NUMERICAL INVESTIGATION OF AIR FLOW INSIDE AN OFFICE ROOM UNDER VARIOUS VENTILATION CONDITIONS

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1 PAMUKKALE ÜNİ VERSİ TESİ MÜHENDİ SLİ K FAKÜLTESİ PAMUKKALE UNIVERSITY ENGINEERING COLLEGE MÜHENDİ SLİ K B İ L İ MLERİ DERGİ S İ JOURNAL OF ENGINEERING SCIENCES YIL CİLT SAYI SAYFA : : 12 : 1 : NUMERICAL INVESTIGATION OF AIR FLOW INSIDE AN OFFICE ROOM UNDER VARIOUS VENTILATION CONDITIONS Şenol BAŞKAYA, Emre EKEN Gaz Unversty, Faculty of Engneerng and Archtecture, Department of Mec. Eng., 657/Ankara Gelş Tarh : ABSTRACT Ar flow characterstcs nsde an offce room, contanng one person and offce furnture, was nvestgated numercally under varous ventlaton condtons. Computatonal Flud Dynamcs (CFD) was used for the soluton of the steady two-dmensonal conservaton equatons. Results were presented n the form of velocty vectors and temperature contours together wth quanttatve velocty and temperature dstrbutons. Effects due to the occupants, nlet/outlet locatons, nlet velocty, and wnter/summer condtons on the arflow were examned. From the present numercal predctons t can be shown that occupants sgnfcantly alter the ndoor ar movement and hence affect comfort condtons. Key Words : Ventlaton, Ar flow, CFD, Room BİR OFİS ODASI İÇİNDEKİ HAVA AKIŞININ DEĞİŞİK HAVALANDIRMA ŞARTLARI ALTINDA SAYISAL OLARAK İNCELENMESİ ÖZET İçnde br nsan ve ofs moblyaları bulunan br ofs odası çndek hava akış karakterstkler değşk havalandırma şartları altında sayısal olarak araştırılmıştır. Zamandan bağımsız, k boyutlu korunum denklemler sayısal akışkanlar dnamğ (SAD) kullanılarak çözülmüştür. Sonuçlar hız vektör dağılımları ve sıcaklık konturlarına lave olarak ncelksel hız ve sıcaklık dağılımları olarak sunulmuştur. Oda çndek nesneler, grş/çıkış konumları, grş hızı ve kış/yaz şartlarının hava dağılımına etkler ncelenmştr. Mevcut sayısal tahmnlerden oda çnde bulunan nesnelern öneml derecede ç hava hareketlern değştrdğ ve bunun netcesnde komfor şartlarını etkledğ gösterleblr. Anahtar Kelmeler : Havalandırma, Hava akışı, SAD, Oda 1. INTRODUCTION In order to obtan maxmum comfort, t s necessary to equp offces wth mechancal ventlaton and ar condtonng systems. Because supply ar propertes and ts dstrbuton play a vtal role n the determnaton of room condtons, correct estmaton of the ar dstrbuton nsde the occuped space s vtal for the desgn of an affectve ar dstrbuton system. Arflow patterns can be predcted expermentally or by usng numercal smulaton. Snce the 197 s, Computatonal Flud Dynamcs (CFD) has been a relable tool for the estmaton of ndoor ar movement and ndoor thermal comfort evaluaton. Numerous studes can be found n the lterature regardng ndoor ar dstrbutons. A bref 87

2 presentaton of these nvestgatons relevant to the present study s gven below. Thermal comfort analyss of a room wth a cooled roof was made by Nu and Koo (1994). Yamamoto et al. (1994) nvestgated the ar dstrbuton nsde an enclosure wth an nlet from the celng and an outlet from the bottom wall. Chow and Wong (1999) presented ar velocty data collected from occuped watng halls of seven tran statons, whch were mechancally ventlated. A two-dmensonal k-ε turbulence model was used by Xue and Shu (1999) for estmaton of ar velocty, temperature, and turbulent knetc energy dstrbutons nsde a room wth an ar nlet from the celng. Dfferent openng confguratons of a room were studed usng CFD by Ayad (1999). The mxed convecton arflow obtaned by two wall jets at dfferent temperatures was nvestgated numercally and expermentally by Costa et al. (1999). Natural convecton from heated room surfaces was studed expermentally by Awb and Hatton (2). Gan (2) developed a numercal method for the determnaton of the effectve depth of fresh ar dstrbuton n rooms. Velocty and temperature dstrbutons were nvestgated by Snha et al. (2) nsde a heated room for varous dfferent nlet and outlet locatons. A CFD computer program was used by Lam and Chan (21) for the nvestgaton of velocty and temperature dstrbutons nsde a gymnasum wth one cold ar supply and four dfferent ext locatons. A heated mannequn, desk, and other heat sources were used by Xng et al. (21), n ther expermental measurements nsde a ventlated room. They also performed numercal smulatons on the same confguraton. As can be seen, research n the lterature has been manly on empty spaces, stuatons wth occupants have not attracted much attenton. 2. NUMERICAL MODEL Fgure 1 shows a two-dmensonal schematc drawng of the offce room nvestgated. All necessary nformaton regardng the room s clearly ndcated on ths drawng. As can be seen from ths fgure the offce room s equpped wth standard offce materal. Ths has been kept to a mnmum wth one person, a char, a desk, and a shelf. Numerous dfferent alternatves are possble, however, the choce made s suffcent for the purpose of ths nvestgaton. Fgure 1. Schematc llustraton of the room (measures n cm) Governng Equatons Arflow and heat transfer nsde the offce room s assumed to be, steady-state, two-dmensonal, turbulent, and a mxed convecton problem. Hence, the problem was formulated wth two-dmensonal equatons of conservaton of mass, momentum, energy, turbulent knetc energy and ts dsspaton rate. For a steady, ncompressble, two-dmensonal flow, the conservaton of mass can be expressed n the followng form. ( ρu ) = (1) The turbulent momentum equaton can be formulated as gven below. P ( ρ uu j) = j j u u µ + ρ ρ ( ) j ef g ref j (2) Here, g s the gravtatonal acceleraton, ρ ref s the reference densty, µ ef s the effectve dynamc vscosty. In the above equaton, the g(ρ- ρ ref ) term s the buoyancy force. The turbulent energy equaton can be formulated as: j ( ) T ρu = Γ jt ef j j (3) The turbulence knetc energy equaton s expressed as: Mühendslk Blmler Dergs 12 (1) Journal of Engneerng Scences 12 (1) 87-95

3 µ σ k ef ( ρu k) = + G + G ρε k K B (4) Here, σ k s a turbulence model constant, G K s the rate of shear producton of k and G B s the rate of buoyancy producton of k. G K and G B are defned as gven below: G G u u u j K =µ t + j xj x (5) µ t 1 ρ B = g σ t ρ x The dsspaton rate of turbulence knetc energy s gven as: µ ef ε ( ρuε ) = σε ε + 1ε K + 3ε B ρ 2εε k ( C G C G C ) (6) mechancs, chemcal reacton and smlar physcal phenomena (Rosten and Spaldng, 1987). Ths program provdes numercal teratve approxmatons to the soluton of non-lnear partal dfferental equatons. The numercal soluton procedure appled s an mproved verson of the wdely used SIMPLE algorthm. For the dscretzaton of convectve-dffusve transport, the hybrd scheme s used. The dscretzed equatons are solved by the TDMA (Tr-Dagonal-Matrx- Algorthm). Dstrbuton of the computatonal grd used s shown n Fgure 2. Tral solutons were obtaned wth a wde range of cell number combnatons for grd ndependency checks. The fnal number of cells used was 1 for the x- drecton and 8 for the y-drecton. Fner grd dstrbutons are employed at locatons wth larger antcpated gradents. Further detals about the soluton procedure appled can be found n Rosten and Spaldng (1987), and Patankar (198) and smlar publcatons. Where, σε, C1ε, C 2ε, C3ε are turbulence model constants. These defntons were made accordng to the standard k-ε turbulence model. In ths model the turbulence model quanttes are gven as (Costa et al., 1999): µ ef = µ t + µ ; k µ t = ρc ; Γ ef = (7) ε σ µ t µ Where, µ t s the turbulent vscosty, ρ flud densty, C µ turbulence model constant and Γ ef the effectve exchange coeffcent. Values of the turbulence model constants are gven below. σ t = 1. ; C µ =. 9 ; σ k = 1. ; σ ε = 1. 3 ; C 1 ε = 1.44 ; C 2 ε = ; C 3 ε = 1. (8) Here, C µ, σk, σ ε,c1ε, C2ε, C 3ε are turbulence model constants. σ t s the turbulent Prandtl number Soluton Method The fnte volume based PHOENICS code was used for the soluton of the transport equatons defnng the problem. PHOENICS s a commercal code used for smulatons of heat and mass transfer, flud 2 t Fgure 2. Dstrbuton of the computatonal grd Boundary Condtons Veloctes at the walls are zero because of the noslp condton. Supply ar nlet veloctes u n and v n, nlet temperatures T n and wall temperatures T w were taken as gven n Table 1. At the outlet the pressure s fxed to the ambent pressure and all varatons of temperature, turbulent knetc energy and ts dsspaton were taken to be zero. The no-slp boundary condton was also appled to the surfaces of the person, shelf, desk, and char. The shelf, desk, and char were assumed adabatc, because at steady operaton these surfaces wll be at approxmately the same temperature as the flud surroundng them. The sttng person s outer surface temperature was taken to be a calculated cloth temperature of 25.4 o C. The nlet and outlet duct dmensons were taken as.2 m. Doors, wndows, etc. are not taken nto account. Smulated are only ventlaton ducts, for whch the second dmenson s not taken nto account as a result of the two-dmensonal smulatons. Mühendslk Blmler Dergs 12 (1) Journal of Engneerng Scences 12 (1) 87-95

4 The logarthmc law of the wall was used n regons close to the wall surfaces. Equatons used for the turbulence nlet boundary condtons are gven below (Costa et al., 1999). 2 2 kn = 1.5It U n c 1/2 2 2 It = n ( u v )/2 + /U c ε n = (9) 3/2 k n / L ε ; L ε = d / 2 (1) Here, k n s the turbulent knetc energy at the nlet, I tn turbulence ntensty at the nlet, U c characterstc velocty scale, u and v average fluctuatng components of velocty, ε n dsspaton rate of k at the nlet, L ε characterstc length and d s the jet slot wdth (Costa et al., 1999) Convergence and Grd Independency The crteron of convergence of the numercal soluton s based on the absolute normalzed resduals of the equatons that were summed for all cells n the computatonal doman. Convergence was consdered as beng acheved when these resduals become less than 1-3, whch was the case for most of the dependent varables. False tme step relaxaton for the three velocty components and temperature, and lnear relaxaton for pressure was used to obtan convergence that s more rapd. Relaxaton factors ranged between.1 and.4. The smulatons exhbted dvergence f no relaxaton was appled. Grd ndependency checks were made and the fnal smulatons were acheved wth 1x8 cell numbers n the x-y coordnate drectons. a temperature of o C. Expermental results from the above mentoned paper was compared wth the present CFD soluton method. These verfcaton results are reported n detal by Kuas et al. (). Examnaton of ths publcaton shows that the results of the CFD solutons obtaned from the present method are n good agreement wth the expermental results by Costa et al. (1999) Results and Dscusson Table 1 lsts ventlaton alternatves studed dependng on nlet/outlet confguratons, summer/wnter condtons and nlet velocty/temperature values. Table 1. Condtons of the Cases Studed Case No: Inlet/Outlet Locatons Summer/ Wnter u n, v n (m/s) T n (ºC) T w (ºC) 1 Inlet 4, Outlet 5 Summer Inlet 4, Outlet 5 Wnter Inlet 4-5, Outlet 1-3 Summer Inlet 4-5, Outlet 1-3 Wnter Inlet 1, Outlet 3-5 Summer Inlet 1, Outlet 3-5 Wnter Inlet/outlet locaton numbers are shown n Fgure 3. The results are analyzed n detal for varous profles of the room. The profles chosen are numbered n Fgure 4. These profles are chosen around the sttng person, because the person s comfort condtons are mportant. 3. NUMERICAL SIMULATION RESULTS Verfcaton of the CFD Method For the verfcaton of the problem defntons used for the present study usng the PHOENICS code, results reported by Costa et al. (1999) were used. Costa et al. (1999) nvestgated room arflow where a horzontal cold ar jet and a vertcal warm ar jet were used at the left wall and an ext duct at the bottom of the rght wall. The velocty of the cold jet s u n =.8 m/s and of the warm jet v n =.87 m/s. The dmensons of the nvestgated closed volume are x x 7 mm. The wdths of the supply nlet ducts are d = 2 mm, outlet duct mm, cold jet nlet temperature s o C, warm jet nlet temperature s 35 o C. All the walls of the room are at Fgure 3. Inlet/outlet locatons studed K6 Fgure 4. Profles chosen for presentaton of results Mühendslk Blmler Dergs 12 (1) Journal of Engneerng Scences 12 (1) 87-95

5 Fgure 5 shows predcted temperature contours and Fgure 6. shows velocty vector plots for the summer cases studed. On these plots, one can see that there s a sgnfcant effect of the occupants (see relevant lterature) on the temperature dstrbuton. In addton, changes of nlet/outlet locatons sgnfcantly alter the temperature dstrbutons. Case 6 Fgure 5. Temperature contour dstrbutons for all the cases studed Case 1 Case 1 Case 2 Case 2 Case 3 Case 3 Case 4 Case 4 Case 5 Case 5 Mühendslk Blmler Dergs 12 (1) Journal of Engneerng Scences 12 (1) 87-95

6 Fgure 4. These quanttatve fgures wll enhance the conclusons drawn from the mostly qualtatve contour and vector plots. Case 6 Fgure 6. Velocty vector dstrbutons for the summer cases studed Velocty vector plots are shown n order to better make sense of the temperature dstrbutons. From the sze of the vectors, supply duct and exhaust vent locatons are clearly vsble on these plots (see Fgs. 5-6). Overall, full recrculaton n the sze of the room s prevented because of the occupants. Instead, the supply ar stream creates smaller recrculatons at dfferent locatons (manly around the spaces of the occupants) before beng expelled at the outlet. If Fgs. 5 and 6 are analyzed jontly, one can see that the temperature dstrbuton develops because of arflow characterstcs resultng from the varaton of nlet/outlet locatons and the occupant s sze and locaton. In other words temperature and velocty dstrbutons are coupled as a result of the convecton nature of the flow. Occupants change the arflow patterns, thus, changng the temperature dstrbuton. In addton, from Fgs. 5 and 6, one can see that there s a substantal dfference between the arflow characterstcs of the coolng ar dstrbutons for the summer cases and the heatng ar dstrbutons for the wnter cases. Ths s due to the dfferent behavor of the supply ar jets, because of the temperature dfference between average room temperature and supply ar temperature. The cold and heaver cool ar jets wll drop faster than compared to the hotter and lghter heatng jets whch tend to rse or at least penetrate further nto the room. Hence, the recrculaton and ar movement s wholly dfferent n summer and wnter condtons. Furthermore, some parts of the room exhbt a stratfed temperature varaton. The stratfcaton occurs mostly close to the celng for the summer cases, and close to the floor for the wnter cases. It s possble to contnue wth further cross sectonal plots of sotherms and velocty vectors, however, t s also mportant to see the actual dstrbutons of temperature and velocty components for certan relevant profles chosen at mportant locatons. Hence, the next fgures show temperature and velocty dstrbutons at some profles shown n Velocty dstrbutons for profles and are shown n Fgs. 7-8, and temperature dstrbutons for profles, and are shown n Fgs All fgures use full lengths of the room for the x and y axs. The gaps n the profles are due to the occupants. From Fgs. 7-8 the largest varatons can be seen for profles along the supply ar jet and exhaust locatons, hence, makng these locatons vsble. The ar jet velocty decay characterstcs of the hot and cold jets dffer as a result of buoyancy forces whch become more domnant n regons wth lower veloctes. Supply ar veloctes decay to very small velocty values, whch domnate the flow n the center regon of the room. Values of veloctes are also very mportant from a comfort-condton pont of vew. The nspecton of values of veloctes near the person shows that they all are lower than the values assocated wth the comfort condtons gven n the lterature (Anon, 1997a; b). Nevertheless, the rsk of draft s present near the nlet and outlet sectons. 1,8,6,4,5 1 1,5 2 2,5 3 3,5 4 4,5 5-1,8,6,4 -,4 Case 1,5 1 1,5 2 2,5 3 3,5 4 4,5 5 -,3,1 - -,3 Case 2,5 1 1,5 2 2,5 3 3,5 4 4,5 5 -,1 Case 3 Fgure 7. Velocty profles along and for Cases 1, 2 and 3 Mühendslk Blmler Dergs 12 (1) Journal of Engneerng Scences 12 (1) 87-95

7 ,5,4,3,1 -,1,5 1 1,5 2 2,5 3 3,5 4 4, ,3 -,4 -,5 x[m] Case 4 3 2,5 1 1,5 2 2,5 Case 1 1,8,6,4,5 1 1,5 2 2,5 3 3,5 4 4, ,4 Case ,5 1 1,5 2 2,5 Case 2,8,7,6,5,4,3,1 -,1,5 1 1,5 2 2,5 3 3,5 4 4,5 5 Case 6 3 2,5 1 1,5 2 2,5 Case 3 Fgure 8. Velocty profles along and for Cases 4, 5 and 6 Fgure 9. Temperature vs. room heght along, and for Cases 1, 2 and 3 Inspecton of Fgs. 9-1 shows that the temperature values along the dfferent profles are mostly smlar and close to each other. As can be seen from Fgure 4, to are chosen around the person, and Fgs. 9-1 show that temperatures do not dffer much between these profles. Effects due to the cold and hot supply ar jets are also vsble. The temperature dfferences between certan elevatons above the floor are an mportant comfort crtera. From Fgure 8 t can be seen, that overall temperature dfferences are not very large. However, for certan cases the values are probably exceedng comfort condtons ,5 1 1,5 2 2,5 Case 4 Mühendslk Blmler Dergs 12 (1) Journal of Engneerng Scences 12 (1) 87-95

8 ,5 1 1,5 2 2,5 Case 5,5 1 1,5 2 2,5 Case 6 Fgure 1. Temperature vs. room heght along, and for Cases 4, 5 and 6 4. CONCLUSIONS Computatonal flud dynamcs smulatons were undertaken for the determnaton of the characterstcs of coolng/heatng ar sent to an offce room that contans a person and other occupants. Effects of the occupants under dfferent nlet/outlet and summer/wnter confguratons on the arflow have been analyzed for two dfferent supply jet veloctes. In order to determne arflow characterstcs nsde the room, velocty vectors, velocty profles, temperature contours and temperature profles for varous cross sectons of the room have been nvestgated. Compared to studes wth empty rooms, a full crculaton proportonal to the sze of the room does not occur. The crculaton s beng nterrupted and changed by the occupants; resultng n smaller recrculatons, flow reversals and other varous dfferent flows. Furthermore, the occupants are hnderng penetraton of the supply ar jets. These effects can result n poor mxng and ventlaton. As can be understood from these comparsons, arflow n a room s hghly nfluenced by persons and occupants present n the room space, as well as nlet/outlet confguratons. In addton, approprateness of nlet/outlet locatons n terms of comfort condtons vares wth coolng/heatng confguratons. 5. REFERENCES Anonymous, 1997a. ASHRAE Handbook Fundamentals. Amercan Socety of Heatng, Refrgeratng and Ar-Condtonng Engneers, Inc., Atlanta, Georga, U.S.A.. Anonymous, 1997b. CIBSE. Natural Ventlaton n Non-Domestc Buldngs, CIBSE Applcatons Manual AM1, London. Ayad, S. S Computatonal Study of Natural Ventlaton, J. of Wnd Engneerng and Industral Aerodynamcs 82, Awb, H. B. and Hatton, A. 2. Mxed Convecton from Heated Room Surfaces, Energy and Buldngs, Chow, W. K. and Wong, L.T Local Ar Speeds Measurement n Mechancally Ventlated Spaces, Buldng and Envronment 34, Costa, J. J., Olvera, L. A. and Blay, D Test of Several Versons for the k-ε Type Turbulence Modellng of Internal Mxed Convecton Flows, Int. J. of Heat and Mass Transfer 42, Gan, G. 2. Effectve Depth of Fresh Ar Dstrbuton n Rooms wth Sngle-Sded Natural Ventlaton, Energy and Buldngs 31, Kuas, G., Başkaya, Ş. and Svroğlu, M.. Numercal analyss of effects of occupants on forced ventlaton nsde a room, V. Internatonal HVAC+R Technology Symposum, Aprl 29 May 1, İstanbul, Turkey. Lam, J. C. and Chan, A. L. S. 21. CFD Analyss and Energy Smulaton of a Gymnasum, Buldng and Envronment 36, Nu, J. and Koo, J Indoor Clmate n Rooms wth Cooled Celng Systems, Buldng and Envronment 29 (3), Patankar, S. V Numercal Heat Transfer and Flud Flow, Hemsphere, New York. Rosten, H. and Spaldng, B PHOENICS Begnners Gude, CHAM/TR1, London. Snha, S. L., Arora, R. C. and Roy, S. 2. Numercal Smulaton of Two-Dmensonal Room Ar Flow wth and wthout Buoyancy, Energy and Buldngs, Mühendslk Blmler Dergs 12 (1) Journal of Engneerng Scences 12 (1) 87-95

9 Xng, H., Hatton, A. and Awb, H. B. 21. A Study of the Ar Qualty n the Breathng Zone n a Room wth Dsplacement Ventlaton, Buldng and Envronment 36, Xue, H. and Shu, C Mxng Characterstcs n a Ventlated Room wth Non-Isothermal Celng Ar Supply, Buldng and Envronment 34, Yamatomo, T., Ensor, D.S. and Sparks, L.E Evaluaton of Ventlaton Performance for Indoor Space, Buldng and Envronment 29 (3), Mühendslk Blmler Dergs 12 (1) Journal of Engneerng Scences 12 (1) 87-95