• Transactions of the Korean Society of Mechanical Engineers
• /
• 제11권5호
• /
• pp.810-818
• /
• 1987

A new method is proposed to identify two-phase flow regimes in horizontal gas-liquid flow, based upon a statistical analysis of instantaneous pressure drop curves at an orifice. The probability density functions of the curves indicate distinct patterns depending upon the two-phase flow regime. The transition region also could be identified by the distribution shape of the probability density function. The statistical properties of the pressure drop are analyzed for various flow regimes and transitions. Finally, the data of flow patterns determined by the proposed method are compared with the flow pattern maps suggested by other investigators.

• Nuclear Engineering and Technology
• /
• 제44권8호
• /
• pp.871-888
• /
• 2012

This paper aims at presenting the state of the art, the recent progress, and the perspective for the future, in the modelling of two-phase flow in the horizontal legs of a PWR. All phenomena relevant for safety analysis are listed first. The selection of the modelling approach for system codes is then discussed, including the number of fluids or fields, the space and time resolution, and the use of flow regime maps. The classical two-fluid six-equation one-pressure model as it is implemented in the CATHARE code is then presented and its properties are described. It is shown that the axial effects of gravity forces may be correctly taken into account even in the case of change of the cross section area or of the pipe orientation. It is also shown that it can predict both fluvial and torrential flow with a possible hydraulic jump. Since phase stratification plays a dominant role, the Kelvin-Helmholtz instability and the stability of bubbly flow regime are discussed. A transition criterion based on a stability analysis of shallow water waves may be used to predict the Kelvin-Helmholtz instability. Recent experimental data obtained in the METERO test facility are analysed to model the transition from a bubbly to stratified flow regime. Finally, perspectives for further improvement of the modelling are drawn including dynamic modelling of turbulence and interfacial area and multi-field models.

• Journal of the Korean Society of Visualization
• /
• 제10권1호
• /
• pp.15-20
• /
• 2012

PCV(Positive Crankcase Ventilation) system is designed to remove blowby gas. In this system, a PCV valve is attached in a manifold suction tube to control the flow rate of blowby gas which generates various operating conditions of an automotive engine. As this valve plays a crucial role, the demand in its design is high owing to the small size and high velocity. For this reason, a numerical investigation was carried out to understand both the spool dynamic motion and internal fluid flow characteristics. As a result, the spool dynamic characteristics(i.e. displacement, velocity, acting force), increase in direct proportion to the magnitude of the pressure difference and indicate periodic oscillating motions. Moreover, the velocity at the orifice region decreases according to the increase in differential pressure due to energy loss caused by the sudden decrease of flow area at the orifice region and the increase of flow volume in front of the spool head. Finally, the mass flow rate at the outlet decreases with the increase of spool displacement.

• Nuclear Engineering and Technology
• /
• 제47권7호
• /
• pp.804-813
• /
• 2015

The electrical-impedance method has been widely used for void-fraction measurement in two-phase flow due to its many favorable features. In the impedance method, the response characteristics of the electrical signal heavily depend upon flow pattern, as well as phasic volume. Thus, information on the flow pattern should be given for reliable void-fraction measurement. This study proposes an improved electrical-conductance sensor composed of a three-electrode set of adjacent and opposite electrodes. In the proposed sensor, conductance readings are directly converted into the flow pattern through a specified criterion and are consecutively used to estimate the corresponding void fraction. Since the flow pattern and the void fraction are evaluated by reading conductance measurements, complexity of data processing can be significantly reduced and real-time information provided. Before actual applications, several numerical calculations are performed to optimize electrode and insulator sizes, and optimal design is verified by static experiments. Finally, the proposed sensor is applied for air-water two-phase flow in a horizontal loop with a 40-mm inner diameter and a 5-m length, and its measurement results are compared with those of a wire-mesh sensor.

• Nuclear Engineering and Technology
• /
• 제21권2호
• /
• pp.123-133
• /
• 1989

The pressure drops through contractions in horizontal single and two-phase flow were investigated. A total of 167 measurements were made for four different entrance shapes to study the effects of the entrance shape on the pressure drop through a contraction in horizontal single and two-phase flow. From this data, pressure drops were calculated and compared with the pressure drops predicted by analytical models for single and two-phase flow. For single phase How the agreement between the data and predictions is within $\pm$25%, whereas for two-phase flow Hoopes model, which gives a better agreement than the homogeneous model, underpredicts the data as much as 45% In addition, the effects of void fraction and liquid phase mass velocity on the pressure drop through the sudden How channel contraction were investigated for two-phase flow.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
• /
• 제16권5호
• /
• pp.414-420
• /
• 2004

The heat transfer and pressure drop characteristics associated with the gas cooling of the supercritical carbon dioxide in a horizontal tube have been investigated experimentally. This problem is of particular interest in the design of a gas cooler of cooling systems using $CO_2$refrigerant. The test section is consisted of 6 series of 455 mm in length, 4.15 mm ID copper tube, respectively. The effects of the inlet temperature, pressure and mass flow rate on the heat transfer and pressure drop of $CO_2$in a horizontal tube is studied in detail. The heat transfer coefficient of $CO_2$is varied by temperature, inlet pressure, and mass flow rate of $CO_2$. This has maximum value at near the pseudocritical temperature. The pressure drop is changed by inlet pressure and mass flow rate of $CO_2$. The results have been compared with those of previous work. The heat transfer correlation at the supercritical gas cooling process is also suggested.

• The KSFM Journal of Fluid Machinery
• /
• 제5권4호
• /
• pp.32-39
• /
• 2002

The purpose of this 3-D numerical simulation is to calculate and examine the complex 3-D stall phenomena on the rotor blade and wake distribution of the wind turbine. The flow characteristics of 500kW Horizontal Axis Wind Turbine (HAWT) are compared with the calculated 3-D stall phenomena and wake distribution. We used the CFX-TASCflow to predict flow and power characteristics of the wind turbine. The CFD results are somewhat consistent with the BEM (Blade Element Momentum) results. And, the rotational speed becomes faster, the 3-D stall region becomes smaller. Moreover, the pressure distribution on the pressure side that directly gets the incoming wind grows high as it goes toward the tip of the blade. The pressure distribution on the blade's suction side tells us that the pressure becomes low in the leading edge of the airfoil as it moves from the hub to the tip. However, we are not able to precisely predict on the power coefficient of the rotor blade at the position of generating complex 3-D stall region.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
• /
• 제10권6호
• /
• pp.732-742
• /
• 1998

Prediction method for heat transfer coefficients in a horizontal smooth tube with forced convection condensation is proposed. In this paper, the analogy between momentum and heat transfer was applied to an annular flow regime and the logarithmic velocity distribution is applied to describe the velocity profile within the liquid film. Prediction results are compared with those of experimental ones. The test refrigerants are R113, R22, R134a, R407C(R33/R125/R134a, 23/25/52 wt%), R410A(R32/R125, 50/50 wt%) and R134a＋R123(R134a/R123, 85.5/14.5 wt%) which are used under operating conditions in a condenser of air-conditioner. The proposed prediction method shows good agreement with experimental data within$\pm 30%$ for pure refrigerants. For the mixture refrigerants including the ternary mixture refrigerant R407C, condensation heat transfer from this study are higher than those from experiments. By correcting the constant in two-phase frictional multiplier, the predicated heat transfer coefficients become similar to the experimental results.

• Journal of Mechanical Science and Technology
• /
• 제18권3호
• /
• pp.513-525
• /
• 2004

This paper reports an experimental study on flow boiling of pure refrigerants R l34a and R l23 and their mixtures in a uniformly heated horizontal tube. The flow pattern was observed through tubular sight glasses with an internal diameter of 10㎜ located at the inlet and outlet of the test section. Tests were run at a pressure of 0.6 MPa in the heat flux ranges of 5-50㎾/㎡, vapor quality 0-100 percent and mass velocity of 150-600㎏/㎡s. Both in the nucleate boiling-dominant region at low quality and in the two-phase convective evaporation region at higher quality where nucleation is supposed to be fully suppressed, the heat transfer coefficient for the mixture was lower than that for an equivalent pure component with the same physical properties as the mixture. The reduction of the heat transfer coefficient in mixture is explained by such mechanisms as mass transfer resistance and non-linear variation in physical properties etc. In this study, the contribution of convective evaporation, which is obtained for pure refrigerants under the suppression of nucleate boiling, is multiplied by the composition factor by Singal et al. (1984). On the basis of Chen's superposition model, a new correlation is presented for heat transfer coefficients of mixture.

• KSBB Journal
• /
• 제18권6호
• /
• pp.506-510
• /
• 2003

The artificial sewage was treated using a horizontal flow zeolite-filled reed bed. The artificial sewage was fed into the reed bed for 10 minutes every 6 hours at the hydraulic load of 314 L/㎡$.$day. The filtering height of the reed bed was 100 cm and the zeolite mixture was filled in the reed bed. The mixttjre consisted of the same volume of two types of zeolite: 0.5∼l mm and 1∼3 mm in diameter. Annual average removal efficiency was CODcr 95.8%, T-N 56.5％, NH$\_$4/$\^$＋/-N 99.4％ and T-P 61.3%. T-N removal efficiency decreased remarkably from 84 to 32％ with the operating time.