Air-water Flow Research Articles

Determination of soil-water retention curve from a transient air-water two-phase outflow experiment

Knowledge of soil-water retention curve (SWRC) is essential in modeling water and air movement in soils. A new method based on air-water two-phase flow inverse solution was proposed to rapidly determine the SWRC from a transient outflow experiment. The experiment was designed to produce coupled air and water flow in soil pores, and the outflow evolution was measured during the transient processes. Air-water two-phase flow simulations combined with grid search parameter estimation algorithm were developed to determine the optimal parameters in the van Genuchten model by minimizing the difference between predicted and measured outflow data. The proposed method was validated against the transient outflow experiment and a multistep outflow test reported in the literature. The results showed that the cumulative outflow predicted from the optimal parameters matched well with the measured data. The good agreements concluded that the proposed method was reliable and feasible to determine the SWRC for unsaturated soils.

Mechanism of loop seal clearance occurring at U-shaped pipe with air-water two-phase flow

Main purposes of the LOCAL (LOop seal Clearance separate effect test of ATLAS) facility are to investigate the effect of cross-over leg (COL) diameter at the loop seal clearance (LSC), to evaluate the two-phase flow regime through high speed visualization, and to suggest the mechanistic models for the LSC prediction at Small Break-Loss of Coolant Accident (SB-LOCA) in Light Water Reactors (LWRs). In this paper, we introduce a key mechanism of the onset of the LSC occurring at U-shaped COL with air-water two-phase flow condition. The onset of LSC is coupled with the flow regime transition at vertical upward and horizontal section of COL. Slug flow with single Taylor bubble results in decrease of pressure difference at vertical upward section and causes rapid flow regime transition at horizontal section from the stratified to the slug. The critical velocity of gas leading to flow regime transition at horizontal section can explain the onset of LSC and high speed visualization can provide the clear understanding of LSC phenomena at the local point of view.

Analysis of Pressure and Flow Patterns on Two-Phase (Air-Water) Flow in Horizontal Pipes With Scalloped Groove

The groved surfaces are widely researched to reduce pressure drop and control a flow on the surface. In applicative fluids flow is turbulen. So, the grove addition capable of affecting the boundary layer condition near the wall pipes. This will greatly determine the pressure drop and the flow patterns that occur during the fluids flow. The purpose of this researched is to analyse the impact of the number of grooves in pipes against the pressure drop and the occurred flow patterns. In this researched, we used 3 pipes, which total of grooves were 4,8,16 and the pipes without grooves that used as comparator. The observed flow was two phases flow ( air-water). The researched pipes used acrylic pipes with diameter of 1 inch and length of 100 cm with the groove used was type of scalloped groove. Pressure drop occurs in all of the grooves ; 4,8, and 16. The highest pressure drop decrease occurred in groove 16. When there was drop in pressure drop in every single of groove, in low air discharge, bubbly that firmed was more tightly without making slug flow. When the debit was getting bigger, the bubbly was reduced and followed by the emergence of slug flow. The changes in water debit in the decrising of pressure drop in every single of drop, shows the dencity between the bubbly and slug flow that firmed

Vertical upward and downward churn flow: Similarities and differences

Investigation of downward two-phase flows received less attention compared to the vertical upward flows. Downward flows are found in many industries and facilities including; oil/gas production, nuclear industry and petroleum refinery/production facilities such as evaporators, chemical reactors and distillation towers. A thorough understanding of the flow dynamic characteristics occur in such transportation lines, process plants and units is very crucial in terms of design, operation, production and safety.In the current work, air-water two-phase churn flow in a 34 mm I.D. pipe was investigated for two configurations of vertical upward (51 cases) and downward (48 cases). Several conductance probes and pressure transducers were used to measure cross-sectional averaged void fraction time series, and pressure drop along the pipe, respectively.The main objectives of the work were to investigate the similarities and dissimilarities between vertically upward and downward churn flow and specifically understand how gravity could affect the behavior of liquid structures present within the flow. To quantify this, different parameters such as Probability Density Function, distribution coefficient in the drift-flux model, structural velocity, slippage number, dimensionless pressure gradient etc. were used.It was noticed that in both configurations, dimensionless pressure gradient and slippage number demonstrated a strong correlation with the mixture Froude number. There were, however, discrepancies in Probability Density Functions (PDFs) and structural velocities of flow in the two orientations.

Development and evaluation of data-driven modeling for bubble size in turbulent air-water bubbly flows using artificial multi-layer neural networks

In the present study, we consider a new reliable model of the bubble size based on multi-layer artificial neural networks (ANN). A multi-layer ANN is used to establish a function for the bubble size without any assumption on the form. In the training procedure, the proposed ANN is trained using data sets collected from open literature and experiments performed in the present study. An excellent agreement was obtained between the trained ANN and experimental data in the bubble size. Also, sensitivity analyses along with principal component analysis and random forest method provide important physical parameters for the bubble size. Next, in order to rigorously evaluate the prediction capability of the present model, flow simulations were conducted for turbulent bubbly flows, for which experimental data are available. The present validation results show that a regime-adaptive data-driven model for the bubble size achieves successful estimation for both wall and core peaking regimes.

Numerical study of air-water two-phase flow in a two-dimensional vertical helical channel

In this study, air-water two-phase flow patterns in a two-dimensional vertical helical channel are investigated. The simulations are performed using OpenFOAM software. The single-fluid method and Eulerian−Eulerian approach are used to solve the governing equations. The paper focuses on different flow patterns based on the change in the apparent velocity of water and air, and the influence of volume fraction of the air at the entrance of the channel. The variations of turbulent kinetic energy, volume fraction and velocity profile are presented. The range of air and water apparent velocities at the channel entrance is 0.05–10 m s−1 and 0.1–1.5 m s−1, respectively. According to the results, four flow patterns including bubbly, churn, turbulent, and annular are observed. It is demonstrated that for the bubbly regime, the values of volume fraction of phases are close to unity. When the air inlet velocity is very high, the two-phase flow forms the annular flow pattern.

Experimental Investigations on the Inner Flow Behavior of Centrifugal Pumps under Inlet Air-Water Two-Phase Conditions

Centrifugal pumps are widely used and are known to be sensitive to inlet air-water two-phase flow conditions. The pump performance degradation mainly depends on the changes in the two-phase flow behavior inside the pump. In the present paper, experimental overall pump performance tests were performed for two different rotational speeds and several inlet air void fractions (αi) up to pump shut-off condition. Visualizations were also performed on the flow patterns of a whole impeller passage and the volute tongue area to physically understand pump performance degradation. The results showed that liquid flow modification does not follow head modification as described by affinity laws, which are only valid for homogeneous bubbly flow regimes. Three-dimensional effects were more pronounced when inlet void fraction increased up to 3%. Bubbly flow with low mean velocities were observed close to the volute tongue for all αi, and returned back to the impeller blade passages. The starting point of pump break down was related to a strong inward reverse flow that occurred in the vicinity of the shroud gap between the impeller and volute tongue area.

Experimental investigation of the surfactant effect on liquid removal in vertical pipes

In order to investigate the effect of surfactant on liquid removal in vertical pipes, an experimental study has been conducted in a 50 mm inner diameter vertical pipe. The superficial gas velocities ranged from 0.06 m/s to 27 m/s to include bubble, slug, churn, and annular flow. The superficial liquid velocities were 0.01 m/s and 0.1 m/s. The surfactant concentration was 2000 ppm. In the presence of surfactant, foaming postpones film reversal dramatically and makes the flow much more regular at low superficial gas velocities, due to the decrease of film density and increase of interfacial-friction factor. However, the slug-churn and bubble-slug transitions are barely influenced with surfactant injecting. At high superficial gas velocities, adding surfactant in the air-water flow creates an increased frictional pressure gradient, leading to an increase in the total pressure gradient. When the superficial gas velocity decreases to some point, there is a limit where the pressure gradients in the flow with and without surfactant are equal. A higher superficial liquid velocity leads to a smaller value for this equality. Thus, proper timing for surfactant injection is of significant importance in gas wells. As the superficial gas velocity decreases, the lifting efficiency of the surfactant increases at first and subsequently decreases dramatically, due to the interaction effect of liquid holdup and hydrodynamic agitation. An optimal area in lifting efficient is in the churn flow near the slug-churn transition. Furthermore, a decrease in lifting efficiency is observed with the increase of superficial liquid velocity and thus we infer that the transition to dispersed bubble flow is the limit for superficial liquid velocity to prevent foaming.

A weakly compressible SPH method for violent multi-phase flows with high density ratio

The weakly compressible SPH (WCSPH) method is known suffering from low computational efficiency, or unnatural voids and unrealistic phase separation when it is applied to simulate highly violent multi-phase flows with high density ratio, such as that between water and air. In this paper, to remedy these issues, we propose a multi-phase WCSPH method based on a low-dissipation Riemann solver and the transport-velocity formulation. The two-phase Riemann problem is first constructed to handle the pairwise interaction between fluid particles, then modified for the fluid-wall interaction to impose the solid wall boundary condition. Since the method uses the same artificial speed of sound for both heavy and light phases, the computational efficiency increases greatly. Furthermore, due to the transport-velocity formulation employed for the light phase and application of the two-phase Riemann problem, the unnatural voids and unrealistic phase separation are effectively eliminated. The method is validated with several 2- and 3D cases involving violent water-air flows, and demonstrates good robustness, improved or comparable accuracy, respectively, comparing to previous methods with the same choice of sound speed or those with much less computational efficiency.

Preferential Paths of Air-water Two-phase Flow in Porous Structures with Special Consideration of Channel Thickness Effects

Accurate understanding and predicting the flow paths of immiscible two-phase flow in rocky porous structures are of critical importance for the evaluation of oil or gas recovery and prediction of rock slides caused by gas-liquid flow. A 2D phase field model was established for compressible air-water two-phase flow in heterogenous porous structures. The dynamic characteristics of air-water two-phase interface and preferential paths in porous structures were simulated. The factors affecting the path selection of two-phase flow in porous structures were analyzed. Transparent physical models of complex porous structures were prepared using 3D printing technology. Tracer dye was used to visually observe the flow characteristics and path selection in air-water two-phase displacement experiments. The experimental observations agree with the numerical results used to validate the accuracy of phase field model. The effects of channel thickness on the air-water two-phase flow behavior and paths in porous structures were also analyzed. The results indicate that thick channels can induce secondary air flow paths due to the increase in flow resistance; consequently, the flow distribution is different from that in narrow channels. This study provides a new reference for quantitatively analyzing multi-phase flow and predicting the preferential paths of immiscible fluids in porous structures.

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.

Gas-Liquid Flow Through an Inclined T-Junction with a Vertical-Upward Branch

The present work studies the behavior of air-water flow mixtures across a 10∘-inclined T-junction. A detailed experimental campaign is carried out to yield data on the global and local properties of the gas and liquid phases, including the mass flow rates at the entrance and exits, pressure at several locations, lengths and velocities of the gas bubbles and mean and turbulent statistics of the continuous field. Particle Image Velocimetry and the Shadow Sizer technique are used simultaneously to provide information on the discrete and continuous fields. The gas and liquid flow rates are varied in the ranges 0.07 m3h− 1 ≤ QG ≤ 0.27 m3h− 1 and 6.07 m3h− 1 ≤ QL ≤ 13.65 m3h− 1 to define nine different test conditions. Concerning the phase separation efficiency of the investigated T-junction, the observed highest fraction of separated gas was 93%, but with fractions of carried liquid as high as 15% for some conditions.

축류형 및 엘보우형 게이트펌프장 모델의 공기흡입 유동특성

The gate pump has recently received considerable attention as a type of effective drainage equipment. Air can be easily inhaled by the suction pipe, which could cause damage to the pumping system. Therefore, to improve the hydraulic performance of the gate pump, the flow behavior of the pump, including the pump-sump, must be investigated. This study focused on the investigation of the air entrainment and water flow behavior in the horizontal and elbow gate pump-sump models using a computational fluid dynamics analysis. A three-dimensional fluid domain of the horizontal and elbow gate pump models was constructed. Subsequently, the commercial code of ANSYS CFX was adopted to conduct numerical simulations. The unsteady simulations of the two-phase flow (air and water) were performed using the shear stress transport turbulence model. A total of six cases of numerical simulations were implemented. The details of the air-water two-phase flow characteristics in the pump-sump, including the pump models, were then investigated. The improvement of the air entrainment reduction using the elbow gate pump model was discussed.

Effect of water supply pressure on atomization characteristics and dust-reduction efficiency of internal mixing air atomizing nozzle

As a new type of nozzle, the internal mixing air atomizing nozzle has been widely used in the field of dust reduction via spraying. In this study, the effect of water supply pressure on the atomization characteristics and dust-reduction efficiency of the internal mixing air atomizing nozzle was investigated. Firstly, the FLUENT software was used to simulate the flow field inside and outside the nozzle under different water supply pressures. The numerical simulation results showed that as the water supply pressure increased, the internal pressure and water flow velocity in the mixing chamber of the nozzle increased while the air flow rate decreased sharply, resulting the continuous decrease in the relative velocity between gas and liquid. Meanwhile, as the water supply pressure increased, the fragmentation scale of the liquid jet at the outlet of the nozzle was prolonged and the atomization of the liquid was limited. Secondly, based on the custom-developed dust reduction experimental system via spraying, the atomization characteristics of the nozzle were investigated. According to the experimental results, when the water supply pressure increased, the water flow rate and air flow rate of the nozzle had exponential increase and decrease, respectively. As the water supply pressure increased, the range, droplet volume fraction, droplet size, and velocity all increased, while the atomization angle first increased and then decreased. Finally, the dust reduction experiment via spraying was performed under different water supply pressures. The results showed that with the increase of water supply pressure, the dust-reduction efficiency for both the total dust and the respirable dust first increased and then decreased.

Airside thermal and hydraulic characteristics of compact bare tube heat exchanger under dry and wet conditions

An experimental study was conducted to investigate the thermal and hydraulic performance of a compact bare tube heat exchanger, under both dry and wet condition. Chilton–Colburn j and f power law correlations were developed for Reynolds number range of 562–1266. Discrepancies between experimental data and new correlations are within ±2%. In wet mode, experimental conditions were varied by two inlet air humidity levels, three air flow rates and three water flow rates. From the experimental results, it was found that the heat transfer performance (j factor) and friction (f factor) decrease as air flow rate increases. Heat transfer performance decreases and friction increases slightly as inlet air humidity increases. But the changes are insensitive. Mass transfer (jm factor) slightly decreases as air humidity increases. The effect of inlet air humidity, air flow rate and water flow rate on total heat transfer capacity, sensible heat, latent heat and pressure drop are discussed. Chilton–Colburn j, jm and f power law correlations were developed for Reynolds number range of 315–1080 under wet condition, with maximum deviation of ±10%, ±7% and ±7%.

Experimental Study of the Two-Phase Flow Patterns of Air-Water Mixture at Vertical Bend Inlet and Outlet

Air-water two-phase flow in pipes introduces a noticeable challenge due to the complexity of the fluids. Thus, to estimate the best design and reasonable financing cost of the transportation pipelines where the bends are presenting a part of their accessories, the investigators should have been able to estimate the flow regime occurring at different directions. An experiment was carried out by using a 90o bend fixed with two pipes where the flow was upstream from a vertical to a horizontal pipe which were representing the bend inlet and outlet respectively. Two wire-mesh sensors were used for obtaining the data of the void fractions (α) at water superficial velocities (Usl) which changed from 0.052 to 0.419m/s, and air superficial velocities (Usg) from 0.05 to 4.7m/s. Furthermore, the characterization of flow regimes of the air-water flow at both bend inlet and outlet were competed accurately by using void fraction analysis of the time series, Power Spectral Density (PSD), tomographic images observed by the sensor program, and the Probability Density Function (PDF) method. The flow regimes of vertical flow lines at the bend inlet were observed as bubbly, cap-bubble, slug, and churn flow, whereas the flow regimes of the horizontal flow line at the bend outlet were characterized as having stratified, stratified wavy, bubbly, plug, slug, wavy annular, and semi-annular flow due to the gravity and bend effects.

A numerical procedure for estimating the sand erosion of elbows in annular flow with the complete thickness distribution of the liquid film

A numerical procedure is carried out to forecast the penetration rates of metal elbows caused by silica sand particles under air-water annular flow conditions. The procedure aims at overcoming the disadvantage of the existing numerical methods, which oversimplified the thickness distribution of the liquid film flowing through the elbows. The numerical procedure contains the steps of solving the flow field, tracking the particle response and calculating the collision damage. The core mixture in the pipe is taken as one homogeneous phase when solving the flow field. The drag force and turbulent dispersion force are calculated when tracking the particles in the core region. When tracking the particles in the liquid film, the specific film thickness at every given position of the elbow is introduced into the calculation. After calculating the particle-wall impact information, the penetration rate of the elbow is obtained by introducing the correlation which can calculate the collision damage of various metal materials. The numerical procedure is verified through the comparison between the typical experimental data and the predicted results. Several empirical and semi-empirical erosion prediction models are compared with the present procedure on the accuracy. The effects of the main factors on the penetration rate are investigated in the parametric studies. The thickness distribution and velocity of the liquid film considerably influence the effective collision intensity of the particles. By using the complete thickness information of the liquid film, the present procedure shows good accuracy in estimating the erosion of elbows under annular flow conditions.

On the Hydraulics of Downward Sloping Pipes With Entrapped Air Pockets

Abstract In this study, air–water flow in a downward sloping pipe subsequent to the entrapping of an air pocket is investigated both numerically and experimentally. A transient, two-dimensional computational fluid dynamics model is applied to study the different possible flow regimes and their associated phenomena. The numerical model is based on the Reynolds-averaged Navier–Stokes (RANS) equations and the volume of fluid (VOF) method. Both numerical and experimental investigations provide visualization for the hydraulic jump, the blowback regime, and the full gas transport regime. The numerical results predict that the flow structure in the pipe downstream the toe of the hydraulic jump is subdivided into three distinct regions including the jet layer, the shear zone, and the circulation region, which agrees qualitatively with the previous investigations of the hydraulic jump characteristics in open channel flow. Numerical results are in reasonable agreement with the experimental measurements of the circulation length and the hydraulic jump head loss.

Experimental Evaluation of Optimum Water to Air Flow Ratio in an Induced Draft Wet Cooling Tower

Cooling towers are devices used to dissipate waste thermal heat to the ambient environment. Appropriate cooling water and air flow rates are necessary to ensure optimum cooling power and cooling efficiency. Also, a simple design is required for cost effectiveness and minimal maintenance issues. This paper experimentally evaluates the cooling power, cooling efficiency, as well as the optimum water to air flow ratio in a spray type induced draft wet cooling tower. The cooling tower, 6 kW cooling capacity, was developed to operate without packings. The experiments were conducted for three different air flow rates and six different water flow rates. Four different inlet water temperatures of 35, 40, 45 and 50 oC were used. The temperature range is a typical range for inlet water temperature to the cooling tower for an absorption cooling system. For each of the inlet water temperatures, air and water flow rates were varied. The effects of this variation on cooling power and cooling efficiency were studied. Effect of varying water to air flow ratio on cooling power and cooling efficiency were studied. Results showed that the cooling power increased with increasing water flow rate, while the cooling efficiency decreased with increasing water flow rate. Decreasing the air flow rate was seen to cause a decrease in both cooling power and cooling efficiency. Maximum cooling power and cooling efficiency of 5.33 kW and 63% respectively were obtained. An optimum water to air flow ratio of 1.6 was obtained. The cooling tower was seen to have operated satisfactorily without packings. Keywords— cooling tower, cooling power, cooling efficiency, flow ratio, thermal energy

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.