• Nuclear Engineering and Technology
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• 제47권6호
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• pp.669-677
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• 2015

A mechanistic model for void fraction prediction with improved interfacial friction factor in nearly horizontal tubes has been proposed in connection with the development of a condensation model package for the passive auxiliary feedwater system of the Korean Advanced Power Reactor Plus. The model is based on two-phase momentum balance equations to cover various types of fluids, flow conditions, and inclination angles of the flow channel in a separated flow. The void fraction is calculated without any discontinuity at flow regime transitions by considering continuous changes of the interfacial geometric characteristics and interfacial friction factors across three typical separated flows, namely stratified-smooth, stratified-wavy, and annular flows. An evaluation of the proposed model against available experimental data covering various types of fluids and flow regimes showed a satisfactory agreement.

• 대한기계학회논문집B
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• 제34권1호
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• pp.9-18
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• 2010

2 상 유동 압력강하에 대한 모델은 균질유동모델과 분리유동모델 두 가지가 있다. 많은 선행 연구자들은 마이크로채널에서의 2 상 유동 압력강하에 대한 상관식을 제시하였고, 대부분은 분리유동모델에 해당하는 Lockhart- $Martinelli^{(27)}$의 수정된 상관식에 기초하고 있다. 본 연구에서는 사각 마이크로채널에서의 압력강하에 대한 연구를 위해서 액상의 물과 기상의 질소를 사용하여 사각 마이크로채널에서의 실험을 수행하였다. 2 상 마찰 압력강하는 2 상 유동양식에 큰 연관성을 가지고 있는 결과를 확인할 수 있었다. 6 가지의 2 상 점성 모델을 포함한 균질유동 모델 ($Owen^{(21)}$'s, $MacAdams^{(22)}$'s, Cicchitti et ${al.}^{(23)}$'s, ${al.,}^{(24)}$ Beattie and ${Whalley,}^{(25)}$ Lin et ${al.}^{(26)}$)과 6 가지의 분리유동 모델 (Lockhart and $Martinelli,^{(27)}$ ${Chisholm,}^{(31)}$ Zhang et ${al.,}^{(15)}$ Lee and ${Lee,}^{(5)}$ Moriyama and ${Inue,}^{(4)}$ Qu and $Mudawar^{(8)}$)에 대한 평가를 실험결과와 비교를 통해 수행하였다. 가장 우수한 2 상 점성 모델은 Beattie and Whalley 의 모델이었고, 가장 우수한 분리유동 모델은 Qu and Mudawar 의 상관식이였다. 균질유동모델과 분리유동모델 모두에 대해서 2 상 유동양식에 종속성을 나타내었다. 그러므로, 본 연구에서는 2 상 유동 양식에 기초한 새로운 상관식을 균질유동모델과 분리유동모델에 대해 각각을 제시하였다.

• 한국전산유체공학회지
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• 제4권3호
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• pp.44-51
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• 1999

A modified version of SST turbulence model is suggested to simulate unsteady separated flows over oscillating airfoils. The original SST model, which shows good performance in predicting various steady flows, often results in oscillatory behavior of aerodynamic loads in large separated flow regions. It is shown that this oscillatory behavior is due to the adoption of the absolute value of vorticity in generalizing the original model. As a remedy, a modification is made such that the vorticity in the original SST model is replaced by strain rate. The present model is verified for a mild separated airfoil flow at fixed angle of incidence and for unsteady flowfields about oscillating airfoils. The results are compared with BSL model and original SST model. It is illustrated that the present model gives a better agreement with the experimental results than other two models.

• 대한기계학회논문집B
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• 제24권3호
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• pp.467-476
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• 2000

An unsteady numerical simulation was performed for locally-forced separated and reattaching flow over a backward-facing step. The local forcing was given to the separated and reattaching flow by means of a sinusoidally oscillating jet from a separation line. A version of the $k-{\varepsilon}-f_{\mu}$ model was employed, in which the near-wall behavior without reference to distance and the nonequilibrium effect in the recirculation region were incorporated. The Reynolds number based on the step height (H) was fixed at $Re_H=33000$, and the forcing frequency was varied in the range $0{\leq}St_H{\leq}2$. The predicted results were compared and validated with the experimental data of Chun and Sung. It was shown that the unsteady locally-forced separated and reattaching flows are predicted reasonably well with the $k-{\varepsilon}-f_{\mu}$ model. To characterize the large-scale vortex evolution due to the local forcing, numerical flow visualizations were carried out.

• 대한기계학회논문집
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• 제19권11호
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• pp.3014-3021
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• 1995

This study is carried out in order to evaluate the performances of the Reynolds stress turbulence models such as SSG and GL models in the calculation of separated flow over backward-facing stepp.In addition, two slow return-to-isotropy models, YA and Rotta models combined with rapid part of SSG model are also tested. The finite volume method is used to discretize the governing differential equations, and the power-law scheme is used to approximate the convection terms. The SIMPLE algorithm is used for pressure correction in the governing equations. The results show that SSG model gives the better prediction near the reattachment point than GL model. In cases that the rapid term of SSG model is combined with Rotta and YA slow models, the results show the better predictions of stress components in recirculation zone, but indicate inaccuracy in the predictions of mean velocity.

• 대한기계학회논문집B
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• 제25권1호
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• pp.87-95
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• 2001

A numerical study was made of heat transfer in locally-forced turbulent separated and reattaching flow over a backward-facing step. The local forcing was given to the flow by means of sinusoidally oscillating jet from a separation line. A Rhee and Sung version of the unsteady $\kappa$-$\varepsilon$-f(sub)u model and the diffusivity tensor heat transfer model were employed. The Reynolds number was fixed at Re(sub)H=33,000 and the forcing frequency was varied in the range 0$\leq$fH/U(sub)$\infty$$\leq$2. The condition of constant heat flux was imposed at the bottom wall. The predicted results were compared and validated with the experimental data of Chun and Sung and Vogel and Eaton. The enhancement of heat transfer in turbulent separated and reattaching flow by local forcing was evaluated and analyzed.

• 해양환경안전학회지
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• 제24권5호
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• pp.611-618
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• 2018

본 연구에서는 이수의 유동 특성을 분석하기 위한 기초 연구로서 상용 코드인 ANSYS CFX 14.5를 이용하여 고체-액체 2상 유동에 대한 수치해석적 연구를 수행하였다. 고체-액체 2상 유동 현상을 모사하기 위해서 균질류 모델과 분리류 모델을 사용하였다. 분리류 모델에서는 Gidaspow의 항력모델을 적용하였으며, 고체 입자에 운동 이론 모델을 적용하였다. 기존의 실험 결과를 기반으로 본 연구에서 사용한 수치해석 모델의 유효성을 검토하였으며, 수치해석은 직경 54.9 mm, 길이 3 m의 수평관에서 체적 분율 0.1~0.5, 속도 1~5 m/s 범위에서 수행되었다. 그리고 압력강하와 고체 입자의 체적 분율 분포를 확인하였으며, 압력강하는 균질류 모델과 분리류 모델이 각각 MAE 17.04 %, 8.98 % 이내에서 실험결과를 잘 예측하였다. 관의 하부에서 높은 체적 분율이 나타나며, 상부로 갈수록 체적 분율은 감소하였다. 그리고 속도가 증가할수록 높이 변화에 따른 체적 분율 분포의 변화는 감소하였으며, 수치해석 결과는 이러한 유동 특성을 잘 예측하였다.

• 수산해양교육연구
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• 제25권3호
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• pp.625-631
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• 2013

Two-phase pressure drop experiments were performed during flow boiling to deionized water in a microchannel having a hydraulic diameter of $500{\mu}m$. Tests were made in the ranges of heat fluxes from 100 to $400kW/m^2$, vapor qualities from 0 to 0.2 and mass fluxes of 200, 400 and $600kg/m^2s$. The frictional pressure drop during flow boiling is predicted by using two models; the homogeneous model that assumes equal phase velocity and the separate flow model that allows a slip velocity between two phases. From the experimental results, it is found that the two phase multiplier decreases with an increase in mass flux. Measured data of pressure drop are compared to a few available correlations proposed for macroscale and mini/microscale. Among the separated flow models, the correlation model suggested by Lee and Garimella predicted the frictional pressure drop within MAE of 47.2%, which is better than other correlations.

• 한국군사과학기술학회지
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• 제14권3호
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• pp.397-404
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• 2011

The aerodynamic characteristics of a separated captive body in flow field around aircraft are studied to observe aerodynamic stability for safe separation from aircraft. Since the captive body separated from aircraft is initially exposed to unsteady flow pattern, the change of aerodynamic forces and moments should be measured to analyze how the flow pattern affects on the captive body at the vicinity of aircraft. Aerodynamic forces and moments of the separated captive body are measured at selected positions along predictable dropping trajectories. The measuring trajectories, generated by the free drop test of the dropping model in the wind tunnel, are consisted of 9 possible lines by free dropped trajectories. Experimental results show that the aerodynamic forces and moments are significantly varied with the distance between the captive body and aircraft. In conclusion, the change of aerodynamic characteristics within flow field around aircraft should be considered to simulate trajectories of the separated captive body from aircraft.

• Journal of Mechanical Science and Technology
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• 제19권9호
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• pp.1790-1800
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• 2005

The examination on the operating mechanism of a pulsating heat pipe (PHP) using visualization revealed that the working fluid in the PHP oscillated to the axial direction by the contraction and expansion of vapor plugs. This contraction and expansion is due to the formation and extinction of bubbles in the evaporating and condensing section, respectively. In this paper, a theoretical model of PHP was presented. The theoretical model was based on the separated flow model with two liquid slugs and three vapor plugs. The results show that the diameter, surface tension and charge ratio of working fluid have significant effects on the performance of the PHP. The following conclusions were obtained. The periodic oscillations of liquid slugs and vapor plugs were obtained under specified parameters. When the hydraulic diameter of the PHP was increased to d=3mm, the frequency of oscillation decreased. By increasing the charging ratio from 40 to 60 by volume ratio, the pressure difference between the evaporating section and condensing section increased, the amplitude of oscillation reduced, and the oscillation frequency decreased. The working fluid with higher surface tension resulted in an increase in the amplitude and frequency of oscillation. Also the average temperature of vapor plugs decreased.