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Experimental investigation of mixing of non-isothermal water streams at BWR operating conditions
KTH, School of Engineering Sciences (SCI), Physics, Reactor Technology.ORCID iD: 0000-0001-8743-7157
KTH, School of Engineering Sciences (SCI), Physics, Reactor Technology. Warsaw University of Technology, Poland.ORCID iD: 0000-0001-5595-1952
2017 (English)In: Nuclear Engineering and Design, ISSN 0029-5493, E-ISSN 1872-759X, Vol. 317, p. 158-176Article in journal (Refereed) Published
Abstract [en]

In this experimental investigation, wall surface temperatures have been measured during mixing of three water streams in the annular gap between two coaxial stainless-steel tubes. The inner tube, with an outer diameter of 35 mm and a thickness of 5 mm, holds six K-type, ungrounded thermocouples with a diameter of 0.5 mm, which measured surface temperatures with a sampling rate of either 100 Hz or 1000 Hz. The tube was rotated from 0 to 360° and moved in a range of 387 mm in the axial direction to allow measurements of surface temperatures in the whole mixing region. The outer tube has an inner diameter of 80 mm and a thickness of 10 mm to withstand a water pressure of 9 MPa. A water stream at a temperature of either 333 K or 423 K and a Reynolds number between 1657 and 8410 rose vertically in the annular gap and mixed with two water streams at a temperature of 549 K and a Reynolds number between 3.56E5 and 7.11E5. These two water streams entered the annulus radially on the same axial level, 180° apart. Water pressure was kept at 7.2 MPa. Temperature recordings were performed at five axial and eight azimuthal locations, for each set of boundary conditions. Each recording lasted 120 s to provide reliable data on the variance, intermittency and frequency of the surface temperature time series at hand. Thorough calculations indicate that the uncertainty in the measured temperature is of 1.58 K. The mixing region extends up to 0.2 m downward of the hot inlets. In most cases, measurements indicate non-uniform mixing in the azimuthal direction, because of asymmetries in either geometry or mass flow rates at the hot inlets. Due to the measurement accuracy and a relatively simple geometry, an experimental database has been obtained for validation of computational methods to predict thermal mixing and fatigue. Furthermore, these data can provide new insight into turbulent mixing at BWR operating conditions and, more generally, into mixing coupled to the dynamics, also termed level-2 mixing.

Place, publisher, year, edition, pages
Elsevier, 2017. Vol. 317, p. 158-176
Keywords [en]
Wall surface temperature, Temperature measurement, Mixing, Uncertainty analysis, BWR control rods
National Category
Energy Engineering
Identifiers
URN: urn:nbn:se:kth:diva-205197DOI: 10.1016/j.nucengdes.2017.03.034ISI: 000401117400015Scopus ID: 2-s2.0-85017113636OAI: oai:DiVA.org:kth-205197DiVA, id: diva2:1087600
Projects
THEMFE
Funder
Swedish Radiation Safety Authority, 47385
Note

QC 20170419

Available from: 2017-04-08 Created: 2017-04-08 Last updated: 2018-10-20Bibliographically approved
In thesis
1. Experimental and analytical study of thermal mixing at reactor conditions
Open this publication in new window or tab >>Experimental and analytical study of thermal mixing at reactor conditions
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

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.

Abstract [sv]

Termisk högcykelutmattning på grund av turbulent blandning av flöden av olika temperaturer är en tvärvetenskaplig fråga som har bäring på säkerhet, underhåll och livstidsförlängning av kärnkraftverk i drift tillsammans med utvecklingen av nya reaktorer. Det är utmanande att modellera delskador och termiska belastningar som härrör från denna blandning.

För att samla stora datamängder för validering av koder modellerande turbulent termisk blandning vid reaktordriftförhållanden, mättes temperaturer vid den inre ytan av den vertikala annulära (ringformade) kanalen mellan två koncentriska rör i 316LN rostfritt stål. Denna annulära kanal representerar den verkliga geometrin mellan styrstavsledrör och styrstavsförlängare i svenska kokvattenreaktorer (BWR) Oskarshamn 3 och Forsmark 3. Vid dessa reaktorer uppvisade ett antal styrstavsförlängare sprickbildning till följd av termisk utmattning under år 2008. I den annulära kanalen strömmade kallare vattenflöden vid 333 K, vid axiell nivå z = 0.15 m. Flödena rörde sig uppåt och blandades med varmare vattenflöden, vilka strömmade i den annulära kanalen vid 549 K, vid z = 0.80 m. Trycket sattes till 7.2 MPa. 333 K, 549 K och 7.2 MPa överensstämmer med BWR-förhållanden. Termoelement fastlödda vid det inre röret kunde endast mäta innerytans temperatur på sex platser. För att kunna mäta temperaturen i hela blandningsområdet kunde röret roteras från 0° till 360° och förflyttas vertikalt över en sträcka av 387 mm.

Blandningsinhomogenitet uppskattades från sådana mätningar. I de undersökta fallen visar innerytans temperatur från områden där blandningen är som minst homogen dominerande frekvenser som är lägre än tio gånger inversen till experimenttiden.

Osäkerheten i dessa temperaturmätningar visar sig vara 1.58 K.

En storvirvelsimulering (LES) av blandning i den annulära kanalen utfördes. Experimentella randvillkor applicerades. Den konjugerade värmeöverföringen mellan vatten och rör modellerades. De små virvlarna (oupplösta skalor) approximerades med hjälp av en turbulensmodell kallad WALE, som ger rätt asymptotiskt beteende för turbulent viskositet nära väggen. En finitelementanalys (FEA) av det inre röret utfördes med LES-tryck och temperatur som belastningar. Totala delskador uppskattades från FEA-spänning/tid kurvor. För detta ändamål applicerades regndroppsmetoden. De totala delskadorna når sina högsta värden mellan z = 0.65 m och z = 0.67 m. Sprickinitiering förväntas inträffa efter 97 timmar. Innerytans CFD-temperatur är i en rimlig överensstämmelse med experimentella data med avseende på medelvärde, omfång, blandningsinhomogenitet och kritiska svängande modalfunktioner som utgör temperaturtidsserier i områden som är utsatta för utmattning. Innerytans CFD-temperatur från områden med högsta totala delskador visar dominerande frekvenser som är lägre än tio gånger inversen till simuleringstiden.

En robust, effektiv, iterativ algoritm för att beräkna det transienta temperaturfältet i det inre röret utifrån överflödiga randdata implementerades och verifierades. Temperaturberoende egenskaper inkluderades. Initiala förhållanden och överflödiga randdata i det inversa problemet stördes av gaussiskt brus för att undersöka lösningsmetodens robusthet mot brus.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2018. p. 113
Series
TRITA-SCI-FOU ; 2018:45
Keywords
High-cycle thermal fatigue, Hilbert-Huang transform, large eddy, WALE, rainflow, inverse heat conduction, adjoint conjugate gradient
National Category
Energy Engineering
Research subject
Energy Technology; Physics
Identifiers
urn:nbn:se:kth:diva-236530 (URN)978-91-7873-001-8 (ISBN)
Public defence
2018-12-06, FB53, AlbaNova University Center, Roslagstullsbacken 21, Stockholm, 13:00 (English)
Opponent
Supervisors
Funder
Swedish Radiation Safety Authority, 47385
Note

QC 20181022

Available from: 2018-10-22 Created: 2018-10-20 Last updated: 2018-11-13Bibliographically approved

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