Study On Interfacial Transport Characteristics Of Two Phase Flow In Narrow Channel

The interface characteristics of two-phase flow are very important for its flow and heat transfer characteristics.For the narrow rectangular channel,considering the safe operation of the nuclear reactor and heat exchanger,interface characteristics are of great significance for the safe design of the nuclear reactor and heat exchanger.The two phase flow pattern represents the change law of the steady state and macro phase interface.The thickness of liquid film under different flow patterns represented the change of phase interface in the direction of narrow side of the narrow rectangular channel.In addition,the interfacial area transport model can describe the physical process of phase interface change from the mechanism,but the application of the interfacial transport model depends on the division of flow patterns.In this paper,the two phase flow patterns in vertical narrow rectangular channels and the thickness of liquid film under different flow patterns are studied.At the same time,the interfacial parameter characteristics and transport model under bubbly flow condition were analyzed.In this paper,the gas-liquid two-phase flow patterns in a vertical narrow rectangular channel with a cross-sectional area of 1.9mm×68mm was observed by high speed camera.The flow patterns can be divided into bubbly flow,slug flow,churn flow and annular flow.Based on different flow pattern transition mechanisms,a new global flow pattern transition criterion model for vertical narrow rectangular channels was established under considering the influence of channel size and fluid physical properties.The comparison of the new criterion model with the experimental data in this paper and in the literature shows that the new criterion model can predict the flow patterns transition well.A PCB liquid film thickness sensor suitable for narrow rectangular channel was designed by the present study based on the conductivity method and printed circuit board technology(PCB).The liquid film thickness under different flow regimes was measured by the PCB liquid film thickness sensor.The bubble liquid film interface is relatively flat under bubbly flow condition,and the liquid film thickness of different bubbles is random.The variation of the slug flow liquid film thickness is relatively regular.The measurement results show that the film thickness of the slug bubble increases first and then tends to be stable with the increasing capillary number.In this paper,the liquid film thickness is related to the Weber number,Capillary number and Bond number by considering the effects of gravity,surface tension and inertia force.And a new prediction correlation for the liquid film thickness of slug flow was proposed.Compared with the experimental data in this paper and 198 data in the public literature,the results show that the prediction error of the new correlation is within ±35%.The regularity of liquid film thickness fluctuation is poor under churn turbulent flow condition.The fluctuation of liquid film thickness in annular flow is relatively random.The mean thickness of liquid film in annular flow is affected by the flow condition.The existing prediction correlations of liquid film thickness in annular flow can not be applied to narrow rectangular channel,so a new prediction correlation was proposed in this paper.By comparing the new correlation with the experimental data in the public literature and present paper data,the results show that the prediction error of the new correlation is within ±25%.In addition,the velocity of disturbance wave in annular flow was measured by high speed camera.It is found that the velocity obeys a certain probability density distribution and the average velocity is affected by the flow condition.According to the analysis results of interfacial shear stress of annular flow,it is found that the interfacial shear stress increases with the increase of two-phase superficial velocity.The pressure drop plays a leading role in the influence of shear stress,followed by droplet entrainment and deposition,while the influence of gravity and expansion is small.The interfacial friction factor decreases with the increase of gas velocity but the decreasing trend becomes gentle gradually,and it has a larger trend with the increase of liquid velocity.The interfacial friction factor can be regarded as a single value function of the void fraction.In this paper,a new prediction correlation of the interfacial friction factor was proposed by fitting the experimental data.The average liquid film thickness of bubbly flow can be correlated with the average bubble diameter,and the evaluation results show that the liquid film thickness has a greater effect on the void fraction and a smaller effect on the interfacial area concentration.Based on the experimental results,it is found that there is a strong correlation between the void fraction and the interfacial area concentration.The lateral distribution of the two parameters presents a central peak distribution near the entrance and becomes a flat peak distribution with the development of flow.The axial variation of interfacial parameters presents different changes with different flow conditions,which can be attributed to bubble breakup and coalescence.The change trend of interfacial parameters of Group-1 bubble and Group-2 bubble is opposite.Considering the application of Group-1 bubble and Group-2 bubble void fraction in the bubble size distribution model,this paper proposed a correlation of Group-2 bubble void fraction.In addition,the influence of aspect ratio is introduced to fit a new correlation for predicting average interfacial concentration.The bubble size of bubbly flow in a narrow rectangular channel presents a lognormal distribution.The experimental results show that the position parameters and logarithmic standard deviation which determine the shape of the lognormal distribution are affected by the superficial liquid velocity and the void fraction.The position parameters and logarithmic standard deviation at the low void fraction can be regarded as a single value function of the superficial liquid velocity,while the position parameters and logarithmic standard deviation in the transition region are only related to the void fraction.In present study,the computational correlations of position parameters and logarithmic standard deviation are proposed to predict the distribution of bubble size based on the experimental results.By comparing with the experimental data,it is found that the new correlation performed betterIn this paper,a bubbly flow interfacial area transport model suitable for narrow rectangular channel is developed based on the existing bubble interaction mechanism and model,considering the bubble limitation in narrow rectangular channel.The comparison between the predicted value of the new development model and the experimental data shows that the new development model has good applicability.In addition,through the analysis of the axial changes of the interfacial area concentration of bubbly flow,it is found that the wake entrainment coalescence is the main mechanism of the change of interfacial area concentration and void fraction,while other bubble interaction mechanisms have little influence.