Air-water Test Research Articles

Interfacial area transport modeling of air–water bubbly two-phase flows in inclined orientations

The present work develops closure models for the interfacial area transport equation for inclined-upward bubbly two-phase flows. This includes frictional pressure drop through a Lockhart-Martinelli method; void fraction and void-weighted gas velocity through a drift-flux analysis; and covariance of bubble interaction mechanisms through a novel void-fraction distribution reconstruction method. Bubbly flow data collected in a 25.4 mm air–water test facility for five angles (0°, 30°, 60°, 80°, and 90°) is used for the development and evaluation of the models. The novel void reconstruction models are able to predict the covariance of bubble interaction terms within ±30%, and the transport of interfacial area can be predicted within 9 %.

Salinity and flow pattern independent flow rate measurement in a gas-liquid flow with optimum feature selection and novel detection geometry using ANNs

This paper presents a study on the salinity determination and flow rate estimation in a two-phase air-water system. For this purpose, an automated air-water test loop capable of generating different flow patterns in a horizontal pathway was utilized to do numerous experiments at varying flow rates. A nuclear measuring setup, comprising Cs-137 and Am-241 as radiation sources, one NaI (Tl) scintillation detector to register transmission counts of Cs and three scintillator detectors for registering transmission and scattering counts from Am, was prepared. A pressure drop pipe was also employed for flow rate measurement, with MLPs were selected as the processing element. Notable innovations of this paper can be considered from two perspectives. Firstly, a novel paradigm in nuclear measurement geometry. This novelty uses the benefits of three scintillator detectors to extract features with the maximum potential for classifying and determining salinity. Secondly, there is a new method in data processing and utilizing optimum features to achieve the best performance in predicting flow rates. Results in flow rate prediction independent of salinity and flow regime indicate that the proposed paradigm and method are reliable for using in industrial fields related to multi-phase metering.

Temporal-spatial mode competition in slug-flow induced vibration of catenary flexible riser in both in plane and out of plane

The slug-flow induced vibration (SIV) of a catenary flexible riser with an aspect ratio of 360 is experimentally investigated in this work. Four different flow rates for the internal gas-liquid slug flow are considered in the tests that carried out at a fixed gas-liquid ratio of 3.0 in a small-scale air-water test loop. The temporal-spatial mode competition is discussed in terms of the response amplitudes, frequencies, modal weights, transferred energy between the riser and flow, pressure fluctuations and slug flow characteristics. Experimental results indicate that the modal weights of higher modes increase with the flow rate although the in-plane response is dominated by the fundamental mode, resulting in the intense mode competition. The occurrence of mode transition first emerges at the response trough, presenting the higher second modal weight. The distinct switching between the first and second mode is observed for the pipe section between the response peak and trough, and such mode switching becomes more pronounced as the flow rate grows. The distribution and lengths of liquid slugs passing through the riser affect the mode switching in terms of the distribution and magnitude of fluid forces. The riser storages the energy extracted from the fluid when it approaches to the equilibrium position, while releases the energy as it departs the equilibrium position. The occurrence of out-of-plane response is found when the mixture velocity exceeds 2.0 m/s. The out-of-plane frequencies appear in the in-plane response, and vice versa, manifesting the interaction between the in-plane and out-of-plane responses.

An Experimental Performance Evaluation for a Swirl-Vane Separator Using an Air-Water Test Facility

The most important design parameters of a moisture separator to optimize the efficiency of a nuclear power plant are the moisture carry over and pressure drop. An air-water test facility was constructed to evaluate the design parameters for various types of conventional moisture separators in Korea. In this study, a half scale of swirl-vane separator was designed, and test conditions were defined based on the scaling law to cover a wide range of operating conditions. The moisture carry over was measured using an isokinetic sampling system for 34 different flow conditions. The MCO was in range of 0.30–7.05% according to the test conditions. Single and two-phase flow experiments were also carried out to identify the two-phase pressure drop characteristics. Based on the obtained results, the proposed empirical correlation model was extended to a submerged separator and applying these results to a full scale system is discussed in this paper.

Severe slug flow-induced nonlinear dynamic behavior of a flexible catenary riser

An experiment is conducted in a small-scale air–water test loop to investigate the severe slug flow-induced vibration of a flexible catenary riser of aspect ratio (the riser length over its internal diameter) 200. The vibration displacement of the catenary riser as well as the internal flow features is simultaneously captured by high-speed cameras. Three stages are observed during a cycle of severe slugging in the riser, including the slug formation, gas blowout, and transition stages. The spatial-temporal dynamic behavior of the flexible catenary riser is closely related to the stage of severe slug flow, liquid slug length, and liquid inventory along the riser, presenting a resonance between the oscillation and the fluid pressure fluctuation.

증기발생기의 표준형 습분분리기 내부 유동 특성에 관한 수치해석

In the present study, a series of numerical analysis was performed to identify the flow characteristics inside the CE-type moisture separator applied to domestic steam generators. To investigate the similarity between normal operation condition of the prototype separator and experimental condition using air-water test facility with real scale and 1/2 scale test sections, single phase and two-phase calculations were carried out. The results of the single phase calculations showed that the outward flows through the flow holes on the wall turned out to be similar trend regardless of the working fluid and inlet conditions on real scale geometry. However, the pressure loss coefficient for the 1/2 scale geometry was relatively larger than that for the real scale geometry. In the two-phase calculation, Eulerian-Lagrangian method was adapted to investigate the behavior of the droplets. Two extreme conditions for the post impingement behavior of the droplet, which were escape and rebound conditions; were applied and the results between them were compared.

Mode switching and standing-travelling waves in slug flow-induced vibration of catenary riser

An experiment in a small-scale air-water test loop is performed to investigate the flow-induced vibration behaviors of a flexible catenary riser conveying slug flow over a wide range of the gas-to-liquid flow rate ratios from 1.0 to 4.5. The clamped-clamped riser model has an inner diameter of 4 mm and an aspect ratio with the length over the inner diameter of about 360. The non-intrusive high-speed cameras are employed to simultaneously capture the catenary riser dynamics and the internal unsteady slug flow characteristics. The space-time varying features of the slug flow-induced vibration are presented and discussed in terms of the response amplitudes, frequencies, modal components, and associated flow-induced forces. Slug flow properties including the translational velocities, lengths, fluctuation frequencies and pressure changes are reported. When increasing the ratio of the gas-to-liquid flow rates, experimental results highlight how a variation of the slug length and frequency contributes to a mode-switching phenomenon over time. This switching from the dominant first-mode frequency to the second-mode frequency leads to the associated transition from the standing wave to the travelling wave of the catenary riser response. The response amplitudes are amplified by the increased translational velocity and slug length, entailing the increased flow-induced momentum and gravitational forces, as the gas-to-liquid flow rate ratio is increased. Consequently, higher vibration modes participate into the riser dynamics exhibiting a strong multi-mode competition.

Effect of part-length rods for different BWR subchannel geometries

Despite the beneficial effects of part-length rods (PLRs) in BWR fuel, they also introduce inhomogeneities to the coolant flow. These effects are hard to simulate and might affect cooling efficiency, dryout margin and CRUD deposition. So far, no fundamental experimental research was conducted to explore the details of their influence to the liquid film behavior in the dominating annular flow regime.To investigate the fluid dynamical effects of PLRs, we analyzed three different types of subchannel geometries. We varied the positions of PLRs in a pair of adjacent subchannels as well as the liquid and gas flow rates, staying within the annular flow regime. The adiabatic air-water test section was equipped with two types of sensors measuring the liquid film thickness distribution on half a fuel rod and on a fuel element wall. The measurement frequency was 10 kHz and the lateral resolution in the two-dimensional measuring domain was 2 mm. This corresponds to 64 × 16 and 16 × 32 measuring points, for rod and wall sensor respectively.Closely downstream of a PLR, we found a recirculation zone with less shear driving the liquid film. This is reflected by an increase of the average film thickness and a decrease of the wave velocity. A cross-flow of gas and liquid towards the open space downstream of the PLR was detected. The side of neighboring fuel rods facing this open space sees an increase of liquid flow. On the PLR-averted side, the coolant flow decreases, reducing the dryout margin. Further downstream, more gas tends to traverse into the open space left behind the PLR, which again increases the driving shear stress on the liquid film. The film becomes thinner and faster. The precise channel geometry featuring PLRs have a strong and complex influence on the local average liquid film thickness and the wave velocities. Their qualitative distribution at different flow conditions however, is similar.

Experimental investigation on the flow-induced vibration of a free-hanging flexible riser by internal unstable hydrodynamic slug flow

Unstable hydrodynamic slug flow is extensively encountered in marine risers, resulting in internal flow-induced vibration, which is quite different with the vortex-induced vibration caused by a cross flow. Experiments were performed in an air-water test loop to study the internal slug flow-induced vibration of a free-hanging flexible riser with aspect ratio (the ratio of the length to the diameter of a riser) of 158. A non-intrusive technique of high speed imaging method was employed to record the vibration displacements of the riser model and the flow regime of two-phase mixture in the riser simultaneously. The results show that an in-plane vibration is created by the internal slug flow, and the responses along the two directions in the plane are interdependent to each other. In the considered gas-liquid ratio (the ratio of air flow rate to water flow rate), the second-order mode dominates the response. The hydrodynamic behaviors of slug flow, including the gas-liquid ratio, liquid slug length and superficial velocity, affect the local pressure fluctuation and hence influence the vibration amplitude. The fluid-structure interaction is reflected from the same frequencies presenting in the vibration and pressure fluctuation.

Characterization of horizontal air–water two-phase flow in a round pipe part I: Flow visualization

As a part of characterizing the bubble interaction mechanisms and flow regime transition processes in horizontal gas–liquid two-phase flow, a flow visualization study is performed in an air–water test facility constructed from 3.81cm inner diameter clear acrylic round pipes. The test section is approximately 250 diameters in length to allow for development of the flow. Flow visualizations are performed using a high-speed video camera at 80 and 245 diameters downstream of the inlet to observe the development of the flow structures. A total of 27 flow conditions including bubbly, plug, slug, stratified, wavy, and annular flows are characterized in the present study.In highly turbulent bubbly flow conditions it is found that the distribution becomes more uniform with increasing development length through a turbulence penetration process that counters the effect of buoyancy. It is also found that plug bubbles form below a layer of small bubbles rather than at the upper pipe wall where the bubbles are most packed. In fact, it is consistently found that turbulence-based bubble interactions do not occur in the most densely packed regions as the eddies there are not large enough to effect the bubbles. Rather, bubble packing-induced coalescence occurs in these regions and contributes to the formation of plug bubbles. The newly formed plug bubbles move faster than, and ultimately pass, the smaller bubbles above due to the effect of the wall. These small bubbles are subsequently overtaken by the following plug bubble and coalesce with the nose region through a process denoted as drag-induced coalescence.

Three-dimensional time-averaged void fraction distribution measurement technique for BWR thermal hydraulic conditions using an X-ray CT system

We have developed a void fraction distribution measurement technique using the three-dimensional (3D) time-averaged X-ray computed tomography (CT) system to understand two-phase flow behavior inside a fuel bundle for boiling water reactor (BWR) thermal hydraulic conditions of 7.2 MPa and 288 °C. As a first step, we measured the 3D void fraction distribution in a vertical square (5 × 5) rod array that simulated a BWR fuel bundle in the air–water test. A comparison of the volume-averaged void fractions evaluated by the developed X-ray CT system with those evaluated by a differential pressure transducer showed satisfactory agreement within a difference of 0.03. Thus, we confirmed that the developed system could be used to get 3D imaging of the vertical square rod array used in the test under the BWR operating pressure condition. In the next step, we did a verification test using the vertical pipe (11.3 mm ID) for BWR thermal hydraulic conditions. A comparison of the cross-sectional-averaged void fractions evaluated by the X-ray CT system with those evaluated by the drift-flux model showed good agreement within a difference of 0.05. We confirmed that the evaluated void fraction distribution forms in the horizontal cross section changed with the quality in response to the flow regime transition.

Experimental Investigation on the Effects of a Spacer Grid on Single- and Two-Phase Flows

Experiments are performed in a scaled 1x3 rod bundle adiabatic air-water test facility to investigate the effects of a spacer grid on singleand two-phase flows. A four-sensor conductivity probe is used to obtain detailed measurements of local time-averaged two-phase flow parameters, including the void fraction and interfacial area concentration, throughout the flow cross-section at four axial locations. The local two-phase flow data indicates that the spacer grid causes redistribution of the bubbles. In the transition from bubbly to slug flows, the spacer grid is found to promote bubble coalescence. In the slug to churn-turbulent transition, however, the grid breaks up the larger bubbles, causing an increase in the interfacial area concentration across the spacer grid. Additionally, local velocity measurements in single-phase liquid flow are performed with a laser Doppler anemometer to investigate the change in the turbulence structure across the grid. The axial turbulence intensity measurements show the influence of the spacer grid dimples, indicating an increased mixing effect from these structures.

Towards the optimization of a pulsatile three-stream coaxial airblast injector

A large-scale parametric air–water test stand (AWTS) study involving more than 40 evaluations was carried out for the purposes of three-stream airblast reactor feed injector characterization and optimization; a subset of seven air stream combinations is discussed here. The role of CFD as a supplement to, or a replacement for, air–water testing is of great industrial interest. To this end a set of CFD simulations was carried out to complement the AWTS study. Pressure responses, spray opening characteristics near the feed injector face, and spray distribution were primary measures for both the AWTS and CFD programs. It was found that, over the range of variables studied, there was somewhat of a match between CFD and AWTS results. A self-exciting, pulsatile spray pattern was achieved in CFD and AWTS (frequencies between 75 and 600 Hz), and an interesting transition in spray bursting character occurred at moderate inner air flows. The oscillatory flow pattern mimics prior work in terms of the energy of the fluctuations, but the fact that the present fluctuations occur at an order of magnitude lower frequency is apparently related to the comparatively low gas/liquid momentum ratio in the current study. Overall, it is shown that the CFD method contained herein can be used to supplement, but not replace, air–water testing for said injector configuration.

Recent Moisture Separator Reheater Design Technologies

The moisture separator reheater (MSR) is a key piece of equipment in reheat systems in nuclear steam turbines that use saturated main steam, where it helps improve turbine efficiency and suppress flow-accelerated corrosion. Fundamental to achieving a compact, reliable MSR design are methods for predicting mist separator vane performance and suppressing tube drainage instability. First, we devised a method for predicting separator performance based on the observation of mist separation behavior under an air-water test. We then developed a method for predicting performance under steam conditions from air-water test data and verified it by means of a comparison with the actual results of a steam condition test. The instability of tube drainage associated with both subcooling and temperature oscillation at turbine partial load, which might adversely affect the seal welding of the tubes to the tube sheet due to thermal fatigue, was measured on an existing unit to clarify the behavior. We then developed a technique for increasing venting steam, which had been operating at a constant flow rate, to suppress instability and verified its effectiveness. Both methods were applied to current MSR models, which were adopted for nuclear power plant turbines commercially placed in service from 1984 to 2009, and the effectiveness of the methods was demonstrated. The separator vane mist carryover rate was less than 0.1%, and tube drainage instability was suppressed, demonstrating the effectiveness of the simple design concept of a two-flow U-tube instead of the prevailing four-flow U-tube design. We put forth a new concept in the design of MSRs for 1700 MW class advanced pressurized water reactor (APWR) units based on associated technologies, along with advanced technology for the compact design of pressure vessels and multidisciplinary optimum design for evaluating heat exchanger tube bundles.

Advanced DVI for ECC direct bypass mitigation

An ECC direct bypass fraction during a late reflood phase of a LBLOCA is strongly dependent on the characteristics of the cross flow and the geometrical configuration of a DVI in the downcomer of a pressurized light water reactor. The important design parameters of a DVI are the elevation, the azimuthal angle, and the separator to prevent a steam–water interaction. An ECC sub-channel to separate or to isolate an ECC water from a high-speed cross flow is one of the important design features to mitigate the ECC bypass phenomena. A dual core barrel cylinder as an ECC flow separator is located between a reactor vessel and a core barrel outer wall in the downcomer annulus. A new narrow gap between the core barrel and the additional dual core barrel plays the role of a downward ECC flow channel or an ECC flow separator in a high-speed cross flow field of the downcomer annulus. The flow zone around a broken cold leg in the downcomer annulus has the role of a high ECC direct bypass due to a strong suction force while the wake zone of a hot leg has the role of an ECC penetration. Thus, the relative azimuthal angle of the DVI nozzle from the broken cold leg is an important design parameter. A large azimuthal angle from a cold leg to a hot leg needs to avoid a high suction flow zone when an ECC water is being injected. The other enhancing mechanism of an ECC penetration is a grooved core barrel which has small rectangular-shaped grooves vertically arranged on the core barrel wall of the reactor vessel downcomer annulus. These grooves have the role for a generation of a vortex induced by a high-speed cross flow. Since the stagnant flow in a lateral direction and rotational vortex provides the pulling force of an ECC drop or film to flow down into the lower downcomer annulus by gravity, the ECC direct bypass fraction is reduced when compared to the current design of a smoothed wall. An open channel of grooves generates a stagnant vortex, while a closed channel of grooves creates an isolated ECC downward flow channel from a high-speed lateral flow. In this study, new design concepts for a dual core barrel cylinder, grooved core barrel, and a reallocation of the DVI azimuthal angle are proposed and tested by using an air–water 1/5 scaled air–water test facility. The ECC direct bypass reduction performances of the new design concepts have been compared with that of the standard type of a DVI injection. The azimuthal angle of the DVI nozzle from a broken cold leg varies from −15° to +52° toward a hot leg. The test results show that the azimuthal injection angle is an effective parameter to reduce the ECC direct bypass fraction. The elevation of the DVI nozzle is also an important parameter to reduce the ECC direct bypass fraction. The most effective design for reducing the ECC direct bypass fraction is a dual core barrel. The reduction fraction when compared to the standard DVI is about −30% for the dual core barrel while it is −15% for the grooved core barrel.

Moisture Separator Reheater for NPP Turbines

This paper introduces the development of the current model Moisture Separator Reheater (MSR) for nuclear power plant (NPP) turbines, commercially placed in service in the period 1984-1997, focusing on the mist separation performance of the MSR along with drainage from heat exchanger tubes. A method of predicting the mist separation performance was devised first based on the observation of mist separation behaviors under an air-water test. Then the method was developed for the application to predict under the steam conditions, followed by the verification in comparison with the actual results of a steam condition test. The instability of tube drainage associated with both sub-cooling and temperature oscillation might adversely affect the seal welding of tubes to tube sheet due to thermal fatigue. The instability was measured on an existing unit to clarify behaviors and the development of a method to suppress them. Both methods were applied to newly constructed units and the effectiveness of the methods was demonstrated.

Experimental investigation and theoretical modeling of liquid entrainment in a horizontal tee with a vertical-up branch

This article describes a comprehensive literature review of liquid entrainment in horizontal pipes with vertical-up branches. Deficiencies in the available data and correlations were identified. The Air–water Test Loop for Advanced Thermal–hydraulic Studies (ATLATS) was constructed and entrainment onset and steady-state data were collected for a wide range of flow conditions. Using new insights gained from experimental testing, the authors developed a model for predicting the onset of entrainment and steady-state entrainment rate. Previously published correlations, along with the new model, are compared against all available data. The new model shows very good agreement with the onset data, but is not very good at predicting branch quality at high liquid flow rates.

Flow pattern of air–water and two-phase R-134a in small circular tubes

Experimental investigation of two-phase flow patterns for refrigerant R-134a and air–water in horizontal tubes with inside diameter from 1.0 to 3.0 mm was performed. The air–water test results agree very well with previous work. However, R-134a flow leads to a shift in the slug to annular transition to lower value of gas velocity. The locations of bubble to plug and slug flow transition are also significantly affected by the working fluids properties. We concluded that, in addition to buoyant force and turbulent fluctuations, surface tension force is also an important parameter for flow pattern determination in small tubes. Surface tension force causes the system to act to minimize its interfacial area. It tends to keep bubbles retaining its circular shapes and also to keep the liquid holdup between the tube walls to retard the transition from slug to annular. Since the surface tension of air–water is much larger than that of R-134a, it makes the intermittent to bubble flow transition occurs earlier for air–water than for R-134a. And also leads to a shift in the slug to annular transition to lower value of gas velocity for R-134a.

Decrease of Pressure Loss in BWR Steam Separator (1)

Numerical analysis and air-water test were performed in order to evaluate gas-liquid separation characteristics in a BWR steam separator. Centrifugal force and flow quality were simulated in the test. Simulation of plant operation conditions was not sufficient at low volumetric flux due to large effect of gravity force. Pressure loss coefficient agreed with the plant conditions within 10%. Carryover agreed well with plant conditions for a current separator but was under-estimated for a separator with small vane angle. Calculated gas-liquid behavior agreed well with plant condition data in the 1st stage but calculated results over-estimated carryover. Combination of calculated results in the 1st stage and simulation test results in the 2nd and 3rd stages gave good prediction for carryover.

ＢＷＲ用セパレータの圧力損失低減 （２） 圧力損失低減方法の検討

Numerical analysis and air-water test were performed in order to decrease pressure loss in a BWR steam separator. A current swirler and two low pressure-loss swirlers with a small swirl angle and with a small hub diameter were evaluated. Calculated results showed that the swirler with a small hub diameter had better gas-liquid separation than the swirler with a small swirl angle because of larger circumferential velocity and centrifugal force. It was also confirmed by the air-water simulation test. In case of the combination of the swirler with a small hub diameter and the current upper structure, the target of carryover was satisfied and its pressure loss was about 45% of the current separator. In case of a 3rd stage high-swirl type, which had the same barrel diameter as the 2nd stage pick-off ring diameter, carryover was as low as the current separator and pressure loss was about 60% of the current.