scholarly journals Numerical Analysis of General Trends in Single-Phase Natural Circulation in a 2D-Annular Loop

2008 ◽  
Vol 2008 ◽  
pp. 1-10 ◽  
Author(s):  
Gilles Desrayaud ◽  
Alberto Fichera
Keyword(s):  
Natural Convection ◽  
Numerical Analysis ◽  
Single Phase ◽  
Closed Loop ◽  
Dynamic Properties ◽  
Natural Circulation ◽  
Flow Behavior ◽  
Pitchfork Bifurcation ◽  
Chaotic Flow ◽  
Over The Top

The aim of this paper is to address fluid flow behavior of natural circulation in a 2D-annular loop filled with water. A two-dimensional, numerical analysis of natural convection in a 2D-annular closed-loop thermosyphon has been performed for various radius ratios from 1.2 to 2.0, the loop being heated at a constant flux over the bottom half and cooled at a constant temperature over the top half. It has been numerically shown that natural convection in a 2D-annular closed-loop thermosyphon is capable of showing pseudoconductive regime at pitchfork bifurcation, stationary convective regimes without and with recirculating regions occurring at the entrance of the exchangers, oscillatory convection at Hopf bifurcation and Lorenz-like chaotic flow. The complexity of the dynamic properties experimentally encountered in toroidal or rectangular loops is thus also found here.

Development of Prevention Method for Air Ingress During a Depressurization Accident of the VHTR

2016 ◽  
Author(s):  
Tetsuaki Takeda ◽  
Shumpei Funatani
Keyword(s):  
Natural Convection ◽  
Numerical Analysis ◽  
Storage Tank ◽  
Molecular Diffusion ◽  
Fluid Layer ◽  
Natural Circulation ◽  
Flow Characteristics ◽  
Mixing Process ◽  
Vertical Slot ◽  
Air Ingress

A depressurization accident is a design-basis accidents of a very high temperature reactor. When a depressurization accident occurs, air is expected to enter the reactor pressure vessel from the breach and oxidize in-core graphite structures. Therefore, it is important to know a mixing process of different kind of gases in the stable and unstable stratified fluid layer. Especially, it is also important to examine an influence of localized natural convection and molecular diffusion on mixing process from a viewpoint of safety. In order to predict and analyze the phenomena of air ingress during a depressurization accident, therefore, it is important to develop the method for prevention of air ingress during the accident. We have carried out an experiment and a numerical analysis using three-dimensional computational fluid dynamics (3D CFD) to obtain the mixing process of two component gases and flow characteristics of the localized natural convection. This study is also to investigate a control method of natural circulation of air by injection of helium gas. The numerical model consists of a storage tank and a reverse U-shaped vertical slot. They are separated by a partition plate. One side of the left wall of the left side vertical slot is heated and the other side was cooled. The right side vertical slot is cooled. The procedure of the experiment and the numerical analysis is as follows. Firstly, the storage tank was filled with heavy gas and the reverse U-shaped vertical slot was filled with light gas. In the left side vertical slot, the localized natural convection was generated by the temperature difference between the vertical walls. The flow characteristics were obtained by the experiment and steady state analysis. The unsteady state experiment and analysis were started after the partition plate was opened. The result obtained in the experiment was simulated by the numerical analysis quantitatively. The gases were mixed by molecular diffusion and natural convection. After the time elapsed, natural circulation occurred. When the temperature difference of the left vertical fluid layer was set to 100K and the combination of the mixed gas was helium and nitrogen, natural circulation produced after 110 minutes elapsed.

Experiment and Numerical Analysis of Mixing Process of Two Component Gases in Vertical Fluid Layer

2017 ◽  
Author(s):  
Tetsuaki Takeda
Keyword(s):  
High Temperature ◽  
Natural Convection ◽  
Numerical Analysis ◽  
Molecular Diffusion ◽  
Natural Circulation ◽  
Flow Characteristics ◽  
Mixing Process ◽  
High Temperature Side ◽  
Two Component ◽  
Air Ingress

A depressurization accident is the one of the design-basis accidents of a Very-High-Temperature Reactor (VHTR). When a depressurization accident occurs, air is expected to enter into the reactor pressure vessel from the breach and oxidize in-core graphite structures. Therefore, it is important to understand the mixing processes of different kinds of gases in the stable and unstable stratified fluid layers. In particular, it is also important to examine the influence of localized natural convection and molecular diffusion on the mixing process from a safety viewpoint. Therefore, in order to predict or analyze the air ingress phenomena during a depressurization accident, it is important to develop a method for the prevention of air ingress during an accident. We carried out experiments and numerical analysis using three-dimensional (3D) CFD code to obtain the mixing process of two-component gases and the flow characteristics of localized natural convection. This study also investigated a control method for the natural circulation of air through the injection of helium gas. The numerical model consists of a storage tank and a reverse U-shaped vertical rectangular passage. They are separated by a horizontal partition plate. One sidewall of the high-temperature side vertical passage is heated and the other sidewall is cooled. The low-temperature vertical passage is cooled by ambient air. The storage tank is filled with heavy gas and the reverse U-shaped vertical passage is filled with a light gas. In the vertical passage of the high-temperature side, localized natural convection is generated by the temperature difference between the vertical walls. The results obtained from the experiments were quantitatively simulated using 3D numerical analysis. The two component gases were mixed via molecular diffusion and natural convection. After some time elapsed, natural circulation occurred through the reverse U-shaped vertical passage. These flow characteristics are the same as those of phenomena generated in the passage between a permanent reflector and a pressure vessel wall of the VHTR.

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