• Fire Science and Engineering
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• v.32 no.3
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• pp.42-50
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• 2018

The effect of sub-grid turbulence and combustion models on the mean flame height in a heptane pool fire according to the Fire Dynamics Simulator (FDS) version (5 and 6) based on Large Eddy Simulation (LES) was examined. The heat release rate for the fire simulation was provided through experiments performed under identical conditions and the predictive performance of the mean flame height according to FDS version was evaluated by a comparison with the existing correlation. As a result, the Smagorinsky and Deardorff turbulence models applied to FDS 5 and 6, respectively, had no significant effects on the mean flow field, flame shape and flame height. On the other hand, the difference in pool fire characteristics including the mean flame height was due mainly to the difference in the mixture fraction and Eddy Dissipation Concept (EDC) combustion models applied to FDS 5 and 6, respectively. Finally, compared to FDS 6, FDS 5 provided the predictive result of a significantly longer flame height and more consistent mean flame height than the existing correlation.

• Journal of the Korean Society of Fisheries and Ocean Technology
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• v.49 no.2
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• pp.144-152
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• 2013

In an effort to find the optimum porous of Taewoo through the mathematical model 2 - dimensional tank water experiment among the approached to a problem related to ocean engineering, this study analyzed the porosity by dividing it into 9 cases. As the wave penetrates through the longitudinal porous of the Taewoo model, it was found that there is a wave energy loss because of the phenomenon of the separation of the porous due to the eddy. Looking into the general tendency based on the wave-height meter (probe) data, it was found that the shorter wavelength and higher frequency area, the more reflection coefficients increased, but in contrast, the longer wavelength and lower frequency area, the transmission coefficients showed the increasing trend and energy dissipation was in a similar way with reflection coefficients. In addition, it was found that the bigger the porosity was, the narrower distribution range of reflection coefficients was, and the more its average value decreased. On the other hand the transmission coefficients in direct opposition to reflection was found to show the wider range and the more gradual increase in the average value as porosity was the bigger around the average value. In contrast, energy dissipation rate was found to increase linearly as porosity increased the more around the porosity of 0.2518 but it decreased gradually around the peak point. Through the above results, it is judged that the porous of optimum in the longitudinal direction of the Taewoo model perforated plate was about 2.6cm because it was found that the porosity which produced the lowest reflection and transmission coefficient and the highest energy dissipation. As a result of comparing this to the case where there was no porosity at all, it showed the function of wave absorbing about 31.60%.

• International Journal of Aeronautical and Space Sciences
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• v.2 no.1
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• pp.12-19
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• 2001

Study of turbulent mixing layers has been a popular subject from the point of view of both practical application and phenomenological importance in engineering field. Turbulent mixing layers can be applied in many fields where rapid transition to turbulence is desirable in order to prevent boundary layer separation or to enhance mixing. The ability to control mixing, structure and growth of the shear flow would obviously have a considerable impact on many engineering applications. In addition to practical applications, free shear flows are one of the simplest flows to understand the fundamental mechanism in the transition process to turbulence. After the discovery of large-scale vortical structure in free shear flows many researchers have investigated the physical mechanism of generation and dissipation processes of the vortical structure. This study investigated the role of the large-scale vortical structures in the turbulent mixing layer using LES(Large-Eddy Simulation). The result shows that the pairing interaction of the vortical structure plays an important role in the growth rate of a mixing layer. It is found that the turbulence quantities depend strongly on the velocity ratio. It is also found that the vorticity in the high-velocity-side can extract energy from the mean flow, while the vorticity in the low-velocity-side lose energy by the viscous dissipation. Finally the results suggest the guideline to obtain the desired flow by control of the velocity ratio.

• 한국연소학회:학술대회논문집
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• 2003.12a
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• pp.65-74
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• 2003

Air pollution included Nitric Oxide(NOx) from heating boilers is increased by pursuing better life. Development of low NOx emission boiler is strongly needed. However commercial burner for heating boiler is also asked to be thermal efficient and low-cost manufactuable in addition to low NOx emission. Small space for combustor including burner is usually allowed. In this study, parametric study of compact low NOx burner for heating boiler was done using numerical analysis and experiments. Commerical computational fluid dynamic(CFD) program named CFX 5-6 was used for numerical analysis of low NOx burner using turbulent diffusion flame. Comparison of outlet NO and outlet temperature under various equivalence ratio and fuel flow rate was performed between experiment and numerical analysis.

• Proceedings of the KSME Conference
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• 2008.11b
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• pp.1967-1972
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• 2008

Reburning is an efficient combustion technology to reduce nitrogen oxide by injecting hydro-carbon fuel to the downstream of the main combustion. In this paper LPG has been used not only as main fuel but also as reburn fuel and air was used as an oxidizer with 15kW swirl burner. Experimental studies have been done to evaluate effect of reburning for NOx reduction. Also to examine the effect of the amount of burnout air for complete combustion by reburn fuel on NOx reduction, test was conducted by reducing the amount of burnout air. Computational fluid dynamic (CFD) simulation was performed using the commercial CFD code FLUENT 6.3 to simulate experimental results and investigate the thermo-chemical characteristics. An evaluation of reaction models for swirl burner has been carried out for propane-air with two step finite-rate eddy-dissipation model in FLUENT.

• Atmosphere
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• v.28 no.2
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• pp.223-232
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• 2018

The Global-Korean aviation Turbulence Guidance (G-KTG) system is developed using the operational Global Data Assimilation and Prediction System of Korea Meteorological Administration with 17-km horizontal grid spacing. The G-KTG system provides an integrated solution of various clear-air turbulence (CAT) diagnostics and mountain-wave induced turbulence (MWT) diagnostics for low [below 10 kft (3.05 km)], middle [10 kft (3.05 km) - 20 kft (6.10 km)], and upper [20 kft (6.10 km) - 50 kft (15.24 km)] levels. Individual CAT and MWT diagnostics in the G-KTG are converted to a 1/3 power of energy dissipation rate (EDR). 12-h forecast of the G-KTG is evaluated using 6-month period (2016.06~2016.11) of in-situ EDR observation data. The forecast skill is calculated by area under curve (AUC) where the curve is drawn by pairs of probabilities of detection of "yes" for moderate-or-greater-level turbulence events and "no" for null-level turbulence events. The AUCs of G-KTG for the upper, middle, and lower levels are 0.79, 0.69, and 0.63, respectively. Comparison of the upper-level G-KTG with the regional-KTG in East Asia reveals that the forecast skill of the G-KTG (AUC = 0.77) is similar to that of the regional-KTG (AUC = 0.79) using the Regional Data Assimilation and Prediction System with 12-km horizontal grid spacing.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2003.05a
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• pp.244-245
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• 2003

고체나 액체 추진로켓에 비하여 하이브리드 추진 시스템은 작동조건의 안정성과 안전함등의 많은 장점을 가지고 있다. HTPB와 같은 고체연료는 제작 및 저장, 운송 그리고 장착상의 안정성을 가지고 있으며 하이브리드 로켓의 고체연료로의 산화제의 유입을 제어하면서 추력의 변화와 엔진내부의 연소중단과 재 점화를 용이하게 할 수 있다. 이러한 이유로 인하여 하이브리드 엔진은 좀 더 경제적인 장치로 기대를 모으고 있다. 그러나, 기존의 하이브리드 로켓 엔진은 고체 추진 로켓에 비하여 낮은 연료 regression 율과 연소효율을 가지는 단점이 있다. 이러한 단점을 해결하고 요구되어지는 추력값과 연료유량을 증가시키기 위하여 고체연료의 표면적을 증가시킬 필요가 있다. 기존의 하이브리드 엔진에서는 연료 그레인에 다수의 연소포트를 만들어 표면적을 증가시켰으나 이는 비 활용 공간의 증가와 추진제의 질량 및 체적분율의 상당한 감소를 초래한다. 지난 수십년간에 걸쳐 하이브리드 엔진에서 연료의 regression 특성 및 엔진 성능 향상을 위한 연구가 계속되어 왔으며 최근에 엔진의 체적 규제를 경감시키고 연료의 regression율을 향상시키기 위하여 선회유동을 이용하는 하이브리드 로켓 엔진들이 제안되고 있다. 이러한 선회유동을 가지는 하이브리드 로켓은 고체연료 그레인에 대하여 평행하게 유입되는 기존의 하이브리드 로켓에 비하여 고체연료 벽면에서의 대류열전달이 현저하게 증가하게 되어 아주 높은 고체연료의 regression율을 얻을 수 있는 이점이 있다. 선회유동 하이브리드 로켓의 연소과정은 고체 연료의 열분해과정, 대류 열전달, 난류 혼합, 난류와 화학반응의 상호작용, soot의 생성 및 산화과정, soot 입자 및 연소가스에 의한 복사 열전달, 연소장과 음향장의 상호작용 등의 복잡한 물리적 과정을 포함하고 있다. 이러한 물리적 과정 중 난류연소, 고체연료 벽면 근방에서의 대류 열전달 및 연소과정에서 생성되는 soot 입자로부터의 복사 열전달, 그리고 고체연료 열 분해시 표면반응들은 고체연료의 regression율에 큰 영향을 미친다. 특히 고체연료의 난류화염면의 위치와 폭, 그리고 비 예혼합 난류화염장에서 생성되는 soot의 체적분율의 예측은 난류연소모델, 열전달 모델, 그리고 regression율 모델에 의해 크게 영향을 받기 때문에 수치모델의 예측 능력 향상시키기 위하여 이러한 물리적 과정을 정확히 모델링해야 할 필요가 있다. 특히 vortex hybrid rocket내의 난류연소과정은 아래와 같은 Laminar Flamelet Model에 의해 모델링 하였다. 상세 화학반응 과정을 고려한 혼합분율 공간에서의 화염편의 화학종 및 에너지 보존 방정식은 다음과 같다. 화염편 방정식과 혼합분률과 scalar dissipation rate의 관계식을 이용하여 혼합분률과 scalar dissipation rate에 따른 모든 reactive scalar들을 구하게 된다. 이러한 화염편 방정식들을 mixture fraction space에서 이산화시켜서 얻은 비선형 대수방정식은 TWOPNT(Grcar, 1992)로 계산돼 flamelet Library에 저장되게 된다. 저장된 laminar flamelet library를 이용하여 난류화염장의 열역학 상태량 평균치는 presumed PDF approach에 의해 구해진다. 본 연구에서는 강한 선회유동을 가지는 Hybrid Rocket 연소장내의 난류와 화학반응의 상호작용을 분석하기 위하여 Laminar Flamelet Model, 화학평형모델, 그리고 Eddy Dissipation Model을 이용한 수치해석결과를 체계적으로 비교하였다. 또한 Laminar Flamelet Model과 state-of-art 물리모델들을 이용하여 선회 유동을 갖는 하이브리드 로켓 엔진의 연소 및 Soot 생성 및 산화과정을 살펴보았으며 복사 열전달이 고체 연료 표면의 regression율에 미치는 영향도 살펴보았다. 특히 swirl강도, 산화제의 유입위치 그리고 선회유동의 형성방식이 하이브리드 로켓의 연소특성 및 regression rate에 미치는 영향을 상세히 해석하였다.

• Transactions of the Korean Society of Mechanical Engineers
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• v.16 no.10
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• pp.1940-1954
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• 1992

Fine grid computations were attempted to analyze the turbulent flows in the near wall low Reynolds number region and the numerical analyses were incorporated by a finite-volume discretization with full find grid system and low Reynolds number k-.epsilon. model was employed in this region. For the improvement of low Reynolds number k-.epsilon. model, modification coefficient of eddy viscosity $f_{\mu}$ was derived as a function of turbulent Reynolds number $R_{+}$ and nondimensional length $y^{+}$ from the concept of two length scales of dissipation rate of turbulent kinetic energy. The modification coefficient $f_{\epsilon}$ in .epsilon. transport equation was also derived theoretically. In the turbulent kinetic energy equation, pressure diffusion term was added in order to consider low Reynolds number region effect. The main characteristics of this low Reynolds number k-.epsilon. model were founded as : (1) In high Reynolds number region, the present model has limiting behavior which approaches to the high Reynolds number model. (2) Present low Reynolds number k-.epsilon. model dose not need additional empirical constants for the transport equations of turbulent kinetic energy and dissipation of turbulent kinetic energy in order to consider wall effect. Present low Reynolds number turbulence model was tested in the pipe flow and obtained improved results in velocity profiles and Reynolds stress distributions compared with those from other k-.epsilon. models.s.s.

• 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.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.24 no.10
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• pp.1368-1379
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• 2000

A numerical study on swirling pulverized coal combustion in an axisymmetric enclosure is carried out by applying the 2-phase 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 and RNG k-${\varepsilon}$ turbulence model, whereas the Lagrangian approach is used for the particulate phase. The eddy-dissipation model is employed for the reaction rate for gaseous mixture, and the single-step and two-step first-order reaction model for the devolatilization process for coal. Special attention is given to establish the thermal boundary conditions on radiative transfer equation By comparing the numerical results with experimental ones, the radiation model used here is confirmed and found to provide an alternative for simulating the radiative transfer.