• 한국연소학회:학술대회논문집
• /
• 1999.10a
• /
• pp.155-163
• /
• 1999

A numerical study for simulating a swirling pulverized coal combustion in axisymmetric geometry is done here by applying the weighted sum of gray gases model (WSGGM) approach with the discrete ordinate method (DOM) to model the radiative heat transfer equation. In the radiative transfer equation, the same polynomial equation and coefficients for weighting factors as those for gas are adopted for the coal/char particles as a function of partial pressure and particle temperature. The Eulerian balance equations for mass, momentum, energy, and species mass fractions are adopted with the standard ${\kappa}-{\varepsilon}$ turbulence model, whereas the Lagrangian approach is used for the particulate phase for soot. The eddydissipation model is employed for the reaction rate for gaseous mixture, and the single-step first-order reaction model for the devolatilization process for coal. By comparing the numerical results with experimental ones, the models used here are confirmed and found to be one of good alternatives for simulating the combustion as well as radiative characteristics.

• Journal of Power Electronics
• /
• v.13 no.5
• /
• pp.829-840
• /
• 2013

The modified sinusoidal pulse-width modulation (SPWM) is one of the PWM techniques used in three-phase AC-DC buck converters. The modified SPWM works without the current sensor (the converter is current sensorless), improves production of sinusoidal AC current, enables obtainment of near-unity power factor, and controls output voltage through modulation gain (ranging from 0 to 1). The main problem of the modified SPWM is the huge starting current and voltage (during transient) that results from a large step change from the reference voltage. When the load changes, the output voltage significantly drops (through switching losses and non-ideal converter elements). The single-input single-output (SISO) approach with minor-loop voltage feedback controller presented here overcomes this problem. This approach is created on a theoretical linear model and verified by discrete-model simulation on MATLAB/Simulink. The capability and effectiveness of the SISO approach in compensating start-up current/voltage and in achieving zero steady-state error were tested for transient cases with step-changed load and step-changed reference voltage for linear and non-linear loads. Tests were done to analyze the transient performance against various controller gains. An experiment prototype was also developed for verification.

• Journal of Power Electronics
• /
• v.18 no.6
• /
• pp.1659-1669
• /
• 2018

An improved circulating current suppression control method is proposed in this paper. In the proposed controller, an outer loop of the average capacitor voltage control model is used to balance the sub-module capacitor voltage. Meanwhile, an individual voltage balance controller and an arm voltage balance controller are also used. The DC and harmonic components of the circulating current are separated using a low pass filter. Therefore, a multiple quasi-proportional-resonant (multi-quasi-PR) controller is introduced in the inner loop to eliminate the circulating harmonic current, which mainly contains second-order harmonic but also contains other high-order harmonics. In addition, the parameters of the multi-quasi-PR controller are designed in the discrete domain and an analysis of the stability characteristic is given in this paper. In addition, a simulation model of a three-phase MMC system is built in order to confirm the correctness and superiority of the proposed controller. Finally, experiment results are presented and compared. These results illustrate that the improved control method has good performance in suppressing circulating harmonic current and in balancing the capacitor voltage.

• The Journal of the Acoustical Society of Korea
• /
• v.24 no.7
• /
• pp.402-409
• /
• 2005

In this paper. we propose a coding method using a matching pursuit algorithm in a strongly periodic highband signal. Also. we propose an efficient quantizer for the estimated parameters : spectral magnitude and phase. Based on the error concealment principle and sinusoidal model. the MP algorithm requires the high-precision pitch period estimation. To estimate more accurate pitch period. the refined pitch obtained from lowband speech is used. which increases the efficiency of bit allocation. The spectral magnitude parameters are quantized by the method which is combined with MDCT (Modified Discrete Cosine Transform) and multi-stage structure. The spectral phase quantizer uses the $2{\pi}$ modular characteristic of phases and the weighted function by spectral magnitudes. To evaluate the efficiency of the proposed method. we applied it to analysis-by-synthesis system. Furthermore we suggest the possibillity of scalable wideband speech codecs based on band-split structure.

• Transactions of the Korean Society of Mechanical Engineers B
• /
• v.35 no.5
• /
• pp.491-501
• /
• 2011

We performed numerical simulation using a DPM (discrete phase model) to identify the optimal operation ranges in two representative PM burners widely used in domestic 500-MW pulverized coal-fired power plants. Recently there has been an increased utilization of low-cost coals such as sub-bituminous coal. We investigate the effects of coal blends on the distribution ratio of coal to air by varying the mass flow rates of pulverized coal and primary air and the particle size. We present and discuss optimal conditions for the distribution ratio of coal to air in PM burners.

• Tribology and Lubricants
• /
• v.32 no.3
• /
• pp.101-105
• /
• 2016

Trap effect of groove is evaluated in a lubricating system using computational fluid dynamics (CFD) analysis. The simulation is based on the standard k-ε turbulence model and the discrete phase model (DPM) using a commercial CFD code FLUENT. The simulation results are also capable of showing the particle trajectories in flow field. Computational domain is meshed using the GAMBIT pre-processor. The various grooves are applied in order to improve lubrication characteristics such as reduction of friction loss, increase in load carrying capacity, and trapping of the wear particles. Trap effect of groove is investigated with variations in cross-sectional shape and Reynolds number in this research. Various cross-sectional shapes of groove (rectangular, triangle, U shaped, trapezoid, elliptical shapes) are considered to evaluate the trap effect in simplified two-dimensional sliding bearing. The particles are assumed to steel, and defined a single particle injection condition in various positions. The “reflect” boundary condition for discrete phase is applied to the wall boundary, and the “escape” boundary condition to “pressure inlet” and “pressure outlet” conditions. The streamlines are compared with particles trajectories in the groove. From the results of numerical analysis in the study, it is found that the cross-sectional shapes favorable to the creation of vortex and small eddy current are effective in terms of particle trapping effect. Moreover, it is found that the Reynolds number has a strong influence on the pattern of vortex or small eddy current in the groove, and that the pattern of the vortex or small eddy current affects the trap effect of the groove.

• Coupled systems mechanics
• /
• v.7 no.6
• /
• pp.649-668
• /
• 2018

In this paper, we present a numerical model for fluid-structure interaction between structure built of porous media and acoustic fluid, which provides both pore pressure inside porous media and hydrodynamic pressures and hydrodynamic forces exerted on the upstream face of the structure in an unified manner and simplifies fluid-structure interaction problems. The first original feature of the proposed model concerns the structure built of saturated porous medium whose response is obtained with coupled discrete beam lattice model, which is based on Voronoi cell representation with cohesive links as linear elastic Timoshenko beam finite elements. The motion of the pore fluid is governed by Darcy's law, and the coupling between the solid phase and the pore fluid is introduced in the model through Biot's porous media theory. The pore pressure field is discretized with CST (Constant Strain Triangle) finite elements, which coincide with Delaunay triangles. By exploiting Hammer quadrature rule for numerical integration on CST elements, and duality property between Voronoi diagram and Delaunay triangulation, the numerical implementation of the coupling results with an additional pore pressure degree of freedom placed at each node of a Timoshenko beam finite element. The second original point of the model concerns the motion of the outside fluid which is modeled with mixed displacement/pressure based formulation. The chosen finite element representations of the structure response and the outside fluid motion ensures for the structure and fluid finite elements to be connected directly at the common nodes at the fluid-structure interface, because they share both the displacement and the pressure degrees of freedom. Numerical simulations presented in this paper show an excellent agreement between the numerically obtained results and the analytical solutions.

• Journal of the Korea Society for Simulation
• /
• v.21 no.2
• /
• pp.19-30
• /
• 2012

In recent decades, it has been known that the Discrete Event System Specification, or DEVS, formalism provides sound semantics to design a modular and hierarchical model of a discrete event system. In spite of this benefit, practitioners have difficulties in applying the semantics to real-world systems modeling because DEVS needs to specify a large size of sets of events and/or states in an unstructured form. To resolve the difficulties, this paper proposes an extension of the DEVS formalism, called the Structured DEVS formalism, with an associated graphical representation, called the DEVS diagram, by means of structural representation of such sets based on closure property of set theory. The proposed formalism is proved to be equivalent to the original DEVS formalism in their model specification, yet the new formalism specifies sets in a structured form with a concept of phases, variables and ports. A simplified example of the structured DEVS with the DEVS diagram shows the effectiveness of the proposed formalism which can be easily implemented in an objected-oriented simulation environment.

• Proceedings of the KIEE Conference
• /
• 2008.11a
• /
• pp.65-67
• /
• 2008

A new pulse width modulation method based on predictive current control strategy is proposed to modulate matrix converters. The predictive current controller utilizes a discrete-time model to predict the future values of output currents and generates proper duty-ratios ta minimize the output current errors. The proposed method uses continuous carrier and establishes a predictive current controller to predetermine duty ratio signal for directly generating gating signals an thus is named "predictive current control PWM(PCCPWM)". The modulation algorithm nd the required equations are derived by using average concept over one switching period. Thus it can be easily extended to other matrix converter topologies, especially with neutral connections, such as sing le-phase ad two-phase matrix converters. The feasibility and validity of the proposed strategy are verified by computer simulation and experimental results.

• Journal of computational fluids engineering
• /
• v.2 no.2
• /
• pp.89-96
• /
• 1997

High injection pressure system has been developed as a measure to reduce harmful exhaust gases. In order to understand the effect of pressure on diesel spray injection process, wide range of high injection pressure was tested. The gas phase is modelled by the Eulerian continuum conservation equations of mass, momentum, energy and fuel vapour fraction. The liquid phase is modelled following the discrete droplet model approach in Lagrangian form. The droplet distributions, vapor fractions and gas flows are analyzed in various injection pressure cases. The distributions of spray and vapor increase and the Sauter mean diameter decreases with increasing injection pressure quickly in a low pressure area but slowly in a high pressure area.