蛇形微通道内泄漏流特性

微流体系统通常具备极大的比表面积、易于控制等优势,在气-液相传质、传热、反应等方面具有良好的应用前景。本文考察了6个气液相体系在矩形截面蛇形微通道中的气液两相泰勒流流动情况以及气泡和液弹的动态行为,以气泡截面形状的几何模型为基础,得到了微通道中净泄漏流的量化方程。同时发现在较大的操作区间内,蛇形微通道对泄漏流的可控性优于直形微通道。并且详细分析了不同气液相流量、液相物性(表面张力和黏度)和气泡长度对蛇形微通道主通道净泄漏流的具体影响。

Identification Method of Gas-Liquid Two-phase Flow Regime Based on Image Multi-feature Fusion and Support Vector Machine

The knowledge of flow regime is very important for quantifying the pressure drop, the stability and safety of two-phase flow systems. Based on image multi-feature fusion and support vector machine, a new method to identify flow regime in two-phase flow was presented. Firstly, gas-liquid two-phase flow images including bub- bly flow, plug flow, slug flow, stratified flow, wavy flow, annular flow and mist flow were captured by digital high speed video systems in the horizontal tube. The image moment invariants and gray level co-occurrence matrix texture features were extracted using image processing techniques. To improve the performance of a multiple classifier system, the rough sets theory was used for reducing the inessential factors. Furthermore, the support vector machine was trained by using these eigenvectors to reduce the dimension as flow regime samples, and the flow regime intelligent identification was realized. The test results showed that image features which were reduced with the rough sets theory could excellently reflect the difference between seven typical flow regimes, and successful training the support vector machine could quickly and accurately identify seven typical flow regimes of gas-liquid two-phase flow in the horizontal tube. Image multi-feature fusion method provided a new way to identify the gas-liquid two-phase flow, and achieved higher identification ability than that of single characteristic. The overall identification accuracy was 100%, and an estimate of the image processing time was 8 ms for online flow regime identification.

Time-Frequency Signal Processing for Gas-Liquid Two Phase Flow Through a Horizontal Venturi Based on Adaptive Optimal-Kernel Theory

A time-frequency signal processing method for two-phase flow through a horizontal Venturi based on adaptive optimal-kernel （AOK） was presented in this paper.First,the collected dynamic differential pressure signal of gas-liquid two-phase flow was preprocessed,and then the AOK theory was used to analyze the dynamic differ-ential pressure signal.The mechanism of two-phase flow was discussed through the time-frequency spectrum.On the condition of steady water flow rate,with the increasing of gas flow rate,the flow pattern changes from bubbly flow to slug flow,then to plug flow,meanwhile,the energy distribution of signal fluctuations show significant change that energy transfer from 15-35 Hz band to 0-8 Hz band;moreover,when the flow pattern is slug flow,there are two wave peaks showed in the time-frequency spectrum.Finally,a number of characteristic variables were defined by using the time-frequency spectrum and the ridge of AOK.When the characteristic variables were visu-ally analyzed,the relationship between different combination of characteristic variables and flow patterns would be gotten.The results show that,this method can explain the law of flow in different flow patterns.And characteristic variables,defined by this method,can get a clear description of the flow information.This method provides a new way for the flow pattern identification,and the percentage of correct prediction is up to 91.11%.

DNS analysis of incipient drop impact dynamics using an accurate level set method

A series of 2D direct numerical simulations were performed with an accurate level set method for single drop impacts.The adopted ACLS method was validated to be efficient with perfect mass conservation in both normal and oblique impacts.A square-root correction for neck bases was modified in accuracy as well as scope of applications.In addition,process of jet formation and evolution was studied to reveal internal dynamics in drop impacts.It's found that pressure gradient and vortex are coexisting and completive reasons for jet topology while the inclined angle has a significant effect on them.Mechanisms of jet formation and evolution are different in the front and back necks.With the help of PDF distribution and correction calculation,a compromise in the competition is observed.This work lays a solid foundation for further studies of dynamics in gas-liquid flows.

Investigation on Liquid Holdup in Vertical Zero Net-Liquid Flow

Zero net-liquid flow (ZNLF) is a special case of upward gas-liquid two-phase flow. It is a phenomenon observed as a gas-liquid mixture flows in a conduit but the net liquid flow rate is zero. Investigation on the liquid holdup of ZNLF is conducted in a vertical ten-meter tube with diameter of 76 mm, both for Newtonian and nonNewtonian fluids. The gas phase is air. The Newtonian fluid is water and the non-Newtonian fluids are water-based guar gel solutions. The correlations developed for predicting liquid holdup on the basis of Lockhart-Martinelli parameter are not suitable to ZNLF. A constitutive correlation for the liquid holdup of vertical ZNLF was put forward by using the mass balance. It is found that the liquid holdup in ZNLF is dependent on both the gas flow rate and the flow distribution coefficient.

Numerical Simulations on Countercurrent Gas-Liquid Flow in a PWR Hot Leg with Air-Water Flow in a 1/15th Scale-Model

To clarify the countercurrent flow in a PWR hot leg under reflux condensation, numerical simulations of countercurrent air-water flow for a 1/15th scale model of the PWR hot leg were conducted using the two-fluid model implemented in CFD software. In this paper, the effect of expansion of the inclined pipe, which is the actual plant geometry, was evaluated. When increasing the air velocity, CCFL characteristics and the mechanism of flow pattern transition had significant differences between the case with and without expansion of the inclined pipe. CCFL characteristics were mitigated in the case with expansion. The effect of computational grid size was also discussed. When the supplied water velocity was small, the predicted flow pattern transition point agreed well with the measured data by increasing the number of cells. On the other hand, when the air velocity was decreasing, there were no significant differences in each case.

Measurment of gas-liquid two-phase slug flow with a Venturi meter based on blind source separation

We propose a novel flow measurement method for gas–liquid two-phase slug flow by using the blind source separation technique. The flow measurement model is established based on the fluctuation characteristics of differential pressure(DP) signals measured from a Venturi meter. It is demonstrated that DP signals of two-phase flow are a linear mixture of DP signals of single phase fluids. The measurement model is a combination of throttle relationship and blind source separation model. In addition, we estimate the mixture matrix using the independent component analysis(ICA) technique. The mixture matrix could be described using the variances of two DP signals acquired from two Venturi meters. The validity of the proposed model was tested in the gas–liquid twophase flow loop facility. Experimental results showed that for most slug flow the relative error is within 10%.We also find that the mixture matrix is beneficial to investigate the flow mechanism of gas–liquid two-phase flow.