CFD VALIDATION OF STRATIFIED TWO-PHASE FLOWS IN A HORIZONTAL CHANNEL

Transcription

1 CFD VALIDATION OF STRATIFIED TWO-PHASE FLOWS IN A HORIZONTAL CHANNEL 1. Introducton Chrstophe Vallée and Thomas Höhne In dfferent scenaros of small break Loss of Coolant Accdent (SB-LOCA), stratfed twophase flow regmes can occur n the man coolng lnes of pressurzed water reactors. Because these flow patterns cannot be predcted wth the requred accuracy and spatal resoluton by the one-dmensonal system codes, the stratfed flows are ncreasngly modelled wth computatonal flud dynamcs (CFD) codes. In CFD, closure models are requred that must be valdated, especally f they are to be appled to reactor safety ssues. Slug flow s a challengng flow regme for CFD, because of the acceleraton of the gaseous phase and of the transton of the fast lqud slugs, whch carry a sgnfcant amount of lqud wth hgh knetc energy. Further, t s potentally hazardous to the structure of the system due to the strong oscllatng pressure levels formed behnd the lqud slugs as well as the mechancal momentum of the slugs. CFD calculatons of slug flow were performed and were compared wth optcal observatons captured at the Horzontal Ar/Water Channel (HAWAC) of the Forschungszentrum Dresden-Rossendorf (FZD). It s the am of the conducted smulatons to valdate the predcton of slug flow wth the exstng multphase flow models avalable n the commercal code ANSYS CFX [1]. Further, t s of nterest to prove the understandng of the general flud dynamc mechansm leadng to slug flow and to dentfy the crtcal parameters affectng the man slug flow parameters (lke e.g. slug frequency and propagaton velocty).. The Horzontal Ar/Water Channel (HAWAC) Experments were carred out at the Horzontal Ar/Water Channel (Fg. 1), whch s devoted to co-current flow experments []. The 8 m long test-secton has a rectangular cross-secton of 100 x 30 mm² (heght x wdth), leadng to a length-to-heght L/h = 80. ar nlet ar outlet V & G 8 m V & L nlet devce pump Fg. 1: Schematc vew of the horzontal ar/water channel (HAWAC) 33

2 A specal nlet devce (Fg. ) was desgned to provde defned boundary condtons at the channel nlet. Therefore, ar and water have to be njected separately nto the test-secton: the ar flows through the upper part and the water through the lower part of the nlet devce. In order to provde homogenous velocty profles at the test-secton nlet, 4 wre cloth flters are mounted n each part of the nlet devce. Ar and water come n contact at the edge of a 500 mm long blade that dvdes both phases downstream of the flter segment. The free nlet cross-secton for each phase can be controlled by nclnng ths blade up and down. In ths way, the perturbaton caused by the frst contact between gas and lqud can be ether mnmsed or, f requred, a perturbaton can be ntroduced (e.g. hydraulc jump). Both, flters and the nclnable blade, provde well-defned nlet boundary condtons for the CFD model and therefore offer very good valdaton possbltes. ar nlet water nlet wre cloth flters nclnable blade lft rod Fg. : The nlet devce The maxmum superfcal veloctes acheved n the test-secton are m/s for the water and 8 m/s for the ar. A flow pattern map (Fg. 3) was establshed on the bass of vsual observatons of the flow structure at dfferent combnatons of the gas and lqud superfcal veloctes. The observed flow patterns are: stratfed flow, wavy flow, elongated bubble flow and slug flow. 10 elongated bubble flow slug flow 1 JL [m/s] 0,1 stratfed flow wavy flow 0,01 0,01 0, JG [m/s] Fg. 3: Flow pattern map for the rectangular channel (nlet blade n horzontal poston) 34

3 3. CFD model of the channel The channel wth rectangular cross-secton was modelled usng ANSYS CFX [1]. The model dmensons are 4000 x 100 x 30 mm³ (length x heght x wdth), whch corresponds to the frst half of the test-secton. The grd conssts of 6 x 10 5 hexahedral elements. A slug flow experment at a superfcal water velocty of 1.0 m/s and a superfcal ar velocty of 5.0 m/s was chosen for the CFD calculatons. In the experment, the nlet blade was n horzontal poston. Accordngly, the model nlet was dvded nto two parts: n the lower 50% of the nlet cross-secton, water was njected and n the upper 50% ar. An ntal water level of y 0 = 50 mm was assumed for the entre model length. In the smulaton, both phases have been treated as sothermal and ncompressble, at 5 C and at a reference pressure of 1 bar. A hydrostatc pressure was assumed for the lqud phase. Buoyancy effects between the two phases are taken nto account by the drected gravty term. At the nlet, the turbulence propertes were set usng the Medum ntensty and Eddy vscosty rato opton of the flow solver. Ths s equvalent to a turbulence ntensty of 5% n both phases. The nner surface of the channel walls has been defned as hydraulcally smooth wth a non-slp boundary condton appled to both gaseous and lqud phases. The channel outlet was modelled wth a pressure controlled outlet boundary condton. As t was the goal of the CFD calculaton to nduce surface nstabltes, whch are later generatng waves and slugs, the nterfacal momentum exchange and also the turbulence parameters had to be modelled correctly. Wthout any specal treatment of the free surface, the hgh velocty gradents at the free surface, especally n the gaseous phase, generate too hgh turbulence throughout the two-phase flow when usng the dfferental eddy vscosty models lke the k-ε or the k-ω model [1]. Therefore, certan dampng of turbulence s necessary n the nterfacal area because the mesh s too coarse to resolve the velocty gradent n the gas at the nterface. On the gas sde of the smooth free surface, ths dampng should be smlar to that used near a sold wall. Moreover, on the lqud sde the advanced model should take the ansotropy between the normal and the tangental Reynolds stresses nto account. Yegorov [] proposed a smple grd dependent symmetrc dampng procedure. Ths procedure provdes for the sold wall-lke dampng of turbulence n both gas and lqud phases. It s based on the standard ω -equaton, formulated by Wlcox [1] as follows: t ρ ω & t [ ω t ω] (1) k ( ρ ω) + ( ρ U ω) = α τ S β ρ ω + ( μ + σ μ ) where α = 0.5 and β = are the k-ω model closure coeffcents of the generaton and the destructon terms n the ω-equaton, σ ω = 0.5 s the nverse of the turbulent Prandtl number for ω, τ t s the Reynolds stress tensor, and S & s the stran-rate tensor. In order to mmc the turbulence dampng near the free surface, Yegorov [] ntroduced the followng source term n the rght hand sde of the gas and lqud phase ω-equatons (1): 6 μ A Δy β ρ B () β ρ Δn 35

4 Here A s the nterface area densty, Δn s the typcal grd cell sze across the nterface, ρ and μ are the densty and vscosty of the phase. The factor A actvates ths source term only at the free surface, where t cancels the standard ω-destructon term of the ω-equaton r β ρ ω and enforces the requred hgh value of ω and thus the turbulence dampng. ( ) The parallel transent calculaton of 5.0 s of smulaton tme on 4 processors lasts 4 days. A hgh-resoluton dscretzaton scheme was used. For tme ntegraton, the fully mplct second order backward Euler method was appled wth a constant tme step of dt = s and a maxmum of 15 coeffcent loops. A convergence n terms of the RMS values of the resduals to be less then 10-4 could be assured most of the tme. 4. Results Optcal measurements were performed wth a hgh-speed vdeo camera. In the followng pcture sequences (Fg. 5 and 6), a comparson s presented between CFD calculaton and experment: the calculated phase dstrbuton s vsualzed and comparable camera frames are shown. Fg. 5: Calculated sequence of vod fracton at J L = 1.0 m/s and J G = 5.0 m/s (depcted part of the channel: 1.4 to 4 m after the nlet) Fg. 6: Measured pcture sequence at J L = 1.0 m/s and J G = 5.0 m/s wth t = 50 ms (depcted part of the channel: 0 to 3. m after the nlet) 36

5 In both cases, a slug s generated. The sequences show that the qualtatve behavour of the creaton and propagaton of the slug s smlar n the experment and n the calculaton. In the CFD calculaton, the slug develops at approxmately t = 1.30 s after the begnnng of the smulaton, nduced by nstabltes. The sngle effects leadng to slug flow that can be smulated are shown n detals n Fg. 7. These phenomena are: Instabltes and small waves are randomly generated by the nterfacal momentum transfer (Fg. 7-a). As a result bgger waves are generated. The waves can have dfferent veloctes and can merge (Fg. 7-b and c). Bgger waves roll over (Fg. 7-c) and can close the channel cross-secton (Fg. 7-d). a) t = 1.7 s b) t = 1.37 s c) t = 1.47 s d) t = 1.51 s Fg. 7: Detals of the slug generaton calculated wth ANSYS CFX at dfferent smulaton tme The needed entrance length for slug generaton was defned as the length between the nlet and the locaton nearest the nlet where a wave closes nearly the entre cross-secton. Ths was observed at about 1.5 m n the experment and.5 m n the calculaton. In contrast to the measurement, the stratfed flow after the slug calculated wth ANSYS CFX s too smooth whch defers the generaton of the next slug. Snce the slug cleared an mportant amount of water from the channel, the next slug appears after the channel s slowly flled up agan by the transport of lqud from the nlet. Ths process takes approxmately 1.5 s. In the experment, small waves are generated mmedately after the slug and create the next one wthn 0.3 to maxmum 0.7 s. Sources of nstabltes are not only the hgh ar velocty but also the pressure surge created by the slugs, partcularly when they leave the channel. Ths effect has not been proper smulated snce just the half of the channel was modelled. Therefore, the slug frequency cannot be compared at ths stage of the smulaton. 37

6 5. Summary and conclusons For the nvestgaton of co-current two-phase flows, the horzontal ar/water channel (HAWAC) was bult at Forschungszentrum Dresden-Rossendorf (FZD). A specal nlet devce provdes well defned as well as varable boundary condtons, whch allow very good CFD-code valdaton possbltes. Optcal measurements were performed wth a hgh-speed vdeo camera. The water level hstory can be extracted from the mage sequences by an nterface capture method. A pcture sequence recorded durng slug flow was compared wth the equvalent CFD smulaton made wth the code ANSYS CFX. The two-flud model was appled wth a specal free surface treatment. Due to the nterfacal momentum transfer, t was possble to generate slugs based on nstabltes. The behavour of slug generaton and propagaton at the expermental setup was qualtatvely reproduced, whle devatons n the slug frequency requre further work. The creaton of small nstabltes due to pressure surge or ncrease of nterfacal momentum transfer should be analysed n the future. Furthermore, pressure and velocty measurements should be performed n the HAWAC channel to allow quanttatve comparsons. Due to the success and promsng future research actvtes, the HAWAC was chosen as an OECD Benchmark test faclty and as a reference test faclty for the German CFD network program. References [1] ANSYS Inc., 006, ANSYS CFX-10.0 User Manual [] Yegorov, Y Contact condensaton n stratfed steam-water flow, EVOL- ECORA D 07. Acknowledgements Ths work s carred out n the frame of a current research project funded by the German Federal Mnstry of Economcs and Labour, project number