Two-phase flow instability induced by flashing in natural circulation systems: An analytical approach

Author(s):  
Akshay Kumar Khandelwal ◽  
Mamoru Ishii
Author(s):  
Wenchao Zhang ◽  
Sichao Tan ◽  
Puzhen Gao

Two-phase natural circulation flow instability under rolling motion condition was studied experimentally and theoretically. Experimental data were analyzed with nonlinear time series analysis methods. The embedding dimension, correlation dimension and K2 entropy were determined based on phase space reconstruction theory and G-P method. The maximal Lyapunov exponent was calculated according to the methods of small data sets. The nonlinear features of the two phase flow instability under rolling motion were analyzed with the results of geometric invariants coupling with the experimental data. The results indicated that rolling motion strengthened the nonlinear characteristics of two phase flow instability. Some typical nonlinear phenomena such as period-doubling bifurcations and chaotic oscillations were found in different cases.


Author(s):  
Jingjing Li ◽  
Tao Zhou ◽  
Mingqiang Song ◽  
Yanping Huang

The gas-liquid two phase flow oscillation in vertical parallel natural circulation channels was performed by the best estimate system computer code Relap5. The effects of symmetry and dissymmetry degree of heated power to flow oscillation and the effects of symmetry and dissymmetry throttling to flow oscillation were researched. The results says that when the twin channels under the same conditions of geometry and boundary, the parameters of the twin channel such as flow are the same. So under these conditions the twin channels can be researched as single channel. It is more possible of flow oscillation for the channels under condition of dissymmetry heating. The use of throttling will make the channels more stable, it is more stable when the throttling coefficient increases. With the implement of dissymmetry throttling, the system is possible for out of phase flow oscillation.


Author(s):  
Jong Chull Jo ◽  
Frederick J. Moody ◽  
Kyu Sik Do

A PWR incorporates a passive auxiliary feedwater system (PAFS), a closed natural circulation loop which is aligned to feed condensed water to its corresponding steam generator (SG). During its operation, saturated steam in the SG secondary side moves up due to buoyancy force and passes through a steam line, and then flows into a tube-tank type passive condensation heat exchanger where steam is condensed inside the tubes while the tube outer surfaces are cooled by the pool water. The condensate water is passively fed into the SG economizer by gravity. Because a natural circulation loop is susceptible to two-phase flow instability, it is requisite to confirm the system is designed adequately to avoid the potential challenges to its operational safety due to the instability. This paper presents an analytical approach for assessing if the PAFS has possible thermal and fluid mechanical characteristics which could lead to an undesirable unstable or oscillating condensate water level in the vertical pipe section. Both steady and unsteady analytical solutions for a simplified natural circulation loop model of the PAFS were derived in terms of the condensate water level and velocity in the vertical pipe section. From the solutions, the criteria for determining a potential for two-phase instability in the system were obtained.


Author(s):  
Xianbing Chen ◽  
Puzhen Gao ◽  
Qiang Wang ◽  
Yinxing Zhang ◽  
Jiawei Liu

Natural circulation has been widely used in some evolutionary and innovative nuclear power plants. Natural circulation systems are susceptible to flow instabilities which are undesirable in the nuclear power devices. An experimentally investigation of two phase flow instability in up-flow boing channel under natural circulation is presented in this paper. Flow instability with and without flow reversal have been found. A pulse signal of water temperature at the inlet of the test section can be detected when the channel suffers from flow reversal. Single phase and two phase flow alternate in the channel regardless of the occurrence of flow reversal. Periodic oscillations with multiple high-order harmonic waves are confirmed by applying Fast Fourier Transform to the time traces of flow rates. Period of flow instability which is the reciprocal of the frequency with the largest amplitude in the amplitude-frequency plane are obtained. Period of flow oscillation presents a nonlinear change with the increase of mass flux. Period of flow instability increases rapidly with the increase of mass flux and decreases slowly when it reaches the maximum value.


2020 ◽  
Vol 139 ◽  
pp. 107269
Author(s):  
Zahraddeen Abbati ◽  
Jiarui Chen ◽  
Kun Cheng ◽  
Fulong Zhao ◽  
Fenglei Niu ◽  
...  

Author(s):  
Zheng Huang

Natural circulation is one of the most important mechanisms of passive safety systems in the advanced NPP design. By using RELAP5, steady-state behaviors of a typical natural circulation loop (NCL) were investigated. The Influences of various system parameters, such as pressure, cooling heat transfer area and loop height were studied. The results show that the heat transfer power of the NCL (i.e. the heat transferred from the heater to the tank by NCL) is linear with respect to the temperatures of boundary heat source and sink, excluding the phase transition region. The NCL operation mode (i.e. two-phase flow or one-phase flow) is mainly determined by the loop pressure. It is found that with the decrease of the pressure, the two-phase flow operation range is widened. The heat transfer power of two-phase flow case is always higher, and varies more substantially with pressure, than that of one-phase flow case under the same boundary temperatures. However, flow instability may occur for two-phase flow case. By increasing the area of heat transfer surface between loop cooling section and tank, the temperature difference between the inlet and the outlet of loop heating section as well as the heat transfer power of the NCL will increase. But the influence is insignificant when the tank temperature is relatively high. It is also observed that for both one-phase flow and two-phase flow, the change of loop height has little effect upon the heat transfer power of the NCL under the given boundary condition in this study.


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