Numerical Simulation Of Gas-liquid Two-phase Flow In Vertical Rectangular Narrow Channel

Gas-liquid two-phase flow system is widely present in many industrial processes,such as nuclear power, petrochemical, power industrial processes and so on. As thevariability and complexity of gas-liquid two-phase flow, with the change of differentparameters, there will be different flow patterns and characteristics in the channel. Theflow pattern plays a fundamental role in analysis of heat transfer, flow instability andcritical heat flux in two-phase flow. Meanwhile the two-phase flow pattern reflectsphase distribution features, to provide a theoretical basis to determine the fluid heattransfer characteristics and pressure drop characteristics. Therefore, the study ofgas-liquid two-phase flow in rectangular narrow channel is of great significance.In this paper, using CFD method, based on the VOF (Volume of Fluid) multiphasemodel under the commercial software platform of Fluent, a numerical simulation oftwo-phase flow in vertical narrow and small rectangular channels have beeninvestigated. The results reveal that the main flow patterns of gas-liquid two-phase flowin the channel are bubbly flow, slug flow, churn flow and annular flow. The simulationagrees well with the experimental results, the correctness of the model is verified.The void fraction and pressure drop distribution characteristic are analyzed underdifferent flow patterns. Study found that there is a congruent relationship between thevoid fraction and pressure drop distribution and flow pattern; it can qualitativelydistinguish the flow pattern type. The void fraction of bubble flow is very low, thefrequency and magnitude of the pressure drop fluctuations are also smaller; due to thegas and liquid slug of slug flow pattern is intermittently distribution in the channel, thevoid fraction of central region is evenly distributed in the channel, the frequency andamplitude of the pressure drop fluctuations are increased; bubbles of churn flow arevery unstable in the channel, the distribution of void fraction is uneven, the frequencyand amplitude of the pressure drop fluctuations are larger; the gas ring formed in thecenter area of annular flow, the distribution of void fraction is even, the frequency andamplitude of the pressure drop fluctuations are smaller.Meanwhile, the flow pattern maps in vertical narrow and small rectangularchannels are drafted under different conditions, and the map shows the distribution ofvarious flow patterns and the transition boundary between flow patterns. At certainliquid mass flow rate, with the increasing of gas mass flow rate, the flow patterns are followed by bubbly flow, slug flow, churn flow and annular flow. The differences of thetwo-phase flow pattern in narrow rectangular channel are studied; the effects ofdifferent cross-section shape and size factors to flow pattern and its transformationcharacteristics are also analyzed. In narrow rectangular channel as the Kok surfacetension and fluid surface tension are increased, bubbles are prone to collisions,aggregation, and grew up, which lead to the large change of bubble and slug flowpattern. While the smaller the slit width is, the sooner the flow pattern transition will beoccurred. The results reveal the transformation mechanism between the various flowpatterns. In two-phase flow system, the resistance and heat transfer characteristics areclosely related to flow pattern, so it is very important to study the gas-liquid flowpattern.