purpose, the initial parison geometry was experimentally determined by a measurement set-up, then the inflation process was simulated on this real parison. A good agreement was obtained between thicknesses of part in the experiments and simulations.

Reducing fossil fuel sources together with tighter environmental laws to control the engine exhaust emissions makes the use of cleaner and renewable fuels inevitable. Therefore, the use of biodiesel fuel as a strategy to conserve energy and reduce emissions is becoming increasingly important in engines. On the other hand, biodiesel fuels increase NOx emissions in the engines, which necessitate the use of water additives to reduce the combustion temperature. To compensate for the negative effect of water addition by reducing combustion quality and thus reducing thermal and exergy efficiency, the use of metal-based nano-particles additive can be a reliable solution. In this study, the effect of adding different concentrations of nano-particles on improving efficiency of the first and second laws as well as fuel consumption of a single-cylinder engine with different fuel combinations with BXWYNZ formula (diesel fuel with X% biodiesel mass, Y% water mass%, and Zppm nano-particles), has been studied experimentally. The results of this study show that adding 60ppm nano-particles to B0W5 will improve about 3% efficiency in the first law and 2.5% efficiency in the second law compared to pure diesel fuel. These values were about 4 and 3.8% for 90ppm nano-particles, and 5 and 4.7% for 120ppm nano-particles, respectively. In addition, based on the experimental results, the B15W5N120 has 7.5% higher first-law efficiency and 7% higher second-law efficiency than pure diesel fuel.

The present study investigates the influence of three different microstructure features including volume fraction of α phase (A), thickness of α phase (B), and aspect ratio of primary α (C) on tensile properties of Ti-6Al-4V alloy, by response surface methodology with central composite design (CCD). The experimental data required for the design of experiment (DOE) and analysis of variance (ANOVA) is predicted using the artificial neural network (ANN). First using the experimental data of other researchers, the ANN with two hidden layers by the error propagation algorithm was trained. The main objective of this study is to compare the two feedforward and feedback neural networks in as well as examine the influence of microstructure on the mechanical properties of the Ti-6Al-4V alloy. The results showed that the feedback neural network has higher accuracy than the feedforward neural network to predict the values of yield strength and elongation. Besides, according to ANOVA and response surface method, C, B2, AB2, and A2C factors and A, C, B2, BC, and A2B factors have more significant effects on yield strength and elongation in Ti-6Al-4V alloy, respectively.

In the current study, experimental and numerical methods have been used to investigate the pressure drop and the separation efficiency of wire mesh demisters in an air-water system. Using the designed and manufactured experimental model, various parameters such as air velocity, packing density, and wire diameter in plastic and metallic demisters have been studied. Numerical simulation was carried out in two-dimensional and transient form using K-epsilon (k-ε) turbulence model in commercial software ANSYS Fluent and validated against experimental results. The Eulerian-Lagrangian discrete phase model was also used to simulate the water droplet trajectory at diameters of 0.2 and 0.05mm. The numerical simulation results are sufficiently accurate compared to the experimental data. The numerical solver predicts separation efficiency with error of about 20% and pressure drop with error of less than 20% compared to experimental data. The numerical simulation results show that increasing the diameter of water droplets at higher air velocities and higher packing densities is more effective and increases the separation efficiency up to 36%. Also, increasing the packing density increases the separation efficiency for droplets with a diameter of 0.2mm and decreases the separation efficiency for droplets with a diameter of 0.05mm. The results show that the separation efficiency of plastic demister is more than the separation efficiency of metallic demister and in lower packing densities, the use of plastic demister is advisable.

In the present study, the experimental study and regression analysis of the dynamic response of circular plates under uniform and localized blast loading were investigated. To this end, several experiments were performed on steel plates under different conditions in the experimental section. In order to complete the database and perform a comprehensive analysis, fourteen series of experiments and 562 data in the open literature were added to the experimental results of the present study. Subsequently, the effect of the radius and thickness of the plate, the impulse of applied load, the mechanical properties of the plate, the loading radius, and the standoff distance on the maximum deflections of circular plates were simultaneously investigated using the Design-Expert software package and response surface methodology. In order to find a significant model, the confidence level of 95% was considered in the analysis. Two separate analyses were done based on the types of loading. The values of R2 for uniform and localized blast loading are equal to 0.9712 and 0.9548, respectively. The results show that the predicted values of the models are in good agreement with the experimental data and the presented models are suitable. Optimal conditions for the minimum deflection of the circular plates under dynamic loading with uniform and local distribution were also presented.

The composites derived from the bioactive glasses, such as BG/polysulfone, have better mechanical properties than pure materials and their characteristics are closer to human bone. In this study, the unknown fracture behavior of 58s BG/PSF composite has been investigated. The extended finite element method (XFEM) was used, in order to model the fracture behavior of 58s BG/PSF composite with greater accuracy. The XFEM doesn’t require remeshing at each step and achieves the precise approximation of singularities by incorporating discontinuity behavior into the elements using the enrichment functions. The aim of using the XFEM was to obtain stress intensity factors, displacements, stress and strain around the crack tip, fracture toughness as well as strain energy release rate. Moreover, the 58s BG/PSF composite with 30% bioactive glass particles was synthesized using solvent casting method and the bending failure test was performed according to the relevant standard. Also, to demonstrate the quality of the interface between the glass particles and polysulfone, SEM investigation was performed on the fracture surface. The obtained fracture toughness was in the range of 1.4 to 1.6 , and the strain energy release rate was in the range of 1600 to 1900 J.m-2, which was comparable to the same properties of natural human bone. Besides, the stress intensity factors and strain energy release rates were calculated by coding in MATLAB and modeling in ABAQUS, and the numerical results were validated with the analytical and experimental data and it was revealed that the numerical results were in great coordinance with the analytical and experimental results.

Nowadays, the interaction between gas bubbles and oil droplets plays an important role in the efficiency of many industrial processes. Therefore, it is of great importance to study the influencing factors on these processes. So, in the present paper, the effect of droplet and bubble size on the drainage time of the trapped intervening film between droplet and bubble was investigated. Six series of experiments were conducted for various sizes and three characteristic time scales including drainage time, coverage time, and rupture time were measured. Each of these experiments was repeated at least five times. The results showed that the drainage time changed independently of the droplet/bubble size. Moreover, it was observed that due to the nature of the phenomenon, the measured drainage times in each equivalent size are notably scattered, which means that the microscopic interactions in the water film and between bubble-droplet interfaces have significant impacts on the drainage time. Also, in the current experiment, it was found that the volume of the intervening film between droplet and bubble has no vital role in the drainage time of the mediate water film.

The present study is devoted to experimental and numerical investigation of in-situ tensile tests to recognize the mechanisms of ductile fracture under different stress states. The GTN model, which is a micromechanical based damage model, has used for numerical simulations. The parameters related to this model for St12 steel were identified by response surface method (RSM) through minimizing the difference between numerical and experimental results of the tensile test on a standard specimen. The void related parameters of GTN model were determined 0.00107, 0.00716, 0.01, and 0.15 for ff, fc, fN, f0, respectively. After calibrating the damage model for the studied material, the tensile tests were carried out on the in-situ specimens with different geometries. The fractographic analysis was performed to identify the ductile fracture under a wide range of stress states and two failure mechanisms were observed. The calibrated damage model was applied to FE simulations of in-situ tensile specimens for numerical study of the experimentally observed fracture phenomenon. The extracted numerical results showed a good agreement with experimental observations comparing load-displacement plots with a margin of error within 5%. The location of fracture initiation, crack growth orientation, and the displacement at fracture zone in numerical studies also showed close correspondence with experiments.

In the present study, the instantaneous velocity profile behind an airfoil at two different Reynolds numbers has been measured experimentally. Data are used to study the wake profile and the corresponding drag coefficient force of the airfoil in different conditions. In the conventional and common methods for calculation of the drag force coefficient through the velocity measurement behind an airfoil, turbulence velocity terms of the momentum equation are ignored. However at moderate to high angles of attack where the flow becomes turbulent and separation occurs, the nature of the flow becomes three dimensional and disregarding the components of the fluctuation of velocity (in three dimensions) in calculation of the drag coefficient of airfoil may result in erroneous information. In the present study, in order to increase the accuracy of the experimental drag coefficient of the airfoil for moderate to high angles of attack, turbulence velocity terms in experimental drag coefficient calculation are considered and this causes an acceptable compatibility between experimental and numerical results whereas for low angles of attack, disregarding the effects of turbulence velocity terms in experimental drag coefficient calculation will improve the accuracy of the experimental drag coefficient and a desired compatibility between experimental and numerical data will be established.

In this experimental study, the condensed water in the evaporator coil was injected using the nebulizer technology into the condenser inlet air and its effect on the performance coefficient was studied. The testing equipment employed in the present study consisted of an air conditioner tunnel with the dimensions of 200×35×35cm, which had the compression refrigeration cycle of 1-ton refrigeration with R404a refrigerant. A data logger with pressure and temperature sensors installed at various points of the unit accurately recorded the measured data. The results indicated that the use of a nebulizer would reduce the compressor outlet pressure and compressor power consumption and also increase the performance coefficient. By increasing the air temperature to the condenser from 21 to 36°C, the use of a nebulizer reduced compressor outlet pressure by more than 27%, decreased compressor power consumption by more than 8%, and increased the performance coefficient more than 64%. The results demonstrate the nebulizer technology could be used on hot days of the year by recycling wastewater from air conditioning systems, as a practical and low-cost method to increase the efficiency of direct expansion air conditioning systems.

RCCI as low temperature combustion is one of the common methods for reducing nitrogen oxides and soot pollutants. In this study, the effect of exhaust gas recirculation on combustion and emission of an RCCI engine, fueled with diesel and CNG was investigated. The investigated engine is a single-cylinder engine with diesel direct injection to the combustion chamber as high-reactivity fuel and a port fuel injection of CNG fuel as low-reactivity fuel. The start of injection, the injection shape, and the injection duration of both injectors are controlled by the developed ECU. Since the engine tested has good stability in the premix ratio of 60% and is capable of operating with high EGR percentage, it was selected for investigation. The results of this study show that with an increase of the exhaust gas recirculation rate from 0 to 34%, the amount of IMEP and thermal efficiency decrease by about 18%. As the EGR increases, the start, middle, and end of the combustion are delayed due to the decrease in oxygen content inside the combustion chamber. With the increase of EGR, the temperature of the combustion chamber decreased so that increasing CO and UHC production, showing an increase of 86 and 300%, respectively, while NOx decreases by 350%.

One of the important challenges of the aerospace industry is the use of magnetic bearings and generating the electromagnetic flux in motor to increase its speed of rotation and angular momentum. In this paper, the passive magnetic bearing for the reaction wheel actuator which is used to modify the status of space satellite is designed and analyzed using the COMSOL software. The performance of constructed reaction wheel in various modes is evaluated. In the passive magnetic bearing system, when the rotor exits the center position of the rotational axis, the return force that results from repulsion between the poles of the same permanent magnet directs the rotor to the center axis position. In the paper, the axial passive magnetic bearing is designed, and the distribution of magnetic flux density and static force of the bearing is estimated using simulation in the software and the stiffness coefficient is obtained from the static properties. To reduce the power consumption of the reaction wheel, various layouts were investigated. Then, based on design and analysis results, the appropriate bearing to achieve the maximum rotational speed and the minimum power consumption is introduced. The results of the FEM analysis clarified the effects of the magnetic stacking structure on the force and magnetic stiffness of the bearing and finally, the experiments proved that the rotational speed and momentum of the reaction wheel are increased in the combined use of the mechanical and passive magnetic bearings.

The vortex tube is one of the widely used cooling systems in the industry. Investigating the effect of all input variables on the outlet cold temperature difference in laboratory state is time-consuming and costly. To this purpose, in the current study, attempts were made to model and predict the effect of all input variables on the outlet cold temperature difference of air and inlet air using adaptive neuro-fuzzy inference system (ANFIS) method. The ANFIS method was designed with three structures of fuzzy inference systems called subtractive clustering (SC) algorithm, fuzzy c-means (FCM), and grid partition (GP) with four types of input membership functions including trimf, gaussmf, gbellmf, and pimf. For model training and testing, 326 laboratory data were used. The developed models were compared using statistical parameters of correlation coefficient, mean absolute relative deviation, standard deviation, and root mean square error (RMSD) together with general desirability function. The results showed that GP algorithm with input pimf membership function with the greatest value of correlation coefficient (0.9975) and lowest value of RMSD for test data (0.4199) and general desirability function value of 0.71 is the best method to predict outlet cold temperature difference. Using the above-mentioned method, the most optimum state of vortex tube performance for industrial applications was found to be the use of 3 or 6 nozzels, at the pressure range of 0.55 to 0.6MPa and the nozzle angle of 20 to30 degrees, and for laboratory applications was obtained to be the use of 6 nozzles, at the pressure range of 0.55 to 0.6MPa, and the nozzle angle of 25 to 35 degrees.

The use of two-layer sheets to improve mechanical properties such as ductility and strength and to improve chemical properties such as corrosion resistance has led to an increasing number of such materials in the industry. In this study, the formability of aluminum-copper two-layer sheets at a high strain rates is investigated by electromagnetic forming method. The simulation of electromagnetic forming of the two-layer sheet was performed at high strain rate using Maxwell and Abaqus software. By making coil and die and using sheets with different geometries and grids on the sheets, the forming limit diagrams (FLD) was also extracted experimentally. The simulation results showed that the electromagnetic pressure applied on the sheet in CA lay-up was 19% higher than in AC lay-up. Using the second derivative of strain criterion, the FLD of aluminum-copper two-layer sheet was derived. The FLD of aluminum-copper two-layer sheet with an initial thickness of 0.5mm is 30% higher in the AC lay-up than in CA lay-up. The reason for this improvement is that in the AC lay-up the sheet with more ductility (copper) is in the outer layer and has greater resistance to tensile stress and necking. The outer layer with better ductility can improve the ductility of the two-layer sheet. The FLD of aluminum-copper two-layer sheets has improved 120% in right-hand side and 55% in left-hand side at high strain rates compared to static conditions. There is about a 6% differences between the simulation and experimental results for forming limit diagram.

Single point incremental forming is a cost-effective process with high flexibility and as a result, would be a proper selection for low-batch and high-customized production compared to traditional processes such as pressing. The target market of this process usually consists of medical, automotive, and aerospace industries in which metals with high strength to weight are highly in demand. These materials are usually formed at elevated temperatures due to their low formability at room temperature. In this study, the AA6061 aluminum sheet was homogeneously heated at 25-400°C. In addition, the effects of important process variables of heat-assisted SPIF including temperature, vertical pitch, feed rate, and three types of lubricants were investigated on formability of truncated cones with various wall angles. According to the results, despite the inability of local heating in enhancing the formability of the AA6061 sheet (37% improvement of formability under optimal conditions), the homogenous heating approach which was used in this article leads to a significant improvement in formability (528%). Temperature is the most important parameters effective on the formability, while lubricant and vertical pitch are ranked as the second and third parameters, respectively and the effect of feed rate is negligible. The critical wall angle increases from 60 to 65 degrees with increasing the temperature from 25 to 400°C. In order to choose a suitable set of parameters, the surface roughness should be taken into account, which may alter the results from 1.18 to 4µm as the best and worst surface conditions, respectively. Furthermore, a truncated cone with a wall angle of 65 degrees was successfully formed to 44mm depth using an appropriate combination of process parameters. This demonstrates an outstanding improvement in formability.