Böttcher, M.

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Fügen Sie auf der Masterfolie ein frei wählbares Bild ein (z.B. passend zum Vortrag) KIT – Universität des Landes Baden-Württemberg und nationales Forschungszentrum in der Helmholtz-Gemeinschaft Institut für Neutronenphysik und Reaktortechnik, INR www.kit.edu           CFD studies of heat tansfer at structured surfaces     Michael Böttcher Institut für Neutronenphysik und Reaktortechnik Abteilungs-, Fakultäts-, Institutsbezeichnung 2 Content Facts about the L-Star test facility CFD models RANS simulations (entire test section) LES (parts of test section) Results Smooth heater rod Heater rod with solid rings Heater rod with perforated rings Summary and outlook CFD Verbundtreffen 4.-5.3.2015 Otterfing M.Böttcher, KIT-INR Abteilungs-, Fakultäts-, Institutsbezeichnung 3 L-Star test facility CFD Verbundtreffen 4.-5.3.2015 Otterfing M.Böttcher, KIT-INR Luft-Stab-Abstandshalter-Rauhigkeiten Experiments related to GEN IV Gas Cooled nuclear reactors Coolant: air   Intententions: Improvement of heat transfer by surface roughness Qualification and improvement of turbulence models Parameter Value Test section length              3.7 m Design electrical power              24 kW (750°C) Gas temperature  range             RT to 250 °C Maximum operating pressure             3 bar (abs) Maximum mass flow rate             0.33 kg/s Maximum operating Re             2∙105 Abteilungs-, Fakultäts-, Institutsbezeichnung 4 L-Star test section composition CFD Verbundtreffen 4.-5.3.2015 Otterfing Measurement equipment: 48 thermocouples   4 pressure sensors LDA system at optical window or Rings as surface roughness: Optical  windows M.Böttcher, KIT-INR Flow straightener for surface rough- ness experiments Abteilungs-, Fakultäts-, Institutsbezeichnung 5 Experiments - overview CFD Verbundtreffen 4.-5.3.2015 Otterfing Flow straightener installed:  Heater rod with solid rings: P0 = 1.5 bar / 6.32-50 g/s /  Re=6000 … 35000 / 0.58 – 1.5 KW Fluctuations by LDA for heated and unheated standard case  Heater rod with perforated rings: P0 = 1.5 bar  /6.32-50 g/s / unheated experiments Fluctuations by LDA  (in progress)  Without flow straightener: Smooth heater rod: P0 = 1.5 bar / 6.32 – 50.5 g/s / Re=6000 … 35000 / 0.339 – 2.031 KW  Pressure and temperature measurements only Standard case: P0 = 1.5 bar 26.15 g/s / Re=16500 / 1KW  M.Böttcher, KIT-INR Abteilungs-, Fakultäts-, Institutsbezeichnung 6 Flow configuration at solid rod surface structures CFD Verbundtreffen 4.-5.3.2015 Otterfing M.Böttcher, KIT-INR Flow direction Recircula-tion zone Boundary layer Develop. zone Front recirc.zone Reattach.  point RANS steady state solution LES instantaneous dist. Flow Phenomena: Separation and reattaching Secondary flow in corners Local Jets (perforated rings) stagnation regions Conjugate heat transfer+radiation  difficult to predict by RANS methods  and  Re analogy!! Abteilungs-, Fakultäts-, Institutsbezeichnung 7 L-Star : the solid ring case  CFD Verbundtreffen 4.-5.3.2015 Otterfing Testsection Flow straightener 33.6 mio hex 38 mio total  Y+<1 (heater rod) Y+<80 (flow straight)  BSL EARSM (SST) Turb. Model  Monte Carlo  Radiation Model  Heat conduction in channel wall, heater rod and flow straight. components M.Böttcher, KIT-INR Abteilungs-, Fakultäts-, Institutsbezeichnung 8 Axial rod surface temperature profile     𝜀Hex       0.25    𝜀rod       0.63    Roughness type: Solid rings M.Böttcher, KIT-INR CFX SST Prt=0.85 CFX EARSM Prt=0.65 Mass flow   26.1 g/s     Heating   1018.9 W   Pressure   1.51 bar            Tin    16.6° C CFD Verbundtreffen 4.-5.3.2015 Otterfing Abteilungs-, Fakultäts-, Institutsbezeichnung 9 Comparison of smooth and structured rod CFD Verbundtreffen 4.-5.3.2015 Otterfing M.Böttcher, KIT-INR The advantage of a better heat transfer is paid by increased pressure loss:  Nu: +200% ∆p: +300% Rough case: Parameters obtained for the base case at Re~16000 are used for the calculation of other experiments (Prt=0.65)  Smooth case: SST (instead of EARSM) Prt=0.85  Solid rings Solid rings (DNS) Abteilungs-, Fakultäts-, Institutsbezeichnung 10 LES Models for solid and perforated rings CFD Verbundtreffen 4.-5.3.2015 Otterfing M.Böttcher, KIT-INR solid rings Perforated rings Mesh size [mio] 1.44 (5 rings) 8.92 (3 rings) Mesh type structured structured Aspect ratio  <50 <80 Angle min.  42° 20° Y+ <1 <1 ∆t 5*10-5s 2.5*10-5s Cou av./max 0.9 / 10 0.7 /10 Comp. time [d] 4 (14 CPU´s) 19 (30CPU´s) Simulated time 0.2s /4000 steps 0.25s 10000 steps LES Smagorinsky   periodic boundary conditions Isothermal flow Abteilungs-, Fakultäts-, Institutsbezeichnung 11 Spectral analysis of LES model data  CFD Verbundtreffen 4.-5.3.2015 Otterfing ~ f-1 ~f-5/3 Production spectrum Kolmogoroff- spectrum ~f-7 Heisenberg- spectrum, dissipation region Energy carrying vortices The spectral analysis indicates a complete coverage of the energy carrying region. Furthermore, the noise indicates a rather short physical integration time. Re≈16500 Axial velocity M.Böttcher, KIT-INR Full turbulent flow  Abteilungs-, Fakultäts-, Institutsbezeichnung 12 Unheated case – solid rings CFD Verbundtreffen 4.-5.3.2015 Otterfing Yw 26.15 g/s 1.5 bar 20°C M.Böttcher, KIT-INR main stream 0.15 s av. time Perhaps not enough !  Abteilungs-, Fakultäts-, Institutsbezeichnung 13 Unheated case – perforated rings CFD Verbundtreffen 4.-5.3.2015 Otterfing Yw 26.15 g/s 1.5 bar 20°C M.Böttcher, KIT-INR main stream Z=1.5mm Z=5mm Z=24mm Abteilungs-, Fakultäts-, Institutsbezeichnung 14 Perforated rings – LES simulation CFD Verbundtreffen 4.-5.3.2015 Otterfing M.Böttcher, KIT-INR Abteilungs-, Fakultäts-, Institutsbezeichnung 15 Summary and Conclusions and Outlook CFD Verbundtreffen 4.-5.3.2015 Otterfing For the L-STAR experiments CFD RANS models for the entire test section as well as LES models for unheated experiments and smaller parts of the test section were developed.  By surface structures the heat transfer can be improved significantly, but this is paid by increased pressure loss .  The comparison with experimental heat transfer data for structured surfaces was successfull, but only by modification of the turbulent Prandtl number, which obviously is Re dependent.  A comparison of turbulent quantities for isothermal flow obtained by LDA measurements demonstrated the inaccuracies of presently available RANS models such as SST, RS or EARSM. LES fits better but is more expensive (up to 500 times ) and grid dependent. Furthermore, boundary conditions are a problem !   The consequence for RANS turbulence models should be the modification of model coefficients with dependency of Re, Pr and the local pressure gradient, which should be the topics of future investigations .  M.Böttcher, KIT-INR The experimental work was supported within the framework of the European THINS project. Abteilungs-, Fakultäts-, Institutsbezeichnung 16 CFD Verbundtreffen 4.-5.3.2015 Otterfing Acknowledgements to Rodrigo Gomez, who provided the experimental data basis, and to all members of the L-Star team ! Are there any questions ? M.Böttcher, KIT-INR Abteilungs-, Fakultäts-, Institutsbezeichnung 17  CFD Verbundtreffen 4.-5.3.2015 Otterfing Abteilungs-, Fakultäts-, Institutsbezeichnung 18 Solid rings or perforated rings ? CFD Verbundtreffen 4.-5.3.2015 Otterfing Idea:  smaller separation region improvement of heat transfer  reduced pressure loss ? First results: See next slights ! 25.3 g/s 1.0 bar T0 = 20°C isothermal M.Böttcher, KIT-INR Abteilungs-, Fakultäts-, Institutsbezeichnung 19 Reynolds analogy concept CFD Verbundtreffen 4.-5.3.2015 Otterfing 𝑣𝑘´ 𝑣𝑗´ =  −ѵ𝑡𝜕𝑣𝑗 𝜕𝑥𝑘  𝑇´ 𝑣𝑘´ = −𝑎𝑡𝜕𝑇 𝜕𝑥𝑘 Reynolds stress tensor   Eddy viscosity approach By time averaged momentum equation: Ѵt solved by turbulence models (k-ε, RS, etc) By time averaged energy equation: 𝑷𝒓𝒕 =ѵ𝒕𝒂𝒕= 𝝆𝒄𝒑ѵ𝒕𝝀𝒕 Reynolds heat flux at solved by Re analogy  turbulent Prandtl number  = 0.9 ?? Pr>>1 Oils Pr=1 Gases Pr<<1 Liquid metals Not valid for stagnation and separation regions, liquid metal flow ! Momentum and thermal  boundary layers: M.Böttcher, KIT-INR INR Seminar 8.12.2014 CFD V rbundtreffen 4.-5.3.2015 Otterfing Abteilungs-, Fakultäts-, Institutsbezeichnung 20 Reynolds analogy concept CFD Verbundtreffen 4.-5.3.2015 Otterfing 𝑣𝑘´ 𝑣𝑗´ =  −ѵ𝑡𝜕𝑣𝑗 𝜕𝑥𝑘  𝑇´ 𝑣𝑘´ = −𝑎𝑡𝜕𝑇 𝜕𝑥𝑘 Reynolds stress tensor   Eddy viscosity approach By time averaged momentum equation: Ѵt solved by turbulence models (k-ε, RS, etc) By time averaged energy equation: 𝑷𝒓𝒕 =ѵ𝒕𝒂𝒕= 𝝆𝒄𝒑ѵ𝒕𝝀𝒕 Reynolds heat flux at solved by Re analogy  turbulent Prandtl number  = 0.9 ?? Pr>>1 Oils Pr=1 Gases Pr<<1 Liquid metals Not valid for stagnation and separation regions, liquid metal flow ! Momentum and thermal  boundary layers: M.Böttcher, KIT-INR INR Seminar 8.12.2014 CFD V rbundtreffen 4.-5.3.2015 Otterfing Abteilungs-, Fakultäts-, Institutsbezeichnung 21 Why to use radiation radiation ? CFD Verbundtreffen 4.-5.3.2015 Otterfing 40,060,080,0100,0120,0140,0160,0180,00 500 1000 1500 2000 2500T_Surface - T_Inlet (K) Position (mm) ExperimentCFX 14.0 Monte CarloKIT CFX14.0 NoRadExperiment: Smooth rod 25.3 g/s 1.5 bar 20°C  M.Böttcher, KIT-INR Abteilungs-, Fakultäts-, Institutsbezeichnung 22 Pressure losses in test section CFD Verbundtreffen 4.-5.3.2015 Otterfing 26.15 g/s 1016 W p2 p1 M.Böttcher, KIT-INR Abteilungs-, Fakultäts-, Institutsbezeichnung 23 Pressure loss for the entire section CFD Verbundtreffen 4.-5.3.2015 Otterfing 26.15 g/s 1016 W p4 p3 M.Böttcher, KIT-INR Abteilungs-, Fakultäts-, Institutsbezeichnung 24 Turbulenzmodelle Explizites algebraisches Reynolds Stress Modell  numerisch Aufwand zwischen 2-Gleichungsmodellen und RS-Modell  keine zusätzlichen Transportgleichungen für RS Komponenten  𝑢𝑖𝑢𝑗 = 𝐹[(𝜕𝑢𝑖𝜕𝑥𝑗+𝜕𝑢𝑗𝜕𝑥𝑖), (𝜕𝑢𝑖𝜕𝑥𝑗−𝜕𝑢𝑗𝜕𝑥𝑖)]   als Kombination mit k-ε oder k-ω  anisotrope Strömungen  Korrektur für stark gekrümmte Stromlinien  höhere numerische Stabilität als RS Modell  SST Shear Stress Transport Modell  2-Gleichungsmodell, Kombination von k-ε und k-ω  Blendingfunktionen: k-ω in Wandnähe und k-ε im Aussenbereich  hat k-ε als Standardmodell abgelöst  CFD Verbundtreffen 4.-5.3.2015 Otterfing M.Böttcher, KIT-INR Abteilungs-, Fakultäts-, Institutsbezeichnung 25 Re analogy concept in CFD Turbulent energy equation:    In analogy with the turbulent viscosity a turbulent heat flux appears and thus a turbulent eddy heat diffusivity eH =lt /(r cp) can be defined, the ratio is called the turbulent Prandt number Prt:        Consequences: Prt is a tensor depending on a couple of variables and difficult to measure directly, especially in liquid metals. Re analogy can be  applied only if Prt=o(1) !! yTyTvcyTyyTvxTuc tpp )('' llrlr??.,''''constyuyTTvvuRy Gr,Pr,RefPrHMt eeCFD Verbundtreffen 4.-5.3.2015 Otterfing M.Böttcher, KIT-INR Abteilungs-, Fakultäts-, Institutsbezeichnung 26 Gittergenerierung und Modellierung CFD Verbundtreffen 4.-5.3.2015 Otterfing Geometrie und Physik CAD Modell Lokale Auflösung, y+ ? Unstrukturiert, strukturiert oder hybrid ? RANS, LES oder DNS Memory PC und Rechner  Voraussichtliche Rechenzeit Best Practice Guidelines  Arbeitszeit (Gitterqualität) Auswahl physikalischer Modelle und Parameter Verwendbares CAD Modell Größe, Gaps, Mehrfachbe- legung  Postprocessing Publikation der Ergebnisse INR Seminar 8.12.2014 Optimierter Ablauf: Konvergenzkriterien abschwächen (falls möglich) Kriterien Nicht erfüllt Simulationsläufe Konvergenz, physi-kalische Plausibilität M.Böttcher, KIT-INR Abteilungs-, Fakultäts-, Institutsbezeichnung 27 The k-ε turbulence model CFD Verbundtreffen 4.-5.3.2015 Otterfing M.Böttcher, KIT-INR Proposal for modification !! Source: ANSYS 15.0 CFX solver manual Cμ=0.09 Abteilungs-, Fakultäts-, Institutsbezeichnung 28 Pressure losses in test section CFD Verbundtreffen 4.-5.3.2015 Otterfing 26.15 g/s 1016 W p2 p1 M.Böttcher, KIT-INR Abteilungs-, Fakultäts-, Institutsbezeichnung 29 Pressure loss for the entire section CFD Verbundtreffen 4.-5.3.2015 Otterfing 26.15 g/s 1016 W p4 p3 M.Böttcher, KIT-INR Abteilungs-, Fakultäts-, Institutsbezeichnung 30 Mass flow 13.41 g/s Heat flux 510.4 W Pin 1.51 bar Tin 11.6 °C Mass flow 38.41 g/s Heat flux 1524.3 W Pin 1.50 bar Tin 20 °C  Simulations capture the form of the rod temperature distribution quite accurately.   However, temperature deviations are increasing up to 5° . Axial rod surface temperature profile - Other cases - Case 4 solid rings CFX EARSM Case 4 solid rings CFX EARSM M.Böttcher, KIT-INR INR Seminar 8.12.2014 CFD Verbundtreffen 4.-5.3.2015 Otterfing Abteilungs-, Fakultäts-, Institutsbezeichnung 31 Structured grid for perforated rings CFD Verbundtreffen 4.-5.3.2015 Otterfing 

CFD studies of heat transfer at structured surfaces

https://publikationen.bibliothek.kit.edu/230100284/3816783

CFD studies of heat transfer at structured surfaces

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