Interface Of Two-phase Flow Research Articles

Numerical investigation on dynamic characteristics of oil-water annular flow-induced vibration in curved pipes

To improve the stability of water-lubricated transportation and reliability of pipes in service, the fluid–structure interaction numerical model has been established to investigate the dynamic characteristics of oil-water annular flow-induced vibration in curved pipes and the effect of main parameters in this study. The results illustrate that a single vibration mode of curved pipes is excited by oil-water annular flow, and there exists no multi-modality due to the limited pulsation of pressure and interface of two-phase flow. The change of velocities and oil-water ratios leads to change of flow pattern, making the dynamic response severer. When the oil-water ratio is larger than 2.035, its effect is greater than velocity. The physical properties of oil have a significant effect on the dynamic response. The root-mean-square dimensionless displacement AY,RMS/D and maximum dimensionless displacement AY,MAX/D of fuel oil-water annular flow-induced vibration decrease to 75.6% and 76.5% respectively, which means that the increase of dynamic viscosity reduces the fluid force and suppresses the vibration. AY,RMS/D and AY,MAX/D increases by 1.62 times when the bending angle θ increases from 30° to 90°, indicating that the increase of bending angles also leads to the severer dynamic response.

An enhanced momentum conservation treatment for FDM simulation of two-phase flows with large density ratio

The differences of the fluid properties across a fluid interface in two-phase flow often bring difficulties into computational simulations, as the conservation of mass, momentum and energy requires careful consideration at the interfacial region. Velocity advection and unsynchronised variables lead to loss of conservation of momentum across the interface, which results in an unphysical interface deformation and spurious interfacial currents. In this study, we investigate the numerical errors and instabilities in the interfacial region, and propose a new algorithm with strong temporal coupling and momentum-based velocity reconstruction, to enhance its conservation properties. The capability of the proposed algorithm is demonstrated with two idealised cases including a one-dimensional convection case of a dense droplet and a standing wave case, and one laboratory dambreak case. Results are compared with theoretical results, experimental data or existing simulations, which demonstrate the advantages of the proposed algorithm on the conservation of mass, momentum and energy, and the mitigation of unphysical interfacial transport. Without modification of any numerical methods or discretization schemes, the algorithm keeps its simplicity and can work with the existing methods, and it is straightforward to implement.

Reflection and Refraction of Waves Across an Interface of Two-phase Flow

Reflection and Refraction of Waves Across an Interface of Two-phase Flow

Applications of An Eddy-Viscosity Eliminator Based on Sigmoid Functions in Reynolds-Averaged Navier-Stokes Simulations of Sloshing Flow

Reynolds-averaged Navier-Stokes (RANS) turbulence modeling can lead to the excessive turbulence level around the interface in two-phase flow, which causes the unphysical motion of the interface in sloshing simulation. In order to avoid the unphysical motion of the interface, a novel eddy-viscosity eliminator based on sigmoid functions is designed to reduce the excessive turbulence level, and the eddy-viscosity eliminator based on polynomials is extracted from the cavitation simulations. Surface elevations by combining the eddy-viscosity eliminators and classical two-equation closure models are compared with the experiments, the ones by using the adaptive asymptotic model (AAM) and the ones by using the modified two-equation closure models. The root-mean-squared error (RMSE) is introduced to quantify the accuracies of surface elevations and the forces. The relation between the turbulence level in the transition layer and RMSEs of surface elevations is studied. Besides, the parametric analysis of the eddy-viscosity eliminators is carried out. The studies suggest that (1) the excessive turbulence level in the transition layer around the interface has a significant influence on the accuracies of surface elevations and the forces; (2) the eddy-viscosity eliminators can effectively reduce the excessive turbulence level in the transition layer to avoid the unphysical motion of the interface; (3) the k − ω SST model combined with the eddy-viscosity eliminators is appropriate for predicting surface elevations and forces in RANS simulations of sloshing flow.

Reynolds-averaged modeling of turbulence damping near a large-scale interface in two-phase flow

The two-fluid Euler-Euler model can be used for the description of co-existing stratified and dispersed multiphase flow within one flow domain. For realistic engineering applications, turbulence is often modeled in the Reynolds-averaged Navier-Stokes (RANS) framework where closure of the Reynolds stresses is mostly achieved using the turbulent viscosity formulation. It is a well-known problem that at large-scale interfaces between two phases such turbulence modeling breaks down as turbulent viscosity in the vicinity of an interface is over-predicted. To address this issue, we adopt the Egorov approach (Egorov et al., 2014) which locally damps turbulence near the interface. This model is based upon the idea that at a large-scale interface the lighter phase may see the heavier phase much like a solid wall, suggesting a wall-like treatment of turbulent dissipation at the interface. The implementation of the model inside a two-phase formulation of the k–ω model is discussed, and shown to give good predictions of interfacial turbulence in co-current stratified two-phase flow. The Egorov approach is extended to the k–ε model, which may be relevant for a large array of engineering applications in which the k–ε model is more effective than the k-ω model. It is shown that the non-dimensional Egorov approach coefficient is grid dependent. We introduce a new formulation of the interfacial damping term in the two-fluid Euler-Euler model which gives more consistent results for different computational grids in comparison to the original formulation of the Egorov approach. This feature, as well as its straightforward implementation in both the k–ω and k–ε models, make the new model useful to a large array of multiphase engineering problems in which interfacial turbulence damping is relevant.

Initial Study of a Wire Mesh Tomography Sensor for Liquid/Gas Component Investigation

Experimental studies have been carried out to study the principle operation of the conductive type wire-mesh tomography sensor and analyse the wire-mesh tomography sensor for the liquid/gas two-phase flow interface and void fraction distribution in a process column. The measurement of the two-phase flows in the process column is based on the cross-sectional local instantaneous conductivity. The sensor consists of two planes of parallel electrode wires with 16 electrodes each and was placed orthogonally with each plane. The sensor electrode wires were made of tinned copper wire with an outer diameter of 0.91 mm which stretched over the sensor fixture. Therefore, this result in the mesh grid size with 5.53×5.53mm². The wire-mesh sensor was tested in a horizontal liquid/gas two-phase flows process column with nominal diameter of 95.6 mm and the sampling frequency of 5882.3529 Hz. The tomogram results show that the wire-mesh tomography provides significant results to represent the void fraction distribution in the process column and estimation error was found in the liquid/gas interface level.

Numerical Analysis on Two-phase Flow Characteristics at Convection Microfluidic Y-junctions

Numerical simulation on two-phase flow characteristics at convection microfluidic Y-junctions based on the two-phase flow interface tracking method. It is found that the Y-angle of the microfluidic Y-junctions, capillary number of continuous phase, and the flux of two phases have a great influence on droplet generation time, rate and size. The decreasing of continuous phase capillary number and Y-angle results in the increasing of the droplet volume. At the same time, with the increasing of flux ratio of the dispersed phase and continuous phase, the influence on the droplet volume is becoming smaller. But the flow flux ratio cannot infinite increase or decrease. Because the dispersed phase can only takes the form of liquid column or silk and it unable to generate the droplet when the ratio is greater than 0.5 or no more than 0.05. When the dispersed phase flux is increasing, the corresponding droplet formation rate is almost proportional increasing; the influence of the dispersed phase flow on the length of the droplets evolution is greater than the continuous phase. In addition, in the formation process of droplet, the filling time becomes longer and the necking time becomes shorter with the increasing of the Y-angle. The droplet detachment mechanism is mainly under the effect of normal stress of the continuous phase.

Molecular Dynamics Simulation of Interfacial Phenomena in Two-Phase Flow: A Review

An attractive feature of the application of Molecular Dynamics (MD) simulation to the liquid-vapor interface in two-phase flow is that the method, well suited for the small length-scale region which may be size-affected, can be applied in multi-scale modeling together with continuum approach for the rest of the domain. Various studies have been reported in literature where the method has been utilized to analyze the interfacial regions in two-phase flow systems. In this article, the major investigations involving Molecular Dynamics analysis applied to interfacial phenomena in two phase flow and heat transfer systems are reviewed, with a focus on the thermodynamic, fluid dynamic and structural properties of the liquid-vapor interface. Discussions on the nature and premises of the reported work and a compilation of the salient features and major results are presented.

Interaction of a solitary wave and a front step simulated by level set method

As a new method, the Level Set method had been developed to compute the interface of two-phase flow. The basic mathematical theory and the detailed method to solve the free surface hydrodynamic problem had been investigated. By using the Level Set method , the transformation of a solitary wave over a front step was simulated. The results were in good agreement with laboratory experiments.

A Mesh-Free Numerical Method for Direct Simulation of Gas-Liquid Phase Interface

A numerical method is presented for mesh-free calculation of moving interface problems in two-phase flow. In this method, the moving particle semi-implicit (MPS) method is combined with the meshless advection using flow-directional local-grid (MAFL) method, for an arbitrary Lagrangian-Eulerian calculation. Moving interfaces are directly traced in Lagrangian coordinates, while fixed boundaries such as inlet and outlet flows are calculated in Eulerian coordinates. The phase interface in two-phase flow is clearly calculated by tracing the computing points on the bubble surface. A calculation model for surface tension force is presented using the curvature of radius. The volume decrease in a gas bubble due to surface tension force is successfully computed. A kernel function with a variable kernel size is introduced to allow local concentration of computing points. A two-dimensional heat diffusion problem is solved using the variable kernel size model, and the result agrees with the exact solution. The void generation process in superheated liquid is simulated using the present method. The calculated bubble growth rate exactly coincides with the analytical solution. Rising bubble shapes in viscous liquid are also simulated in two dimensions, and the estimated shapes show good agreement with those of experiments.