Upward Air-water Two-phase Flow Research Articles

Effect of inlet gas and liquid velocity profiles on one-group interfacial area transport equation in a vertical large rectangular channel

Bubbly flows appear in many heat and mass transfer equipment. The interfacial area concentration is a critical parameter in determining the performance of the heat and mass transfer systems. The one-dimensional one-group interfacial area transport equation (IATE) was developed based on area-averaged bubbly flow data collected under uniform inlet flow boundary conditions. The applicability of the IATE to the flow systems with non-uniform inlet flow boundary conditions was not comprehensively examined. This study investigated the effect of inlet liquid and gas velocity profiles on measured interfacial area concentration changes along the flow direction. The data used in this study were collected for vertical upward air-water two-phase flow in a rectangular channel with a gap length of 10 mm and width of 200 mm. The test conditions were the superficial gas velocity ranging from 0.0870 to 0.288 m/s and the superficial liquid velocity ranging from 0.515 to 2.53 m/s. The interfacial area concentrations with uniform and non-uniform inlet flow boundary conditions measured in a large vertical rectangular channel were compared under the same area-averaged flow rate conditions. Then, the predictive capability of the IATE to the bubbly flow test data with uniform and non-uniform inlet flow boundary conditions was assessed. Some non-uniform inlet flow boundary conditions, including channel-center peaking of liquid flow (CPF), channel-center peaking of gas flow (CPG), single-side-wall peaking of liquid flow (SPF), and single-side-wall peaking of gas flow (SPG), significantly affected the interfacial area concentration changes along the axial location at relatively high superficial gas or liquid velocity conditions. The IATE's predictive capability deteriorated for several non-uniform inlet flow boundary conditions, including double-side-wall peaking of gas flow (DPG) and single-side-wall peaking of liquid flow (SPF), specifically at relatively high superficial liquid velocity conditions. The maximum percentage error of the interfacial area concentration prediction by the IATE for these non-uniform inlet flow boundary conditions reached 75%.

The research on interfacial area transport equation in rectangular channels: Experimental research and models development of sink and source terms

Interfacial area concentration is a crucial constitutive parameter to close the two-fluid model, the accuracy of whose prediction is vital, so the interfacial area transport equation (IATE) has been proposed. According to the analysis of experimental results involving bubble interaction mechanisms in the rectangular channel from the bubbly-to-slug flow, new sink and source terms in a one-group IATE are put forward in this study. The new source term aroused by random collision is considered with relative velocity inducing collision and the effect of gap size on coalescence efficiency. Besides, the number density of small and large bubbles is considered respectively to model the source term of wake entrainment. Combined with the source terms of break up brought by turbulent impact and expansion caused by the pressure gradient, a new one-group IATE applicable to rectangular channels are completed. The coefficients in the newly-derived model are determined by experimental results of 4 × 66 mm and 8 × 66 mm vertical rectangular channels under co-current upward air-water two-phase flow, and the average relative error is 11.9%. Furthermore, the new model is also testified by the experimental result in the 6 × 66 mm rectangular channel independently, and the average relative error is 14.4%. Generally, the new model is feasible to predict the change of the interfacial area concentration in different rectangular channels.

Investigation on Local Gas Content For Upward Air–Water Two-Phase Flow And Comparison With Classical Void Fraction Calculation Models

Investigation on Local Gas Content For Upward Air–Water Two-Phase Flow And Comparison With Classical Void Fraction Calculation Models

Two-group phase distribution characteristics for air-water flow in 5 × 5 vertical rod bundle channel with mixing vane spacer grids

The phase distribution in rod bundle channel was associated with the heat transfer coefficient, flow resistance and critical heat flux, which was of great significance for the design and safe operation of nuclear reactor and steam generator. In order to clarify the phase distribution characteristics and reveal the effect of mixing vane spacer grid (MVSG) in two-group air-water flow in rod bundle channel, the adiabatic upward air-water two-phase flow experiments were conducted in 5 × 5 rod bundles with MVSGs and the detailed local two-phase flow parameters were acquired with miniaturized four-sensor conductivity probe (MFSCP). Some interesting phenomena were reported: Group I and II void fraction profiles showed the core-peak features; Group I void fraction almost stayed at the region from 0.2 to 0.3 for two-group air-water flow at current flow conditions; the dissipation length of MVSG was larger than 32.1 L/D at current two-group flow conditions; and the influencing region of MVSG was divided into at least three regions along the axial direction with the flow development for current flow conditions.

High-resolution high-speed void fraction measurements in helically coiled tubes using X-ray radiography

Helical coil heat exchangers are known for their superior heat transfer, compactness and robustness against thermal expansion compared to systems with straight pipes. Due to all these benefits, they became quite popular in the last few decades in various industries such as pharmaceutical, chemical or nuclear. Generally, they are used in a single-phase environment or with evaporation taking place on the shell side. However, in most of the steam generator designs adopted in advanced nuclear reactors, the two-phase mixture flows upward within the vertically oriented helical coils. In addition, physical two-phase phenomena taking place inside helical tubes differ significantly from those in straight pipes due to inherent centrifugal and torsion effects. To improve the computational models used to predict the thermal hydraulic performance of such steam generators, experimental characterization and identification of different two-phase flow patterns in helical coils are essential. In the present paper, high-resolution measurements of the void fraction distribution for an adiabatic upward air–water two-phase flow were conducted in a transparent helically coiled acrylic pipe by means of high-speed X-ray radiography at ambient conditions. Various void fraction based features such as probability density function estimates, profiles over the height of the pipe cross-section and temporal evolutions were used to identify and characterize flow regimes. A total of 13 measurements are presented in this paper, classified into six flow regimes, namely: bubbly, plug, wavy, slug, slug-annular and annular flow. The obtained flow regimes are in accordance with flow regime characterizations and transition criteria already reported in literature.

Experimental investigation on local/global void distribution of air-water two-phase flow in a 3 × 3 rod bundle channel under low-flow conditions

In this study, the vertical upward air–water two-phase flow tests have been carried out to investigate the distribution of local and global void fractions in a rod bundle channel under low-flow conditions. The height of the rod bundle channel is about 5 m, and the channel cross section is comprised of a 52 × 52 mm rectangular casing and 3 × 3 circular rods. The rods with a diameter of 11.5 mm and a pitch of 15.4 mm were set as a rectangular array. Four sets of non-intrusive conductivity sensors were built to measure the local and global void fractions of various gaps within the rod bundle channel. The tests were operated at low flow conditions: jg= 0.016–0.92 m/s and jf = 0.02–0.05 m/s, which covered the possible flow regimes of bubbly, cap-bubbly and cap-turbulent flows. The measured global void fraction were benchmarked with existing one-dimensional drift-flux model, and the local void fractions have been summed up by regions and compared with the global void fraction for validation. The dynamic characteristics of local/global void fractions have been analyzed with probability density function (PDF) and fast Fourier transform (FFT), and the distributions and variations of averaged void fraction against test conditions have been presented. Furthermore, the empirical correlations for estimating the local void fractions have been proposed, which can successfully estimate the local void distribution from global void fraction with limited errors. The present results and correlations can be a reference for future development and modification of detailed two-phase flow models in the rod bundle geometry.

Experimental Investigation for Flow Regime Identification Using Probability Density Function of Void Fraction Signals

Abstract In this study, upward air–water two-phase flow tests were carried out in a 3 cm diameter pipe under atmospheric pressure, and over 3000 data points were collected from a wide range of superficial gas and liquid velocities (⟨jg⟩ ≈ 0.02–30 m/s and ⟨jf⟩ ≈ 0.02–2 m/s) for the investigation of flow regime identification. The probability density function (PDF) of transient void fraction signals and its full-width at half-maximum (FWHM) were obtained and used for analysis and data classification. Considering the features of PDF profiles, the flow conditions can be classified into four regions, which show a fair agreement with the existing flow regime maps in general trends. Furthermore, by examining the FWHM distributions, two more regions with high-FWHM (HF) values were identified as the transitions of higher-flow bubbly-to-slug and slug-to-churn flows as well as most portion of churn flow, and a valley region next to the HF regions can express the transition of churn-to-annular flows. Overall, six groups of flow conditions can be classified based on the present methodology, and each group can be corresponding to specific flow regimes or transition regions. This study can provide a simple and efficient way for flow regime identification.

Characterisation and classification of gas-liquid two-phase flow using conductivity probe and multiple optical sensors

Attempts have been made for identification and classification of regimes for upward air-water two-phase flow fusing the response from multiple sensors. A needle type conductivity probe and different combinations of optical probes have been employed. The statistical signatures of the signals have been used for identification of the type of flow and revealing some information regarding the interfacial structure. The signals have further been processed using Principal Component Analysis (PCA) and used to classify the flow regime by k-mean clustering algorithm. Classifications have been made using signals from different individual sensors as well as combining them. Finally, the performance of the predictions made by these sensor combinations have been made using confusion matrix.

Flow structure of bubbly to slug transition flow in a small pipe

To develop a mechanistic constitutive model to characterize the flow structure development in more than one flow regime, the detailed measurement of the flow structure is indispensable. In this study, the void fraction, interfacial area concentration, and bubble velocity profiles of upward air-water two-phase flow has been measured using the four-sensor conductivity probe in a 25.4 mm ID pipe. The test matrix has two features that distinguish it from previous studies, which is the void fraction-based flow condition selection and the wide range. The test matrix consists of 24 flow conditions with 3 different superficial liquid velocities (0.5, 1.0, 2.0 m/s) and 8 different void fractions (0.10, 0.15, 0.20, 0.25, 0.30, 0.35, 0.45, 0.60). The results indicate that the flow characteristics and flow structures not only depend on the void fraction but also significantly depend on the superficial mixture velocity. The velocity effects on various flow characteristics are discussed such as void distribution, cap bubble generation, interfacial area. Besides, a velocity correlation has been developed to predict the bubble velocity radial distribution, with the mean absolute percentage error of 10.05% for the current data.

Experimental investigation on the thermal and entropic behavior of a vertical helical tube with none-boiling upward air-water two-phase flow

Helical tubes have great compact structure, two-phase flow stability and well thermal performance. The increment of applications which use two-phase flow in helical tubes, necessitates the investigation on thermal and entropic characteristics of two-phase flow in helically coiled tubes. The present study provides experimental results of heat transfer characteristics of upward air-water none-boiling two-phase flow in a vertical helically coiled tube. The helical tube was put under constant heat flux. The inlet, outlet, wall and ambient temperature of helical tube were measured for calculating the heat transfer coefficient and entropy generation. The water superficial Dean number (Desl) was between 1000 and 4800. Also, the gas (air) superficial Dean number (Desg) was between 45 and 235. The VF (volume fraction) fraction was between 0.11 and 0.55. Almost all investigated cases were at the range of slug and plug flow regimes. Results revealed that the air-water two-phase flow could increase the heat transfer coefficient. Increment of air superficial Dean Number increased the heat transfer rate and entropy generation. A maximum increment of 35% and %26 were observed for heat transfer coefficient and entropy generation respectively. Also it was revealed that increment of VF increases the Witte-Shamsundar efficiency.

Experimental study of interfacial characteristics of vertical upward air-water two-phase flow in 25.4 mm ID round pipe

An adiabatic upward air-water flow experiment was performed on a vertical pipe with 25.4mm inner diameter to investigate the interface structure and characteristics. Local experiment parameters, including time-averaged void fraction, interfacial area concentration, bubble interfacial velocity and Sauter mean diameter were measured using a four-sensor conductivity probe with a data acquisition frequency of 100kHz. Experiment data for 19 flow conditions was collected at 3 axial locations (z/D=15, 78, and 141) and 13 radial positions from r/R=−0.84 to 0.7. In the experimental study, the measuring range of superficial liquid and gas velocity were from 0.3 to 1.0m/s and 0.03 to 11.0m/s, respectively. The flow regimes ranged from bubbly flow to the transition of churn-turbulent to annular flow. The local measurement result agreed well with the drift flux model by Ishii (1977) and Kataoka (1987). The different interfacial structures in different flow regimes and two group bubble interfacial transfer were observed from the experiment result. This data will be used for the evaluation and extension of the current interfacial area transport equation.

Experimental investigation on dominant waves in upward air-water two-phase flow in churn and annular regime

This paper describes new measurements on the characteristics of waves in churn and annular flow. The measurements were for air-water flow in an 11mm diameter vertical tube, a diameter close to the equivalent diameter of nuclear reactor channels. These measurements extend the parameter space of previous publications into the low and gas and liquid mass flow rates important for nuclear reactor fault studies. Detailed measurements of the flow characteristics are reported, including the identification of a new regime we designate ‘pre-annular flow’, in which the liquid film between the waves appears to be stationary. The results of these measurements at low mass fluxes are compared with predictions in this low mass flux region using the principal literature correlations, all of which were actually obtained from elsewhere in the parameter space. Reassuringly, the broad conclusion is that despite this, these correlations obtained from elsewhere are safe to continue to be used in this low mass flux region. A new interpretation of the criteria for onset of entrainment was also obtained from the study.

Experimental investigation and identification of the transition boundary of churn and annular flows using multi-range differential pressure and conductivity signals

In this study, an upward air-water two-phase flow experiment was carried out with a transparent circular tube to investigate the flow characteristics of churn and annular flows. A simple and novel method utilizing the transient differential pressure and conductivity signals were proposed to identify the transition boundary of churn and annular flows. The test range covers the flow conditions of superficial gas and liquid velocities of Jg≈5.1–30m/s and Jf≈0.04–0.4m/s under the atmospheric pressure, and the statistical techniques including PDF, FFT and STD mesh plots were used to present the flow characteristics. According to the present experimental results, the churn flow may transform to annular flow at the transition boundary of Jg≈12.6–15.7m/s under the low liquid flow condition of Jf⩽0.4m/s. The present results were benchmarked with the existing flow regime maps including Mishima-Ishii and Taitel-Dukler maps, and the identified transition boundary of churn to annular flows shows a good agreement with the existing maps. The present method can provide a simple and efficient way to identify the flow regimes.

水平円柱下流の気液界面積濃度分布の遷移

Measurements of the interfacial area concentration and the void fraction in a vertical upward air-water two-phase flow downward of single horizontal tube and horizontal tube bundle were carried out by a double sensor electrical conductivity probe. The test section comprised a 50mm × 50mm square channel partially obstructed by the single horizontal tube and the horizontal tube bundle inside. Five types of the test sections which the horizontal tube arrangement is different were used to provide various distributions of air-water two-phase flow downstream of the test section. The superficial gas velocity and superficial liquid velocity ranged from 0.02 to 0.20 m/s and from 0.13 to 0.53 m/s, respectively. Local flow measurements were performed at the three axial locations of z (z') = 110 (70), 340 (300), and 640 (600) mm and the transverse locations from r = 3 to 25 mm. The single horizontal tube and the horizontal tube bundle effect the distributions of the interfacial area concentration and the void fraction at z (z') = 110 (70). This effect was decreased along the flow direction due to the diffusion of bubble toward horizontal direction. At high gas flow rate conditions, void fraction increase, while interfacial area concentration decrease with increasing gas flow rate because of the bubble coalescence.

Effect of a Polymer Additive on Heat Transfer and Pressure Drop Behaviors of Upward Air–Water Flow in an Inclined Smooth Circular Tube

This article presents an experimental study of the effect of polyacrylamide (PAM) on heat transfer and frictional pressure drop behaviors of upward air–water two-phase flow in an inclined smooth circular tube with an upward inclination angle of 8 degrees from the horizontal direction. The test tube has an inside diameter of 40 mm and an outside diameter of 48 mm. A PAM water solution with a concentration of 300 ppm was used in the experiments. The liquid phase superficial velocities are 0.52, 1.02, and 1.46 m/s and the gas-phase superficial velocities are from 1.79 to 6.54 m/s. Heat transfer tests were performed by cooling the air–water flow inside the test tube through its wall using the cooling water with a heat transfer length of 3 m, and two-phase pressure drops were measured over a length of 3.1 m. Results show that the air–water two-phase frictional pressure drops can be reduced from 26.2% to 42.7%, while the two-phase heat transfer coefficients can be reduced from 39.7% to 80.8% with addition of PAM. Furthermore, new proposed physical mechanisms of the two-phase frictional pressure drop and heat transfer reductions are used to explain the experimental results.

Investigation of one-dimensional interfacial area transport for vertical upward air–water two-phase flow in an annular channel at elevated pressures

The interfacial area transport of vertical, upward, air–water two-phase flows in an annular channel has been investigated at different system pressures. The inner and outer diameters of the annular channel were 19.1mm and 38.1mm, respectively. Twenty three inlet flow conditions were selected, which covered bubbly, cap-bubbly, and churn-turbulent flows. These flow conditions also overlapped with twelve conditions of a previous study for comparison. The local flow parameters, such as void fractions, interfacial area concentrations (IAC), and bubble interface velocities, were measured at nine radial positions for the three axial locations and converted into area-averaged parameters. The axial evolutions of local flow structure were interpreted in terms of bubble coalescence, breakup, expansion of the gas-phase due to pressure drop and system pressure. An assessment of interfacial area transport equation (IATE) was made and compared with the experimental data. A discussion of the comparison between model prediction and the experimental results were made.

Developing structure of two-phase flow in a large diameter pipe at low liquid flow rate

In order to develop the interfacial area transport equation for the interfacial transfer terms in the two-fluid model, accurate data sets on axial development of local parameters such as void fraction, interfacial area concentration, interfacial gas velocity and Sauter mean diameter are indispensable to verify the modeled source and sink terms in the interfacial area transport equation. From this point of view, local measurements of both group 1 spherical/distorted bubbles and group 2 cap/slug bubbles in vertical upward air–water two-phase flow in a large diameter pipe with 200mm in inner diameter and 26m in height were performed at three axial locations of z/D=41.5, 82.8 and 113 as well as 11 radial locations from r/R=0–0.95 by using four-sensor probe method. Here, z, r, D and R are the axial distance from the inlet, radial distance from the pipe center, pipe diameter and pipe radius, respectively. The liquid flow rate and the void fraction ranged from 0.0505m/s to 0.312m/s and from 1.98% to 32.6%, respectively in the present experiment. The flow condition covered extensive region of bubbly flow, cap turbulent flow as well as their transition. The extensive analysis on the radial profiles of local flow parameters and their axial developments demonstrate the development of interfacial structures along the flow direction due to the bubble coalescence and breakup and the gas expansion. The significant decrease in void faction and interfacial area concentration and the increase in Sauter mean diameter and interfacial velocity were observed when the gradual flow regime transition occurred. Finally, the net change in the interfacial area concentration due to the bubble coalescence and breakup was quantitatively investigated in the present paper to reflect the true transfer mechanisms in observed two-phase flows.

Flow regime development analysis in adiabatic upward two-phase flow in a vertical annulus

In this work radial and axial flow regime development in adiabatic upward air–water two-phase flow in a vertical annulus has been investigated. Local flow regimes have been identified using conductivity probes and neural networks techniques. The inner and outer diameters of the annulus are 19.1 mm and 38.1 mm, respectively. The equivalent hydraulic diameter of the flow channel, D H , is 19.0 mm and the total length is 4.37 m. The flow regime map includes 1080 local flow regimes identifications in 72 flow conditions within a range of 0.01 m/s < 〈 j g 〉 < 30 m/s and 0.2 m/s < 〈 j f 〉 < 3.5 m/s where 〈 j g 〉 and 〈 j f 〉 are, respectively, superficial gas and liquid velocities. The local flow regime has been classified into four categories: bubbly, cap–slug, churn-turbulent and annular flows. In order to study the radial and axial development of flow regime the measurements have been performed at five radial locations. The three axial positions correspond to z/ D H = 52, 149 and 230, where z represents the axial position. The flow regime indicator has been chosen as some statistical parameters of local bubble chord length distributions and self-organized neural networks have been used as mapping system. This information has been also used to compare the results given by the existing flow regime transition models. The local flow regime is characterized basically by the void fraction and bubble chord length. The radial development of flow regime shows partial and complete local flow regime combinations. The radial development is controlled by axial location and superficial liquid velocity. The radial flow regime transition is always initiated in the center of the flow channel and it is propagated towards the channel boundaries. The axial development of flow regime is observed in all the flow maps and it is governed by superficial liquid velocity and radial location. The prediction results of the models are compared for each flow regime transition.

Distribution parameter and drift velocity for two-phase flow in a large diameter pipe

In view of practical importance of the drift flux model for two-phase flow analysis in general, and in the analysis of nuclear reactor transients and accidents in particular, the distribution parameter, and the drift velocity have been studied for two-phase flow in a vertical large diameter pipe. In this, study, local measurements were performed on flow parameters, such as void fraction, gas velocity and, liquid velocity in a vertical upward air–water two-phase flow in a pipe of 200mm inner diameter and, 25m in height by using the local sensor techniques such as hot-film probes, optical multi-sensor, probes and differential pressure gauges. Two-phase flow regimes in a vertical large diameter pipe, were classified into bubbly, churn and slug flows according to the visual observation. The values of the, distribution parameter and the mean drift velocity were determined directly by their definition using experiment data of the local flow parameters in a two-phase flow in a large diameter pipe. Various existing drift flux correlations were compared with the present experimental results and experimental data obtained by other researchers. A detailed discussion on the problems of these correlations was presented in this paper.

熱線流速計による垂直大口径管内の気液二相流の計測

In this study, local measurements of flow parameters, such as void fraction, liquid velocity and turbulence intensity were performed in a vertical upward air-water two-phase flow in a 200 mm inner diameter and 24 m in height pipe by using a hot-film anemometer. The measured mean void fraction and mean liquid velocity agreed well with those by the differential pressure method and the liquid Venturi flow meter respectively. The experimental results are summarized as follows: (1) The radial profile of void fraction changes from a wall peak shape to a core peak shape and the radial profile of liquid velocity changes from a flat shape to a core peak shape as the flow develops. (2) When the average void fraction increases, the shape of void fraction radial profile changes from a wall peak to a core peak and the radial profile of liquid velocity shows a gradually increasing core peak shape. (3) The turbulence intensity increases with the increase in void fraction, flow direction and radial direction. The predicted liquid velocities by using existing algebraic turbulence models of Sato et al. and Kataoka and Serizawa were compared with the present experimental data. The comparison indicates that those models do not reasonably predict the liquid velocity distribution in a two-phase flow in a large diameter pipe mainly due to the defect in the predictive model of the liquid shear stress. It is necessary for us to establish a new model for the radial pressure distribution when we predict the liquid velocity distribution by using the measured void fraction distribution and algebraic turbulence models in a large diameter pipe. (author)