Browsing by Department "Sharif University of Technology"

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    Effect of oxygen enrichment in spectral thermal radiation in an unconfined turbulent bluff-body flame
    (Elsevier Limited, 2020-05-01) Darbandi, Masood; Barezban, Mohammad Bagher; Fatin, Ali; Bordbar, Hadi; Sharif University of Technology; Department of Civil Engineering
    Oxy-fuel combustion is a modern carbon capture and storage (CCS) technique that improves the combustion process and reduces the environmental penalty of many combustion systems. Evidently, the accurate radiative calculation of oxy-fuel combustion is very important to obtain more improved combustion system designs with less environmental drawbacks. In the present study, a small scale unconfined turbulent bluff-body flame is numerically simulated to calculate the gas radiative properties using three different approaches of ignoring radiation, applying a modified version of the weighted sum of gray gases (WSGG) model, and employing the spectral line based weighted sum of gray gases (SLW) model. First, the selected bluff-body flame is validated against experimental data. The early outcome is that the simulation results of three chosen approaches are very close if there is no oxygen enrichment. Next, the effect of oxygen enrichment is carefully investigated imposing the aforementioned spectral radiation approaches. The achieved results indicate that the predicted gas temperature becomes more sensitive to the implemented radiative approach as the oxygen concentration in the oxidizer increases. In very high oxygen enrichment condition, the gas temperature predicted by SLW model shows up to 155 K differences with that of ignoring radiation approach. The simulation results also show that the oxygen enrichment would raise the CO 2 and H 2O volume fractions in the flame zone. Therefore, the non-grayness of gases becomes more significant in such conditions and the accurate radiative calculation becomes more essential. This is investigated carefully in this study.
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    Effect of sulfonating agent and ligand chemistry on structural and optical properties of CuSbS2 particles prepared by heat-up method
    (ROYAL SOC CHEMISTRY, 2018) Moosakhani, Shima; Sabbagh Alvani, Ali Asghar; Mohammadpour, Raheleh; Sainio, Jani; Ge, Yanling; Hannula, Simo Pekka; Department of Chemistry and Materials Science; Amirkabir University of Technology; Sharif University of Technology; Department of Applied Physics
    Chalcostibite copper antimony sulfide (CuSbS2) is a promising candidate for application in solar cells. The functionality of CuSbS2 particles depends on particle size and morphology and controlling these two parameters during synthesis is of utmost importance. In this study, CuSbS2 particles were prepared by a facile heat-up synthesis method utilizing sulfur powder (Su) and thiourea (Tu) to investigate the effect of the sulfur source on the structural and physical properties of CuSbS2 particles. Different morphologies were observed when Su and Tu were employed. The results demonstrated that the shape uniformity can be improved by applying a coordinating sulfur precursor (Tu). Moreover, nanoplatelet- and nanobrick-shaped particles were obtained by changing the ligand chemistry, i.e., by using a different combination of oleylamine (OLA), 1-octadecene (ODE), and oleic acid (OL). Band gap calculations showed that CuSbS2 had direct and indirect bandgaps with a small difference of 0.2 eV. Composition analysis of samples obtained from the Tu precursor revealed that antimony contents varied resulting in differences of the lattice parameter c. Moreover, valence band (VB) and conduction band (CB) positions determined by cyclic voltammetry (CV) suggested that this material based on its composition can have dual applications: first, as an absorber in nanocrystalline solar cells and second, as a hole transport material in perovskite solar cells.
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    A general framework for voltage sag performance analysis of distribution networks
    (Multidisciplinary Digital Publishing Institute (MDPI), 2019-07-22) Safdarian, Amir; Fotuhi-Firuzabad, Mahmud; Lehtonen, Matti; Department of Electrical Engineering and Automation; Sharif University of Technology
    The proliferation of more sensitive loads has obliged distribution companies to pay greater attention to the voltage sag mitigation potential of different design alternatives in network planning studies. In doing so, a company has to have effective tools for estimating the voltage sag performance of its network. In this regard, this paper establishes a three-step framework for evaluating voltage sag performance of a distribution network. The first step, designated as state selection, is to select a network state in which voltage sag is likely. Although voltage sags have various causes, those that originated from faults in distribution networks are considered in this paper. The stochastic nature of fault location, type, resistance, and duration as well as the response of the protection system are taken into account. The second step, called state evaluation, deals with sag characteristics during the fault clearing time and the protection system response. The third step, named index calculation, is to estimate indices reflecting the sag performance of the network. A number of indices are proposed in this paper to reflect both system and load point-oriented issues. In light of the indices, companies may find effective solutions for voltage sag mitigation and customers choose appropriate solutions to provide ride-through support for their critical processes.
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    Improved Resonant Converter for Dynamic Wireless Power Transfer Employing a Floating-Frequency Switching Algorithm and an Optimized Coil Shape
    (IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, 2022) Ghohfarokhi, Shahriar Sarmast; Tarzamni, Hadi; Tahami, Farzad; Kyyra, Jorma; Sharif University of Technology; Department of Electrical Engineering and Automation
    This paper offers a new EF-class converter for dynamic wireless power transfer application. The proposed high-frequency converter employs a floating-frequency switching algorithm to control the converter in a continuous frequency range, eliminate the requirement to any additional operational data from the secondary (receiver) side, accelerate the load impedance match while moving, maximize the transferred power rate, reduce charging interval and compensate power transfer tolerances. Moreover, an optimized super elliptical shape coil is designed to cope with lateral misalignment, enhance coil coupling, and increase efficiency. In the proposed converter, (i) soft switching is implemented to increase switching frequency, decrease passive components size, and improve power density, (ii) undesired voltage harmonics are attenuated to reduce peak voltage stress of the power switch in a wide frequency range, (iii) the receiver side is enabled for higher mobility with stable power transfer, and (iv) the resonant frequency is updated to compensate non-accurate values of passive components in experimental prototyping. In this study, the operational analytics, compensation method, control algorithm, coil design and converter optimization are followed with some comparisons to present the converter capabilities. In addition, simulation and experimental results are provided under different degrees of misalignment to verify the accuracy of theoretical analytics.
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    Numerical study on NOx reduction in a large-scale heavy fuel oil-fired boiler using suitable burner adjustments
    (PERGAMON-ELSEVIER SCIENCE LTD, 2020-05-15) Darbandi, Masood; Fatin, Ali; Bordbar, Hadi; Sharif University of Technology; Department of Civil Engineering
    A numerical framework was carefully developed to simulate the combustion of heavy-fuel-oil (HFO) in a large-scale boiler. The present numerical solutions were compared with the measured data of a laboratory benchmark test and on-site operational data of the chosen HFO-fired boiler. Next, the developed framework was used to perform the sensitivity analyses aiming to reduce the NO emission from the HFO-fired boiler without any adverse effect on its combustion performance. Practically, this study focused on re-adjustments of 24 working burners, which could control the combustion in the HFO-fired boiler. The early outcome showed that the boiler NO emission and its combustion performance could be controlled via the proper adjustments of air distributions within the three burners’ stages and the swirl intensity. Although bigger mean droplet sizes and higher injection velocities reduced the NO emission considerably, it adversely led to much lower boiler’s combustion efficiency. The present study eventually arrived at an optimal adjustment for the burners by reconsideration of the air distributions within the three burners’ stages, the flame swirl intensity magnitude, and the fuel injection quality. The achieved optimal adjustment reduced the amount of NO emission by 30%, while the combustion efficiency would remain unaffected.
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    On the MILP Modeling of Remote-Controlled Switch and Field Circuit Breaker Malfunctions in Distribution System Switch Placement
    (IEEE, 2023) Jooshaki, Mohammad; Karimi-Arpanahi, Sahand; Millar, Robert; Lehtonen, Matti; Fotuhi-Firuzabad, Mahmud; Geological Survey of Finland; University of Adelaide; Department of Electrical Engineering and Automation; Sharif University of Technology; Department of Electrical Engineering and Automation
    Installing sectionalizing switches and field circuit breakers (FCBs) is vital for the fast restoration of customer electricity supply in distribution systems. However, the high capital costs of these protection devices, especially remote-controlled switches (RCSs) and FCBs, necessitate finding a trade-off between their costs and financial benefits. In this study, we propose a mixed-integer linear programming (MILP) model for optimizing switch planning in distribution systems. The proposed model determines the optimal allocation of manual switches, RCSs, and FCBs to minimize the costs of switches and the reliability-oriented expenses. While the former includes the costs of installing and operating the switches, the latter consists of the distribution company’s lost revenue due to the undelivered energy and the regulatory incentives (or penalties) associated with service reliability indices. Two penalty-reward mechanisms are used to account for the financial benefits of increasing the service reliabilitythrough reducing the duration and frequency of interruptions. Proposing a novel reliability assessment model, we consider the possibility of malfunctions in both RCSs and FCBs in a highly efficient manner, which is a major contribution of this work. The proposed MILP model is applied to three test networks to validate its applicability and efficacy. The results show the importance of considering the possibility of switch malfunctions in distribution networks.
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    Optimal power flow problem considering multiple-fuel options and disjoint operating zones
    (Elsevier Limited, 2019-12) Pourakbari-Kasmaei, Mahdi; Lehtonen, Matti; Fotuhi-Firuzabad, Mahmud; Marzband, Mousa; Mantovani, José Roberto Sanches; Power Systems and High Voltage Engineering; Department of Electrical Engineering and Automation; Sharif University of Technology; Northumbria University; São Paulo State University
    This paper proposes a solver-friendly model for disjoint, non-smooth, and nonconvex optimal power flow (OPF) problems. The conventional OPF problem is considered as a nonconvex and highly nonlinear problem for which finding a high-quality solution is a big challenge. However, considering practical logic-based constraints, namely multiple-fuel options (MFOs) and prohibited operating zones (POZs), jointly with the non-smooth terms such as valve point effect (VPE) results in even more difficulties in finding a near-optimal solution. In complex problems, the nonlinearity itself is not a big issue in finding the optimal solution, but the nonconvexity does matter and considering MFO, POZ, and VPE increase the degree of nonconvexity exponentially. Another primary concern in practice is related to the limitations of the existing commercial solvers in handling the original logic-based models. These solvers either fail or show intractability in solving the equivalent mixed integer nonlinear programming (MINLP) models. This paper aims at addressing the existing gaps in the literature, mainly handling the MFOs and POZs simultaneously in OPF problems by proposing a solver-friendly MINLP (SF-MINLP) model. In this regard, due to the actions that are done in the pre-solve step of the existing commercial MINLP solvers, the most adaptable model is obtained by melting the primary integer decision variables, associated with the feasible region, into the objective function. For the verification and didactical purposes, the proposed SF-MINLP model is applied to the IEEE 30-bus system under two different loading conditions, namely normal and increased, and details are provided. The model is also tested on the IEEE 118-bus system to reveal its effectiveness and applicability in larger-scale systems. Results show the effectiveness and tractability of the model in finding a high-quality solution with high computational efficiency.
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    Platelet CuSbS2 particles with a suitable conduction band position for solar cell applications
    (ELSEVIER SCIENCE BV, 2018-03-15) Moosakhani, Shima; Alvani, Ali Asghar Sabbagh; Mohammadpour, Raheleh; Hannula, Pyry-Mikko; Ge, Yanling; Hannula, Simo-Pekka; Amirkabir University of Technology; Sharif University of Technology; Department of Chemical and Metallurgical Engineering; Department of Chemistry and Materials Science
    Single crystalline platelet chalcostibite (CuSbS2) particles with good shape and size uniformity were successfully prepared using a hot injection method. In this synthesis, sulfur powder in oleylamine (OLA) was employed as a sulfonating agent. The synthesized CuSbS2 had an orthorhombic structure with a plate-like morphology. Selected area electron diffraction (SAED) patterns confirmed their single crystal nature. Band gap calculation from diffuse reflectance data revealed that it had both direct and indirect band gaps of 1.52 eV and 1.46 eV, respectively. Moreover, valence band (VB) and conduction band (CB) positions were determined by cyclic voltammetry (CV) characterization. Optical and structural properties of CuSbS2 indicate its potential applicability for solar cell applications. (C) 2017 Elsevier B.V. All rights reserved.
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    Portfolio Value-at-Risk and expected-shortfall using an efficient simulation approach based on Gaussian Mixture Model
    (Elsevier, 2021-12) Seyfi, Seyed Mohammad Sina; Sharifi, Azin; Arian, Hamidreza; School Services, BIZ; Sharif University of Technology; Department of Finance
    Monte Carlo Approaches for calculating Value-at-Risk (VaR) are powerful tools widely used by financial risk managers across the globe. However, they are time consuming and sometimes inaccurate. In this paper, a fast and accurate Monte Carlo algorithm for calculating VaR and ES based on Gaussian Mixture Models is introduced. Gaussian Mixture Models are able to cluster input data with respect to market’s conditions and therefore no correlation matrices are needed for risk computation. Sampling from each cluster with respect to their weights and then calculating the volatility-adjusted stock returns leads to possible scenarios for prices of assets. Our results on a sample of US stocks show that the Gmm-based VaR model is computationally efficient and accurate. From a managerial perspective, our model can efficiently mimic the turbulent behavior of the market. As a result, our VaR measures before, during and after crisis periods realistically reflect the highly non-normal behavior and non-linear correlation structure of the market.