| In passive safety technologies,natural circulation plays an important role.And it is widely used in 3rd and 4th generation reactors.Natural circulation systems have complex self-feedback procedures,which can cause flow instability phenomena under rolling conditions and severe accidents.Flow instability with complex thermal-hydraulic mechanism is harmful to reactor safety,and simulation on flow instability is a difficult problem in engineering.Flow instability is always a quasi-periodical phenomenon and shows weak chaotic characteristics,and is sensitivity to tiny disturbances.As a basis theory of chaotic neural network,research on chaotic evolution characteristics of natural circulation flow instability plays an important role in simulation and prediction work of flow instability phenomenon based on artificial intelligence technology.The aim of this research is to reveal chaotic revolution routes of flow inatability in natural circulation systems.In this paper,flow instability phenomena of natural circulation under rolling condition and that thermal-hydraulic scenario under heat of sink loss accident(LOHS)are studied by experimental research,and the thermo-hydraulic mechanism and chaotic evolution route of them are analyzed.Natural circulation flow instability meets Devaney chaos definition,and is a kind of deterministic chaotic motion.In order to analyze chaotic characteristics of non-stationary signal,the windowed maximal Lyapunov exponent algorithm was developed originally in this paper.Natural circulation pressure drop oscillation(PDO)has different thermal-hydraulic and chaotic evolution phenomena while under different rolling periods.The experimental study on composite oscillations of natural circulation PDO and rolling motion shows that when the ratio of PDO natural frequency to rolling frequency is 1.5:1-1.2:1,it follows a chaotic evolution route of frequency-locking.There is an evolution of thermal-hydraulic phenomenon is thermal-hydraulic stability,frequency-locking oscillation,and rolling effect weakening region.It is found that chaotic broadband phenomenon and high-order harmonic phenomenon would occur in spectrum.The experiment of co-frequency oscillations of natural circulation PDO and rolling motion is carried out,and it is found that while PDO natural frequency to rolling frequency are close to each other,chaotic evolution route becomes chaotic resonance oscillation.And evolution route of thermo-hydraulic phenomena is thermo-hydraulic stability,bimodal oscillations and chaotic resonance oscillations.The bimodal oscillation phenomenon is the precursor phenomenon of chaotic resonance oscillation,and its thermal-hydraulic phenomenon is analyzed.Compressible volume in pressurizer can significantly influence thermal-hydraulic mechanism and stability boundary of flow instability.Natural circulation PDO experiment under rolling condition with low compressible volume condition is carried out.Low compressible volume condition can reduce maximal Lyapunov exponent greatly,but thermo-hydraulic mechanism and chaotic evolution route would not be changed.In this paper,LOHS accident experiments in natural circulation system are established.The evolution phenomena of natural circulation LOHS accident are: single-phase flow,onset of flow instability,PDO region,two-phase stability region,post-annular-flow instability and intermittent CHF.PDO is caused by slug flow pattern in heating channel,and has two kinds of wave modes: unimodal oscillation and bimodal oscillation.PDO phenomena cannot last forever.When annular flow pattern is generated at the outlet of heating channel,high void fraction in heating channel is able to absorb flow oscillation,so that PDO could gradually disappear.After the two-phase stability region lasting for 40-150 s,the phenomenon of post-annular-flow instability occurs,then intermittent CHF phenomenon occurs.In this paper,a new kind of flow instability phenomenon is discovered in natural circulation LOHS accident,which is named as post-annular-flow instability.Through experiment research,the instability boundary of post-annular-flow instability is described,and its thermal-hydraulic mechanism and spectral characteristics are analyzed.Post-annular-flow instability is caused by bubble aggregation in heating channel and is always accompanied by Kelvin-Helmholtz instability.The mean period of post-annular-flow instability is about 4.55 s,and flow oscillation can cause the advance of CHF.The occurrence of post-annular-flow instability requires low flowrate condition,long heating channel condition and annular flow as precursor phenomenon.This study uses windowed maximal Lyapunov exponent algorithm to analyze chaotic evolution phenomenon of PDO in natural circulation LOHS accident.It is found that it belongs to intermittent chaos evolution route,in which maximal Lyapunov exponent have significant sudden increase and decrease.During the evolution of PDO in natural circulation LOHS accident,the quasi-periodic oscillation is destroyed.Flow wave mode may be unimodal,bimodal or irregular oscillations,and could transform to each other.The chaotic evolution of PDO is a determined chaotic phenomenon,but dominated by stochastic motion. |
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