• 한국추진공학회지
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• 제15권4호
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• pp.65-72
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• 2011

수직온도구배를 가진 유체 내의 기포유동을 수치해석적 방법으로 연구하였다. 본 연구의 목적은 Eulerian-Lagrangian 방정식모델을 적용하여 온도가 수직으로 층상화된 기-액 2상류(two phase flow)의 대류유동을 정확하게 해석할 수 있는 프로그램의 개발과 온도가 층상화된 유체의 기포에 의한 온도혼합과정의 가시화 그리고 유체역학적 특성을 이해하는 것이다. 또한, 기포반경, 보이드율, 그리고 유량이 기포에 의해 야기된 대류유동에 미치는 영향을 함께 검토하였다.

• 설비공학논문집
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• 제13권10호
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• pp.1034-1041
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• 2001

A numerical model which simulates the flow boiling process of the ammonia/water solution within a plate type generator for ammonia/water absorption refrigerators was developed. The ammonia/water solution flows downward under gravity and the ammonia/water vapor generated by flow boiling flows upward. The heating medium flows counter to the ammonia/water solution. The flow pattern within the generator was assumed to be a bubbly flow, and the liquid and vapor phases were assumed to be saturated. It was shown that the boiling of ammonia occurred mainly in the upper part of the generator. The effects of the heating medium inlet temperature, the mass flow rate of the heating medium and the mass flow rate of ammonia/water solution into the generator on the generation of ammonia/water vapor were investigated.

• 한국수자원학회논문집
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• 제38권10호
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• pp.841-849
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• 2005

유사 입자가 포함된 흐름이나 공기 방울이 포함된 흐름과 같은 이상류 (two-phase flow) 는 유체의 속도와 입자의 속도와 같은 두 가지 서로 다른 속도 분포가 존재한다. 이러한 이상류의 속도장 분석을 위해서는 두 가지 속도 분포를 별도로 측정할 수 있는 기법을 이용해야 한다. 공기 방울이 포함된 흐름에 대해서는 입자영상 유속계(PIV)나 입자추적유속계(PTV)를 이용하여 비교적 타당한 정도로 유속 분포를 측정하여 왔다. 그러나 자연 모래를 포함한 흐름 영상의 속도 분포 해석에서는 PIV나 PTV가 그다지 성공적이지 못했는데, 그것은 흐름 중에 있는 유사 입자가 영상을 해석하기 어렵게 만들기 때문이다. 유사 흐름의 속도 분석을 위해 다양한 영상 분석 기법을 결합한 방법을 제시하였다. 입자 추출 알고리듬으로 역치값, 경계 추출 알고리듬, 세선화 알고리듬을 조합한 새로운 방법을 제안하였다. 또한 입자의 이동 변위 계산을 위해서 PIV와 PTV를 조합한 새로운 방법을 개발하였다. 이 새로운 알고리듬은 다음과 같은 기능을 가지고 있다. (1) 새알고리듬은 유사 입자, 특히 자연 모래와 같이 불규칙한 형태를 갖는 입자의 경계를 정확히 찾아낼 수 있다. (2) 필요한 정보를 잃어버리지 않고, 반사광이나 난반사에 의한 영상을 효율적으로 제거할 수 있다. (3) 추적 입자가 유사입자 가까이에 있어 유사의 난반사 영역이 들어 있어도 이를 분리해 낼 수 있다.

• Nuclear Engineering and Technology
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• 제48권1호
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• pp.64-71
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• 2016

Void fraction is an important parameter in the oil industry. This quantity is necessary for volume rate measurement in multiphase flows. In this study, the void fraction percentage was estimated precisely, independent of the flow regime in gas-liquid two-phase flows by using ${\gamma}-ray$ attenuation and a multilayer perceptron neural network. In all previous studies that implemented a multibeam ${\gamma}-ray$ attenuation technique to determine void fraction independent of the flow regime in two-phase flows, three or more detectors were used while in this study just two NaI detectors were used. Using fewer detectors is of advantage in industrial nuclear gauges because of reduced expense and improved simplicity. In this work, an artificial neural network is also implemented to predict the void fraction percentage independent of the flow regime. To do this, a multilayer perceptron neural network is used for developing the artificial neural network model in MATLAB. The required data for training and testing the network in three different regimes (annular, stratified, and bubbly) were obtained using an experimental setup. Using the technique developed in this work, void fraction percentages were predicted with mean relative error of <1.4%.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2001년도 춘계학술대회논문집D
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• pp.304-310
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• 2001

A numerical model which simulates the heat and mass transfer processes within a counter-current plate type generator for ammonia/water absorption refrigerators was developed. Ammonia/water solution flows downward under gravity and ammonia/water vapor generated by flow boiling flows upward. The flow pattern within the generator was assumed to be a bubbly flow, and the liquid and vapor phase were assumed to be saturated. It was shown that the boiling of ammonia occurred mainly in the upper part of the generator. The effects of the generator length, the wall temperature and the mass flow rate of ammonia/water solution into the generator on the generation of ammonia/water vapor were investigated.

• Journal of Mechanical Science and Technology
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• 제15권12호
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• pp.1801-1807
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• 2001

In multiphase flne the bubble size and velocity. To achieve this, one of approaches is to utilize laser phase-Doppler anemometry. However, it was found that the second order refraction has great impact on PDA sizing method when the relative refractive index of media is less than one. In this paper, the problem of second order refraction is investigated and a model of phase-size correlation to eliminate the measurement errors is introduced for bubble sizing. As a result, the model relates the assumption of single scattering mechanism in conventional phase-Doppler anemometry. The results of simulations based on this new model by using Generalized Lorenz Mie Theory (GLMT) are compared with those based on the conventional method. An optimization method for accurately sizing air-bubble in water has been suggested.

• 소음진동
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• 제4권2호
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• pp.199-208
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• 1994

Two-phase cross-flow exists in many shell-tube heat exchangers such as nuclear steam generators, condensers and reboilers. An understanding of damping and of flow-induced vibration excitation mechanisms in necessary to avoid problems due to excessive tube vibration. In this paper, we present the results of experiments on normal-triangular tube bundles of pitch to tube diameter ratio, p/d, 1.22, 1.32 and 1.47. The bundle were subjected to air-water mixtures to simulate realistic mass fluxes and vapour qualities corresponding to void fractions from 5 to 99%. Damping, fluidelastic instability and turbulence- induced excitation are discussed. The behaivior of damping and two vibration mechanisms are different for intermittent flows from for bubbly flows. The effect of pitch to tube diameter ratio and void fraction is dominant on damping and fluidelastic instability.

• Journal of Mechanical Science and Technology
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• 제17권3호
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• pp.429-439
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• 2003

Counter-current two-phase flows of air- water in narrow rectangular channels with offset strip fins have been experimentally investigated in a 760 mm long and 100 mm wide test section with 3.0 and 5.0 mm gap widths. The two-phase flow regime, channel-average void fractions and two-phase pressure gradients were studied. Flow regime transition occurred at lower superficial velocities of air than in the channels without fins. In the bubbly and slug flow regimes, elongated bubbles rose along the subchannel formed by fins without lateral movement. The critical void fraction for the bubbly-to-slug transition was about 0.14 for the 3 mm gap channel and 0.2 for the 5 mm gap channel. respectively. Channel-average void fractions in the channels with fins were almost the same as those in the channels without fins. Void fractions increased as the gap width increased, especially at high superficial velocity of air. The presence of fins enhanced the two-phase distribution parameter significantly in the slug flow, where the effect of gap width was almost negligible. Superficial velocity of air dominated the two-phase pressure gradients. Liquid superficial velocity and channel gap width has only a minor effect on the pressure gradients.

• 한국자동차공학회논문집
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• 제4권1호
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• pp.187-197
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• 1996

Many investigations have been made to determine the pressure drop and heat transfer characteristics for single phase flow in tape generated swirl flow. But few studies have been carried out to investigate the heat transfer in two component, two phase swirl flow with non-boiling. An experimental study has been conducted to determine the effects of tape twist ratios on two phase convective heat transfer coefficients, pressure drop, and void fraction distribution in a non-boiling, air-water, two phase flow. The flow conditions were both swirl and non swirl flows. The internal diameter of the test section is 42.5mm. The tape twist ratios of pitch to diameter ratio varied from 4.0 to 10.6. The heating conditions were isothermal and nonisothermal. The flow patterns identified with experiments were bubbly, bubbly-slug, slug, and slug-annular flow in up-flow. This study has concluded that no significant difference in void fraction distribution were observed both isothermal and nonisothermal conditions, the pressure drop for two phase flow with twisted tape swirler increase as the tape twist ratio decrease, and that values of two phase heat transfer coefficient increase when the tape twist ratio decreases.

• Nuclear Engineering and Technology
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• 제54권4호
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• pp.1495-1507
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• 2022

The improvement of thermal-hydraulic analysis techniques is essential to ensure the safety and reliability of nuclear power plants. The one-dimensional two-fluid model has been adopted in state-of-the-art thermal-hydraulic system codes. Current constitutive equations used in the system codes reach a mature level. Some exceptions are the partition method of wall friction in the momentum equation of the two-fluid model and the interfacial drag force model for a horizontal two-phase flow. This study is focused on deriving the partition method of wall friction in the momentum equation of the two-fluid model and modeling the interfacial drag force model for a horizontal bubbly flow. The one-dimensional momentum equation in the two-fluid model is derived from the local momentum equation. The derived one-dimensional momentum equation demonstrates that total wall friction should be apportioned to gas and liquid phases based on the phasic volume fraction, which is the same as that used in the SPACE code. The constitutive equations for the interfacial drag force are also identified. Based on the assessments, the Rassame-Hibiki correlation, Hibiki-Ishii correlation, Ishii-Zuber correlation, and Rassame-Hibiki correlation are recommended for computing the distribution parameter, interfacial area concentration, drag coefficient, and relative velocity covariance of a horizontal bubbly flow, respectively.