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Session Overview
8.05-2: Experimental & Numerical Studies - II
Tuesday, 17/Mar/2020:
1:45pm - 3:15pm

Session Chair: Ulrich Bieder, CEA, France
Location: R-3014

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Pre-test analysis of the H2P4 test in HYMERES-2 project

Dmitry Grishchenko, Pavel Kudinov

Royal Institute of Technology, KTH

Pressure suppression pool (PSP) is an important element of safety system for a boiling water reactor. In case of the primary coolant system depressurization, whether in normal operation or during an accident, the steam is directed into the PSP via blowdown pipes and spargers for direct condensation. The PSP is expected to mitigate containment pressurization and serve as filter for fission products. However, PSP may fail if thermal stratification is developed in the pool. In the Fukushima accident thermal stratification in the PSP lead to increased rate of pressure rise, which would be much slower if pool would be completely mixed.

OECD/HYMERES-2 project test series H2P4 aims to address integral phenomena of the containment behavior with pressurization caused by pool stratification. Specifically, a Fukushima type scenario i.e. SBO with steam injection into wetwell at relatively small rates is considered. The goal of this paper is to provide pre-test analysis for the H2P4 tests to define conditions at which thermal stratification and containment pressurization phenomena can be addressed considering also possible effects of other factors, e.g. activation of spray.

In the paper we demonstrate how specific goals for code validation, definition of relevant phenomena, similarity scaling with Fukushima unit 3 accident and pre-test simulations are used to select experimental conditions and minimize the effect of irrelevant and not measured in the experiment phenomena. We develop and validate a simplified fast running model and then apply it for the selection of experimental conditions such as pool depth, sparger submergence, steam injection rate and free gas volume. In addition, we carry out a set of refined simulations to address phenomena that may be affected by coarse mesh of the simplified model. We discuss the results of the pre-test analysis and compare those to available experimental data.

Non-linear stability analysis of Ledinegg Instability

Alok Kumar, Md Emadur Rahman, Munendra Pal Singh, Suneet Singh

Indian Institute of Technology Bombay

In several industrial flows, particularly in nuclear reactors, boiling two-phase flow is very often seen. The instabilities in a two-phase flow system may cause mechanical vibrations, boiling crisis, control problems and even burnout at the wall surface and hence deemed to be dangerous. One of the most common static instability that occurs in most of the flows is Ledinegg instability (LI). Although, a lots of research has been carried out to study this instability, the advent of open source bifurcation tools has led to several interesting insights into this phenomena. The present work aims to study co-dimension two bifurcation (where two parameters of the system are simultaneously varied) in a two-phase flow heated channel.

The characteristics of the pump and internal resistance in mass flow rate and pressure drop space are called external and internal characteristic curves respectively. The internal characteristic curve is interpolated as a third-degree polynomial of mass flow rate, whose coefficients correspond to the parameters of the system like heat flux, pressure head. A mathematical model consisting of two first-order nonlinear ordinary differential equations is constructed via the lumped parameter method and verified through a linear stability analysis. The system has either three equilibrium points or one equilibrium point based on the values of the parameters. This transition by changing the parameters leads to LI which is identified by saddle-node bifurcation. For a fixed internal characteristic (i.e. given heat flux), the variation of external characteristic leads to two saddle-node points for the system. However, as heat flux in the channel is varied these two saddle bifurcation lines intersect at a point, known as Cusp point beyond which LI does not exist. Such identification of stable region in parameter space is important of the safe operation of these systems.

Non-linear Stability Analysis of Parallel Channels System under Supercritical Pressure Condition

Munendra Pal Singh, Alok Kumar, Suneet Singh

Indian Institute of Technology Bombay

The present work is devoted to studying the linear as well as nonlinear stability characteristics of Supercritical fluids, using a reduced order nodalized model for a parallel heated channels system. The supercritical fluids are most promising working fluids in generation IV nuclear reactors. To fulfill the primary objective of the present study, auto-adaptive schemes has been where the node sizes automatically adjust according to variation in enthalpy change in the channel. The stability thresoul drawn in the pseudo-phase chane (N_tpc) and pseudo-subcooling (N_spc) space. The linear stability analysis is carried out with help of the eigenvalues of the Jacobian matrix, which is created after linearize the system around the steady state and the non-linear analysis includes detailed study of dynamic and static instabilities. Different types of bifurcation phenomena are observed as indicating various features of the dynamic instabilities. Corresponding to that first Lyapunov coefficient has been calculated to distinguish between sub-critical and super-critical Hopf bifurcations. Whereas in static instability; Ledinegg excursive phenomena, which is characterized as a saddle-node bifurcation, is observed. Additionally on saddle-node bifurcation curve, Bogdanov-Takens bifurcation points (as an interaction with Hopf bifurcation) appear. These bifurcations lead to complex dynamics in the system therefore, several numerical simulations have been carried out around the stability threshold. This analysis helps to enhance the heat transfer characteristics during oscillatory flow in system.

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