Year:2004 | Volume:38 | Issue:4 (86)|Papers Count:14

In the present study, enhancement of heat transfer during condensation of R-134a inside horizontal coiled wire inserted tubes has been investigated experimentally. Test condenser was a double pipe counter-flow water cooled heat exchanger which refrigerant flows inside the internal tube, while water flows in the annulus. Coiled wires with different wire diameters and coil pitches were utilized in full length of test condenser. It is found that the use of helically coiled wire increased the condensing heat transfer by as much as 85 percent above the plain tube values on a nominal area basis. Influence of variety of parameters such as vapor quality and coiled wire geometry on the heat transfer augmentation was also investigated. A correlation for the prediction of heat transfer coefficients inside the coiled wire inserted tubes was developed by utilizing the gained results for the rough tubes, on the basis of Boyko and Kruzhilin (1967) correlation.

In this research we investigate the role of normal modes of internal waves in generation of layered structure in a part of Persian Gulf where river plume inters, and in some laboratory experiments. The model prediction and observations show that layers so formed have a thickness of about 2m based on salinity variations with depth, but layers (about 5m) based on horizontal velocity profiles. Laboratory experiments with a plume outflow in a "filling box" profile also generate normal mode layered structure with where is layer thickness and is the plume depth. In these experiments the of the flow is smaller than the of field flow; hence, the normal modes are substantially dissipated with depth. The comparison between the field study and the experimental observation shows that the two are, within the experimental limitations, in agreement.It should be emphasized that field observation is for semi-enclosed seas but the laboratory experiments are for enclosed region. Hence some of the discrepancies in the results of two cases are inevitable.

The structure of the leading edge vortices over highly swept wings at various angles of attack has been studied. Some semi-empirical relations have been used to investigate the behavior of these vortices before and after burst. Further the normal force and drag coefficients and vortex burst location have been predicted for Delta wings with the sweep angles of 60 to 75 degrees for both static and dynamic cases. The results show that for dynamic motions, increasing the reduced frequency and oscillation amplitude and decreasing the wing sweep angle will increase the normal force and drag coefficients. The aerodynamic forces during upstroke motion are higher than their static counterparts. This phenomenon reverses during down stroke motion, creating the Hysteresis loop.

The removal of dissipated heat from electronic devices, particularly used in navigation systems of high-speed combat aircraft is of a great importance. A kind of heat sinks proposed to be used in the vicinity of such devices is helically grooved heat pipe (HGHP). The combat aircraft is anticipated to maneuver in air. This maneuver may cause several transverse accelerations in different directions and even high magnitudes, simultaneously. So in order to obtain an acceptable performance of a HGHP, it is necessary to study the effects of these various accelerations, which influence significantly their cooling capacity. In this work a mathematical model for a copper HGHP with Ethanol as its working fluid, has been presented. By using this model, the capillary limit heat transfer rate for each groove, dry out phenomenon that may happen in some grooves as well as overall heat removal capacity of HGHP have been studied. In the previous works only one acceleration was applied to the heat pipe, hence to the aircraft, in which the heat pipe was installed. But here we consider both single distinct transverse accelerations separately, as well as their combination. The results are expressed in terms of the type of accelerations, their magnitude and working adiabatic temperatures. It is shown that the combination of revolving and rotating motions of the aircraft in which HGHP is proposed to be used (in the vicinity of its electronic packages in trailing edge flap actuators or ailerons), has a noticeable effect on the increase of the capillary limit of the HGHP and its cooling capability. In addition, boiling limitation and entrainment limitation as two important heat transport limitations of heat pipes are calculated and compared with total capillary limit heat transfer of the HGHP in terms of temperature, to get a better view of the total performance of this kind of heat pipes.

The two-dimensional laminar air jet issuing from a nozzle of half which terminates at height above a flat plate normal to the jet is numerically studied to determine the effects of buoyancy on the flow and thermal structure of the region near impingement. The impinging surface is maintained at a constant heat flux condition. The full Navier-Stokes and energy equations are solved by a finite difference method to evaluate the velocity profiles and temperature distribution. The governing parameters and their ranges are: Reynolds number Re, 10-50, Grashof number Gr, 0-50, Richardson number Ri =Gr/Re2, Non-dimensional nozzle height H, 2-3. Results of the free streamline, local friction factor and heat transfer coefficient are graphically presented. It is found that enhancement of the heat transfer rate is substantial for high Richardson number conditions. Although the laminar jet impingement for isothermal condition has been already studied, however the constant heat flux has not been studied enough. The present paper will analyze a low velocity air jet, which can be used for cooling of a simulated electronics package.

Electrochemical machining is a suitable & effective method of machining the complex Shapes that can be used for hard materials. Machining of the surfaces to which access is hard or impossible shall be considered as one of the method's advantages. Therefore, the method is one of the suitable procedures for machining of the pipe gun's rifles with moderate or high calibers. The aim of the paper is to simulate the machined surfaces by the method and use of the mathematical model, which has been obtained from electrochemical sciences. Finally, the governed equations which are non-linear differential type, have been solved by Rung kuta. Furthermore, experimental results which have been obtained by the method, are described in terms of parameters of machining. In summary, the experimental and model results have been compared with each other and the comparison shows that the model conforms well with experimental results.

During previous research studies, Pb- 2Sn-0.08Ca alloy was found to be the most suitable composition among Lead-Tin-Calcium compositions for fabricating anode electrodes for use in copper electro winning cells from the viewpoints of mechanical properties and corrosion resistance. In this paper the electro-catalytically behavior of the selected lead alloy is studied within a set range of applied current densities together with kinetics and reversibility capabilities of oxidation-reduction processes and the effect of CO3+ ions on reduction of over voltage of H2O anodic reaction during copper electro winning process using cyclic voltammeter technique (1 to 7 cycles), in applied potential range of -1300 to 2600 mV (vs. SCE) and at various scan rates. Then, the chemical phases of oxide layers formed over the Lead Anode surface within the applied potential range when inside 180 g/l H2S04 solution, was determined via XRD analysis. The obtained results indicate that the electrolyte resistively increases simultaneously, as the porous complex of Oxy- Sulphate Lead film is formed. Also at potentials more than 1.4V (vs. SCE), PbO2 film forms which has a good electro-catalytic property and high electrical conductivity. It is also true to say that this film (PbO2) is responsible for protecting the base metal from corrosion during copper electro winning process. In addition it has been shown that the kinetic of oxidation process, which yields to the formation of protective oxide layer is faster than the reduction process responsible for degradation of the protective film. Also the presence of Co3+ ions at 100ppm concentration can reduce the over voltage of H2O anodic reaction to as much as 200 mV.

Activity coefficients of solutes in a solution containing Fe as solvent and several solutes could be calculated by mathematical relations between the interaction coefficients of the solutes measured in a ternary Fe based solution, but these mathematical relations could be used only for dilute solutes (the solutes with concentration of 1% or less). In this work, a new method has been developed in which the calculation of the activity coefficients of non-dilute solutes as well as dilute solutes in a solution containing Fe as solvent and several solutes could be possible by using the interaction coefficients of the solutes measured in a ternary solution containing Fe as solvent, but only in a concentrate range in which the experimental relation between Ln Ki and Xj is linear (i and j are solutes and K is activity coefficient).  

Mn-Zn ferrites with chemical formula (Mn-Zn)Fe2O4 have a widespread applications in the electrical and electronics industries. These ferrites are synthesized generally by conventional method. This method which contains calcinations of raw materials, is affected by several parameters such as composition and purity of the raw materials and temperature, time and atmosphere of the process. Optimization of these parameters is very important because these parameters affect the magnetic and electrical properties of the material. In this work, first, the ratio of the raw materials for production of high purity ferrite has been optimized and then the effects of calcinations temperature and atmosphere on the composition have been investigated. The magnetic properties have also been measured. Finally, suitability of the optimized process parameters for other compositions has been studied.

Nano-crystalline particles of barium hex ferrite (BaFe12O19) have been prepared by a sol-gel combustion technique using nitrate-citrate gels. Ammonia and sodium hydroxide were used as adjusting agents. The results revealed that the nitrate-citrate gels in both cases exhibit a self-propagating behavior after ignition. The thermal decomposition process of the nitrate citrate gels and as-burnt powders was investigated by XRD and DTA/TG techniques. The results indicated that the single phase of Ba- hex ferrite forms at a relatively low temperature of 900 C when ammonia is used as a pH adjusting agent. Also, the SEM investigations and VSM measurements revealed that the barium hex ferrite particles processed in the presence of ammonia exhibit a larger mean particle size and higher magnetic properties in comparison with those of processed using sodium hydroxide.

The aim of the present work is to assess the compatibility of Sarcheshmeh molybdenite concentrate with native bacteria of the area. The strain of thiobacillus ferrooxidans was isolated from Sarcheshmeh mine drainage and the test work were performed in shake flasks. Effect of increasing pulp density from 1 to 5%, changing the culture medium from 9K to Norris and addition of surfactant Tween 80 on the performance of the bioleaching was investigated. The results were compared to the chemical leaching process. It was revealed that Mo dissolution in bioleaching of molybdenite was 6 times greater than chemical leaching. It was also found that thiobacillus ferrooxidans was able to leach pyrite and copper sulfide at higher rate compared to molybdenite, and consequently reduced the Fe and Cu content of molybdenite concentrate. Application of Norris culture medium or addition of Tween 80 to 9K medium for dissolution of Cu and Fe content of molybdenite can reduce the time of bioleaching and its process cost.