Investigatioin On The Charapteristics Of Flow And Its Induced Vibration For Gas-liquid Two-phase Flow In A U-tube

U-tubes are widely used as basic flow and heat transfer components in two-phase flow heat exchangers such as evaporators,condensers,steam generators,and steam-water heat exchangers.Different interactions among centrifugal,gravitational,viscous,and buoyant forces lead to different flow patterns and secondary vortex for gas-liquid two-phase flows in the U-bend,which are more complicated than the flows in the straight tubes.Different flow patterns also induce the vibration of heat transfer tubes.These phenomena have great influence on the pressure drop,heat transfer and safety performance of heat exchangers.Therefore,it is essential to study the flow and resistance and flow-induced performance for the two-phase flow in the U-tubes.Because the results can provide an important theoretical basis for the design and safety operation of a U-tube heat exchanger.The flow and induced vibration characteristics of two-phase flows in U-bends were investigated experimentally and theoretically.The main results are as follows:An experimental system for the flow of air-water two-phase flow in a transverse U-tube was constructed.In this system,the flow images of air-water two-phase flow in the U-bend section and the inlet fully developed section were captured using a high-speed camera.The axial pressure gradient and pressure pulsation propagation signals in straight pipe sections of the U-tube were measured with dynamic pressure transducers.From the measurement results,it is firstly found that the pressure gradients in the adjacent straight sections are significantly higher than those in the fully developed sections.And then the dimensionless perturbation length Lp/D for the upstream and downstream straight pipes was determined through taking the attenuation law of the segmental straight pipe pressure gradient along the axial direction into account,indicating that the maximum value of Lp/D is less than 60.At the same time,the factors affecting the dimensionless interference length were analyzed,showing that with the increase of the air volume fraction ?,the Lp/D increases slowly and then sharply decreases,and reaches the peak point when the ? is about equal to 0.9.The effects of two-phase mixing Froude number Frm,two-phase Webb number Wem,and two-phase Dean number Dnm on the Lp/D are similar to the above variation trend.In addition,the dimensionless parameters ??Frm?Wem and Dnm are taken into account to develop the mathematical model of the perturbation length of U-bend to the upstream and downstream straight pipes within the scope of the experiment.Finally,through analyzing the propagation law of pressure pulsation in the straight tube of U-tube,it is found that the pressure pulsation of the gas-liquid two phase flow is propagated in the form of wave,and the fluctuation frequency of the wave is constant in the process of propagation,but the energy of the pulsation gradually attenuates.The visual observation technique and the analysis of the corresponding pressure drop fluctuations were used to objectively identify the flow regimes of the air-water two-phase flow in a transverse U-bend unit.A total of five flow regimes,i.e.,stratified-plug(stratified-churn for upward oriented flow),plug-bubble,slug-wavy,annular-wavy and annular-dispersed flows are identified within the scope of the experiment.They are different from the flow regimes in a horizontal or vertical straight tube.Two flow pattern maps for downward and upward two-phase flow in the U-bend unit were presented based on the superficial gas and liquid velocities.Stratified-plug flow in downward-oriented flow and stratified-churn flow in upward-oriented flow occurred at low superficial gas and liquid velocities,due to the fact that gravity force plays a more important role than centrifugal force under these flow conditions.They are different from the stratified flow in a horizontal straight tube and bubble/slug flow in a vertical straight tube.These unique flow patterns may occur in other curved channels at low gas and liquid velocities.The skewness of PSD(SKEPSD)and the rate of MSE(RMSE)were obtained from the press drop time series.A new method for recognizing flow regimes in U-tube quantitatively was proposed based on the SKEPSD-RMSE,which can be useful for online identification of flow patterns in other non-transparent U-bends.The characteristics of the pressure drop for the gas-liquid two-phase flow in the U-tube were investigated experimentally.And the influence factors of the pressure gradient variation in the U-tube were analyzed.When the gas superficial velocity Ug and liquid superficial velocity Ul are fixed,the pressure gradient of gas-liquid ntwo-phase flow in the U-tube decreases with the tube diameter increasing.Besides,when the Ui is fixed,the pressure gradient in the U-tubes increases with the increase of Ug.For a given Ug,the pressure gradient in the U-tube increases linearly with the increase of Ul.The rate of linear growth increases with the increase of Ug.The flow direction has a significant influence on the pressure gradient in the U-tube at low Ug and Ul,and the pressure gradient in the downward flow is larger than that in the upward flow.This is mainly due to the throttling effect caused by the fact that the liquid column flows down through the channel between the plug bubble and the tube wall.The effect of flow direction on pressure gradient in U-bend unit is directly proportional to the length and diameter of plug bubbles at low superficial liquid and gas velocities.The flow direction has little effect on the pressure gradient in the U-tube at high superficial gas velocity.The experimental pressure drop values of the U-tube were compared with the widely used classical prediction correlation.The frictional pressure drop correlations of U?bend in the existing literatures cannot accurately predict the pressure drop of gas-liquid two-phase flow in the transverse U-bend.Based on the experimental results of pressure drop and the analysis of the influencing factors,a mathematical prediction model of pressure drop for gas-liquid two-phase flow in the U-tube has been proposed.The dynamic response characteristics of the gas-liquid two-phase flow induced vibration(hereinafter referred to as GLTP-FIV)in a transverse U-tubes was experimentally studied.The intensity and form of induced vibration under different flow patterns were obtained.It is found that the GLTP-FIV in the transverse U-tube is mainly the flapping phenomenon in plane direction at low frequency.The order of the vibration response intensity(hereinafter referred to as VRI)is slug-wavy flow,annular-wavy flow,annular dispersed flow,plug-bubble flow and plug-stratified flow.Dynamic response distribution of the U-tube was analyzed.The amplitude of the spectrum presents the approximate symmetrical parabola distribution along the axial measured points.And the VRI at the 90-degree position of the U-tube reaches the maximum.The effect of superficial liquid velocity U1,superficial gas velocity Ul and volumetric gas fraction ? on the VRI were discussed.For a given superficial liquid velocity,the VRI presents a trend of approximately flat growth and then a sharp increase with the increase of Ug.The boundary of the sharp increase is about Ug =1m/s.For a given Ug,VRI increases with the rise of Ul.he main reasons are the strength of two-phase turbulence and the impact of the periodic liquid slug.For a given low volumetric gas fraction ?VRI presents a very slow growth trend with the increasing o ?.However,when P increase to a certain value,VRI increases greatly with the rise of ?.The certain value decreases with the increasing of superficial liquid velocity.The above research and results can be important reference for the development,design and operation of relative heat exchangers and transport pipes that involve a U tube as basic component.