Mechanism Of Two-Phase Flow Regime Transition Based On Bubble Dynamics

Gas liquid two-phase flow is widely encountered in many industrial applications such as nuclear power plant,chemical reactor,food production,aviation engine etc.Thus,much attention has been attracted to this area.As the foundation of all the two-phase flow issues,flow regime,representing the characteristics of two phase distribution in space and time scale,has strong influences on heat and mass transfer characteristics in specific flow conditions.However,flow regime is influenced by many factors,resulting in the complicated mechanisms in terms of its formation and transition.Although many researchers tried to explore the mechanisms that can provide the acceptable prediction of two-phase flow behavior,most of such exploration was based on the phenomenology with little consideration of physical analysis in terms of two-phase interaction.Thus,these phenomenological transition mechanisms can only be applied to specific flow conditions.In addition,it's difficult to precisely and objectively identify the flow regime during the research,especially when the flow rate is high,causing rather discrepancies on the published results of flow regime map.Through the literature review,it can be identified that the research on the flow regime identification and transition mechanisms is insufficient and needs to be paid much attention.In present research,the impedance void meter,four-sensor conductivity probe and high-speed video camerasd were applied to obtain local and global two-phase flow data for further conducting the study of mechanism of two-phase flow regime transition.The experimental work contained four part,i.e.,objective flow regime identification method development,experimental study of single bubble rising in stagnant pure water,experimental study of local bubble dynamical characteristics in bubbly flow in 25.4 mm round pipe,experimental study of cross-section averaged phase distribution in bubbly to slug flow transition area,and experimental study of phase distribution characteristics and its development in slug flow and slug to churn-turbulent transition.To objectively identify the flow regime,the present research developed the ReliefF-FCM algorithm to precisely identify the gas-liquid flow regime in the 25.4 mm round pipe at three differnet axial positions,i.e.,L/D=25,85,145,respectively.Based on the identification results,the flow regime map was obtained as the experimental basis for further study on flow regime transition mechanism.The entrance effects on the flow regime transition was also discussed.Comparison of flow regimes at three different axial positions was conducted to analyze the effects of axial development.To thoroughly analyze the bubble dynamics in transiton process,the single bubble rising characteristics in stagnant pure water were experimentally studied.The bubble with the equivalent diameter of 2-12mm(bubble Reynolds number 300-3300)rising process was carefully studied by applying high-speed video camera with new image processing algorithm based on Perona-Malik nonlinear filter and Canny edge detection algorithm.was developed.The present research studied the bubble oscillation characteristics during the rising process and conducted the frequency analysis in terms of the trajectory and fluctuation of rising velocity and aspect ratio.Three bubble rising modes based on the oscillation characteristics were proposed and the relations between rising velocity,trajectory and aspect ratio in each mode were also discussed.For bubble size ranging from 2-6 mm,which is involved in bubbly slug transiton process,the aspect ratio correlation based on Eotvos number and Galileo number was developed.Furthermore,drag coefficient with the aspect ratio into consideration was also proposed and compared it with the results of other researches.Bubble swarm dynamical characteristics in round tube was experimentally studied by using four-sensor conductivity probe.The local parameters of bubble chord length,bubble rising velocity,interfacial area concentration and void fraction were obtained for further exploring the flow regime transition from bubbly to slug flow.From the experimental research,hindrance effects were observed due to bubble interaction,which enhanced its influences as the bubble size increased in the transition from bubbly to slug flow.A general correlation of drag coefficient of bubbles in bubbly flow and flow regime transition region is proposed based on the experimental measurements.The next step of the research is to analyze the relative motion of two phases and its development with the flow regime transition from bubbly to slug flow.It was found that the velocity ratio decreased with the increasing superficial gas velocity in bubbly flow at the constant liquid superficial velocity.The velocity ratio always reached its minimum during the flow regime transition and then increased in slug flow.Based on all the experimental phenomena observation and bubble dynamical analysis,new bubbly to slug flow regime transition criterion was proposed based on extremum condition of velocity ratio of two phases and applied to predict the transition in flow channels with different size and shape to verify its applicability.New transition criterion was developed from the single bubble dynamical characteristics and the interactions between gas phase and flow fields were also considered.It can properly explain the experimental phenomena happened in the transition process and thus has wide range of application.In present research,the phase distribution in slug flow and slug to churn-turbulent flow transition was also experimentally studied by using impedance void meter in 25.4 mm round tube.The research was focused on the length and void fraction of Taylor bubble and liquid slug in slug unit.These parameters evolvement with the flow regime changing were also carefully discussed.It was found that new structure called turbulent slug units played important roles in the slug to churn-turbulent flow transition.The research also discussed the triggers of slug/churn-turbulent and slug/annular transition and found that the happening of flooding may decide the transition process.By analyzing the published transition criterion from slug to churn-turbulent flow,the mechanisms of stability of liquid slug in slug units provided the most acceptable explanation according to the present experimental phenomenon.Two structure parameters,void fraction of liquid slug and length ratio of Taylor bubble to liquid slug,monotonously increased as the gas flow rate increased with constant liquid flow rate,which were applied for transition criterion modeling.