Air-water Countercurrent Flow Research Articles

Volume of Fluid simulation of air–water co-current and Counter-current flow with variable density turbulence formulation

Co-current and counter-current air–water flows are studied by Volume of Fluid (VOF) simulation, with emphasis on turbulence modelling near the interface. The relevance of constant density (ρ-con) and variable density (ρ-var) formulations of the turbulence transport equations in two classical two-equation turbulence models, k−ϵ RNG and k−ω SST models, is discussed. The Reynolds Stress Tensor (RST) model approach is also investigated and literature results with the two-fluid model are included for comparison. Our results with VOF show that the widely used constant density formulation underestimates the turbulence dissipation without capturing the turbulence jump across the interface, which also affects the interface liquid height, the velocity profiles of both phases and the pressure drop along the domain. In contrast, the variable density formulation significantly improves the results by increasing the turbulence dissipation at the interface. Although the best solutions have been obtained using the RST model, the ρ-var and k−ϵ RNG gave quite good agreement with experimental data. Despite the narrowness of the domains, three-dimensional simulations have been in better agreement than the two-dimensional ones. Both the VOF results obtained with either the RST and the ρ-var turbulence formulations were found to be superior to those reported with the TF model using damping corrections specifically calibrated for the tests. This study demonstrates that VOF simulation combined with ρ-var two equations turbulence models is a suitable method for modelling large-scale interface segregated flows.

The effect of the radial heat loss, upper reservoir temperature, and corrugated wall channel on the flooding critical heat flux

This study measured the flooding critical heat flux (flooding CHF) for two vertical annular channels with different channel gaps. The experiments were conducted at atmospheric pressure and with no flow. The effects of the upper reservoir temperature within 30–80 °C and radial heat loss have been studied. It was found that the upper reservoir temperature has a small effect on the flooding CHF values, and it varied in the range of up to 10% compared to the saturated conditions. On the other hand, a significant effect of the radial heat loss was observed on the measured flooding CHF values (up to 100%). The flooding CHF experimental results were compared with numerous empirical correlations from the literature, some initially based on air-water counter-current flow experiments (flooding experiments with no heating). It was found that based on the onset flooding velocity experiments, the flooding CHF results can be successfully predicted. The effect of the wall corrugation on the flooding CHF was also examined in the simple air-water counter-current flow setup. This study shows that the air flooding velocity in a corrugated channel is higher than in a smooth channel with the same cross-section geometry and dimensions. Therefore, using a smooth channel flooding CHF value for a corrugated one guarantees a conservative result.

Heat and mass transfer of countercurrent air-water flow in a vertical tube

Heat and mass transfer of countercurrent air-water flow in a vertical tube

Development of a dimensionless flooding correlation based on experimental study on air-water countercurrent flow limitation in a vertical tube

The flooding phenomenon has an important influence on the safety of equipment, especially the nuclear power plant. A new semi-empirical correlation for flooding was proposed based on the experimental investigation of the physical characteristics of the flooding in our previous work. The model was derived from the momentum equations in the annular flow model. The liquid volume fraction, the wall and interfacial friction have been taken into account. The interfacial friction factor is emphatically discussed in this study since the interfacial friction has a significant influence on flooding. A modified model of interfacial friction factor was presented containing the Bond number since the Bond number can remove the influence of the geometry. The predicting error was basically within 20% compared with the existing experimental data obtained in different geometries and sizes.

Instability of stratified two-phase flows in inclined rectangular ducts

The effect of inclination on instability of two-phase stratified flow in inclined rectangular ducts is studied for the first time via a rigorous linear stability analysis that accounts for all possible infinitesimal three-dimensional disturbances. The results of stability analysis of concurrent downward, concurrent upward, and countercurrent air-water flows are presented in the form of stability diagrams in the coordinates of critical superficial velocities for each of the cases at a characteristic set of other governing parameters. The multiplicity of base-flow solutions predicted for inclined flows is addressed. This study is the first to show that within the rigorous formulation of the stability problem in a realistic duct geometry, the concurrent upward two-phase flows allow for multiple stable flow states. While the main bulk of results is obtained for ducts with a square cross-section, the effect of the duct aspect ratio on the onset of instability is studied for several representative examples. It is shown that in most cases the flow becomes unstable due to different modes of long-wave disturbances that are associated with either interface or shear instability mechanisms. The latter arises mainly due to the duct bottom and top walls. The results substantiate the premise that the simpler two-plate geometry can be used to estimate the effect of the system parameters on the flow stability. The three-dimensional finite-wavelength critical disturbances are visualized by a novel approach of divergence-free projections on the coordinate planes.

Experimental study on air-water countercurrent flow limitation in a vertical tube based on measurement of film thickness behavior

The gas-liquid counter-current flow limitation (CCFL) is closely related to efficient and safety operation of many equipment in industrial cycle. Air-water countercurrent flow experiments were performed in a tube with diameter of 25 mm to understand the triggering mechanism of CCFL. A parallel electrode probe was utilized to measure film thickness whereby the time domain and frequency domain characteristics of liquid film was obtained. The amplitude of the interface wave is small at low liquid flow rate while it becomes large at high liquid flow rate after being disturbed by the airflow. The spectral characteristic curve shows a peak-shaped distribution. The crest exists between 0 and 10 Hz and the amplitude decreases with the frequency increase. The analysis of visual observation and characteristic of film thickness indicate that two flooding mechanisms were identified at low and high liquid flow rate, respectively. At low liquid flow rate, the interfacial waves upward propagation is responsible for the formation of CCFL onset. While flooding at high liquid flow rate takes place as a direct consequence of the liquid bridging in tube due to the turbulent flow pattern. Moreover, it is believed that there is a transition region between the low and high liquid flow rate.

Deciphering the heat and mass transfer behaviors of staggered tube bundles in a closed wet cooling tower using a 3-D VOF model

This article presents a three-dimensional CFD model to describe the counter-current air-water flow across the staggered tube bundles in a bench-scale closed wet cooling tower by combining VOF multiphase method and SST k-ω turbulence model. The proposed model can investigate the interfacial heat-mass transfer and complex interaction mechanisms among multiple tubes. Through adding the shear force term and water evaporation in the model, the circumferential distribution of the falling film and air humidity fields are obtained, and the effects of the spray density and the air velocity on the falling film flow mode, air humidity, and transfer process are studied. The results show that, the thinnest water film and maximum evaporation rate are obtained when the circumferential angle is 75° and the spray density is 0.048 kg m−1 s−1, respectively; the air Reynolds number has a greater influence on multiphase heat and mass transfers than that of the spray water Reynolds number.

Pengaruh Kemiringan Saluran Terhadap Fenomena Flooding Dengan Gangguan Pada Aliran Udara–Air Dan Udara–Minyak Tanah

Some equipments which are used in petroleum industries, chemical industries, steam power-plants and nuclear reactors must avoid the flooding phenomena in order to work at optimal condition. The purpose of this research is to investigate the effects of inclination and obstacle on flooding onset at counter-current air-water and air-kerosene flows with porous inlet and sharp outlet. The model of obstacle was conformed to industrial application i.e .pipe joining or pipe scaling. Experimen
 ts were conducted in a plexyglass pipe of 25.4 mm diameter and 2500 mm length of test section at annular flow pattern. The variation of inclination of ducts are 90o, 80o and 60o. The geometry of the obstacle is a ring with rectangular cross sectional which is installed with a distance 600 mm or 1200 mm from the inlet. The result of the research indicated that the effects of inclination and obstacle of ducts are significant toward flooding onset. Flooding phenomena was indicated with drastic increase of pressure gradient above the liquid inlet. Flooding phenomena happened earlier of larger inclination of the ducts and nearer obstacle of the inlet. This phenomena also early happened at kerosene which have higher viscosity and lower surface tension than water

The Characteristic of Two-Phase Flow Pattern on Air-Water Countercurrent Flow in Vertical Pipe

Two-phase flow is widely encountered in several engineering and industrial facilities. The purpose of this study is to investigate the characteristic of slug countercurrent flow pattern in a vertical small diameter pipe. An experimental investigation in this study has been performed toward a transparent tube of 19 mm diameter (outside diameter 22 mm, length 200 mm), water and air as the liquid and gas fluid in the vertical pipe followed by taking a picture of flow pattern in the transparent pipe using the high-speed camera. Two differential pressure sensors were set in transparent tube in order to receive slug countercurrent flow rate associated with the acquisition data as media to record data and then displayed on the screen. The two-phase flow was always preceded by the instability of flow rate and bubbles appearance. It occurred after the steady flow data acquisition rate was turned on for recording the differential pressure. The result showed that the flow pattern characteristic of two-phase flow (air-water) in the vertical pipe caused by the superficial velocity of air.

Counter-Current Gas-Liquid Film Cross-Flow for the Staggered Tube Bundles in Closed Wet Cooling Towers

This work establishes a three-dimensional CFD model that is capable of describing the counter-current air-water flow across a staggered tube bundle in a bench-scale wet cooling tower (CWCT) by combining the VOF multiphase method and the SST k-ω turbulence model. One important feature of the proposed model is that it investigates the interaction mechanisms among multiple tubes in the staggered tube bundle, which can reduce the gap between simulation and experiment. Through adding the air-water shear force term and water evaporation equation in the model, the effects of the spray density and the inlet air velocity on the falling film flow mode, air temperature in CWCT were studied. The numerical results show that, the inlet air velocity can largely influence the flow mode in CWCT. Under the spray density of 0.048 kg∙m-1∙s-1 and 0.060 kg∙m-1∙s-1 in which condition the Rew is greater than 288, the falling film flow develops into column mode. The air temperature changes logarithmically along height direction, and their increase rates enlarge with the increment of spray water, but decrease with the increase of ua,in. These results provide an insight into the complex mechanisms of heat-mass transfer for CWCTs.

Interfacial behavior of the air-water counter-current two-phase flow in a 1/30 scale-down of pressurized water reactor (PWR) hot leg

The study on the interfacial phenomena of flooding during the counter-current flow of gas-liquid in a complex pipe was carried-out experimentally. It consists of a combination of the horizontal pipe, elbow and inclined pipes to simulate the hot leg pipe in a PWR. The inner diameter was 25.4 mm as scale down of real hot leg pipe of German Konvoi type reactor. The visual observations were performed using high-speed video cameras. The measured parameters were the liquid film thickness and the pressure difference between reactor pressure vessel (RPV) simulator and steam generator (SG) simulator. The film thickness inside the test pipe was determined by using the processed image of visual data.As a result, three different mechanisms at the onset of flooding in counter-current were clarified, and are strongly affected by liquid superficial velocity. At low liquid superficial velocity (JL), the flooding mechanism is triggered by the presence of wavy interface along the horizontal pipes in the form of a liquid blockage on the elbow. At the medium JL, the slug was detected along the horizontal pipe. However, the slug collapses immediately since the water is insufficient to maintain the blockage. At high JL, the slug occurs and travels along the horizontal pipe until the elbow. Moreover, the effect of the length of horizontal pipe on the interfacial behavior of air water counter current flow also can be explained. As the length of the horizontal pipe decreases, the onset of flooding occurs at a higher gas liquid mass flow rate.

Experimental Investigation of Air-Water CCFL in the Pressurizer Surge Line of AP1000

Countercurrent flow limitation (CCFL) may occur under certain flow conditions in the surge line, restricting the draining of water from the pressurizer and thus affecting the coolant inventory and water level in the reactor pressure vessel (RPV). The complexity of the AP1000 pressurizer surge line structure makes predicting CCFL fairly difficult, and there are still not enough CCFL studies on this complex structure. Based on an extensive literature survey, the authors of this paper are particularly aware of the need for improved CCFL models for the pressurizer surge line of AP1000. To investigate the CCFL phenomenon in the surge line assembly fixture of AP1000, a whole-visual test model of the surge line is designed with a scaling ratio of 1:4, and a test loop is established to carry out visualization experiments with air-water countercurrent flow (CCF). The whole-visual test section made of acrylic material is composed of a pressurizer simulator, a surge line tube, a hot leg T-type tube, and an RPV simulator. The air-water CCF experiments are conducted at atmospheric pressure and room temperature with the pressurizer simulator water level varying from 150 to 900 mm. The visual CCF experimental processes and CCFL phenomena are filmed by a high-speed camera and analyzed in detail. The pressure drops at different CCFL locations are measured and evaluated to explore the relationships between the CCFL characteristics and flow patterns in the surge line. The development process of the CCFL is defined as the CCFL region, which can be divided into different regions according of the changes in water mass flow and CCF flow behavior. The CCFL data are analyzed and compared using the air and water superficial velocities to study the effects of hysteresis and water level. Small discrepancies are found between the data of different water levels, reflecting the small but not-negligible influence of the upper tank water level. Empirical models for the CCFL in the surge line assembly fixture are explored preliminarily using Kutateladze-type correlation and Froude-Ohnesorge correlation. Deficiencies still exist in the present semiempirical models, inspiring a more in-depth study on the empirical models for CCFL in the surge line assembly fixture that considers the complex two-phase flow behaviors in the upper tank and near the joint between the upper tank and surge line tube. The present CCFL data are compared broadly and in detail with groups of CCFL data of similar former experiments to demonstrate the applicability of the present air-water CCFL data to the development of a CCFL prediction model for the prototype large-diameter surge line assembly fixture of the AP600/AP1000. We will perform much more experimental and theoretical work to study the detailed mechanism of these special phenomena and to develop a more applicable CCFL model for the geometry and conditions of the prototype large-diameter surge line assembly fixture.

Prediction Method of Countercurrent Flow Limitation in a Pressurizer Surge Line and Its Evaluation for a 1/10-Scale Model

The method for predicting countercurrent flow limitation (CCFL) and its uncertainty in an actual pressurizer surge line of a pressurized water reactor (PWR) using 1/10-scale air–water experimental data, one-dimensional (1D) computations, and three-dimensional (3D) numerical simulations was proposed. As one step of the prediction method, 3D numerical simulations were carried out for countercurrent air–water flows in a 1/10-scale model of the pressurizer surge line to evaluate capability of the 3D simulation method and decide uncertainty of CCFL characteristics evaluated for the 1/10-scale model. The model consisted of a vertical pipe, a vertical elbow, and a slightly inclined pipe with elbows. In the actual 1/10-scale experiment, air supplied into the lower tank flowed upward to the upper tank and water supplied into the upper tank gravitationally flowed downward to the lower tank through the pressurizer surge line. In the 3D simulation, however, water was supplied from the wall surface of the vertical pipe to avoid effects of flooding at the upper end (the 3D simulation largely underestimated falling water flow rates at the upper end). Then, the flow pattern in the slightly inclined pipe was successfully reproduced, and the simulated CCFL values for the inclination angle of θ=0.6 deg (slope of 1/100) agreed well with the experimental CCFL data. The uncertainty among air–water experiments, 1D computations, and 3D simulations for the 1/10-scale model was dC=±0.015 for the CCFL constant of C=0.50. The effects of θ (θ=0,1.0 deg) on CCFL characteristics were simulated and discussed.

Countercurrent Air-Water Flow in a Scale-Down Model of a Pressurizer Surge Line

Steam generated in a reactor core and water condensed in a pressurizer form a countercurrent flow in a surge line between a hot leg and the pressurizer during reflux cooling. Characteristics of countercurrent flow limitation (CCFL) in a 1/10-scale model of the surge line were measured using air and water at atmospheric pressure and room temperature. The experimental results show that CCFL takes place at three different locations, that is, at the upper junction, in the surge line, and at the lower junction, and its characteristics are governed by the most dominating flow limitation among the three. Effects of inclination angle and elbows of the surge line on CCFL characteristics were also investigated experimentally. The effects of inclination angle on CCFL depend on the flow direction, that is, the effect is large for the nearly horizontal flow and small for the vertical flow at the upper junction. The presence of elbows increases the flow limitation in the surge line, whereas the flow limitations at the upper and lower junctions do not depend on the presence of elbows.

Effect of air inlet geometry on flooding in a large diameter vertical tube

The effect of air inlet geometry on flooding during air–water countercurrent flow in a large diameter vertical tube is studied experimentally. Two different air inlet geometries, i.e. a straight tube inlet and a multi-hole inlet were investigated. The results show that for the same air inlet momentum, flooding may not be achieved with the multi-hole inlet whereas it is observed with the straight tube inlet. The magnitude of air flow rate required to achieve flooding with a multi-hole inlet is considerably greater than that required for a single tube inlet for any given water flow rate. The results further show that the axial velocity decays much faster with a multi-hole inlet as compared to the single tube inlet and thus requires a greater air flow rate to induce flooding.

ＰＷＲホットレグを模擬した矩形流路での気液対向流（２）（気液対向流制限の解析的評価）

In the previous study, flow patterns under countercurrent air-water flow were observed in a rectangular channel simulating a PWR (pressurized water reactor) hot leg and close interrelations between flow patterns and countercurrent flow limitation (CCFL) were indicated. In this study, simple calculations of CCFL characteristics based on a momentum equation and numerical simulations using the thermal hydraulic calculation code FLUENT6.3.26 were conducted. The CCFL characteristics by the simple calculations agreed with the measured results but the constant m in the Wallis correlation was underestimated in the region of large air flow rates. In the numerical calculations, flow patterns in the simulated hot leg were not reproduced by 2D calculations due to the limitation of capability to predict the flow pattern transition from stratified flow to wavy-mist flow but were successfully reproduced by 3D calculations because of the interrelation between CCFL characteristics at the inlet and outlet of the simulated hot leg. Falling water flow rates predicted by 3D calculations agreed with the data in the region of large air flow rates.

Onset of water accumulation in the upper plenum with a perforated plate

This paper describes an experimental study on the onset of water accumulation above a perforated plate in a vertical air-water countercurrent flow. We experimentally investigate the effects of holes with the relatively large diameter (5 cm), the number of holes (4 holes and 12 holes), the thickness of plate (1 cm and 4 cm), asymmetric water injection, and the location of the air vent line on the onset of water accumulation. The present results indicate that the onset of water accumulation is promoted by a smaller diameter of hole, a smaller number of holes, and symmetric water injection whereas it has negligible dependence on the location of air vent line in the upper plenum. It turns out that the nondimensional superficial velocity, j k * , fits the data better than either the Kutateladze number, K k , or the interpolation parameter, H k * . We develop a Wallis-type correlation for the onset of water accumulation when the diameter of the holes ranges between 0.7 and 5 cm, and the number of holes is greater than six.

Air–water counter-current slug flow data in vertical-to-horizontal pipes containing orifice type obstructions

This paper presents experimental counter-current air–water flow data on the onset of flooding and slugging, the slug propagation velocity, the predominant slug frequency and the average void fraction collected by using different size orifices installed at two locations in a horizontal pipe. For the flow conditions covered during these experiments, it was observed that there is no significant difference between the onset of flooding and the onset of slugging when an orifice is installed in the horizontal run. However, a difference was observed for the experiments carried out without orifices. Furthermore, the position of the orifice with respect to the elbow does not affect the onset of flooding and slugging. When an orifice is installed in the horizontal run, it was observed that slugs occur due to the mutual interaction (constructive interference) of two waves traveling in opposite directions. This means that a completely different mechanism seems to govern the formation of slugs in counter-current two-phase flows in horizontal partially blocked pipes. This is in contrast to that described for the slugging phenomena in co-current flow, where wave instability seems to be the principal mechanisms responsible of bridging the pipe. The mutual interaction of waves traveling in opposite directions seems to control the behaviour of the slug propagation velocity, the slug frequency and average void fraction with increasing the gas superficial velocity.

Countercurrent flow distribution in structured packing via computed tomography

A newly developed γ-ray computed tomography scanner was implemented to study liquid flow distribution in a 30.48-cm-diameter column packed with corrugated structure using countercurrent air–water flow. Validation experiments confirm that spatial resolution of ∼2.5 mm can be achieved by the new CT unit. Within the gas (0–10 cm/s superficial velocity) and liquid (0.6–2.5 cm/s superficial velocity) flow ranges investigated, a good liquid distribution was observed at all conditions, as manifested by uniformity factor in excess of 70%. The liquid saturation increases with increasing superficial liquid velocity as well as down the column height. Within the conditions studied, the effect of gas velocity was, in general, found to be minimal.

Direct numerical simulation of turbulence in a sheared airwater flow with a deformable interface

Direct numerical simulation has been performed to explore the turbulence near a freely deformable interface in a countercurrent air–water flow, at a shear Reynolds number Re� = 171. The deformations of the interface fall in the range of capillary waves of waveslope ak =0 .01, and very small phase speed-to-friction velocity ratio, c/u� .T heresults for the gas side are compared to open-channel flow data at the same shear Reynolds number, placing emphasis upon the influence of the waves in the interfacial viscosity-affected region, and away from it in the outer core flow. Comparison shows a similarity in the distribution of the turbulence intensities near the interface, confirming that for the range of flow conditions considered, the lighter phase perceives the interface like a flexible solid surface, at least in the limit of non-breaking waves. Overall, in a time-averaged sense, the interfacial motion affects the turbulence in the near-interface region; the most pertinent effect is a general dampening of the turbulent fluctuating field which, in turn, leads to a reduction in the interfacial dissipation. Furthermore, the turbulence is found to be less anisotropic at the interface than at the wall. This is confirmed by the analysis of the pressure– rate-of-strain tensor, where the effect of interfacial motion is shown to decrease the pressure strain correlation in the direction normal to the interface and in the spanwise direction. The analysis of the turbulent kinetic energy and Reynolds stress budgets reveals that the interface deformations mainly affect the so-called boundary term involving the redistribution of energy, i.e. by the action of pressure, turbulent fluctuations and molecular viscosity, and the dissipation terms, leaving the production terms almost unchanged. The non-zero value of the turbulent kinetic energy at the interface, together with the reduced dissipation, implies that the turbulent activity persists near the interface and contributes to accelerating the turbulent transfer mechanisms. Away from the interface, the decomposition of the fluctuating velocity gradient tensor demonstrates that the fluctuating rate-of-strain and rate-of-rotation at the interface influence the flow throughout the boundary layer more vigorously. The study also reveals the streaky structure over the deformable interface to be less organized than over a rigid wall. However, the elongation of the streaks does not seem to be much affected by the interfacial motion. A simple qualitative analysis of the quasi–streamwise vortices using different eduction techniques shows that the interfacial turbulent structures do not change with a change of boundary conditions.