Sharif Digital Repository

In this study, natural convection heat transfer in a vertical flat-plate solar air heater of 2.5 m height and 1 m width, with one- and two-glass covers was studied experimentally. Totally six cases of airflow (two for air heater with one glass cover and four for air heater with two-glass covers) were considered. These cases included states that air could flow within spaces between absorber plate and glass covers or air was enclosed in such spaces. Absorber plate temperature, back-plate temperature, glass cover temperatures, mass flow rates of air within channels and the solar radiation were measured. The following relations are suggested:. For channels in which air could flow:Nu = 0.7362... 

This paper focuses on the use of quadratic time-frequency distributions (QTFDs) to characterize the temporal variation of low-frequency electromechanical oscillations (LFEOs) using ringdown data. A refined Margenau-Hill distribution (MHD) and a procedure to extract the instantaneous frequencies and damping factors as well as initial phase angles of system low-frequency modes when the analyzed signal is multi-component are proposed. The refinement process enjoys a fast-converging iterative algorithm and without loss of generality it can be applied to other QTFDs for using in other applications. A non-stationary synthetic signal as well as simulated ringdown data of a 5-area, 16-machine power... 

A simple method for the formation of inclusion complex of [60]fullerene with two O2N2-donor aza-crown macrocyclic ligands was introduced. The products were characterized using UV-vis and IR spectroscopies as well as HPLC, and ESI mass spectrometry. The ESI mass and elemental analysis data revealed that the ratio of macroring:[60]fullerene were not the same. The binding capability of the macrorings as well as the number of the macroring addends to [60]fullerene were investigated using thermogravimetric analysis (TG) and differential scanning calorimetry (DSC). Considering percentage mass loss in different steps of TG and the enthalpy changes in DSC found for each of these products, the number... 

The stress field of a multi-phase spherical/cylindrical inhomogeneity with arbitrary interface bonding conditions, subjected to both a uniform temperature change and a uniform hydrostatic tension, is presented. Imperfect bonding conditions are modeled using linear spring model and coherent interface model. In nanosize inhomogeneities, since the surface energy is not negligible with respect to the bulk energy, the effect of surface stresses is incorporated into the formulation. Accurate estimates for the thermal stresses of a functionally graded coated inhomogeneity with perfect and /~or imperfect interfaces are given. The influence of coating stiffness, coating thickness and interface... 

The determination of the thermo-mechanical stress field in and around a spherical/ cylindrical inhomogeneity surrounded by a functionally graded (FG) coating, which in turn is embedded in an infinite medium, is of interest. The present work, in the frame work of Boussinesq/Papkovich-Neuber displacement potentials method, discovers the potential functions by which not only the relevant boundary value problems (BVPs) in the literature, but also the more complex problem of the coated inhomogeneities with FG coating and sliding interfaces can be treated in a unified manner. The thermo-elastic fields pertinent to the inhomogeneities with multiple homogeneous coatings and various combinations of... 

Substations and switching stations are critical segments in an electric power system. Their impacts on system operation and the consequential effects of their components failure are usually more dominant in overall power system reliability. There is always a degree of uncertainty in the practical input data used in reliability analysis. This paper presents a conceptual possibilistic approach using fuzzy set theory to manage the uncertainties in the reliability input data such as failure rate, repair time and operation of protective devices. Suitable fuzzy numbers are proposed for components reliability data. Load of feeders are also represented by fuzzy numbers and their impact on ENS... 

This work presents the optimization of a solar collector surface in solar desiccant wheel cycle which for cooling process with typical configuration naming desiccant wheel, heat exchanger and water spray evaporative cooler. In this cooling cycle the thermal solar energy has used to heat the regeneration air of desiccant wheel cycle. The optimum solar collector surface has determined by taking into account of design parameters such as velocity of air, wheel speed, thickness of the desiccant and hydraulic diameter of the desiccant wheel and also operating conditions such as outside temperature, outside relative humidity, regeneration air temperature and total solar irradiance. For this... 

In this paper, we propose a data offloading algorithm in a network comprised of a macro base station and a few number of WiFi access points with overlapped coverage areas. The main goal of our algorithm is how to assign offloaded mobile stations (MSs) to access points (APs). Our approach in proposing the algorithm is based on mapping the problem onto finding a suitable matching algorithm, in order to minimize the average delay of the offloaded packets as one of the main QoS parameters on the one hand and to maximize the utility of the APs on the other hand. The utility of each AP is considered to be a combination of monetary benefit due to offloading service and the cost due to extra energy... 

Let γs′ (G) be the signed edge domination number of G. In 2006, Xu conjectured that: for any 2-connected graph G of order n (n ≥ 2), γs′ (G) ≥ 1. In this article we show that this conjecture is not true. More precisely, we show that for any positive integer m, there exists an m-connected graph G such that γs′ (G) ≤ - frac(m, 6) | V (G) |. Also for every two natural numbers m and n, we determine γs′ (Km, n), where Km, n is the complete bipartite graph with part sizes m and n. © 2008 Elsevier B.V. All rights reserved  

A new criterion for the appearance of hot tears in metallic alloys is proposed. This model introduces a critical solid fraction (fcr) to simulate feeding. A hot-cracking sensitivity (HCS) index according to the temperature and solidification time is defined and then is compared with other hot tearing criteria. This new criterion indicates the higher cracking sensitivity for higher casting velocity and higher sensitivity in billet center in the casting and finally the calculated values are in fairly good agreement with the experimental results. © 2007 Elsevier B.V. All rights reserved  

This paper proves that if G is a graph (parallel edges allowed) of maximum degree 3, then χc′(G) ≤ 11/3 provided that G does not contain H1 or H2 as a subgraph, where H1 and H2 are obtained by subdividing one edge of K23 (the graph with three parallel edges between two vertices) and K4, respectively. As χc′(H1) = χ c′(H2) = 4, our result implies that there is no graph G with 11/3 < χc′(G) < 4. It also implies that if G is a 2-edge connected cubic graph, then χc′(G) ≤ 11/3. © 2005 Wiley Periodicals, inc  

We study the structure of a minimum dominating set of C2n+1 n, the Cartesian product of n copies of the cycle of size 2n + 1, where 2n + 1 is a prime  

Suppose we have a family F of sets. For every S∈F, a set D⊆S is a defining set for (F,S) if S is the only element of F that contains D as a subset. This concept has been studied in numerous cases, such as vertex colorings, perfect matchings, dominating sets, block designs, geodetics, orientations, and Latin squares. In this paper, first, we propose the concept of a defining set of a logical formula, and we prove that the computational complexity of such a problem is Σ2-complete. We also show that the computational complexity of the following problem about the defining set of vertex colorings of graphs is Σ2-complete: INSTANCE: A graph G with a vertex coloring c and an integer k.QUESTION: If... 

Circular chromatic number, χc is a natural generalization of chromatic number. It is known that it is NP-hard to determine whether or not an arbitrary graph G satisfies χ(G)= χc(G). In this paper we prove that this problem is NP-hard even if the chromatic number of the graph is known. This answers a question of Xuding Zhu. Also we prove that for all positive integers k ≥ 2 and n ≥ 3, for a given graph G with χ(G)= n, it is NP-complete to verify if χc(G)≤ n -1/k. © 2004 Wiley Periodicals, Inc  

Molybdenum disilicide (ModSi 2) is an attractive structural material due to its unique combination of properties, namely excellent oxidation resistance, high melting point (2030°C) and relatively low density (6.3g.cm -3). The present work investigates joining of two MoSi 2 parts through Cusil/Zr/Cusil interlayer with Cusil being a commercial eutectic of Cu-Ag alloy. The joining operation was implemented in an inert gas tube furnace by brazing. The brazing temperature ranged at 830-930°C while the operation lasted for 1hr. Interfacial microstructure was studied by Scanning Electron Microscope (SEM), Energy Dispersive Spectroscopy (EDS) and X-Ray Diffraction (XRD) techniques. Applying the... 

The present work investigates joining of two MoSi2 parts through Cusil/Zr/Cusil interlayer with Cusil being a commercial eutectic of Cu-Ag alloy. The joining operation was implemented in an inert gas tube furnace by brazing. The brazing temperature ranged from 800 to 930°C while the operation lasted for 60min. Evaluation of joints strength through shear loading identified the maximum strength 60.31MPa for the brazed sample at 830°C. Interfacial microstructure was studied by Scanning Electron Microscopy (SEM), Energy Dispersive Spectroscopy (EDS) and X-ray Diffraction (XRD) techniques. Applying the temperature of 830°C was led to a uniform dense joint consisting of various phases with... 

In this research work, graphene/cobalt nanocomposites are functionalized with polyethylene glycol (PEG) to be a platform for theranostics application and antibacterial activity. The non-covalent functionalization of PEG on the surfaces of nanocomposites improved their stability and diminished their cytotoxicity. The PEGylated nanocomposites are demonstrated to allow simultaneous administration of two cancer therapy methods such as magnetic fluids hyperthermia (MFH) which is carried out by converting magnetic energy into heat through ferromagnetic cobalt nanoparticles and heat generation through near-infrared optical absorption by the reduced graphene oxide. A concise simulation is carried... 

We study the 2-center problem with outliers in high-dimensional data streams. Given a stream of points in arbitrary d dimensions, the goal is to find two congruent balls of minimum radius covering all but at most z points. We present a (1.8+ε)-approximation streaming algorithm, improving over the previous (4+ε)-approximation algorithm available for the problem. The space complexity and update time of our algorithm are poly(d,z,1/ε), independent of the size of the stream. © 2016 Elsevier B.V