• Title/Summary/Keyword: k-$\varepsilon$model

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Effect of Flow Distribution on the Combustion Efficiency In an Entrained-Bed Coal Reactor (분류층 석탄반응로에서 유동분포가 연소성능에 미치는 영향)

  • CHO, Han Chang;SHIN, Hyun Dong
    • Transactions of the Korean Society of Mechanical Engineers B
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    • v.23 no.8
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    • pp.1022-1030
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    • 1999
  • A numerical study was carried out to analyze the effect of flow distribution of stirred part and plug flow part on combustion efficiency at the coal gasification process in an entrained bed coal reactor. The model of computation was based on gas phase eulerian balance equations of mass and momentum. The solid phase was described by lagrangian equations of motion. The $k-{\varepsilon}$ model was used to calculate the turbulence flow and eddy dissipation model was used to describe the gas phase reaction rate. The radiation was solved using a Monte-Carlo method. One-step parallel two reaction model was employed for the devolatilization process of a high volatile bituminous Kideco coal. The computations agreed well with the experiments, but the flame front was closer to the burner than the measured one. The flow distribution of a stirred part and a plug flow part in a reactor was a function of the magnitude of recirculation zone resulted from the swirl. The combustion efficiency was enhanced with decreasing stirred part and the maximum value was found around S=1.2, having the minimum stirred part. The combustion efficiency resulted from not only the flow distribution but also the particle residence time through the hot reaction zone of the stirred part, in particular for the weak swirl without IRZ(internal recirculation zone) and the long lifted flame.

The Characteristics of Turbulent Diffusion Flame Impinging on the Wall (벽면 충돌 난류 확산화염의 특성)

  • Park, Yong Youl;Kim, Ho Young
    • Transactions of the Korean Society of Mechanical Engineers B
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    • v.23 no.2
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    • pp.175-184
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    • 1999
  • A theoretical study on the turbulent round jet diffusion flame impinging on the wall was carried out to predict the characteristics and structure of Impinging jet flame and heat transfer to the wall. Finite chemistry via Arrhenius equation and eddy dissipation model was adopted as a combustion model, and the Favre averaging and $k-{\varepsilon}$ model were Introduced In the theoretical modeling. The SIMPLE algorithm was applied to the calculation. All the transport properties were considered as the variable depending on the temperature and composition. For the parametric study, the distance from nozzle to impinging wall and Reynolds number at nozzle exit were chosen 88 the major parameters. As the results of the present study, the characteristics of flow fields, the distributions of main variables and each chemical species and the flame shapes were obtained. The heat transfer rate from the flame to the wall and the effective heating area were calculated to investigate the Influences of the major parameters on the heat transfer characteristics.

A Study on the Flow Fields of Bubble Trap of Turbidimeter Using the Multiphase Model (다상모델을 이용한 탁도계 버블트랩 내부 유동장에 관한 고찰)

  • Lee, Kye-Bock;Kim, Young-Do
    • Transactions of the Korean Society of Mechanical Engineers B
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    • v.31 no.11
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    • pp.963-970
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    • 2007
  • The objectives of this study are to examine a bubble trap mechanism of the turbidimeter for low turbidity and to acquire it's technology concerned. Reynolds-Averaged Wavier-Stokes equation and Laglangian discrete phase model were applied to analyze a flow field in the bubble trap. 3D hybrid grid system was used to simulate the flow field of bubble trap and numbers of it's node point are about 110,000. From the comparison between the standard $k-{\varepsilon}$ model and the laminar state, it was found that the former estimated less the velocity in the outlet of bubble trap than the latter did, and that the former estimated more the shear stress at the wall of bubble trap than the latter did. And, it was possible to visualize the path of bubbles in the bubble trap and to copy the removal process of bubbles out bubble trap. Also, it was found that nearly most of bubbles in the bubble trap disappeared.

Numerical Simulation of Self-excited Combustion Oscillation in a Dump Combustor with Bluff-body (둔체를 갖는 연소기에서 자려 연소 진동에 관한 수치해석)

  • Kim, Hyeon-Jun;Hong, Jung-Goo;Kim, Dae-Hee;Shin, Hyun-Dong
    • Transactions of the Korean Society of Mechanical Engineers B
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    • v.32 no.9
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    • pp.659-668
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    • 2008
  • Combustion instability has been considered as very important issue for developing gas turbine and rocket engine. There is a need for fundamental understanding of combustion instability. In this study, combustion instability was numerically and experimentally investigated in a dump combustor with bluff body. The fuel and air mixture had overall equivalence ratio of 0.9 and was injected toward dump combustor. The pressure oscillation with approximately 256Hz was experimentally obtained. For numerical simulation, the standard k-$\varepsilon$ model was used for turbulence and the hybrid combustion model (eddy dissipation model and kinetically controlled model) was applied. After calculating steady solution, unsteady calculation was performed with forcing small perturbation on initial that solution. Pressure amplitude and frequency measured by pressure sensor is nearly the same as those predicted by numerical simulation. Furthermore, it is clear that a combustion instability involving vortex shedding is affected by acoustic-vortex-combustion interaction. The phase difference between the pressure and velocity is $\pi$/2, and that between the pressure and heat release rate is in excitation range described by Rayleigh, which is obvious that combustion instability for the bluff body combustor meets thermoacoustic instability criterion.

Convective Heat Transfer Correlations for the Compact Heat Exchanger with Circular Tubes and Flat Tubes-Plate Fins (원형관 및 납작관-평판휜 형상의 밀집형 열교환기에 대한 대류열전달 상관관계식)

  • Moh, Jeong-Hah
    • Transactions of the Korean Society of Mechanical Engineers B
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    • v.34 no.3
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    • pp.291-299
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    • 2010
  • Aspect-ratio-based numerical analysis is carried out to investigate the air-side convective heat transfer characteristics in compact heat exchangers with circular tubes and flat tubes-plate fins. The RNG $k-{\varepsilon}$ model is adopted for turbulence analysis. The numerical analysis is carried out for aspect ratios ranging from 3.06 to 5.44 and for Reynolds numbers ranging from 1,000 to 10,000. The calculated results indicate a correlation between the friction factor and Colburn j factor in the compact heat exchanger system for the range of aspect ratios under consideration. The results obtained for circular tubes and flat tubes-plate fins in this study can be utilized to realize the optimal design of an air conditioning system.

Modeling on Ratio-Dependent Three-Trophic Population Dynamics Responding to Environmental Impacts (외부 환경영향에 대한 밀도비 의존 3영양단계의 개체군 동태 모델)

  • Lee, Sang-Hee;Choi, Kyung-Hee;Chon, Tae-Soo
    • Korean Journal of Ecology and Environment
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    • v.37 no.3 s.108
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    • pp.304-312
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    • 2004
  • The transient dynamics of three-trophic populations (prey, predator, and super predator) using ratio-dependent models responding to environmental impacts is analyzed. Environmental factors were divided into two parts: periodic factor (e.g., temperature) and general noise. Periodic factor was addressed as a frequency and bias, while general noise was expressed as a Gaussian distribution. Temperature bias ${\varepsilon}$, temperature frequency ${\Omega}$, and Gaussian noise amplitude ${\`{O}}$ accordingly revealed diverse status of population dynamics in three-trophic food chain, including extinction of species. The model showed stable limit cycles and strange attractors in the long-time behavior depending upon various values of the parameters. The dynamic behavior of the system appeared to be sensitive to changes in environmental input. The parameters of environmental input play an important role in determining extinction time of super predator and predator populations.

Modeling of internal wave generation near a shelf slope by ocean finite element method

  • Lee, Kwi-Joo;Joa, Soon-Won;Eom, Ki-Chang
    • Journal of the Korean Society of Fisheries and Ocean Technology
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    • v.42 no.1
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    • pp.38-43
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    • 2006
  • The 3-D modeling of ocean finite element method(OFEM) using $k-{\varepsilon}$ turbulent model and tetrahedron grids has been used to investigate the internal wave generation during the expansion of the deep water from the open sea to the shelf with a simple shape, which can be widely used in the fields of submarine development, ocean environment and meteorology, etc. In this paper, the detailed configuration of internal wave with its length and height and also the distribution of salinity and turbulent kinematic energy, etc. were derived. It is hoped that this OFEM method can be successfully applied to the numerical calculation of internal wave for and the oceanographic problems (tidal flows around underwater hill, plateau, Georges Bank, etc.) and ocean engineering problems(flow past artificial sea reefs) in future.

Conceptual design and numerical simulations of a vertical axis water turbine used for underwater mooring platforms

  • Wenlong, Tian;Baowei, Song;Zhaoyong, Mao
    • International Journal of Naval Architecture and Ocean Engineering
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    • v.5 no.4
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    • pp.625-634
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    • 2013
  • Energy is a direct restriction to the working life of an underwater mooring platform (UMP). In this paper, a vertical axis water turbine (VAWT) is designed to supply energy for UMPs. The VAWT has several controlled blades, which can be opened or closed by inside plunger pumps. Two-dimensional transient numerical studies are presented to determine the operating performance and power output of the turbine under low ocean current velocity. A standard k-${\varepsilon}$ turbulence model is used to perform the transient simulations. The influence of structural parameters, including foil section profile, foil chord length and rotor diameter, on the turbine performance are investigated over a range of tip-speed-ratios (TSRs). It was found that turbine with three unit length NACA0015 foils generated a maximum averaged coefficient of power, 0.1, at TSR = 2.

Analysis on Characteristic of Pressure Fluctuation in Hydraulic Turbine with Guide Vane

  • Shi, FengXia;Yang, JunHu;Wang, XiaoHui
    • International Journal of Fluid Machinery and Systems
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    • v.9 no.3
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    • pp.237-244
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    • 2016
  • An unsteady three-dimensional simulation based on Reynolds time-averaged governing equation and RNG $k-{\varepsilon}$ turbulence model, was presented for pump-as-turbine, the pressure fluctuation characteristic of hydraulic turbine with guide vane was obtained. The results show that the time domains of pressure fluctuation in volute change periodically and have identical cycles. In volute tongue and inlet pressure fluctuations are light, while in dynamic and static coupling interface pressure fluctuations are serious; In impeller blade region the pressure fluctuation of pressure surface are lighter than that of suction surface. The dominant frequencies of pressure fluctuation concentrate in low frequency region, and concentrate within 2 times of the blade passing frequency.

A Study on Combined Heat Transfer in a Enclosure with a Block (밀폐공간내의 피가열체 존재시 복합열전달에 관한 연구)

  • Hong, Seong-Kook;Ryou, Hong-Sun;Hong, Ki-Bae;Chae, Soo
    • Journal of the Korean Society of Safety
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    • v.15 no.1
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    • pp.19-27
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    • 2000
  • This paper numerically deals with combined heat transfer in a enclosure with a block. The block affected by hot wall is located centrally in the enclosure with a radiating gray gas. The discrete ordinate method(DOM) was used for solving the radiative transfer equation. Both laminar and turbulent cases were investigated for various Rayleigh number and standard k-$\varepsilon$ model was adopted to turbulent case. The effects of optical thickness, wall emissivity and fluid-solid thermal conductivity ratio are investigated on the flow and temperature fields. This study shows that as the wall emissivity decreases, the temperature distribution gradually becomes uniform and the heat transfer is reduced in enclosure. It is expected that this study can help to design the energy system related to the combined heat transfer and operate it safely.

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