Sharif Digital Repository

The present study is undertaken to calculate the rate of change of pressure and residual air mass in die casting for vacuum venting under choked flow conditions. In these calculations, the influence of friction factor, due to roughness and vent air velocity change through the Mach number, has been taken into account. The results show that there is a critical area ratio below which the pressure and vent inlet Mach number increase with time and above which decrease with time. In addition, for an area ratio less than the critical area, the rate of change of residual air mass seems to be more changed at the late stages than at the early stages of the filling time. The picture is reversed for a... 

In the casting process, as a mould is filled with molten metal, air escapes through the vents. Air pressure in the mould cavity has serious effects upon the filling behaviour such as surface profile of the molten metal and filling time. In this project a computational model was developed for calculation of air pressure during the mould filling. A 3D single phase code based on the SOLA-VOF algorithm was used for the prediction of the fluid flow. The ideal gas assumption, conservation of mass equation and Bernoulli law were used for the calculation of air pressure. A new algorithm was developed to interpolate air pressure on the surface cells. The creation of air pressure was correlated with... 

Due to increase of per capita water usage, water supply costs and population growth rate; optimal usages of available resources are vital. This is especially important in developing countries. If we add water resource deficits to the above problems, its importance duplicates. One of the ways of using resources optimally is utilizing transferred water in distribution systems properly and preventing its loss. Unaccounted for water (UFW) is an indicator of water loss in most cities. A big part of UFW is due to leakage of pipeline in distribution systems. Researchers have found direct relation between leakage and pressure in distribution systems. Earlier works have shown a series of laboratory... 

In this study, the ultrafiltration of produced water was studied using a two-stage ultrafiltration process. In the first stage, the influences of operating parameters, including transmembrane pressure, temperature, and cross-flow velocity on the amount of flux decline caused by membrane fouling, were investigated using a polymeric membrane. In order to design the experiments and optimize the experimental results, the Taguchi method was applied. L9 (33) orthogonal array for experimental planning and the smaller-the-better response category was selected to obtain optimum conditions because the lowest flux decline was our aim. Analysis of variance was used to determine the most important... 

It has been known that applying pressure and doping carbon nano-tubes (CNTs) can each affect the behavior of high temperature superconductors. Here, bridging these two effects in order to see how they can interplay, we investigate how the dc resistivity varies in both pure and CNT-doped polycrystalline YBa2Cu3O7-δ (Y-123) compounds under hydrostatic pressure. We demonstrate that the broadening of the resistivity transition in the CNT-doped samples (of 0.7 wt%) is smaller than the undoped samples. In addition, by increasing applied pressure, the doped samples exhibit a higher rate of the increase of the transition temperatures Tcon and Tcmid than the undoped samples. Interestingly as well,... 

A series of experiments were carried out to study the unsteady wakes behind an oscillating airfoil. The airfoil is a section of a wind turbine blade oscillating in pitch about the quarter chord axis at various reduced frequencies, oscillation amplitude and mean angles of attack. Real time velocity profiles were obtained using total and static pressure at 35 vertically aligned points behind the airfoil via two similar rakes. The rakes were located at a distance of 1.5 chord length behind the model. The results show great influence of oscillation amplitude and reduced frequency on the wake velocity profiles. Copyright © 2007 by ASME  

In this research, deformation of material in equal channel angular extrusion (ECAE) process with outer curved corner is analyzed using an upper bound solution. The effects of die angle, friction between the sample and the die walls, and the angle of the outer curved corner, on the extrusion pressure are all considered in the analysis. It is found that the extrusion pressure decreases with increasing both the die angle and the outer curved corner angle and increases with increasing the friction coefficient. Moreover, a good agreement is found between the predicted and experimental results of extrusion pressure relating to two dies of different outer curved corner angles used in ECAE tests of... 

The physical processes responsible for bubble formation in microchannels are not well understood and lack fundamental understanding. Experimental results are not exactly in agreement with each other and there are no definite theories to explain the possible effects of different parameters. Among different parameters the microchannel hydraulic diameter can affect the bubble formation mechanism in microchannels strongly. In this paper the effect of the time dependent pressure difference between inside and outside of the bubble on bubble dynamics in microchannels have been investigated. The source of this time dependency can be the emergence of bubble embryos which produces a density... 

Full potential density functional theory (DFT) calculations have been performed for the MgB2 superconductor. Results show that applying positive (negative) pressure leads to a decrement (increment) in the DOS at the Fermi level that this calculations suggest a decrement in Tc under application of positive pressure. In the Γ - A path of momentum space, the band which has the dominant role in conduction properties, moves upward when c increases or a decreases, and moves downward when c decreases or a increases. By relaxation of the system under the plane strain, we have studied the behavior of axial lattice parameter c. Our results show that changes in the axial lattice constant c is one third... 

The well-known Rayleigh-Plesset [Formula presented] equation is the basis of almost all hydrodynamical descriptions of single-bubble sonoluminescence [Formula presented]. A major deficiency of the [Formula presented] equation is that it accounts for viscosity of an incompressible liquid and compressibility, separately. By removing this approximation, a new modification of the [Formula presented] equation is presented considering effect of compressional viscosity of the liquid. This modification leads to addition of a new viscous term to the traditional bubble boundary equation. Influence of this new term in the dynamics of a sonoluminescing bubble has numerically been studied considering... 

Two new correlation functions with three adjustable parameters for the calculation of thermal conductivity of gases, liquids, and refrigerants over wide temperature-pressure ranges are presented. It is shown that by using suitable mathematical expressions for the correlation functions, the number of adjustable parameters can be limited to three, which is less than for similar functions. The average and maximum deviation of the correlation compared with the measured data is about 1% and 8%, respectively  

The production of reducing gas is usually a bottleneck to capacity increase in direct reduction plants. Modelling and simulation of reducing gas production result in better design and operation of these plants. In the present work, a mathematical model is discussed that relates to a Midrex reformer, which is different from conventional steam reformers in some respects. Both process side and furnace side have been included in an integrated model. Simulation results have been tested against available data from an actual plant. The model predictions have been found to be reasonably accurate. The effects of three process variables, feed gas temperature, feed gas pressure and excess air, on... 

In theories of multiphase flow through porous media, capillary pressure-saturation and relative permeability-saturation curves are assumed to be intrinsic properties of the medium. Moreover, relative permeability is assumed to be a scalar property. However, numerous theoretical and experimental works have shown that these basic assumptions may not be valid. For example, relative permeability is known to be affected by the flow velocity (or pressure gradient) at which the measurements are carried out. In this article, it is suggested that the nonuniqueness of capillary pressure-relative permeability-saturation relationships is due to the presence of microheterogeneities within a laboratory... 

Dissolved air precipitation/solvent sublation (DAP/SS) was used for treatment of simulated and real oilfield produced water to generate very fine bubbles which are necessary for effective separation. In this method micro bubbles produced by saturation of air in a pressurized packed column were released in an atmospheric column leading the bubbles to raise resulting trapped contaminants in the Gibbs layer around them to be removed by a layer of immiscible solvent at the top of column. The method was conducted to solutions including Benzene, Toluene and Chlorobenzene (ClB) as part of BTEX contaminants in produced water, mixture of them as simulated produced water and real oilfield produced... 

This Paper presents simulation of Electroosmotic phenomenon in a capillary using an extended hybrid finite-volume-element method. Most of the last electroosmotic phenomenon studies in microchannels do not take into consideration the effect of reservoirs and pressure drops occurred at the inlet and outlet of the microchannel. This neglect may lead to a few drawbacks. For example, the incorrect pressure drops can significantly alter the flow patterns and mass flow rates in microchannels. As a remedy the use of reservoirs at the two ends of a microchannel has been recently practiced as a novel strategy to overcome this drawback. However, this remedy needs careful attention because the resulting... 

A plunge pool is one of the energy dissipater structures for high head dams, which ensures stability of the dam under extreme hydrodynamic pressure, due to falling jet impacts. The excess energy of the jet is dissipated, together with significant impact pressure exerted on the floor and pool walls. Thus, appropriate assessment of jet dynamic characteristics in the plunge pool is essential. This research presents experimental results focusing on. the distribution of mean and extreme pressure fluctuations on the sidewalls, due to circular and rectangular plunging jets, and the effects of pool dimension, especially in narrow valleys. The experimental variables are discharge, geometry of jet,... 

Dyestuffs removal from industrial wastewater requires special advanced technologies, since dyes are usually difficult to remove by biological methods. In this study nanofiltration process was used for removal of different dyestuffs from solutions. The rate of dye removal by spiral wound nanofiltration membrane in film thin composite MWCO=90 Dalton, was evaluated for four classes of dyes acidic, disperse, reactive and direct in red and blue dyes medium. Dye absorbance was measured by spectrophotometric method (2120 Standard Method 1998). Effects of feed concentration, pressure and total dissolved solids concentration were also studied. Results showed that increasing dye concentration lead to... 

Solid rocket performance during rapid pressure excursions differs greatly from predictions based on steady state burning rate data. Rapid pressurization following a chamber filling interval produces indicated burning rate overshoots. Transient internal ballistics of a solid propellant rocket motor during rapid pressurization part of chamber filling phase has been considered in this work. Quasi one-dimensional unsteady Euler equations with a transient propellant burning model that accounts for the effects of time rate of change of the chamber pressure on the burning rate have been used to simulate the internal ballistics of rocket motors. The compressible convective flow solver used in this... 

This paper presents a state of the art design for the blade tip injection. The design includes the means, to inject high-pressure gas jet directly into a circumferential casing groove formed in the shroud adjacent to the blade tip. The casing groove is positioned over the blade tip and exceeds 30% of the blade axial chord beyond the impeller to both upstream and downstream directions. In order to validate the multi block model used in the tip gap region, main flow characteristics are verified with the experimental data for smooth casing with a design clearance of 0.5% span. Three arbitrary mass flow rates (1.75%, 2.45%, and 4.35% of choked mass flow) have been studied. The results indicate... 

Unsymmetrical dimethyl hydrazine (UDMH) is a strong propellant, which due to its very good physical properties and high power of repellency has been utilized as a liquid fuel for fighter jet engines for so many years. There are different methods for production of this material. One of the more efficient ones which results in higher yields compared to others is the catalytic hydrogenation of Nitroso Dimethylamine (NDMA). In this work hydrogenation of NDMA to UDMH on a 5% Pd/C in aqueous solution of NDMA was studied experimentally. Experiments were carried out in a Semi-batch three phase STR reactor under constant pressure and temperature in the range of 40 to 70°C, pressures of up to 15 bar...