Earticle

The present study experimentally investigated the effect of flow direction and other flow parameters on two-phase flow distribution of refrigerants at a T-junction, and also suggested a prediction model for refrigerant in a T-junction by modifying previous model for air-water flow. R-22, R-134a, and R-410A were used as test refrigerants. As geometric parameters, the direction of the inlet or branch tube and the tube diameter ratio of branch to inlet tube were chosen. The measured data were compared with the values predicted by the models developed for air-water or steam-water mixture in the literature. We propose a modified model for application to the reduced T-junction and vertical tube orientation. Among the geometric parameters, the branch tube direction showed the biggest sensitivity to the mass flow rate ratio for the gas phase, while the inlet quality showed the biggest sensitivity to the mass flow rate ratio among the inlet flow parameters.

Chemical mechanical polishing (CMP) is a very precise planarization technique where a wafer is polished by a slurry-coated pad. A slurry is dropped on the rotating pad surface and is supplied between the wafer and the pad. This research aims at reducing the slurry consumption and removing waste particles quickly from the wafer. To study the roles of grooves, slurry flows were simulated using the volume of fluid method (two-phase model for air and slurry) for pads with no grooves, and for pads with circular grooves.

We carried out numerical studies to investigate the single- and two-phase flow characteristics in the single- and multi-channels. We used the finite volume method to solve the mass and momentum conservation equations. The volume of fluid model is used to predict the two-phase flow in the channel. We obtained the distribution of velocity fields, pressure drop and air volume fraction for different water mass flow rates. We also calculated the distribution of mass flow rates in the multi-channels to understand how the flow is distributed in the channels. The calculated results for the single- and two-phase flow are partly compared with the present experimental data both qualitatively and quantitatively, showing relatively good agreement between them. The numerical scheme used in this study predicts well the characteristics of single-and two-phase flow in a multi-channel.

The aerodynamic effects of leading-edge accretion can raise important safety concerns since the formulation of ice causes severe degradation in aerodynamic performance as compared with the clean airfoil. The objective of this study is to develop a numerical simulation strategy for predicting the particle trajectory around an MS-0317 airfoil in the test section of the NASA Glenn Icing Research Tunnel and to investigate the impingement characteristics of droplets on the airfoil surface. In particular, predictions of the mean velocity and turbulence diffusion using turbulent flow solver and Continuous Random Walk method were desired throughout this flow domain in order to investigate droplet dispersion. The collection efficiency distributions over the airfoil surface in simulations with different numbers of droplets, various integration time-steps and particle sizes were compared with experimental data. The large droplet impingement data indicated the trends in impingement characteristics with respect to particle size ; the maximum collection efficiency located at the upper surface near the leading edge, and the maximum value and total collection efficiency were increased as the particle size was increased. The extent of the area impinged on by particles also increased with the increment of the particle size, which is similar as compared with experimental data.

Pulsatile flow of a two-phase model for blood flow through arterial stenosis is analyzed through a mathematical analysis. The effects of pulsatility, stenosis, peripheral layer and non-Newtonian behavior of blood, assuming the blood in the core region as a Herschel-Bulkley fluid and the plasma in the peripheral layer as a Newtonian fluid, are discussed. A perturbation method is used to solve the resulting system of non-linear quasi-steady differential equations. The expressions for velocity, wall shear stress, plug core radius, flow rate and resistance to flow are obtained. It is noticed that the plug core radius and resistance to flow increase as the stenosis size increases while all other parameters held constant. The wall shear stress increases with the increase of yield stress while keeping other parameters as invariables. It is observed that the velocity increases with the axial distance in the stenosed region of the tube upto the maximum projection of the stenosis.

본 연구는 수평관 내 물-공기 이상류에서 측정된 유속변동성분에 대한 주파수 분석을 수행하고, 이를 통해 단면 내 유동구조와 유효흐름단면 변화를 해석하였다. PIV 기법으로 측정된 유속자료를 이용하여 지점별 유속변동성분의 에너지스펙트럼밀도(Power Spectral Density, PSD)를 산정하였으며 저주파 대역의 적분값을 기반으로 에너지스펙트럼지표를 정의하였다. 분석 결과, 흐름 내 액체 지배영역과 기체 지배영역 간 상이한 스펙트럼 특성이 나타났으며 흐름양상에 따라 변동에너지의 분포와 특성위치에 차이가 나타났다. 이를 기체점유율 기준 상 경계와 비교한 결과, 두 기준은 서로 다른 물리적 관점을 반영하여 상 경계의 절대적 위치에는 차이를 보였으나 흐름양상에 따른 공간적 변화 경향은 유사하게 나타났다. 이러한 결과는 유동구조를 해석하는 기준에 따라 상 경계의 해석 결과가 달라질 수 있음을 보여주며 공기 혼입에 따라 실제 액체흐름에 기여하는 통수 단면이 변화함을 시사한다. 따라서 관로시스템의 설계 및 통수능 산정 시 공기 혼입에 따른 유동구조 변화를 고려할 필요가 있다.

Frequency analysis was conducted on velocity fluctuations measured in air-water mixture flowing through a horizontal pipe. The analysis was used to interpret cross-sectional flow structure and effective flow area. The velocity field was measured using Particle Image Velocimetry (PIV), and the Power Spectral Density (PSD) of the velocity fluctuations was estimated at each measurement location. A spectral energy index was defined based on the integral of PSD over the low-frequency range. The results show that the liquid-dominated and gas-dominated regions exhibited intrinsic spectral characteristics. The distribution of fluctuation energy and the characteristic location varied depending on the flow regime. Comparison with the phase boundary defined by the void fraction revealed that the two criteria reflect different physical perspectives. Although the absolute position of the phase boundary differs between the two criteria, comparable spatial variations with the flow regime were observed. These results reveal that the location of the phase boundary depends on the analysis criterion applied in the two approaches. The effective flow area contributing to actual liquid conveyance can also vary with entrained air volume. Thus, the influence of air entrainment on the flow structure should be considered in the design and hydraulic capacity evaluation of conduit systems.

본 연구에서는 수평관을 흐르는 물-공기 이상류에 대한 실험을 통해 흐름의 주기적 특성을 고려한 기체점유율(void fraction)과 유효흐름단면을 산정하였다. 실험은 내경 100 mm 수평관에서 플러그 및 슬러그 흐름 조건을 대상으로 수행되었으며 PIV 기법을 이용한 유속구조 실험과 영상분석기법을 적용한 기포거동 실험을 각각 수행하였다. 실험결과를 통해 흐름 내 기체점유율 분포는 유량 조건보다 흐름양상 특성에 지배되는 것을 확인하였다. 플러그 흐름에서는 기체가 관 상부에 집중되어 관 내 기체점유율 분포에서 급격한 변화를 보였으며 슬러그 흐름에서는 기체가 단면 전반으로 확산되며 상대적으로 완만한 분포를 관찰할 수 있었다. 기체점유율이 높은 상부 영역에서는 PIV 유효 벡터비가 감소하였으며 해당 구간이 시간평균 유속장과 순간 유속장 간 차이가 발생하는 경계로 작용함을 확인하였다. 이에 따라 해당 구간을 기체 점유 영역의 경계로 정의하여 흐름양상별 유효흐름단면을 산정하였다. 그 결과 플러그 흐름과 슬러그 흐름 간 유효흐름단면에는 최대 20 % 차이를 확인하였으며 이는 흐름양상에 따라 실제 액체수송에 기여하는 유효단면적의 변화가 발생할 수 있음을 의미한다. 이러한 결과는 이상류가 발생하는 대심도 터널과 같은 대구경 관로에서 흐름양상에 따라 실제 배수능력이 설계 성능에 미치지 못할 수 있음을 의미한다. 따라서 관로시스템 설계 및 운영 시 공기 혼입에 따른 유효흐름단면 감소와 그에 따른 통수능 변화를 고려할 필요가 있다.

A series of experiments has been conducted to analyze the void fraction and effective flow area of two-phase flow in a horizontal pipe. Experiments were performed in a 100 mm diameter horizontal pipe under plug and slug flow conditions. The velocity field was measured using Particle Image Velocimetry (PIV) and bubble behavior was analyzed using an image processing technique. The in-pipe distribution curve of void fraction was governed by the flow regime rather than the flowrate. For plug flow, a sharp change in the cross-sectional distribution curve of void fraction can be observed due to the accumulation of air in the upper part of a pipe. While the more even distribution of air across a relatively wider area, in slug flow, results in a gradual shape for void fraction distribution profile. The upper part, which shows high void fraction and low PIV valid vector ratio, coincides with the boundary where the difference between the time-averaged and instantaneous velocity fields appears. This boundary can be defined as the edge of the region occupied by gas, this implies that the change of effective flow area contributing to actual liquid conveyance depends on the flow pattern. The effective flow area differed by up to 20 % between plug and slug flows, showing a decrease in the effective liquid flow area depending on the flow pattern. These results suggest that air entrainment should be considered in the design and operation of large-diameter conduit systems to ensure stable hydraulic performance.

양광시스템은 심해저 광물자원 개발의 성패를 좌우하는 중요한 요소이다.한국지질자원연구원(KIGAM)에서는 지금까지 양광시스템 개발연구를 수행해오고 있으며 최근에는 30 m 급 육상 양광시험시스템을 구축하였다.본 연구는 양광관내에서 고-액 유동특성을 규명하고 향후 실해역 채광시스템의 설계자료를 획득하기 위해 수력펌핑양광실험을 수행하였다. 기수행된 소규모 수력펌핑양광실험 결과에 기초하여 30 m 급 육상 수력펌핑 양광시스템이 설계되었다.본 연구에서 수행된 고-액 2상 유동실험은 실제 망간단괴와 동일한 밀도를 가진 직경 20 mm의 모조단괴가 직경 100 mm의 양광관내를 물과 함께 유동할 때 최대 유속은 5 m/sec, 최대 고체체적점유율은 20%의 조건에서 수행되었다.본 실험으로부터 양광관 내에서의 고체체적점유율과 유동특성 간의 관계를 해석하였다.본 연구결과는 향후 수행될 근해역 채광실험과 실해역 채광시스템의 설계자료로 이용될 것이다.

The lifting technology plays a key role in the success of deep-sea mineral development. KIGAM (Korea Institute of Geoscience and Mineral Resources) has striven to develop it, and recently performed 30meter scale on-land lifting experiments. In the study, the hydraulic pumping test has been conducted to understand the flow characteristics of solid-liquid mixture in a lifting pipe and to acquire the design data for real offshore test that will be performed in the future. Firstly, an on-land hydraulic pumping test system of 30 meters in height was designed on the basis of the results in the small scale hydraulic pumping test previously performed. In the experiment of this study, we performed solid-liquid two phase flow tests on the condition that the maximum flow velocity was 5 m/sec and maximum solid volume fraction was 20 %,where lifting pipes were 100 mm in diameter together with 20 mm synthetic manganese nodules of the same density asthe real ones. From the tests we could analyze the relationship between solid volume fraction and flow characteristics in pipes. The results of this study will be used as a basis for the off-shore mining field experiments in the near future and for the design of the deep-sea mining system.

In this study, the visual integrated model that is available to predict the future productivity of production well has been developed. This model which is based on conventional decline curve analysis method is for two phase decline curve analysis. This model is composed of three modules. First one is a decline type curve module that can analyze reservoir characteristics such as permeability, skin factor et al. using transient production data. The second is statistical-decline curve module which is able to predict productivity with high confidence level by using production data in depletion stage. The third one is the two phase (oil-water) decline curve module for the future performance of high WOR well. For the utilization of these modules, a user-friendly visual model of the integrated decline curve analysis was developed in this study. In order to validate the developed model, the decline curve analysis on the production data containing large amount of water in SES producing oil field, Indonesia, was carried out. As a results, in case of the well showing a conventional decline curve trend, more reliable prediction could be obtained through the statistical decline curve analysis module. In the analysis of the well producing large amount of water in waterflooded reservoir, the result of decline curve analysis based on two phase flow shows relatively excellent producing performances even though the enhanced production data is not included in the analysis. Also, the radius of investigation, permeability and skin factor at both transient and depletion stage were calculated from the model. It is found that the matching result between actual data and type curves is correct.

침투능을 계산하기 위한 공식으로서 Green-Ampt 공식은 식의 간편성과 정확성으로 인하여 많은 연구의 대상이 되어왔다. 그러나 이 공식의 변수인 습윤전선(Wetting Front)에서의 모세관 압력수두항은 연구자에 따라 해석이 많이 다르다. 침투능해석을 위하여 2개 유체의 흐름방정식의 해는 Green-Ampt 공식과 같은 형태를 가지므로서 Green-Ampt 공식의 모세관 압력 수두항을 결정할 수 있게 되었다. 이러한 2개 유체 해석에 의한 침투능 산정공식의 유도는 1개층으로만 구성되어 있는 경우에 이루어 졌으나 특성이 각각 다른 여러 층의 흙으로 구성 되어 있는 경우에는 아직 이루어진 바 없다. 본 논문에서는 이러한 2개 유체의 흐름 해석에 의하여 여러층의 흙인 경우 침투능 산정공식을 유도하였다.

The Green-Ampt equation for infiltration has been intensively investigated by many researchers because of its simplicity and adequacy for fitting experimental data to theoretical one. The infiltration equation derived from the theory of two phase flow coincides with the Green-Ampt equation except the viscouse resistance correction factor. This approach clearly defines variables in the Green-Ampt equation and also encounters the effect of viscosity of two fluids. A new equation for infiltration into multilayered soil is derived from the theory of two phase flow and compared with conventional equation. The new equation shows lower infiltration rate than that of conventional one and it is believed that this caused from the inclusion of viscosity in the derivation.

Due to the advantages of scale invariant feature transform (SIFT) feature points on the invariant to image scale, brightness, rotation, occlusion, noise and so on, this paper proposes a Particle Tracking Velocimetry (PTV) method, based on SIFT feature points matching for velocity measurement of oil-water two-phase flow in horizontal pipelines. The oil-water two-phase flow with large droplet diameter, oil droplets overlap, ununiform lighting, and the centroid position of oil droplets can’t be obtained only by using traditional PTV methods through morphological processing. However, in this paper, the algorithm can directly achieve the average velocity of the flow field according to the positions of correctly matched SIFT feature points, and there is no need to extract the centroid coordinates of each oil droplet. The experimental results show that the proposed algorithm not only can be used in the average velocity measurement of oil-water two-phase flow in horizontal pipelines, but also can reach 95% in the measuring accuracy when the matching feature points are enough sufficient.

A semi-solid forming technology has some advantages compared with conventional forming processes such as die casting, squeeze casting and hot/cold forging. In this study, the numerical analysis of semi-solid filling has been studied with solid fraction fs = 30% of A356 aluminum alloys. The finite difference program of two-phase flow model of Navier Stokes' equation coupled with heat transfer and solidification has been developed to predict a filling pattern, liquid segregation and temperature distribution of semi-solid metals. It gives die filling patterns and final solidification area. It can predict mechanical properties of semi-solid forming processes.

Recently, coal-fired power plants are experiencing many problems that they have never experienced before due to an increase in flexible operation. In particular, a two-phase flow accelerated corrosion on water wall tubes in a boiler has not been detected overseas or domestically. There is no response plan to deal with such corrosion problem either. However, oxide film damage and tube material corrosion due to a two-phase flow accelerated corrosion are being discovered on water wall boiler tubes of domestic coal-fired power plants recently. If this situation is severe, it can cause enormous damage such as tube rupture. Therefore, in this paper, in order to prepare a response plan for a two-phase flow accelerated corrosion on water wall tubes in the future, differences between a two-phase flow accelerated corrosion and a single-phase flow accelerated corrosion were investigated and an example of discovery of a two-phase flow accelerated corrosion on water wall tubes was presented.

Marine applications involving attached bubbles on propellers and underwater vehicles are obvious examples of cavitation phenomena. For naval architecture issues, cavitation is always a very important topic relating to high-speed vessels. However, the high-speed propulsors including water jet system, super-cavitating propeller and surface-piercing propeller are facing critical challenge from cavitation problems. For this reason, it is necessary to make a thorough study of cavitation phenomena. Due to the complexities and difficulties of theoretical analysis of cavitation, most of the conducted researches are based on experimental approaches. With the fast development of computational fluid dynamics techniques, computations of turbulent flow for cavitation problems seem to become feasible. In order to simulate sheet cavitation flows, the presented method started from a two-dimensional flow code capable of computing turbulent hydrofoil flows. In order to take cavitation region into account, a two-phase flow scheme was further implemented in this code. The proposed approach treats gas and liquid phases as an effective fluid with density varied in space. The flow field is computed in both phases with vapor pressure recovered inside the cavitation cavity via a pressure-velocity-density coupling relation. The computation was iteratively conducted, until the shape of the sheet cavitation became stable. The steady sheet cavitation flow of a two-dimensional hydrofoil was simulated using a two-phase approach. The size and shape of sheet cavitation, and the lift and drag coefficient of a cavitating hydrofoil were numerically predicted, and compared with existing experimental measurements. The applied numerical scheme was too diffusive due to a first order approximation scheme. Further improvement of accuracy is required.

Spiral tubes are used in a wide range of applications and it is significant to understand the vibration introduced by two-phase flow in spiral tubes. In this paper, the numerical method is used to study the vibration induced by the gas-liquid two-phase flow in a spiral tube with different flow regimes. The pressure fluctuation characteristics at the pipe wall and the solid vibration response characteristics are obtained. The results show that the motion of small bubbles in bubbly flow leads to small pressure fluctuations with low-frequency broadband (0-50 Hz). The motion of the gas plug in the plug flow causes small amplitude periodic pressure fluctuation with a shortened low-frequency broadband (0-15 Hz) compared to the bubbly flow. The motion of the gas slug in the slug flow causes large periodic fluctuations in pressure with a significant dominant frequency (6-7 Hz). The wavy flow is very stable and has a distinct main frequency (1-2 Hz). The vibration regime in the bubbly flow and wave flow are close to the first-order mode, and the vertical vibrating component is dominant. The plug flow and slug flow excite higher-order vibration modes, and the lateral vibration component plays more important part in the vibration response.

Narrow rectangular channels are employed in nuclear research reactors that use plate-type nuclear fuels, high heat-flux compact heat exchangers, and high-performance micro-electronics cooling systems. Two-phase flow in narrow rectangular channels is important, and it needs to be better understood because it is considerably different than that in round tubes. In this study, mechanistic models were developed for the flow regime transition criteria for various flow regimes in co-current air-water two-phase flow for vertical downward flow inside a narrow rectangular channel. The newly developed criteria were compared to a flow regime map of downward air-water two-phase flow inside a narrow rectangular channel with a 2.35-mm gap width under ambient temperature and pressure conditions. Overall, the proposed model showed good agreement with the experimental data.

A new instrument, an average BDFT (Birectional Flow Tube), was proposed to measure the flow rate in single and two phase flows. Its working principle is similar to that of the Pilot tube, wherein the dynamic pressure is measured. In an average BDFT, the pressure measured at the front of the flow tube is equal to the total pressure, while that measured at the rear tube is slightly less than the static pressure of the flow field due to the suction effect downstream. The proposed instrument was tested in air/water vertical and horizontal test sections with an inner diameter of 0.08m. The tests were performed primarily in single phase water and air flow conditions to obtain the amplification factor(k) of the flow tube in the vertical and horizontal test sections. Tests were also performed in air/water vertical two phase flow conditions in which the flow regimes were bubbly, slug, and churn turbulent flows. In order to calculate the phasic mass flow rates from the measured differential pressure, the Chexal drift-flux correlation and a momentum exchange factor between the two phases were introduced. The test results show that the proposed instrument with a combination of the measured void fraction, Chexal drift-flux correlation, and Bosio & Malnes' momentum exchange model could predict the phasic mass flow rates within a $15\%$ error. A new momentum exchange model was also proposed from the present data and its implementation provides a $5\%$ improvement to the measured mass flow rate when compared to that with the Bosio & Malnes' model.