Thermal Striping Phenomena Research Articles

Numerical Simulation of Thermal Striping Phenomena in a T-Junction Piping System Using Large Eddy Simulation

At the Japan Atomic Energy Agency (JAEA), the simulation code “MUGTHES (MUlti Geometry simulation code for THErmal-hydraulic and Structure heat conduction analysis in boundary fitted coordinate)” has been developed to evaluate thermal striping phenomena that are caused by the turbulence mixing of fluids at different temperatures. In this paper, numerical schemes for thermal-hydraulic simulation employed in MUGTHES are described, including the LES model. A simple method to limit numerical oscillation is adopted in energy equation solutions. A new iterative method to solve the Poisson equation in the BFC system is developed for effective transient calculations. This method is based on the BiCGSTAB method and the SOR technique. As the code validation of MUGTHES, a numerical simulation in a T-junction piping system with the LES approach was conducted. Numerical results related to velocity and fluid temperature distributions were compared with existing water experimental data and the applicability of numerical schemes with the LES model in MUGTHES to the thermal striping phenomenon was confirmed.

Crack Detectability in Vertical Axis Cooling Pumps During Operation

The problem which is faced in this article is the analysis of the effects of a transverse propagating crack on the vibrational behavior of a vertical axis cooling pump. The crack is assumed to develop in a section between the impeller and a seal, preventing the hot water to flow upwards along the rotor shaft. The pressurized seal is fed with an injection of cold water, and crack initiation may be due to a thermal striping phenomenon. Afterwards, crack growth could be driven by a combination of thermal and mechanical loads, causing alternate cyclic stress in the shaft. Cracking instances of this type have been reported worldwide in several machines of similar design. In this article, the fact is emphasized that the crack behavior is likely to be influenced by the thermal field and by the water pressure in the cracked area. A dynamical lineshaft model, integrated by an original representation of the crack, has been developed to investigate the possible vibratory symptoms related to a crack propagation. The vibrations are generally measured in correspondence of a rigid coupling which connects the motor shaft to the pump shaft, in a position which is rather far away from the crack. 1 × rev., 2 × rev., and 3 × rev. vibration components, which are generally displayed by the machine condition monitoring system and are the most significative symptoms of the presence of a transverse crack in a rotating shaft, are calculated.

Flows in T-Junction Piping System (2nd Report, Numerical Analysis of Vortex Street Formed by Branch Pipe Flow)

Thermohydraulic analyses for a fundamental water experiment simulating thermal striping phenomena at T-junction piping systems were carried out using a quasi-direct numerical simulation code DINUS-3. Calculated results were compared with the experimental results on the flow patterns in the downstream region of the systems, the arched vortex structures under the deflection jet condition, the generation frequency of the arched vortex in the various conditions ; i.e., diameter ratio α, flow velocity ratio β and Reynolds number. From the comparisons, it was confirmed that (1) the DINUS-3 code is applicable to the flow pattern classifications in the downstream region of the T-junction piping systems, (2) the arched vortex characteristics with lower frequency components can be estimated numerically by the DINUS-3 code, and (3) special attentions should be paid to the arched vortex generations with lower frequency components of fluid temperature fluctuations, which might induced high-cycle thermal fatigue for the design of T-junction systems.

E114 流体温度ゆらぎがある平行平板間流路内の非定常熱伝達

Fluid temperature fluctuation of low frequency, known as the thermal striping phenomena, occurs in the mixing tees of nuclear components with hot and cold fluids. In this study, in order to elucidate the heat transfer characteristic of the thermal striping phenomena, fluid temperature fluctuation is developed by the periodic large scale vortices in a channel flow with rectangular cylinder, and the heat transfer coefficients were measured by the thin film heat flux sensor which was installed in the channel wall. The fluctuations of Nusselt number and fluid temperature are antiphase in the periodic temperature fluctuation field. And the time averaged Nusselt number in the fluid temperature fluctuation field is almost equal to that of the constant fluid temperature, although the R. M. S. of Nusselt number increases with fluid temperature fluctuation.

1423 高速炉のサーマルストライピングに関する研究 : 構造材への温度変動伝達特性の評価

1423 高速炉のサーマルストライピングに関する研究 : 構造材への温度変動伝達特性の評価

Structural response function approach for evaluation of thermal striping phenomena

A rational analysis method for thermal stress induced by fluid temperature fluctuation is developed, by utilizing frequency response characteristics of structures. High frequency components of temperature fluctuation are attenuated during a transfer process from fluid to structures. Low frequency components hardly induce thermal stress, since temperature homogenization in structures. Based on investigations of the frequency response mechanism, a frequency response function of structures was derived, which can predict stress amplitudes on structural surfaces from fluid temperature amplitudes and frequencies. It is formulated by multiplication of the effective heat transfer and the effective thermal stress functions. The frequency response function was applied to fatigue analysis of nuclear components, and clarified relation of fatigue damage to thermal hydraulic and structural design parameters.

Numerical Analysis of Nonstationary Thermal Response Characteristics for a Fluid-Structure Interaction System

Nonstationary thermal response analysis for a fundamental sodium experiment simulating thermal striping phenomena was carried out using a quasi-direct numerical thermohydraulics simulation code with a third-order upwind scheme for convection terms and a boundary element method code for thermal response evaluation of structures due to random sodium temperature fluctuations developed at Power Reactor and Nuclear Fuel Development Corporation (PNC). Discussions centered on an applicability of the numerical method for the damping effects of the temperature fluctuations in the course of heat transfer to the inside of structures from the fully turbulent region of sodium flows through the comparisons with the experiment. From these comparisons, it was confirmed that the numerical method has a sufficiently high potential in accuracy to predict the damping effects of the temperature fluctuations related to the thermal striping phenomena. Consequently, it is concluded that the numerical prediction by the method developed in this study can replace conventional experimental approaches using 1:1 or other scale model aiming at the simulation of the thermal striping phenomena in actual liquid metal fast breeder reactor (LMFBR) plants. Furthermore, economical improvements in the FBR plants can be carried out based on the discussions of optimization and rationalization of the structural design using the numerical methods.

Development of Thermohydraulics Computer Programs for Thermal Striping Phenomena

Thermal striping phenomena characterized by stationary random temperature fluctuations are observed in the region immediately above the core exit of liquid-metal-cooled fast reactors (LMFRs) due to the interactions of cold sodium flowing out of a control rod (C/R) assembly and hot sodium flowing out of adjacent fuel assemblies (F/As). Two thermohydraulics computer programs AQUA and DINUS-3, which are represented by both time- and volume-averaged transport analysis and direct numerical simulation of turbulence, respectively, were developed and validated for the evaluation of thermal striping phenomena. These codes were incorporated with higher order difference schemes to approximate the convection terms in conservation equations and adaptive time-step size control systems based on the fuzzy theory to eliminate numerical instabilities. From validation analyses with fundamental experiments in water and sodium, it was concluded that (a) thermal striping conditions such as spatial distributions of the intensity and the frequency of the fluid temperature fluctuations can be estimated efficiently by a combined approach incorporating the AQUA code and the DINUS-3 code, and (b) the thermal striping phenomena for the in-vessel components of actual liquid-metal-cooled fast reactors can be evaluated by the numerical method without conventional approaches such as large scale model experiments using sodium.

サーマルストライピング現象に対する解析的評価手法の開発

A numerical method, which is represented by both time- and volume-averaged transport analysis and direct numerical simulation of turbulence, was developed for thermal striping phenomena. The phenomena are characterized by a stationary random temperature fluctuation occurring in the region immediately above the fast breeder reactor (FBR) core due to a temperature difference of the core outlet coolant between subassemblies. The thermal striping phenomena are recognized as one of the key problems from the standpoint of high-cycle thermal fatigue of the in-vessel components such as the upper core structure, flow guide tube, etc. Fundamental experiments using water and sodium to simulate these thermal striping phenomena were calculated using the method developed in this study. Calculated results by the method were compared with the data under wide experimental conditions on the amplitude and frequency of the temperature fluctuations. Furthermore, the thermal striping phenomena in a 1:1 scale mock-up model sodium experiment simulating the outlet region of the FBR core were calculated by the method, and were compared with the calculational data. From these comparisons with the experimental data, it was confirmed that the numerical method has a sufficiently high potential in accuracy and efficiency to predict the amplitude and frequency of the temperature fluctuations related to the thermal striping phenomena. Consequently, it is concluded that the numerical prediction by the method developed in the present study can replace conventional experimental approaches using 1:1 or other scale model aiming at the simulation of the thermal striping phenomena in actual FBR plants. Furthermore, economical improvements in the FBR plants can be carried out based on the discussions of optimization and rationalization of the structural design using the numerical method.

Development of Analytical Method for Thermal Striping Phenomena.

A thermal striping phenomenon characterized by a random temperature fluctuation occurs in the region immediately above the fast breeder reactor core due to the temperature difference of the core outlet coolant between subassemblies. Then the random temperature fluctuation applies a thermal stress to in-vessel structures. In this paper, we have investigated the intensity of the temperature fluctuation in water by implementing the algebraic stress turbulence model (ASM) including the second-order momentum of turbulence and the fuzzy controller to determine an optimum time step size in a general-purpose thermohydraulic analysis code. From the analysis, it is concluded that the characteristics of the temperature fluctuation intensity can be estimated efficiently by the ASM.

Hydraulic similarity in the temperature fluctuation phenomena of non-isothermal coaxial jets

The effects of working fluids on temperature fluctuation phenomena have been studied, with special reference to similarity rules for the thermal striping phenomena in LMFBRs. Temperature measurements were made in non-isothermal coaxial jets using sodium, water and air as working fluids. These experimental results indicated the dependence of the characteristics of average and root-mean-square values for the temperature and peak-to-peak amplitudes and periods of temperature wave-form signals on Reynolds and Peclet numbers. It may be concluded that the characteristics of temperature fluctuation due to turbulent mixing in LMFBRs can be evaluated from the results of model tests using water and/or air if the Reynolds and Peclet numbers are sufficiently large.

Application of the Vortex Method to Analysis of Coolant Temperature Fluctuation

The two-dimensional vortex method is applied to the analysis of coolant temperature fluctuations caused by turbulent mixing to examine its applicability to the analysis of thermal striping in liquid-metal fast breeder reactors (LMFBRs). A coaxial jet flow having different temperatures streaming into a stagnant pool is simulated with the method. The calculated velocities are compared with the measurements of an air jet flow. Large scale eddy structures in the mixing region, which cause dynamic large temperature fluctuations, are observed in the numerical simulations. This vortex method can analyze temperature fluctuations caused by large eddies, and is shown to be a useful method for analyzing thermal striping phenomena in LMFBRs.