Sharif Digital Repository

The aim of this experimental work is to study the effect of grooved surfaces in the entrance region of annular flows on local heat transfer. The outer stationary cylinder is grooved and the inner rotating cylinder is smooth. This configuration is applicable in industrial applications such as rotating heat pipes for cooling of superconducting machines or motors rotor, electrical generators where heat generates in the grooves containing wires, transient heating of axial compressor rotor drams, combustion chamber in turbojets, air-cooled axial-flux permanent-magnet machines. The experimental tests were performed based on aspect ratio of the groove, effective Reynolds number and Taylor number.... 

A numerical analysis has been performed to simulate flow structure, heat transfer, and pressure drop of turbulent flow in an annulus with a few longitudinal cavities on the outer stationary cylinder. The cross sections of cavities are rectangular, closed and open trapezoidal shapes. This kind of annular flow is applicable to industries applications such as electrical generators where heat generates in the cavities containing wires, heating of axial compressor rotor drams, rotating heat pipes for cooling of superconducting machines or motor rotor. The governing equations of turbulent flow are solved by using Renormalization group (RNG) k–ε model for Reynolds and Taylor numbers in the range of... 

The electrodeposition of cobalt, nickel and Ni-Co alloys was studied by electrochemical techniques. Cyclic voltammetry and current transient measurements were used to characterize the Co-Ni system in other to obtain the nucleation and growth mechanism. The cyclic voltammetry results clearly showed that electrodeposition of cobalt, nickel and Co-Ni alloy is diffusion-controlled process with a typical nucleation mechanism. The redox potentials of the Co and Ni are shifted to more cathodic potentials in the Co-Ni alloy system. In addition, the current transients revealed that nucleation mechanism is instantaneous with a typical three-dimensional (3D) nucleation and growth process. The number of... 

Herein, we report a modified two-step method to construct a uniform and pinhole-free polycrystalline perovskite film with large grains up to the microscale using lead mixed-halide (PbI2-PbCl2) precursor solutions to guarantee the device functioning. Commonly used sequential deposition methods based on dip- and spin-coatings are combined with a vapor-assisted solution process to improve the perovskite morphology and tune the residual lead mixed-halides. These can enhance the optical absorption and efficiency of perovskite solar cell (PSC) devices. The impact of the Cl content in the lead mixed-halide precursors and the employed preparation methods on the photovoltaic performance of PSCs are... 

Herein, we demonstrate a facile low temperature approach to prepare a gel polymer electrolyte (GPE) based on the triiodide/iodide redox couple in a quasi-solid-state dye-sensitized solar cell (DSSC). Only one polymeric agent (i.e., hydroxypropyl cellulose (HPC)) is used for the preparation of a non-volatile GPE system for the standard liquid electrolyte containing 1-methyl-3-propylimidazolium iodide (MPII). The prepared GPE completely penetrates into a TiO2 photoanode at room temperature due to the uniform distribution of Ti, Ru and I elements in the cross-section of the electrode, forming an interpenetrating GPE/TiO2 network to access all dye molecules. The GPE with the optimum HPC : MPII... 

Using the Shan-Chen lattice Boltzmann multi-phase model, we investigate the effect of adding microstructured grooves to the walls of a 2D parallel-plate channel on the pressure drop in the channel. The effects of the size of the grooves on the pressure drop in the channel were considered. It was observed that the pitch of the grooves has a considerable effect on the pressure drop in the channel, and even for some values of the pitch we observe an increase in the pressure drop. As the pitch decreases, a lower pressure drop is achieved. The results also show that as the ratio of the solid-liquid contact surface to the whole surface is decreased, the pressure drop decreases. It is also observed... 

The preparation and characterization of a new type of nanocomposite polyelectrolyte membrane, based on DuPont Nafion/imidazole-modified nanosilica (Im-Si), for direct methanol fuel cell applications is described. Related to the interactions between the protonated imidazole groups, grafted on the surface of nanosilica, and negatively charged sulfonic acid groups of Nafion, new electrostatic interactions can be formed in the interface of Nafion and Im-Si which result in both lower methanol permeability and also higher proton conductivity. Physical characteristics of these manufactured nanocomposite membranes were investigated by scanning electron microscopy, thermogravimetry analysis,... 

An efficient electrocatalyst for Pt-free I−/I3 − reduction has been synthesized by high temperature treatment of graphene oxide-cobalt phthalocyanine mixtures. The graphitic material was characterized by various techniques and was found to consist of reduced graphene oxide carrying CoN3 entities, obtained by thermal decomposition of cobalt phthalocyanine. This material had an ionization potential equal to 4.9±0.1 eV and thus it was judged appropriate to act as reduction electrocatalyst for the I−/I3 − redox electrolyte. It was subsequently deposited as thin film on FTO electrodes, which were employed as counter electrodes for dye-sensitized solar cells. Its capacity to reduce I−/I3 − redox... 

The Cu surface segregation is investigated in the electrodeposited Ni-Fe layers using X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), secondary ion mass spectroscopy (SIMS) and atomic force microscopy (AFM). The results indicate that Cu segregation and accumulation take place in areated and deareated baths and the amount of segregated copper increases after air exposure. This phenomenon is explained by lower interfacial tension of the Cu in comparison with Ni and Fe. Our results reveal more surface segregation in the electrodeposit than vacuum reported results. This should be due to interface charging and higher surface diffusion in applied potential. The effect of... 

This paper investigates numerical simulation of one-dimensional homogeneous adiabatic gas-liquid two-phase flow in a rectangular microchannel with one boundary porous wall under the assumption of hydrophobic condition. Gas enters the microchannel with a uniform velocity and liquid is injected through the porous side wall. The present approach is to simulate water injection effects and developing mechanism of two-phase flow. The modeling and solution of the conservation equations provide pressure drop, vapor quality, void fraction and tow-phase mixture velocity for different water injection rates. The results show that velocity and pressure drop significantly perturbed when the water... 

This paper developed a new mathematical model to investigate the heat transfer as well as wick's thickness of a heat pipe. The model was established by conservative equations of continuity, momentum, and energy in the thermal boundary layer. Using a similarity variable, the governing equations were changed to a set of ordinary differential equations and were solved numerically by the forth-order Runge-Kutta method. The flow variables, such as velocity components, wick's thickness, and Nusselt number, were obtained. The results show that the Nusselt number is proportional to the square root of the Darcy-modified Rayleigh number and to the distance from the edge of the condenser surface.... 

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 this paper, an analytical model is presented for forced convective film condensation inside smooth vertical tubes, in which Prandtl mixing length theory, Van Driest hypothesis and Reynolds analogy are used. Comparison of this model with the available experimental data and three other correlations reported in the literature demonstrates that the results are in satisfactory and close agreement  

This paper introduces a stochastic scheduling problem. In this problem a directed acyclic graphs (DAG) represents the precedence relations among m tasks that n workers are scheduled to execute. The question is to find a schedule Σ such that if tasks are assigned to workers according to Σ, the expected time needed to execute all the tasks is minimized. The time needed to execute task t by worker w is a random variable expressed by a negative exponential distribution with parameter λ wt and each task can be executed by more than one worker at a time. In this paper, we will prove that the problem in its general form is NP-hard, but when the DAG width is constant, we will show that the optimum... 

Computing the visibility polygon, VP, of a point in a polygonal scene, is a classical problem that has been studied extensively. In this paper, we consider the problem of computing VP for any query point efficiently, with some additional preprocessing phase. The scene consists of a set of obstacles, of total complexity O(n). We show for a query point q, VP(q) can be computed in logarithmic time using O(n 4) space and O(n 4 logn) preprocessing time. Furthermore to decrease space usage and preprocessing time, we make a tradeoff between space usage and query time; so by spending O(m) space, we can achieve O(n 2 log √ m/n)/√ m)query time, where n 2≤m≤n 4. These results are also applied to... 

In this research, the modal parameters of a beam in free-free condition are extracted by performing different experiments in the laboratory. For this purpose, two different techniques are employed. The first methodology is considered as a time domain method in Operational Modal Analysis. The other one is frequency domain impact hammer test which is categorized as an Experimental Modal Analysis method and can be regarded as the most common method in modal analysis. Checking the results obtained by the two methods, one can notice a distinct inconsistency in modal damping ratios extracted by each method. However, based on recent publications on the subject, it can be inferred that the time... 

In this research, formation of aluminide/silicide diffusion coatings on γ -TiAl[Ti-48Al-2Nb-2Cr (at.%)] alloy using gasphase diffusion pack cementation process has been investigated. The application of powder mixtures with various chemical compositions in the pack cementation process performed at 1000°C for 6 hours in order to achieve simultaneous diffusion of Al and Si, showed that the composition of the powder mixture could have a significant effect on the structure and thickness of the aluminide/silicide coatings. The identification and analysis of aluminide/silicide microstructures formed as a result of simultaneous diffusion of Al and Si, which was comprehensively and qualitatively done... 

The fractal study of thin layer films has been concerned by numerous studies, but it is a novel idea to use this method for interpretation of layer formation during electrocrystallization, simultaneously. In present study, Scharifker's equations were derived for instantaneous and progressive nucleation and 3D growth of hemispherical centers under diffusion-controlled condition to calculate in situ change of fractal dimension of surface. It was assumed that the layer could be formed completely when fractal dimension of surface inclined to 2. Moreover, the fractal analysis of AFM images has confirmed the presumed model. © 2006 Elsevier Ltd. All rights reserved