Modeling in Engineering
Year:2018 | Volume:16 | Issue:52|Papers Count:33

In order to perform reliability analysis of steel structures subject to sea waters and thus to corroding marine environments، it is necessary to use: a) a model describing physical behavior of corrosion in structures placed in these circumstances and b) a stochastic model describing probabilistic behavior of corrosion phenomenon along time. In this study، Melcher's model is employed as a physical corrosion model. Furthermore، a probabilistic model is proposed in which the corrosion process is discretized into a few steps each of which representing a jump/an increment in corrosion quantity. Despite having a pre-defined deterministic small duration، each step is modeled with an independent random variable following a gamma distribution. In this paper، in contrast to the conventional gamma process، the shape and scale parameters of the involving gamma distributions are different for each step. This will result in an accumulating corrosion process which is a sum of n different gamma distributed random variables in any arbitrary point of time. Herein، in addition، a new application of "maximum likelihood method" for estimating of model parameters is proposed. Panama Canal data for carbon and low-alloyed steels is used for the purpose of parameter estimation. Reliability analysis of a structure subjected to an environment similar to that of Panama Canal is then investigated. Implementation of the model proposed herein to data available indicates that it could be sufficiently flexible to show the mean and variance of the marine corrosion processes.

Young's modulus of blends of thermoplastic polyurethane (TPU) and acrylonitrile butadiene styrene (ABS) are measured with different weight percentage (blend ratio). The results of the different micromechanical models prediction of Young's modulus، based on both droplet matrix and co-continuous morphology، are compared with experimental results. Both two-dimentisional models like series، parallel، Maxwell، Halpin-Tsai، Takayanagi، Davis، Coran and Patel، and three-dimensional models like Kolarik، Barentson and Nijhof are used for Young's modulus prediction of polymer blends. In this work، an emphasis was given to the effect of weight percentage on morphology and mechanical properties of the blend. Scanning electronic microscopy shows droplet matrix morphology in the presence of less than 20 wg% ABS in TPU matrix but in 70/30 TPU/ABS blend ratio، ABS phase dispersed like elongated elliptical and phase inversion happened. In the 95/5، 90/10 and 80/20 blend ratio which ABS droplets dispersed uniformly throughout the TPU matrix، Parallel Takayanagi and Barentson series model of parallel parts، could predict Young's modulus with good accuracy. In the 70/30 blend ratio which phase inversion was observed and both phases are somehow continuous، Coran-Patel، series Nijhof and Kolarik models were accurate.

In this paper، the effect of the tool? s material and the workpiece rotational speed on the near dry electrical discharge machining (N. D-EDM) has been studied. In order to، three levels of the workpiece rotational speed and the discharge current are used for evaluating material removal rate (MRR)، tool wear rate (TWR) and surface roughness (SR). After doing the experimental study and getting the appropriate speed، a comparison has been done between the fixed and rotary workpiece N. D-EDM. Finally، another comparison has been done between the fixed workpiece N. D-EDM with CO2 gas and the submerged EDM. The results show that the rotational speed of 600 rpm cause to increase in material removal rate، tool wear rate and the reduction in surface roughness. In addition to، the comparison between rotary and fixed workpiece N. D-EDM explain that rotary state have the higher rank of efficency. Due to the low level of electrical resistivity of copper tool، it has the highest MRR and by increasing the discharge current، the highest TWR than other tools. Also، the surface roughness of the workpiece in the fixed N. D-EDM is less than submerged EDM.

This study presents a non-destructive method for detecting location and depth of crack in beams. The method utilizes Hilbert-Huang Transform (HHT) as a time-series analysis technique. Crack is considered to be open and has been modeled by rotational spring. First، the natural frequencies of the cracked beam are calculated using Timoshenko? s beam theory. Then by using the vibration signals corresponding to the beam، experimental natural frequencies are calculated by Fast Fourier Transform (FFT) and HHT. Finally، with the help of Artificial Bee Colony، the location and depth of the crack are predicted by minimization of an objective function. The objective function is constructed by the weighted sum of the squared errors between the theoretical and experimental natural frequencies of the cracked beams. To investigate the feasibility of proposed method، cracks in different locations and depths are introduced in steel beams and crack parameters are predicted. The results show that both the location and depth of the crack can be predicted well through the proposed method. Moreover، by using the experimental natural frequencies obtained by HHT method، cracks (especially small depth) can be detected with a better precision than FFT method.

Laser forming is a modern and non-contact method for sheet metal bending. Flexibility and rapidness of this forming procedure on one hand and controllability of that on the other hand make this method as one of the most popular manufacturing method. Determination of the effective and optimize parameters for achieving the highest bending angle with no surface melt is one the most important parts of the laser forming process. In this paper، steel sheets have been laser formed in different powers، velocities and beam diameters by a continuous Nd: YAG laser. The results depicted that in the higher powers and smaller beam diameters، the "bending angle-velocity" curve has been shifted to right-side. Furthermore، it has been shown that in smaller beam diameter the trend of bending is reasonably linear and in the higher amounts it is relatively non-linear. In the following، numerical study of laser forming has been done. The results has showed that by increasing of power and decreasing of velocity، beam diameter and work piece thickness the amount of bending angle has been increased. Finally، comparison between numerical and experimental results has showed that they are in a good agreement.

In this paper، the exergy analysis of a direct-expansion solar-assisted heat pump system (DX-SAHP) has been presented considering the effects of the pressure drop associated with the ï ? ow of R134a refrigerant through the condenser، collector/ evaporator and connection pipes. In order to calculate the two phase refrigerant pressure drop through the evaporator and condenser، the homogeneous model of pressure drop inside horizontal pipes has been used. The DX-SAHP system mainly employs a bare ï ? at-plate solar collector with a surface area of 4 m2، a hot water tank with the volume of 150L، a rotary-type hermetic compressor and a thermostatic expansion valve. Furthermore، the effect of various parameters including ambient temperature، solar radiation، collector area and compressor speed have been analyzed on the exergy efficiency and exergy loss of the system. The results of the simulation have good agreement with the experimental results and show that the increase in solar collector/evaporator pressure drop adversely affects the system COP and the solar collector efficiency. During the operation of the DX-SAHP system within a year، it is concluded that the largest value of the exergy efficiency of the system is 22. 7 percent and the lowest value of the exergy loss is 12. 6 kW. The obtained values are particularly due to the minimum amount of solar radiation in December. Results indicate that the solar collector has the minimum value exergy efficiency amounted to 12. 6 percent and the maximum value of exergy loss amounted to 2. 19 kW، among all of the system components.

Voltage and reactive power control in distribution networks is one of the most basic tasks of a distribution network operator. This is usually controlled by on-load tap-changer (OLTC)، substation capacitors، and feeder capacitors. This paper first investigates a local voltage and reactive power control in a distribution system and then how the presence of induction machine based DG affects it. In the following a proper coordination among OLTC and capacitors to minimize losses in the distribution system، with and without DG، is formulated. The results showed that in the presence of induction generator، local control was not optimal، and sometimes operating constraints are violated. Finally three methods for controlling voltage and reactive power in the presence of induction generator to reduce losses and comply with the limitations operation during the day are proposed. Induction generator can be both fixed and variable be evaluated. Simulation studies on a distribution network of 10 bus with 70/10kV voltage has been made.

This paper proposes a new topology for three phase cascaded multilevel converter based on multilevel modules (MLM) and three-phase bridge. The main advantages of the proposed topology، compared to the other topology converter are significantly reduction of the number of power switches and diodes as the number of output voltage levels increases. To achieve the proper levels in the output voltage، an algorithm is introduced to determine the value of DC sources. In the other part of this study، is introduced the determining the minimum values of switches، diode، DC sources، so on to gain the maximum levels at the output voltage. Also، how specifying the switching interval in order to reduce the total harmonic distortion (THD) is proposed that using this switching method، the output voltage THD is minimized. To show the validity of proposed topology، a 125 level three-phase inverter is designed and simulated using PSpice. In addition، to verify the performance of proposed configuration in desired output voltage generation، a sample output voltage containing 3th and 5th harmonic is generated using this topology and the obtained results are evaluated.

For the first time، a new structure is proposed for tunneling CNTFET (T-CNTFET). In this new structure، the drain region is divided into two parts، and the part near the channel has a linear doping profile. This new structure is called T-CNTFET with linear doping in drain (LD-T-CNTFET). The obtained results using a non-equilibrium Green? s function (NEGF) method show that the LD-T-CNTFET compared with conventional T-CNTFET leads to increase in ON current، reduction in OFF current، increasing current ratio، reducing sub-threshold swing، reducing power consumption and increasing the transistor speed. In addition to the mentioned benefits، the LD-T-CNTFET structure increases the cutoff frequency in comparison with conventional T-CNTFET structure. Therefore، it can be said that LD-T-CNTFET is a proper structure for the applications with low power consumption and high speed.

In this paper، using pattern recognition method all fault type is classified. Firstly، feature vectors obtained from sequence components of current and/or voltage signals are normalized by efficient technique. Afterwards، the proposed supervising function applies Kernel Naive Bayes classifier. The classification method through tuning of kernel function bandwidth s suitable for a complex and non-linear feature spaces. The signal processing procedures is done by using minimum sampling frequency hence the output of conventional current and voltage transformers can be utilized. Moreover، the performance of proposed pattern recognition methodology is evaluated from different point of views. The achieved results indicate that the proposed fault classifier has acceptable performance even in the noisy conditions.

In this paper a method for reducing the interactions among power system stabilizers (PSS) in a multi-machine large scale power system and hence improving the small signal stability is proposed. Dynamic Relative Gain Array (DRGA) matrix is used for identifying input-output pairs that are strongly coupled to decompose a MIMO system to several SISO systems. By using the DRGA، an efficient and simple method for collecting information about how PSSs interact among themselves in different frequencies related to electromechnical modes of the power system is introduced. This information is then employed to design a post processor controller that can eliminate the negative interaction among stabilizers by managing the output signals. In order to increase the flexibility and reduce the sensitivity of the stabilizers toward the system changes، stabilizers and the post processor are designed based on fuzzy logic. The proposed method is evaluated on the two area test system in MATLAB environment. The obtained results show that the post processor، by minimizing the negative interactions among the power system stabilizers، can satisfactorily improve the small signal stability and decrease the sensitivity of these stabilizers regard to variations in operating conditions simultaneously.

In this paper a new magnetic-solar engine for vehicles is proposed. Each cylinder of this engine includes two permanent magnets، which their magnetic forces drive the piston. One of these two permanent magnets acts as the piston of the corresponding cylinder. The other permanent magnet of each cylinder rotates cyclically such that the same and opposite poles of the two magnets are placed against each other periodically. The attraction/repulsion forces generated in this way inside the cylinders are coordinated to drive the crankshaft of the engine. The rotatable permanent magnet of each cylinder is rotated by a DC motor، which is fed by the solar panels of the proposed magnetic-solar engine. One of important advantages of the proposed engine is elimination of expensive and heavy induction motors and their associated equipment. The proposed engine is also equipped with batteries to store/retrieve solar power، which increases the useable time of the engine. The constructed solar-magnetic engine is registered as a patent in the Iran? s patent office with the number of 85138.

This paper presents a method for calculation of bar? s currents and its magneto motive force (MMF) in a squirrel cage induction machine. For this propose، at first multiple coupled circuit modeling of cage induction motor is introduced. Then with the aim of dynamic equations، an analytic expression for bar? s currents will be derived. This equation can introduce the instantaneous values of currents in all bars. At next section، this equation is simplified for the steady state operation of machine. At the end، the equation of MMF produced by rotors will be derived based on analytic bar? s equation and winding function theory.

In this paper، L1 adaptive control strategy for stabilizing chaotic systems in the presence of model uncertainty is proposed. In order to design controller، first، system dynamics is divided into two linear and nonlinear parts. The linear part is converged by placement feedback to the reference model behavior. The uncertain nonlinear part is compensated through adaptive control based on projection adaptive algorithm. This part includes a unknown vector multiplied at infinity norm of state vector and a vector as offset. A state observer also describes reference model behavior. In addition، first order pre-filters with unity gain are used to increase the stability margin. The main property of L1 adaptive control is encountering both parametric and nonparametric uncertainties. Stability analysis of the closed loop system is presented based on Lyapunov theory، and the control system performance is compared and evaluated with one of the conventional adaptive control methods. The results indicate the desirable performance of the proposed method for stabilizing the chaotic system in the presence of model uncertainty.

For over three decades، hydrologists were recommended multivariate models to describe and modeling complex hydrology data. While recently the multivariate models in hydrology is discussed. In multivariate models، the modeling and predicting various parameters can improve by involving other factors. In this study، using both univariate and multivariate periodic ARMA models for modeling monthly discharge in Nazloochaei River in West Azerbaijan Province during the period of 1962-2011 were compared. The results of evaluation and verification models showed that the multivariate periodic ARMA models by involving the climatic parameters such as temperature and precipitation of the basin، the more accurately than univariate periodic ARMA models. Also the result showed that the selected models، maximum and minimum points of discharge to the appropriate model.

Over the last decades، optimization of structures has been the subject of extensive investigations and optimization using Genetic Algorithms (GA? s) has been studied intensively. Many researchers have been focused on the weight optimization of buildings mainly on steel structures rather than reinforced concrete (RC) frames. One of the challenges in multi-objective optimization of steel and RC buildings is to determine the balance between objective functions in order to find the Pareto-front. In this study، weighted sum method (WSM) optimization technique is utilized to solve RC frames optimization problems to minimize simultaneously cost and displacement، as objective functions، subject to the stress، displacement، size and other related constraints، based on the ACI 318-11 code. For this purpose، first، using WSM technique the two objective functions is converted in to single objective function and then GA applied for optimization procedure. Three design models of 4، 8 and 12-story RC frames are introduced as representative for short، mid and high rise buildings. Cross section of beams and columns are employed as design variables. The Pareto front for building models is obtained. To validate the results، they are compared to those obtained by ant colony optimization (ACO). The results illustrate the accuracy and rapidity of the proposed method.

Awareness of salinity of soil layers under drains، particularly in areas with shallow saline groundwater such as Khozestan leads to design the best depth and spacing drain. In this study the application of artificial neural network modeling to predicting of changes in groundwater salinity under drain pipes have been tested. In order to calibrate and validate the model results، data collected from experimental model with 1. 8 m long، 1 m wide and 1. 2 m high were used. In the model، drains were installed at 20، 40 and 60 cm depths and spacing of 60، 90 and 180 cm. In the method of artificial neural network، LevenbergMarquardt learning algorithm with SigmoidAxon transfer function was used. After statistical analysis and calculation of RMSE، the standard error and correlation coefficient، adjustment between measured and simulated values of changes in groundwater salinity was calculated. The value of these product indexes 5. 27 ds/m، 0. 12 and 0. 96 was estimated respectively. Changes in drains salinity in different depths and spaces over time with discharges of 0. 07، 0. 11 and 0. 14 lit/s are 0. 34 dS/m، 0. 09 and 0. 99، respectively. The results showed that artificial neural network method on simulating of changes in groundwater salinity under drain pipes and also changes in drain water salinity in difference depths and spaces of drains have reasonable accuracy.

The demand for FRP materials has been increased over the past years for retrofitting or strengthening of reinforced concrete members. Several recent studies focused on the shear strengthening of reinforced concrete deep beams. In deep beams، due to the high ratio of shear span-to-depth and shear deformation، the ultimate strength is generally controlled by shear rather than flexure with lower shear ductility. Shear strengthening with FRP sheets is one of the most effective methods to overcome the weaknesses of deep beams. In this paper، the behavior of 104 deep beam specimens strengthened with different schemas of FRP sheets which was investigated. Specimens were modelled through the Non Linear finite elements method and analyzed under Push over static loading. The efficiency of FRP installing schema and the effect of some parameters such as shear span-to-depth ratio، compressive strength of concrete and the arrangement and ratio of shear reinforcement were evaluated. The results showed an average increase of shear capacity varied between 15 through 65 percent in each schemas of installed FRP sheets. Another aim of this study is to define and evaluation of shear ductility index for deep beams. The results show that the shear ductility index depends on the dimensions of the deep beam، compressive strength of concrete and the scheme of installed FRP sheet as using the layers of FRP can increase the shear ductility of strengthened deep beams about 5 to 30 percent.

The tunnel form buildings provide some advantages for medium to high-rise buildings that constructed by using a special tunnel form technique are commonly built in countries facing a substantial seismic risk. There are no special requirements for seismic design of this type of buildings، and some provisions of the concrete bearing wall system are used. None of the seismic codes addresses the R factor for tunnelâ ? گform buildings directly. This study presents R factor of this structural system based on the nonlinear pushover analyses of 7 case studies using the ABAQUS software. The impacts of shear wall reinforcement ratio and its detailing on system ductility، load-carrying capacity and failure mechanism under seismic forces are evaluated. Influences aspect ratio and wall openings on the response are also discussed.

One of the connections that are less use by designers due to the difficult implementation is welded unreinforced flange welded web connection. Almost can be said that this type of connection can be use without design. Studies on this connections indicated presence of plastic strains in near the beam to column connection causes decreased levels of confidence this connections. In this article studied on I shape beam to column moment connections with welded unreinforced flange-welded web (WUF-W)، welded top and bottom flange plate (WFP) and reduced beam section (RBS). For this propose for each specimen design connection with ST37 material properties according to Iranian national Cone No-10 (Design Steel Structures specifications). With the Abaqus finite element analysis software studied and compares hysteresis curves and performance of each connection under cyclic loading. Finally recommended details for Increase safety level of WUF-W moment connection. This study has shown that energy dissipate of WFP 20 and 52 present respectively was more than WUF-W and RBS moment connection. Load capacity of WFP 19 and 54 present respectively was more than WUF-W and RBS moment connection. Reinforce WUF-W with haunch had best results in energy dissipate، rigidity and load capacity that increase respectively 10. 5، 18. 5 and 8 present parameters.

Over the last two decades، extensive research has been conducted on structural health monitoring and damage detection in order to reduce the life-cycle cost of structures and improve their reliability and safety. These methods are divided into modal-based and signal-based approaches. Recent advances in the field of sensor technologies have facilitated the use of signal-based methods as practical solution to detect damages in structures. After a sever earthquake، usually there is no possibility for visiting the individual structures. Therefore، application of methods that can detect damage of structures only by using signals recorded at the time of the earthquake is noteworthy. Many existing methods، especially methods based on signal processing are not able to determine the damage severity. This article presents a signal-based seismic structural health monitoring technique for damage detection and evaluating damage severity of a multi-story frame subjected to an earthquake event. As a case study، this article is focused on IASC? ASCE benchmark problem to provide possibility for side-by-side comparison. First three signal processing techniques including EMD، HVD and LMD، which are categorized as instantaneous time-frequency methods، have been compared to find a method with the best resolution in extracting frequency responses. Based on the results EMD has proved to outperform than the others. Second، EMD is used to extract the acceleration response of the sensors. Results show that by taking advantage of signal processing and artificial intelligence techniques in this research، damage detection of structures was carried out for three levels including damage occurrence، damage severity and location of the damage.

Until now، all of the calculations about determining of water content around the particles in unsaturated soil have done with supposing the spherical shape of particles and most of quantities such as matric suction and interparticle stress due to water meniscus were calculated by using that simple assumption. In this paper، three packing methods of ellipsoidal particles are studied and by considering changes in meniscus geometry، matric suction and water content volume are evaluated as functions of meniscus radii for soil particles under the pore-water regime. This analysis provides a theoretical basis for describing several well-known phenomena in unsaturated soil behavior. A series of rigorous analytical equations was developed for describing the total volume of menisci between contacting ellipsoidal soil grains by treating the zero contact angle. Matric suction characteristic curves modeled using the analytical solutions displayed behavior similar to real unsaturated soils by using the ellipsoidal particles. The results show that by increasing the filling angle، the volumetric water content of horizontal arrangement is about 2 to 5 times higher than other states، while by changing in the radius ratio of the lens، this amount is more for vertical alignment. So، the matric suction for elliptical particles is quantified with little inclination towards the water content change which achieved four times larger for double diameter ratio.

Steel plate shear walls have been used as lateral resistant systems at several buildings particularly at high rise building during past three decades. This system contains appropriate rigidity to control displacements and also have ductile failure and high energy dissipation mechanism، so it can be used for new buildings or strengthening existing structures at seismic prone areas. Generally steel plate shear walls are consisting of steel plate walls and two boundary columns and horizontal floor beams. In this paper، initially a steel plate shear wall of a 5-floor building at high seismic area was designed according to AISC 341-05، then ABAQUS finite element program was used to investigate the seismic behavior of structure and get hysteresis curves under cyclic loading and also investigate the several parameter effects on local instability and non-linearity of material at designed typical shear wall and also the other new walls with different opens and rigidities. Numerical analysis results indicate that rigidity، shear capacity، and energy dissipation of wall were increased whenever column dimensions and length to height ratio was increased. Moreover، the main failure mode as global buckling was happened at the base of the column with increasing of the number of floors، so indicating the decreasing capacity and rigidity. And also the results showed that more open dimensions caused the higher ductility، decreasing capacity and rigidity and dissipation. The higher capacity، rigidity، energy dissipation، and ductility were observed at shear wall consisting of the more stiff nesses.

One of the main reasons for lack of competitiveness in automotive production in our country is mismanagement in supply of primary parts and transporting them to the domestic auto companies. This paper presents an integrated approach in order to make decisions to schedule orders of production and transportation، including assigning orders to suppliers and vehicles، sequencing production orders for suppliers and the way they are transported to Iran Khodro company. A shared transportation navigation is also considered in the problem. In order to optimize decisions in the proposed model، a genetic algorithm is used. In comparison to integrated and non-integrated models and the developed genetic algorithms used recently in the literature، the algorithm suggested in this paper gives a better performance. Furthermore، when compared to a real case، the proposed algorithm in this paper showed superior results.

Kanban control systems used to optimize inventory levels just in time (JIT) production systems. Evaluation of kanban systems are based on flow control principles and it plays a very important role in production systems. This article presents a new dynamic card control methodology in order to keep a high level of performance measures. The main advantage of the proposed method is easy to adjust parameters. A simulation model for generalized kanban control system with dynamic adjustment of kanban size is proposed. Simulation experiments designed to test the effect of kanban sizes on performance measures such as total production time and amount of work in process، as well. The results of the simulation model indicate the improvement in production rate and reduction of backlog demand by automatic and dynamic adjustment of kanban size.

Headways (i. e. the time period between the departure times of two consecutive transportation vehicles) is an important issue for urban railway companies. In this problem we are facing with two conflicting objectives، average passenger travel time and rate of carriage fullness. Until now different multi-response optimization procedure for solving this problem is studied but these approaches do not consider responses variance and covariance between them in optimization process. Therefore this research presents a modeling and solution approach based on discrete-event simulation and response surface methodology that not only puts average of response in satisfactory region but also try to minimize the variance of responses relative to noise variable and also consider covariance between them. In order to evaluate the performance of the proposed approach، Tehran metro line 4 has been assessed. The results show that the proposed approach is superior to existing techniques.

In this paper، heat transfer rate from these surfaces were studied by computational fluid dynamics. The effect of nanoparticles concentrations and kinds on the rate of heat transfer were investigated numerically. Different models were used in order to access optimum results in simulations. The results of these simulations were compared with the well-known empirical relationships and showed a good adaptation. Finally، the RNG k-&epsilon model was chosen to simulate the heat transfer from spherical surface in the turbulent regimes. The results showed that with 5 times increasing in nanoparticles concentration، heat transfer rate increased 2. 6 times. At low Reynolds number (Re=11. 2)، the enhancement of heat transfer rate in Ag nanofluids was 2. 8 fold greater than that in Al2O3 nanofluids. Nanoparticles effects were negligible at high Reynolds number so that at Re=11190 heat transfer at three different kinds of nanofluids، which were contained Ag، Al2O3 and Cu nanoparticles، was equal.

Adsorption and desorption of substances in the surface of adsorbent materials are very important in various chemical industries. In this work، at first، desorption of carbon dioxide on activated carbon fixed bed were examined، then by using of a two dimensional geometry and finite elements method with COMSOL multiphysics software the desorption phenomena were simulated numerically. By using of the proposed numerical model the effects of some effective parameters were studied theoretically. Modeling of activated carbon fixed bed was performed for two fixed and mobile phases in the bed and in stationary and time dependent regimes. Based on the reported results in this work، the mass transfer coefficient and the outlet flux of carbon dioxide from the particles of bed are effective parameters on the desorption phenomenon.

Molecular dynamics simulations are used to study the mechanical properties of single-walled carbon nanotube reinforced polyvinyl pyrrolidone matrix. The effects of nanotube diameter and chirality on the elastic moduli of carbon nanotube reinforced nanocomposites are studied. It is shown that zigzag nanotube reinforced polymers have higher longitudinal elastic modulus than their armchair counterparts. For example، embedding (5، 5) and (9، 0) SWCNTs whose diameters are close to eachother in polymer matrix lead to the elastic modulus of 78. 43 and 81. 55 GPa، respectively. Besides، increasing diameter results in decreasing longitudinal Young? s modulus. Because of disability of the existing finite element approaches to study the behavior of polymers containing atom types other than carbon، based on molecular dynamics simulations، a finite element method is proposed. The results of the proposed method are in good agreement with the results of molecular dynamics simulations. The correlation coefficient of diameter and Young's modulus obtained from molecular dynamics simulations is equal to-0. 8375. Moreover، the correlation coefficient of diameter and Young's modulus computed by finite element method is obtained as-0. 8781.

In this study، a comprehensive force balance model is developed to estimate the equilibrium size of agglomerates formed during the fluidization of nanoparticles with considering the importance of hydrogen bond، van der Waals and gravitational forces. Also، the influence of vapor of different polar materials (including methanol، ethanol، 1-propanol، 2-propanol، 1-butanol، 2-butanol and ammonia) on the size of hydrophilic silica nanoagglomerates and consequently their fluidization behavior is studied by experiments in a gas-solid fluidized bed. The results show that using vapor of these polar materials (with hydroxyl groups in their formula) improves the fluidization behavior of hydrophilic silica nanoparticles significantly and results in a higher bed expansion. To justify the improving effect of different vapor of polar materials on fluidization behavior، the electrostatic repulsion force is added to the model and size of agglomerates are calculated in the presence of this force. It is obtained that the results of model are in good agreement with the experimental observations، so that، among all used materials methanol، 2-propanol and ethanol have the most effective impact on fluidization improvement and the smallest size of agglomerates is estimated using physical properties of these three alcohols as well. Finally، the size of agglomerates calculated by the model shows error less than 11% compared with the size of agglomerates measured experimentally by laser. This error is lower than the previous reported ones in the literature.

Direct catalytic oxidation of methane to methanol is a novel technique which has eliminated the intermediate and expensive process of synthesis gas production. This technology can be used for low deposit، remote، low-pressure fields، and abundant resources of unconventional gas، without need to manufacture of expensive units of gas-to-liquid (GTL) process. In present study، modeling and simulation of the single step (direct) conversion of methane to methanol has been investigated in a fluidized-bed reactor packed with V2O5/SiO2 particles as the reaction catalyst. Firstly، the reactor was simulated at steady-state conditions and the effect of important parameters such as feed temperature and residence time of reactants within the reactor on methane conversion and product's selectivity was studied. In the next step، dynamic simulation of the process was performed and the effect of disturbances of effective parameters on reactor performance was investigated. In steady state conditions، for the residence time of 9 seconds maximum yield of methanol production was obtained at 50 bar and 773 K and methane conversion was 32. 2 and methanol selectivity was 42. 1. Results showed that by increasing reactor temperature and residence time، methane conversion increases but methanol selectivity decreases. Cooling fluid temperature and residence time were found to have the greatest effect on reaction rate and rector performance. Also results showed reasonable agreement with the experimental data reported in the literature for fixed-bed reactor.

A helical double-pipe heat exchanger filled with a porous material under asymmetric heat flux is studied analytically. Momentum and energy equations are transformed to the Germano? s coordinate and then expressions representing the velocity and temperature fields are obtained based on the powers of dimensionless curvature using perturbation analysis. To be more insightful، contours of velocity and temperature fields are presented. Results reveal an asymmetric axial velocity profile arising from the curved geometry. The Nusselt number increases with respect to the dimensionless curvature increment as well as the inner to outer radius ratio increase.