Year:2001 | Volume:35 | Issue:2 (72)|Papers Count:11

Heat recovery of heat exchangers with gas as one of the streams depend on the use of segmented finned tubes to compensate for the inherently low heat transfer coefficient. Segmented fins are of great practical importance in compact heat exchangers, finned tubes, waste heat recovery boilers, etc. For a given fin weight" the fin can dissipate various quantities of heat depending on its shape and geometry. Optimizing the fin, namely finding the shape that would dissipate the maximum heat for a given weight, is an important requirement in fin design. In this paper the temperature profile and the efficiency of four different shapes of segmented fins (rectangular, hyperbolic and parabolic) with variable heat transfer coefficient h, are determined by solving numerically the relevant governing differential equations, It's found that the parabolic fin has the best performance among the four examined shapes, although it has more manufacturing problems.

In Synchronous motors, in the range of several mega watts, operated at low switching frequencies, it is advantageous to use off-line optimal PWM, to reduce the switching losses and undesired frequency harmonic components. But its low transient performance may restrict the use of such commands. Considering the feed forward structure of PWM modulator, it is tried in this paper to compensate the undesired modulation error transients that may cause high over currents. An 80-KW setup is used to simulate the experiments in laboratory sizes. IGBT based, two-point voltage source inverter is used, and pperated at low switching frequencies to simulate GTO-based inverters that are commonly used to feed motors in this power range.  

In this paper, Constrain-Move strategy is generalized for a team of four object-lifting robots. The effects of redundancy in the number of cooperative robots on the system and robot-object contact stability are also studied. In addition, a distributed coordination method is proposed to ensure the contact and system stability. In the presented cooperation strategy, the robots support the load on its bottom face and there is no lateral artificial constraint on the object. The distributed coordination protocol is designed in such a way that with minimum volume of communication and implementing their own local sensors, the robots can lift the object while its Euler angles and robot-object contact are controlled. Stability condition for the system is attained mathematically. Simulation results are given to support the applicability of the presented system.

In this paper condensation heat transfer inside horizontal elliptical tubes is investigated for a stratified; concurrent two-phase flow of vapor and liquid. The analysis takes into account the effects of interfacial shear, axial pressure gradient, saturation temperature level and the development of the stratified angle associated with the accumulated condensate layer at the bottom of the tube. The governing equations are solved numerically with finite difference method. The results are presented for two cases; i.e. with and without fins. Also different values of ellipticity (b/a) are considered and discussed. At first, the results for special case of circle (b/a=l) are compared with available data in the literature that shows good agreement. Also, comparing of obtained results for different values of ellipticity shows that for ellipticity values of less than one, condensation heat transfer is greater than circular tubes; both above case's show greater transfer than those corresponding to ellipticity values bigger than one

BIMEVOX solid electrolytes are produced by partly substitution of vanadium of bismuth vanadate, Bi4V2O11, by different metals (ME: Cu, Ni, Ti, Zr,..;), and have tetragonal crystal structure at room temperature with the highest ionic conductivity at moderate temperature. BICUVOX.1 in which 10 atomic percent of vanadium of Bi4V2O11 has been substituted by Cu is the most famous member of the mentioned oxide ion conductors and at 400oC its ionic conductivity (0.02 S/cm) is more than 50 to 100 times of stabilized zirconia (the most famous solid electrolyte) at the same temperature. In the present work, BICUVOX.1 compound has been synthesized by solid-state method and its crystal structure and probable phase transformations at different temperatures were determined by X-ray diffraction and thermal analysis (DTA/TG) methods. Ionic conductivity of the BICUVOX.I dense samples was determined in the range of 100-750oC using impedance spectroscopy method. The results showed that BICUVOX.1 synthesized at 650oC with tetragonal crystal structure is stable up to the melting point and just ordering/disordering transformation of oxygen vacancies occurs at around 400- 500oC. Ionic conductivities at moderate temperatures (300-500oC) were obtained more better than stabilized zirconia, with lower activation energies (0.4 eV versus 1.1eV).

Petroleum resin belongs to a class of synthetic polymeric products and is derived from a mixture of linear and cyclic olefin monomers. The latter are byproducts of the cracking of natural gas, gas oil or petroleum naphtha. Dependent upon the usage, various petroleum resins are produced with different feed. In IRAN, the sources of petroleum resin feedstock are found in petrochemical complexes and composed of various reactive and non reactive aliphatic and aromatic components. In this research, first thorough investigation was done on suitable feedstock. Then polymerization was carried' out using a catalyst concentration of 1 to 3 percent by weight of feed, at a temperature range of 0 - 100 C under atmospheric pressure, with a reaction time of 0.5 to 3 hours. The effect of reaction temperature, residence time, catalyst concentration, speed of mixing, etc. on the condition of reaction was studied and maximum yield was obtained. This is a first project of its kind in IRAN to produce petroleum resin.

Two hydrotreating catalysts which are markedly different in terms of textural and physical properties, are investigated and compared in hydrodesulphurization reaction of thiophene added to gas oil. The catalyst attributed (A) is a microporous and the other catalyst (U), is a mesoporous type. An empirical model of reaction rate was derived for two catalysts in a micro tubular reactor in high pressure. The kinetic parameters were determined in high pressures, i.e. more than 1.2 Mpa, and temperature range of 250 to 320°C with high space velocity of 5 to 7.5 hr-1 The activity tests were carried out on pellet catalysts in different temperatures to derive the effectiveness factor of the catalysts. Using the mathematical modeling of the pellet and reactor thiele modulus, effective diffusivity and tortusity factor of the catalysts were calculated. It is inferred that catalyst (U) with spherical shape and higher pore size is more suitable for HDS reaction of gas oil than catalyst (A). The kinetic and physical properties of these catalysts can be used for modeling of HDS process in pilot plant scales.

This paper considers the problem of finding the shortest path from the source node to the sink node in networks of queues in steady conditions. Some nodes in the network contain service stations with either one or infinite number of servers. The arrival process is assumed to be Poisson and also the arc lengths are assumed to be mutually independent random variables. The paper introduces a method, which transforms each node that contains a service station to a stochastic arc corresponding to the waiting time in that node. The stochastic network is then transformed to a bicriteria network by computing the expected value and the variance of the waiting times and augmenting those to the new arc. Finally, by defining the proper utility function, dynamic programming is used to obtain the shortest path.

Every year, a vast amount of waste materials are produced and remained in rice, sugar cane, wheat, cotton,... farms in Iran, which are suitable to be used as a raw materials in plywood board industries. The existence of this abundant and cheap raw materials which are available to industries proves the value of conducting a research in this field. This research shows how the waste materials remained in farms can be used to produce high quality plywood boards without current adhesives in the plywood board industry and only through using the mineral compounds available in Iran (hydrochloric acid and magnesium oxide). The plan not only creates job and added value, but also due to the simplicity of the production process and reductions in the amount of investment, matches the environmental and economical conditions of Iran. The research has been conducted in lab scale and on the waste materials of rice farms. By continuing the work and making slight changes in production process, it might be applied for similar raw materials as well.

Bilevel programming, a tool for modeling decentralized decision problem, consists of the objective of the leader at its first level and that of the follower at the second level. Bilevel programming has been proved to be Np hard problem. Numerous algorithms have been developed for solving bilevel programming problem. These algorithms have not enough efficiency for solving a real problem. In this paper an attempt has been made to develop an algorithm based on Tabu Search (TS) approach. This algorithm can be efficient to finding a near optimal solution. In this paper, the proposed method and that of Mathieu and et.al [1] are also compared by solving different problems.