재순환유동 예측을 위한 k-ɛ 난류모델 개선에 대한 연구

이영모(Y.M. Lee),김철완(C.W. Kim) 한국전산유체공학회 2016 한국전산유체공학회지 Vol.21 No.2

The standard k-ɛ and realizable k-ɛ models are adopted to improve the prediction performance on the recirculating flow. In this paper, the backward facing step flows are used to assess the prediction performance of the recirculation zone. The model constants of turbulence model are obtained by the experimental results and they have a different value according to the flow. In the case of an isotropic flow situation, decaying of turbulent kinetic energy should follow a power law behavior. In accordance with the power law, the coefficients for the dissipation rate of turbulent kinetic energy are not universal. Also, the other coefficients as well as the dissipation coefficient are not constant. As a result, a suitable coefficients can be varied according to each of the flow. The changes of flow over the backward facing step in accordance with model constants of the k-ɛ models show that the reattachment length is dependent on the growth rate(λ) and the k-ɛ models can be improved the prediction performance by changing the model constants about the recirculating flow. In addition, it was investigated for the curvature correction effect of the k-ɛ models in the recirculating flow. Overall, the curvature corrected k-ɛ models showed an excellent prediction performance.

재순환유동 예측을 위한 난류모델 개선에 대한 연구

이영모,김철완 한국전산유체공학회 2016 한국전산유체공학회지 Vol.21 No.2

The standard and realizable models are adopted to improve the prediction performance on the recirculating flow. In this paper, the backward facing step flows are used to assess the prediction performance of the recirculation zone. The model constants of turbulence model are obtained by the experimental results and they have a different value according to the flow. In the case of an isotropic flow situation, decaying of turbulent kinetic energy should follow a power law behavior. In accordance with the power law, the coefficients for the dissipation rate of turbulent kinetic energy are not universal. Also, the other coefficients as well as the dissipation coefficient are not constant. As a result, a suitable coefficients can be varied according to each of the flow. The changes of flow over the backward facing step in accordance with model constants of the models show that the reattachment length is dependent on the growth rate() and the models can be improved the prediction performance by changing the model constants about the recirculating flow. In addition, it was investigated for the curvature correction effect of the models in the recirculating flow. Overall, the curvature corrected models showed an excellent prediction performance.

단일공급 스월 혼합챔버 내의 와류유동에 대한 실험적 연구

김형민(Hyungmin Kim),윤웅섭(Woongsup Yoon) 한국추진공학회 2011 한국추진공학회 학술대회논문집 Vol.2011 No.11

금속분말을 연소시키기 위한 스월 연소기 설계의 기초단계로써 단일 접선 공급유로를 갖는 스월 혼합챔버를 제작하고 Paticle Image Velocimetry(PIV)를 사용하여 스월 혼합챔버의 내부 유동장 측정실험을 수행하였다. 상온의 공기를 작동유체로 사용하였으며 접선 공급유량이 증가하는 경우의 스월 혼합챔버 내 축방향 및 접선방향 성분 속도를 획득하였다. 측정된 유동장을 바탕으로 스월유동과 역압력 구배로 인해 발생하는 외부 유입유동간의 혼합특성을 평가 하였다. Swirling flows inside a swirl mixing chamber are investigated for simple configuration where swirl is produced by a tangential entry type swirl generator. The flow downstream of the swirl generator has been quantified by measurements two velocity components and their corresponding mean values along axial and radial direction using Particle Image Velocimetry(PIV). The mass flow rate of the tangential entry is increased in order to study their effect on the flow field. From the measurement profile of velocity and vorticity, flow mixing characteristics in a swirl mixing chamber are evaluated.

호흡량에 따른 사람의 기도 내 유동 특성에 관한 수치 해석적 연구

성건혁(K.H. Sung),노경철(K.C. Ro),유홍선(H.S. Ryou) 한국전산유체공학회 2012 한국전산유체공학회 학술대회논문집 Vol.2012 No.5

The inspiratory rate of a human is changed with the amount of the workload. The flow field is affected by the inspiratory flow rate. In the flow filed, the wall shear stress is important to predict the inflammation or a injury in airway wall. In addition, the turbulence intensity and secondary flow affect the deposition pattern of particle which is important for the drug-aerosol targeting. Thus the analysis of the flow characteristic in a human airway is important. The purpose of this study is to investigate the effects of the inspiratory flow rate on the flow characteristics in a human airway. The tubular airway is consistent with the oral cavity, pharynx, larynx and trachea. The constant inspiratory flow rate is used at each case of human states regarding the workload. By the effect of geometric airway changes, transition to turbulent airflow after the larynx can occur with relaminarization further downstream. Consequently, the low Reynolds number k-ω turbulence model is used for analysis with flow regime. As the inspiratory flow rate is larger, the wall shear stress and the turbulence intensity increase in airway. On the other hand, the area of recirculation zone is smaller.

다공배플 두께가 마이크로 연소기의 유동 및 혼합특성에 미치는 영향

김원현(Won Hyun Kim),박태선(Tae Seon Park) 대한기계학회 2017 大韓機械學會論文集B Vol.41 No.9

다공배플을 가진 마이크로연소기에 레이놀즈응력 난류모형을 이용하여 난류유동 및 혼합특성에 대한 수치해석 연구를 수행하였다. 다공배플은 연소실 내부에 다수의 3차원 와유동을 발생시키는 기하학적 특징을 가지고 있다. 그러한 형상특징 중에서 배플두께를 변화시킬 경우 와유동구조의 다양한 변화가 얻어졌다. 여러 와유동중에서 연료유동으로부터 생성된 와유동은 혼합도 증가에 결정적 역할을 하였다. 연소실 내부의 3차원 와유동구조는 배플두께 변화에 따른 유동의 발달상태에 의존하였다. 특히, 배플두께가 연료유입구 직경보다 작을수록 배플구멍 제트유동의 속도분포는 포물선형태에서 안장모양의 형태로 변하였다. 연소실내부의 재순환영역크기 및 혼합도는 이러한 제트유동구조에 밀접한 상관관계를 가졌다. Flow structure and mixing characteristics in a micro combustor with a multi-hole baffle were numerically studied using the Reynolds stress model. The multi-hole baffle has geometrical features to produce multiple three-dimensional vortices inside combustion chamber. When the thickness of the baffle"s geometrical factors changes, variations of vortical structures occur variously. Among these vortices, the vortex generated from the fuel stream exerts a critical influence on the mixing enhancement. The three-dimensional vortical structure, in its development state, was strongly dependent on the baffle thickness. In particular, as the baffle thickness decreases to values less than the diameter of the fuel hole, the jet stream in baffle holes changes from the parabolic to saddleback profile type. The sizes of recirculation zones inside combustion chamber and the mixing state were closely affected by the structure of the jet streams.

자동차용 고분자 연료전지 수소 재순환 시스템의 이상 유동해석

곽현주(Hyun Ju Kwag),정진택(Jin Taek Chung),김재춘(Jae Choon Kim),김용찬(Yongchan Kim),오형석(Hyung Seuk Oh) 대한기계학회 2008 大韓機械學會論文集B Vol.32 No.6

The purpose of this paper is to analyze two-phase flows of the hydrogen recirculation system. Two-phase flow modeling is one of the great challenges in the classical sciences. As with most problems in engineering, the interest in two-phase flow is due to its extreme importance in various industrial applications. In hydrogen recirculation systems of fuel cell, the changes in pressure and temperature affect the phase change of mixture. Therefore, two-phase flow analysis of the hydrogen recirculation system is very important. Two-phase computation fluid dynamics (CFD) calculations, using a commercial CFD package FLUENT 6.2, were employed to calculate the gas-liquid flow. A two-phase flow calculation was conducted to solve continuity, momentum, energy equation for each phase. Then, the mass transfer between water vapor and liquid water was calculated. Through an experiment to measure production of liquid water with change of pressure, the analysis model was verified. The predictions of rate of condensed liquid water with change of pressure were within an average error of about 5%. A comparison of experimental and computed data was found to be in good agreement. The variations of performance, properties, mass fraction and two-phase flow characteristic of mixture with resepct to the fuel cell power were investigated.

유량변화에 따른 모델 가스터빈 연소기 내 Counter-rotating 선회유동 특성

이상호(Sangho Lee),진유인(Yuin Jin),유경원(Gyongwon Ryu),이지근(Jeekeun Lee) 한국추진공학회 2015 한국추진공학회 학술대회논문집 Vol.2015 No.5

본 연구는 가스터빈 내에서 연소 효율과 화염의 안정성에 영향을 미치는 중요한 요인인 선회유동 특성에 관한 연구이다. 가스터빈 연소기 내 선회유동의 특성을 관찰하고자 가스터빈 연소기를 직사각형으로 모사하였다. 모델 연소기 내의 선회유동을 생성하기 위하여 Counter-rotating 스월러가 사용되었다. PIV 기법을 이용하여 선회유동의 속도 성분을 측정하였고, 다수의 유동 추적 입자 이미지를 획득한 후 평균을 취하여 평균속도와 난류 특성을 계산하였다. 작동유체는 증류수를 사용하였으며, 재순환 영역, corner vortex와 모델 연소기의 벽을 타고 흐르는 벽면 유동을 포함하는 선회유동의 특성을 Reynolds 수에 따라 관찰하였다. In the present study, the characteristics of swirling flow, which is one of the most important phenomenon for efficiency and flame stabilization in gas turbines, were studied. The gas turbine combustor was simplified as rectangular shape to examine the swirling flow. A counter-rotating swirler was applied to generate the swirling flow inside the model combustor. A PIV system was used to measure velocity components of swirling flow, and the mean and turbulent components were calculated by averaging the acquired many seeding particle images. The pure water was used as a working fluid. The characteristics of swirling flow, including the formation of a central recirculation zone, corner vortex, and the wall flow along the model combustor wall, were investigated with Reynolds numbers.

모델 가스터빈 연소기 내 연소실 형상비의 변화가 Counter-rotating 선회유동에 미치는 영향

이상호(Sangho Lee),진유인(Yuin Jin),유경원(Gyongwon Ryu),이지근(Jeekeun Lee) 한국추진공학회 2015 한국추진공학회 학술대회논문집 Vol.2015 No.11

모형 가스터빈 연소기 연소실 형상비 (aspect ratio)가 연소실 내 선회유동 특성에 미치는 영향이 실험적으로 조사되었다. 모형 연소실은 아크릴 재질의 직사각형 형상으로 구성되었고 연소실 폭을 변화시켜 형상비를 조정하였다. 축류형 counter-rotating 선회기를 사용하여 연소실 내 선회유동장을 형성하였으며, 2D PIV 시스템을 이용하여 선회 속도 성분을 측정하였다. 작동유체는 물을 사용하였으며 공기와의 유사성을 고려하여 Reynolds 수를 설정하였다. PIV 기법을 이용하여 다수의 유동 추적 입자 이미지를 획득했으며, 각 이미지의 속도 성분을 분석하여 평균을 취한 후 선회유동 특성을 관찰하였다. 이를 통해 선회기에 의해 형성되는 재순환영역, corner vortex, 모델 연소기의 벽면을 타고 흐르는 벽면 유동 등, 모델 연소기 내에서 형성되는 각 유동들에 현소기 형상비의 변화가 미치는 영향을 조사하였다. The gas turbine combustor was modeled as rectangular shape with acrylic material and the aspect ratio of model combustor was decided by changing chamber width. Swirling flow in the model combustor was created with axial counter-rotating swirler and 2D PIV system was used to measure the velocity components of the swirling flow. Water was used as working fluid and Reynolds number was selected by considering similarity between air and water. A number of pictures of seeding particle were captured with PIV system to examine the characteristics of swirling flow by averaging obtained pictures. The effect of chamber aspect ratio on the structures of swirling flow, such as recirculation zone, corner vortex, wall flow was investigated.

회전원판형 CVD 장치의 유동 재순환을 억제하는 출구부 형상 설계를 위한 전산해석

박장진(J.J. Park),김경진(K. Kim),곽호상(H.S. Kwak) 한국전산유체공학회 2013 한국전산유체공학회지 Vol.18 No.4

A numerical design analysis is conducted to search for an optimal shape of outlet in a rotating-disk CVD reactor. The goal is to suppress flow recirculation that has been found in a reactor having a sudden expansion of flow passage outside of the rotating disk. In order to streamline gas flow, the sidewall at which the flow in the Ekman layer is impinged, is tilted. The axisymmetric laminar flow and heat transfer in the reactor are simulated using the incompressible ideal gas model. For the conventional vertical sidewall, the flow recirculation forming in the corner region could be expanded into the interior to distort the upstream flow. The numerical results show that this unfavorable phenomenon inducing back flow could be dramatically suppressed by tilting the sidewall at a certain range of angle. The assessment of deviation in deposition rate based on the characteristic isotherm illustrates that the sidewall tilting may expand the domain of stable plug-like flow regime toward higher pressure. A physical interpretation is attempted to explain the mechanism to suppress flow recirculation.

회전원판형 CVD 장치의 유동 재순환을 억제하는 출구부 형상 설계를 위한 전산해석

박장진(J.J. Park),김경진(K. Kim),곽호상(H.S. Kwak) 한국전산유체공학회 2013 한국전산유체공학회지 Vol.18 No.4

A numerical design analysis is conducted to search for an optimal shape of outlet in a rotating-disk CVD reactor. The goal is to suppress flow recirculation that has been found in a reactor having a sudden expansion of flow passage outside of the rotating disk. In order to streamline gas flow, the sidewall at which the flow in the Ekman layer is impinged, is tilted. The axisymmetric laminar flow and heat transfer in the reactor are simulated using the incompressible ideal gas model. For the conventional vertical sidewall, the flow recirculation forming in the corner region could be expanded into the interior to distort the upstream flow. The numerical results show that this unfavorable phenomenon inducing back flow could be dramatically suppressed by tilting the sidewall at a certain range of angle. The assessment of deviation in deposition rate based on the characteristic isotherm illustrates that the sidewall tilting may expand the domain of stable plug-like flow regime toward higher pressure. A physical interpretation is attempted to explain the mechanism to suppress flow recirculation.