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Numerical methods for the prediction of thermal fatigue due to turbulent mixing

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Abstract

Turbulent mixing of hot and cold flows is one of the possible causes of thermal fatigue in piping systems. Especially in primary pipework of nuclear power plants this is an important, safety related issue. Since the frequencies of the involved temperature fluctuations are generally too high to be detected well by common plant instrumentation, accurate numerical simulations are indispensable for a proper fatigue assessment. In this paper, a link is made between two such numerical methods: a coupled CFD–FEM model and a sinusoidal model. By linking these methods, more insight is obtained in the physical phenomenon causing thermal fatigue due to turbulent mixing. Furthermore, useful knowledge is acquired on the determination of thermal loading parameters, essential for reducing overconservatism, as currently present in simplified fatigue assessment methods.

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... In consequence, the effect of the thermal interaction between the fluid flow and the solid walls is a relevant subject of study in the academic research and engineering field (Johnson et al., 1998;Li et al., 2019;Khalesi and Sarunac, 2019;Zhang, 2018). Lately, research studies are focused on the influence of fluid temperature fluctuations that cause stress in the solid (Hannink and Blom, 2011;Guo et al., 2018;Nakamura et al., 2015) that may lead to cracks as in CIVAUX I nuclear power plant (Chapuliot et al., 2005). Therefore, the understanding and correct modeling of temperature fluctuations is a relevant topic to prevent this type of fatigue (Stephan et al., 2002;Duff et al., 2007;Monod et al., 2012), especially, in Fast Breeder Reactor (FBR), where the coolant is a metal (Cao et al., 2017). ...
... Wall temperature fluctuations originated by turbulent mixing are difficult to measure experimentally (Hannink and Blom, 2011). There are some methods to model them as the sinusoidal method (Hannink and Blom, 2011) that in general leads to over-conservative results in the determination of thermal fatigue (Hannink and Timperi, 2011). ...
... Wall temperature fluctuations originated by turbulent mixing are difficult to measure experimentally (Hannink and Blom, 2011). There are some methods to model them as the sinusoidal method (Hannink and Blom, 2011) that in general leads to over-conservative results in the determination of thermal fatigue (Hannink and Timperi, 2011). In this context, numerical analysis is a tool that, when properly used, can provide reliable temperature fluctuations. ...
... The simulated fluid temperature fields at the wall boundary can be used as a thermal boundary condition in heat transfer analyses of the pipe wall employing a heat transfer coefficient assumption (Chapuliot et al., 2005). A more advanced approach for load determination uses CFD-LES analyses with conjugate heat transfer (CHT) (Hannink and Blom, 2011;Kloeren and Laurien, 2011;Kuhn et al., 2010). CHT stands for simulations where the heat conduction in the wall and the fluid convection are computed simultaneously. ...
... Therefore, the 1D methods intrinsically omit the global response of the structure. Hence, they are not able to explain phenomena such as crack propagation, three-dimensional (3D) response of the structure to large-scale flow instabilities (cold or hot spots moving on the component's surface (Hannink and Blom, 2011;Kamaya and Nakamura, 2011)), or the conditions close to the discontinuities in a 3D structure (Chapuliot et al., 2005). ...
... Studies of thermal response of pipe wall under thermal loads from turbulent mixing in T-junctions have shown that temperature fluctuation fields of the pipe's inner surface consist of cold and hot spots following mainly an axial movement with velocity quite close to main flow velocity (Blom et al., 2007;Hannink and Blom, 2011). . ...
Article
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.
... Thus, it is clear that the highest thermal stresses, which might cause fatigue damage, occur at intermediate frequencies; namely, from 0.1 to 10 Hz. Even though sinusoidal methods are known to yield overly conservative estimates of the fatigue lifetime (Hannink and Blom (2011)), it is evident that performing a proper spectral analysis of surface temperatures can help to predict the risk of thermal fatigue cracking. Anyway, given the complexity of such loads in the case of turbulent mixing, 3-D coupled finite volume/finite element analyses or factors accounting for plasticity at geometric discontinuities are usually taken into account (Dahlberg et al. (2007)). ...
... This approach is very common because most studies aim at assessing whether the results obtained agree with Kolmogorov's power-law scaling of turbulence in the inertial subrange -that is, E(κ) ∝ κ − 5 /3 , where E is the energy spectrum function and κ is the wavenumber -and at finding a number of frequencies for the evaluation of thermal stresses in the adjacent walls. The former purpose was pursued in Ayhan and Sökmen (2012) and Timperi (2014), while the latter was explored in Radu et al. (2009) and Hannink and Blom (2011). In the latter case, either the bulk or the inner-surface temperature is assumed to vary sinusoidally with time in order to evaluate stresses in the wall, consistently with the so-called "sinusoidal method". ...
Thesis
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High-cycle thermal fatigue arising from turbulent mixing of non-isothermal flows is a key issue associated with the life management and extension of nuclear power plants. The induced thermal loads and damage are not fully understood yet. With the aim of acquiring extensive data sets for the validation of codes modeling thermal mixing at reactor conditions, thermocouples recorded temperature time series at the inner surface of a vertical annular volume where turbulent mixing occurred. There, a stream at either 333 K or 423 K flowed upwards and mixed with two streams at 549 K. Pressure was set at 72E5 Pa. The annular volume was formed between two coaxial stainless-steel tubes. Since the thermocouples could only cover limited areas of the mixing region, the inner tube to which they were soldered was lifted, lowered, and rotated around its axis, to extend the measurement region both axially and azimuthally. Trends, which stemmed from the variation of the experimental boundary conditions over time, were subtracted from the inner-surface temperature time series collected. An estimator assessing intensity and inhomogeneity of the mixing process in the annulus was also computed. In addition, a frequency analysis of the detrended inner-surface temperature time series was performed. In the cases examined, frequencies between 0.03 Hz and 0.10 Hz were detected in the subregion where mixing inhomogeneity peaked. The uncertainty affecting such measurements was then estimated. Furthermore, a preliminary assessment of the radial heat flux at the inner surface was conducted.
... 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. ...
... The cited papers in the above paragraph use 1D analyses mainly due to industry application reasons (Fontes et al, 2012). On the other hand, pipe regions with higher stress oscillations are those were the fluid temperature changes spatially, meaning cold or hot spots near the pipe surface, and with low frequencies (Kamaya and Nakamura, 2011;Hannink and Blom, 2011). Spatial fluid temperature differences generate heat fluxes within the pipe wall which can't be reproduced with 1D methods. ...
Article
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.
... Due to its balance of numerical accuracy and efficiency, IDDES-SST is often favoured in industrial applications where a compromise between computational resources and numerical simulation fidelity is necessary. Many studies successfully adopted the IDDES-SST model to predict phenomena associated with thermal stripping and its performance has been validated against experimental measurements (Hannink & Blom, 2011;Rohrig, Jakirlic, & Tropea, 2015;Lampunio, Duan, Eaton, & Bluck, 2021;Lampunio, Duan, & Eaton, 2022). ...
Article
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This study focuses on analysing thermal mixing in T-junctions with varying momentum and Reynolds number ratios, utilizing computational fluid dynamics (CFD) simulations. The T-junction is a critical component of the primary nuclear thermal-hydraulic circuit within a pressurized water-cooled reactor (PWR). The T-junction connects the pressurizer (PRZ) with the steam generator (SG) and the reactor pressure vessel (RPV). Water from the PRZ and the SG are at different temperatures and incomplete thermal mixing occurs when these two fluid streams meet at the T-junction. This incomplete thermal mixing can induce thermal stratification of the water within the T-junction as well as thermal striping phenomena. Thermal striping phenomena can lead to fluctuations of the temperature at the inner pipe wall of the T-junction. Thermal stratification and thermal striping phenomena can induce thermo-mechanical fatigue and eventual pipe failure which can affect the safety of the reactor. Therefore, a high-fidelity, mechanistic understanding of the turbulent thermo-fluid mixing within T-junctions of PWRs might lead to improvements in component reliability and safety within nuclear power plants (NPPs). The primary aim of the research, presented in this paper, is to understand and quantify the effect of variations in the momentum ratio on the turbulent fluid flow within T-junctions. This is achieved by either varying the branch pipe diameter while keeping the inlet velocity constant (part one) or by adjusting the branch pipe inlet velocity while maintaining a constant diameter (part two). Despite the different variations in momentum ratios, the specific momentum ratios under consideration in both parts of the study remain consistent (namely 98 and 66.4). It is also noteworthy that the momentum ratios considered in the paper can be classified as wall-jet and impinging-jet, according to the definition in (Hosseini, Yuki, & Hashizume, 2008). It should be noted that the momentum ratio is manipulated by adjusting the flow parameters, leading to variations in the Reynolds number ratio between the main pipe inlet and the upstream branch pipe at the T-junction. The turbulent flows in the cases that are considered are simulated using the Improved delayed detached eddy simulation (IDDES-SST) model. The numerical results from these simulations indicate, for the considered momentum ratios, that maintaining the same momentum ratio does not produce similar mean flow behaviour and turbulent quantities of interest (QoI). For instance, the size of the flow recirculation zone is more likely linked to the diameter of the branch pipe. Moreover, the turbulent QoI and temperature fluctuations at the determined locations are likely affected by the changed flow recirculation zone as well as the Reynolds number of the branch pipe flow.
... The field of fluid-solid coupling mechanics has experienced significant advancements in recent years. Researchers have become increasingly intrigued by the quantitative correlation that exists between temperature fluctuations and high-cycle thermal fatigue [16][17][18]. Fluid-structure interactions are the subject of scholarly investigation through both experimental methods and numerical simulation techniques [19][20][21][22]. Zughbi et al. [23] discovered that thermal mixing occurs more rapidly over a shorter distance when the branch pipe is inclined at angles of 45°and 60°. ...
Article
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In this paper, the numerical simulation was done for a cylindrical tee by establishing a steady-state simulation to examine the mixing performance. The temperature of the fluid at the hot inlet was chosen as 36 °C and 19 °C for the cold inlet. The numerical simulation was done for a short tee of 192 mm and a long mixing tee of 262 mm at a variety of momentum ratios. The geometry was meshed in FLUENT before solving the domain. For the meshing, the faces were initially named hot inlet, cold inlet, outlet, and walls. The triangular method was chosen to generate a mesh for the flow domain. The size of the cell in meshing was taken as 0.1 m. In this work, the SST k–ω models were selected to perform the computations. The analytical values of temperature were used to validate the numerical results. Results show that the thermal mixing was done effectively using the CFD ANSYS software package. Results show that the size of the mixing area is the same hence there is not much of a difference between the long tee and the short tee in that particular sector. The thermal mixing was found better when the velocity at the vertical inlet (y-axis) becomes greater and the average temperature is lower. Also, the increase in the pipe's length causes the average temperature to drop since the fluid mixes better the farther along it travels, while also slightly increasing the velocity.
... This method is popular as most researchers strive to show that their data tally with the Kolmogorov law in the inertial subrange -i.e., E(κ) ∝ κ − 5 /3 , where κ is the wavenumber and E is the energy spectrum function -and to determine a range of frequencies for the assessment of thermal stresses in the adjoining walls. The former goal was pursued in Ayhan and Sökmen (2012) and Timperi (2014), the latter in Radu et al. (2009) and Hannink and Blom (2011). In the latter case, either the inner-surface or the bulk temperature is conjectured to change sinusoidally with time so as to compute stresses in the wall, compatibly with the so-called sinusoidal method (Dahlberg et al. (2007)). ...
Thesis
Full-text available
High-cycle thermal fatigue due to turbulent mixing of streams at distinct temperatures is an interdisciplinary issue affecting safety and life extension of existing reactors together with the design of new reactors. It is challenging to model damage and thermal loads arising from the above mixing. In order to collect vast data sets for the validation of codes modeling turbulent thermal mixing under reactor conditions, temperatures were sampled at the inner surface of the vertical annular volume between two concentric 316LN stainless steel tubes. This annulus simplifies that between control-rod guide tube and stem in Swedish boiling water reactors (BWRs) Oskarshamn 3 and Forsmark 3. In 2008, several stems there were reported as broken or cracked from thermal fatigue. Cold water entered the annulus at 333 K, at axial level z = 0.15 m. It moved upward and mixed with hot water, which entered the annulus at 549 K, at z = 0.80 m. Pressure read 7.2 MPa. Hot and cold inlet temperatures and pressure match BWR conditions. The thermocouples attached to the inner tube could only acquire inner-surface temperatures at six locations, so the inner tube was translated and rotated about the z-axis to expand the measurement zone. Mixing inhomogeneity was estimated from such measurements. In the cases examined, the inner-surface temperatures from areas with the highest mixing inhomogeneity show dominant frequencies lower than ten times the inverse of the experiment time. The uncertainty of this temperature measurement appears to be equal to 1.58 K. A large eddy simulation (LES) of mixing in the above annulus was conducted. Experimental boundary conditions were applied. The conjugate heat transfer between water and tubes was modeled. The wall-adapting local eddy viscosity (WALE) subgrid model was adopted. A finite element analysis (FEA) of the inner tube was performed using LES pressure and temperature as loads. Cumulative fatigue usage factors (CUFs) were estimated from FEA stress histories. To this end, the rainflow cycle-counting technique was applied. CUFs are highest between z = 0.65 m and z = 0.67 m. Cracking is predicted to initiate after 97 h. LES and experimental inner-surface temperatures agree reasonably well in relation to mean values, ranges, mixing inhomogeneity, and critical oscillation modes in areas sensitive to fatigue. LES inner-surface temperatures from areas with the highest CUFs show dominant frequencies lower than ten times the inverse of the simulation time. A robust, effective iterative algorithm for reconstructing the transient temperature field in the inner tube from redundant boundary data was implemented and verified. Temperature-dependent properties were included. Initial conditions and over-specified boundary data in the inverse problem were perturbed with Gaussian noise to check the robustness of the solving method to noise.
... This is clearly visible in Fig. 10b. It is important to note that this difference is proportional to the thermal stress [13]. It has been reported by [2], the dominant amplitudes wich have frencencies in the range from 1 to 10 hz, are changed and converted to heat stress. ...
Article
Turbulent mixing of two fluids having a temperature difference can lead to temperature fluctuations in the T-junctions, particularly in the primary cooling circuit of nuclear power plants. Temperature changes produce thermal stresses causing thermal fatigue. This is a key topic related to nuclear security. This article presents the numerical results of two thermal turbulent mixing simulations, applied to a T-junction (OECD-NEA-Vattenfall T-junction). These simulations apply, respectively, to the case of a flow with and without rotational movements into the cold side. The numerical predictions were made using the large eddy simulation with the commercial code Fluent. The numerical results obtained in the first case were compared with the available experimental data and were found in an agreement satisfactory to the profiles of average variables and the spectrum of the temperature near the wall. The comparison with RMS profiles was less satisfactory in the center of the main tube, but was of good quality in the near-wall region. In the second simulation, the main movement of the fluid is affected by the spiral paths created by the swirl velocity caused at the cold inlet, which improves the mixing of hot and cold fluids downstream of the T-junction. From the analysis of these results, it clearly appears that the temperature fluctuations decrease near the wall in this case which leads to less high thermal loads on the tube, and hence a lesser thermal fatigue.
... Simulations of thermal mixing and wall structural response using CHT have been conducted by a number of researchers [12][13][14][15][16][17][18][19]. Timperi [18] studied the mixing in a T-junction by LES and CHT between the fluid and pipe solid wall. ...
Conference Paper
Full-text available
In a nuclear reactor, thermal striping, stratification, and cycling take place as a result of mixing of pressurized hot and cold water streams. The fluctuating thermal load generated by such unsteady mixing may result in fatigue damage of the associated structures. Generally, thermal fatigue is considered to be a long-term degradation mechanism in nuclear power plants. This is significant, especially for aging power plants, and improved screening criteria are needed to reduce risks of thermal fatigue and improve methods to determine the potential significance of fatigue. Though fluid mixing and thermal fatigue have been studied separately, a number of issues related to complex interaction between turbulent mixing and the mechanical structure of the Light Water Reactor (LWR) have not yet been resolved. The primary objective of the study reported here was to advance the use of numerical modeling techniques for reactor safety determination by developing a proof-of-concept benchmark simulation that demonstrates that computational methods can be used to address turbulent mixing-induced thermal fatigue in the context of LWR operations. In addition, the structure of turbulence in the T-junction was investigated. A computational method comprised of Large Eddy Simulation (LES) and Unsteady Reynolds Averaged Navier Stokes (URANS) modeling was used to simulate turbulence and capture the coherent structures and turbulence scales. In addition, Conjugate Heat Transfer (CHT) analyses were performed to predict the thermal field and temperature distribution in the solid piping material of the T-junction. Finally, the corresponding thermal stress in the solid pipe was estimated based on a simplified one-dimensional model to assess the thermal-structure degradation. Fatigue calculations show that the initiation of cracking due to thermal fatigue primarily depends on the temperature difference between the hot and cold streams.
... A factor 2 on this value has also been proposed in recent thermal fatigue assessments [9] based on an experimental study of heat transfer during turbulent mixing [28]. In a recent numerical study, coupled CFD analysis with conjugate heat transfer of fluid mixing in T-junctions has shown values of the heat transfer coefficient varying between 3000 and 7000 W/m 2 K [29]. In this case, the authors considered the Vattenfall benchmark facility conditions. ...
Article
Full-text available
Large sets of fluid temperature histories and a recently proposed thermal fatigue assessment procedure are employed in this paper to deliver more accurate statistics of predicted lives of pipes and their uncertainties under turbulent fluid mixing circumstances. The wide variety of synthetic fluid temperatures, generated with an improved spectral method, results in a set of estimated distributions of fatigue lives through linear one-dimensional heat diffusion, thermal stress estimates and fatigue assessment codified rules. The results of the fatigue analysis indicate that, in order to avoid the inherent uncertainties due to comparatively short fluid temperature histories to the estimated fatigue lives, a conservative safe design against thermal fatigue could be attempted with the lower bounds of the predicted life distributions, such as the 5% failure probability life (5% of samples fail). The impact of the convection heat transfer coefficient on the predictions is also studied in a sensitivity analysis. This represents a detailed attempt to correlate the uncertainties in the physical fluid mixing conditions and heat transfer to the estimated fatigue life using spectral methods.
... Simulations of thermal mixing and wall structural response using CHT have been investigated by a number of researchers [12][13][14][15][16][17][18][19]. Timperi [18] studied the mixing in a T-junction by LES and CHT between the fluid and pipe solid wall. ...
Conference Paper
Full-text available
Thermal striping generally is recognized as a significant long-term degradation mechanism in the primary cooling water circuit of nuclear power plants (NPPs). This phenomenon occurs by mixing of hot and cold water streams in the primary coolant loop. Depending on the flow configuration, the turbulent mixing process can lead to thermal striping, temperature fluctuations in the T-junction region, thermal fatigue, and crack generation in the associated structure. The objective of this study is to provide an in-depth look into the underlying physics for thermal fatigue to determine appropriate screening criteria and risk significance for the regulatory safety evaluation process. In addition, the structure of turbulence in the T-junction also is investigated. The computational method comprised of Large Eddy Simulation (LES) modeling to simulate turbulence and Proper Orthogonal Decomposition (POD) analysis to capture the coherent structures and turbulence scales. In addition, Conjugate Heat Transfer (CHT) analyses have been performed to predict the thermal field and temperature distribution in the solid piping material of the T-junction. Finally, the corresponding thermal stress in the solid pipe is estimated based on a simplified one-dimensional model to assess the thermal-structure degradation. Copyright © 2015 by ASME Country-Specific Mortality and Growth Failure in Infancy and Yound Children and Association With Material Stature Use interactive graphics and maps to view and sort country-specific infant and early dhildhood mortality and growth failure data and their association with maternal
... Thermal fatigue is studied experimentally to guide strategies for limiting component damage of pressurized water reactors (e.g., Fissolo et al. 2009). In addition, computational fluid dynamics (CFD) tools are being used to simulate fluid/structure interactions to improve understanding and management of thermal cycling in nuclear power system components (Galpin and Simoneau 2011;Hannink and Blom 2011). Broader acceptance of these tools for plant design and licensing will require additional validation against experimental data, especially for cases involving complex interactions between interdependent physical phenomena. ...
Article
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This paper introduces the use of a Rayleigh backscatter-based distributed fiber optic sensor to map the temperature field in air flow for a thermal fatigue application. The experiment involves a pair of air jets at 22 and 70 °C discharging from 136 mm hexagonal channels into a 1 × 1 × 1.7 m tank at atmospheric pressure. A 40 m-long, ϕ155 µm fiber optic sensor was wound back and forth across the tank midplane to form 16 horizontal measurement sections with a vertical spacing of 51 mm. This configuration generated a 2D temperature map with 2800 data points over a 0.76 × 1.7 m plane. Fiber optic sensor readings were combined with PIV and infrared measurements to relate flow field characteristics to the thermal signature of the tank lid. The paper includes sensor stability data and notes issues encountered using the distributed temperature sensor in a flow field. Sensors are sensitive to strain and humidity, and so accuracy relies upon strict control of both.
... Three dimensional steady state simulations are performed using k-ε turbulence model to numerically predict the temperature and velocity fields, which are found to be in good agreement with the experimental data. Hannink and Blom (2011) performed numerical investigation of turbulent mixing of hot and cold fluids in a T-junction by linking two numerical models, namely, coupled CFD-FEM model and sinusoidal model. LES was used for CFD modeling. ...
... Hannink and Blom [2] investigated numerical methods for the prediction of thermal fatigue due to turbulent mixing in a T-junction. Simulations are performed using a coupled CFD-FEM model and a sinusoidal model. ...
Conference Paper
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High Cycle Fatigue (HCF) is caused in a T-junction piping system as a result of temperature fluctuations occurring in the near wall region due to the turbulent mixing of fluids at different temperatures. The paper presents numerical computations of the fluid mixing in a horizontal T- junction piping system using the Large Eddy Simulation (LES) method. Wall Adaptive Local Eddy Viscosity (WALE) sub-grid scale model is used. The numerical results show that the mixing behavior of the flow is characterized by a wavy pattern of thermal stratification. The normalized mean and rms temperature distribution obtained from the LES results at locations 5D and 6D (D-inner diameter of main pipe) downstream of the T- junction is compared with the corresponding experimental data to assess the quality of LES predictions. The mixing behavior of fluids is strongly dependent on the inflow conditions of temperature and velocity.
... The study is carried out by using Ansys Fluent 12.1, which is widely used in investigations of thermal-hydraulic problems in nuclear reactors (e.g. Chang and Tavoularis, 2008;Hannink and Blom, 2011;Yan and Rizwan-uddin, 2011;Ganesan et al., 2013). ...
Article
With the development of nuclear power and nuclear power safety, high-cycle thermal fatigue of the pipe structures induced by the flow and heat transfer of the fluid in pipes have aroused more and more attentions. Turbulent mixing of hot and cold flows in a T-pipe is a well-recognized source of thermal fatigue in piping system, and thermal fatigue is a significant long-term degradation mechanism. It is not an easy work to evaluate thermal fatigue of a T-pipe under turbulent flow mixing because of the thermal loads acting at fluid–structure interface of the pipe are so complex and changeful. In this paper, a one-way Computational Fluid Dynamics-Finite Element Method (CFD-FEM method) coupling based on the ANSYS Workbench 15.0 software has been developed to calculate transient thermal stresses with the temperature fields of turbulent flow mixing, and thermal fatigue assessment has been carried out with this obtained fluctuating thermal stresses by programming in the software platform of Matlab based on the rainflow counting method. In the thermal analysis, the normalized mean temperatures and the normalized root mean square (RMS) temperatures are obtained and compared with the experiment of the test case from the Vattenfall benchmark facility to verify the accuracy of the CFD calculation and to determine the position which thermal fatigue is most likely to occur in the T-junction. Besides, more insights have been obtained in the coupled CFD-FEM analysis and the thermal fatigue damage assessment, and the normalized thermal fatigue damage rate D∗τ is found to be positively related to the normalized RMS temperature TRMS/ΔT to some extent. The method proposed in this paper is the inheritance and development of the research studying thermal fatigue of the T-junction under turbulent fluid mixing of hot and cold fluid by analysing the temperature fluctuations, and may provide a promising way for the quantitative assessment of high-cycle thermal fatigue of the pipe structure induced by complex flow and heat transfer in practical engineering of the nuclear power plants (NPPs).
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Temperatures were measured at the inner surface of an annulus between two coaxial tubes, where three water streams mixed. These temperatures were sampled at either 100 Hz or 1000 Hz. The acquisition time was set to 120 s. Two water streams at 549 K, with a Reynolds number between 3.56 × 10^5 and 7.11 × 10^5, descended in the annular gap and mixed with a water stream at 333 K or 423 K, with a Reynolds number ranging from 1.27 × 10^4 to 3.23 × 10^4. Water pressure was kept at 7.2 MPa. Inner-surface temperatures were collected at eight azimuthal and five axial positions, for each combination of boundary conditions. To better analyze these temperatures and mixing in the vicinity of the wall, scalars estimating the mixing intensity at each measurement position were computed from detrended temperature time series. Fourier and Hilbert–Huang marginal spectra were calculated for the time series giving rise to the highest values of a mixing estimator of choice. The relationship between temperature and velocity was explored by examining the results of an LES simulation using the same boundary conditions as in one of the experimental cases.
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Temperature fluctuations occur in the region where hot and cold fluids mix turbulently in the nuclear power plants. Temperature fluctuations cause thermal fatigue of piping systems. In the design of generation IV nuclear power plants, supercritical fluids are supposed to be used widely. This paper investigated the thermal striping phenomenon caused by the turbulent mixing in a supercritical water Tee. There are two key issues in the study of thermal striping phenomenon: One is to find the region which experiences the peak temperature fluctuation; the other is how to attenuate it. Porous media was used to attenuate the temperature fluctuations in this paper. The results show that porous media with proper parameters in a tee can reduce the temperature fluctuations magnificently.
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Incomplete mixing of fluids of different temperatures generates thermal striping, which is to date difficult to accurately predict. For metal piping the induced temperature fluctuations are harmful, because they lead to thermal fatigue. This phenomenon is significant in nuclear power plant safety considerations, in particular in connection with aging of the equipment. This work contributes to the current research effort by presenting an array of resistive temperature detectors (RTDs) based on a micro patterned thin film platinum resistor for experimental studies of the thermal impact to the wall. The sensors are located on an aluminium substrate, on which the platinum resistors and the electrical contact leads are embedded. The operational temperature range is below 0 °C up to 180 °C (in air), the temperature coefficient of resistivity (TCR) is 0.0014 1/°C with a high linear correlation coefficient. The thermal time constant has been measured and studied using a one-dimensional finite element method. This sensor was applied in a test facility where two water streams of different temperatures and the same flow speed blend downstream of the rear edge of a splitter plate in a horizontal rectangular channel. It was used to measure and characterize in terms of frequency and amplitude the thermal fluctuations generated by the contact of the mixing patterns developed in the bulk of the flow with the channel side walls.
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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.
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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.
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Turbulent mixing of hot and cold fluids may lead to high-cycle thermal fatigue in piping of nuclear power plants. In this work, the mixing in a T-junction experiment is studied by large-eddy simulation (LES). Conjugate heat transfer (CHT) between fluid and pipe wall is studied by replacing the plexiglass pipe of the experiment with a steel one. Different inlet and wall boundary conditions are first considered. Steady and turbulent inlets are compared, as well as adiabatic and CHT wall conditions. The turbulent inlets are created by using the vortex method which is validated for fully developed flow. The inlets are shown to have only small effect in bulk of the flow, but non-negligible effect near walls. The adiabatic and CHT cases show practically no difference in the logarithmic layer and upward, whereas near walls the difference becomes significant due to thermal inertia of the pipe wall. In bulk of the flow, the mean and fluctuating quantities show good agreement with the experiment. CHT simulations by using different meshes and flow velocities are then considered. A coarse mesh is found to yield qualitative agreement but significant errors in e.g. Reynolds stresses near walls. Temperature fluctuation intensity at the pipe inner surface is fairly similar for both meshes. Normalized profiles and spectra for different velocities are qualitatively similar, but some differences exist e.g. in distributions of wall temperature fluctuation intensity.
Article
Temperature fluctuations occur due to thermal mixing of hot and cold streams in the T-junctions of the piping system in nuclear power plants, which may cause thermal fatigue of piping system. In this paper, three-dimensional, unsteady numerical simulations of coolant temperature fluctuations at a mixing T-junction of equal diameter pipes were performed using the large eddy simulation (LES) turbulent model. The experiments used in this paper to benchmark the simulations were performed by Hitachi Ltd. The calculated normalized mean temperatures and fluctuating temperatures are in good agreement with the measurements. The influence of the time-step ranging from 100 Hz to 1000 Hz on the numerical simulation results was explored. The simulation results indicate that all the results with different frequencies agree well with the experimental data. Finally, the attenuation of fluctuation of fluid temperature was also investigated. It is found that, drastic fluctuation occurs within the range of less than L/D = 4.0; the fluctuation of fluid temperature does not always attenuate from the pipe center to the wall due to the continuous generation of vortexes. At the top wall, the position of L/D = 1.5 has a minimum normalized mean temperature and a peak value of root-mean square temperature, whereas at the bottom wall, the position having the same characteristics is L/D = 2.0.
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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 a T-junction. In the present work, turbulent mixing in a T-junction is investigated by a coupled approach of fluid-flow calculations and mechanical calculations. The flow characteristics and the temperatures in the pipe wall are obtained using Large-Eddy Simulations (LES). The corresponding structural stresses, causing the fatigue loading, are determined by means of finite element calculations. In general, LES is very well capable of capturing the mixing phenomena and the accompanied turbulent flow fluctuations in a T-junction. Previous work based on a direct comparison with experimental results (8, 9) showed the accuracy of LES predictions for thermal fatigue assessment in case of adiabatic flow. In this paper, also the effect of heat transfer through the pipe walls is included in the simulations. Temperatures are solved in both fluid and structure. Subsequently, the temperatures in the structure are transferred from the fluid-flow model to the finite element model. Using the finite element method, a mechanical calculation is performed to determine the structural deformations and stresses due to these thermal loads. Based on the resulting stresses, the fatigue life of the T-junction is finally predicted using fatigue curves from a design code. A strategy has been developed to assess thermal fatigue of complex mixing problems in nuclear power plants by coupling LES modelling, finite element modelling and code assessment. A description of this approach along with its application to a specified test case is presented here.
Article
A potential cause of thermal fatigue failures in energy cooling systems is identified with cyclic stresses imposed on a piping system. These are generated from temperature changes in regions where a cold flow is intensively mixed with a hot flow. Typical locations of such thermal mixing are T-junctions in nuclear reactor cooling systems. Turbulent mixing in a T-junction is investigated here using large-eddy simulation (LES). In general, LES can capture very well the mixing phenomena and the accompanying turbulent flow fluctuations that occur in a T-junction. Through a direct comparison with experimental results, an assessment of the accuracy of LES predictions is made for the Smagorinsky and the Vreman models. It is shown that the results obtained with the Vreman model are closest to the experimental results. The Smagorinsky model is found to provide the least accurate results. This is particularly detected in the near-wall regions that are of great importance in thermal fatigue predictions. Detailed numerical validation was performed with simulations using five different spatial mesh resolutions. These simulations show that computational meshes must resolve important turbulence length scales in order to obtain sufficiently accurate results. This accuracy assessment and error quantification are based on the integral and Taylor length scales of turbulence. For the investigated cases, the mesh resolution with average cell sizes of the order ofλ/3 (three times smaller than the Taylor microscale length) is sufficient to give very similar results to those obtained on much finer meshes. An engineering estimation of the minimal mesh resolution gives an initial guideline for construction of computational meshes that allow for accurate predictions of turbulent mixing in a T-junction using LES. Additionally, analysis of the temperature measurement data at specified probe locations is presented along with a quantification of an error introduced by the applied LES method.
Article
Thermal fatigue is a significant long-term degradation mechanism in nuclear power plants (NPP). In particular, as operating plants become older and life time extension activities are initiated. Operators and regulators need screening criteria to exclude risks of thermal fatigue and methods to determine significant fatigue relevance. In general, the common thermal fatigue issues are well understood and controlled by plant instrumentation at fatigue susceptible locations. However, incidents indicate that certain piping system Tee's are susceptible to turbulent temperature mixing effects that cannot be adequately monitored by common thermocouple instrumentations. Therefore, a European project on thermal fatigue evaluation of piping system Tee-connections THERFAT has been initiated. In THERFAT, a collation of existing field experience has been conducted leading to a selection of Tee-connections for further investigations. The load determination covers experimental tests and advanced thermo-hydraulic flow simulations. The integrity evaluation work package comprises stress/fatigue analyses and fracture mechanics assessments supported by targeted verification damage tests. Proposals will be made for improved load thermal fatigue assessment procedures, screening criteria to determine lower non fatigue significant threshold values and for establishing a European methodology on thermal fatigue. [GRAPHICS]
Article
Thermal striping is identified as one of the causes of thermal fatigue failure in nuclear power plants. Numerical studies of thermal striping require three-dimensional, unsteady turbulent modeling that resolves both large and small-scale turbulent motions. Benchmark studies were carried out using the LES turbulence model solved by the commercial CFD code FLUENT. Two types of mixing tee configurations were modeled to evaluate the performance of the CFD code. The simulation results presented in normalized average temperature and normalized fluctuating temperatures are in good agreement with measurements.
Article
Large Eddy Simulations are performed in a T-junction to analyze the suitability of wall-functions in accurately predicting the thermal fluctuations acting on the pipe walls due to turbulent mixing. The WALE sub-grid-scale model used in the LES solver is validated with existing experimental data. In order to reduce the computational costs, Reynolds number scaling is performed while preserving the essential flow features. While the wall-function based simulation showed good agreement with the wall-resolved approach for the bulk velocity and temperature field, the corresponding RMS components were consistently under-estimated close to the wall boundaries. The same was true for the RMS fluctuations of the wall heat-flux. As a consequence, it is suggested that wall-functions should be used with caution, especially for the considered nuclear application.
Article
The paper deals with T-junction mixing experiments carried out with wire-mesh sensors. The mixing of coolant streams of different temperature in pipe junctions leads to temperature fluctuations that may cause thermal fatigue in the pipe wall. This is practical background for an increased interest in measuring and predicting the transient flow field and the turbulent mixing pattern downstream of a T-junction. Experiments were carried out at a perpendicular connection of two pipes of 51 mm inner diameter. The straight and the side branches were supplied by water of different electrical conductivity, which replaced the temperature in the thermal mixing process. A set of three wire-mesh sensors with a grid of 16 × 16 measuring points each was used to record conductivity distributions downstream of the T-junction. Besides the measurement of profiles of the time averaged mixing scalar over extended measuring domains, the high resolution in time and space of the mesh sensors allow a statistic characterization of the stochastic fluctuations of the mixing scalar in a wide range of frequencies. Information on the scale of turbulent mixing patterns is obtained by cross-correlating the signal fluctuations recorded at different locations within the measuring plane of a sensor.
Article
A potential cause of thermal fatigue failures in energy cooling systems is identified with cyclic stresses imposed on a piping system. These are generated due to temperature changes in regions where cold and hot flows are intensively mixed together. A typical situation for such mixing appears in turbulent flow through a T-junction, which is investigated here using Large-Eddy Simulations (LES). In general, LES is very well capable in capturing the mixing phenomena and accompanied turbulent flow fluctuations in a T-junction. An assessment of the accuracy of LES predictions is made for the Vreman model through a direct comparison with available experimental results. The integral and Taylor length scales were used to provide the accuracy assessment and error quantification in perfomed simulations. It is shown that the mesh resolution with the average cell-sizes three times smaller than the Taylor microscale length is sufficient to give very similar results to these obtained on much finer meshes. This may serve as an initial engineering guideline for construction of computational meshes that allow for an accurate prediction of turbulent mixing. Additionally, detailed analysis of the temperature measurement data at specified probe locations is presented along with a quantification of an error introduced by LES modelling.
Article
This paper covers work carried out by the CEA to study the mechanisms leading to cracking of piping as a result of thermal loading in flow mixing zones. The main goal of the work is to analyse, by calculation, the thermal loading caused by turbulent mixing in tees and to understand the mechanism of initiation and propagation of cracks in such components. This work is supported by IRSN. This thermal fatigue phenomenon is still not fully understood. One of the main obstacles to its understanding resides in the multi-domain nature of the loading and associated damage, involving three complementary scientific disciplines: thermal-hydraulic field, thermo-mechanical field and materials science.
Thermal mixing in a T-junction A simplified method to predict thermal fatigue in mixing tees of nuclear reactors Hydrothermal–mechanical analysis of thermal fatigue in a mixing tee
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Development of a European procedure for assessment of high cycle thermal fatigue in light water reactors: final report of the NESC-thermal fatigue project
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Dahlberg, M.K.-F., Nilsson, et al., 2007. Development of a European procedure for assessment of high cycle thermal fatigue in light water reactors: final report of the NESC-thermal fatigue project. Technical Report EUR 22763 EN. Joint Research Centre, The Netherlands.