نشریه مهندسی مکانیک و ارتعاشات
سال چهارم شماره 3 (پیاپی 12، پاییز 1392)

The Microcellular foams are a group of foams which have a lot of micro cells in the size of about 10 microns. In these foams, neutral gases (Nitrogen or Carbon dioxide) are used as a foaming material. Because of their small sizes, these foams have better mechanical and physical properties (such as fatigue strength, yield strength and de-electrical and thermal properties) than non-foamed plastics. In this study, the mechanical properties of polystyrene microcellular foams have been studied. The rate of relative density, the size of the cells, the structure of the foams and yield and fatigue strength have been studied and measured. To achieve an optimal structure, the results show that temperature and time of foaming should be controlled carefully. This condition is in the temperature of about 100° c and the time of 100 second. Also, the tensile strength of the foamed sample has been reduced and the rate of energy absorption has been significantly increased.

In this study, numerical mixed convection heat transfer in a horizontal concentric annulus in the presence of water-based fluid with nanoparticles of aluminum oxide, copper, silver and titanium dioxide discussed. Numerical solution using finite volume method based on SIMPLE algorithm is done and discretization of equations generally is first order. Inner and outer cylinders have constant temperature and inner cylinder temperature is higher than outer one. Both of the cylinders can be rotated in either direction with constant angular velocity. The effect of parameters such as Rayleigh, Richardson, Reynolds number and volume fraction of nanoparticles on heat transfer and flow pattern discussed. Results showed that by increasing Rayleigh number increases heat transfer, heat transfer rate also increases with increasing volume fraction of nanoparticles and these increase is about 8.25 percent for 5 percent nanoparticles by volume. Rotation of cylinders reduces the overall heat transfer rate. Different rotation has large impact on the flow pattern and heat transfer. Adding nanoparticles has not significant impact on the flow patterns and isotherms but is effective in quantity. The results has good agreement with the similar work has been done before.

Today, apart from a design perspective structures, The application of advanced technology due to economic considerations, The need to reduce the time and improve the performance of various retrofit projects on the level of risk selection and execution easy to run in cases where there are restrictions Or the need for uninterrupted use of the intended structure is more felt. This study is aimed to evaluate the simultaneous effect of seismic isolator and tuned mass damper on reducing the effect of moment in seismic behavior of steel structures. Therefore 2 model structures of 10 & 15 stories with fixed base, base isolated and a combination of them are evaluated under time history analysis through SAP2000 software using 7 far fault record acceleration and 7 near fault record acceleration scaled to 0.5 g results show that hybrid controlling system is lead to 80% reduction of stories moment in record acceleration of far and near fault earthquakes

In this paper, investigation the parameters that affect the coefficient of performance (COP) and features of thermoelectric cooling devices to help a number of recent research in this field has been provided. Specific aspects included practical considerations on figure of merit, cooling capacity and coefficient of performance respectively. This paper starts by thematic categories from recent articles, and then describes in detail the parameters of cooling capacity and coefficient of performance. In addition, the cooling capacity for single-stage and multi-stage thermoelectric cooler is mentioned. Concerning the COP, its dedicated expressions are constructed starting from the classical formulation and introducing additional factors or modifications in order to take into account the Thomson effect. Finally, has been investagted the coefficient of performance in different applications of thermoelectric cooling.

In the common practice design of structures, Often the effect of vertical component of earthquake is ignored, but this component is important parameter especially in near field Earthquakes. The objective of this paper is study the effect of vertical and horizontal component of earthquake ground motion in steel braced frames. First verification of numerical models through finite element software package (ABAQUS) is carried out. Comparison of the result of load displacement curve for numerical and experimental model shows matching result. After this, two concentrically braced frames with X-configuration are selected from previous researches. Four earthquake records have been used for this study. Kobe, Northridge, Tabas and Imperial Valley. Two components of earthquake are applied simultaneously to frames under nonlinear time history analysis. The results that extract from the analysis content of vertical and horizontal displacement of roof and plastic dissipated energy during earthquakes. the intensity of the former parameters is depend of earthquake intensity and frequency content of them. Assessing the result reveal that in the three story frame, the ratio of maximum vertical displacement to maximum horizontal displacement is crucial parameter with respect to seven story braced frame. Also the time of plastic energy dissipated for frames is short because of nature of near field earthquake.