NUMERICAL STUDY OF CIRCULAR JET DIFFUSER FOR TASK VENTILATION OF UNDER-FLOOR AIR SUPPLY SYSTEM IN TROPICS. Bin Yang 1 and S.C.

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1 NUMERICAL STUDY OF CIRCULAR JET DIFFUSER FOR TASK VENTILATION OF UNDER-FLOOR AIR SUPPLY SYSTEM IN TROPICS Bn Yang 1 and S.C. Sekhar 1 1 Department of Buldng, Natonal Unversty of Sngapore, Sngapore , Sngapore ABSTRACT Bases on the concept of task/ambent ventlaton, fresh ar can be decoupled from re-crculated ar so as to mprovng ventlaton effectveness n breathng zone. Celng mounted hgh velocty crcular jet dffusers, whch are regarded as remote personalzed ventlaton ar termnal devces (PV ATDs) wthout affectng room aesthetc effects, can be utlzed to supply fresh ar wthout causng draft ratng because tropcally acclmatzed occupants prefer slghtly hgher ar movement. Under-floor ar dffusers are used to supply re-crculated ar. Correspondng mxng ventlaton system by only utlzng underfloor ar dffusers are compared wth ths task/ambent ventlaton system. Numercal smulaton usng a valdated computatonal flud dynamcs (CFD) model for crcular jet dffusers s conducted to nvestgate the dfference between task/ambent ventlaton systems and mxng ventlaton systems. Locaton weghted personal exposure effectveness s utlzed as an evaluaton ndex for evaluatng ndoor ar qualty (IAQ). Ar velocty and temperature are utlzed as evaluaton ndces for evaluatng thermal comfort level. KEYWORDS Task/ambent ventlaton, Crcular jet dffuser, Mxng Ventlaton, Under-floor ar supply INTRODUCTION Indoor ar qualty (IAQ) and thermal comfort become the most mportant characterstcs of ndoor envronment because people spend more than 90% tme ndoors. The crteron for evaluatng a mechancal ventlaton and ar condtonng system s whether t can provde a comfortable and healthy ndoor envronment for the occupants n an energy effcent manner. Mxng ventlaton and task/ambent ventlaton are the two man ventlaton strateges for ventlaton system. For mxng ventlaton, t can be categorzed nto normal celng supply mxng ventlaton (MV), dsplacement ventlaton (DV) and under-floor supply mxng ventlaton (UV). The man feature of MV s the dluton of the ar. The cool, clean ar s forced nto ndoor space and mxed wth old ndoor ar. The am s to make the mxed ar have good qualty n the shortest tme. However, the exstence of the ar exhaust mples a porton of outdoor ar s wasted and f the ndoor nducement s ntroduced, the nvolved system equpments must deal wth a lot of ar whch has been dsposed. DV dscharges ar at relatvely low velocty near the floor. The suppled cool and clean ar, whch s 3-4 C lower than the old ndoor ar temperature, spreads and, because of ts low temperature and hgh densty, forms a pool of condtoned ar over the floor. When ths ar meets a heat source, a convectve thermal plume s generated due to the temperature dfference and resultant buoyant force, whch acts as a channel through whch the warm and polluted ar moves. The stratfcaton, upper level (warm and polluted) and lower level (cool and clean), s the most obvous characterstcs n DV. As a result, IAQ n occuped zone can be effcently controlled. UV dstrbutes ar drectly to the occuped zone that s substantally closer to the occupants. The supply ar velocty s slghtly hgher than that of DV. Ventlaton effcency at breathng zone s mproved especally for rooms wth parttoned furnture, whereas MV may lead to poor ar crculaton at parttoned areas. Supply ar temperature of at least 17 C s requred to avod draft and room ar s often re-crculated nto underfloor plenum to attan ths temperature. For all these mxng ventlaton systems, t s dffcult to make full use of outdoor ar to dlute ndoor pollutants, such as carbon doxde, n breathng zone because of premxng of outdoor ar and a re-crculated ar n varable ar volume (VAV) box of each zone. Task/ambent ventlaton s a method of provdng occupants wth control over a local supply of ar so that they can adjust ther ndvdual thermal envronment and ar qualty. The task/ambent ventlaton system provdng 100% outdoor ar to the breathng zone of the occupants s termed as personalzed ventlaton (PV) system (Fanger 2000). Clean, cool and dry outdoor ar, termed as personalzed ar, s served nto breathng zone drectly by extended PV ar ducts. The majorty of PV-related studes, especally those wth human subjects nvolved, were conducted n temperate clmate. Few smlar studes n tropcs show that tropcally acclmatzed occupants prefer cool and hgh local ar movement, whch suggests that ar movement mght be an mportant and postve factor n mprovng both ar qualty and thermal comfort n

2 tropcs (Sekhar et. al. 2003, Sekhar et. al. 2003, Tham et. al. 2004, Tham et. al. 2004, Sekhar et. al. 2005). After that, objectve measurements of three PV ATDs, whch nclude hgh turbulence crcular perforated panel (Hgh-Tu CPP), low turbulence crcular perforated panel (Low-Tu CPP) and desk mounted grll (DMG), have been conducted by usng breathng thermal mankn n tropcs (Zhou 2005). Correspondng subjectve responses of two of these three PV ATDs, Hgh-Tu ATD and Low-Tu ATD, have been conducted by usng tropcally acclmatzed subjects (Gong 2006). In order to avod affectng aesthetc effects by extended PV ar ducts, celng mounted crcular jet dffusers, whch can be regarded as remote PV ATDs, are utlzed for task ventlaton and correspondng hgher PV ar veloctes are kept. More fresh ar can be suppled nto breathng zone n an energy effcent manner wthout causng draft ratng (Yang and Sekhar 2007). As a result, celng mounted crcular jet dffuser has potental to be task ventlaton ATD and accompaned wth MV, DV or UV as ambent ventlaton system. In recent years, ventlaton strategy studes have been conducted wth the ad of computatonal flud dynamcs (CFD) tools. Nelsen was one of the poneers n usng CFD methods to predct ar movement and heat transfer n buldngs (Nelsen 1974). CFD has been used to model dfferent sze of enclosures, dfferent ar change rate, ntake and exhaust locaton, human bodes and offce furnture. Some successful cases show the feasblty of CFD smulaton. Awb and Gan used CFD modelng to conduct predctons of ar movement n a mechancally ventlated offce module and a naturally ventlated offce and obtaned accurate results (Awb and Gan 1994). Dfferent ventlaton strateges, whch nclude MV, DV, UV and PV, were wdely smulated. Cheong et al used CFD to predct arflow pattern n an ar-condtoned semnar room n tropcs and also got close results compared wth expermental results (Cheong et. at. 1999). A computatonal model for celng mounted crcular jet dffuser based on commercal CFD program was developed and tested usng expermental data (Yang and Sekhar 2007). Some conclusons that CFD codes can be appled successfully to buldng arflow predcton but must be carred out wth care and sound engneerng judgment are gven (Jones and Whttle 1992) and some uncertantes nvolved n CFD are hghlghted (Chen 1997). Ths paper attempts to compare and evaluate mxng ventlaton strategy and Task/ambent ventlaton strategy numercally. There are 4 cases n the comparson, (1)MV, (2)MV wth jet dffusers, (3)UV, (4)UV wth jet dffusers. The study s based on a feld envronment chamber (FEC) of the Natonal Unversty of Sngapore. Commercal software FLUENT s used to smulate ar velocty, temperature and fresh ar percentage n flow feld. METHODS Physcal model and boundary condtons Table 1 Geometry and boundary condtons Enttes Dmensons(m) Flow BC Thermal BC Fgure 1 Geometry of UV wth jet dffusers Speces BC ( percentage) FEC Interor wall Non-slp adabatc celng Non-slp adabatc floor Non-slp adabatc exteror wall Non-slp T=303K exteror wndow Non-slp T=303K Occupants Non-slp 110W 8 Lamps Non-slp 54W 22 Computers Non-slp 110W 8 MV-Jet MV dffusers Velocty nlet T=293K 0 z=0.58m/s x=0.58m/s z=-0.58m/s x=-0.58m/s Jet dffusers D=0.04 Velocty nlet T=293K 1 v=5m/s (Normal to boundary) UV-Jet UV dffusers D=0.21, d=0.11 Velocty nlet T=293K 0 v=0.54m/s (Normal to boundary) Jet dffusers D=0.04 Velocty nlet T=293K 1 v=5m/s (Normal to boundary) Total ar flow rate=1800m 3 /h, Fresh ar flow rate=180m 3 /h (6.25l/s.person) Based on the FEC, three dmensonal physcal models were created. The dmensons of FEC are

3 7 2.7m 3. 8 workstatons were allocated to smulate typcal publc offce. 6 celng mounted square four-way dffusers or 33 under-floor dffusers are used to supply ambent ar for MV or UV. 8 celng mounted crcular jet dffusers are used to supply task outdoor ar for 8 workstatons respectvely. 6 celng mounted return ar grlles are used to crculate ndoor ar. The thermal boundary condtons of each wall except one exteror wall are adabatc, ncludng celng and ground. The thermal boundary condtons of exteror wall and exteror wndow are gven as certan fxed temperatures. Because the occupants, lamps and other enttes n the FEC have relatvely complex shapes, some smplfcatons of dfferent enttes are made. Human bodes are smplfed to be cubods, whose dmensons are smlar to that of seated people. The lamps, computers and furnture are also smplfed to be cubods. Tables are regarded as thn surfaces wthout thckness, whose thckness s defned n FLUENT later. The enttes n offce are also the heat sources. Numercal method It s assumed that the flow s three dmensonal, steady state, turbulent, Newtonan and ncompressble wth constant physcal propertes. Forced convecton s the man flow type and, at the same tme, the effect of natural convecton s also taken nto consderaton. Fnte volume method s used to transfer partal dfferental equatons to dfference equatons. Sem-mplct pressure lnked method (SIMPLE) s used to solve pressure and velocty coupled problem. Boussnesq assumpton s taken nto consderaton to smulate buoyancy effect. Renormalzaton (RNG) k-turbulent model s used. Speces model s actvated to smulate outdoor ar percentage n the flow feld. Grd and converge crtera The whole space of FEC, created by occupants, computers and other enttes, forms to be an rregular volume. As a result, the drect meshng to such doman s much too dffcult. As a result, nonunform grds are appled and the grd s fne near sold wall, ar nlet and outlet. The typcal total grd number s about 2 to 3 mllon cells wth the frst grd sze at 0.03 to wall, whch s fne enough to obtan mesh-ndependent soluton (Zha and Chen 2004). The converged soluton s obtaned when the followng crtera are satsfed for dependant varables: ; = U, V, W, k,, C 10 6 ; = T (1) (2) Smulated quanttes By utlzng experment method, tracer gas sulfur hexafluorde (SF 6 ) s dosed nto re-crculated ar duct and the concentraton of SF 6 n PV ATD, nhaled pont and return ar grll s measured to calculate personal exposure effectveness (Melkov et. al. 2002). CR, SF 6 CI, SF 6 p = (3) CR, SF 6 CPV, SF 6 Based on the smulaton results, outdoor ar percentage near human mouth, jet dffuser and return ar grll s obtaned to calculate smulaton based personal exposure effectveness for each occupant approxmately because respraton flow s not smulated. The expresson of the formula s as follows. PR, RA PI, RA PI, PR, p = = (4) P P P P R, RA jet, RA jet, R, Evaluaton ndex The man purpose of usng crcular jet dffusers as task ventlaton s to supply more outdoor ar nto breathng zones. Several ndces, ncludng ventlaton effectveness, pollutant removal effcency, personal exposure ndex and personal exposure effectveness (Melkov et. al. 2002), can be used to descrbe the ventlaton ablty quanttatvely. For ths celng mounted crcular jet dffusers whch are regarded as remote PV ATD, flow feld changes not only n breathng zone but n the whole feld. For MV jet system, coanda effect s also affected by the jet flow. As a result, the locaton of crcular jet dffusers and ambent ar dffusers nfluence the ar flow so as to affectng personal exposure effectveness. One new ndex whch s called locaton weghted personal exposure effectveness wll be used as evaluaton ndex for evaluatng ar qualty n breathng zones. The expresson of t s as follows. n = n (5) p p / = 1 Temperature and velocty dstrbutons are manly utlzed to analyze the thermal comfort level. RESULTS Velocty dstrbuton 1800 m 3 /h of total volume ar, whch has 10% fresh ar, s suppled nto FEC for all these 4 cases. The dfference between cases wth jet dffusers or not s whether 10% outdoor ar s suppled by jet dffusers separately or mxed wth re-crculated ar and

4 suppled together. For cases wth jet dffusers, 10% outdoor ar s suppled by jet dffusers at mean dscharge velocty 5m/s. One of the surface s chosen to show the velocty feld. The core regons of jet flow can be extended to breathng zone wthout nducng much re-crculated ar for the cases wth jet dffusers. For MV jet case, coanda effect s affected because of the ntervenng dstrbuton of square 4- way dffusers and jet dffusers. For UV jet case, the core regon of jet flow becomes shorter than that of MV jet case because of the opposte flow drectons of jet flow and underfloor ar supply. For all the cases wth jet dffusers, the ar velocty n breathng zone s m/s, whch has reached or exceeded the threshold value (0.25m/s) of causng draft ratng. Accordng to prevous research results (Sekhar et. al. 2003, Sekhar et. al. 2003, Tham et. al. 2004, Tham et. al. 2004, Sekhar et. al. 2005), ar movement s an mportant and postve factor n mprovng thermal comfort for tropcally acclmatzed occupants. Fgure 4 Temperature dstrbuton for MV jet Fgure 2 Velocty dstrbuton for MV jet Fgure 3 Velocty dstrbuton for UV jet Temperature dstrbuton The supply ar temperature of jet dffusers, square 4 way dffusers and DV cylnders s 20 C. Room ar temperature s kept at 24 C averagely for all 4 cases. For MV jet case, ar temperature at breathng zone s slghtly hgher than that of UV jet case because the supply ar cools down the upper space of FEC frst. Fgure 5 Temperature dstrbuton for UV jet Personal exposure effectveness For MV and UV cases, outdoor ar and re-crculated ar are fully mxed before supply. As a result, the outdoor ar percentage s almost the same (10%) n all flow felds. From Fgure 6 and Fgure 7, outdoor ar percentage ncreases twce n breathng zone for both MV jet case and UV jet case. The core regon of jet flow can be kept to breathng zone wthout nducng much re-crculated ar so as to mprove the outdoor ar percentage n breathng zone. The nteracton between jet flow and other flows become obvous because of the long dstance between jet dffuser and breathng zone. As a result, locaton weghted personal exposure effectveness (Table 2) s utlzed to evaluate the average effects of nhaled ar qualty for each person. The range of personalzed exposure effectveness s from 0 to 1. For MV and UV cases, t almost equals to 0 because of fully mxed outdoor ar and re-crculated ar. Hgher the value of personalzed exposure effectveness s, the more outdoor ar can be suppled nto breathng zone. The value of personalzed exposure effectveness for jet dffuser s lower than that for normal PV (Melkov et. al. 2002) because of long dstance between jet outlet and breathng zone. Many factors, whch nclude locatons of jet dffusers, other dffusers,

5 return ar grlls and workstatons, wll affect the result. Fgure 6 Fresh ar percentage dstrbuton for MV jet Fgure 7 Fresh ar percentage dstrbuton for UV jet MV jet percentage p Table 2 Locaton weghted personal exposure effectveness CONCLUSION UV jet percentage p A B C D E F G H Average Celng mounted crcular jet dffusers can be utlzed as a knd of remote PV ATDs wthout nfluencng room esthetcal effects. Slghtly elevated ar velocty n breathng zone wll be a postve factor for mprovng thermal comfort for tropcally acclmatzed occupants. Coanda effect s affected by jet flow because of ntervenng dstrbuton of jet dffusers and square 4 way dffusers for MV jet case. Underfloor ar supply has some offset effects for jet flow because of opposte flow drectons. MV jet case has slghtly hgher temperature n breathng zone because supply ar for thermal load cools down upper space of FEC frst. A new method, based on smulaton results, s utlzed to calculate the value of locaton weghted personal exposure effectveness. The value of personalzed exposure effectveness for jet dffuser s lower than that for normal PV because of long dstance between jet outlet and breathng zone. REFERENCES Fanger PO Indoor ar qualty n the 21st century: research for excellence, Indoor Ar. 10: Sekhar SC, Gong N, Maheswaran CRU, Cheong KWD, Tham KW, Melkov A and Fanger, PO Prelmnary fndngs of a plot study of personalzed ventlaton n a hot and humd clmate, Proceedngs of Healthy Buldngs, Sngapore 2003, 2: Sekhar SC, Gong N, Maheswaran CRU, Cheong KWD, Tham KW, Melkov A and Fanger PO. Energy effcency potental of personalzed ventlaton system n the tropcs, Proceedngs of Healthy Buldngs, Sngapore 2003, 2: Tham KW, Sekhar SC, Cheong KWD and Gong N. Expermental study of draft percepton of tropcally acclmatzed subjects under personalzed ventlaton, Proceedngs of Roomvent 2004, Combra: DEM-FCT, Unv. of Combra. Tham KW, Sekhar SC, Cheong KWD and Gong N. Thermal sensaton of tropcally acclmatzed subjects under fxed ar flow of personalzed ventlaton, Proceedngs of Roomvent 2004, Combra: DEM-FCT, Unv. of Combra. Sekhar SC, Gong N, Tham KW, Cheong KWD, Melkov A, Wyon DP and Fanger PO Fndngs of personalzed ventlaton studes n a hot and humd clmate, Internatonal Journal of Heatng, Ventlatng, Ar-Condtonng and Refrgeratng Research. 11(4):

6 Zhou W Performance evaluaton of ar termnal devces for personalzed ventlaton n the tropcs, Master Thess, Natonal Unversty of Sngapore (Sngapore), 205 pages. Gong, N Human percepton of local ar movement n the tropcs, Ph.D. Thess, Natonal Unversty of Sngapore, (Sngapore), 153 pages.. Yang B and Sekhar SC Three-dmensonal numercal smulaton of a hybrd fresh ar and recrculated ar dffuser for decoupled ventlaton strategy, Buldng and Envronment. 42(5): Nelsen PV Flow n ar-condtoned rooms, Ph.D. Thess, Techncal Unversty of Denmark, (Denmark). T temperature U x drecton velocty component V y drecton velocty componen W z drecton velocty component wldcard character dsspaton of turbulent knetc energy p personal exposure effectveness p locaton weghted personal exposure effectveness n number of measurng ponts. Awb HW and Gan G Predctng ar flow and thermal comfort n offces, ASHRAE Journal. 36(2): Cheong KWD, Sekhar SC, Tham KW and Djunaedy E. Ar flow pattern n ar-condtoned semnar room, Proceedngs of the 8th nternatonal conference on ndoor ar qualty and clmate, Scotland, 1999, 2: Jones PJ and Whttle GE Computatonal flud dynamcs for buldng ar flow predcton-current status and capabltes, Buldng and Envronment. 27(3): Chen Q Computaton flud dynamcs for HVAC: successes and falures, ASHRAE Transactons. 102: Zha ZQ and Chen Q Numercal determnaton and treatment of convectve heat transfer coeffcent n the coupled buldng energy and CFD smulaton, Buldng and Envronment. 39: Melkov AK, Cermak R and Majer M Personalzed ventlaton: evaluaton of dfferent ar termnal devces, Energy and Buldngs. 34: NOMENCLATURE C outdoor ar concentraton k turbulent knetc energy outdoor ar percentage n nhaled ar I OA re-crculated ar percentage n nhaled ar I RA outdoor ar percentage n jet dffuser jet OA re-crculated ar percentage n jet dffuser jet RA P R, OA outdoor ar percentage n return ar grll re-crculated ar percentage n return ar grll R RA