Interfacial Transport Characteristics And Mechanism Model For Two Phase Flow In Rod Bundles

The complexity of the two-phase flow comes from the existence of the interface between the gas-liquid and the transfer of mass,momentum and energy through the interface.For rod bundles,the interface characteristics are essential for the flow boiling heat transfer with considering the safe operation of the reactor.The interface study of two-phase flow can be divided into two types:(1)Steady-state characteristics,which mean two-phase flow regimes.Current researches focus on the flow regime transition criteria.(2)Transient characteristics.Interface area transport model can describe the physical processes of bubble coalescence,breakup,expansion and phase change of the two-phase interface region.Thus,the dynamic characteristics of the interface can be modeled.Since the interface area transport model is divided into two groups,the two-phase flow pattern can provide the boundary conditions for the interface area transport model.In this paper,a visual experimental study in rod bundles was carried out to explore the two-phase flow pattern transition mechanism of the bundle beam channel Based on experimental research and theoretical analysis,the one-dimensional interface transport model was proposed to provide the theoretical basis for practical engineering applications.For the study of two-phase flow regime of rod bundles,various flow regimes maps have been obtained with different spacer grids and at different axial heights by visual measurement system,respectively.On the basis of experimental research,this paper redefines the flow regimes division for rod bundles,including bubbly flow,cap bubbly flow,dispersed bubbly flow,cap turbulent flow,churn flow and annular flow.This study has proposed a new flow regime transition criteria model based on the analysis of the underlying physics of the upward two-phase flow behaviour in the vertical rod bundles.A reliable drift-flux correlation to predict void fraction in the vertical rod bundle developed recently has been used in modeling the flow regime transition criteria.A fairly good agreement with some discrepancies has been obtained between the newly developed transition criteria and the measured flow regime maps.The experimental results show the effects of different structures of spacer grids on parameter distributions are significant in rod bundles.Under flow condition of low void fraction,the prototypical spacer grids can delay the flow regime transition due to the strong mixing effect.It is beneficial to optimize the two-phase flow heat transfer of the flow channel.For the transition from cap turbulent flow to churn flow,the bubbles can immediately coalescence due to the sufficient gas phase and strong mixing even if the bubbles are sheared.Thus,the prototypical spacer grids have less influence on flow regime transition with high void fraction.At different axial locations,the flow regime transition boundaries are also significantly different.The higher the axial locations,the more likely the flow regime transition occurs when void fraction is low.However,the influence of locations on the flow regime transitions is weakened with high void fraction.No significant flow regime transitions occurred in advance.For the study of the interface area transport model of rod bundles,many experimental data of two-phase flow have been obtained in the radial direction and the axial direction,incudling interfacial area concentration,void fraction and interfacial velocity.The distribution of the two-phase parameters along the radial direction shows alternating changes in peaks and troughs based on the experimental data.An area-averaged model for parameters has been proposed.The variation characteristics of the two-phase parameters along the axial direction are quantitatively analyzed.Experimental data obtained in the axial direction indicate spacer grids produce two mechanisms on axial distribution of void fraction,“swirling effect” and “bubble breaker effect”.If the “swirling effect” is larger than the “bubble breaker effect”,the void fraction distribution tends to become a core-peak type,resulting in a decrease in gas phase velocity.Conversely,if the “bubble breaker effect” is larger than the “swirling effect”,the void fraction distribution tends to be a wall-peak type,resulting in an increase in gas phase velocity.One dimensional interfacial area transport model has been proposed for bubbly flow in rod bundles with spacer grids.New model re-determines the bubble breakup model and the bubble coalescence model in the source and sink terms with considering the characteristic of channel structure and spacer grids.The results indicate that the new model can predict the interfacial area concentration with the relative error of 19.9%.In this paper,the dynamic characteristics of the phase interface have been investigated based on experimental research and theoretical analysis.Accurate measurement of the bubbles distribution under bubble flow conditions has been realized.The flow regime transition criteria mechanisms and interfacial area transport mechanism have bee revealed which provide clear boundary conditions and theoretical model methods for calculation of reactor system program and safety design.