树脂流动对气泡运动特性的影响

为了排除复合材料成型过程中的气泡, 建立了气泡运动可视化装置, 研究了树脂流动状态和流动速度对气泡运动速度的影响, 并在此基础上建立了气泡运动模型。研究结果表明: 树脂流动对气泡运动有明显的带动或阻碍作用。当树脂流动方向与气泡运动方向相同时, 随着树脂流速的增加, 气泡的运动速度明显增大; 而流动方向相反时, 随着树脂流速的增加, 气泡的运动速度呈明显下降的趋势。所建立的气泡运动模型与实验结果基本吻合。该研究结果将为热压成型过程中气泡运动模型的建立奠定基础。      

Dissolution of bubbles in glassmelts with equilibrium redox reactions: approximations for a moving bubble boundary

The solutions of the diffusion equations with a moving boundary for bubble dissolution (or growth) accompanied by a chemical reaction, were examined. From the viewpoint of the interaction between the convective transport and the chemical reactions, some approximations introduced to solve numerically the equations governing the bubble dissolution (or growth) were studied. For bubble dissolution in glassmelts with equilibrium redox reactions due to refining agents, the applicability of the convection transport approximations employed in the literature was discussed. It was found that our previously proposed model, in which the convective transport is neglected, agrees well with the rigorous finite-difference solution and is more reasonable compared with the approximate models proposed in the literature. Comparisons between our approximate model and the rigorous numerical solution indicate better agreement for dissolution of a bubble by simultaneous mass transfer and chemical reaction, compared with that by only mass transfer. © 1998 Chapman & Hall     

Study on bubble formation at a submerged micro‐hole by a two‐stage model

Abstract

In order to increase the accuracy of detached bubble volume prediction and to correct the theoretical weakness in models proposed previously, a modified two‐stage spherical bubble formation model is proposed in this study. This modified model takes account of an important factor, the length‐to‐diameter ratio of the micro‐hole by calculating the orifice constant with an entrance flow effect and expands the applicability of the modified model. Also, experimental tests were conducted to form bubbles at a submerged micro‐hole with diameters ranging from 60 to 1200 μm under variable pressure conditions due to continuous liquid drainage with drain rate ranging from 0.006 to 0.100 ml/s. The improved model indeed increases the ability to predict the detached bubble volume in the present study. Moreover, the results show that the condition for bubble formation in the present study could be shifted from a constant flow condition to a constant pressure condition depending on the orifice constant.     

Simulation of chemical transformation wave propagation through a flow reactor with a microbubble medium

A numerical simulation is carried out of the exothermal heterogeneous reaction of cumene oxidation on the basis of a homogeneous model of bubble liquid. The thermal properties of the bubble medium are determined according to the proposed model. The influence of different factors is investigated on an oxidation level with a volume gas content value of more than 0.5.     

An experimental study of bubble behavior in a temperature gradient near a heated wall under low-gravity and microgravity conditions aboard NASA DC9 airplane

The behavior of a single bubble and a pair of bubbles under microgravity conditions has been investigated using the NASA-DC9 aircraft in order to understand the effects of various parameters and to control the bubble behavior in space. Silicone oil was used as the test liquid, and a nitrogen gas bubble was injected from the top wall under different experimental conditions. In an isothermal case, two different microgravity conditions were achieved by either fixing the experimental apparatus to the aircraft floor or freely floating the apparatus in the aircraft cabin. The bubble behavior was found to be clearly influenced by the quality of the microgravity environment, and variations of the bubble aspect ratio with the Bond number were presented. The results indicate that there is a critical Bond number of the order of 10⁻¹ which determines the bubble shape deformation. In the free-floating experiments, a temperature gradient was imposed on the liquid around the bubble near the heated top wall. Marangoni convection was expected to occur around the bubble and the bubble behavior was studied under various temperature gradients. The bubble aspect ratio was found to decrease with an increase in the Marangoni number. A theoretical model for the relation between the Marangoni flow around the bubble and the aspect ratio is proposed based on simple assumptions. Visualization of Marangoni convection around the bubbles using the photochromic dye activation method was successfully performed. The aspect ratios predicted by the model agreed with the experimental results reasonably well. Direct measurements of surface velocity are, however, necessary to further evaluate the validity of the model.     

Use of compromise-based local porosities for coarse grid DEM simulation of bubbling fluidized bed with large particles

In discrete element method (DEM) for simulations of gas–solid fluidized beds, coarse grid calculation can lead to excessive smoothing of local porosities for correlating drag on individual particles. This article tries to calculate local porosities with consideration of the heterogeneous particle distribution within a coarse grid. The compromise-based local porosity (CLP) model is set up and incorporated into coarse grid DEM simulations of gas–solid fluidized beds with large particles. Simulated bubble size and shape are in good agreement with experimental results. Various complex bubbling phenomena are described such as bubble eruption, bubble rupture and bubble coalescence. The fluctuations of solid volume fraction, relative pressure and bed layer height simulated at the fine-tuned parameters are in general agreement with the experimental data. These results indicate that the proposed CLP model is suitable to characterize distinct heterogeneities within coarse grids.     

Influence of model particle size and spatial resolution in coarse-graining DEM-CFD simulation

The discrete element method (DEM) coupled with computational fluid dynamics (CFD) is a powerful tool for exploring the detailed behaviors of dense particle–fluid interaction problems such as fluidized beds. Coarse-graining models have been proposed to decrease the computational cost by increasing the model particle size. In this study, we examine the influence of the model particle size and the spatial resolution on the average size and number of bubbles in coarse-graining DEM-CFD calculations of bubbling fluidized beds. Calculation results indicate that the bubble size is scaled by the model particle size if parameters are following similarity laws defined in a particle scale, as well as the geometric similarity of the whole system is maintained. The usage of coarse spatial resolution increases the bubble size and decreases the number of bubbles. The countervailing influence of the model particle size and the spatial resolution in a practical coarse-graining scenario results in nearly the same bubble size.     

Cavitation bubble dynamics in a polar liquid

It is suggested to introduce a new term into the equation describing the cavitation bubble dynamics in a polar liquid that takes into account a finite relaxation rate of dipole molecules of the liquid during rapid collapse of the bubble. It is shown that solutions of the modified equation provide significantly lower values of the thermodynamic parameters of the gas at the end of the collapse, so that thermal mechanisms of the high-energy processes accompanying cavitation cannot be operative. A substantial difference between commonly accepted notions and the proposed physical mechanism of accumulation of the acoustic field energy is demonstrated, whereby the energy is concentrated in the electrostatic field of oriented dipoles rather than in the form of the energy of compressed gas in the cavitation bubble. Experimental data are presented that confirm the proposed model.     

泡罩药品包装视觉检测系统图像滤波算法

目的有效滤除泡罩药品包装视觉检测系统中的图像噪声,以保证后续泡罩包装药品的图像分割、特征提取等任务顺利开展。方法分析引导滤波的数学模型,并在此基础上对其进行改进,提出一种自适应高斯引导的新型泡罩药品包装图像滤波算法。采用更加理性的参数优化算法,且用高斯窗口来代替传统的盒窗口。结果自适应高斯引导滤波算法能够有效滤除图像中的噪声,同时最大程度地保留原始图像中的边缘和细节等信息。结论该算法对后期泡罩药品包装的图像处理和分析有一定的指导意义。