Numerical Analysis of Natural Ventilation System in a Studio Apartment in Bangladesh

Transcription

1 Numercal Analyss of Natural Ventlaton System n a Studo Apartment n Bangladesh K.M Arful Kabr 1, a) Md. Rakbul Hasan 2, b) and Md. Abdul Hakm Khan 1 1 Bangladesh Unversty of Engneerng and Technology, Dhaka-1000, Bangladesh 2 Southeast Unversty, Dhaka-1213, Bangladesh a) k.arful@yahoo.com b) md.rakbul_hasan@yahoo.com Abstract. The study of temperature and ar flow for natural ventlaton system has been nvestgated numercally. A fnte element model for studo apartment was developed wth the am of achevng detal energy allocaton n the real buldngs durng the transent process n the walls and nternal ar. A tool of computatonal flud dynamcs (CFD) s employed to assst the process. In the tropcal regons most of the energy s consumed by the heatng, coolng and ventlaton applances. Therefore, the optmze ventlaton system wll be a sutable and vald opton for the savng of energy from the household sector to ncrease coolng performance and ensurng thermal comfort as well. A mathematcal exploraton s carred out on full scale dwellng and small scale model and ndcaton s gven on the relevance of such a comparson. Calculatons are carred out wth household heat sources for calm and wndy perod, but wthout any human. As expected, for wndy perods, the wnd s the man drvng force behnd the nternal ar flow. However, n calm perods for unsteady flow the nternal arflow looks lke more complexes through observaton. INTRODUCTION Consumpton of energy s an acute proclams and has become a great anxety durng the last eras. In developed countres buldngs are responsble for one-thrd of all energy consumpton [1]; atmospherc polluton and global warmng are ncreasng due to ncrease of fossl-based energy. The energy requrement has ncreased at a rate of 2 % per year for the past 25 years and t wll ncrease contnuously at the same rate f current energy patterns persst, accordng to the Internatonal Energy Agency (IEA). Energy s used for heatng and coolng purposes n most of the buldng, and approxmately 51% of energy consumpton n resdental buldngs n London [2].Whereas n Bangladesh, the consumpton patterns was gven below: Domestc (47.21%), Industry (36.56%) and Commercal (9.49%) [3] and t s ncreasng day by day. One of the ways of savng costs of energy s the use of natural ventlaton, whch supples and removes ar to the buldng wthout usng engneerng systems. Natural ventlaton has sgnfcant role to decrease operatng costs to condton buldngs whle mantanng acceptable ndoor ar qualty [4-5]. Whle ntroducng a natural ventlaton system, the nsde ar qualty must be consdered. For decreasng the energy use n buldngs and keepng a healthy ndoor envronment an alternatve energy effcent soluton s Natural ventlaton [8-9]. Tamam Kusuda, showed about the heat transfer n buldngs [10]. Davd Park, solve the heat transfer and ar flow n a room numercally [11]. There are manly two types of natural ventlaton: cross ventlaton and sngle-sded ventlaton, and occur by wnd and buoyancy forces, respectvely. The buoyancy forces are nduced from gradents of densty occurrng due to temperature gradents. The arflow and thermal condtons n the buldng can be altered by heat sources such as fres, electronc components, electrcal heaters, and even from people. Usually cross ventlaton s used for coolng purposes, whch depends on outsder wnd to transport the cool ar nto an nlet (wndow, door, etc.) and to transport the nsder warm ar through an outlet (wndow, door, etc.). Sngle-sded ventlaton nvolves a room wth one openng. The dfferences between the ndoor and the outdoor temperatures create a pressure dfference, and for these reason the ar s ventlated through an openng. It s a greater challenge to

2 desgn a naturally ventlated buldng than desgnng a mechancally ventlated buldng because natural ventlaton affects by weather, whch changes contnuously. On the other hand, for a mechancally ventlated buldng, ambent condtons outsde the buldng are not as mportant. Wthout any openngs, the room can be mantaned at comfortable thermal condtons. However for a naturally ventlated buldng, fluctuatng ambent temperature, humdty, wnd speed, and wnd drecton must be consdered [12-13]. When desgnng a naturally ventlated buldng layout of the buldng also must be measured. The sze and poston of nlet, heat sources, or obstacles are mportant ssues that affect the arflow nsde the buldng. Expermental nvestgaton s sutable to study practcal arflow patterns, but t s costly and lengthy. Moreover, t s not easy to study of the arflow patterns n a buldng n detals. An alternatve way to model natural ventlaton n a buldng s CFD, whch s a numercal analyss to solve the Naver-Stokes equatons and calculate the arflow wthn the buldng. In compare to expermental nvestgaton, CFD s cost-effectve and easy to nvestgate on the flow due to geometry change. So t s easy to use the powerful tool CFD to model natural ventlaton. MATHEMATICAL MODEL The problem s solved usng a set of equaton for compressble, unsteady flud flow wth standard k-ε model. The equaton for a general varable has the well-known form, t ) x ( u) x ( ) S x ( (1) Where, the varable substtutes velocty components u, v, w temperature T and knetc energy of turbulence k model of turbulence are as follows, k and ts rate of dsspaton. The equatons for x k ef ( uk) ( ) P PB e (2) x k x ( u) x x ef ( The major heat balance equaton for ths problem s, 2 ) C1( )( P C3PB ) C3( ) x k k (3) ( k. T ) Cp.. T C p. T Q t Where, k means heat conductvty, T s the room temperature, s densty, tme, Q s heat source. (4) MODEL DETAIL C p s specfc heat, s speed, t s In the present study, a typcal flat studo s modelled and smulated usng three dmensonal CFD and Heat Transfer. The net length s 7.5m, wdth s 5m and heght s 3m of the apartment. There s 1 ktchen, 1 bathroom and 1 lvng room as shown n FIGURE 1(a).The studo apartment space s flled wth ar and one burner n ktchen, one refrgerator and one Televson n lvng room as heat sources as shown n FIGURE 1.The studo apartment s employed wth three doors, two wndows n lvng room and one n ktchen of typcal sze and a vent n bathroom as shown n FIGURE 1. Every room has an open door to an nteror crculaton area, whch n our model s consdered as an open boundary, lke as [14]. Hgh resoluton second order accurate advecton scheme was used to dscretze the equatons for the flow, turbulent knetc energy, and turbulence eddy dsspaton. Kajtar et al [15], smulated dfferent cases usng FLOVENT by employng k-ε turbulence model. The CFD module s the platform for smulatng devces and systems that nvolve sophstcated flud flow and heat transfer models. Intally, 0 (zero) velocty s used nsde the flat. Here nlets are taken n ktchen, bathroom and lvng room (north sde wndow).

3 (a) (c) (d) FIGURE 1. (a) 3D Geometry of Studo Apartment model, 2D Geometry of Studo Apartment model wth detals, (c) 3D meshng of Studo Apartment model and (d) 2D meshng of Studo Apartment model Then, a complete combuston occurs n presence of ar nsde the flat. There are two outlets; one s lvng room s Southsde wndow and other s lvng room s door. Both outlets are open durng the smulaton. The buoyancy drven ar flow resultng from temperature dfference between the gas combuston and the surroundng ar was also accounted n the smulatons. An unstructured trangular shape grd s adopted for the entre doman as shown n FIGURE 1 (c) and (d) for dfferent cross-sectonal plans. An approprated attenton s gven n the combuston zone where gradent of the flow varables are expected to be sgnfcant. Snce the combuston s modeled and heat s convctng nto the space due to burnng, a very smooth grd transton from fner to coarser was mantaned so that the nformaton can be transmtted wth reasonable accuracy. The study does not take nto account radaton, humdty and external pressure. For smplcty, velocty and temperature are assumed accordng to the clmate condton of Bangladesh. In future, further study wll be performed usng radaton, humdty, pressure and acoustc wth dfferent postons and places. RESULT AND DISCUSSION In ths work natural ventlaton of the studo apartment s smulated. Numercal analyss based on fnte element method s used to solve three dmensonal unsteady flows n enclosed space and resultng temperature dstrbuton and ar velocty feld wth dfferent heat sources. In the followng fgure, t s vsble that the cold ar s pushed to the exteror face. Accordng to the present poston of the nlets, the temperature nsde the flat wll not be equally dstrbuted. Analyss shows strong nfluence of natural ventlaton and room geometry on thermal comfort. Accordng to the ANSI/ASHRAE Standard [16] the mean values of temperature and ar flow velocty

4 obtaned n ths study are satsfactory for ndoor ar qualty and thermal comfort. FIGURE 2 (a) and descrbe the temperature effect n an apartment due to natural ar flow for dfferent tmes. Velocty magntude s shown for 0.02 mnute and 1 mnute n FIGURE 3 (a) and. In FIGURE 4 (a) to (d), the pressure dstrbutons are depcted to show the nternal Temperature dstrbuton of dfferent tmes for sotherm s shown n FIGURE 5 (a) and. (a) FIGURE 2. (a) Temperature dstrbuton at tme t=0.01 mn and Temperature dstrbuton at tme t=01 mn (a) FIGURE 3. (a) Velocty profle at tme t=0.02 mn and Velocty profle at tme t=01 mn From the examned fgure t can be explaned that small volume of ar follows the ventlaton ar flow whle large volume of ar may behave dfferently and settle. Internal re-crculaton of such volumes nto the convectve heat source may ncrease ar exposures n the breathng zone. The contnuous settlng of volumes durng transport n the man flow drecton leads to an accumulaton of partcles that are brought nto the breathng zone by the convectve heat sources. The exposures are ncreased by dstance from supply wall wth dsplacement ventlaton. Due to the fact that the ar s not dstrbuted equally some volumes of ar n the room wll reman colder. Ventlaton system performance was ndcated well acceptng result [17] for ndoor ar velocty profle for ncdence angle of 0 o. In

5 TABLE1, nlet and outlet wnd velocty are shown for two cases A 1 (U = 1 m/s) and A 2 (U = 3 m/s) to valdate our numercal data only for velocty profle. (a) (c) (d) FIGURE 4. (a) Pressure dstrbuton at tme t=0 mn, Pressure dstrbuton at tme t=0.02 mn, (c) Pressure dstrbuton at tme t=0.5 mn and (d) Pressure dstrbuton at tme t=01 mn. TABLE 1 Case [17] Result (nlet wnd)/(outlet wnd) (nlet wnd)/(outlet wnd) A / /4.01 A / /3.73

6 (a) FIGURE 5. (a) Isothermal Contour at tme t=0.05 mn and Isothermal Contour at tme t=01 mn CONCLUSION Ventlaton system performance was nvestgated wth respect to a studo apartment room and can contnue wth multple scenaros was examned here. The results of the analyses provded the nformaton about the nternal ar movement nsde the studed studo apartment and sensble areas whch are not reached by the outer ar. Ths area may cause the form of problems regardng nhabtants comfort. Comfort and ndoor ar qualty are dependent on many factors, ncludng thermal regulaton, control of nternal and external sources of pollutants, supply of acceptable ar, removal of unacceptable ar, occupants actvtes and preferences, and proper operaton and mantenance of buldng systems. The lack of ventlaton n some area nsde the studo may create condton for a very unwanted phenomenon. Though the result of the study s satsfactory wth respect to ASHRAE Standard [17], results obtaned through ths study could not explan all the condtons and possbltes due to not consderng all the factors. At Dhaka n Bangladesh, the average ar velocty and temperature are 1.12 m/s and 25 C respectvely [6-7]. Usng ths natural resources, dependng on mechancal system can be reduced n Bangladesh. ACKNOWLEDGEMENT The authors wsh to acknowledge the techncal and fnancal support provded by the Bangladesh Unversty of Engneerng and Technology (BUET). REFERENCES [1] Energy Informaton Admnstraton, State Energy Data Report 1995, 3-7 tables, [2] Energy Consumpton Gude 19, Energy Effcency n Offces, Energy Effcency Offce/HMSO, London, [3] Bangladesh Power Development board, Annual Report , page-56. [4] Vladmr Gerlch, Modellng Of Heat Transfer In Buldngs, 25 th European Conference On Modellng And Smulaton, ESE-51. [5] R. Zhao, Y. Xa, Effectve non-sothermal and ntermttent ar movement n human thermal responses, Room vent 2 (1998) [6] M. Roungu Ahmmad, Statstcal Analyss of the Wnd Resources at the Importance for Energy Producton n Bangladesh, nternatonal Journal of u- and e- Servce, Scence and Technology,Vol.7, No.2 (2014), pp [7] Hossan Mohuddn, Md. Musfqur Rahman Bhuya and Mustofa Mahmud Al Mamun, An analyss of the temperature change of Dhaka cty, Proceedngs of 5 th Internatonal Conference on Envronmental Aspects of Bangladesh [ICEAB 2014], [8] J.F. Busch, A tale of two populatons: thermal comfort n ar condtoned and naturally ventlated offces n

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