Impedance Void Meter Research Articles

Air-water two-phase flow regime and transition criteria in vertical upward narrow rectangular channels

Upward two-phase flow in two different vertical narrow gaps was investigated experimentally. For better understanding the characteristics of air-water flow in rectangular channels, the visual images were captured through high-speed cameras and the void fraction results were obtained through three impedance void meters. The flow pattern was identified through ReliefF-FCM clustering algorithm to avoid the subjectivity of human observation. The flow regime maps were plotted in two different forms, superficial velocities and void fraction and compared with the previous model in the open literature. Through the analysis of the experimental results, the air-water flow in narrow rectangular channels is characterized by the limitation of bubble distribution, coalescence and movement. The probability of bubble coalescence in narrow rectangular channels is increased compared with those in conventional channels due to the restriction of the narrower side walls. New transition criteria was obtained according to the drift-flux model and the force balance analysis which shows satisfactory agreements with existing experimental data for gaps of 1–6 mm.

Two-phase interfacial area characteristics and transport under the transition from subcooled boiling to saturated boiling flows

This study focuses on the analysis of the transition from the subcooled boiling to saturated boiling flow. During this process, vapor generates drastically in a short developing length, which makes it difficult to predict the interfacial characteristics of two-phase flow. To investigate the characteristics of this process, experiments were performed in a vertical annulus with 19.1 mm inner diameter and 38.1 mm outer diameter. The two-phase interfacial parameters including time-averaged local vapor fraction, interfacial area concentration, and bubble interfacial velocity are obtained using high-temperature miniature four-sensor electrical conductivity probes. In this study, the conductivity probe signal is processed using the newly-developed signal processing algorithm developed for small spherical bubbles by Shen and Nakamura (2014) , instead of the algorithm used in the previous subcooled boiling experimental studies Ozar (2009) and Kumar et al. (2019), [3] for large bubbles developed by Kataoka et al. (1986). The flow conditions cover a wide range of inlet subcooling temperature at 12.9–21.2 °C, heat flux at 57.9–197.3 kW/m2, and inlet flow rate at 0.265–0.536 m/s. The vapor fractions are cross-validated with impedance void meter measurements. The state-of-art two-group interfacial area transport equations (IATE) [6], (Ozar, 2009) and (Brooks and Hibiki, 2016) is evaluated with the experimental data. The result shows that the two-group IATE gives fairly good predictions on the transition phenomena. Besides, a defect was identified in the condensation model (Park et al., 2007) used in IATE, which is the bubble boundary diameter classifying the two condensation mechanisms: heat-transfer-controlled and inertia-controlled condensation. A rigorous approach to estimate this boundary diameter is established in this study.

Investigation of the effect of the electrode distance on the impedance void meter performance in the two-phase flow measurement

Experimental study on the effect of electrode distance on the performance of impedance void meter is presented. The study includes multiple approaches. In the numerical simulation approach, based on the mathematical similarity between electric and temperature fields, the electric fields with different electrode distances are simulated using ANSYS Fluent to generate temperature fields. In the experimental study, an adiabatic air-water experiment is performed to compare two impedance meters with different electrode distances. The test conditions cover the flow regimes from bubbly flow to annular flow. Gamma densitometer and multi-single sensor conductivity probe are utilized for the void fraction comparison. The effect of the electrode distance on the flow regime identification using the artificial neural network is investigated, the signals of the different impedance meters are applied as the inputs for the artificial neural network. It is found that impedance meter with a properly large electrode distance has better performance in void fraction measurement, especially at high superficial velocity.

Measurement of subchannel void fraction in 5 × 5 rod bundles using an impedance void meter

In order to improve the ability to predict nuclear reactor behaviour, reliable measurement of the void fraction in a subchannel is essential. A subchannel impedance void meter (SCIVM) is developed and applied in air–water two-phase flow in 5 × 5 rod bundles, which consists of a six-strip emitter electrode and six-strip receiver electrode to cover the whole subchannel. Because of the lack of accurate void fraction calculation in the subchannel, the developed impedance void meter is calibrated by a four-sensor conductivity probe with a quartic polynomial function. The void fractions in different subchannels are measured with a sampling frequency of 10 kHz to capture the dynamic features of the rapidly changing phase distribution in two-phase flow. The phase distributions in rod bundles are discussed based on the void fractions measured in different subchannels, and among all flow regimes, the cap turbulent flow has the largest void fraction differences between the inner, side and corner subchannel on the same cross-section due to the effect of the casing tube. Moreover, the dynamic features of the subchannel void fraction are analysed for different flow regimes, including bubbly, cap bubbly, cap turbulent, churn turbulent, and annular flow. It is demonstrated that the newly designed subchannel impedance void sensor is an effective tool to measure the void fraction in the subchannel and figure out its dynamic features.Highlights• A novel SCIVM is designed to measure the void fraction in the subchannel.• The measured subchannel void fraction is calibrated with a four-sensor conductivity probe and verified with empirical drift-flux correlations.• The differences in void fraction are discussed for different subchannels on the same cross-section.• The dynamic features of the subchannel void fraction are analysed for different flow regimes.

Experimental study of vertical co-current slug flow in terms of flow regime transition in relatively small diameter tubes

In order to investigate the evolution characteristics of structure parameters during flow regime transition process in gas-liquid slug flow, the vertical upward slug flow in a wide range of liquid superficial velocity (0.03< jl<1.6m/s) are studied in a tubular test section with the inside diameter of 25.4mm. Impedance void meters are employed to measure the time series of cross-sectionally averaged void fraction of separated two parts corresponding to Taylor bubble and liquid slug. With increasing flow gas flow rates at constant liquid flow rate, the present research studies the evolution of length and void fraction of each part in slug unit. New structure of turbulent slug unit in strong relation with transition process is observed. It is found that such special structure plays an important role in transition from slug flow to churn-turbulent flow. It is also found that the flooding determines whether the flow regime evolves into churn-turbulent flow or directly into annular flow. By studying the evolution of flow regimes, two structure parameters, void fraction of liquid slug and length ratio of Taylor bubble to liquid slug, can be used as the transition criterion. New criterion for the transition from slug flow to churn-turbulent flow was also proposed based on these two parameters. It is found that the current results agree well with the existing flow regime map for relatively small diameter tubes in literature.

Drift-flux model of sub-channel in vertical rod bundles with spacer grids

Drift-flux model has been one of the most popular models predicting two phase flow parameters in nuclear reactor and other industrial equipment since proposed. Most of researches are based on the void fraction measurement on the whole cross-section of the void fraction, which is under the influence of casing tube and might be unfit for sub-channel. Therefore, experiments have been performed to measure the void fraction in sub-channels with the newly designed impedance void meter, based on which the existing drift-flux correlations have been evaluated. However, few correlations have good performance in bubbly and cap bubbly flow. In order to improve the accuracy and consider the physical significance, the new correlation of distribution parameter is proposed by adopting Julia’s drift velocity correlation. The proposed correlation could predict the present data, Yang’s data and Clark’s data with the mean relative error of 13.96%, 16.23% and 24.9% respectively. Furthermore, the L/D effect on distribution parameter has been analyzed with different spacer grids. The mixing vane spacer grid (MVSG) results in a larger value of distribution parameter than simplified spacer grid in low void fraction region for the stronger recirculating flow.

Sub-channel flow regime maps in vertical rod bundles with spacer grids

The accurate prediction of thermal-hydraulic parameters is based on the flow regime maps for rod bundle, which is important for the safety of nuclear reactor. An air-water two phase flow experiment has been performed to study the sub-channel flow regime (SCFR) maps in 5 × 5 rod bundles section with two distinct spacer grids, including simplified spacer grid (SSG) and mixing vane spacer grid (MVSG). To obtain the objective SCFR maps in rod bundles, a sub-channel impedance void meter is newly developed to measure the time series void fraction in sub-channels. Besides, the random forest clustering algorithm has been adopted to identify the sub-channel flow regimes objectively based on a training sample and 13 selected feature values of time series void fraction. The feature values include mean value, standard deviation value, sample entropy value and 10 proportion values. In this way, the objective sub-channel flow regime maps are obtained at four different locations with different spacer grids. Distinct features have been observed for different SCFRs. As for the SCFR transitions in different sub-channels over the same cross-section, almost all of them arise in the inner sub-channel firstly for the effect of casing tube. Moreover, the dissipation length of spacer grid is larger than 19.5 L/D. In the influencing region of spacer grid, the transition from cap bubbly to cap turbulent flow occurs in the corner sub-channel at 19.5 L/D downstream of spacer grid firstly and then at 6.8 L/D for low liquid velocity, while firstly occurs at 6.8 L/D for high liquid velocity. The magnitude of the spacer grid effect on SCFR transition depends on the superficial liquid and gas velocity, as well as the structure of spacer grid for current flow conditions. Only Liu and Hibiki’s model is applicable for the transition from bubbly to cap bubbly flow in sub-channel. Therefore, new transition models or correlations should be developed for other sub-channel flow regime transitions.

Experimental investigation of horizontal forced-vibration effect on air-water two-phase flow

In order to investigate the potential seismic vibrations effect on two-phase flow in an annular channel, experimental tests with air-water two-phase flow under horizontal vibrations were carried out. A low-speed eccentric-cam vibration module capable of operating at motor speed of 45–1200rpm (f = 0.75–20Hz) was attached to an annular channel, which was scaled down from a prototypic BWR fuel sub-channel with inner and outer diameters of 19.1mm and 38.1mm, respectively. The two-phase flow was operated in the ranges of 〈jf〉 = 0.25–1.00m/s and 〈jg〉 = 0.03–1.46m/s with 27 flow conditions, and the vibration amplitudes controlled by cam eccentricity (E) were designed for the range of 0.8–22.2mm. Ring-type impedance void meters were utilized to detect the area-averaged time-averaged void fraction under stationary and vibration conditions. A systematic experimental database was built and analyzed with effective maps in terms of flow conditions (〈jg〉-〈jf〉) and vibration conditions (E-f and f-a), and the potential effects were expressed by regions on the maps. In the 〈jg〉-〈jf〉 maps, the void fraction was found to potentially decrease under vibrations in bubbly flow regime and relatively lower liquid flow conditions, which may be explained by the increase of distribution parameter. Whereas and the void fraction may increase at the region closed to bubbly-to-slug transition boundary under vibrations, which may be explained by the changes of drift velocity due to flow regime change from bubbly to slug flows. No significant change in void fraction was found in slug flow regime under the present test conditions.

Development of impedance void meter for evaluation of flow symmetry

Often it is useful or even necessary to know whether the void fraction is evenly distributed in two-phase flows. In industrial systems, unevenly distributed flows can be a factor in vibrations and other undesirable occurrences. In bubble column chemical reactors, uneven distribution of the void can result in reduced reaction efficiency. In experiments, it is often desirable to have symmetrically distributed void for local measurements since this allows for more rapid data collection. Current systems for determining the symmetry of the void distribution in distributed flows either suffer from very long measurement time requirements or cause significant disturbances to the flow. To address the need for approximate evaluation of flow symmetry without causing excessive disturbance to the flow, a new impedance void meter design has been developed and tested under a variety of conditions. The ability of the new design to evaluate the void distribution symmetry has been tested through comparison with conductivity probe measurements of local void fraction, and the accuracy of the impedance meter has been verified through comparison with the conductivity probe measurements and with typical impedance void meter measurements.

Modified distribution parameter for churn-turbulent flows in large diameter channels

Two phase flows in large diameter channels are important in a wide range of industrial applications, but especially in analysis of nuclear reactor safety for the prediction of BWR behavior and safety analysis in PWRs. To remedy an inability of current drift-flux models to accurately predict the void fraction in churn-turbulent flows in large diameter pipes, extensive experiments have been performed in pipes with diameters of 0.152m, 0.203m and 0.304m to collect area-averaged void fraction data using electrical impedance void meters. The standard deviation and skewness of the impedance meter signal have been used to characterize the flow regime and confirm previous flow regime transition results. By treating churn-turbulent flow as a transition between cap-bubbly dispersed flow and annular separated flow and using a linear ramp, the distribution parameter has been modified for churn-turbulent flow. The modified distribution parameter has been evaluated through comparison of the void fraction predicted by the drift-flux model and the measured void fraction.

Design and performance prediction of an impedance void meter applied to the petroleum industry

A novel methodology based on numerical simulation is developed for designing a gas–liquid void fraction meter operating on the principle of electrical impedance. The numerical simulations employ software based on the finite volume method. The analysis allowed heuristic optimization of the electrodes’ geometry and also the establishment of the calibration curve according to the electrical properties of the fluids in question. Using dimensional analysis it is possible to reduce the electrical impedance estimates into simple algebraic expressions relating the void fraction to the dimensionless voltage output. Dynamic tests are performed using air and water in order to validate the computational study and verify the performance of the sensor operating at different flow patterns.

Measurement and modeling of two-phase flow parameters in scaled 8 × 8 BWR rod bundle

The behavior of reactor systems is predicted using advanced computational codes in order to determine the safety characteristics of the system during various accidents and to determine the performance characteristics of the reactor. These codes generally utilize the two-fluid model for predictions of two-phase flows, as this model is the most accurate and detailed model which is currently practical for predicting large-scale systems. One of the weaknesses of this approach however is the need to develop constitutive models for various quantities. Of specific interest are the models used in the prediction of void fraction and pressure drop across the rod bundle due to their importance in new Natural Circulation Boiling Water Reactor (NCBWR) designs, where these quantities determine the coolant flow rate through the core. To verify the performance of these models and expand the existing experimental database, data has been collected in an 8×8 rod bundle which is carefully scaled from actual BWR geometry and includes grid spacers to maintain rod spacing. While these spacer grids are ’generic’, their inclusion does provide valuable data for analysis of the effect of grid spacers on the flow. In addition to pressure drop measurements the area-averaged void fraction has been measured by impedance void meters and local conductivity probes have been used to measure the local void fraction and interfacial area concentration in the bundle subchannels. Experimental conditions covered a wide range of flow rates and void fractions up to 80%.

Electrical impedance-based void fraction measurement and flow regime identification in microchannel flows under adiabatic conditions

Electrical impedance of a two-phase mixture is a function of void fraction and phase distribution. The difference in the specific electrical conductance and permittivity of the two phases is exploited to measure electrical impedance for obtaining void fraction and flow regime characteristics. An electrical impedance meter is constructed for the measurement of void fraction in microchannel two-phase flow. The experiments are conducted in air–water two-phase flow under adiabatic conditions. A transparent acrylic test section of hydraulic diameter 780μm is used in the experimental investigation. The impedance void meter is calibrated against the void fraction calculated using analysis of images obtained with a high-speed camera. Based on these measurements, a methodology utilizing the statistical characteristics of the void fraction signals is employed for identification of microchannel flow regimes. A self-organizing neural network is used for classification of the flow regimes.

Flow Regime Identification Under Adiabatic Upward Two-Phase Flow in a Vertical Rod Bundle Geometry

Flow regime maps were obtained for adiabatic air-water two-phase flow through a flow channel with 8 × 8 rod bundle, which simulated a typical rod bundle in a boiling water reactor. Impedance void meters were used to measure the area averaged void fraction at various axial locations in the flow channel. The Cumulative Probability Distribution Functions of the signals from the impedance meters were utilized along with self organizing neural network methodology to identify the flow regimes. The flow regimes were identified at five axial locations in the channel in order to understand the development of the flow regimes in axial direction. The experimental flow regime transition boundaries for bubbly to cap-bubbly and part of the cap-turbulent to churn-turbulent agreed with the theoretical boundaries of bubbly to slug and slug to churn-turbulent in round pipes. In addition, the two impedance void meters located across a spacer grid, revealed the nature of change in the flow regime across the spacer grid.

An application of the wavelet analysis technique for the objective discrimination of two-phase flow patterns

This paper presents an application of the wavelet analysis technique for two-phase flow pattern identification by using the void fraction signals obtained from a multi-channel Impedance Void Meter (IVM) in a vertical-upward air–water flow. A new method for the objective discrimination of the two-phase flow pattern has been developed to provide information regarding the local energy of void fraction signals at a given scale on the joint time–frequency diagram. The void signals are processed with Continuous Wavelet Transform (CWT) to get the local wavelet energy coefficients map on the time–frequency diagram. The effective local wavelet energy and the effective scale are then calculated. Then the criteria for flow pattern identification are, finally, obtained. A series of void fraction measurements were conducted over a wide range of air–water vertical-upward flow condition to provide an extensive database to cover several types of flow patterns. The results show that the proposed method has a high precision for characterizing different flow regimes in two-phase flow, and is considerably more promising for the online recognition of two-phase flow patterns due to the short time of data processing.

Void fraction and flow regime in adiabatic upward two-phase flow in large diameter vertical pipes

In pipes with very large diameters, slug bubbles cannot exist. For this reason, the characteristics of two-phase flow in large pipes are much different than those in small pipes. Knowledge of these characteristics is essential for the prediction of the flow in new nuclear reactor designs which include a large chimney to promote natural circulation. Two of the key parameters in the prediction of the flow are the void fraction and flow regime. Void fraction measurements were made in a vertical tube with diameter of 0.15 m and length of 4.4 m. Superficial gas and liquid velocities ranged from 0.1 to 5.1 m/s and from 0.01 to 2.0 m/s, respectively. The measured void fractions ranged from 0.02 to 0.83. Electrical impedance void meters at four axial locations were used to measure the void fraction. This data was verified through comparison with previous data sets and models. The temporal variation in the void fraction signal was used to characterize the flow regime through use of the Cumulative Probability Density Function (CPDF). The CPDF of the signal was used with a Kohonen Self-Organized Map (SOM) to classify the flow regimes at each measurement port. The three flow regimes used were termed bubbly, cap-bubbly, and churn flow. The resulting flow regime maps matched well with the maps developed previously through other methods. Further, the flow regime maps matched well with the criteria which were proposed based on Mishima and Ishii's (1984) criteria.

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&lt;⟨jg⟩&lt;30 m/s and 0.2 m/s&lt;⟨jf⟩&lt;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.

Interfacial Structures and Regime Transition in Co-Current Downward Bubbly Flow

The vertical co-current downward air-water two-phase flow was studied under adiabatic condition in round tube test sections of 25.4-mm and 50.8-mm ID. In flow regime identification, a new approach was employed to minimize the subjective judgment. It was found that the flow regimes in the co-current downward flow strongly depend on the channel size. In addition, various local two-phase flow parameters were acquired by the multi-sensor miniaturized conductivity probe in bubbly flow. Furthermore, the area-averaged data acquired by the impedance void meter were analyzed using the drift flux model. Three different distributions parameters were developed for different ranges of non-dimensional superficial velocity, defined by the ration of total superficial velocity to the drift velocity.

Interfacial structures and interfacial area transport in downward two-phase bubbly flow

An adiabatic, air–water, co-current, vertically downward bubbly flow was studied to gain a better understanding of interfacial structures and flow characteristics. The experimental test sections were round pipes with internal diameters of 25.4 and 50.8 mm. Flow regime map was obtained using characteristic signals obtained from an impedance void meter, and a neural network-based identification methodology to minimize the subjective judgment in determining the flow regimes. A four-sensor conductivity probe was used to measure the local two-phase flow parameters that characterize the interfacial structures. The parameters measured were: void fraction, interfacial area concentration, bubble velocity, and bubble Sauter mean diameter. Furthermore, a laser Doppler anemometer (LDA) system was used to measure local axial liquid velocity and turbulence. The local profiles of these parameters as well as their axial development revealed the nature of the interfacial structures and the bubble interaction mechanisms occurring in the flow. Based on previous study of interfacial area transport for upward flows, the interfacial area transport equation applicable to downward flow was developed with certain modifications in bubble interaction terms.

Cap bubbly-to-slug flow regime transition in a vertical annulus

The transition of the cap bubbly-to-slug flow regime was studied by using impedance void meters in a vertical annular flow channel. The characteristics of various flow regimes were demonstrated by the acquired impedance signals. The statistical parameters from the impedance signals were then fed into a self-organizing neural network for pattern categorization. Based on flow visualization, the classified patterns were translated into corresponding flow regimes. In addition, an analytical model was developed to predict the regime transition from cap bubbly to slug flows. Good agreement was obtained between the results by the neural network and the analytical model. Furthermore, comparisons of the model with other researchers’ experimental data also showed satisfactory results.