Effect Of Non-uniform Heating On Flow Instability Under Natural Circulation Condition

In order to enhance the inherent safety of nuclear power system,passive safety technology plays a more and more prominent role.Natural circulation served as a one of the passive safety technologies,has rather extensive application prospects in nuclear power and nuclear supply department.However,the flow instability which occurred in natural circulation system makes the system face many hazards,such as mechanical fatigue,control failure and thermal shock.It restricts the application of natural circulation.Therefore,it has important significance to investigate the thermal-hydraulic characteristics and local heat transfer characteristics of natural circulation system during flow instability.It is worth mentioning that the particularity of the power distribution of reactor heat release poses a new challenge to the application of natural circulation technology in nuclear power system.In this paper,both experimental and numerical simulation methods are used to study the natural circulation flow instability.Meanwhile,the effect of axial non-uniform heating on the natural circulation flow instability is discussed as well.Initially,a tubular test section which can realize the axial cosine heating is designed and manufactured.The design scheme is optimized step by step according to the principle of resistance heating and the level of processing technology.Accordingly,the outer diameter profile of the channel is modified.Through theoretical derivation and experimental study,the wall temperature distribution law of axial non-uniform heating channel is investigated under natural circulation steady state.It includes the location of peak wall temperature,the influence of excess temperature and the non-linear distribution of fluid temperature along the channel.A series of natural circulation experiments on single-phase flow and heat transfer characteristics are carried out in axial non-uniform heating channel.The simulation capability of axial cosine heat flux distribution in the designed channel is verified through the experiments.Based on the steady-state heat conduction differential equation of a circular tube channel with uniform heat flux distribution,the wall temperature field under unsteady boundary conditions is derived theoretically and extended to the experimental channel with axial cosine heating.According to the theoretical deduction results,it is pointed out that the influence of the axial variation of wall thickness on Fourier number is the primary reason for the difference of temperature amplitude and phase between inner and outer walls under unsteady boundary conditions.In two channels with uniform and non-uniform axial heat flux distributions,experimental studies on flow instability of low-pressure closed natural circulation are carried out respectively.In the uniform heat flux channel,two typical flow instability modes,including geysering flow instability and compound dynamic flow instability,are summarized according to the characteristics of parameters during flow instability.The local heat transfer characteristics are studied in depth under unstable operating conditions.The heat transfer types and laws in each stage of flow fluctuation are analyzed from the view of flow pattern transition,and the mechanism explanation of the local heat transfer characteristics is given.The results show that the heat flux is the direct criterion for the two phenomena,while the fundamental reason for the difference in the instability of the two phenomena is that the local characteristics and the system characteristics have different effects on the natural circulation.The experiments are conducted in order to investigate the effects of non-uniform heating conditions on flow instability in natural circulation.The mechanism of flow instability is explained from the view of local heat transfer characteristics and system characteristics.The onset and disappeared point of flow instability in this channel are obtained by continuous experiments of heating power upstream and downstream processes.The stability boundaries of natural circulation system under uniform and non-uniform heating modes are given.Several typical conditions including steady state,variable power and periodic oscillation are investigated in low pressure and closed natural circulation experimental system.According to the primary principle of natural circulation,the driving forces are refined.With the help of RELAP5 code,it is clear that inertia force is the main reason which causes the numerical simulation deviation of natural circulation flow instability.By comparing the relationship between driving forces and kinetic energy when various kinds of flow instability occur,the effects of flashing and condensation in the long straight riser is illustrated under different conditions.The results show that when the heat flux is lower than a critical value,the flashing will be enhanced with the increase of the heat flux.On the other hand,if the heat flux in the heating section is higher than the critical value,flashing is suppressed with the increase of the heat flux.Based on RELAP5/MOD3.4 code,natural circulation thermal-hydraulic characteristics under steady and transient conditions are analyzed.The discrete problem which induced by rough mesh in the process of modeling is solved by a criterion according to the experimental results.A revised criterion of independence verification is proposed.It is pointed out that the accuracy and stability of transient natural circulation simulation can be effectively improved by limiting the maximum Courant number of all components in the loop.The capability of RELAP5 for analyzing low-pressure two-phase natural circulation is evaluated.