Investigation On Flow Characteristics Of Low Height-Difference Natural Circulation System At Low Pressure

As a new type of natural circulation system,a low-height-difference natural circulation system has broad application prospects in floating nuclear power plants at low pressure.Compared with the conventional natural circulation systems,this kind of natural circulation system has a lower height difference between the heat source and heat sink and uses infinite volume cold source such as ocean as its heat sink.This natural circulation system,as a passive residual heat removal system,not only further simplifies the system structure but can achieve long-term effective heat export.At present,research of the low-height-difference natural circulation system at low pressure mainly focuses on the systemic heat removal performance in single-phase condition,and the local physical processes are not paid enough attention,especially the direct contact condensation of steam and subcooled water in two-phase condition.In order to fully understand the single-phase and two-phase flow characteristics of low-height-difference natural circulation system at low pressure,this paper has carried out and completed the following research work.To systematically study the flow characteristics of the low-height-difference natural circulation system at low pressure,an experimental facility was designed and built based on the passive residual heat removal system of floating nuclear power plant.Based on the facility,the experiments were carried out to study single-phase natural circulation flow characteristics.The results show that cold water can flow back into the hot pipe section in single-phase condition,resulting in thermal stratification.Moreover,the effects of natural circulation flow rate,temperature difference between cold water and hot water and pipe diameter on the backflow length of cold water in the hot pipe section were clarified.On the basis of experimental research,the reverse flow model of cold water was established by using CFD software star CCM +,and the internal mechanism of the reverse flow phenomenon was analyzed.Based on Reynolds number Re,density Froude number Fo and Atwood number At,the general expression of dimensionless reverse flow length of cold water was obtained.Under the conditions of heating power from 29 ～ 74 kW,water temperature from 10 ～26 ℃,and inlet resistance coefficient from 85 ～ 138,this paper firstly performed the experiment on two-phase flow characteristics of the natural circulation system.The results show that there are three flow modes: sinusoidal oscillation,intermittent oscillation and condensation oscillation.In the condensation oscillation mode,the key thermal-hydraulic phenomenon of steam condensation induced water hammer(CIWH)occurs in the hot pipe section.By analyzing the formation process of CIWH under different pipe diameters,the influence of this local physical phenomenon on the systemic flow characteristics was clarified.In addition,the effects of heating power,the water temperature of the heat sink,fluid subcooled degree,and inlet resistance coefficient on the systemic flow characteristics were investigated in detail.Through the analysis of the amplitude and discrete characteristics of the natural circulation flow rate,the boundary and criterion of the stable zone and CIWH were obtained.The visualization experiments on the formation mechanism of steam slug in horizontal hot pipe section were performed at heating power from 50 ～ 74 kW.The results show that there are three kinds of formation mechanisms of steam slug,which are:(1)the increase in two-phase flow velocity leads to the transition of flow pattern from stratified flow or wavy flow to slug flow;(2)the formation,propagation,collision,and fusion of solitary wave and interface wave in the heat pipe section lead to an increase in wave height,which induces Kelvin-Helmholtz instability to form steam slug;(3)the pressure wave generated by the CIWH event intensifies the disturbance of upstream two-phase interface and increases the wave height,resulting in the formation of steam slug.By analyzing the statistical characteristics of CIWH events,a new understanding of the randomness of CIWH was given.In order to establish the theoretical analysis model of pressure peak caused by the CIWH event under natural circulation conditions,the visual experimental study was carried out on the transient characteristics of water hammer induced by the isolated steam slug under different working conditions.Based on the experimental results,the growth laws of the liquid columns on both sides of the steam slug were analyzed and obtained during the condensation process of the steam slug,and the growth model of the liquid column was established.The deviation between the model and the experimental results is within-17% and 22%.On this basis,combined with the interface mass transfer theory,the theoretical analysis model of the pressure peak caused by CIWH was established by using Joukowsky’s formula of water hammer.The deviation of the pressure peak obtained from the model and the experiment is mainly concentrated within-35% and 20%.