Air-water Mixture Research Articles

TWO-PHASE FRICTION FACTOR IN GAS-LIQUID PIPE FLOW

An improved friction factor prediction model for two-phase gas-liquid pipe flow is proposed. The model is based on a previous no-slip formulation where a mixture Reynolds number was defined. In this study, the mixture Reynolds number is modified by introducing slip-ratio information through the inclusion of void-fraction and flow-pattern dependent models. An experimental database reconstituted from the available literature and new frictional pressure-drop data for air-water horizontal flow in an I.D. 0.0204m pipe are also presented. The full database considers several different flow conditions for horizontal two-phase flow of refrigerants and air-water mixtures. It was compared to predictions of models from the literature as well as the new proposed model. We found that the proposed and Müller-Steinhagen-and-Heck methods provide better agreement for the current experimental database. It is shown that the inclusion of void-fraction information on the previous mixture Reynolds definition improves the friction-factor prediction

Influences of Operating Variables on Hydrodynamic Performance of Plunging Water Jet Downflow Bubble Column

The hydrodynamics of a co-current down flow bubble column has been investigated with air – water system. A Perspex bubble column of 5cm in diameter and 1.5m height is used as a test contactor using nozzles of 7, 8 and 9 mm diameter for air-water distributing. The column is provided with three electro-resistivity needle probes for bubble detection.Experimental work is carried out with air flow rates from 0.09 to 0.45 m3/hr and liquid flow rates from 0.65 to 1.1m3/hr in order to study the effects of superficial gas velocity, nozzle diameter and liquid flow rate on the characteristics of hydrodynamic interactions viz. gas hold up, bubble diameter and bubble velocity by using two technical methods, direct height measurements for air-water mixture in the column and resistivity probe techniques.Gas hold up is found to be progressively increased with increasing superficial gas velocity and with decreasing liquid flow rate. Lower gas hold up is obtained with smaller nozzle diameter. However, gas hold up in two-phase zone is considerably higher than the corresponding value in mixing zone.The mean bubble velocity is increased with increasing superficial gas velocity, liquid flow rate and nozzle diameter for both mixing and two phase zones. Experimental data are found to be fairly fitted with the Drift Flux model of Zuber and Findly.The bubble diameter is considerably increased with increasing superficial gas velocity and with decreasing liquid flow rate, whereas it is slightly influenced by nozzle diameter. However, the bubbles in two-phase zone are relatively bigger than those observed in mixing zone. Finally, mathematical correlations have been developed from the experimental data to describe the gas hold up and bubble velocity in the uniform two-phase zone.

A compressible multiphase flow model for violent aerated wave impact problems

This paper focuses on the numerical modelling of wave impact events under air entrapment and aeration effects. The underlying flow model treats the dispersed water wave as a compressible mixture of air and water with homogeneous material properties. The corresponding mathematical equations are based on a multiphase flow model which builds on the conservation laws of mass, momentum and energy as well as the gas-phase volume fraction advection equation. A high-order finite volume scheme based on monotone upstream-centred schemes for conservation law reconstruction is used to discretize the integral form of the governing equations. The numerical flux across a mesh cell face is estimated by means of the HLLC approximate Riemann solver. A third-order total variation diminishing Runge–Kutta scheme is adopted to obtain a time-accurate solution. The present model provides an effective way to deal with the compressibility of air and water–air mixtures. Several test cases have been calculated using the present approach, including a gravity-induced liquid piston, free drop of a water column in a closed tank, water–air shock tubes, slamming of a flat plate into still pure and aerated water and a plunging wave impact at a vertical wall. The obtained results agree well with experiments, exact solutions and other numerical computations. This demonstrates the potential of the current method to tackle more general wave–air–structure interaction problems.

Theoretical and experimental study on conveying behavior of a twin-screw multiphase pump

In this paper, the conveying behavior of a twin-screw multiphase pump is investigated when it pumps either pure water or gas–liquid mixtures with gas void fractions varying from 20% to 90%. A prototype of the twin-screw multiphase pump is developed and set up in a laboratory for this purpose. A theoretical model is established to evaluate backflow rates in the twin-screw pump clearances, total pump volume flow rates, and power consumptions at various pressure differences and gas void fractions. Results show that the predictions from the model agree well with experimental data. For pumping pure water, the power consumption increases by 45.3% when the pressure difference between inlet and outlet of the twin-screw multiphase pump increases from 0.6 to 1.0 MPa. However, the effect of the pressure difference on the total pump flow rate is negligible. For pumping air–water mixtures, the pressure difference has a significant effect on both power consumption and total volume flow rate of the multiphase pump. The pump power consumption increases by more than 40% when the pressure difference increases from 0.4 to 1.0 MPa. On the contrary, the pump volume flow rate decreases between 10% and 30% varying with the gas void fraction value. However, at the fixed pressure differences of 0.4 MPa and 1.0 MPa, the pump power consumption does not show much difference with the change of gas void fractions from 20% to 90% although the total pump volume flow rate reduces by 18.7% and 25.7%, respectively.

Velocity measurement of two-phase liquid-gas flow in a horizontal pipeline using gamma densitometry

This paper presents application of gamma-ray absorption method to liquid-gas flow investigation in a pipeline. In the described measurement two sealed 241Am radioactive sources and probes with NaI(Tl) scintillation crystals have been used. For the analysis of digital signals provided by detectors, a traditional cross-correlation function (CCF), and modified correlation methods based on the quotient of CCF and average magnitude difference function (AMDF), as well as the quotient of CCF, and average square difference function (ASDF) have been proposed. Exemplary results of the mean velocity determination of the gaseous phase transported by a liquid in the water-air mixture flow were demonstrated and the evaluation of its uncertainty have been presented.

Detailed analysis of phase distributions in a vertical riser using wire mesh sensor (WMS)

This paper looks into the results of an experimental study concerned with the phase distributions of gas–liquid multiphase flows experienced in a vertical riser. Scale experiments were carried out using a mixture of air and silicone oil in a 6m long riser pipe with an internal diameter pipe of 67mm. A series of pipe flow experiments were performed for a range of injected air superficial velocities over the range 0.05–4.73m/s, whilst the liquid superficial velocities ranged from 0.05 to 0.38m/s. Measurements of cross-sectional void fraction and radial time averaged void fraction across a pipe section located 4.92m from the pipe flow injection were obtained using a capacitance wire mesh sensor (WMS). The data were recorded at a frequency of 1000Hz over an interval of 60s. For the range of flow conditions studied, the average void fraction was observed to vary between 0.1 and 0.83. An analysis of the data collected concluded that the observed void fraction was strongly affected by the gas superficial velocity, whereby the higher the gas superficial velocity, the higher was the observed average void fraction. The average void fraction distributions observed were in good agreement with the results obtained by other researchers. The accuracy and performance of void fraction correlations were carried out in terms of percentage error and Root Mean Square (RMS) error. Reasonably symmetric radial void fraction profiles were obtained when the air–silicone oil was fully developed, and the shape of the symmetry profile was strongly dependent on the gas superficial velocity. The data for air/water and air/silicone oil systems showed reasonably good agreement except at gas superficial velocity of 0.05m/s. A comparison of the experimental data was performed against a published model to investigate the flow structure of air–water mixtures in a bubble column. A satisfactory report was observed for radial void fraction profile (mean relative error is within 5.7%) at the higher gas superficial velocities.

Advancements in free surface RANSE simulations for sailing yacht applications

The analysis of yacht hulls performance using RANSE based free surface simulations has become an accepted approach over the last decade. Access to this technology has been eased by the development of user-friendly software and by the increase of computational power. Results are widely accepted as superior to previous non-viscous approaches and have to compete with towing tank results in terms of accuracy. However, many practical applications suffer from a numerical smearing of the free surface interface between air and water which can be described as numerical ventilation. This problem occurs when the intersection between bow and calm water surface forms an acute angle and is further pronounced if the stem is rounded or blunt. It is therefore especially linked to sailing yacht applications. The problem manifests itself as a non-physical suction of the air–water mixture under the yacht hull, causing a significant under-prediction of viscous resistance. While this is the easily observable appearance of the problem, a second issue is its effect on wave resistance. It can be shown that wave damping is significantly increased, causing a prediction of wave resistance which is also too low. The paper provides a review of the Volume-of-Fluid method. It discusses the resultant implications for practical applications. A remedy to circumvent the problem is described and its impact on the accuracy of the result is shown. Simulations on an identical appended hull with and without interface smearing are compared. Effects on free surface visualization and numerical accuracy are shown. The paper finishes with a thorough verification and validation of a fully appended yacht in accordance with ITTC standards.

Optimum air-demand ratio for maximum aeration efficiency in high-head gated circular conduits.

Oxygen is an important component of water quality and its ability to sustain life. Water aeration is the process of introducing air into a body of water to increase its oxygen saturation. Water aeration can be accomplished in a variety of ways, for instance, closed-conduit aeration. High-speed flow in a closed conduit involves air-water mixture flow. The air flow results from the subatmospheric pressure downstream of the gate. The air entrained by the high-speed flow is supplied by the air vent. The air entrained into the flow in the form of a large number of bubbles accelerates oxygen transfer and hence also increases aeration efficiency. In the present work, the optimum air-demand ratio for maximum aeration efficiency in high-head gated circular conduits was studied experimentally. Results showed that aeration efficiency increased with the air-demand ratio to a certain point and then aeration efficiency did not change with a further increase of the air-demand ratio. Thus, there was an optimum value for the air-demand ratio, depending on the Froude number, which provides maximum aeration efficiency. Furthermore, a design formula for aeration efficiency was presented relating aeration efficiency to the air-demand ratio and Froude number.

Choking conditions inside plunging flow dropshafts

Severe rain events can subject our existing drainage systems to capacity. While the conveyance capacity of sewers can be increased by temporarily tolerating a full pipe flow condition, the complex dynamics of water–air mixture occurring in vertical dropshafts can largely restrict the overall conveyance capacity of drainage systems. In this study, the hydraulic performance of plunging flow dropshafts is investigated experimentally with respect to their conveyance capacity. The model dropshaft consisted of an upstream horizontal inflow pipe, a vertical circular shaft, and an outflow pipe discharging to the atmosphere. Five setups were built with Ds/Difrom 1.0 to 3.0 for the range of drop heights H/Ds= 3.0 to 21.0, where Diis the inlet pipe diameter, Dsthe shaft diameter, and H the drop height. A wide range of discharges up to Q* = 25 was tested, where Q* = Q/(gDi5)(1/2). Flow patterns recognized within the shaft (free flow, surface roller, plug flow, slug flow, and full pipe flow) help to assess the two stages of choking: the incipient and fully choked state. Criteria to predict choking occurrences were developed experimentally. The flow is considered to reach its conveyance capacity when an incipient choking state is visible. For a typical dropshaft with Ds/Di= 2, Q* was found to be approximately 5.

Experimental and Numerical Studies on Laminar Premixed Flames of Ethanol–Water–Air Mixtures

Laminar burning velocities of premixed ethanol–water–air flames over a range of equivalence ratios from 0.7 to 1.6 at 0.1 MPa and 383 K were determined experimentally at different water contents in a combustion chamber with central ignition. An ethanol oxidation mechanism was selected to simulate one-dimensional planar flames of ethanol–water–air mixtures under the same conditions to observe the effect of the water addition on the planar flame structure, the sensitivity of laminar burning velocity, and the net reaction rates of the elementary reactions. The physical effect of water was separated from its chemical effect by designing a type of fictitious water in the simulation. Results show that unstretched flame speeds and laminar burning velocities of the flames decrease with increasing the water content. When the water content was elevated, the peaks of the mole fractions of the main radical species gradually decrease and the net reaction rates of the elementary reactions with positive sensitivity coefficients decrease more than those of the elementary reactions with negative sensitivity coefficients. Both physical and chemical effects of water suppress laminar burning velocities of hydrous ethanol–air mixtures, and the former dominates. The chemical effect of water promotes production of OH and has a much more remarkable influence on the reaction rates of the elementary reactions with negative sensitivity coefficients than on those of the elementary reactions with positive coefficients. The physical effect of water has an inhibiting effect on both the production of the radicals and the reaction rates of the elementary reactions.

Characterisation of the hydraulic maldistribution in a heat exchanger by local measurement of convective heat transfer coefficients using infrared thermography

A methodology was developed to characterise the heat exchangers' performance decrease due to two-phase flow maldistribution. It consists in measuring the spatial distribution of the local heat transfer coefficients with a rapid, non-invasive and fluid independent method. The method is based on the infrared (IR) thermography measurement of the temperature response to an oscillating heat flux. The amplitude of the measured temperatures is compared to the solution of an analytical model. The problem is solved iteratively to obtain the heat transfer coefficients. This method has been applied to evaluate the uneven phase distribution of an air–water mixture in a compact heat exchanger. The exchanger is composed of seven multiport flat tubes, a vertical downward header and horizontal channels. Experiments were performed for mass flux from 29 kg m−2 s−1 to 116 kg m−2 s−1 and for quality from 0.10 to 0.70.

The modeling of oxygen transfer efficiency in gated conduits by using genetic expression programming

Oxygen transfer is the process by which oxygen is transferred from the gaseous to the liquid phase. The oxygen transfer efficiency depends almost entirely on the amount of surface contact between the air and water. This surface contact can be increased by gated conduits that involve air–water mixture flow. In reality, the physical structure of the air–water interface is complex and still awaits clarification. In recent years, different soft computing systems have been successfully employed for the solution of complex problems. Genetic expression programming (GEP) is an example of soft computing systems. This study presents the use of GEP based on a genetic algorithm to predict oxygen transfer efficiency in high−head and free−surface gated conduits. The comparison of experimental results with the results of GEP models revealed that correlation coefficients (R2) are very high and mean square errors (MSE) are very small. Therefore, GEP models are a fairly promising approach for the prediction of oxygen transfer efficiency in gated conduits.

Experimental investigation of air–water two-phase flow through vertical 90° bend

The behaviour of two-phase air–water mixture flowing from the horizontal to the vertical through a 90° bend has been investigated experimentally. Cross sectional void fraction at nine positions, three upstream and six downstream of the bend have been measured using a conductance probe technique. The bend, manufactured from transparent acrylic resin has a diameter of 34mm and a curvature (R/D) equal to 5. The superficial velocity of the air was varied between 0.3 and 4m/s and that for the water between 0.21 and 0.91m/s. The characteristics signatures of Probability Density Function (PDF), the Power Spectral Density (PSD) of the time series of cross sectionally average void fraction and visual observations have been used to characterise the flow behaviour. For the experimental conditions, plug, slug and stratified wavy flow pattern occurred in the horizontal pipe while slug and churn flow patterns were present in the vertical pipe. The void fraction increased with the gas superficial velocity. The correlation of Nicklin et al. predicted the structure velocity for the slug flow in both horizontal and vertical pipes reasonably accurately. With regards to the frequency of the periodic structures present, some conditions showed little change from upstream to downstream the bend whilst others showed an increasing in the structure frequency from horizontal to vertical pipe. The slug length increased by passing through the vertical bend.

A new Spool Piece for horizontal two-phase flow measurement

This works presents the characterization of a Spool Piece (SP), made up of a Classical Venturi and a Wire Mesh Sensor (WMS), that are installed in a horizontal test section, in which an air-water mixture flows. The test section consists of a horizontal Plexiglas pipe of internal diameter 19.5 mm and total length of about 7 m. The flow quality ranges from 0 to 0.73 and the superficial velocity ranges from 0.14 to 32 m/s for air and from 0.019 to 2.62 m/s for water; the pressure ranges from atmospheric pressure to 4 bar depending on the experimental conditions. The observed flow patterns are stratified-bubbly-slug/plug-annular. The instruments response is analyzed and discussed. From the signal analysis the mass flow rate of each phase is obtained. The developed model allows the evaluation of the mass flow rate with an accuracy higher than 20% in the 84% of the cases and with an accuracy higher than 10% in the 73.3% of the cases. Finally the introduction of the estimated parameters in the SP model is considered and discussed.

Pressure drop in a horizontal, equal-sided, sharp-edged, combining tee junction with air–water flow

Experiments were conducted on the two-phase pressure drop through a horizontal, equal-sided, sharp-edged, combining tee junction of diameter 37.8mm with air–water mixtures at a nominal absolute pressure of 150kPa. The new experimental data were used to assess the performance of existing models and to develop a new empirical model for the pressure-drop prediction. The experiments were conducted in a new facility with 41 pressure taps distributed over the three legs of the tee to provide accurate measurements of the pressure drop. The test matrix covered wide ranges of mixture qualities on the three sides of the junction and of distributions of liquid and gas flows between the two inlets. It is shown that previous modelling methods are not capable of consistent prediction of the data and a new empirical model is proposed based on the present data and using a new approach in formulating the irreversible component of the pressure drop. The new model produced excellent agreement with the data.

Trajectories of Air-Water Bubbly Jets in Crossflows

Trajectories of bubbly jets in crossflows by injecting air-water mixtures vertically through a circular nozzle were experimentally studied. Bubbles separated from the water jets in strong crossflow, similarly to that of multiphase plumes in crossflow reported in the literature. Based on dimensional analysis, a semiempirical relation was developed to predict the separation height for bubbly jets with strong initial momentum. Centerline trajectories of both the water and air phases of bubbly jets were examined. After the separation, the air-phase centerlines can be treated as straight lines. The spreading rates of half-thickness and half-width of the air phase were studied along the air-phase centerline. They decreased substantially after the bubble separation from the water jets and their terminal values appeared to increase with mean bubble diameter.

Hydraulic Features of Air-Water Mixture Flow on a Staircase with Rest Platforms

AbstractTo investigate hydraulic features of the flooding flow on a staircase with a rest platform, experiments were conducted on a 1:2-scale physical model of a staircase with a middle rest platform between two stair segments. The transition or skimming flow occurring on the first segment is similar to that on a stepped chute. However, a jet flow forms immediately downstream of the rest platform, followed by a redeveloping transition flow on the second segment. The measured profiles of air concentrations in those flow regions agree well with that in the skimming and transition flow known from stepped spillways. The amount of air entrained into the free-falling jet increases dramatically because of a higher air concentration in the approaching flow. Rapid rises of the air-water mixture depth and the flow velocity in part of the jet and the transition flow downstream were observed. These features could affect human maneuverability on a flooding staircase with rest platform(s) and make it more difficult for...

A two-phase damping model on tube bundles subjected to two-phase cross-flow

An analytical model is developed to estimate the two-phase damping ratio for upward cross-flow through horizontal tube bundles. The present model is formulated based on Feenstra’s model (2000) for void fraction and various models (homogeneous, Levy, Martinelli-Nelson and Marchaterre) for two-phase friction multiplier. The analytical results of drag coefficient on a cylinder and two-phase Euler number are compared with the experimental results by Sim-Mureithi (2013). The correlation factor between frictional pressure drop and the hydraulic drag coefficient is evaluated by considering the experimental results. The two-phase damping ratios given by the analytical model are compared with existing experimental results. The model based on Marchaterre’s model is suitable for air-water mixture, whereas the Martinelli-Nelson’s model is suitable for steam-water and Freon mixtures. The two-phase damping ratio is independent of pitch mass flux for air-water mixture, but is more or less influenced by the mass flux for steam-water/Freon (134) mixtures. The two-phase damping ratios given by the present model agree well with experimental results for a wide range of pitch mass ratio, quality, and p/d ratios.

Characterization of Two-Phase Flow Patterns in Small Round Tubes

To investigate flow characteristics by high speed video systems, experimental study was conducted to gas-liquid two-phase flow in horizontal round small tube with diameter of 5.5 and 2.6 mm, the typical flow pattern images were obtained, but stratified flow of the conventional size horizontal channel had not been discovered. Gas and liquid superficial velocities range from 0.1 to 100 ms-1, and 0.01to 10.0 ms-1 respectively. Flow patterns for co-current flow of air-water mixtures in horizontal round tubes are determined by high-speed video analysis to develop flow regime maps and the transitions between these flow regimes. Comparisons with the relevant literatures show that diameter and surface tension effects play an important role in determining the flow patterns and transitions between them.

Numerical Study of the Effect of the Relative Depth on the Overtopping Wave Energy Converters According to Constructal Design

The conversion of wave energy in electrical one has been increasingly studied. One example of wave energy converter (WEC) is the overtopping device. Its main operational principle consists of a ramp which guides the incoming waves into a reservoir raised slightly above the sea level. The accumulated water in the reservoir flows through a low head turbine generating electricity. In this sense, it is performed a numerical study concerned with the geometric optimization of an overtopping WEC for various relative depths: d/λ = 0.3, 0.5 and 0.62, by means of Constructal Design. The main purpose is to evaluate the effect of the relative depth on the design of the ramp geometry (ratio between the ramp height and its length: H1/L1) as well as, investigate the shape which leads to the highest amount of water that insides the reservoir. In the present simulations, the conservation equations of mass, momentum and one equation for the transport of volumetric fraction are solved with the finite volume method (FVM). To tackle with water-air mixture, the multiphase model Volume of Fluid (VOF) is used. Results showed that the optimal shape, (H1/L1)o, has a strong dependence of the relative depth, i.e., there is no universal shape that leads to the best performance of an overtopping device for several wave conditions.