http://www.dva-portal.org Postprnt Ths s the accepted verson of a paper presented at 13th Internatonal Conference on Indoor Ar Qualty and Clmate, Indoor Ar 214, 7-12 July 214, Hong Kong. Ctaton for the orgnal publshed paper: Cehln, M., Karmpanah, T., Larsson, U. (214) Unsteady CFD smulatons for predcton of arflow close to a supply devce for dsplacement ventlaton. In: Indoor Ar 214-13th Internatonal Conference on Indoor Ar Qualty and Clmate (pp. 47-54). N.B. When ctng ths work, cte the orgnal publshed paper. Permanent lnk to ths verson: http://urn.kb.se/resolve?urn=urn:nbn:se:hg:dva-19218
Topc B6: Predcton & measurement UNSTEADY CFD SIMULATIONS FOR PREDICTION OF AIRFLOW CLOSE TO A SUPPLY DEVICE FOR DISPLACEMENT VENTILATION Mathas CEHLIN, Tagh KARIMIPANH *, and Ulf LARSSON Faculty of Engneerng and Sustanable Development, Department of Buldng, Energy and Envronmental Engneerng, Unversty of Gävle, Sweden * Correspondng emal: tkh@hg.se Keywords: Unsteady CFD, URANS, Dsplacement ventlaton SUMMARY Modern dffusers appled n the feld of ventlaton of rooms are often complex n terms of geometry, ncludng perforated plates, dampers, gude rals, curved surfaces and other components nsde the dffuser, wth the ntenton to create satsfyng thermal comfort for the occupants. Also connectng ducts can be dfferent for the same dffuser n dfferent stuatons, affectng the supply velocty profle. It s obvous that smulaton of arflow and ar temperature partcularly n rooms wth dsplacement ventlaton s very troublesome, partcularly f the near-zone of the dffuser s of nterest. Experments commonly ndcate very hgh turbulence ntenstes n the near-zone of dsplacement ventlaton supply devces, especally close to the floor where hgh mean flow gradent occurs. Ths ndcates that the ar flow from nlet devces desgned for dsplacement ventlaton mght be very unstable; the poston of the stream leavng the dffuser and enterng the room s changng wth tme, hence dffuson of momentum and temperature are ncreased. Ths effect s not captured n RANS smulatons, snce t s performed wth the assumpton of tme-ndependent condtons. In ths paper URANS smulatons were performed for predcton of velocty and temperature dstrbuton close to a complex ar supply devce n a room wth dsplacement ventlaton. The presented study show that unsteady smulatons wth the realzable turbulence k-ε model generates too hgh eddy vscosty and therefore damps out the unsteadness of the flow especally nsde the dffuser. INTRODUCTION Because of transent behavor of almost all exsted flows n nature, the real lfe flow modelng and smulatons are not easy task. By ncreasng the computatonal capactes transent flow modelng has been used n many applcatons e.g. water dstrbuton technques (Wood et al 25), pulsng flow ventlaton, dsplacement ventlaton, resdence tme of a pollutant n a room and some turbo-machnery flows etc. The flow ssued from a dsplacement supply devce s a complex flow and one cannot be sure how developed t s. In addton, the perforated plate at the ext,.e. forced changng n geometry, causes more unsteadness, see fgure 1.
In dsplacement ventlaton systems, the ar s suppled at floor level n the zone of occupaton. The ar s suppled wth a lower temperature than the ambent ar and the ventlaton ar s therefore both a source of momentum and buoyancy. Close to the dffuser, ar s acceleratng n the vertcal drecton and the ar undergoes a free fall. Rooms wth dsplacement ventlaton are often assocated wth complants about drafts and dssatsfacton due to relatvely hgh veloctes and low temperature at the ankle level (Wyon and Sandberg 1989). In addton, the turbulence ntensty and the domnant frequency of the flow fluctuaton affect the draft (Fanger et al 1988, Fanger and Pedersen 1977). Envronmental parameters are not only nfluenced by the type of ventlaton system but also dffuser characterstcs. It s obvous that the lmtng factor for dsplacement ventlaton, n partcular schools and offces, seems to be thermal comfort. Fgure 1. Ar flow pattern wth perforated panel dffusers, see Bulter (22) Predcton of room arflows s a challengng task because of the complex nature of turbulence. Smulaton of arflow n rooms wth dsplacement ventlaton are partcular troublesome, because ar s suppled under-tempered at low veloctes. In ventlaton strateges, the commonly used approach s tme averagng and steady state calculatons. In case of dsplacement ventlaton supply devces these approxmatons may cause some uncertantes close to the supply outlets. Ths may be suspected case n near-zone regons,.e. close to the supply, where we know that the flow stuaton so called gravty current occurs. Ths transent behavor causes also draught problem n the regon and the occupant may not be able to seat there and that s to say practcally one meet a useless space n workplace. Therefore we may solve ths stuaton by a tme dependent (unsteady) arflow soluton technques. Solvng the Naver-Stokes equatons wth no approxmatons, Drect Numercal smulaton (DNS), s extremely memory ntensve. It s more or less nfeasble to performed DNS for room arflows wth the capacty and speed of present computers. Therefore turbulence s often modeled, and these models can be dvded nto two major groups: Large Eddy Smulaton (LES) and Unsteady Transent Reynolds Average Naver Stokes (URANS) smulaton. In LES large vortces are resolved by the computatonal grd and the smaller turbulent scales are completely modelled. LES has rarely been appled to modelng ndoor ar flows because of restrctons n computer power due to the requrement of a very fne (Peng and Davdson 2, Chen et al 27). Here because of tme cost and relevance to ventlaton practce we used URANS method. The purpose of the paper was to study the use of URANS smulatons to acheve satsfyng predcton of velocty and temperature dstrbuton close to a complex ar supply devce n a room wth dsplacement ventlaton. The predctons were compared wth expermental data.
METHODOLOGIES Physcal model and expermental setup The room under consderaton had the followng dmensons, length by wdth by heght, 3.4m 4.1m 2.7m, see Fgure 2. The ar was suppled through a flat low-velocty dffuser wth dmensons, wdth by heght,.49m.4 m, located.9 m above the floor level at one of the walls. The dffuser had a perforated plate wth 49 24 round openngs wth dameters of 6.8 mm. The frst row of openngs was located.1 m above floor level. The total free area of the dffuser plate was 427.9 cm 2. y outlet z travers e 2.7 m nlet screen 4.1 m x IR camera 3.4 m Fgure 2. Layout of the physcal model. Experments were performed wth hot-wre anemometer, thermocouples and nfrared thermography, see Cehln and Moshfegh (21) for detaled nformaton. A CFD model was bult n a way geometrcally smlar to the room and the supply dffuser, see Fgure 3. Only half of the dffuser was ncluded n the CFD model due to symmetry. All 25 24 openngs were for smplcty modeled as squares of 6. 6. mm, except for the openngs along the symmetry plane whch was modeled by 3. 6. mm. Ths resulted n a total free area of 211.68 cm2, whch was.9% lower than n the experments. The velocty profle of the ar enterng the dffuser from the duct was defned at the top of the dffuser. The dscretzaton of the governng equatons was based on the fnte volume method, and the computatons were carred out usng Ansys Fluent 14.5. The SIMPLE algorthm was used for couplng veloctes and pressure. The grd dependence was tested wth dfferent grd denstes. The smulatons were performed on a computer wth two parallel 2.4GHz processors each havng 6 cores. A non-conformal mesh setup was appled between the nlet openngs and the room. Ths nterface makes t possble to have separate cell-szes for the nlet openngs and the rest of the room. The rato between the cell-szes at the nterface was 1/5. The fnal 3-D model conssted of 5,384,88 hexahedral cells (majorty of the cells located nsde the dffuser) where each openng for the perforated plate conssted of 25 elements. The mesh has a hgher densty near the walls, especally at the perforated plate, n order to capture n detal the arflow pattern leavng the dffuser wth hgher densty near the nlet dffuser. The enhanced wall functon
was used and n the fnal mesh y+ values were below 1 nsde n dffuser (nvokng two-layer zonal model) and below 5 n the room. The smulaton was declared converged when the parameters and the resduals were no longer changng wth successve teratons and the resduals for all varables were less than 1-8. The tme-step was set to 1-4 s wth a maxmum of 2 teratons per tme-step. 5mm connectng duct 2l/s 1764 openng 4mm 4mm 49mm 6mm 1mm 9mm Fgure 3. Full 3-D model of the dffuser. The boundary condtons of the smulatons duplcated measurements n the test-room, see Fgure 4. The buoyancy effect was modeled based on the ncompressble deal gas law. Wall 2 Wall 1 y symmetry plane z Wall 3 x wall 1 wall 2 wall 3 floor celng nlet Temp. ( o C) 21.7 22. 22.2 21.5 23. 16.9 Fgure 4. Temperature boundary condtons for the walls and the nlet. For unsteady, three dmensonal, non-sothermal, ncompressble turbulent flow and usng the deal gas law, the contnuty, Naver-Stokes and energy equaton are gven by: U x U t = ( ρ ρ ) ( U ju ) P U + = + ν u u j + x j ρ x x j x j ρ g (1) (2) T ( U jt ) T + = α u θ t x j x j x j (3)
where α s thermal dffusvty and u, u θ consttute the second-moments statstcal u j correlatons or so-called Reynolds stresses and turbulent heat fluxes. These unknowns must be modelled n order to close the system of equatons. The turbulence model used n ths study s the Realzable k-ε model, see Ansys Fluent Manual 213. RESULTS AND DISCUSSION Fgure 5 shows the mean velocty contours and the mean temperature feld at the symmetry plane close to the supply dffuser. Three strong recrculaton bubbles can be observed n the fgure, where two are located nsde the dffuser affectng flow development and one just beneath the supply. The resolved and modeled turbulent fluctuatons at z = m are presented n fgure 6. As one can clearly see the resolved fluctuatons are much lower than the modeled one especally nsde the dffuser ndcaton too much modeled dsspaton (too large turbulent vscosty whch dampens resolved fluctuatons). Too much eddy vscosty damps out the unsteadness of the flow and showng the weakness of the turbulent model used n ths study. It s worth mentonng that more sophstcated lke the v2f model, see Parnex and Durbn (1997), and non-lnear eddy-vscosty models lke the realzable Reynolds stress algebrac model (Shh et al 1993) could overcome ths shortcomng. Fgure 5. Mean velocty dstrbuton (left) and mean temperature dstrbuton (rght) at z = m. Fgure 6. Resolved turbulent fluctuatons, v at symmetry plane (left). Modeled turbulent fluctuatons, v at symmetry plane (rght). Fgure 7 compares the predcted mean velocty profles wth measured values at correspondng dstances from the dffuser for the symmetry plane. As mentoned before, because of the dsspatve characterstc of the turbulence model used the predcted mean veloctes are slghtly under-predcted. The trend s however captured pretty well but not the levels as s expected due to the restrcton of the used turbulence model. The realzable k-ε turbulence model used here assumes sotropc turbulence whch s not really true n ths case due to mpngement, curvature effects and separaton at the upstream regon.
3 A Measurements turbulence<3% 3 B 3 C 25 turbulence >3% 25 25 2 CFD 2 2 15 15 15 1 1 1 5 5 5.2.4.6.8 1.2.4.6.8 1.2.4.6.8 1 3 D 3 E 3 F 25 25 25 2 2 2 15 15 15 1 1 1 5 5 5.2.4.6.8 1.2.4.6.8 1 = measured mean veloctes wth turbulence ntensty > 3% = measured mean veloctes wth turbulence ntensty > 3%.2.4.6.8 1 Fgure 7. Mean velocty profles at z = m at dfferent dstances from the supply devce. A: x=.5m; B: x=.1m; C: x=.2m; D: x=.3m; E: x=.4m; F: x=.5m. 4 A 4 B 4 C 3 3 3 2 2 2 measurements CFD 1 1 1 16 17 18 19 2 21 22 23 16 17 18 19 2 21 22 23 16 17 18 19 2 21 22 23 4 D 4 E 4 F 3 3 3 2 2 2 1 1 1 16 17 18 19 2 21 22 23 16 17 18 19 2 21 22 23 16 17 18 19 2 21 22 23 Fgure 8. Mean temperature profles at z=m at dfferent dstances from the supply devce. A: x=.5m; B: x=.1m; C: x=.2m; D: x=.3m; E: x=.4m; F: x=.5m. Fgure 8 compares the predcted mean temperature profles wth measured values at correspondng dstances from the dffuser for the symmetry plane. The trend s captured well and also the levels are predcted farly well after.3 m downstream the dffuser,.e. when the
jet leaves the near-zone regon because of the known gravty current phenomena n dsplacement ventlaton systems. The predcted temperature profles very close to the supply devce have some dscrepances compare to measurements because of falure of the turbulence model to fully predct the flow complexty nsde the dffuser. As can be observed form fgure 9, taken by an nfrared camera, the flow pattern close to the supply outlet cause an nstable regon so called for the gravty current. In our ongong project, we are gong to verfy sophstcated non-lnear models wth ths type of magng. For more detaled mage analyss and successes n usng nfrared camera magng for CFD valdaton see Karmpanah et al (214). 5mm Fgure 9. Temperature feld maged by nfrared camera for near-zone regon (close to the supply termnal). CONCLUSIONS Ths effort shows that the smulaton of arflow n rooms wth dsplacement ventlaton s stll troublesome. It s also ponted out that for dsplacement ventlaton supples usng the undertempered ar at low veloctes the flow wll be nfluenced by both momentum and buoyancy forces. The small detals of a supply ar dffuser have an obvous nfluence on the envronmental parameters and the thermal comfort n partcular dsplacement type of ventlaton. The ar dffuson n a room s also at least partly domnated by the supply ar parameters, such as nlet velocty, arflow rate, turbulence ntensty and nlet temperature. The presented applcatons show that unsteady smulatons wth the realzable turbulence k-ε model generates too hgh eddy vscosty and therefore damps out the unsteadness of the flow especally nsde the dffuser. Physcal effects can be predcted accurately (almost qualtatvely accurate) by URANS. Weak predcton of flow unsteadness caused by ansotropy of turbulence nsde the supply devce s notced n connecton wth nfluence of perforated plate.
The resolved fluctuatons are observed to be lower than the modeled fluctuatons ndcatng too hgh turbulent vscosty whch causes dampng effect on the unsteadness of the flow. It was suggested that n future nvestgatons one should use more sophstcated nonlnear turbulence models. ACKNOWLEDGEMENT The authors are thankful for the fnancal support from the Unversty of Gävle. The authors gratefully acknowledge the support receved by personnel at the laboratory of Bult Envronment n Unversty of Gävle. REFERENCES Butler, D. (22) Ar condtonng usng dsplacement ventlaton to maxmse free coolng. In: CIBSE Conference 22. Cehln M. and Moshfegh B. (21) Numercal modelng of a complex dffuser n a room wth dsplacement ventlaton. Buldng and Envronment 45.1 (21): 224-2252. Fanger P.O., Melkov, A.K., Hanzawa, H., and Rng, J. (1988). Ar turbulence and Sensaton of Draught. Energy and Buldnngs, vol. 12, pp. 21-39. Fanger, P.O. and Pedersen, C.J.K. (1977). Dscomfort Due to Ar Veloctes n Spaces. Proceedngs of the Meetng of Commsson B1, B2, E1 of the Internatonal Insttute of Refrgeraton, vol.4, pp.289-296. Karmpanah T., Larsson U. and Cehln M. (214) Investgaton of flow pattern for a confluent-jets ventlaton system on a workbench of an ndustral space, wll be presented n IndoorAr 214, Hongkong. Parnex, S. and Durbn, P. (1997) Numercal smulaton of 3D turbulent boundary layers usng the V2F model. Annual Research Brefs, 1997, 135-148. Peng, S. and Davdson, L. (2). The Potental of Large Eddy Smulaton Technques for Modelng Indoor Ar Flows. Proceedngs RoomVent 2, vol. 1, pp. 295-3. Shh T.H., Zhu J., and Lumley J. L. (1993) A realzable Reynolds stress algebrac equaton model. Techncal memo 15993, NASA. Wood et al (25) Numercal Methods for Modelng Transent Flow n Dstrbuton Systems, Journal of Amercan Water Works Assocaton (AWWA) 97:7. Wyon, D and Sandberg, M. (1989). Thermal Mankn Predcton of Dscomfort due to Dsplacement Ventlaton. ASHRAE Transactons, vol. 95, part 1. Chen, Q., Zhang, Z., Zuo, W. (27). COMPUTATIONAL FLUID DYNAMICS FOR INDOOR ENVIRONMENT MODELING: PAST, PRESENT, AND FUTURE. The 6th Internatonal Conference on Indoor Ar Qualty, Ventlaton & Energy Conservaton n Buldngs IAQVEC 27, Oct. 28-31 27, Senda, Japan.