Sharif Digital Repository

Heat exchanger network synthesis (HENS) is one of the most efficient process integration tools to save energy in chemical plants. In this work, a new optimization framework is proposed for the synthesis of HENS, based on a genetic algorithm (GA) coupled with a commercial process simulator through the ActiveX capability of the simulator. The use of GA provides a robust search in complex and nonconvex spaces of mathematical problems, while the use of a simulator facilitates the formulation of rigorous models for different alternatives. To include the most common heat exchanger structures in the model, a promising superstructure has been used. Allowing nonisothermal mixing of streams in the new... 

Stepped planing hulls enable the feasibility of running at relatively low Drag-Lift ratio by means of achieving more optimal trim angle at high speeds. Currently, there is no precise method to analyze these hulls over the full range of operating speeds. In this study, a three-dimensional computational fluid dynamics (CFD) model using volume of fluid (VOF) approach is presented for examining the characteristics of a planing hull having one transverse step. A procedure is presented to transform a series of fixed-position simulations into a free to heave and pitch simulation. Resistance, lift, running draft, dynamic trim angle, and wetted area are compared with available experimental data and... 

Local velocities of bubbles rising in four different nanosilica solutions were investigated experimentally. Also, the density, viscosity, and surface tension of fumed nanosilica and modified nanosilica solutions were measured. Heat treatment and chemical functionalization were used to modify the properties of silica nanoparticles. It was found that the addition of nanosilica affected the hydrodynamics of the rising bubble by increasing the drag friction at the interface. However, environmentally responsive nanosilica particles behaved like surfactant molecules, due to the interfacial activity of hydrophilic and hydrophobic chains. Silica nanoparticles coated with both hydrophilic and... 

Memory-intensive workloads operate on massive amounts of data that cannot be captured by last-level caches (LLCs) of modern processors. Consequently, processors encounter frequent off-chip misses, and hence, lose significant performance potential. One of the components of a modern processor that has a prominent influence on the off-chip miss traffic is LLC's replacement policy. Existing processors employ a variation of least recently used (LRU) policy to determine the victim for replacement. Unfortunately, there is a large gap between what LRU offers and that of Belady's MIN, which is the optimal replacement policy. Belady's MIN requires selecting a victim with the longest reuse distance,... 

The kinetics of coalescence is studied experimentally using a new technique for tracking the process in the bulk phase. For this aim, aqueous solutions of KSCN (colorless) and FeCl3 (light yellow) are used to make individual W/O emulsions. Any coalescence occurred between drops containing KSCN solution and those containing FeCl3 solution would combine these solutions. This leads to a reddish brown solution due to the formation of iron(III)thiocyanate. The intensity change of this red color with time represents the dynamics of coalescence occurring between drops of different emulsified aqueous droplets. The detector response to any changes in the system is recorded as a function of time. In... 

In this work, a microstrip transmission line with a planar left-handed structure, is investigated. Negative μ is achieved by creating periodic capacitive slots in the conducting strip of line. By using the transmission matrix method for periodic structures, these slots are analyzed. To this end, the unit cell is considered as a combination of two series capacitors and a section of transmission line, and then the transmission matrix of this unit cell is calculated. The dispersion relation verifies the existence of a stop band with a negative magnetic permeability (μ 0;< 0;0) below the cut-off frequency. The width of stop band can be controlled by changing the structure dimensions  

For many years the model of a dynamic adsorption layer (DAL) is well established as explanation for the behavior of rising bubbles in surfactant solutions. This model explains the velocity profile and the evolution of the shape of a rising bubble based on the hypothesis of the balance between the drag force and the structure of the adsorbed layer governed by Marangoni convection. However, direct measurements of interfacial properties of the bubble during rising are a real challenge. Here we present a new experimental protocol called "bubble in flow field" suitable for direct measurements of dynamic interfacial properties of a bubble surface using the capillary pressure tensiometry under... 

In this study, a hybrid of silica nanosphere/multiwall carbon nanotube (MWCNT) has been synthesized by wet chemical method at room temperature. The effect of MWCNTs, silica nanospheres and hybrid nanostructures (80% silica nanosphere/20% MWCNT and 50% silica nanosphere/50% MWCNT) on the thermal conductivity of distilled water has been investigated. SDBS was used as the dispersant to stabilize nanomaterials in the aqueous suspension and its concentration was 1.5 times of the concentration of nanomaterials. As results show, by increasing the concentration of nanomaterials, effective thermal conductivity of nanofluids increased. The most and the least enhancement in the effective thermal... 

The main objective of the present study was to develop a valid and appropriate model to evaluate companies' efficiency and environmental performance, concerning health, safety, and environmental management system principles. The proposed model overcomes the shortcomings of the previous models developed in this area. This model has been designed on the basis of a mathematical method known as Data Envelopment Analysis (DEA). In order to differentiate high-performing companies from weak ones, one of DEA nonradial models named as enhanced Russell graph efficiency measure has been applied. Since some of the environmental performance indicators cannot be controlled by companies' managers, it was... 

L1 instruction(L1-l) cache misses are a source of performance bottleneck. While many instruction prefetchers have been proposed, most of them leave a considerable potential uncovered. In 2011, Proactive Instruction Fetch (PIF) showed that a hardware prefetcher could effectively eliminate all instruction-cache misses. However, its enormous storage cost makes it impractical. Consequently, reducing the storage cost was the main research focus in instruction prefetching in the past decade. Several instruction prefetchers, including RDIP and Shotgun, were proposed to offer PIF-level performance with significantly lower storage overhead. However, our findings show that there is a considerable... 

In the present work, adsorption of hydrogen molecules over a metal organic framework (MOF-5) has been investigated by using first principles density functional theory (DFT). Different strategies have been applied for improving hydrogen storage, i.e. metal doping, boron substitution and functionalization. The metal atoms used for enhancing hydrogen adsorption include Li, Ca and Sc. It is found that the binding energy between these metal atoms and MOF is not enough to prevent clustering. Therefore a number of carbon atoms are substituted by boron atoms and it is indicated that boron substitution enhances the binding energies, significantly. Also the results reveal that boron substituted MOF... 

In the present work the interaction of hydrogen molecules with perfect, defective and scandium doped polycyclic aromatic hydrocarbon (PAH) has been evaluated. At first the potential barrier for the penetration of hydrogen molecules through PAH structures has been investigated and then the adsorption of hydrogen molecules over PAH structures has been studied. To model the graphene surface for barrier calculations, it is shown that coronene can successfully estimate the graphene monolayer. The barrier height is calculated for perfect and two different defective PAH structures including Stone-Wales (SW) and 585. It is found that PAH even with small defects is impermeable to hydrogen molecules.... 

In the present work, the interaction of hydrogen molecules with defective graphene structures doped by transition metal (TM) atoms is investigated by using first principles density functional theory (DFT). Defective graphene structures include Stone-Wales (SW), 585 and 555-777 and transition metals include early TMs, i.e. scandium (Sc), titanium (Ti) and vanadium (V). It is found that in comparison with the pristine graphene, presence of defects significantly enhances the metal binding. Among three defects, 585 divacancy leads to the strongest binding between graphene and metal. Hydrogen adsorption is then evaluated by sequential addition of hydrogen molecules to the system. The results... 

Hydrogen appears as one of the most promising energy carriers. Hydrogen has the advantage of being the cleanest fuel and as a result it can efficiently help limiting the greenhouse effect. However one of the major barriers for the large scale use of hydrogen, especially in the transportation sector, is hydrogen storage issue. There are four different technologies for storing hydrogen which are as follows: (1) Cryogenic liquid hydrogen (LH); (2) compressed gas storage; (3) metal hydride storage technology; (4) Physisorption methods. Physisorption of hydrogen in novel carbon nanostructures such as carbon nanotubes and graphene seems to be a safe and promising method for storage of hydrogen.... 

The use of more efficient energy consuming devices, which are closely associated with reduction of environmental pollution, has gained significant interest in the recent decades. The reduction of drag coefficient also improves safety and durability of environmental structures subjected to high-velocity fluid flow, and causes the noise and vibration to decrease as well. This paper describes the efficiency improvement in energy management by means of reducing drag coefficient in a practical divergent tubular- truncated cone. Extensive numerical simulations with emphasis on the shape optimization study were performed in order to find minimum drag coefficient for both laminar and turbulent flows... 

This study presents a method for global estimation of joint velocities in robotic manipulators. The authors consider a non-minimal model of a robotic manipulator and design an adaptive observer capable of handling uncertainties in robot dynamics. Smoothness of the dynamics of the proposed observer allows its easy implementation in comparison with non-smooth observers. Dimension of the proposed observer is shown to be at least 3n where n stands for the manipulator's degrees of freedom. This number is less than the dimension of most existing globally convergent adaptive observers. Global asymptotic convergence of state estimates to their true values is achieved under no persistency of... 

This paper gives the design and analysis approaches for the class-E power amplifier with a shunt inductor under the nominal conditions, i.e., zero-current switching (ZCS) and zero-current derivative switching (ZCDS), with taking into account the MOSFET nonlinear output parasitic capacitance at any duty ratio. Although, the class-E ZCS/ZCDS conditions obtained high-efficiency, but the switch-current waveform affected by the slope of the voltage across the MOSFET nonlinear drain-to-source parasitic capacitance during the switch-off state, which restricted the operating frequency. On the other hand, the duty ratio is an adjustment parameter to obtain high-frequency operation. Therefore, the...