Air Entrainment Mechanisms Research Articles

Air-Entrainment Mechanisms in Plunging Jets and Breaking Waves

Air entrainment in liquids is a complex phenomenon that has important applications in industry and the environment. This article addresses recent research on how air is entrained during the impact of a liquid stream on a pool of the same material in a variety of scenarios. At the fundamental level, these scenarios include the prototype flows of impacting stationary laminar and turbulent steady jets, the transient impact of isolated masses of liquid, the impact of jets with organized disturbances, and translating steady and transient jets. Although significant advances have been made recently, the complexity of this multiphase, three-dimensional, and frequently turbulent flow phenomenon leaves many unanswered questions. To help elucidate the problems still to be addressed in future research, the final section of the article examines air entrainment in the more complex application of plunging breaking waves and points out the many parts of this process that are poorly understood.

Effects of entrained air manner on cavitation damage

Early in 1953 the experiments by Peterka proved that air entrainment has effects on decreasing cavitation damage. This technology has been widely used in the release works of high dams since the inception of air entrainment in the Grand Goulee Dam in 1960. Behavior, mechanism and application of air entrainment for cavitation damage control have been investigated for over half century. However, severe cavitation damage happened due to complex mechanism of air entrainment. The effects of air entrainment are related to many factors, including geometric parameters, hydraulic parameters and entrained air manners. In the present work an experimental set-up for air entrainment was specially designed, the behavior of reducing cavitation damage was experimentally investigated in the three aspects of entrained air pressure, air tube aera and air tube number. The results show that magnitude of reduction of cavitation damage is closely related to the entrained air tube number as well as entrained air pressure, air tube aera, and that the effect through three air tubes is larger than that through single air tube although the entrained air tubes have the same sum of tube aera, that is, 1+1+1>3. Therefore, it is important to design an effective manner of air entrainment.

Doorway flow from a reduced scale isothermal air/helium approach

This paper deals with an experimental study of buoyancy-induced gas flow at a doorway-like opening as part of a fire smoke propagation application. The experimental approach is based on a reduced scale air/helium protocol, which allows a wide range of density conditions to be studied. The flow at the opening has been studied for wide ranges of density (0.17–1.22) kg/m 3, flow rate (20–600) l/min and doorway width (0.4‑14) cm usually associated with smoke propagation in life-size applications. The data set consists of measurements of flow thicknesses, spilling angles and laser tomography visualizations. The results show that flow behaviour is in harmony with the basic Bernoulli theory commonly applied to this type of flow. The value of the flow coefficient obtained with several density conditions corresponds to the behaviour found by Emmons for a given flow density. The magnitude of the spilling angle has been investigated and its dependency on the Froude number has been demonstrated. The opening width has a significant effect on flow behaviour and the associated spill plume. The reduction of the opening width enhances the mechanism of air entrainment and modifies the position of the transition height at which plume cross sections change from being rectangular to circular in shape.

A quantitative sub-grid air entrainment model for bubbly flows – plunging jets

The accurate prediction of air entrainment is critical in simulating various important multiphase (air/water) flows. In this paper, we present a sub-grid air entrainment model that quantitatively predicts the rate of air entrainment and subsequent disperse bubbly flow for a plunging jet. The derivation of this model is based on the two-stage (i.e., low and high liquid jet velocity) air entrainment mechanisms suggested by Sene [Sene KJ. Air entrainment by plunging jets. Chem Eng Sci 1988;43(10):2615–23]. This model was validated against extensive experimental data for water jets in air over a wide range of liquid velocities (from around 1 to 10 m/s) for the total rate of air entrainment. It was then implemented into an Eulerian/Eulerian two-fluid computational multiphase fluid dynamics (CMFD) model, wherein the liquid and the bubbles are modeled as two distinct continua. This multiphase model, supplemented by the new sub-grid air entrainment model, was used to predict the void fraction distribution underneath plunging water jets at different depths and water jet velocities. It was found that this approach yields results that match the experimental observations very well.

Prediction of aeration efficiency on stepped cascades by using least square support vector machines

It is important to predict aeration efficiency in stepped cascades because they are used in most water treatment applications for re-oxygenation. The flow conditions on stepped cascades have been classified into nappe, transition and skimming flows. Due to the different mechanisms of air entrainment in the nappe, transition and skimming flow conditions, the aeration efficiencies of the three flow conditions differ significantly from each other. In this paper, two intelligent models were created to predict flow condition and aeration efficiency in stepped cascades using critical flow depth, step height and channel slope information. Least square support vector machine (LS-SVM) was used as intelligent tool. The performances of LS-SVM models were evaluated by 3-fold cross validation test method. The correlation between observed and predicted flow condition is 0.99 and the correlation between measured and predicted aeration efficiency is 0.89. The test results indicated that the LS-SVM can be used successfully in predicting flow condition and aeration efficiency in stepped cascades.

Flow Measurements in a Suction Sump by UVP

In the present study, to reveal the air entrainment mechanism into a suction pipe in a suction sump, the authors conduct flow-velocity measurements by UDM (Ultrasonic Doppler Method). Here, we consider the simplest geometry as a suction sump, that is, a straight channel with rectangle-cross section and a simple suction pipe near the end of the channel. Ultrasonic transducers are fixed outside the side, bottom and back walls with right/near-right angles and, we get three-dimensional time-mean velocity distributions and equi-vorticity contours. At first, measurement accuracy is checked by comparing velocity profiles by UDM with hydrogen bubble method. As a result, the authors show typical flow fields in the sump, and show the relation between flow pattern and air entrainment. Especially, we compare two cases where the air entrainment is often observed.

Flow Measurements in a Suction Sump by UVP

In the present study, to reveal the air entrainment mechanism into a suction pipe in a suction sump, the authors conduct flow-velocity measurements by UDM (Ultrasonic Doppler Method). Here, we consider the simplest geometry as a suction sump, that is, a straight channel with rectangle-cross section and a simple suction pipe near the end of the channel. Ultrasonic transducers are fixed outside the side, bottom and back walls with right/near-right angles and, we get three-dimensional time-mean velocity distributions and equi-vorticity contours. At first, measurement accuracy is checked by comparing velocity profiles by UDM with hydrogen bubble method. As a result, the authors show typical flow fields in the sump, and show the relation between flow pattern and air entrainment. Especially, we compare two cases where the air entrainment is often observed.

Air entrainment processes in a full-scale rectangular dropshaft at large flows

Rectangular dropshafts, commonly used in sewers and storm water systems, are characterized by significant flow aeration. New detailed air–water flow measurements were conducted in a near-full-scale dropshaft at large discharges. In the shaft pool and outflow channel, the results demonstrated the complexity of different competitive air entrainment mechanisms. Bubble size measurements showed a broad range of entrained bubble sizes. Analysis of streamwise distributions of bubbles suggested further some clustering process in the bubbly flow although, in the outflow channel, bubble chords were in average smaller than in the shaft pool. A robust hydrophone was tested to measure bubble acoustic spectra and to assess its field application potential. The acoustic results characterized accurately the order of magnitude of entrained bubble sizes, but the transformation from acoustic frequencies to bubble radii did not predict correctly the probability distribution functions of bubble sizes

Effect of protein additive on properties of mortar

Abstract Test results presented in the article describe the impact of a protein additive (powdered red blood cells) on selected physical properties of cement paste, fresh mortar and physical and mechanical properties of hardened mortar. The protein additive is achieved as a result of industrial processing of animal blood. The research was made in the laboratory of the Institute of Structural Engineering of Poznan University of Technology. The analysis of the test results achieved showed that the protein included in the preparation added to the mortar leads to intensive air entraining, which has a significant effect on the most basic of physical properties both of the fresh mortar (consistence, plasticity, volumetric density) and hardened mortar (bending strength, shrinkage, volumetric density and frost resistance). The intensity of this influence, calculated in relation to the mass of the protein preparation added to the mortar, for most of the tested properties, is much higher than in the case of currently used air entraining agents [Chatterji S. Freezing of air-entrained cement-based materials and specific actions of air-entraining agents. Cem Concr Compos 2003;25:759–65, Du Lianxiang, Folliard KJ. Mechanisms of air entrainment in concrete. Cem Concr Res 2005;35:1463–71, Neville AM. Properties of concrete. 4th and final ed. London: Longman Group Limited, 1995. p. 844].

Two- and three-dimensional CFD applied to vertical display cabinets simulation

A commercial CFD code has been employed to simulate the air flow pattern and the temperature distribution in a frozen food vertical display cabinet. At first the choice of solver parameters has been investigated in a 2D modelisation. 3D simulations have been then performed, and the effects of the cabinet length, of the warm air curtain and of longitudinal ambient air movement have been investigated. The results show that, in short cabinets, 3D secondary vortices at the side walls provide the most important mechanism for hot air entrainment. Comparison with experimental results shows that a 2D simulation is totally inadequate for such configurations, while 3D computations predict refrigeration power within engineering accuracy. Furthermore, the computed refrigerating power shows that even low room air velocity, due to its interaction with the end-wall vortices, has a significant impact on cabinet performance.

Mechanisms of air entrainment in concrete

The advent of air-entraining agents is probably one of the most important technological advances in construction in the last century. It has been widely used to improve the freeze–thaw resistance of concrete, and to a lesser extent, the workability of concrete. Despite the overall successful application of air-entraining agents in concrete, problems in field concrete are not uncommon. The ability to consistently obtain target air-void systems in concrete is not trivial, and changes in raw materials, processing, or construction methods may significantly impact air entrainment. To address these potential problems in the field, a sound understanding of the mechanisms of air entrainment is essential. This paper attempts to synthesize available literature and field experience and provide a framework for understanding the fundamental aspects of air entrainment in concrete. Various parameters and influencing factors, such as concrete temperature, the physical and chemical characteristics of constituent materials, and mixing and placing techniques, are discussed.

Controlling dust emissions and explosion hazards in powder handling plants

This paper presents results from investigations into quantifying and modelling the dust generation and air entrainment mechanisms that occur during free-falling streams of material. Emphasis is placed on the effects of drop height and product temperature. This information is important for the efficient design and operation of dust control systems, as well as minimising the risk and severity of dust explosions.

Transient impact of a liquid column on a miscible liquid surface

The flow induced by a liquid column falling on an undisturbed liquid surface is studied with the aid of a high-speed camera. The falling liquid spreads over the receiving liquid forming a cavity which eventually pinches off due to the action of gravity. It is only at this point that the normal flow pattern in which the impacting liquid penetrates below the free surface is established. The same process—at a scale smaller by four orders of magnitude—is encountered in the jetting behavior of collapsing cavitation bubbles. It is also observed that the cavity dynamics is strikingly similar to that found when a disturbance is induced on a steady jet falling on a liquid. This observation supports a generic mechanism for air entrainment hypothesized in an earlier paper.

Measurements of Air Entrainment by Bow Waves

This paper describes measurements of the air entrained in experiments simulating the breaking bow wave of a ship for Froude numbers between two and three. The experiments and the characteristics of the wave itself are detailed in T. Waniewski, 1999, “Air Entrainment by Bow Waves; Ph.D. theses, Calif. Inst. of Tech.” The primary mechanism for air entrainment is the impact of the plunging wave jet, and it was observed that the air bubbles were entrained in spatially periodic bubble clouds. The void fraction and bubble size distributions were measured in the entrainment zone. There were indications that the surface disturbances described in Waniewski divide the plunging liquid jet sheet into a series of plunging jets, each of which produces a bubble cloud.

Mechanism of air entrainment by a disturbed liquid jet

It was shown in recent work that the crests of surface disturbances on a falling jet are a powerful agent for air entrainment at the free surface of a liquid pool. The paper explores the opposite case in which the jet is disturbed so as to form an axisymmetric trough, rather than a crest. It is found that no air is entrained in this case. The paper concludes with some considerations on the validity of a recently proposed model for air entrainment.

On the mechanism of air entrainment by liquid jets at a free surface

The process by which a liquid jet falling into a liquid pool entrains air is studied experimentally and theoretically. It is shown that, provided the nozzle from which the jet issues is properly contoured, an undisturbed jet does not entrap air even at relatively high Reynolds numbers. When surface disturbances are generated on the jet by a rapid increase of the liquid flow rate, on the other hand, large air cavities are formed. Their collapse under the action of gravity causes the entrapment of bubbles in the liquid. This sequence of events is recorded with a CCD and a high-speed camera. A boundary-integral method is used to simulate the process numerically with results in good agreement with the observations. An unexpected finding is that the role of the jet is not simply that of conveying the disturbance to the pool surface. Rather, both the observed energy budget and the simulations imply the presence of a mechanism by which part of the jet energy is used in creating the cavity. A hypothesis on the nature of this mechanism is presented.

Base Flame Structure and Air Entrainment Mechanisms of Small-Scale Pool Fires

Base Flame Structure and Air Entrainment Mechanisms of Small-Scale Pool Fires

Air Entrainment Processes and Bubble Size Distributions in the Surf Zone

Abstract A new optical instrument was deployed in the surf zone in a trial experiment to measure bubble size distributions and visualize air entrainment and bubble formation mechanisms within breaking surf. Images of bubbles and the evolving air–water mixture inside and beneath breaking wave crests are presented. The images resolve features of the air–water mixture to length scales of hundreds of microns across a 3.7-cm field of view. Two qualitatively different large-scale air entrainment processes are observed. First, intrusions of air and water, thought to be created by jets penetrating the water’s surface, fragment into plumes of bubbles. Second, an air cavity trapped by the overturning wave crest is observed to disintegrate into bubbles. The timescale for the evolution from a compacted air–water mass to individual bubbles was on the order of 90 ms or less for both of these processes. In addition, small-scale air filaments hundreds of microns wide and millimeters long have been discovered beneath wave...

An Experimental Study of Individual Air Bubble Entrainment at a Planar Plunging Jet

At the impact of a plunging liquid jet with a receiving pool, air bubbles may be entrained if the impact velocity exceeds a critical velocity. New experiments were performed in a two-dimensional plunging jet. The flow conditions near the inception of air entrainment were investigated. Two mechanisms of air entrainment were visualized at low jet velocities: by elongated air cavity and by foam bubbles. The breakage of entrained air bubbles was studied also. The results highlight that the bubble breakage process depends critically upon the initial bubble size.

Experimental Investigations of Free-Surface Aeration in the Developing Flow of Two-Dimensional Water Jets

Turbulent water jets discharging into the atmosphere are often characterized by a substantial amount of free-surface aeration. The effects can be detrimental or beneficial. In any case, the knowledge of the air entrainment mechanisms is essential for an optimum design. New experimental data are presented in the developing flow region of two-dimensional water jets discharging into air. The results indicate that the air diffusion takes place rapidly downstream of the nozzle and it is nearly independent of the momentum transfer process. Further, the distribution of air bubble frequency may be related to the air content distribution by a parabolic relationship.