Study On The Mechanism Of Boiling Two-Phase Flow Destabilization Coupling Void Evolution In Parallel Rectangular Narrow Channels

Due to its high heat transfer efficiency and compact configuration,the rectangular narrow channel attracts increasing investigation and application in the design of advanced nuclear reactors with plate type fuel element.When intense boiling occurs in parallel rectangular narrow channels with high wall heat flux,two-phase flow destabilization will take place,which could result in boiling crisis,mechanical and thermal stress damage,or disruption in system control.Therefore,study focusing on the characteristics and triggering mechanism of two-phase flow destabilization in parallel rectangular narrow channels has very important research significance and application value for the thermal-hydraulic safety of nuclear reactor.In this paper,the visual experiments and theoretical study have been carried out to investigate the oscillation of thermal parameters of two-phase during the flow destabilization process and to analyze the triggering mechanism of two-phase flow destabilization based on the behavior of void evolution.The main works of this paper is listed as following:(1)A void distribution parameter model for boiling two-phase flow in rectangular narrow channel has been developed based on theory analysis.Under the high sub-cooled boiling,the prediction of the vapor velocity using present distribution parameter model is in good agreement with the experimental data(the mean relative errors(MRE)is less than ±8.5%)concluded from the bubbles PIV of the visualization images.The artificial neural network has been used to dynamically identify the flow regime evolution in rectangular narrow channel during flow destabilization.Four different flow regimes are observed in our experiment: dispersed/coalesced bubbly flow,cap flow,churn flow and annular flow.(2)According to the characteristics of the mass flux oscillation,the working point with significant increasing oscillation amplitude is defined as the onset of flow destabilization.After flow destabilization occurring,two stages of oscillations take place consequently with gradually increasing heat flux.With the relative lower heat flux,the first stage of oscillation occurs with a relative low amplitude and unclear period;under the high heat flux,the second stage of oscillation(typical density wave oscillation)occurs with a high amplitude and clear period.According to the The visualization of void evolution behavior,it is found that the cluster bubbles coalesce due to strong boiling at the outlet of the parallel channels as the water at channel outlet reaches saturated state,which results in the rapid flow regime transition from churn flow to annular flow and the occurrence of the first stage of oscillation.As the relative long annular flow occurs in channels,the second stage of oscillation takes place with a period-doubling phenomena of the void evolution in the parallel channels.(3)Combining two-fluid model and the kinetic constitutive law of drift-flux model,a mixture model for two-phase flow is developed in this paper.Different from four-equation drift-flux model developed by Ishii et.al.,the presented mixture model has no covariance terms,which makes the the numerical solution of the set of two-phase flow equations easier and more accurate.A JFNK method with a physics-based preconditioner is used to solve the discretization of the two-phase flow equations using the coupled fully-implicit discretization scheme,then an efficient and stable preconditioner is obtained from the semi-implicit discretization of the two-phase flow mixture equations.The verification of the proposed numerical method and validation of the code accuracy are performed by compared with experimental result.(4)The proposed numerical method has been used to investigate the flow instability boundary and the nonlinear stability characteristics of the boiling two-phase flow in the parallel rectangular narrow channels.The prediction of the instability boundary is in good agreement with the experimental results,and the mean relative errors is less than ±7%.A nonlinear characteristics of the system instability is found that the supercritical Hopf bifurcation occurs in high sub-cooled region when the working condition crosses over the flow instability boundary from the stable region toward the unstable region.However,the sub-critical Hopf bifurcation exists in low sub-cooled region.Moreover,the sensitivity analyses of the key parameters on flow instability indicate that the boiling two-phase flow in parallel rectangular channels is more stable with high inlet subcooling,the larger channel gap or greater distribution parameter.(5)Also,the proposed numerical method has been used to study the mechanism of flow destabilization in parallel rectangular narrow channels.Due to the delay of flowing heat transfer and the migration of void fraction,there exists the delayed effect of void fraction against the oscillation of the inlet mass flux.This results in a double-period phenomenon of the void evolution and the two-phase flow in the channels.When the heat flux reaches a relative high value,the high void fraction occurs in channels.In parallel rectangular narrow channels,there exists a phase difference over 180° between the pressure drop oscillations of the two channels,due to the the delayed effect of the void evolution.The coupled delayed effects of the void evolution and the two-phase flow resistance result in the two-phase flow destabilization.In this paper,the experimental and theoretical investigations have been performed to study the characteristics and mechanism of two-phase flow destabilization in parallel rectangular narrow channels,based on the analyses of the void evolution behavior.The conclusions and method of this paper can provide the theoretical basis for the engineering applications in the inhibition of two-phase flow instability.