Yeh-Chan Ahn

Flow measurement with an electromagnetic flowmeter in two-phase bubbly and slug flow regimes

In order to investigate the characteristics of an electromagnetic flowmeter in two-phase flow, an alternating-current electromagnetic flowmeter was designed and manufactured. The signals and noise from the flowmeter under various flow conditions were obtained, and analyzed in comparison with the flow patterns observed with a high-speed charge-coupled device camera. n nAn experiment with void simulators, in which a rod-shaped non-conducting material was used, was carried out to investigate the effect of bubble position and void fraction on the flowmeter. Two-phase flow experiments, encompassing bubbly to slug flow regimes, were conducted with a water–air mixture. n nThe simple relation ΔUTP=ΔUSP/(1−α), relating the flowmeter signal between single-phase flow and two-phase flow, was verified with measurements of the potential difference and the void fraction for a bubbly flow regime. Due to the lack of homogeneity in a real two-phase flow, the discrepancy between the relation and the present measurement increased slightly with increasing void fraction and superficial liquid velocity jf. n nWhereas there is no difference in the shape of the raw signal between single-phase flow and bubbly flow, the signal amplitude for bubbly flow is higher than that for single-phase flow at the same water flow rate, since the passage area of the water flow is reduced. In the case of slug flow, the phase and the amplitude of the flowmeter output show dramatically the flow characteristics around each slug bubble and the position of the slug bubble itself. Therefore, the electromagnetic flowmeter shows a good possibility of being useful for identifying the flow regimes.

A modeling of in-tube condensation heat transfer for a turbulent annular film flow with liquid entrainment

Abstract An analytical model is developed for condensation heat transfer coefficients for a turbulent annular film flow in tubes. This calculation model not only incorporates a new turbulent eddy viscosity profile modified from the Blangetti and Schlunder model but also liquid entrainment effect. In order to assess the prediction capability of the calculation model, an experiment was performed for measurement of condensation heat transfer coefficients of R22 in horizontal smooth tubes. The calculated condensation heat transfer coefficients of R22 were compared with present experimental data and some existing correlations. From the present analysis and experiment it is found that incorporating the effect of liquid entrainment into the calculation model gives better prediction capability for in-tube condensation heat transfer coefficients. Also, the proposed turbulent eddy viscosity model gives better prediction capability for the condensation heat transfer coefficients than that of the Blangetti and Schlunder model.

A current-sensing electromagnetic flowmeter for two-phase flow and numerical simulation of the three-dimensional virtual potential distribution: I. Fundamentals and annular flow

The theory of the current-sensing electromagnetic flowmeter for a high temporal resolution was developed for two-phase flow measurements. To predict the output of the current-sensing flowmeter, the three-dimensional virtual potential distribution G and the newly introduced flow pattern coefficient f were derived and computed. The output of the flowmeter depends on the liquid conductivity (sensitive to temperature) and flow configurations of the two-phase flow with sinusoidal excitation over 100 Hz. The flow pattern coefficient was specially devised to separate the dependence on the flow configuration of the two-phase flow from that on the liquid conductivity which can be expressed with the calibration of single-phase flow. Using the finite-difference method, the three-dimensional virtual potential distributions were computed for an electrode of finite size. By taking the derivative of the virtual potential, the weight functions were evaluated. In addition, the flow pattern coefficients were evaluated for annular flows with various film thicknesses, and compared with the experimental results obtained by impedance spectroscopy. The numerical results agreed well with the experimental data.

An experimental study on the characteristics of electromagnetic flowmeters in the liquid metal two-phase flow

Abstract Based on the previous study for the water-air mixture, the characteristics of AC electromagnetic flowmeter were investigated in the liquid metal two-phase flow. Two-phase flow experiments, encompassing bubbly to slug flow regimes, were conducted with the liquid sodium–nitrogen mixture. The simple relation Δ U TP =Δ U SP (1− α ) , relating flowmeter signal between single-phase flow and two-phase flow, was verified with measurements of the potential differences and the void fraction α for a bubbly flow regime in water-air loop. Whereas there was no difference in the shape of the raw signal between single-phase flow and bubbly flow, the signal amplitude of bubbly flow was greater than that of single-phase flow under the same liquid flow (sodium or water) rates, since the passage area of the liquid flow was reduced. In the case of slug flow, the phase and the amplitude of the flowmeter output showed a change in the flow characteristics around each slug bubble and the position of a slug bubble itself. Therefore, the electromagnetic flowmeter shows a good potential as a useful device for identifying the flow regimes. The void fraction can be simply measured with two electromagnetic flowmeters for a low-void fraction flow such as bubbly flow. Thus, the flowmeters can be used to measure the void fraction for a liquid sodium.

The Performance of Electromagnetic Flowmeters in a Liquid Metal Two-Phase Flow

In order to investigate the characteristics of an electromagnetic flowmeter in a liquid metal two-phase flow, AC electromagnetic flowmeters were designed and manufactured. Two-phase flow experiments, encompassing bubbly to slug flow regimes, were realized with a water-air mixture and a liquid sodium-nitrogen mixture, respectively. The relation ΔU TP =ΔU SP /(1—α), between the flow induced electromotive force (e.m.f.) of a single-phase flow and that of a two-phase flow, was verified by the measured e.m.f.s and the void fraction α for a bubbly flow regime. Whereas there is no difference in the shape of the flow induced e.m.f. between single-phase and bubbly flow, the signal amplitude of the bubbly flow is greater than that of the single-phase flow under the same water or liquid sodium flow rate, since the real liquid velocity is higher than that of the single-phase flow. In the case of a slug flow, the phase and the amplitude of the flowmeter output changed drastically when a slug bubble passed through the flowmeter. Therefore, the electromagnetic flowmeter shows good potential as a useful device for identifying the flow regimes. The void fraction in a bubbly flow can be measured using two electromagnetic flowmeters.

Measurement of the Void Fraction of Slug and Bubbly Flows Using Three-Ring Impedance Meters

Real slug and bubbly flows were measured using a three-ring impedance meter that can efficiently measure the void fraction of two-phase flows in a tube. First, the fitting curves between the signal from the impedance meters and the void fraction were found. The impedance meter had different fitting curves for slug and bubbly flows that had the same void fraction. An impedance meter should choose one of the two fitting curves according to the flow pattern, and the flow patterns can be recognized using the measured void fraction. The velocities and sizes of the bubbles were calculated using the void fraction curves measured by two impedance meters.

Experimental Study of Characteristics of Three-Ring Impedance Meter and Dependence of Characteristics on Electric Conductivity of Fluids

Key Words: 2-Phase Flow(2상유동), Void Fraction(기공률), Impedance Meter(임피던스미터), Temperature(온도), Electric Conductivity(전도도)초록: 2상유동(기체-액체) 현상은 전자기발전, 원자력발전, 철강산업 등 유체 시스템에 자주 나타나는 현상으로, 2상유동을 파악하는 것은 유체 시스템의 안정성 및 성능을 위해 중요하다. 2상유동 특성 중 기공률은 압력강하와 열전달 성능을 결정하는 주요 인자로서 이를 측정하는 기술이 특히 중요하다. 유동의 임피던스를 측정하여 기공률을 산정하는 임피던스법은 전기적 특성을 이용하기 때문에 반응속도가 빨라 실시간 측정이 가능하며, 유동관 벽에 전극을 설치할 경우 유동 교란없이 측정할 수 있는 장점이 있다. Coney는 원형관에 적용할 수 있는 ring 임피던스미터를 이론적 연구하였다. 본 연구에서는 Coney가 이론적으로 제안하고 실험적으로 검증하지 못한 3-ring 임피던스미터의 유체 전기전도도에 대한 독립성을 실험적으로 검증하였다. Abstract: A two-phase (gas-liquid) flow is a common phenomenon in fluidic systems, e.g., fluidic systems in the electro-magnetic or nuclear power generation industry and in the steel industry. The measurement of a two-phase flow is important for guaranteeing the safety of the system and for achieving the desired performance. The measurement of the void fraction, which is one of the parameters of the two-phase flow that determines the pressure drop and heat transfer coefficient, is very important. The time resolution achieved by employing the impedance method that can be used to calculate the void fraction from the impedance of the fluid is high because the electric characteristics are taken into account. Therefore, this method can be employed to accurately measure the void fraction without distortion of flow in real time by placing electrodes on the walls of the tubes. Coney analytically studied a ring-type impedance meter, which can be employed in a circular tube. The aim of this study is to experimentally verify the robustness of a three-ring impedance meter to variations in the electric conductivity of the fluid; this robustness was suggested by Coney but was not experimentally verified.

Theoretical foundation for an electromagnetic two-phase flowmetry

Abstract The theory for the current-sensing electromagnetic flowmeter was newly developed. The current-sensing flowmeter can achieve the measurement with a high temporal resolution so that it can be applied to measure the flows with fast transients like two-phase flow. The signal prediction and the calibration of the current-sensing flowmeter in simplified two-phase flow were conducted, and the given calibration process would be an important step toward the calibration for real two-phase flow. The three-dimensional virtual potential distributions for the electrodes of finite size were computed for single-phase flow, annular flow, and modeled slug flow. With the gradient of the virtual potential, weight functions related to each flow pattern were deduced. A flow pattern coefficient f was introduced to simplify the calibration process for two-phase flow and measured with the impedance spectroscopy method. In order to measure the local mean velocity of a developing flow using the electromagnetic flowmeter, a localization parameter χ was modeled and compared with experimental data.

A Real-Time Measurement of Slug Flow Using Electromagnetic Flowmeter with High frequency Triangular Excitation

In order to investigate the characteristics of two-phase slug flow, an electromagnetic flowmeter with 240Hz triangular AC excitation was designed and manufactured. The signals and noise from the flowmeter were obtained, and analyzed in comparison with the observations with a high speed CCD camera. The uncertainty of the flowmeter under single-phase flow was 2.24% in real-time. For two-phase slug flow, electromagnetic flowmeter provided real-time simultaneous measurements of the mean film velocity around Taylor bubble and the relative location and the length of the bubble. Besides, it is an easier and cheaper method for measuring mean film velocity than others such as photochromic dye activation method or particle image velocimetry.

Flow Measurement in Bubbly and Slug Flow Regimes Using The Electromagnetic Flowmeter Developed

In order to investigate the characteristics of electromagnetic flowmeter in two -phase flow, an AC electromagnetic flowmeter was designed and manufactured. In various flow conditions, the signals and noises from the flowmeter were obtained and analyzed by comparison with the observed flow patterns with a high speed CCD camera. The experiment with the void simulators in which rod shaped non-conducting material was used was carried out to investigate the effect of the bubble position and the void fraction on the flowmeter. Based on the results from the void simulator, two -phase flow experiments encompassed from bubbly to slug flow regime were conducted. The simple relation