Interfacial Area Transport Characteristics Of Two-phase Flow In Large Pipe Under Rolling Condition

As a typical marine condition,rolling motion has a significant effect on a two-phase flow system,which has attracted increasing attention.However,the local interfacial characteristics of two-phase flow system under rolling condition are seldom reported,which makes the two-fluid model difficult to apply in such a situation.The local interfacial structure in two-phase flow systems affected by rolling motion may present some difference with that in steady condition.As a consequence,a study on the interfacial area transport characteristics of two-phase flow under rolling condition as well as the effect of rolling motion on the local interfacial behavior is required for further comprehending the intrinsic mechanism by which some thermal-hydraulic models become applicable under a rolling condition.On the other hand,it is essential to evaluate the performance of flow and heat transfer for a two-phase flow system in such a condition.Experimental investigations on the local interfacial area transport characteristics for gas-liquid two-phase bubbly flow both in steady and rolling conditions were conducted under ambient temperature and pressure.The working fluids in operation are air and tap water.The test sections are transparent round tubes that are made of acrylite with an inner diameter of 50.8 mm and 101.6 mm,respectively.The experiments were mainly performed in the large pipe of the latter one that is fixed vertically on the rolling platform and can rotate with the platform or keep at a vertical or inclined steady state.In the present experiments,four-sensor optical probes that are designed and fabricated in home were applied to obtain the local interfacial parameters,such as the time-averaged void fraction,interfacial area concentration(IAC),Sauter mean diameter,interfacial velocity and bubble frequency.To obtain the local instantaneous interfacial parameters at a rolling angle position,a new method for dealing with the probe signals under rolling conditions was proposed and data processing software was developed.With the method of optical probe and a great deal of local interfacial experiment data obtained both in steady and rolling conditions,a complete dataset was set up,on which equations and models for interfacial transportation were evaluated under different flow conditions.In the vertical two-phase flows,the phase distribution patterns both in small and large pipes could be classified into three basic types,i.e.wall peak,core peak and flat distribution.The phase distribution generally varies from the wall-peaked to core-peaked distribution as the gas flow rate increases;while it varies from the core-peaked to wall-peaked distribution with an increase in the liquid flow rate.In the inclined pipe flows,as the inclination angle increases,the void fraction profile near the upper side wall becomes steeper or its peak value is higher and the site of the peak is more close to the wall.Both the value of the peak and the distance between the peak site and wall increase with the increase in a gas flow rate,whereas the opposite tendency is observed when the liquid flow rate increases.Under the rolling conditions,as the rolling period increases,the void fraction peak position can delay,synchronize or precede with the maximum angle position of the test section,and the effect of the rolling amplitude is similar to that of the rolling period;under the same rolling condition,the peak position is shifted ahead as the gas flow rate increases,whereas the trend is opposite for the decrease in the void fraction with an increase in the liquid flow rate.Additionally,the change of the void fraction profile would lag behind the rolling motion when the test pipe passes through the vertical balance position.With respect to the theoretical study,some efforts were devoted on the closure relations of the two-fluid model,particularly on the interfacial forces of the lift,wall and turbulence dispersion force.Such force models were implemented in CFX of the commercial CFD package and tested against two-phase flows in vertical and inclined pipes.Meanwhile,numerical simulation of two-phase flow under rolling condition was attempted to be conducted.The results of numerical simulation for the evaluation of the interfacial force models show that the combination of the lift force model of Tomiyama(2002),wall force model of Tomiyama(1998)and turbulence dispersion force model of Burns(2004)could provide a reasonable prediction for the local void fraction against the experimental value.Thus they are recommended to be utilized in two-phase flow numerical simulation.For the evaluation of the interfacial area transport models,the model of Ishii-Kim(2001)is the best one for large diameter pipes.Though the model was developed for the small diameter pipes,it also works well for the large pipes.