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Fluctuating stresses imposed on a piping system are potential causes of thermal fatigue failures in energy cooling systems of nuclear power plants (4). These stresses are generated due to temperature fluctuations in regions where cold and hot flows are intensively mixed together. A typical situation for such mixing appears in turbulent flow through...
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... more quantitative description can be found in [8,9]. In Figure 3 the magnitude of the velocity field is presented. Two fluid streams accelerate in the mixing zone and together they form a jet-like structure, which has a recirculation zone in the upper part of the downstream pipe. ...
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... 12 Stress intensity fluctuations at the inner pipe wall at two different times. Figure 13 shows stress intensity fluctuations as a function of time for the same two locations as presented in Figure 11. For comparison, also the magnitude of the temperature fluctuations is plotted here. ...
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... maximum stress intensity in the whole structure, during the simulated period, is 41.4 MPa. Figure 13 Stress intensity fluctuations and temperature fluctuations as a function of time at two arbitrary positions at the inner pipe wall. ...
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... point for the assessment are the stress fluctuations in the pipe, which are calculated as described in the previous section. As can be seen in Figure 13, these stress fluctuations, resulting from turbulent mixing, vary considerably in time, both in amplitude and frequency. To reduce such a complex signal to a set of cyclic loads, application of a cycle counting method is required. ...
Citations
... The simulation test is based on an experimental device, which was used before validating the CFD (Computational Fluid Dynamics) model for adiabatic flow as shown in Figure 2 with two perpendicularly connected pipes. Figure 2. T-junction experimental device [4] Structural temperatures resulting from CFD analysis were used as thermal charges in finite element analysis [4]. We also find the work of [5], where the authors of this research present the finite element analysis of the constraints on T-junction with the I-DEAS program. ...
... The simulation test is based on an experimental device, which was used before validating the CFD (Computational Fluid Dynamics) model for adiabatic flow as shown in Figure 2 with two perpendicularly connected pipes. Figure 2. T-junction experimental device [4] Structural temperatures resulting from CFD analysis were used as thermal charges in finite element analysis [4]. We also find the work of [5], where the authors of this research present the finite element analysis of the constraints on T-junction with the I-DEAS program. ...
... It was concluded that finer mesh was required to capture the temperature fluctuations near the centre line of the pipe of the mixing zone. [4], [5]. www.ijsrp.org ...
... The need of high-fidelity computational tools is a well-known requirement especially for the simulations of abnormal scenarios that can compromise the safe operation, or the integrity of a fusion reactor. European project inside the HORIZON 2020 framework [1,2] and works available in the scientific literature confirm the need of coupled computational tools capable to account for multiple physical phenomena such as fluid/structure interaction [3], neutronics/thermal-hydraulic [4] and multi-scales modelling such as 1D/3D thermal-hydraulic coupling tool [5,6]. ...
The In-Box Loss Of Coolant (LOCA) postulated accident is considered a major concern for the safety connected with the development of EU-DEMO fusion reactor. Relating to the renewed interest in the Water-Cooled Lithium Led blanket concept, an innovative experimental campaign is under development at ENEA Brasimone laboratories aiming at investigating the consequences related to the In-Box LOCA applied to the WCLL breeding blanket. In this frame, a new coupling tool between the SIMMER-III (modified version to implement the PbLi/water chemical interaction) and the RELAP5/Mod3.3 codes (modified version to implement PbLi thermo-physical properties) has been developed together with its preliminary application to simple test cases with water as working fluid. The coupling procedure can be defined as a “two-way”, “non-overlapping”, “online” technique aiming at investigating multi-physics and multi-scales phenomena in support of the development of fusion reactor technologies.
... Therefore, different flow patterns can be reproduced with the experimental data of each case. The calculation of the momentum ratio (Igarashi et al., 2002;Kamide et al., 2009) for the Vattenfall benchmark facility conditions gives a type of flow in between Wall-Jet or Reattached (Hannink, 2008;Westin et al., 2008) and even Deflecting-Jet (Ayhan and Sökmen, 2012). The momentum ratio is sensitive in the ranges of fluid temperatures and flow values considered. ...
There is a need to perform three-dimensional mechanical analyses of pipes, subjected to complex thermo-mechanical loadings such as the ones evolving from turbulent fluid mixing in a T-junction. A novel approach is proposed in this paper for fast and reliable generation of random thermal loads at the pipe surface. The resultant continuous and time-dependent temperature fields simulate the fluid mixing thermal loads at the fluid–wall interface. The approach is based on reproducing discrete fluid temperature statistics, from experimental readings or computational fluid dynamic simulation's results, at interface locations through plane-wave decomposition of temperature fluctuations. The obtained random thermal fields contain large scale instabilities such as cold and hot spots traveling at flow velocities. These low frequency instabilities are believed to be among the major causes of the thermal fatigue in T-junction configurations. The case study found in the literature has been used to demonstrate the generation of random surface thermal loads. The thermal fields generated with the proposed approach are statistically equivalent (within the first two moments) to those from CFD simulations results of similar characteristics. The fields maintain the input data at field locations for a large set of parameters used to generate the thermal loads. This feature will be of great advantage in future sensitivity fatigue analyses of three-dimensional pipe structures.
... To design for the risk of thermal fatigue, which for LMFR is considered higher than for light water reactors, approaches developed for LWRs, as e.g. described in [19] and [20], should be transferred and adapted to LMFRs. As the applied approaches essentially use CFD to determine the thermal fluctuations, basically, the heat transport models need to be updated and validated. ...
Liquid metal cooled reactors are envisaged to play an important role in the future of nuclear energy production because of their possibility to use natural resources efficiently and to reduce the volume and lifetime of nuclear waste. Typically, sodium and lead(-alloys) are envisaged as coolants for such reactors. Obviously, in the development of these reactors, thermal-hydraulics is recognized as a key (safety) challenge. A relatively new technique to deal with thermal-hydraulics issues is Computational Fluid Dynamics (CFD). This technique is used increasingly nowadays for design and safety evaluation purposes. This paper will discuss the development status of CFD application to liquid metal cooled reactors. In addition, the main challenges for future developments will be indicated. Firstly, the technological challenges will be discussed which ask for CFD application. Afterwards, the needs for CFD development and/or validation will be discussed. The discussion will also include the need for accompanying experiments.
... The results of such simulations are coupled with computer codes for thermal analyses of the pipe material and then with finite element models (FEM) to analyze the thermal stresses in the pipes. Examples can be found in References (Chapuliot et al, 2005;Kang, et al., 2011;Hannink and Blom, 2011;Kamaya and Nakamura, 2011;Hannink et al, 2008). Using these 3D analyses, the fluid temperature fields and heat transfer between the fluid and pipe are predicted. ...
Thermal fatigue is a structural damage of materials induced by the cyclic thermal loads that are frequently generated by the changes of fluid temperature inside of pipes. Among the thermal fatigue assessment methods we find the one-dimensional (1D) approach. Thermal, mechanical and fatigue analyses are performed for the pipe wall assuming that the distribution of temperatures only varies along the wall thickness. On the other hand, pipe regions with higher stress oscillations are those where the fluid temperature changes spatially, meaning cold or hot spots near the pipe surface, and with low frequencies. Spatial fluid temperature differences generate heat fluxes within the pipe wall which can’t be reproduced with 1D methods. For this reason, the present work focuses on understanding the wall temperature distributions for different values of heat fluxes and frequencies of fluid temperature. Due to the implication in wall temperature measurements, the heat fluxes and frequencies effects on temperature readings of wall thermocouples are also investigated.
Water experiments were carried out for fluid structure interaction aspects of non-isothermal mixing in a T-junction which is part of a new test facility. The main subject of this paper is firstly to present the new facility and secondly, to demonstrate the Near-Wall LED-Induced-Fluorescence (NWLED-IF) technique, which is a new experimental method for studying fluid–structure interaction under conditions similar to those in LWR. The Fluid–Structure-Interaction-Setup is a closed-loop T-junction facility with a design pressure of 75 bar and a maximum temperature of 280 °C. The water streams are mixed in a horizontally aligned, sharp-edge, 90° T-junction. The forged T-junction is made of austenitic steel 1.4550 (X6 CrNiNb 18-10) with reduced carbon content in accordance with the German KTA 3201.1. It is equipped with 24 thermocouples (1 mm in diameter) in blind holes, which have a surface offset of 1–3 mm. The facility design comprises interchangeable modules, which can be arranged upstream or downstream of the T-junction. Two of these modules provide an optical access to the fluid by means of flanges and tubes made of glass. Additional experiments are conducted in an isothermal T-junction facility and at the Fluid–Structure Interaction Facility. It is demonstrated that the Near-Wall LED-Induced-Fluorescence technique is an image-based measurement method that provides spatially and temporally resolved information of the turbulent flow in the mixing region of the T-junction even under presence of high density differences. In all experiments a buoyancy-driven stratified flow is observed. The light fluid arranges itself on top of the denser which is characterized by a meander-like structure. The experiments are conducted under different fluid-mechanical boundary conditions, yet fluid patterns were similar and the stratification and the meander-like structures were captured by the Near-Wall LED-Induced-Fluorescence technique. The presented experiment is the first of its kind delivering flow information with a spatial and temporal resolution useful to assess even small turbulent structures within the thin near-wall layer.
Turbulent mixing in a T-junction causes thermal striping which is irregular and frequent fluctuation of thermal layer. Thermal striping is a significant thermal problem because it causes unpredicted high cycle thermal fatigue and fatigue cracking in piping systems. Since this phenomenon is hardly detected by common plant instruments due to high frequency and complex mechanism, numerical approaches are indispensable for a precise evaluation. This research was carried out to define a suitable and effective numerical method for evaluating thermal stress and fatigue induced by thermal striping. A three-dimensional hydro-thermo-mechanical analysis was performed based on one-way separate analysis method to find out the characteristics of stress components and its results were compared to the results of a one-dimensional simplified analysis. It was found that the detailed three-dimensional analysis is indispensable because one-dimensional simplified analysis can overestimate or underestimate according to the assumed heat transfer coefficient and cannot estimate the considerable mean stress effects.
Thermal fatigue assessment of pipes due to turbulent fluid mixing in T-junctions is a rather difficult task because of the existing uncertainties and variability of induced thermal stresses. In these cases, thermal stresses arise on three-dimensional pipe structures due to complex thermal loads, known as thermal striping, acting at the fluid-wall interface. A recently developed approach for the generation of space-continuous and time-dependent temperature fields has been employed in this paper to reproduce fluid temperature fields of a case study from the literature. The paper aims to deliver a detailed study of the three-dimensional structural response of piping under the complex thermal loads arising in fluid mixing in T-junctions. Results of three-dimensional thermo-mechanical analyses show that fluctuations of surface temperatures and stresses are highly linearly correlated. Also, surface stress fluctuations, in axial and hoop directions, are almost equi-biaxial. These findings, representative on cross sections away from system boundaries, are moreover supported by the sensitivity analysis of Fourier and Biot numbers and by the comparison with standard one-dimensional analyses. Agreement between one- and three-dimensional results is found for a wide range of studied parameters. The study also comprises the effects of global thermo-mechanical loading on the surface stress state. Implemented mechanical boundary conditions develop more realistic overall system deformation and promote non-equibiaxial stresses.