مجله مهندسی مکانیک
سال چهل و یکم شماره 2 (پیاپی 62، پاییز و زمستان 1390)

In this paper the pressure and positive/negative inclination of transport line effect on initiation and development of hydrodynamic slug instability is studied. In practical case pressure and inclination of the line have a major effect on initiation and development of slug as well as the flow rate of the line. Better understanding of these parameters will help the designers to select a proper size of the line. The base of this methodology is on numerical solution of hyperbolic two fluid model equations by means of a class of high resolution shock capturing methods. The results obtained for slug flow modeling in horizontal duct were compared with two sets of experimental results. The good agreement of the present modeling results with the experimental results of own and other investigators and also grid independency study show the model is capable of slug tracking and slug capturing and the numerical method which is used here can predict with high enough accuracy. It was shown that in low pressures the growth rate of the wave is faster than high pressure condition and higher pressure has damping effect on Kelvin Helmholtz instability. Other parameters such as the position of slug initiation and inclination effect were investigated.

Flowforming or tube spinning is a process with high inherent potential in flexible fabrication of high precision thinwalled tube with high surface finish and improved mechanical properties. In this paper, quality improvement method of flowformed AA7075 aluminium alloy tubes, has been investigated experimentally. Using design of experiment with Taguchi method and analysis of variance, effects of most important input parameters, such as feed rate, percentage of thickness reduction, number of passes, initial thickness, speed of rotation and entrance angle on major quality parameters of tubes, such as geometrical and dimensional accuracy, hardness, surface finish and creation of defect, have been studied. It is found that most effective factors on quality of parts are feed rate, reduction and number of passes. Decreasing the feed rate and increasing the number of passes, result in upper hardness and effective percentage of thickness reduction, better roundness, surface finish and part quality.

Laminar burning velocity is one of the most important thermo chemical properties of any combustible mixture. it relates to some variables such as type of fuel, mixture composition, its temperature and pressure. Laminar burning velocity plays a vital role in evaluating turbulent burning velocity. Evaluating laminar burning velocity at different temperatures and pressures of NG-air mixtures using experimental data and finding out a related correlation are more important. To meet the aims, a significant set of experimental pressure-time data were recorded for combustion of NG-air stoichiometric mixtures at the ranges of initial temperature of 300 to 360K and initial pressure of 3 to 7.5 bar; then they defined as input to a simulator of two zone thermodynamic combustion model and burning velocities in terms of bomb pressure during combustion period were calculated. Error analysis showed that the estimation was faced with large error at the early stage and low error at the other stages of combustion. The selected function used to correlate the laminar burning velocity versus temperature and pressure was in the form of l l 0 0 0 u u T T P P. The least square method was utilized to determine ul0, and over more than 5000 data of burning velocity and their values were found to be 22.55(cm/s), 1.68 and -0.24, respectively, with average deviation of 2.92%.

With the development of technology, metallic production especially steel production is performed through the rolling process. The efficiency and delicacy of rolling production causes this industry to have an important place compared to other metal forming methods. In practice, in most of cold rolling processes especially continuous cold rolling of strip the interstand tension is created between mills. This parameter affects the roll pressure, rolling force, forward and backward slip, and neutral angle. Thus, its calculation is an important step in continuous rolling process design. Currently, there is no analytical formula for calculating the inter-stand tension and most of the available data on this subject is based on the experimental observations. In this research, two analytical formulas for calculating inter-stand tension have been presented on the basis of analytical relations that exist in the rolling theory, the effects of inter-stand tension on rolling parameters, and the effects of difference in roll velocity on the inter-stand tension. To demonstrate the validity of the analytical model, the predicted results are compared with experimental measurements performed at Mobarakeh Steel Complex.

Cooling tower is one of the most important units used in the power plant. Thermal efficiency of the power plant is directly affected by thermal performance of the cooling system. Thermal performance of the Heller type cooling tower is significantly reduced in the wind condition as same as the other dry-cooling towers. Thermal performance of the cooling tower may be reduced up to %40 during high speed wind condition. It is due to separating flow around the radiators and deflected exit plume at the top of the tower stack. In the present research work, a special configuration was proposed for Heller-type cooling tower to reduce the undesirable wind effects. Numerical simulations of the flow field showed that, it could increase the thermal performance of the cooling tower up to %64 regarding the usual design configuration. This proposal is a cooling tower having 20×20 (m2) windbreakers and exit plane making angle of 27o with horizontal direction.

Non-Newtonian laminar flow over rotating cylinder is studied numerically. We have simulated non Newtonian flow (the both shear thinning and shear thickening) using power law model and investigated the variation of important parameters such as power index, rotating speed and inlet Reynolds number effects. Then intensive dependence of thermal and drag coefficients to these parameters is recognized. Increasing Reynolds number results in increasing the thermal and drag coefficients. Also increasing power index, due to boundary layer growth, thermal coefficient decreased but drag coefficient enhanced.

Because of the simple structure and the high efficiency, the solar flat plate collectors are plenty used to heat building as a low enthalpy heat resources. Among the solar flat plate collectors, the compact solar water heaters are the cheapest and the compact of them. Experimental and numerical investigations were performed for evaluating the performance of an innovative compact solar water heater, in this paper. The both black steel sheet with the V shape cross section and the pebbles filled into the V shape spaces have been appeared in the absorber rule. The stored water into the heater tank is able to flow through of the pebble beds and then to contact with the inner transparent cover. The free convection tests were conducted with the three inclination of collector as: 38.2  (local latitude), 33  and 42.2 . In local latitude tests was done distinctly toward south, east and west (with azimuth angles of 10 ). The average daily thermal efficiency for this collector was obtained over than 70%.