Cap Bubbles Research Articles

Study on flow regime characteristics in 3×3 rod bundle channels based on wire mesh sensor

Abstract The characteristics and evolution of two-phase flow are of critical importance to the safety of pressurized water reactors. Based on the double-layer wire mesh sensors in this paper, the air-water two-phase flow experiment of the 3×3 rod bundle channels is carried out at room temperature and pressure. The results show that the critical bubble diameter range for the reversal of lateral lift direction is 4 to 5.8 mm. In addition, the time-averaged void fraction for bubbly flow reveals a wall peak distribution at lower superficial gas velocities and shifts to a core peak as these velocities increase. For cap flow, the cross-distribution of cap bubbles within adjacent subchannels triggers large-scale mixing of the liquid phase between adjacent subchannels. For slug flow, large-sized bubbles develop along the axis and cross subchannel gaps to aggregate into slug-shaped bubbles, with a more pronounced distribution of the central peak of void fraction. The drift-flux models are evaluated against the experimental data, and the Ozaki model demonstrates higher precision in estimating void fraction, exhibiting a deviation of 9.8% on average.

Bubble Characteristics Required for the Complete Removal of Alumina Inclusions from Steel Melts

Gas bubbling can be an effective means to float out alumina inclusions from liquid steel in a ladle. However, large spherical cap bubbles are formed when using porous plugs, as the liquid steel is nonwetting to the porous refractory. These bubbles rise rapidly through the liquid steel, forming a fast‐moving bubble plume, restricting contact times. Sized microbubbles, by contrast, have now been generated in liquid metals by shearing methods, involving linear crossflows to an entering flow of gas, or alternatively by rotational shearing. Combined with these convective shearing forces, local kinetic energy of turbulence can also play an important part in determining final microbubble size distributions. As microbubbles have much smaller rise velocities and present a far greater inclusion capture surface area than those of a single large bubble of the same gross volume, this will allow us to remove sub‐50 μm inclusions from liquid steel. It is expected that this goal will require a redesign of current ladle shrouds.

Direct numerical simulations of microlayer formation during heterogeneous bubble nucleation

In this article, we present direct numerical simulation results for the expansion of spherical cap bubbles attached to a rigid wall due to a sudden drop in the ambient pressure. The critical pressure drop beyond which the bubble growth becomes unstable is found to match well with the predictions from classical theory of heterogeneous nucleation imposing a quasi-static bubble evolution. When the pressure drop is significantly higher than the critical value, a liquid microlayer appears between the bubble and the wall. In this regime, the interface outside the microlayer grows at an asymptotic velocity that can be predicted from the Rayleigh–Plesset equation, while the contact line evolves with another asymptotic velocity that scales with a visco-capillary velocity that obeys the Cox–Voinov law. In general, three distinctive regions can be distinguished: the region very close to the contact line where dynamics is governed by visco-capillary effects, an intermediate region controlled by inertio-viscous effects away from the contact line yet inside the viscous boundary layer, and the region outside the boundary layer dominated by inertial effects. The microlayer forms in a regime where the capillary effects are confined in a region much smaller than the viscous boundary layer thickness. In this regime, the global capillary number takes values much larger then the critical capillary number for bubble nucleation, and the microlayer height is controlled by viscous effects and not surface tension.

Flow visualization experiments of argon injection in a molten salt natural circulation loop

Off-gas systems are implemented in molten salt reactor designs to control the release of gaseous fission products. Two-phase flow in molten salt must be studied to understand how the system will behave in comparison to traditional working fluids like water. Flow visualization experiments and particle image velocimetry measurements were performed for three argon bubble sizes injected into a co-current stream of molten salt in a natural circulation loop facility. Similar bubble sizes were injected in experiments with water to compare the bubble shape, trajectory, and wake flow behavior of the fluids. The bubble region of interest was used to calculate the equivalent diameter and terminal velocity. Results for water showed a wobbling bubble surface and less stable bubble trajectory due to lower surface tension and viscosity compared with molten salt. Particle image velocimetry results demonstrated the increased viscosity of salt dampens turbulent fluctuations for the smaller bubble size. For a cap bubble, turbulent fluctuations were larger and longer lasting than in results for the wake flow of an argon cap bubble in water.

Two-group drift-flux model for dispersed gas-liquid flows in annuli

Interfacial area concentration (IAC) plays a vital role in mass, momentum, and heat transfers between gas and liquid phases in two-phase flows. The IAC of group-one bubbles, including spherical and distorted bubbles, differs from the IAC of group-two bubbles, including cap, slug and churn-turbulent bubbles. The two-group interfacial area transport equation (IATE) can be used as a mechanistic model to obtain the IAC of each group. The equation demands the two-group gas velocity fields, leading to the need to solve an extra momentum equation, which is complicated. The two-group drift-flux model can prevent this complexity by providing two-group gas velocities without introducing the extra momentum equation. Despite the importance of gas-liquid flows in annuli, the drift-flux model for gas-liquid flows is not well-developed like it is for pipe flows. The main objective of this study is to develop a two-group drift-flux model for dispersed vertical flows in annuli. The two-group drift-flux model provides the flow characteristics of group-one and group-two bubbles, such as void fractions and velocities, which strongly affect the prediction of the available interfacial area concentration of the group-one and group-two bubbles for heat and mass transfer. Hence, after comprehensively investigating the available experimental data, this study first improves a one-group drift-flux model for dispersed gas and low-viscosity liquid upward flows in annuli and proposes a new two-group drift-flux model. Then, the models are evaluated using the respective group experimental data. The consistency between one-group and two-group drift-flux models is also investigated. This consistency helps to reduce the two gas momentum equations to one gas mixture momentum equation. The results of the comparison between model predictions and experimental data are satisfactory.

ANALYSIS OF THERMAL PERFORMANCE IN A TWO-PHASE THERMOSYPHON LOOP BASED ON FLOW VISUALIZATION AND AN IMAGE PROCESSING TECHNIQUE

Thermal performance was analyzed based on flow visualization and an image processing technique in a two-phase thermosyphon loop (TPTL) with boiling water as the working fluid at low pressure. The bubble geometries, bubble frequency, and dynamic void fraction were estimated using direct image analysis combined with the power spectrum and statistical analysis. At a heating rate of 400 W and a filling ratio of 0.88, the thermal performance of the TPTL was enhanced with a thermal efficiency of 91% and effective thermal conductivity of 11,336 Wm<sup>-1</sup> °C<sup>-1</sup>. The enhancement was due to the higher frequency cap bubble flow of 11.2 Hz obtained by the direct flow visualization. The bigger Taylor bubbles with slug flow were examined to contribute to the negative effect on the heat transfer rate due to their film boiling regimes. The detailed analysis reveals the mechanism of bubble flow interacting with thermal performance.

Vertically downward gas-liquid flow: Void fraction and pressure drop

Using an air-water mixture and 34 mm ID pipe, a series of pressure drop and void fraction measurements were collected. The observed flow regimes were falling film, annular, bubbly, cap bubble, slug, and churn flows. The temporal fluctuations of void fraction and pressure drop measurements were firstly studied using signals visualization and PDF. It was revealed that both signals can be used to identify the flow regimes. A new flow pattern map is proposed based on the Froude number, calculated using the mixture velocity of the two phases, and the ratio of gas-to-liquid superficial velocities. The behaviors of the total and frictional pressure drops were studied and the influence of the superficial velocities as well as the flow regimes were identified. The experimental results were compared to four existing vertically downward frictional pressure drop models, none of which provided accurate predictions.

Molecular simulation study of methane hydrate decomposition in the presence of hydrophilic and hydrophobic solid surfaces

Natural gas hydrates (NGHs) exist mainly in marine muddy porous media, and it is of great significance to understand the hydrate decomposition in microporous sediments for hydrate exploration. In the paper, molecular dynamics (MD) simulations have been performed for CH4 hydrate decomposition in the slit nanopores of graphite and hydroxylated-silica surfaces. As expected, the higher the temperature, the faster the decomposition rate. Compared with the hydrate decomposition in the free system, the solid surface can enhance hydrate decomposition; the hydroxylated silica surface has better enhancement effect, and this effect is more obvious at lower temperatures. At 330 K, 340 K, 350 K and 360 K, the decomposition rates of hydrate in HS-H systems are 50.8%, 36.4%, 14.7% and 11.7%, respectively, which are higher than those in G-H systems. In the slit-nanopores, hydrate decomposition starts from the surface and then grows towards the bulk region, the hydrate near the pore surface decomposes faster. However, due to the strong ability of the hydrophilic surface to adsorb water molecules, the hydrophilic-silica pore has a more vital role in enhancing hydrate decomposition near the surface in the early stage. The higher the temperature, the stronger the diffusion ability of methane molecules and water molecules, and methane molecules diffuse more easily in pores in directions parallel to the surface than perpendicular to it. Due to the difference in wettability, the contact angles of nanobubbles on the surface are different, resulting in different forms of nanobubbles. For the G-H system, both surface cap bubbles and bulk spherical bubbles were formed at higher temperatures, while only surface cap bubbles were formed at lower temperatures. For the HS-H system, bulk spherical bubbles were formed over the temperature range studied. The formation conditions of the bulk spherical bubbles are more lenient than surface cap bubbles. The hydrates in the graphene pores proceed in an isotropic and uniform decomposition. At 330 K, the liquid methane concentration of HS-H system (0.03181–0.03397) is higher than that of G-H system (0.02452–0.02559) at t = 4–6 ns, while the decomposition rate is higher than that of G-H system, which is due to the formation of bulk spherical bubbles. Bulk spherical bubbles in the hydroxylated-silica pores leads to the uneven decomposition of the hydrates, this inhomogeneity leads to local differences in hydrate dissociation rate, and the difference in hydrate dissociation rates in different regions can be up to 4.3 times. The above findings provide the theoretical basis for the safe and efficient exploitation and transportation of hydrate in marine sediments.

Drift-flux model for upward dispersed two-phase flows in vertical medium-to-large round tubes

The void fraction prediction is critical in nuclear safety analyses. Available one-dimensional computational codes heavily depend on the two-fluid model. The interfacial drag force in the momentum equation of the two-fluid model is currently formulated using the drift-flux model. Thus, the drift-flux model is critical for advanced two-phase flow analysis computational codes. The drift-flux model is characterized by two essential parameters: the distribution parameter and the drift velocity. The distribution parameter expresses the covariance of the area-averaged product of a void fraction and mixture volumetric flux. Flow channel geometry is a significant factor affecting the distribution parameter. The drift velocity expresses the relative velocity between gas and liquid phases. A bubble shape regime affects the drift velocity. The bubble shape regime is susceptible to flow regime dependence on the bubble Reynolds number. Bubbles in a spherical or distorted particle regime (group-1 bubbles) dominate in bubbly flow at a low void fraction, whereas bubbles in a slug or cap bubble regime (group-2 bubbles) play a critical role in beyond-bubbly flow. Whether slug bubbles or cap bubbles appear depends on the channel size. Thus, flow channel size significantly affects the drift velocity. The drift velocity dynamically changes along the two-phase flow, transforming from bubbly to beyond-bubbly flows.The present study first formulates the evolution process of the drift velocity using the two-bubble-group approach. A two-group-based formulation is simplified to model the drift velocity behavior at the bubbly-to-beyond-bubbly flow transition. A scheme is proposed to explicitly calculate the distribution parameter and drift velocity using operating parameters. A drift-flux correlation with the new drift velocity model is evaluated by 750 data collected from 8 different sources. Tested fluid systems are nitrogen-water, air-water, and steam-water systems. Flow channel diameter ranges from 0.0508 to 0.305 m. Test section height (L)-to-channel diameter (D) ratio L/D ranges from 9.41 to 130. Superficial gas velocity is changed from 0.0100 to 11.2 m/s, whereas superficial liquid velocity is changed from 0 (pool condition) to 2.6 m/s. Operating pressure covers from 0.1 to 4.6 MPa. A systematic evaluation demonstrates the validity of the newly developed drift velocity model for a wide range of test conditions. This model is simple enough to be implemented into existing one-dimensional computational codes without any substantial code architecture change.

Comparative Analysis of Riser Base and Flowline Gas Injection on Vertical Gas-Liquid Two-Phase Flow

Gas injection is a frequently used method for artificial lift and flow regime rectification in offshore production and transportation flowlines. The flow behaviour in such flowlines is complex and a better understanding of flow characteristics, such as flow patterns, void fraction/hold up distributions and pressure gradient is always required for efficient and optimal design of downstream handling facilities. Injection method and location have been shown to strongly affect downstream fluid behaviour that can have important implications for pumping and downstream facility design, especially if the development length between pipeline and downstream facility is less than L/D = 50 as reported by many investigators. In this article, we provide the results of an experimental investigation into the effects of the gas injection position on the characteristics of the downstream upwards vertical gas flow using a vertical riser with an internal diameter of 52 mm and a length of 10.5 m. A horizontal 40-m-long section connected at the bottom provides options for riser base or horizontal flow line injection of gas. The flowline gas injection is performed 40 m upstream of the riser base. A 16 by 16 capacitance wire mesh sensor and a gamma densitometer were used to measure the gas-liquid phase cross-sectional distribution at the riser top. A detailed analysis of the flow characteristics is carried out based on the measurements. These include flow regimes, cross-sectional liquid holdup distributions and peaking patterns as well as analysis of the time series data. Our findings show that flow behaviours differences due to different gas injection locations were persisting after a development length of 180D in the riser. More specifically, core-peaking liquid holdup occurred at the lower gas injection rates through the flowline, while wall-peaking holdup profiles were established at the same flow conditions for riser base injection. Wall peaking was associated with dispersed bubbly flows and hence non-pulsating as against core-peaking was associated with Taylor bubbles and slug flows. Furthermore, it was found that the riser base injection generally produced lower holdups. It was noted that the circumferential injector used at the riser base promoted high void fraction and hence low liquid holdups. Due to the bubbly flow structure, the slip velocity is smaller than for larger cap bubbles and hence the void fraction is higher. The measurements and observations presented in the paper provides valuable knowledge on riser base/flowline gas introduction that can directly feed into the design of downstream facilities such as storage tanks, slug catchers and separators.

On the dynamics of a collapsing bubble in contact with a rigid wall

This work reveals that the dynamic response of a spherical cap bubble in contact with a rigid wall depends on the effective contact angle at the instant prior to collapse. This parameter allows us to discriminate between two regimes in which the mechanisms of interaction between the collapsing bubble and its surrounding medium differ significantly: when the contact angle is smaller than $90^{\circ }$ , a classical jet directed towards the wall is observed, whereas if the initial contact angle is larger than $90^{\circ }$ , an annular re-entrant jet parallel to the wall appears. We show that this change of behaviour can be explained using the impulse potential flow theory for small times, which shows the presence of a singularity on the initial acceleration of the contact line when the contact angle is larger than $90^{\circ }$ . Direct numerical simulations show that although viscosity regularises the solution at $t > 0$ , the solution remains singular at $t=0$ . In these circumstances, numerical and experimental results show that the collapse of flat bubbles can eventually lead to the formation of a vortex ring that unexpectedly induces long-range effects. The role of the bubble geometry at the instant of maximum expansion on the overall collapse process is shown to be well captured by the impulse potential flow theory, which can be generalised easily to other bubble shapes. These results may find direct application in the interpretation of geophysical flows as well as the control and design of biomedical, naval, manufacturing and sonochemistry applications.

Experimental study of gas-liquid flow patterns and void fraction in prototype 5 × 5 rod bundle channel using wire-mesh sensor

In this paper, the wire-mesh sensor (WMS) has been successfully introduced into a typical pressurized water reactor (PWR) prototype 5 × 5 rod bundle channel. Combined with the high-speed camera, the typical flow patterns and phase distribution of air–water two-phase flow in the rod bundles are experimentally studied. The gas superficial velocity is 0.020 ∼ 1.102 m/s and liquid superficial velocity is 0.041 ∼ 1.604 m/s. The results indicate that the WMS can accurately identify bubble, cap bubble, cap churn and churn flow. Due to the influence of the wall effect, flow patterns between the subchannels of the rod bundle channel are not completely consistent with the global flow pattern. After evaluating the existing drift-flux models, new correlations of distribution parameter and drift velocity for different flow patterns in rod bundle channel are proposed, which shows a good prediction capability.

Analogy between Vertical Upward Cap Bubble and Horizontal Plug Flow

SummaryThe intermittent gas-liquid flow can be seen in both vertical upward and horizontal pipes. In a vertical pipe, the gas pockets of intermittent flow can be present as cap bubbles (cap bubble flow) or Taylor bubbles (slug flow), while in a horizontal configuration, the intermittent flow can be as plug or slug flows. Extensive literature survey has shown a lack of deep understanding of the difference between the vertical upward cap bubble and horizontal plug flow regimes.This paper explains the hydrodynamic similarities between vertical cap bubble flow and horizontal plug flow regimes. Moreover, the differences between the cap bubble and slug flow in vertical pipes are explained in detail. The study was carried out using a collected database from the open literature of different flow parameters. A comparison between the behavior of the void fraction, bubble structure velocity, slippage number, slug frequency, and slug length demonstrated the similarity between cap bubble and plug flows. It was also demonstrated, from the evolution of the void fraction, that the gas-to-liquid superficial velocities ratio plays a significant role in the cap bubble-to-slug flow transition. These results highlight the existence of an analogy between vertical cap bubble and horizontal plug flow. In addition, the difference between the flow structures and flow parameters behavior between cap bubble and slug flow, demonstrated in this study, highlights the need to differentiate between the two flow patterns.

Statistical characterization of the flow structure of air-water-solid particles three-phase flow in the airlift pump-bubble generator system

The purpose of the present study was to investigate the performance and flow structure of an airlift pump-bubble generator during the lifting of gas-water-solid particles three-phase flow experimentally. Here, a mechanistic model was also developed to predict the performance of an airlift pump bubble-generator on the basis of power balance. In the experimental part, the flow structure was analyzed visually, and taken from the extraction of the differential pressure signal at both the bottom and top test sections. Time series of differential pressure normalization was analyzed by using wavelet transform to determine the wavelet energy distribution. The wavelet energy was used as input the artificial neural network method to clustering the flow regime.The results indicate that under a constant of superficial gas velocity, the discharged both the water and particle increase with the submergence ratio (SR). SR is defined as the ratio of the distance from the injector to the water surface and the distance from the injector to the outlet side. Next, under a constant gas superficial velocity, the increase of SR will increase the solid fraction, but the fractions of both gas and water will decrease. The flow patterns were classified in the clustered bubble, homogeneous bubble, cap bubble, bubbly-stable slug, bubble unstable slug, and slug churn. Furthermore, the bubble flow is indicated by the peak wavelet energy on an eighth-level decomposition of approximation signal (a8), and the energy of the fourth-level decomposition of detail signal (d4) closes to the energy of the fifth-level decomposition (d5). The wavelet energy concentrated on the seventh and/or eighth level decomposition of detail signal (d7 and/or d8) is the indicator of slug flow in the riser pipe. The clustering flow patterns by using the ANN with input from wavelet energy gives a better approach than that of stochastic parameters of time series of the pressure differential. Moreover, the developed mechanistic model shows a good agreement with the experimental data.

Effects of liquid viscosity and bubble size distribution on bubble plume hydrodynamics

The present paper provides a complete databank of bubble plume hydrodynamics in various media characterized by different viscosities in the range 1–100mPa.s. Thus, the influence of the viscosity on bubble plume oscillations and on both liquid and gas velocity fields is studied. The role of the bubble size distribution at different viscosities was also analysed by using two different spargers: a membrane to create ellipsoidal millimetrice bubbles and a slugflow tube to create spherical cap bubbles. Mean velocity profiles, averaged over a large number of plume oscillation periods and corresponding root mean square profiles are discussed, in both phases. Furthermore, void fraction profiles, plume oscillation periods, and bubble size distributions are given for all experimental conditions. Finally, probability density functions and phase averaged profiles in the liquid phase provide additional information. The dependence of bubble plume hydrodynamics on liquid viscosity is huge. At large viscosity, the oscillating plume is damped, regardless of the sparger. Concerning the membrane sparger the viscosity increase modifies the plume oscillation period and its lateral plume expansion, whereas, in the case of the slugflow sparger, only the plume expansion is impacted. Liquid phase hyrdodynamics are weakly affected by the choice of sparger, especially at high viscosities.

From single bubble to global mixing: Added mass, drift, wake

The goal was to present an approach where the micro-scale events related to single-particle motion directly translate onto higher levels and produce large-scale observable phenomena related to transport and mixing. Three multiphase hydrodynamic effects were considered, analysed and discussed: added mass, drift, wake. Although the results are of general validity, the spherical-cap shape (cap bubble) was used as the definite object for demonstration. It was shown how the unsteady inertial effect of added mass (coefficient C), irrelevant for mixing, is related to the steady kinematic concept of Darwin drift (coefficient D) that is directly associated with mixing. It was shown how the inviscid drift and the viscous wake operate and contribute to the resulting circulation and mixing. List of currently available formulas for C was completed, which are usable for estimation of D.

Interfacial area transport for bubbly-to-slug transition flows in small diameter pipes

This study aims to experimentally investigate the two-group interfacial area transport in small diameter pipes. Experimental data focusing on the bubbly to slug transition regime, namely one-group to two-group transport region, are collected in a 12.7 mm vertical pipe under adiabatic, air-water conditions. The result shows the intergroup transfer in the small diameter pipe can be drastic, especially under low superficial liquid velocities. The cause of this phenomenon is mainly due to the large relative bubble size comparing to the pipe cross-sectional area. The wake entrainment effect could be enhanced by the small spherical bubbles that are acting like cap or slug bubbles in a medium-size pipe. Based on the experimental observation, a throughout analysis of the dependence of the drastic intergroup transfer is provided in this study. The models predicting the initiation of drastic intergroup transfer in small diameter pipes in terms of the bubble diameter and the void fraction are developed. These models are compared with the two-phase data among the different pipe sizes and the results show a good agreement. These newly developed models are applied to the IATE wake entrainment model by developing a transition function analogous to the sigmoid function. With the transition function, the revised IATE model is given the new ability on predicting the drastic intergroup transfer phenomenon.

Experimental investigation of two-phase flow evolution in a tight lattice bundle using wire-mesh sensor

Tight lattice fuel assemblies have significant advantages in power density and conversion ratios. However, the dynamic characteristics of two-phase flow in tight lattice bundles, which is very important to safety analysis, are still not clarified. This study presents an experimental investigation of two-phase flow evolution in a double sub-channels tight lattice bundle. The experimental channel was up-scaled (1:2.7) compared with the prototype fuel. Phase distributions at four different axial positions, z/Dh = 28.95, 57.90, 86.86, and 115.8, are measured with a high spatiotemporal resolution by a 16 × 32 wire-mesh sensor. The flow regime includes bubbly flow, cap-bubbly flow, and slug flow. In the bubbly flow, the large-scale pulsations across the gap of two sub-channels were observed, as same as the single-phase flow in a tight lattice bundle. The time-averaged void fraction shows a gap region peak in bubbly flow. In the cap-bubbly flow, the cap bubbles are confined to a single sub-channel due to the small gap. The cap bubbles are alternated arrangements in the two sub-channels and the large-scale pulsations across the gap are caused due to this special flow configuration. The time-averaged void fraction shows a core peak profile in a cap-bubbly flow. In the slug flow, the slug bubbles can across the small gap to span the two sub-channels and the time-averaged void fraction shows a core peak profile. Even though the time-averaged void fraction almost coincides at different axial positions, the phase interface structure shows a significantly different according to the virtual side view of void fraction. The accuracy of the drift-flux models has been evaluated with the experimental data. The Bestion's model underestimated the void fraction in the channel while the Chexal's model overestimated it. Clark's model gets the best prediction with an error of 10.1%.

Statistical Characterization of Flow Structure of Air–water Two-phase Flow in Airlift Pump–Bubble Generator System

The aim of this study is to identify the flow structure in an airlift pump–bubble generator system by using experimentally obtained differential pressure signals. Differential pressure measurements were applied at both the bottom and top test sections. The normalized time variation of the differential pressure data was analyzed using the probability density function (PDF), power spectral density function (PSDF), Kolmogorov entropy, and discrete wavelet transform (DWT). The results indicate that the water movement mechanism in the riser pipe could be divided into three regions, namely fixed liquid, locally moving liquid, and fully moving liquid, depending on the supplied superficial air velocity. Moreover, the chaotic in riser pipe increases with the increase of the submergence ratio at high supplied superficial air velocity. The chaotic level at the bottom of the test section was higher than that at the top of the test section at low supplied superficial air velocity. Finally, the observed flow patterns closely conformed to the previous definition and flow pattern maps. The bubbly flow was classified into clustered, homogeneous, and cap bubble regimes. The slug flow regime was classified into bubbly stable slug, bubbly unstable slug, and slug-churn, wherein the frequencies of both the liquid and air slugs increased with an increase in the air superficial velocity and submergence ratio.

Beyond time-averaged measurement of bubble parameters in steam-water flows with conductivity probes

Two-phase data from flashing experiments are presented beyond the traditional time-averaged approach in this work. Probability density functions of parameters such as bubble duration, velocity, chord length, and number frequency are obtained from four-sensor conductivity probes and presented here. The axial evolution of the distributions is discussed in detail along with high-speed videos for a qualitative description of the flow field. The distributions of bubble parameters are observed to be positive-skewed with a long tail, and the shape of the distributions is analyzed in terms of the skewness and kurtosis. The number frequency of spherical and cap bubbles in the liquid slug is studied, as well as the number ratios of each bubble type in the flow. Parametric studies are carried out to investigate the effects of mass flux and system pressure on the distributions of bubble parameters. Bubble diameter distribution functions such as the Nukiyama-Tanasawa, Weibull, and lognormal distributions are validated with the chord length data of spherical bubbles where the Nukiyama-Tanasawa distribution appears to fit the chord length data the best. Sensitivity study is performed to investigate the effects of the adjusting parameters on the accuracy of the fits. The data are potentially useful for the benchmark and validation of high-fidelity simulation tools and the development of modeling approaches for phase-change flows. This work also provides a template for reporting beyond-time averaged measurements in future datasets generated with conductivity probes.