Numerical simulation for the air entrainment of aerated flow with an improved multiphase SPH model

Aerated flow is a complex hydraulic phenomenon that exists widely in the field of environmental hydraulics. It is generally characterised by large deformation and violent fragmentation of the free surface. Compared to Euler methods (volume of fluid (VOF) method or rigid-lid hypothesis method), the existing single-phase Smooth Particle Hydrodynamics (SPH) method has performed well for solving particle motion. A lack of research on interphase interaction and air concentration, however, has affected the application of SPH model. In our study, an improved multiphase SPH model is presented to simulate aeration flows. A drag force was included in the momentum equation to ensure accuracy of the air particle slip velocity. Furthermore, a calculation method for air concentration is developed to analyse the air entrainment characteristics. Two studies were used to simulate the hydraulic and air entrainment characteristics. And, compared with the experimental results, the simulation results agree with the experimental results well.     

Convective transport of air bubbles in strong hydraulic jumps

A hydraulic jump is a flow singularity characterised by a significant amount of air entrainment in the shear zone. The air is entrapped at the jump toe that is a discontinuity between the impinging flow and the roller. The impingement point is a source of air bubbles, as well as a source of vorticity. Herein the convective transport of air bubbles in the jump roller is re-visited. Some analytical extension is presented and the theoretical results are compared with some laboratory experiments conducted in a large-size facility operating at large Froude numbers. The turbulent air bubble mixing coefficient was found to increase linearly with increasing distance and be independent of the Froude and Reynolds numbers. Overall the study highlighted some seminal features of the air–water shear layer in hydraulic jumps with large Froude numbers (5.1 < Fr₁ < 11.2). The air bubble entrainment in the mixing zone was a convective transport process, although there was some rapid flow de-aeration for all Froude numbers.     

Experimental investigation of the onsets of gas and liquid entrainment from a small branch mounted on an inclined wall

The phenomena of the onsets of liquid entrainment and gas entrainment were investigated experimentally for the case of a flat plane with a circular outlet branch of diameter d (=6.35 mm) at the plane centre. This flat plane was situated in a large tank containing a stratified mixture of air and water under pressure (317 kPa for most experiments and 520 kPa for a few experiments) and at room temperature. The plane was inclined through various angles (θ) in increments of 30°, from the outlet branch orientation being vertically upward through the horizontal to vertically downward. For both onsets the vertical distance between the centre of the outlet branch and the undisturbed gas–liquid interface (h) was measured for various angles of inclination and Froude numbers. Both onsets were observed visually through a large viewing part of the test section. It was found that for both onsets there is a range of inclination angles where the onset h depended on θ and a range where the onset h essentially did not depend on θ. The data were correlated in terms of onset h/d, Froude number, and θ where there was dependence of onset h on the angle of inclination.     

A numerical method for the computation of the effects of an air vessel on the pressure surges in pumping systems with air entrainment

This paper describes an improved numerical method and computational procedure for the implementation of typical air vessel responses and their influence on the pressure transient for unsteady flow in a pipeline system with air entrainment. The proposed numerical method and computational procedure is without the necessity of an excessive iterative procedure as required previously by the conventional approach. The effects of air in the transient fluid system with the air vessel were then studied through the improved numerical computational method. Free and dissolved gases in the transported fluid, and cavitation at vapour pressure, are included. © 1998 John Wiley & Sons, Ltd.     

对流边界层顶部光学湍流的室内模拟研究

对流边界层顶部夹卷层是边界层和自由大气之间的过渡带,夹卷层的湍流结构是其主要特征之一。由于夹卷层的位置较高,进行精细结构观测很难,对此认识较少。利用尺寸为150 cm×150 cm×60 cm室内对流水槽模拟了大气对流边界层的发生和发展。将准直光通过模拟对流边界层,根据光在湍流介质中的传输理论得到混合层顶部夹卷层的温度结构常数的特点,在夹卷层内,归一化的温度结构常数满足相同的规律;在高度0.8zi处,温度结构常数最小,随着高度增加,温度结构常数增大,zi处达最大,然后又随着高度减逐渐小,在高度1.2zi处湍流消失。在边界层顶部zi处,归一化的温度结构常数处于一定的变化范围,归一化温度结构常数处于0.08~0.3之间,这可能与夹卷层的大尺度结构有关。     

Entrainment of cold gas into thermal plasma jets

There is increasing evidence that the entrainment of cold gas surrounding a turbulent plasma jet is more of an engulfment type process rather than simple diffusion. A variety of diagnostic techniques have been employed to determine the development of turbulence in a plasma jet and to measure concentration and temperatures of the cold gas entrained into atmospheric-pressure argon plasma jets in ambient argon or air. The results indicate that the transition to turbulence causes a rapid drop of the axial jet velocity due to entrainment of the cold gas surrounding the plasma jet. Dissipation of the cold engulfed gas bubbles by molecular diffusion is relatively slow if molecular gases (for example air) are entrained, as indicated by conditional sampling and CARS measurements. Temperature measurements using emission spectroscopy and enthalpy probes show strong discrepancies in the jet fringes.     

Modeling Study on the Entrainment of Ambient Air into Subsonic Laminar and Turbulent Argon Plasma Jets

Modeling study is performed to reveal the special features of the entrainment of ambient air into subsonic laminar and turbulent argon plasma jets. Two different types of jet flows are considered, i.e., the argon plasma jet is impinging normally upon a flat substrate located in atmospheric air surroundings or is freely issuing into the ambient air. It is found that the existence of the substrate not only changes the plasma temperature, velocity and species concentration distributions in the near-substrate region, but also significantly enhances the mass flow rate of the ambient air entrained into the jet due to the additional contribution to the gas entrainment of the wall jet formed along the substrate surface. The fraction of the additional entrainment of the wall jet in the total entrained-air flow rate is especially high for the laminar impinging plasma jet and for the case with shorter substrate standoff distances. Similarly to the case of cold-gas free jets, the maximum mass flow-rate of ambient gas entrained into the turbulent impinging or free plasma jet is approximately directly proportional to the mass flow rate at the jet inlet. The maximum mass flow-rate of ambient gas entrained into the laminar impinging plasma jet slightly increases with increasing jet-inlet velocity but decreases with increasing jet-inlet temperature.     

Application of a gas-liquid entraining rotor to supercritical fluid extraction : Removal of iron(III) from water

Supercritical fluid extraction (SFE) of aqueous solutions is often limited by poor mass transport. The performance of a new gas-liquid entraining device was investigated to improve mass transport and thereby increase extraction efficiency. As a test system, iron(III) was extracted from water with a β-diketone chelating agent (HL) and supercritical fluid carbon dioxide. Metal β-diketonate complexes with sufficient solubility in supercritical fluid CO₂ are often poorly extracted from aqueous solutions due to limited mass transport between the water-soluble metal ion and the CO₂-soluble chelating agent. The new entraining device maximizes contact between the ligand-rich CO₂ phase and the metal ion-rich aqueous phase. Iron(III) was extracted from water with the chelating agent 2,2,7-trimethyl-3,5-octanedione (H(tod)) and supercritical fluid CO₂ at 60 °C and 20.8 MPa. With entrainment, 79% of the iron was removed from the aqueous phase. This represents a three-fold increase in iron extraction efficiency over that of a static system.     

Large eddy simulation of a light gas stratification break-up by an entraining turbulent fountain

The erosion process of a stably stratified light gas layer by a vertical turbulent fountain of denser fluid inside a generic containment – for which experimental reference data are available – is studied computationally using large eddy simulation (LES). In addition, various Reynolds averaged Navier–Stokes (RANS) models are applied aiming at a comparative assessment of different computational approaches for the considered case. With the LES methodology included into the present modelling study, a novelty to date is established for fountain-stratification interaction inside generic containments. The high Reynolds number RANS models applied in the framework of this study include both the realisable k–? eddy viscosity model (EVM) as well as the basic Reynolds stress model (RSM). Furthermore, we show that certain regimes of the present configuration can be predicted using an analytically derived scaling approach. Various data beyond the experimentally obtained ones are computationally provided in order to facilitate the calibration of less costly statistical turbulence models and lumped parameter codes, since the presently considered configuration is regarded to be a valuable small-scale equivalent for containment flow applications.     

Foaming and antifoaming in non-aqueous liquids

Investigation into the physics of foaming has traditionally been focused on aqueous systems. Non-aqueous foams, by contrast, are not well understood, but have been the subject of a recent surge in interest motivated by the need to manage foaming across industrial applications. In this review, we provide a comprehensive discussion of the current state-of-the-art methods for characterizing non-aqueous foams, with a critical evaluation of the advantages and limitations of each. Subsequently, we present a concise overview of the current understanding of the mechanisms and methods used for stabilizing and destabilizing non-aqueous foams. We conclude the review by discussing open questions to guide future investigations.