نشریه مهندسی مکانیک مدرس
سال بیستم شماره 11 (آبان 1399)

In recent years, forming a 3D microfluidic channels on the electrical non-conductive material such as Polydimethylsiloxane (PDMS) in the micro-electromechanical system (MEMS) and medical applications is of great interest. Lithography is the most know process to create patterns on the PDMS however there are a few drawbacks to this process such as high operational cost and time, and sidewall angle. In all applications, the quality of the micro-channel surface determines the performance of it. In this research as innovatively the electrochemical discharge milling (ECDM) which is known for lower operational cost and proper material removal rate (MRR; i.e. process time), and is capable of creating patterns on electrical non-conductive material, was used to form a micro-channel on the PDMS. To that end, the effect of process parameters such as electrolyte concentration, feed rate and cutting speed and voltage on the surface roughness and surface integrity deeply investigated. It was observed that ECDM is capable of creating patterns on the PDMS with surface integrity which is comparable with the lithography microchannel. It is also observed that decreasing the rotational speed from 10000 to 0rpm results in increasing the surface roughness 2 to 4 times. This happens due to the increasing the thickness of the gas film around the tool, and subsequently increasing the flying sparks which results in higher surface roughness. Increasing the Voltage from 38 to 42V results in 36% enhancement of surface roughness. The 25% electrolyte concentration results in lower surface roughness.

Today, the use of metallic bipolar plates in the fuel cell industry has attracted the attention of many researchers due to its much lower cost than thick graphite plates produced by machining. The best method for the production of metallic bipolar plates is forming process. Among the different forming methods, the stamping process has a higher production rate, simpler process, and lower production cost. One of the major problems in the formation of the metallic bipolar plates is the springback of the sheet after forming, which causes distortion and non-uniformity in the formed channels. In this study, the effects of geometrical parameters such as draft angle, corner radius, depth of channel and process parameter such as lubricant on filling profile as well as springback of formed sheet made of stainless steel 304 with a thickness of 0.1 mm were investigated. For this purpose, the simulation was performed using ABAQUS finite element software and the results were verified by experimental analysis. Then the outputs were evaluated by changing the input parameters in the simulation. The results showed that the draft angle and channel width had the most influence on the springback value of the formed plates. The results related to the process parameter such as the lubricant effect showed that the springback value is almost independent of the lubricant parameter. However, in quite equal conditions, the stress distribution in the corners and channel walls is much more uniform when using the lubricant.

A new experimental technique has been developed to measure the pressure distribution over the surface of a spinning wind tunnel model. The technique is unique in that all elements of the instrumentation, thus avoiding many of the technical problems and operational limitations associated with previous attempts to measure this effect. Surface pressure distributions were obtained for selected tip speed ratios for different angles of attack (5000rpm). The results obtained from the pressure profiles determine the Magnus forces and make it possible to interpret the boundary layer and the effects of separation. In this method, in addition to determining the Magnet force, its distribution on the model is also obtained. The results show that most of the Magnus force is created at the ends of the projectile. The validity of the data was established by comparing the integrated pressure values with directly measured balance data. Similar results were obtained by the numerical simulations and were compared with the experimental data. This new technique can be applied to a variety of model configurations and Mach number regimes.

In the present study, using a new method, dual-phase (DP) steel with high strength and good ductility was produced from plain carbon steel with 0.16% carbon. The DP steel with ferritemartensite structure was obtained using austenitizing, quenching, asymmetric cold rolling, and intercritical annealing at temperatures of 770 and 800 °C and short holding times of 1 and 5 min. Due to the application of uniform shear strain through asymmetric cold rolling, a uniform distribution of the martensite phase was observed in the RD-TD and RD-ND planes. By increasing the holding time, the volume fraction of martensite increased from 8% to 12% at 770 °C and from 10% to 33% at 800 °C for the holding times of 1 and 5 min, respectively. Hardness and strength improved with increasing temperature and time of intercritical annealing. The sample produced at a temperature of 800 °C and a time of 5 minutes showed excellent mechanical properties such as 244 HV hardness and 1020 MPa strength and 12.5% ductility. In addition, due to the high volume fraction of martensite and the consequent reduction of its carbon content, the hardness of this phase decreased and as a result, it showed significant plastic deformation and high strain hardening. The fracture surface of all produced DP steels mainly included dimples, which indicates ductile fracture behavior.

The main disadvantage of natural draft dry cooling towers is the influence of atmospheric conditions such as ambient temperature and wind speed on the thermal performance. Wind disrupts the natural flow of air inside the tower creating vortices at the back and inside the tower that disrupts the air flow structure. When the wind blows, increasing the velocity of inlet air through the front louvers causes the air to pass through the behind louvers rather than outlet opening. The negative effect of this phenomenon reduces the cooling performance and consequently reduces the turbine production power in power plants. A good solution to this problem is to adjust the Louvers angle correctly. Therefore, in the present study, the thermal performance of the dry cooling tower was evaluated under the conditions of opening and closing the front louvers and changing their angle. In this regard, a natural draft dry cooling tower unit with the dimensions of the cooling tower located in combined cycle power plant was simulated using 3D model in fluent software and the numerical results have been validated by experimental data. The Realizable k-ε turbulent model is used to model the turbulent flow. The performance of the tower has been studied in three modes, including no wind, with the wind and fully open louvers and with the wind and semi-open louvers. According to the results, by partially removing the louvers to 60°, heat transfer and mass flow rate can be increased to 16% and 15% respectively.

In this paper, experimental investigation and regression analysis are investigated on plastic deformation of polyurethane composite sandwich panels reinforced with nanoclay under blast loading. For this purpose, polyurethane sandwich panels were prepared with different percentages of nanoclay and in different densities. The mechanical properties of nanoclayreinforced foams were studied by the tensile-compression test. Explosive shock tube device and C4 explosive material were used for explosive loading. Then, the response surface methodology was used to investigate the effect of significant parameters, the percentage of nanoclay and density of polyurethane foam, on the displacement of composite sandwich panels and to optimize the parameters for minimum deformation. The results obtained from the regression model at 95% confidence level indicate a very good agreement between the experimental results and the values predicted by the model. The high value of the correlation coefficient between the studied parameters and the amount of plastic deformation of the sandwich panel (R2 = 99%) indicate that the proposed model has higher accuracy. Finally, the optimal conditions for achieving the minimum displacement of composite sandwich panels were determined as 1.57% nanoclay content and foam density of 130 kg/m3.