International Journal of Multiphase Flow最新文献

The existence of only a few bubbles could drastically reduce the acoustic wave speed in a liquid. Wood’s equation models the linear sound speed, while the speed of an ideal shock waves is derived as a function of the pressure ratio across the shock. The common finite amplitude waves lie, however, in between these limits. We show that in a bubbly medium, the high frequency components of finite amplitude waves are attenuated and dissipate quickly, but a low frequency part remains. This wave is then transmitted by the collapse of the bubbles and its speed decreases with increasing void fraction. We demonstrate that the linear and the shock wave regimes can be smoothly connected through a Mach number based on the collapse velocity of the bubbles.

Major safety accidents involving silos in industrial processes are closely associated with abnormal wall stresses. The effects of different flow patterns and particle velocity distribution on wall stresses are investigated during silo discharge. The existence of particle velocity retardation layer near the wall boundary in the mass flow was observed by tracer particles and high-speed camera. The silo discharge undergoes a transformation from funnel flow to mass flow at a hopper half angle of 30°. As the outlet diameter increases, the time point of the flow pattern transformation becomes more and more later. The evolution of particle velocity in the central of the funnel flow is related to the outlet velocity wave propagation and the V-shaped surface expansion. The relationship between stress fluctuations and velocity characteristics is established. The flow channel simultaneously expands upwards as the velocity wave propagates and generates a periodic fan-shaped velocity wave at the top. The formation of stress concentration zone at the bin/hopper transition was observed.

In this research, we delve into the intricacies of viscous fluid flow with electric field coupling by employing the Finite Element Method (FEM) in tandem with the level set method. We generate a weak form for satisfying governing equations for electric field and fluid velocity while two phases are tracked by the level set function. The primary focus of this study is the complex interactions between free-falling jet and electric field, and the behavior of droplet encompassing deformation, fission, and fusion under the influence of electric field. The main contribution of this paper is given a new implement by using the P1/P1 scheme to directly solve the weak forms of coupled governing equations, which significantly improves calculation efficiency compared to the P2/P1 scheme, and we open source the code. This implement is verified by comparing with the experimental results of oil droplets deforming under an electric field. Computations are performed by FEniCS open-source packages. The phenomena documented underscore the multifaceted relationship between electrodynamic forces and fluid mechanics, accentuated distinctly under non-uniform electric field conditions.

The dispersed phase in liquid–liquid emulsions and air–liquid mixtures can often be fragmented into smaller sizes by the surrounding turbulent carrier phase. The critical parameter that controls this process is the breakup frequency, which is defined from the breakup kernel in the population balance equation. The breakup frequency controls how long it takes for the dispersed phase to reach the terminal size distribution for given turbulence. In this article, we try to summarize the key experimental results and compile the existing datasets under a consistent framework to find out what is the characteristic timescale of the problem and how to account for the inner density and viscosity of the dispersed phase. Furthermore, by pointing out the inconsistency of existing experimental data, the key important unsolved questions and related problems on the breakup frequency of bubbles and droplets are discussed.

Large-eddy simulations of turbulent heat transfer and solid particle deposition in helically rib-roughened pipe flows have been performed for different Reynolds numbers $Re$ and various particle diameters $D_p$. An Euler–Lagrange approach, using cyclic boundary conditions for the continuous and the dispersed phase, have been applied to achieve a fully developed turbulent flow. An adhesion and removal model have been added to the multiphase large-eddy simulations to take into account the physical effect of particle re-entrainment. The complex interactions between particle-laden turbulent flow and the structured pipe wall in multiple-started helically ribbed pipes are numerically investigated with regard to heat transfer, pressure loss, and particulate deposition. The results of the Nusselt numbers $Nu$, friction factors $f_d$, and particle deposition rates ${\dot{N}}_d$ are presented for each geometry variant. For same Reynolds numbers $Re$, significant differences of those values have been observed for the differently structured pipes.

Linear stability analysis is extensively used for predicting the transition between stratified and slug flow. In the present work, a one-dimensional two-fluid flow is linearly perturbed to evaluate the behavior of the dispersion curves through parameters such as the maximum wave growth rate (${\omega }_{I,max}$), the fastest-growing wave ($k_{max}$) and the wave that makes the problem permanently stable ($k_c$) as a function of the gas and liquid superficial velocities. The novelty of this article relies upon coupling the analysis of the behavior of transition-related parameters to the physical effects that are responsible for stabilizing and destabilizing the flow interface. The coupling of the transition analysis with the physical parameters showed potential as a reliable way of explaining the obtained transition behavior. By doing this, the stabilizing effects of gravity and surface tension are found to be invariable to the superficial velocities of the phases. On the other hand, the destabilizing effect of inertia increased with phase superficial velocities, while the shear stress increases with the liquid superficial velocity and shows a non-monotonic behavior with the gas superficial velocity. Although the overall trend of ${\omega }_{I,max}$, $k_{max}$ and $k_c$ was to increase with the superficial velocities of the phases, they were directly affected by the shear stress behavior, also showing a non-monotonic trend.

This paper presents an efficient implementation of the four-way coupled point-particle direct numerical simulation (PP-DNS) for compressible particle-laden wall turbulence, utilizing the open-source finite-difference compressible Navier–Stokes solver, STREAmS. The proposed design integrates a GPU-based two-phase collision detection algorithm known as the spatial subdivision method, along with specialized storage and MPI communication strategies for Lagrangian particles on multi-GPU platforms. Specifically, a ‘page table’ like data structure is designed to store the particle information compactly and to enable highly parallelized packing and unpacking procedures for GPU-GPU data exchange. These advancements significantly reduce the computational cost of four-way coupled particle-laden flow simulations, enabling efficient simulations involving over $O\left(10^7\right)$ particles (an order of magnitude higher than that in the state-of-the-art simulations) on a single NVIDIA A100 GPU. To validate the proposed implementation, we perform simulations of compressible particle-laden wall-bounded turbulence using canonical configurations such as channel flows and zero-pressure-gradient boundary layers. The example results highlight the effects of inter-particle collisions and flow compressibility. Furthermore, we assess single-GPU performance and scalability by employing up to eight NVIDIA GPU devices. Even for four-way coupled simulations, the elapsed time per step scales approximately linearly with the number of particles (when the number of particles is large enough), and a parallel efficiency of 94.1% is achieved on 8 NVIDIA A100 GPUs.

With the growing demand for hydrocarbon resources and the depletion of conventional oil and gas reservoirs, the development of shale oil will become popular. As the shale oil fields gradually enter the middle and late stages of exploitation, the water content reaches as high as 60–80 %. Analyzing the flow characteristics of shale oi–water flow during high water content can help optimize the range of flow parameters and improve pipeline transportation efficiency. However, research on shale oil–water flow patterns and pressure drop prediction models is still lacking. Therefore, this work conducted shale oil–water flow experiments in the multi-test section pipe flow loop, considering the temperatures (40–70 °C), mixture velocity (0.2–1.2 m/s), and water contents (60–80 %). The flow patterns of shale oil–water flow are studied, and the flow pattern maps are created. Moreover, the influence factors on pressure gradient are analyzed, including pipe diameter, water content, mixture velocity, and temperature. Eventually, the pressure drop prediction models are modified based on the pressure gradient and holdup experimental data. The results show that a thin oil film and an oil-sticking layer are adhered to the pipe wall at 40 °C and 50 °C, respectively. The mixture velocity increased from 0.4 m/s to 1.2 m/s, and the pressure gradient increased by 69.89 % when the temperature and water content are constant. The lubrication coefficient is introduced in stratified flow (ST) and three-phase stratified flow (TPS) models, and the pipe flow friction coefficient is modified in dispersed flow (DF) and intermittent flow (IF) models. Moreover, the average relative deviations between experimental data and calculated values of ST, TPS, DF, and IF modified models are 4.49 %, 4.23 %, 4.40 % and 5.17 %, respectively. Therefore, surface wettability reduces the friction between the oil phase and the pipe wall, and the oil film and oil-sticking layer increase the pipe flow resistance.

The present work studies bubble breakup in gas–liquid centrifugal pumps. Different inlet gas-fraction conditions are considered to provoke bubble-to-bubble, slug-to-bubble, and slug-to-slug pattern transitions. Results are presented for different mixture velocities and pump rotation frequencies. Data on mean equivalent bubble diameter and slug unit properties (length, passage frequency) are introduced. For small bubbles, a simple phenomenological theory is developed for the prediction of mean diameters. A discussion on the dynamics of small bubbles in the impeller region is also presented.

The direct simulation Monte Carlo (DSMC) method was improved for describing the process of acoustic agglomeration assisted by spray droplets. The agglomeration and rebound as consequences of inter-particle collisions were modeled by considering all possible particle types, in particular, mixed-phase particles associated with the immersion and distribution mechanisms. Numerical predictions by the present method were validated by both analytical solutions and experimental data. The dynamic process of the acoustic agglomeration in terms of the size and type evolution as well as the evolution of number concentration of different particle types were examined. Results suggest that the strong interaction between the solid particles and the spray droplets causes the significant enhancement of acoustic agglomeration. Furthermore, the effect of frequency varying from audible to ultrasonic range on the acoustic agglomeration was evaluated. Overall, a better performance of acoustic agglomeration can be achieved at a lower frequency, regardless of addition of spray droplets. Agglomeration efficiency of 64.8 % can be achieve at acoustic frequency of 1000 Hz in case with spray droplets. This study provides not only a numerical model for describing the agglomeration of complex particles in particle-laden gas flows, but also important insights into the acoustic agglomeration assisted by spray droplets.