Basar Ozar

Characteristics of Bubble Departure Frequency in a Low-Pressure Subcooled Boiling Flow

In order to measure the bubble departure frequency, a flow visualization system was set up on a vertical annulus test section with a heater rod by using a high-speed camera. In this study, we developed an efficient methodology of image processing for obtaining the bubble departure frequency data. Bubble nucleation was investigated under various thermal hydraulic conditions of water, which correspond to pressures from 167 to 346 kPa, mass fluxes from 214 to 1869 kg/m2s, heat fluxes from 61 to 238 kW/m2, and subcooling degrees from 7.5 to 23.4 K. The characteristics of bubble departure frequency were analyzed with the present data. The measured data was compared with models available in existing literature and a more plausible model was proposed.

Axial Development of Flow Regime in Adiabatic Upward Two-Phase Flow in a Vertical Annulus

This study has investigated the axial development of flow regime of adiabatic upward air-water two-phase flow in a vertical annulus. The inner and outer diameters of the annulus are 19.1 mm and 38.1 mm, respectively. The hydraulic diameter of the flow channel, DH, is 19.0 mm and the total length is 4.37 m. The flow regime map includes 72 flow conditions within a range of 0.01 m/s<⟨jg⟩<30 m/s and 0.2 m/s<⟨jf⟩<3.5 m/s, where ⟨jg⟩ and ⟨jf⟩ are, respectively, superficial gas and liquid velocities. The flow regime has been classified into four categories: bubbly, cap-slug, churn, and annular flows. In order to study the axial development of flow regime, area-averaged void fraction measurements have been performed using impedance void meters at three axial positions corresponding to z/DH=52, 149, and 230 simultaneously, where z represents the axial position. The flow regime indicator has been chosen to be statistical parameters from the probability distribution function of the area-averaged void fraction signals from the impedance meters, and self-organized neural networks have been used as the mapping system. This information has been used to analyze the axial development of flow regime as well as to check the predictions given by the existing flow regime transition models. The axial development of flow regime is quantified using the superficial gas velocity and void fraction values where the flow regime transition takes place. The predictions of the models are compared for each flow regime transition. In the current test conditions, the axial development of flow regime occurs in the bubbly to cap-slug (low superficial liquid velocities) and cap-slug to churn (high superficial liquid velocities) flow regime transition zones.

Flow Regime Identification in Large Diameter Pipe

Air-water vertical two-phase flow experiments were performed in a 0.15 m diameter and 4.4 m long test section. Superficial liquid velocity was varied from 0.05 m/s to 2.0 m/s and superficial gas velocity was varied to obtain the area averaged void fraction range of 0.1 to 0.7. Exit pressure was close to the atmospheric pressure. In order to study the development of flow structure over the length of test section, area averaged void fraction was measured using impedance meters at four different measuring ports. Pressure drop was also measured between these ports. Since the temporal variation of void fraction signal obtained from the impedance meter and its distribution are characteristic of the flow regime, a Cumulative Probability Distribution Function (CPDF) of the void fraction signal was utilized for the identification of flow regime at each port. The CPDFs of the impedance probe void fraction signal were supplied as an input to the Kohonen Self Organized neural network or the Self Organized Map (SOM) for the identification of the patterns by employing self organized neural network technique. The three flow regimes identified by the neural network are subjectively named as bubbly flow, cap-bubbly flow and cap-turbulent flow.Copyright

Local and Area-Averaged Flow Structure of Air-Water Two-Phase Flow in a Vertical Annulus

The flow structure of gas-liquid two-phase flow has been investigated in a vertical annulus channel. The annulus consisted of a geometry where the inner diameter was 19.1 mm and the outer diameter was 38.1 mm. The total height of the test section was 4.37 m. Experiments were conducted for nineteen inlet flow conditions. These flow conditions covered bubbly, cap-slug, and churn-turbulent flows. The local flow parameters, such as void fraction, interfacial area concentration, and bubble interface velocity, were measured at nine radial positions within the gap of the annulus at z/Dh = 230 of the test section. Radial distributions of these parameters were interpreted in terms of turbulent velocity profile, lift and wall forces. In addition, the local measurements were used to calculate distribution parameter, C0 in drift-flux model, and area averaged interfacial area concentration. Ishii’s (1977) model was modified and a new correlation of C0 was proposed based on the experimentally obtained C0 values. The area-averaged interfacial area concentration (IAC) values were compared with the most widely used models (Ishii and Mishima, 1980; Spore et al., 1983; Hibiki and Ishii, 2002). The advantages and drawbacks of these models were highlighted.Copyright

Study on Drag Coefficients for Two Groups of Bubbles

To apply the two-fluid model to a wide range of flow regimes in gas-liquid two-phase flows, the gas phase is categorized into two groups: small spherical/distorted bubbles as Group 1 and large cap/slug/churn-turbulent bubbles as Group 2 in the modeling of interfacial area transport. The interfacial transfer terms of momentum and energy for the gas phase are then divided into two groups accordingly in the implementation of the two-group interfacial area transport equation to the two-fluid model. Thus, the drag coefficients and the interfacial heat transfer for each group bubbles need to be developed. An approach has been sought for evaluating the drag coefficients of each bubble group based on a comprehensive experimental data base obtained in air-water upward flows in various size round pipes. Comparisons have been made with the theory of the drag coefficients and it was found that the agreement is not very satisfactory although the general trends can be predicted by the current approach.Copyright

Investigation of Hydrodynamic Loads Associated With Pyrotechnic Valve Actuation

Pyrotechnic-actuated valves are utilized for various applications requiring remote actuation with high reliability. One such application is passive safety injection (SI) within the emergency core cooling system (ECCS) within the Generation III+ advanced commercial nuclear power plant designs. The pyrotechnic (explosive) actuation within the valve internals, which opens the valve for water flow, creates a vertical force that must be supported by the surrounding piping restraints. This is a well-known phenomenon that is accommodated in the design.However, there exists also a subsequent, lesser-known axial (horizontal) force that must be accommodated also. A RELAP5/MOD3.3 (patch03) code [1] model for the pyrotechnic valve and the broader injection system was configured to analyze the extent of this water hammer. Typically, the pyrotechnic actuation occurs at relatively low reactor coolant system pressure since the injection itself will eventually be a passive gravity-driven feed. However, even at this low actuation pressure, the RELAP5 analysis demonstrates that the hydrodynamic loads can be substantial. Furthermore, the analysis shows that staggered actuation of a two-valve parallel configuration can exacerbate and magnify the load, compared to a single valve actuation.Copyright

Investigation of Effects of Piping Configuration and Water Supply Pressure on Air Intrusion

An evaluation of the effects of geometry and water supply pressure on the void transport has been performed using RELAP5/MOD3.3 (patch03). Two different piping configurations were considered for a hypothetical nuclear power plant. The cases that were analyzed considered switchover between two different water supplies, i.e. condensate storage tank (CST) and essential service water system (SX) for a safety system that acted as the ultimate heat sink. In addition, two different pressures were considered for the pressure of SX to investigate the effect of supply water pressure on void transport. Results were interpreted based on the differences in the geometries of the piping configurations and supply water pressures.Copyright