Adiabatic Air-water Two-phase Flow Research Articles

Analysis of two-phase flow regimes and parameter distribution in 5 x 5 rod bundle using image processing techniques

With the recent developments in image processing and analysis, this paper presents bubble characteristics distribution in adiabatic air-water two-phase flow through a 5 × 5 rod bundle. The experiment covered water superficial velocities (Jl = 0.012 m/s – 0.421 m/s) and air superficial velocities (Jg = 0.042 m/s – 0.987 m/s) in which three distinct flow regimes were identified. The flow regime map was compared with existing flow regime transition criteria for vertical rod bundles. Distinct features from the two-phase flow images were extracted to train a classifier model to distinguish between regimes from a separate experiment. The model distinguishes between the bubbly flow regime and others accurately. The void fraction and velocity distributions were also extracted from the R–CCN masked images. Bubble-induced turbulence that was dominant in the subchannel at (Jl = 0.28 m/s) shifted to the outer subchannels and gaps when the flow rate increased (Jl = 0.42 m/s). These methods over-predicted the void faction around the surfaces of the inner rods.

Experimental study of two-phase flow structure in churn-turbulent to annular flows

The local phase distribution of adiabatic air-water two-phase flow in churn-turbulent to annular flows was experimentally investigated using the Two-Sensor Droplet-Capable Conductivity Probe (DCCP-2). The experiments were performed in a 2.54 cm inner diameter vertical round pipe at downward flows. Twelve flow conditions in churn-turbulent to annular flows were chosen for the current study. The inlet superficial gas velocity ranged from 5.68 m/s to 17.19 m/s, and the inlet superficial liquid velocity ranged from 1.21 m/s to 2.64 m/s. The DCCP-2 is able to distinguish droplets, liquid ligaments, bubbles, and continuous gas. The radial profiles of volume fraction, frequency, axial interfacial velocity, and chord length of droplets, ligaments and bubbles were measured. The experimental results showed that gas volume fraction had a center-peak distribution, and the maximum gas volume fraction in centerline could exceed 95%. The volume fraction of droplets was very low, only on the order of 0.1%. Droplet concentration also showed a center-peak distribution. Both ligament and bubble volume fraction showed a near-wall peak distribution. The ligament volume fraction in the near wall region could reach 80%, which indicated the existence of a film region near the wall and likely includes surface waves on the film. The volume fraction profile of gas, droplets, ligaments, and bubbles showed the tomography structure in churn-turbulent to annular flows. The effects of flow conditions on phase distribution were also studied. An interesting feature was that both the increase of gas flux and the increase of liquid flux tended to increase the bubble volume fraction. The statistical distribution of droplet size was found to be invariant of radial position. The droplet velocity at centerline was generally larger than the superficial gas velocity, which can be attributed to the gravitational effects at downward flows.

Experimental study on local interfacial parameters in upward air-water bubbly flow in a vertical 6 × 6 rod bundle

This paper presents a complete database of the local two-phase flow parameters for upward adiabatic air-water two-phase flows in a vertical 6 × 6 rod bundle flow channel. A total of 16 flow conditions were selected and investigated at 16 measuring points in the octant triangular measuring region of a cross section at the position with height-to-diameter ratio of 149 (z/DH = 149) in the rod bundle experiment. The local void fraction, interfacial area concentration (IAC), bubble diameter and bubble velocity vector were measured by using the four-sensor optical probe measuring technique. In the measured cross-sectional distributions of local void fraction and IAC, both core-peaking and wall-peaking distribution shapes were observed and the distribution pattern trended to change from the core-peaking shape to the wall-peaking shape with the increase of superficial liquid velocity (〈jf〉) and the decrease of superficial gas velocity (〈jg〉). The radial distributions of local bubble diameter are nearly flat, which explains the similarity of the radial local void fraction and IAC distributions. The measured local bubble diameters increase with the increase of 〈jg〉 and the decrease of 〈jf〉. The bubbles move along the rod bundle flow channel with their velocity component in the main flow direction (vgz) showing a typical radial core-peaking profile and their velocity components in the cross section showing a significant movement from the central region to the channel box wall region especially under high superficial liquid velocities. The cross-sectional area-averaged results of the local parameters were obtained by a measuring-point-based graphical integration. The obtained area-averaged void fraction and IAC are compared with the predictions of the drift-flux model with its available 2 frequently-used distribution parameter and drift velocity correlations and the 2 recently-developed IAC correlations respectively. The applicability of the 2 drift-flux correlations and the 2 IAC correlations to the rod bundle flow channel is evaluated and analyzed.

Flow structure and flow regime transitions of downward two-phase flow in large diameter pipes

Downward two-phase flow in large diameter pipes appears in numerous industrial applications and nuclear reactor accidents. In this study, adiabatic air–water two-phase flow experiments in a 203.2 mm diameter pipe have been conducted to investigate flow regimes and their transitions in downward and horizontal flow. Three flow regimes (cap-bubbly, churn-turbulent and annular flow) were recognized in downward flow, as well three flow regimes (stratified, plug and pseudo-slug flow) were observed in horizontal section. Evolution of void fraction and flow structure along the loop under different flow conditions has been discussed. The Probability Density Function (PDF) and Cumulative Probability Density Function (CPDF) of area-averaged void fraction signals were utilized as the indicators for self-organized neural network (SONN) method to identify horizontal and vertical downward flow regimes, respectively. The downward flow regime maps for 203.2 mm diameter pipes have been proposed and compared with that for different diameter pipes. The results show that the flow regime maps agree well with that of 101.6 mm, but don’t agree well with that of smaller diameter pipes (25.4 mm and 50.8 mm). It is found that the transition between churn-turbulent and annular flow occurs at a certain superficial liquid velocity regardless of superficial gas velocity. A set of new transition criteria have been developed for downward flow regime transitions in large diameter pipes, and validated by the experimental data of 203.2 mm and 101.6 mm diameter pipes. Compared with existing models, these criteria provide more accurate predictions for downward flow regime transitions in large diameter pipes.

Flooding behaviour in countercurrent gas-liquid flow in vertical tubes with turbulence promoters

Counterflow of gas and liquid in tubes is of great interest in many industrial processes. The liquid flowing under gravity forms a thin film coating the inner tube wall. The gas driven by a pressure difference flows up in the core of the tube. Interfacial shear stress is acting on the film surface. The phenomenon of flooding is a complex two-phase flow phenomenon. It is a major limiting factor in design and operation of reflux condensers.Inserts in shape of matrix elements are often used as turbulence promotors for in-tube condensation of mixtures. These inserts are disturbing and thinning the liquid film and the concentration boundary layer. The heat and mass transfer is enhanced significantly. These inserts also have an impact on the flooding behaviour.A test apparatus was built up to study the flooding behaviour of a countercurrent adiabatic air-water two-phase flow in a vertical glass tube. The tube length was 3000mm. Two different inner tube diameters of 15mm and 22mm were used. The superficial velocities of air and water were varied to obtain the flooding curves for the empty tubes with 15mm and 22mm inner diameter as well as for the tubes with the low and high density turbulence promoters. Counterflow is possible below the flooding curve. The flooding curve of the 15mm tube is below that of the 22mm tube. That means flooding occurs at lower superficial gas velocities. The possible region for countercurrent flow is diminished when using turbulence promoters and especially with high density. The increasing water column height in the tube and resulting climbing velocity of the liquid are presented. Correlations from literature are used for comparison.

Experimental study of gas–liquid two-phase flow through packed bed under natural circulation conditions

Dry-out phenomena in packed beds or porous media may cause a significant digression of cooling/reaction performance in heat transfer/chemical reactor systems. One of the phenomena responsible for the dry-out in packed beds is known as the counter-current flow limitation (CCFL). In order to investigate the CCFL phenomena induced by gas–liquid two-phase flow in packed beds inside a pool, a natural circulation packed bed test facility was designed and constructed. A total of 27 experimental conditions covering various packing media sizes (sphere diameters: 3.0, 6.4 and 9.5 mm), packed bed heights (15, 35 and 50 cm) and water level heights (1.0, 1.5 and 2.0 m) were tested to examine the CCFL criteria with adiabatic air–water two-phase flow under natural circulation conditions. Both CCFL and flow reversal phenomena were observed, and the experimental data including instantaneous and time-averaged void fraction, differential pressure and superficial gas–liquid velocities were collected. The CCFL criteria were determined when periodical oscillations of void fraction and differential pressure appear. In addition, the Wallis correlation for CCFL was utilized for data analysis, and the Wallis coefficient, C, was determined experimentally from the packed bed CCFL tests. Compared to the existing data-sets in literature, the higher C values obtained in the present experiment suggest a possibly higher dry-out heat flux for natural circulation debris systems, which may be due to the water supply from both top and bottom surfaces of the packed beds. Considering the effects of bed height and hydraulic diameter of the packing media, a newly developed model for the Wallis coefficient, C, under natural circulation CCFL is presented. The present model can predict the experimental data with an averaged absolute error of ±7.9%.

Air–water two-phase flow in a rolling 3 × 3 rod bundle under stagnant condition

Experimental investigation on adiabatic air–water two-phase flow in a 3×3 rod bundle subjected to rolling motion under stagnant condition was carried out. The gas superficial velocity jg is from 0.024m/s to 0.95m/s, and the rolling period and amplitude range from 8s to 16s and from 5° to 15°, respectively. The results showed that the additional pressure drop introduced by rolling motion could be neglected compared with gravitational pressure drop. Rolling motion leads to the void fraction decreasing and the gas rising velocity increasing at low gas flow rate, while the influence at high gas flow rate is not significant. The transient gas flow rate, void fraction and gas rising velocity oscillate apparently in the region of jg<0.086m/s, whereas their fluctuations in the region of jg>0.086m/s are insignificant. In addition, the increase in rolling amplitude or the decrease in rolling period make the fluctuation amplitudes of the void fraction and gas rising velocity increasing. Though Wilson (1965) model provides fair predictions on the averaged values of the two parameters above, it cannot predict their transient values in rolling motion, for which a new correlation is given based on the Wilson model (1965) in the region of jg<0.086m/s.

Experimental Study of Adiabatic Two-Phase Flow in an Annular Channel Under Low-Frequency Vibration

In order to investigate the possible effect of seismic vibration on two-phase flow dynamics and thermal-hydraulics of a nuclear reactor, experimental tests of adiabatic air-water two-phase flow under low-frequency vibration were carried out in this study. An eccentric cam vibration module operated at low motor speed (up to 390 rpm) was attached to an annulus test section which was scaled down from a prototypic boiling water reactor (BWR) fuel assembly subchannel. The inner and outer diameters of the annulus are 19.1 mm and 38.1 mm, respectively. The two-phase flow operating conditions cover the ranges of 0.03 m/s ≤ 〈jg〉 ≤ 1.46 m/s and 0.25 m/s ≤ 〈jf〉 ≤ 1.00 m/s and the vibration displacement ranges from ±0.8 mm to ±22.2 mm. Steady-state area-averaged instantaneous and time-averaged void fraction were recorded and analyzed in stationary and vibration experiments. A neural network flow regime identification technique and fast Fourier transformation (FFT) analysis were introduced to analyze the flow regimes and void signals under stationary and vibration conditions. Experimental results reveal possible changes in flow regimes under specific flow and vibration conditions. In addition, the instantaneous void fraction signals were affected and shown by FFT analysis. Possible reasons for the changes include the applied high acceleration and induced void/flow structure changes at certain ports under the specific flow and vibration conditions.

One-group interfacial area transport equation and its sink and source terms in narrow rectangular channel

The characteristics of two-phase flow in a narrow rectangular channel are expected to be different from those in other channel geometries, because of the significant restriction of the bubble shape which, consequently, may affect the heat removal by boiling under various operating conditions. The objective of this study is to develop an interfacial area transport equation with the sink and source terms being properly modeled for the gas–liquid two-phase flow in a narrow rectangular channel. By taking into account the crushed characteristics of the bubbles a new one-group interfacial area transport equation was derived for the two-phase flow in a narrow rectangular channel. The random collisions between bubbles and the impacts of turbulent eddies with bubbles were modeled for the bubble coalescence and breakup respectively in the two-phase flow in a narrow rectangular channel. The newly-developed one-group interfacial area transport equation with the derived sink and source terms was evaluated by using the area-averaged flow parameters of vertical upwardly-moving adiabatic air–water two-phase flows measured in a narrow rectangular channel with the gap of 0.993mm and the width of 40.0mm. The flow conditions of the data set covered spherical bubbly, crushed pancake bubbly, crushed cap-bubbly and crushed slug flow regimes and their superficial liquid velocity and the void fraction ranged from 0.214m/s to 2.08m/s and from 3.92% to 42.6%, respectively. Good agreement with the average relative deviation of 9.98% was obtained between the predicted and measured interfacial area concentrations in this study.

Tuberculose : localisations peu communes

In order to establish a reliable rod bundle database under pool conditions and to benchmark the existing models, a well-scaled 8 × 8 rod bundle test loop was designed based on the scaling criteria and a series of experiments was carried out with adiabatic air–water two-phase flow. Experiments for pool conditions covered the area-averaged void fraction 〈α〉 range of 0.12–0.93. Existing models and experimental data including boiling and air–water two-phase flow were compared and analyzed. Experimental results show that differences exist between large and small casing rod bundles as the flow structure changes with the casing scale. In addition, traditional drift-flux models for pipes may reflect the casing scale effect, but cannot be directly applied to the rod bundle geometry in pool conditions. Among the existing models for rod bundles, the Murase et al., 1986, Kamei et al., 2008, Ishizuka et al., 1995 give relatively better predictions in most regions. All benchmark results of existing models are tabulated in terms of void fraction prediction error for the present database.

Adiabatic air–water two-phase flow in circular microchannels

Gas–liquid two-phase flow in microchannels with hydraulic diameters of 100–500 μm exhibits drastically different flow behaviors from its counterpart in conventional macroscopic channels. Of particular interests are the two-phase flow patterns and the two-phase frictional pressure drop for given flow conditions in these microchannels. This paper presents an experimental study of the effects of channel size and superficial phasic velocity on the two-phase flow pattern and pressure drop of air–water mixture in circular microchannels with inner diameters of 100, 180 and 324 μm. Two-phase flow patterns were visualized using high-speed photographic technique. Four basic flow patterns, namely, bubbly flow, slug flow, ring flow and annular flow, were observed. The two-phase flow regime maps were constructed and the transition boundaries between different flow regimes identified. In an effort to unify the flow transition boundary in microchannels of different sizes, a new flow map was developed using the modified Weber numbers as the coordinates. The two-phase frictional pressure gradient in the microchannels was measured and the data were compared with predictions from the separated flow model, the homogeneous flow model and the flow pattern-based phenomenological models. Results show that the flow pattern-based models provide the best prediction of the two-phase pressure drop in the microchannels.

One-dimensional drift-flux model for two-phase flow in pool rod bundle systems

A one-dimensional drift-flux model was developed and benchmarked with existing models in the present study. Different from existing empirical correlations, this newly developed 1-D drift-flux model started from the variations of two-phase flow distribution parameter C 0 for non-dimensional superficial gas velocity 〈 j g + 〉 from low to high, which represents the variation of phase distribution patterns. Due to a combination of small-scale sub-channel effect and large-scale casing effect in a rod bundle, the Hibiki–Ishii’s drift velocity V gj + was introduced to describe the relative motion between two phases. A clear boundary of distribution parameter, C 0, can be found when the non-dimensional superficial gas velocity 〈 j g + 〉 is closed to 0.5. A linearly increasing trend and an exponentially decreasing trend of C 0 were found in the lower and higher gas sides divided by this boundary, respectively. Through the present model, one dimensional area-averaged gas velocity and void fraction can be determined for rod bundles in pool conditions. The prediction results were compared with several existing models and experimental data. The present model shows a good agreement with the existing experimental data and can be applied to both boiling and adiabatic air–water two-phase flows. In order to explain the peak values of C 0 in low superficial gas velocity conditions, several possible superficial velocity and void fraction profiles were examined by considering the recirculation flow/void distributions.

Experimental study of air–water two-phase flow in an 8 × 8 rod bundle under pool condition for one-dimensional drift-flux analysis

In order to establish a reliable rod bundle database under pool conditions and to benchmark the existing models, a well-scaled 8×8 rod bundle test loop was designed based on the scaling criteria and a series of experiments was carried out with adiabatic air–water two-phase flow. Experiments for pool conditions covered the area-averaged void fraction 〈α〉 range of 0.12–0.93. Existing models and experimental data including boiling and air–water two-phase flow were compared and analyzed. Experimental results show that differences exist between large and small casing rod bundles as the flow structure changes with the casing scale. In addition, traditional drift-flux models for pipes may reflect the casing scale effect, but cannot be directly applied to the rod bundle geometry in pool conditions. Among the existing models for rod bundles, the Murase’s model (1986), Kamei’s model (2008) and Ishizuka’s model (1995) give relatively better predictions in most regions. All benchmark results of existing models are tabulated in terms of void fraction prediction error for the present database.

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.

Visualization and void fraction measurement of gas-liquid two-phase flow in a commercial plate heat exchanger by thermal neutron radiography

Adiabatic air-water two-phase flows in a commercial plate heat exchanger were visualized by thermal neutron radiography method as a nondestructive test in order to clarify liquid distributions. Liquid distributions in a single channel and a multichannel plate heat exchanger were investigated. From the results of a single channel, it was shown that gas phase tended to flow at the center of the net-like conduit and liquid fraction at the both side was higher. However, in the case of higher gas volumetric flux above 19 m/s, liquid distributions seemed to be homogenous. Measured average void fractions were lower than those by the homogenous model. On the other hand, from the results of a multichannel, it was shown that liquid distribution into the each channel strongly depended on inlet conditions. In the case of higher liquid flow rate and lower gas flow rate, liquid fraction became higher with the deeper channel due to larger liquid momentum. However, in the case of lower liquid flow rate and higher gas flow rate, the opposite tendency of liquid distribution was observed, that is to say, liquid fraction was lower with the deeper channel.

A Study of the Flow Characteristics of Gas-Liquid Two-Phase Flow in a Plate Heat Exchanger (Visualization and Void Fraction Measurement in a Single Channel)

In order to clarify the gas-liquid two-phase flow characteristics in a plate heat exchanger, gas-liquid two-phase flows in simulated heat exchangers with a single channel placed in a vertical plane were visualized by a neutron radiography method. Adiabatic air-water two-phase flows and R 141 b boiling two-phase flows were visualized, and two-dimensional void fraction distributions were measured from visualized images via some image processing techniques. Moreover, total pressure loss of adiabatic airwater two-phase flows through the test section was measured, and the frictional pressure loss was calculated by using measured results of average void fraction. As a result, it was shown that the phase distributions in heat exchangers were strongly affected by the inlet configuration and the inlet conditions. In the case that the incoming flow was a gas-liquid two-phase flow, stagnant liquid at the test section inlet caused unsymmetrical phase distribution. For high gas volumetric flux, both phases tended to flow straight separately and liquid mixing in the channel with ribs was a little. It was also shows that the average void fractions and the frictional pressure loss of adiabatic air-water two-phase flows in the channel with ribs could be correlated by the drift-flux model and the L-M method, respectively.

Flow characteristics of gas–liquid two-phase flow in plate heat exchanger: (Visualization and void fraction measurement by neutron radiography)

In order to clarify the gas–liquid two-phase flow characteristics in a plate heat exchanger, gas–liquid two-phase flows in simulated heat exchangers with a single channel placed in a vertical plane were visualized by a neutron radiography method. Air–water adiabatic two-phase flows and chlorofluorocarbon R141b boiling two-phase flows were visualized, and two-dimensional void fraction distributions were measured from visualized images via some image processing techniques. Moreover, total pressure loss of adiabatic air–water two-phase flows through the test section was measured, and the frictional pressure loss was calculated by using measured results of average void fraction. As a result, it was shown that the phase distributions in heat exchangers were strongly affected by the inlet configuration and the inlet conditions. When the incoming flow was a gas–liquid two-phase flow, liquid stagnation at the test section inlet caused unsymmetrical phase distribution. For high gas volumetric flux, both phases tended to flow straight separately. Moreover, it was shown that the average void fractions and the frictional pressure loss of adiabatic air–water two-phase flows in the ribbed channel could be correlated by the drift-flux model and the L–M method, respectively.

B173 細管内空気-水二相流の摩擦圧力損失

Frictional pressure drops of horizontal adiabatic air-water two-phase flow in Pyrex grass tubes with inside diameters of around 1,2,4 mm, respectively, were measured. The experimental results are compared with the author's previous experiment on heated air-water vertical flow with stainless steel tube of 2 mm I.D., and good agreement is obtained between the present and previous experimental results. Two-phase multiplier Ф_l^2 is correlated very well by Chisholm-Laird's equation at relatively large liquid and gas superficial velocities, whereas Mishima-Hibiki's modified correlation for small diameter tube is valid for the case where the liquid superficial velocity is low. This depends on Reynolds number of liquid phase. Transition from laminar to turbulent flow is considered to occur at liquid phase Reynolds number of around 1000.