The dynamic behavior and energy transformation mechanism of the multi-period evolution of bubbles collapsing near a wall have been essential considerations in bubble dynamics research. In this study, a compressible two-phase solver considering thermodynamics and phase transitions is developed on OpenFOAM (version v2112). This model is validated via comparison with analytical solutions and experimental results. The dynamics of the multi-period evolution of bubbles collapse process at different dimensionless stand-off distances (γ) were accurately reproduced. The results indicate that the shock wave emitted by the collapse of cavitation bubbles impacts the wall, causing the fluid temperature along the wall to increase. Moreover, the liquid jet has a dual effect on the wall temperature increase, depending on the initial stand-off distance between the bubble and the wall. When γ is small, the jet carries the low-temperature fluid to occupy the high-temperature region, and when γ is large, the jet carries the high-temperature fluid to occupy the low-temperature region. Compared with the mechanisms above of wall temperature increase, the collapse process of cavitation, when directly attached to the wall, increases the fluid temperature along the wall more significantly. Additionally, an energy transformation mechanism is proposed considering the internal bubble energy based on the analysis of the internal bubble energy and acoustic radiation energy with different γ values. Both the internal and acoustic radiation energy initially decreased and subsequently increased with increasing γ values. These findings provide deeper insights into the near-wall collapsing cavitation process mechanism.

关于管道二次流动的系统分析较少.为此,利用CFD仿真模拟弯管冲蚀,计算不同流速以及不同颗粒直径对于管道肘部二次流动冲蚀的影响,分析冲蚀和空蚀耦合时管道肘部的侵蚀情况以及耦合作用对肘部二次流动的冲蚀产生的影响.分析结果表明:管道冲蚀最严重的区域主要集中在弯管肘部靠近出口处的外壁面以及肘部出口直管段内壁面;颗粒直径增加,因二次流动产生的在肘部出口直管段内壁面冲蚀会相应减弱;当流速增大时,受到二次流动驱动的颗粒增多,在肘部出口直管段内壁面产生的冲蚀更加严重;高流速时,出口直管段受二次流动影响的冲蚀中心区域逐渐减小.所得结论可为管道的安全运行及检测提供理论参考.

A compressible two-phase solver considering phase transition and thermodynamic effects is developed on OpenFOAM to numerically investigate the dynamics of single cavitation bubble collapse near the rigid wall. A grid independence analysis is conducted, and the accuracy of the pressure field is verified by comparing the numerical results with the experimental results. The emission process of acoustic energy during the bubble collapse is discussed. Moreover, the acoustic radiation energy in the flow field under different dimensionless bubble-wall distances γ is further studied, which is related to the mechanism of cavitation erosion. The results show that γ−value has a significant effect on the amplitude of acoustic energy. The maximum amplitude of acoustic energy accounts for 8% to 25% of the total energy in the flow field when γ−value ranges from 0.1 to 2.0. The amplitude of acoustic energy decreases with γ in the range 0.1<γ<0.9 while increases with γ in the range 0.9<γ<2.0.

This paper collected the Raman spectral characteristics of polyethylene(PE) microplastics under different detection parameters, and extracted the key characteristic data by mathematical model, which provides a quantitative analysis method of PE microplastics.

In order to explore the factors affecting coal spontaneous combustion, the fractal characteristics of coal samples are tested, and a pore-scale model for oxygen adsorption in coal porous media is developed based on self-similar fractal model. The liquid nitrogen adsorption experiments show that the coal samples indicate evident fractal scaling laws at both low-pressure and high-pressure sections, and the fractal dimensions, respectively, represent surface morphology and pore structure of coal rock. The pore-scale model has been validated by comparing with available experimental data and numerical simulation. The present numerical results indicate that the oxygen adsorption depends on both the pore structures and temperature of coal rock. The oxygen adsorption increases with increased porosity, fractal dimension and ratio of minimum to maximum pore sizes. The edge effect can be clearly seen near the cavity/pore, where the oxygen concentration is low. The correlation between the oxygen adsorption and temperature is found to obey Langmuir adsorption theory, and a new formula for oxygen adsorption and porosity is proposed. This study may help understanding the mechanisms of oxygen adsorption and accordingly provide guidelines to lower the risk of spontaneous combustion of coal.

This study numerically investigates the interactions between a collapsing bubble and a movable particle with a comparable size in a free field, which is associated with the microscopic mechanisms of the synergetic effects of cavitation erosion and particle abrasion on the damages of materials in fluid machineries. A new solver on OpenFOAM based on direct numerical simulations with the volume of fluid (VOF) method capturing the interface of a bubble and with the overset grid method handling the motion of the particle was developed to achieve the fluid–structure interaction (FSI). The results show that bubbles in cases with stand-off parameter χ (defined as (d0−Rp)/R0), where d0 is the initial distance between the centers of the bubble and particle, and Rp,R0 are the particle’s radius and the initial radius of the bubble respectively >1, experience spherical-shaped collapse under the influence of the approaching particle, which is attracted by the collapsing bubble. The bubbles in these cases no longer present non-spherical collapse. Additionally, a force balance model to account for the particle dynamics was established, in which the particle velocity inversely depends on the size of the particle, and approximately on the second power of the initial distance from the bubble. This analytical result accords with the numerical results and is valid for cases with χ>1 only, since it is based on the theory of spherical bubbles. These conclusions are important for further study of the interactions between a bubble and a movable particle near a rigid wall.

Noble metal nanoparticles (NPs) have potential for use in many optical and electronic applications due to their shape- and size-dependent properties. However, it remains a significant challenge to synthesize size- and shape-controlled NPs on a large scale while maintaining their unique properties. We have demonstrated a simple and effective microreactor based on a microfluidic chip and liquid peristaltic pump for the synthesis of monodispersed Au NPs. This reactor allows for the continuous high-flux synthesis of Au NPs with precisely controlled sizes. The effects of reaction temperature, reaction time, flow ratio of the two inlets, and amount of surfactant agent are investigated in this study. The uniform and ultrafine Au NPs were synthesized by directly controlling the temperature, flow rate, surfactant concentration, and flow rate ratio. Moreover, the Au NPs are fabricated controllably, continuously, and stably on a large scale with this system. The proposed system thus contributes to the large-scale industrial production of NPs.

The inception of tip vortex cavitation is very sensitive to water quality. In order to quantify the effect of water quality on the inception of tip vortex cavitation, we develop a motion model to describe the migration and growth of nuclei in water. An analytical solution of migration of nuclei in a vortex flow is obtained so that the capture times of various nuclei can be given out directly. A criterion is built to determine the critical nucleus in a certain nuclei spectra distribution. Tensile strength of the critical nucleus is used to quantify the effect of water quality and correct the tip vortex cavitation inception number. Finally this change of cavitation inception number is compared with experimental results to validate our model.

Tip vortex cavitation occurs on ship propellers which can cause significant noise compared to the wet flow. In order to predict the inception of tip vortex cavitation, numerous researches have been investigated about the detailed flow field around the tip. According to informed studies, the inception of tip vortex cavitation is affected by many factors. To understand the effect of water quality on cavitation inception, the motion of nuclei in an ideal vortex flow, i.e., the Rankine vortex flow, was investigated. The one-way coupling point-particle tracking model was employed to simulate the trajectory of nuclei. Meanwhile, Rayleigh-Plesset equation was introduced to describe the growth of nuclei. The results show that the nucleus size has a significant effect on nucleus’ trajectory. The capture time of a nucleus is approximately inversely proportional to its radius. The growth of nucleus accelerates its migration in the vortex flow and shortens its capture time, especially for the case of explosive growth.

Tip vortex cavitation often occurs on ship propeller. This type of cavitation can cause significant noise compared to the wet flow. Aiming at predicting the inception of tip vortex cavitation, numerous researchers have investigated the detailed flow field around the tip. According to informed studies, the inception of tip vortex cavitation is affected by complicated factors, such as the minimum pressure in the vortex core, the turbulence fluctuation and the water quality. To understand the effect of water quality on cavitation inception, the motion of nuclei in the vortex flow is investigated. The vortex flow is given by the Ranking vortex model. The one-way coupled point-particle-tracking model (PTM) is employed to simulate the trajectory of nuclei. Meanwhile, the theoretical solution of motion of nuclei is obtained and compared to the numerical results.