فهرست مطالب

Science and Technology Transactions of Mechanical Engineering - Volume:39 Issue:2, 2015
  • Volume:39 Issue:2, 2015
  • تاریخ انتشار: 1394/07/18
  • تعداد عناوین: 13
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  • D. Donmez Demir, B. G. Sinir, N. Bildik Pages 283-290
    In this paper, the dynamical behavior of an axially moving string modeled by fractional derivative is investigated. The governing equation represented motion is solved by the method of multiple scales. Considering principal parametric resonance, the stability boundaries for string with simple supports are obtained. Numerical results indicate the effects of fractional damping on stability.
    Keywords: Axially moving string, Fractional differentiation, Method of multiple scales, Principal parametric resonance
  • A. Fadaei, H. Mokhtari Pages 291-307
    Welding is one of the most important assembling methods which are widely applied in different applications. In service conditions, some of the weld joints which meet defects must be repaired. The residual stresses impress the operation and efficiency of the repair weldment. In this study, the temperature and the residual stress fields were investigated by both the finite element simulation and the experimental measurement due to repair butt weld of the two thin St 37 plates. The welding process was simulated using the three-dimensional finite element and the Goldak double-ellipsoid heat source models. The temperature distribution and the residual stresses were computed both in the initial and repair welds. The element birth and death technique was used to simulate the filler metal deposition into the weld pool. The experimental measurements were performed to verify the numerical results in the both initial and repair welds. The residual stresses were measured using the hole drilling method. The experimental results revealed that the finite element simulations obtained in the present work were capable of predicting the residual stresses in the initial and repair welds. Also, the effect of the repair length variation on both magnitudes and distribution of the transverse and longitudinal residual stresses were shown.
    Keywords: Repair butt weld, residual stress, finite element simulation, experimental study, hole drilling method
  • N. Ahmadi, H. Taraghi, M. Sadeghiazad Pages 309-323
    Modeling the heat and mass transport in micro channel is being used extensively in researches and industrial applications. The aim is optimizing fuel cell designs before building a prototype for engineering application. In this study, numerical, three-dimensional, single phase computational fluid dynamics model of a proton exchange membrane fuel cell with both the gas distribution flow channels and the Membrane Electrode Assembly (MEA) has been developed. A single set of conservation equations which are valid for the flow channels, gas-diffusion electrodes, catalyst layers, and the membrane region, are solved by finite volume technique. The present simulated straight channels PEMFC model, accounts for major transport phenomena and the performance. Additionally, the effect of reversing the flow direction at cathode side has been investigated to study the fuel cell performance and species distribution. The results showed that, in the PEMFC with the counter flow channels, the output current density has been decreased and also the kind of species distributions has been influenced by this phenomenon. It is very important to model the back diffusion and electro-osmotic mass flux for determination of ionic conductivity of membrane which affects the performance of fuel cell. Finally, the numerical results are validated by available experimental data.
    Keywords: PEM fuel cells, cell voltage, current density, counter flow, fuel cell performance
  • Y. Bakhshan, A. Hajhosseini Pages 325-335
    In this study, thermal performance of a cylindrical heat pipe is investigated numerically. Three different types of water based nanofluids, namely (Al2O3+Water), (Diamond+Water) and (Multi-Wall Carbon Nano tube (MWCNT) +Water) have been used. The influence of using the simple nanofluids and MWCNT nanofluid on the heat pipe characteristics such as liquid velocity, pressure profile, temperature profile, thermal resistance and heat transfer coefficient of heat pipe have been studied. A new correlation developed by Bakhshan and Saljooghi, for viscosity of nanofluids has been implemented. The results show good agreement with the available analytical and experimental data. Also, the MWCNT based nanofluid has lower thermal resistance, higher heat transfer coefficient and lower temperature difference between evaporator and condenser sections, so it has good thermal specifications as a working fluid for use in heat pipes. The prepared code has capability for parametric studies also.
    Keywords: Heat Pipe, nanofluid, carbon nanotube, thermal performance
  • H. Yazici, M. Akcay, M. Golcu, M.F. Koseoglu, Y. Sekmen Pages 337-349
    Heating and sudden cooling are among the most important processes affecting temper quality in the production period of tempered auto glass. In this study, heat transfer characteristics and cooling times during the sudden cooling process of tempered glass production have been experimentally investigated. Flat glass with dimensions of 50x50x4 mm has been heated up to 680 ºC in the furnace and was then exposed to sudden cooling process until its surface temperature dropped to 70 ºC. In the cooling process two mutually placed 8 cm long air jet nozzles with an internal diameter of 8 mm have been used. Experiments have been conducted for Reynolds numbers in the range of (20000≤Re≤40000) and for dimensionless jet to plate distances in the range of (1≤H/D≤10). Based on the above parameters cooling periods, local (Nux), average (Nuaver) and stagnation point (Nustag) Nusselt numbers have been experimentally determined.
    Keywords: Auto glass tempering, heat transfer, impinging air jet, Nusselt number
  • R.S. Lebelo, O.D. Makinde Pages 351-363
    In this paper, we examine the effects of thermal radiation on CO2 emission and O2 depletion in an exothermic reactive slab of combustible materials with uniform surface temperature. The governing equations for the nonlinear heat and mass transfer problem are derived and solved numerically using Runge-Kutta-Fehlberg method with shooting technique. Numerical expressions for temperature field, CO2 emitted and O2 depleted are derived and utilised to obtain expressions for Nusselt number and Sherwood number at the material surface. The effects of various thermo-physical parameters on the temperature field, CO2 emission and O2 depletion are depicted graphically and discussed quantitatively. Thermo-physical parameters that help to reduce CO2 emission and hence O2 preservation, are identified, including those which help to monitor measures to avoid explosions in spontaneous combustion processes.
    Keywords: Reactive slab, CO2 emission, O2 depletion, Thermal radiation, numerical simulation technique
  • M.J. Kazemzadeh-Parsi Pages 367-387
    The present study is concerned with optimal shape determination of inhomogeneous and temperature dependent domains under steady state heat conduction. Such situations are important in many thermal design problems, especially in shape design of electronic components and chips. In the present paper, we formulate the shape optimization problem based on volume minimization of heat conductive material while limiting maximum temperature. The smoothed fixed grid finite element method which is a new approach based on the non-boundary-fitted meshes is used to obtain temperature field. The boundary parameterization technique using splines is also adopted to manipulate the shape variations. A modified version of the firefly algorithm which is a recently developed metaheuristic optimization technique is proposed as the optimizer. These modifications consist of adding memory, adding newborn fireflies and proposing a new updating formula. To evaluate the applicability of the proposed method five numerical examples are solved and the results are presented.
    Keywords: Shape optimization, nonlinear heat conduction, smoothed fixed grid finite element method, metaheuristic optimization, firey algorithm
  • P. Raghu, N. Nallusamy Pages 389-398
    The spray characteristics of the fuel greatly influence emissions from diesel engines. Spray development plays a vital role in improving the combustion and emission characteristics of fuel because it directly affects the air- fuel mixture formation. The spray characteristics of fuel mainly depend on fuel injection process, fuel density, fuel viscosity, ambient pressure and temperature. Among these, the effect of fuel injection pressure is a very important parameter directly affecting spray structure. This study investigates the effects of fuel injection pressure on the spray characteristics such as spray angle and spray tip penetration in a constant volume chamber under non evaporating conditions by image processing techniques. The macroscopic spray characteristics were quantified using dimensionless analysis by examining the role of the dominating forces associated with liquid jet breakup. The Weber number, Reynolds number and air- to- fluid density ratio were used to capture the primary forces including the inertia, surface tension and aerodynamics drag forces. The Weber number has a more profound effect on the spray penetration and spray cone angle compared to the Reynolds number contribution. This analysis yielded dimensionless correlations for spray cone angle and spray tip penetration that provided important insight into the spray breakup and atomization process.
    Keywords: Biodiesel, spray tip penetration, spray cone angle, Weber number, Reynolds number, spray characteristics
  • H. Shokuhmand, F. Sangtarash Pages 399-412
    Numerical and experimental analyses are performed on multilouvered fin banks in low and medium Reynolds regimes to investigate the effects of dimples and perforations on flow structure and heat transfer capacity. In different Reynolds numbers the flow efficiency has been calculated numerically and experimentally by flow visualization method and the results showed that dimples and perforations can increase flow efficiency more than 5% in fin banks. Additionally, a set of numerical and experimental analysis has been done to evaluate temperature contours and total heat transfer in simple, dimple and dimple-perforation fin banks in different Reynolds Numbers. The simulations revealed that the heat transfer and temperature augmentations occur due to the existence of a circulation region that is created by the dimple. The results showed that adding dimples with perforations increased the heat transfer capacity of the fin banks up to 9% compared with the simple louver fin banks.
    Keywords: Multilouvered fin, dimple, perforation, flow efficiency, heat transfer enhancement
  • M.R. Soltani, J. Sepahi Yoinsi, V. Farajpoor Khanaposhtani Pages 413-426
    The starting behavior of a supersonic axisymmetric mixed compression air intake was numerically investigated. The code solves Reynolds-averaged Navier–Stokes equations using an explicit finite volume method in a structured grid by the Roe flux difference splitting scheme. Further, it uses the Baldwin–Lomax algebraic model to compute the turbulent viscosity coefficient. The correct method of surveying the intake starting problem and effects of several geometrical parameters such as: intake throat area, cowl lip roundness and spike surface curvature upstream of the throat on the starting and performance of the intake were studied. Results showed that correct combinations of the mentioned parameters can suppress the intake unstart problem and in addition can prevent the reduction of the intake efficiency.
    Keywords: Supersonic air intake, starting problem, throat area, cowl lip roundness, spike surface curvature, intake efficiency
  • M. Abid, H. Abdul Wajid, S. Ullah Pages 427-436
    Welding produces residual stresses and distortion, having detrimental effect on structure integrity and service performance of the welded pipe joints. This paper investigates residual stresses and distortion during Gas Metal Arc Welding (GMAW) of pipes of schedule 40 and 60, nominal diameter 100mm and 200mm and thickness 8mm and 10mm with ANSI flanges of class 300#. Welding parameters including voltage, current and heat input are varied to find the optimized set to control stresses and deformations. Stress variation on the flange side is observed prominent compared to the pipe side due to its dimensional variation. Axial flange displacement along 360 degrees is also concluded obvious, hence effecting sealing performance of the gasketed flanged pipe joints.
    Keywords: Welding, geometric parameters, residual stresses, deformations, welded pipe, flange joints
  • P. Thirumal, K.S. Amirthagadeswaran, S. Jayabal Pages 437-450
    Air conditioning is widely applied for the improvement of standard of living in human life. This present investigation focused on the prediction of indoor air quality characteristics of air conditioned car using fuzzy logic algorithm. The conditioned space was selected and the experiments were planned as per design of experiments to study the effect of human load, fresh air supply and air velocity on the human comfort conditions. The mathematical models were developed to predict the comfort conditions, namely temperature, CO2 level and relative humidity over a specified range of input conditions. Carbon dioxide exhalation rate differs person to person based on their body weight and burning rate of calories, etc. Fuzzy logic predicted the intermediate response of IAQ parameters for varying input conditions in this present investigation. The proposed multi response fuzzy model predicted better results comparing with nonlinear regression models. The absolute error percentage of fuzzy model for carbon dioxide level, temperature and relative humidity is 2.05%, 3.81 % & 2.24 % respectively.
    Keywords: Indoor air quality, temperature, relative humidity, carbon dioxide, Fuzzy logic
  • A.R. Zendehboudi, R. Hashemi Pages 451-458
    Due to climate changes in recent years, in order to attain comfort and convenience, demands for heating, ventilating, and air conditioning (HVAC) systems have been increased around the world, which has led to increases in emissions and power. Desiccant cooling systems (DCSs) are designed and built to reduce moisture and power consumption, create comfortable conditions for people in a building, and environmental protection. Evaluation and study of desiccant wheels (DWs) is vital as they are one of the most important components of DCSs and their performances have a direct impact on the total system. So, the primary purpose of this research is to design a DW by focusing on the operational parameters of the wheel. In this regard, an experimental study was conducted to investigate the effect of regeneration temperature, rotation speed and inlet air humidity ratio on the performance of a DW in a hot and humid climate. The experimental results were obtained in the regeneration temperature between 70-100oC, wheel rotation speed 4-12 revolutions per hour (rev/h), 1:2 split between regeneration and process sides and different humidity.
    Keywords: Desiccant wheel, silica gel, regeneration temperature, rotational speed, experimental analysis