Study On Flow And Rupture Characteristics Of Liquid Film On The Wall Of Corrugated Plate Dryer Under Horizontal Shear Of Air Flow

The corrugated plate dryer(CPD)is one of the most commonly used steam-water separation devices in steam generator of secondary loop in nuclear power plants(NPPs).The steam in the evaporator must be dehumidified before participating in power generation,otherwise the airflow with a lower dryness will affect the safety of the NPP.In addition,the efficiency of undried steam in the steam turbine is extremely low,which will also seriously affect the improvement of the economic indicators of the NPP.Therefore,the importance of the CPD in the secondary loop is self-evident.The steam-water separation efficiency(SE)of the corrugated plate(CP)is extremely important for the safety and economic improvement of NPPs.In the real operation of NPP,high-temperature and high-pressure(HTHP)airflow generated in evaporator enters the multi-stage CPD at a certain speed,and the steam-water mixture will flow along the flow channel and hit the wall at the corners of the CP.At this time,larger droplets will be absorbed and captured by the wall of the CP.The increase in the amount of droplets on the wall will form a liquid film(LF)on the wall,which is then discharged from the dryer through the drainage tank.The small droplets that are not captured continue to flow along the flow channel together with the steam to enter the next stage of the CP.The steam dryness can be improved by multi-stage droplet capture.However,there is a phenomenon in the CP that has a great influence on the SE of steam and water,that is,the phenomenon of re-entrainment.The droplets captured by the corrugated wall will flow along the wall and be discharged.When there are many liquid droplets on the wall surface,it will converge into a wall surface LF,and this part of the LF will also flow along the wall surface to the drainage groove and be discharged.However,due to the continuous flow of steam in the dryer,the flow of the LF on the wall will produce certain fluctuations,which will then be sheared and broken by the airflow to form fine droplets(called secondary droplets).At this time,the steam will carry the droplets flowing on the wall and the secondary droplets generated when the LF on the wall ruptures and continue to flow along the flow channel to the next stage of the CP.This phenomenon that the separated droplets and the secondary droplets generated owing to LF breakdown on CP wall are carried by the steam carrying the droplets again is called re-entrainment.The Re-entrainment phenomenon contains a large amount of flow,fluctuation and LF breakdown on the wall.Therefore,in order to improve the working performance(ie SE)of the CP,it is necessary to explore the causes of the rupture of the LF from the source and provide measures to improve the critical airflow velocity(The local airflow velocity at the moment when the LF on the wall ruptures under the action of the sweeping airflow).In addition,related studies have shown that the wave characteristics and return characteristics of the LF on the CP wall when there is no air flow,the rupture position of the LF on the CP wall under the action of air flow,and the size of the secondary droplets generated when the LF on the wall ruptures,problems such as the movement characteristics of the strip-like LF formed in the rupture also have a vital influence on the mechanism research of re-entrainment.In fact,the CPD is not only widely used in NPPs,but also has important applications in two-phase separation related fields such as chemical industry and petroleum.Therefore,the related mechanism research on the LF on the wall of the CPD can provide great reference value for the two-phase separation technology.Due to the need for visualization experiments,HTHP water vapor cannot be used for experiments,thus the HTHP water vapor is replaced by air to realize the cold state simulation of the hot state.In addition,the cold state experiment is a commonly used research method for the CPD,which plays an important role in the optimization and design of dryer parameters.In this paper,the law of LF flow on the CP wall under the action of no airflow and the law of LF rupture on the CP wall under the action of traversing airflow are studied.First,the thickness of the free LF on the wall of the CP without air flow was measured by the planar laser induced fluorescence technology(PLIF),and the law of the change of the liquid film thickness(LFT)with time was obtained.Using many non-linear characteristic analysis methods such as power spectral density method(PSD),probability density method(PDF),wavelet method,chaos theory,BP neural network,the non-linear characteristics of the LFT are analyzed to realize the determination of the characteristics of the LF on the wall.In addition,based on the dimensionless method,the control equation of the LF flow in the two-dimensional curvilinear coordinate system is derived.According to the boundary layer theory and the force of the LF,the formula of critical airflow velocity can be derived theoretically and compared with the experimental results.And an important method to increase the critical airflow velocity is obtained.The rupture process of the LF on the wall surface under the action of air flow is studied through visual experiments.The rupture process of the LF is divided into five stages.For the first time,the SVM+SIFT image recognition method is introduced into the field of nuclear engineering for the LF on the wavy plate wall.The recognition and automatic classification of the images of the rupture process greatly reduces the workload of artificially classifying the images of the rupture process of the LF.Finally,the related mechanism research on the flow and rupture of the LF on the wall of the CP is carried out.The rupture position of the LF on the wall of the CPD under the action of air flow is visually measured and the calculation method of the rupture position of the LF is theoretically deduced.The experiment measured the relative height of the liquid film rupture(LFR)on the wall,fitted it based on the calculation formula of the LFR position,and got the calculation method of the relative height(CMRH)of the LFR.Based on the above CMRH of the LFR position and the LFR,the calculation method of the radius of the secondary droplets produced by the LFR on the wall surface of the CPD is theoretically deduced and it is compared and analyzed with the droplet size measured by the visual experiment.The LF on the wall of the CPD will flow back under the action of a larger airflow.With the further increase of the airflow speed,the LF will break.Visualized experimental measurement of the lateral movement distance of the LF on the wall surface at the moment of liquid film rupture was carried out,and the LF reflux strength(distance)was theoretically deduced,and the calculation method of the LF reflux strength(distance)was obtained.The angle of the band-shaped LF produced when the LF is broken is measured experimentally.Based on the calculation method of the position of the LFR and the relative height of the LFR,a calculation method for the angle of the band-shaped LF is fitted.The above-mentioned research provides important reference for the research on the breaking mechanism of the LF in the CPD and the improvement of the SE.