Year:2006 | Volume:39 | Issue:6 (94)|Papers Count:14

Backward extrusion is one of the metal forming processes that have been developed for use in industries in recent years. In this paper ,an upper bound analysis of the backward extrusion process of tubes of arbitrary shapes from round billets is carried out .A new cinematically admissible velocity field is proposed. Using this velocity field, the upper bound expression is determined and is minimized with respect to optimizing parameters. Results for ellipse and gear shaped punches are shown in diagrams that indicate extrusion load and gauge pressure variations with reduction of area, shape factor and friction factor variations. These results are compared with experimental and theoretical results of other researchers. Agreements of the present work with the experimental results have been observed for higher values of reduction of area. For low values of the reduction of area observations of the present and experimental work show some differences.

In the present work, heat transfer augmentation during condensation of R-134a vapors inside horizontal twisted tapes inserted tubes has been investigated experimentally. Test condenser was a double pipe counter flow water cooled heat exchanger which refrigerant flowed inside the internal tube, while water flowed in the annulus. Twisted tapes with different twisted ratios were used in full length of test condenser. It is found that the use of twisted tape increased the condensing heat transfer rates by as much as 43 percent above the plain tube values on a nominal area basis. Influence of a variety of parameters such as vapor quality and twisted  tape geometry on the heat transfer augmentation was also investigated. An empirical correlation was developed to predict the enhanced heat transfer coefficients.

An efficient work holding plan must include an optimized clamping scheme. This article uses finite element analysis with a supportive combination of neural network and genetic algorithm to compute an optimized set of clamping forces. A frictional finite element model of work part-fixture system under cutting and clamping forces is solved. The results are utilized for training and querying an artificial neural network (ANN). The ANN is employed to recognize a pattern between the clamping forces and contact status in the work piece-fixture system and maximum elastic deformation of the work piece. Using this pattern as fitness function, a genetic algorithm based program determines the optimum values of clamping forces. These forces while hold the component securely, would not cause its excessive deformation or stress. Therefore they can be prescribed confidently for process planning. The advantage of this work against similar studies is manifestation of the state of contact between fixture elements and work part. The study facilitates fixture design task in a design for manufacture environment and contributes to computer aided process planning.

In this paper, the applicability of polymeric coatings to reduce the drag experienced by submerged ax symmetric bodies was investigated experimentally. Two different commercial polymeric coatings were used for this study: 1) Polyacrylamide (PAA), and 2) Polydemethyl siloxane (PDS). Whereas the first polymer is water-soluble; in contrast, the second polymer is hydrophobic. Drag measurements were conducted on a model of the body of interest with a geometrical scale of 1/10. A home-made water column device was used to obtain drag data at Reynolds numbers up to 20,000 (large enough for the annular flow between the exterior of the model and the interior wall of the test cell to be taken as turbulent). Extensive test results obtained using this device revealed that both polymers are effective in reducing the drag coefficient in the range of 20-30 %. The effectiveness of PAA coating, however, was realized not to be steady apparently because of its being washed away by the flowing water. In contrast, steady drag reduction was found to be achievable using the PDS coating.

In this paper, a capacitive type MEMS accelerometer is designed based on surface micromachining technology. The synthesis is limited to the mechanical part not the electronic conditioning circuits. A lateral comb type transducer is selected. The design layout is based on manufacturing limitations of MUMPs; a surface micromachining technology for polysilicon. The sensor parameters were obtained for an automotive application in the airbag systems. To reach the desired bandwidth and good stability, a (force) feedback system was used applying negative stiffness. The comb spring stiffness was derived analytically. A finite element analysis was also conducted to verify the results. Other performance characteristics of the sensor such as shock resistance and transverse sensitivity (which were complicated to drive analytically) were calculated by finite element simulation within an optimization algorithm to meet the required sensor specifications.

A finite element solution procedure is presented for the simulation of transient incompressible fluid flows with natural convection using triangular meshes. The algorithm is based on the artificial compressibility technique in connection with a dual time-stepping approach. A first-order discrimination is employed in real-time while an explicit multistage Runge-Kutta scheme is used to march in the pseudo-time domain. A standard Galerkin finite element method stabilized by using an artificial dissipation technique is used for the spatial discrimination. The performance of the proposed algorithm is demonstrated by solving a couple of transient natural convection for various Rayleigh numbers. Computations show that the proposed method produces accurate results for the range of Rayleigh numbers analyzed here.

Operating of pump as turbine could be considered with two point of view: first in transient operation when flow to be reversed and second in steady operation that pump is used instead of turbine. Although very efforts have been done for obtaining of relations of pump as turbine operation but few efforts have been done according to flow analyzing and losses behavior. In the first phase of this presented paper, with considering of pump as turbine behavior in steady operating, the hydraulic losses of volute and impeller according to the experimental results and theoretical relations have been obtained and in the next section, the new method for prediction of the best operating point has been presented. Because of complexity of pump as turbine behavior, the prediction of performance of pump as turbine is difficult. The existed relations that predicate the pump as turbine behavior is pending of empiric. So the very interests exist for obtaining of theoretical relations. In the present paper, the performance point of pump as turbine is determined with calculation of hydraulic parameters and then the best performance point of pump as turbine is presented. Finally the obtained results are compared with experimental results.

In this study fluid flow and heat transfer in a micro channel in presence of EDL is studied. To avoid the complication due to cross-section geometry and the surface roughness of the flow channels, a micro channel formed by two parallel, smooth silicon plates were used. Solution of Poisson- Boltzman in the channel gives electrical potential distribution in cross section of the channel. Then after using the electrical potential distribution in the channel, the electro-kinetic field has been determined. Electro-kinetic field is used in momentum equation as a term of body force. For solving the momentum equation velocity distribution in the channel is determined. Finally using velocity distribution, energy equation is solved and temperature contour and Nusselt number in channel is calculated.

Explosive compaction of powders is considered as a high energy and rapid process which involves an intense deposition of shock energy on powder particle surfaces. This process is an attractive route for the consolidation of aluminum matrix composite that offers several unique advantages over the static compaction methods in conventional powder metallurgy. The extremely short processing time (microseconds) in explosive compaction can avoid the formation of any interfacial reactions between reinforcing and matrix phase. In this investigation the density of the Al-20vol.%SiCp composite compacts has been studied with respect to the variations of explosive pad thickness as one of the important processing parameters. Microstructure and interface properties of the compacts have been studied by optical microscopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Observations indicated that distribution of SiC particles in Al matrix was different at the periphery and center of compacts. According to the experimental results obtained from SEM, TEM and X-ray diffraction, the surface of SiC particles showed no obvious reaction products such as Al4C3, indicating a clean interface between SiC particles and the aluminum matrix.

Tribological characteristics of iron-based sintered friction materials were studied using a conventional pin-on-disk testing machine. The specimens of different compositions were compacted and sintered under controlled atmosphere in three different cycles. Friction coefficient and wear rates of composite specimens and cast iron disks were determined and worn surfaces were examined using scanning electron microscopy. Results show that the specimens prepared from ball-milled powder and also from large grain size iron powder were destroyed due to the lack of strength and excessive expansion. It is also observed that for other specimens prepared by a double cone mixer, the coefficient of friction and wear rate are strongly dependent on the contact surfaces behavior. Wear rate decreases with increasing load and sliding velocity in these specimens. Among different compositions, those containing Fe, Cu, G, BaSO4 showed better tribological behavior under higher load and sliding velocities.

Some experiments were conducted to investigate the formation of iron bearing intermetallics in 319 aluminum alloy at different levels of iron and manganese. Also, the influence of solution heat treatment at 515oC for various solution zing times up to 26h on the size and morphology of iron bearing compounds was determined. The results showed that in the presence of Mn, α iron bearing intermetallics are formed in Chinese script and polyhedral morphologies and so, the addition of Mn in a sufficient level neutralizes the effect of Fe in the formation of β-needle phase. The dissolution of α-Chinese script and α- polyhedral phase was seen in the alloy solution heat treated at 515oC. Also, it was confirmed that the solution zing of the alloy can change the morphology of α phase from polyhedral to Chinese script. However, the dissolution of α phase occurs by the rejection of Si atoms from α.

A salt roasting-water leaching process was adopted for the recovery of vanadium from steel making converter slag produced at Isfahan steel plant. A mixture of slag and sodium carbonate was palletized and roasted under various conditions. The effects of roasting parameters such as temperature, sodium carbonate content of the mixture, particle size, oxygen content of gas atmosphere and time were studied. The highest efficiency of roasting was achieved at 850°C with salt (sodium carbonate) to slag weight ratio of 0.5 and slag powder particle size of - 246μm. It was observed that the oxygen partial pressure in gas atmosphere has a little effect on the roasting efficiency. This observation is in accordance with the form of vanadium in the slag which is in pentavalent state as revealed by XRD analysis. To extract vanadium compounds, the roasted products were leached in hot water under optimum conditions reported in the literature. Vanadium compounds were precipitated from the solution and was analyzed by XRD and XRF techniques where in β-Na4V2O7 and V3O4 detected.

In the present research, different trends of temperature of eutectic ending and duration of eutectic transformation due to employment of different combinations of five types of inoculants and four types of modularizes, with different fading resistances, were studied. Cooling curves and image analysis were used to characterize the influence of holding time, for 0, 5, 10, 15 and 20 min, on the samples prepared from the melt processed by 2.5 wt% modularize and 0.4 wt% inoculants and kept at 1320ºC under nitrogen atmosphere. Chemical composition of 20 samples out of 100 attained by the means of emission spectrometry. While the dominant graphite morphology is spherical or flake, by succeeding of the fading phenomenon, temperature of eutectic ending and duration of eutectic transformation respectively decrease, increase and decrease again. When there is a combination of spherical, compacted and flake graphite, the above results are reverse. As long as the main graphite morphology is spherical, Mg, S and Al content decrease by advancement of fading, nevertheless after this stage, Mg and Al content reach a steady state whereas S content slightly increases.

Lap joint poly composites of base-clad AA3003/AA4043 alloys have been prepared in various conditions after vacuum furnace brazing at a pressure setting of 5×10-5 mbar. Shear strength of basements were measured under tensile loading. The results show that the bond strength decreases by increasing the length of lap joint. The maximum peeling strength was obtained at a critical length (2mm) of overlap using a clad thickness of 50- 100 μm. Sagging resistance of brazing sheets was independent of reduction in area due to low Si content of braze material. Fracture surfaces of the samples show two different ductile and brittle regions. Due to results of the energy dispersive X-ray analyses (EDX), brittle fracture regions have more Si content than the ductile fracture regions.