Sharif Digital Repository

Turbulent fully developed periodic condensation heat transfer and pressure drop in herringbone-type plate heat exchangers for R134a and steam were numerically investigated. A mixture model was introduced and the equations governing the two phase flow were simplified. Thus a single phase periodic flow with mixture properties was solved for R134a. For Steam a separate empirical model was used. The governing equations were solved numerically by a finite-volume method for elliptic flows in complex geometries using collocated variable arrangement. The influence of mass flux and vapor quality on frictional pressure drop for refrigerant was investigated. The heat transfer was studied using a... 

For a given sequence of nonincreasing numbers, d = (d1, . . ., dn a necessary and sufficient condition is presented to characterize d when its realization is a unique labelled simple graph. If G is a graph, we consider the subgraph G′ of G with maximum edges which is uniquely determined with respect to its degree sequence. We call the set of E(G)/E(G′) the smallest edge defining set of G. This definition coincides with the similar one in design theory  

Let R be a commutative ring with unity. The co-maximal ideal graph of R, denoted by Γ(.R), is a graph whose vertices are the proper ideals of R which are not contained in the Jacobson radical of R, and two vertices I1 and I2 are adjacent if and only if I1 +I2=R- We classify all commutative rings whose co-maximal ideal graphs are planar. In 2012 the following question was posed: If T(R) is an infinite star graph, can R be isomorphic to the direct product of a field and a local ring? In this paper, we give an affirmative answer to this question  

Let H be a group. The co-maximal graph of subgroups of H, denoted by Γ(H), is a graph whose vertices are non-trivial and proper subgroups of H and two distinct vertices L and K are adjacent in Γ(H) if and only if H = LK. In this paper, we study the connectivity, diameter, clique number, and vertex chromatic number of Γ(H). For instance, we show that if Γ(H) has no isolated vertex, then Γ(H) is connectedwith diameter at most h. Also, we characterize all finite groupswhose co-maximal graphs are connected. Among other results, we show that if H is a finitely generated solvable group and Γ(H) is connected, and moreover, the degree of a maximal subgroup is finite, then H is finite. Furthermore,... 

Let D be a t-(v, k, λ) design and let N i(D), for 1≤i≤t, be the higher incidence matrix of D, a (0, 1)-matrix of size(v/i×b), where b is the number of blocks of D. A zero-sum flow of D is a nowhere-zero real vector in the null space of N1(D). A zero-sum k-flow of D is a zero-sum flow with values in {1,..., ±(k-1)}. In this article, we show that every non-symmetric design admits an integral zero-sum flow, and consequently we conjecture that every non-symmetric design admits a zero-sum 5-flow. Similarly, the definition of zero-sum flow can be extended to Ni(D), 1≤i≤t. Let D=t - (v.k,(v-t/k-t))be the complete design. We conjecture that Nt(D) admits a zero-sum 3-flow and prove this conjecture... 

Predominance of nano-scale effects observed in material behavior at small scales requires implementation of new simulation methods which are not merely based on classical continuum mechanic. On the other hand, although the atomistic modeling methods are capable of modeling nano-scale effects, due to the computational cost, they are not suitable for dynamic analysis of nano-structures. In this research, we aim to develop a continuum-based model for nano-beam vibrations which is capable of predicting the results of molecular dynamics (MD) simulations with considerably lower computational effort. In this classical-based modeling, the surface and core regions are taken to have different... 

Two novel heterogeneous hybrid catalysts were successfully synthesized using octa-aminopropyl polyhedral oligomeric silsesquioxane hydrochloride salt (OA-POSS) and ionic liquids, followed by immobilization of Cu and Ag nanoparticles. The obtained hybrid catalysts, Cu@POSS-ILs and Ag@POSS-ILs, were characterized using FT-IR, SEM, EDX, TEM, XRD and TGA. Catalytic studies demonstrated that Cu@POSS-ILs was an excellent catalyst for click reaction in aqueous medium. Furthermore, Ag@POSS-ILs was also exhibited to be an active catalyst for 4-nitrophenol (4-NP) reduction. Simple preparation and easy catalysts recovery and recyclability of Cu@POSS-ILs and Ag@POSS-ILs for at least 3 and 6 runs,... 

Laser ablation is a novel non-mechanical wheel preparation method for optimizing the treatment costs of superabrasive tools. In this study the thermal effects of picosecond laser radiation on CBN superabrasive grinding wheel surface is analytically and experimentally investigated. The analytical approach is intended to find threshold process parameters for selective ablation of cutting grains and bond material. It has been analytically and experimentally shown that, the extent of material degradation is defined by the maximum surface temperature induced by the laser radiation which is in turn defined by the laser beam energy. It is also suggested that, the depth of laser thermal effects is... 

Let G be a graph with n vertices and m edges and assume that f : V ( G ) → N is a function with ∑v ∈ V ( G ) f ( v ) = m + n. We show that, if we can assign to any vertex v of G a list Lv of size f ( v ) such that G has a unique vertex coloring with these lists, then G is f-choosable. This implies that, if ∑v ∈ V ( G ) f ( v ) > m + n, then there is no list assignment L such that | Lv | = f ( v ) for any v ∈ V ( G ) and G is uniquely L-colorable. Finally, we prove that if G is a connected non-regular multigraph with a list assignment L of edges such that for each edge e = u v, | Le | = max { d ( u ), d ( v ) }, then G is not uniquely L-colorable and we conjecture that this result holds for... 

In this paper optical filters based on photonic resonant tunneling effect are analyzed by using the polynomial expansion method. Amplitude and phase response together with their dependency on the physical parameters of the filters are also investigated. These steep-edge filters show low insertion loss amplitude response, and linear phase variation in their passband, a suitable feature for WDM and DWDM applications where constant time delay and dispersion free devices are needed. Two kinds of filters, namely discrete level and continuous profile filters are introduced. These structures can be analyzed and designed by using Transfer Matrix Method. However, this approach suffers from inaccuracy... 

We construct counterexamples to the conjecture of Xu (1990, J. Combin. Theory Ser. B50, 319-320) that every uniquely r-colorable graph of order n with exactly (r-1)n-(r2) edges must contain a Kr. © 2001 Academic Press  

Synthesis of 14-aryl or alkyl-14H-dibenzo[a,j]xanthenes using nano-TiO 2 as eco-friendly and efficient catalyst is reported. Short reaction times, high yields, a clean process, simple methodology, easy work-up and green conditions are advantages of this protocol  

Let G be a graph. A zero-sum flow of G is an assignment of non-zero real numbers to the edges of G such that the sum of the values of all edges incident with each vertex is zero. Let k be a natural number. A zero-sum k-flow is a flow with values from the set {±1,...,±(k - 1)}. It has been conjectured that every r-regular graph, r ≥ 3, admits a zero-sum 5-flow. In this paper we provide an affirmative answer to this conjecture, except for r = 5  

For a graph G, a zero-sum flow is an assignment of non-zero real numbers on the edges of G such that the total sum of all edges incident with any vertex of G is zero. A zero-sumk -flow for a graph G is a zero-sum flow with labels from the set {± 1, ..., ± (k - 1)}. In this paper for a graph G, a necessary and sufficient condition for the existence of zero-sum flow is given. We conjecture that if G is a graph with a zero-sum flow, then G has a zero-sum 6-flow. It is shown that the conjecture is true for 2-edge connected bipartite graphs, and every r-regular graph with r even, r > 2, or r = 3. © 2009 Elsevier Inc. All rights reserved  

In this paper we study the structure of some special bases for the null space of the incidence matrix of a graph. Recently it was shown that if G is a graph with no cut vertex, then G has a {-1, 0, 1}-basis. We generalize this result showing that the statement remains valid for every graph with no cut edge. For the null space of any bipartite graph, we construct a {-1, 0, 1}-basis. For any bipartite graph we obtain the support sizes of all elements in the null space of its incidence matrix. Among other things, we prove that for a graph G, there exists a {-1, 1}-vector for the null space of G if and only if the degree of any vertex of G is even and G has an even number of edges. © 2005... 

In this article we show that for any forest there exists a labelling of the vertices for which the row-reduced echelon form of its adjacency matrix is a {-1, 0, 1}-matrix. This result clearly provides an affirmative answer to the conjecture: The null space of the adjacency matrix of every forest has a {-1, 0, 1}-basis. © 2005 Elsevier Inc. All rights reserved  

The current research attempts to illustrate results of a reliable 3-D numerical simulation to scrutinize the charging process of paraffin as the phase change material (PCM) in an innovative heat exchanger called double spirally coiled tube heat exchanger (DSCTHE) with two separated paths. The water as a heat-transfer fluid (HTF) streams through the double spiral tubes embedded throughout the latent thermal energy storage (LTES) system. The PCM melting characteristics under three different geometries are investigated. After validating the proposed numerical simulation model, the impacts of operational and geometrical factors including inlet temperature of HTF and radius of the inner spiral... 

In this paper, a robust control system is developed for a flexible beam with piezoelectric actuators (called Smart Beam). The active control system of smart beam satisfied desired properties for all admissible measurement and plant noises, disturbances and model uncertainties. Thus it is the best candidate for sensitive industrial, medical and etc. robot arms and bases a new concept in the use of smart structures in robotics. The Frequency Response Function (FRF) of the smart beam was obtained from a Finite Element (FE) model. The corresponding transfer function was derived from the □ synthesis and several control schemes were then designed to suppress the vibration. Results showed the... 

Hexagonal core–shell SiO2[–MOYI]Cl–]Ag nanoframeworks were synthesized via surface modification of hexagonal silica nanoparticles prepared from perlite (EP) as a cheap and abundant raw material. The prepared samples were well characterized by X-ray diffraction powder (XRD), energy dispersive X-ray (EDX), diffuse reflectance spectroscopy (DRS), Brunauer–Emmett–Teller (BET) specific surface area analysis, fourier transform infrared spectroscopy (FTIR), energy-dispersive X-ray spectroscopy (EDX), and transmission electron microscopy (TEM). The XRD patterns confirmed that Ag and AgCl crystalline phases were successfully loaded on the surface. The TEM images were also implied that the... 

Numerous applications in engineering and biotechnology have attracted the attention of many researchers to the analysis of underlying physical phenomena during the droplet pinch-off. In this study, the neck evolution during the formation of a ferrofluid droplet from a capillary is investigated under two types of magnetic field for a drop-on-demand system. The two types are steady and Pulse-Width Modulated (PWM) magnetic fields. First, under steady magnetic field, the necking process is studied for different values of magnetic Bond number and various angles between magnetic coil centerline and gravity. Subsequently, self-similar behavior in the vicinity of the detachment moment is observed....