Hybrid Ventilation in New and Retrofitted Office Buildings

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1 IEA INTERNATIONAL ENERGY AGENCY ENERGY CONSERVATION IN BUILDINGS AND COMMUNITY SYSTEMS Annex35 HybVent Hybrd Ventlatn n New and Retrftted Offce Buldngs Techncal Paper Presented at the Frst Internatnal One day Frum n Natural and Hybrd Ventlatn, HybVent Frum 99, 09/1999, Sydney, Australa Numercal Smulatn f Transent Effects f Wndw Openngs G. V. Fracastr, Dpartment d Energetca del Pltecnc d Trn, Italy E-mal: fracastr@plt.t Marc Pern, Dpartment d Energetca del Pltecnc d Trn, Italy E-mal: pern@athena.plt.t Ths techncal paper s nt an ffcal IEA-ECB&CS Annex 35 publcatn. The vews and judgements expressed are thse f the authrs and d nt necessarly reflect thse f Annex 35 r IEA-ECB&CS.

2 NUMERICAL SIMULATION OF TRANSIENT EFFECTS OF WINDOW OPENINGS G V Fracastr & M Pern Dept. f Energy Technlgy Pltecnc d Trn Crs Duca degl Abruzz, Trn, Italy fax: e-mal: fracastr@plt.t, pern@athena.plt.t ABSTRACT The smulatn f rm arng (ventlatn by means f dr/wndw penng) by means f CFD technques requres a specally sklled user, because a number f dffcultes arse snce the frst stage f smulatns develpment, when the user s asked t chse the calculatn dman and the tme step, and chces whch n prncple appear crrect may frequently lead t meanngless results. Ths wrk s centered n the D, transent analyss f a sngle sde enclsure where the ventlatn s nly due t temperature dfferences. Wnd effect has nt been taken nt cnsderatn. Dfferent runs have been perfrmed varyng: bundary cndtns, wndw szes and calculatn dmans. Feld mdel results have been cmpared t lumped parameter and zne mdel analyses. A check n cnservatn prncples has shwn that CFD results are affected by ntceable naccuraces fr what cncerns the predctn f bth ar temperature and ach s, whch may be partally vercme re-scalng the tme dependence f the phenmenn. 1 INTRODUCTION A detaled lterature revew develped durng the fact-fndng phase f the Annex 35-HybVent actvty has pnted ut the dffcultes that arse when CFD technque s used t smulate naturally ventlated systems. Ths specally apples t arng (ventlatn by means f dr/wndw penng) (Schaeln et al, 199, Elsayed, 1998), a smple actn whch prduces qualtatvely well knwn effects. Hwever, dffcultes start snce the frst stage f smulatns develpment, when the user has t chse the structure f the calculatn dman. An apparently reasnable chce may, n fact, lead t surprsngly meanngless results under the physcal pnt f vew (e g, cld ar enterng the rm thrugh the upper part f the wndw and warm ar extng frm belw), whle resduals values wuld suggest a successful smulatn. Furthermre, the use f smplfed mdels and equatns (see fr examples Etherdge et al., 1996, Andersen, 1996, ASHRAE Handbk f Fundamentals, 1997, Agnlett et al. 1981) s usually straghtfrward, but the user must prvde ne re mre emprcal ceffcents, whse value s nt always knwn a-prr and may depend n the type f the penng as well as the temperature dfference. Mrever, the phenmenn s, by ts nature, unsteady and hence the value f temperature dfference t be used n these frmulas has t be frecasted as an average between ntal and fnal cndtns. In ths frame t has been decded t develp a CFD mdel and nvestgate the pssblty t express the results n cncse terms makng use f nn dmensnal quanttes such as Grashf number.

3 MODELS FEATURES.1 CFD Mdels A tw dmensnal CFD transent analyss has been perfrmed fr a natural sngle sde ventlated enclsure usng a well knwn cmmercal sftware (FLUENT ). Only thermal effects have been taken nt accunt and therefre the wnd speed has been assumed equal t zer. In rder t avd dffcultes n the descrptn f the dman, the gemetry f the rm has been assumed very smple (see sketch belw). h T L 3 Tw dfferent CFD mdels have been mplemented. Bth have been dscretzed usng a nn unfrm grd made f 00 x 00 cells, but n the frst ne nly the ndr envrnment s ncluded (wndw 1), whle n all the ther mdels a strp m wde f utdr envrnment has been mdelled. A ttal f 5 runs have been perfrmed varyng wndw heght and temperature dfferences. Table 1 reprts the characterstcs f the smulatns. The ntal ar temperature has been always taken equal t 0 C. The same value has been adpted fr the wall temperature, cnsdered cnstant. Snce the phenmenn s dmnated by the buyancy effect, the Grashf number has been dentfed as the relevant ndependent varable. It has been calculated as: g β T H Gr = ν Where T = Temperature dfference between wall and utdrs and H = wndw heght. Table 1 Mdel features and bundary cndtns. Mdel Outdr env. Wndw heght [m] 3 T D T [ C] Grashf Number Wndw 1 Nt ncluded Wndw Included Wndw 3 Included Wndw 4 Included Wndw 5 Included The fllwng assumptns were adpted: Turbulence mdel: standard k-ε Interplatn scheme: pwer-law Wall functns: standard lg-law Transent analyss: varable tme steps (values frm 0.5 s up t 60 s). In the frst 0-30 s f smulatn, tme steps larger than 0.5 s lead t numercal nstablty. Number f teratns per tme-step: 1000 Ttal number f smulated tme-steps: abut 100 (equvalent t a tme span f abut 600 s, fr each f the smulated cnfguratns fr wndw and 3, and abut 150 s fr wndw 4 and 5). Cmputatnal tme: abut 3 weeks (fr each smulatn). Hardware: HP Apll 70 RISC WS (54 Mb RAM memry) The slutn phase s crtcal due t numercal nstablty prblems and requres partcular care. Mrever, as fllw frm the data lsted abve, t requres lng tme and resurces.. Engneerng Mdels A sngle-zne mdel has been develped based n the frmula reprted by ASHRAE (1997) cupled wth the cnservatn equatn fr energy:

4 h m& = ρ where the dscharge ceffcent C d s gven by: The tw-zne mdel s descrbed by the fllwng equatns T h ( ) ( ) 1 p1 Ap1 Tp1 T1 = m& cp T1 T + ρ1 V1 cv τ T h ( ) ( ) p Ap Tp T = m& cp T T1 + ρ V cv τ m& = A C p d A p ( T T ) = m& c ( T T ) p A C d ρ + ρ 1 g H ρ ρ β ρ + β ρ 1 = g H T p ( T T ) T T ρ + ρ V c The meanng f the symbls s shwn n Fgure 1 and n the fllwng lst: v T τ h T T 1 L m& A Fg. 1 Tw-zne mdel calculatn scheme. T (Y ) (X) H A = half wndw surface A p = wall surfaces h p = flm ceffcent c v, c p = heat capactes (cnstant vlume/pressure) T p = wall temperature T = ar temperature V = vlume, β = pressure lss ceff. Subscrpts,,1, = utdr, ndr, zne 1, The value f β 1 = β has been determned assumng the ntal ar flw rate t be equal t the ntal flw rate f the sngle-zne mdel. The relatn fr the ar mass flw rate has been derved ntegratng the energy cnservatn equatn alng the ar flw path frm utsde (X) t utsde (Y). It has been assumed that n ar shrt-cut takes place between the ndr ar exhausted frm the wndw and the utdr ar enterng the rm. Furthermre, fr the tw zne mdel the ar flws nly frm the lwer zne (1) and the upper zne (), wthut nternal recrculatn. These tw mdels have been slved numercally, dscretzng the ODE s and emplyng an explct tme ntegratn. The mdel s mplemented by means f a spread-sheet sftware (Excel ). N partcular prblems f numercal nstabltes have been encuntered. Hwever, n rder t acheve accurate slutns (wth precse energy and mass balances) qute small tme steps have t be used, specally n the frst part f the smulatn (0.1 s fr the frst 8 secnds, larger, and ncreasng, tme steps fr the fllwng tme). Furthermre, n the case f sngle zne mdel, Smulnk (a MatLab tlbx fr dynamc system smulatn) was als used fr the mdel smulatn. Ths tl, n fact, mght shw tself extremely useful n the HybVent system analyss, as t allws an easy cuplng f dfferent phenmena calculatng quanttes such as flw rates, pllutant cncentratn, temperatures, and ntrducng als the cntrl strateges. At ths stage f develpment nly the flw rate mdel has been mplemented. In all the tested cases, the cmputatnal tme requred fr the slutn (usng a PC Pentum) s less than 1 s. 4

5 3 RESULTS The use f engneerng mdels s smple and straghtfrward. The nly nnvatn ntrduced by the authrs cnssts n the unsteady state applcatn f frmulas expressng the ar mass flw rate, m&. The ar flw rate, n fact, s evaluated at each ntegratn tme step, adptng fr the calculatn the prevus value f ar temperature. The prfles f ach s and ar mean temperature versus tme (nt shwn here fr brevty) btaned by means f sngle- and tw-zne mdels 1 are qute smlar. The use f ther frmulas (Etherdge, 1996) fr m& has prduced lttle dfferences n the fnal results. In the same way results btaned by means f Excel sftware are dentcal t thse btaned adptng the Smulnk mdel. Table resumes the ndr ar mean temperature and the ach s values when steady state cndtns are practcally reached (and the crrespndng tme requred) determned by means f engneerng mdels. Fr what cncerns the CFD analyss, as mentned n the ntrductn, the chce f the gemetrcal dman has revealed t have a paramunt effect n the relablty f the results. Actually, the results f mdel wndw 1, fr whch the utdr envrnment has been mdelled by means f prper bundary cndtns (fxed pressure bundares) are cmpletely meanngless. After a few secnds when the ar, as expected, flws frm utdrs t ndrs n the lwer part f the wndw and vce versa n the upper part (althugh the neutral level appears t be strkngly lw), there s an nversn f the ar path and the warm ar starts t flw frm ndrs t utdrs n the lwer area f the wndw, cntradctng the cmmn experence. Ths smple example s a further evdence that CFD analyses chces whch appear straghtfrward (specally fr nn expert users) may lead t wrng cnclusns, even when the numercal ndcatrs (resduals) assume satsfactry results. Beng meanngless, the results related t mdel wndw 1 wll nt be ncluded n the fllwng fgures. Results related t mdels wndw thrugh 5, are cnsstent wth the expected ar flw behavur. The analyss f ar velcty and mass flw rate prfles alng the wndw heght shws qute symmetrcal trends that develp snce the frst secnds f smulatn. In the frst tme steps the prfle s slghtly rregular (and the glbal mass balance f the rms s nt perfectly satsfed), but after abut secnds the curves are smth and the mass balance s, practcally, perfect. Fgure shws fr the mdel wndw 4, as an example, the prfles f mass flw rate acrss the wndws at dfferent tme steps. In fgure 3 the ar changes per hur are pltted versus tme fr the dfferent mdels. It must be underlned that the adpted mdel s -D, thus the rm depth s assumed t be equal t 1 m. Cnsequently, the rm vlume V, adpted fr the ach s calculatn, s: V = 4. m x.7 m x 1m = m 3. Frm a physcal pnt f vew the system behaves as f t were an L x h enclsure f nfnte wdth wth a cntnuus strp wndw. The results are strctly applcable nly t ths type f wndw, but they may prbably be extended t ther cnfguratns whether the edge effects f the wndw sdes are neglgble ( e, nt t hgh values f rat heght t wdth). The effect f wndw t rm wdth rat s nt knwn a prr and shuld be nvestgated. In rder t btan the actual value f ach related t a partcular wndw and rm wdth, ne must multply the values shwn n the fllwng fgures by the rat f wndw wdth t rm wdth. Fgure 4 shws the ar temperature (rm mean value) prfle versus tme. In these charts 8 curves are pltted: fur refer t CFD results, fur refer t the tw-zne mdel smulatns. It s pssble t see that there are large dscrepances between the results btaned by means f the tw dfferent classes f mdels. The analyss f the energy balance at each tme step has revealed qute large errrs n the case f CFD mdels. Ths appears unexpected; n fact, durng the slutn phase f all the mdels the resduals related t enthalpy were qute lw. In fgure 5 s pssble t see the entty f pwer mbalance a t varus tme steps (curves wth symbls refer t the man axs n the left 1 In the tw-zne mdel the rm mean temperature s determned as the smple average between zne 1 and ar temperatures. 5

6 Heght [m] t = 1.5 s t = 3.5 s t = 15 s t = 3 s t = 40 s t = 60 s t = 300 s E-0-8.0E E E E E+00.0E E E E E-0 Mass flw rate [kg/s] Fgure Temperature prfle alng the wndw heght fr dfferent tme steps - CFD results Wndw Wnd. - h = 1.5 m - DT = 0 C Wnd. 3 - h = 1.89 m - DT = 10 C Wnd. 4 - h = 1.5 m - DT = 10 C Wnd. 5 - h = 1.5 m - DT = 5 C ach's [1/h] Tme [s] Fgure 3 CFD results Tme hstry f ar change rate. (Pwer mbalance ) H& f the chart) and the relatve errr defned as: E = 100 (symbls - H& refer t the secndary axs n the rght f the chart). An ff-lne study f each term f the energy balance equatn, perfrmed by means f smplfed analytcal calculatns, has pnted ut that the cmpnents f the energy balance due t heat fluxes exchanged between walls and ar and t enthalpy fluxes are apparently well predcted by the cde. Instead, the tme varatns f nternal energy ( e, temperature) seem t be largely underestmated. Everythng happens as f the tme step ntervals adpted fr the 6

7 numercal slutn prcedure d nt cncde wth the actual tme scale f the phenmenn. Startng frm these remarks t has been decded t re-scale all the CFD numercal results n the bass f tme steps, τ r, btaned mpsng the energy balance at each tme step: ρ V c v T τr =. Q& H& Fgures 6 t 9 shw the results f such a prcedure n terms f ach s and mean ar temperature versus tme. It s pssble t see a general substantal mprvement n the predctns, wth a gd agreement wth prfles calculated by means f znal mdels, partcularly fr what cncerns wndw, 4 and 5 mdels. In the case f wndw 3 mdel, nstead, the perfrmances f the prcedure seems t be wrst. Hwever, n ths last smulatn substantal re-crculatn f warm exhaust ar wth fresh enterng utdr ar ccurs. Due t ths mxng, the ar actually enterng the enclsure has a temperature slghtly hgher than the utdr ar. Ths phenmenn s clearly shwn by the CFD smulatn temperature felds f whch fgure 10 s an example (fg. 10a wndw mdel, fg.10b wndw 3 mdel. Tme: abut s after the wndw penng). (a) (b) Fgure 10 Temperature feld after s and man ar flw paths Mdel wndw and wndw 3 The lwer temperature dfferences between ndr and utdr nduce a smaller ar flw rate. It fllws that the tme prfle f ar mean temperature predcted by CFD calculatn dffers frm that btaned by means f the tw-zne mdel. Frm a physcal pnt f vew ths behavr s prbably lnked t the fact that n wndw 3 mdel (that frm a theretcal pnt f vew shuld present the same ach s f wndw, snce bth cases have the same Gr number), the ar velctes thrugh the larger wndw are qute lw and are nfluenced by the great clckwse vrtex that takes place nsde the rm due t ndr thermal gradents (whle fr wndw the ntal vrtex s destryed by the strnger ar current that flws n and ut the rm). Such knd f phenmenn culd never be predcted by znal mdels, as they assume a prr n re-crculatn and mxng. Table Steady state values zne mdels Mdel n [1/h] T [ C] n [1/h] T [ C] t [s] Sngle zne Tw zne Wndw Wndw Wndw

8 Wnd. - h = 1.5 m - DT = 0 C - CFD Wnd. - Thery - zne Wnd. 3 - h = 1.89 m - DT = 10 C - CFD Wnd. 3 - Thery - zne Wnd. 4 - h = 1.5 m - DT = 10 C - CFD Wnd. 4 - Thery - zne Wnd. 5 - h = 1.5 m - DT = 5 C - CFD Wnd. 5 - Thery - zne T [ C] Tme [s] Wndw Wndw -Imbalance Wndw 3-Imbalance Wndw 4-Imbalance Wndw 5-Imbalance Wndw -Errr perc. Wndw 3-Errr perc. Wndw 4-Errr perc. Wndw 5-Errr perc Imbalance [W] Errr [%] Tme step Fgure 4 Ar temperature (rm mean values) versus tme (CFD and -zne mdel). Fgure 5 Pwer mbalance and relatve errrs f CFD smulatns. 8

9 Wnd. - h = 1.5 m - DT = 0 C - CFD Wnd. - Thery - zne Wnd. - CFD re-scaled Wnd. 3 - h = 1.89 m - DT = 10 C - CFD Wnd. 3 - Thery - zne Wnd. 3 - CFD re-scaled T [ C] Fgure 6 Ar temperature (rm mean value) versus tme (CFD, -zne and re-scaled values) Mdels Wndw and Tme [s] Tar [ C] Wnd. 4 - h = 1.5 m - DT = 10 C - CFD Wnd. 4 - Thery - zne Wnd. 4 - CFD re-scaled Wnd. 5 - h = 1.5 m - DT = 5 C - CFD Wnd. 5 - Thery - zne Wnd. 5 - CFD re-scaled Tme [s] Fgure 7 Ar temperature (rm mean value) versus Tme (CFD, -zne and rescaled values) Mdels Wndw 4 and 5. 9

10 Wnd. - h = 1.5 m - DT = 0 C - CFD Wnd. - Thery - zne Wnd. - CFD re-scaled Wnd. 3 - h = 1.89 m - DT = 10 C - CFD Wnd.3 - CFD re-scaled Wnd. 3 - Thery - zne ach's [1/h] Tme [s] Fgure 8 Ar changes per hur versus Tme (CFD, -zne and re-scaled values) Mdels Wndw and 3. ach's [1/h] Wnd. 4 - h = 1.5 m - DT = 10 C - CFD Wnd. 4 - Thery - zne Wnd.4 - CFD re-scaled Wnd. 5 - h = 1.5 m - DT = 5 C - CFD Wnd. 5 - Thery - zne Wnd.5 - CFD re-scaled Tme [s] Fgure 9 Ar changes per hur versus Tme (CFD, -zne and re-scaled values) Mdels Wndw 4 and 5. 10

11 40 4 CONCLUSIONS ach (1/h) E+00.E+09 4.E+09 6.E+09 8.E+09 1.E+10 1.E+10 1.E+10 Grashf Fg. 11 Steady-state ach s vs. Grashf number. ach (1-zne) ach (-zne) The wrk s, actually, stll n prgress. The am f the paper was t verfy the applcablty f CFD analyss t a smple, yet physcally cmplex phenmenn such as the cnsequences f penng a wndw under the mere effect f a temperature dfference between ndrs and utdrs. It has been shwn that CFD shuld be used very carefully, wth a sutable chce f the calculatn dman. Furthermre, there s an apparent cntradctn between the calculatn tme scale and energy cnservatn prncples. Ths cntradctn has been slved re-scalng the tme hstry by frcng the slutn t cmply wth the energy balance. Smplfed analytcal mdels have als been develped, and ther results have been cmpared t the CFD mdel results. After the tme re-scalng, there s a far agreement between CFD and engneerng mdels, except fr wndw 3, fr whch the CFD analyss revealed a certan degree f utdr-ndr ar recrculatn. A relatnshp between ar changes per hur at steady state cndtns and Grashf number has been derved (see fgure 11). Bth curves shw a defnte functnal dependency upn Grashf number and culd be used fr frst attempt predctn. 5 REFERENCES Agnlett L., E. Grava, La ventlazne naturale degl ambent, Cndznament dell Ara, ttbre Andersen K.T., Desgn f natural ventlatn by thermal buyancy thery, pssbltes and lmtatns, 5 th Int. Cnf. On ar dstrbutn n rms, ROOMVENT 96, July 1996, Ykhama. ASHRAE, Handbk f Fundamentals, Ch. 5, Butera F., G. Cannstrar, M. Yaghub, A. Laurtan, Benessere termc e ventlazne naturale negl edfc, HTE Energe Alternatve, ann 11, n. 59, magg-gugn Elsayed M., Infltratn Lad n Cld Rms, HVAC&R Research, Vl. 4 N., Aprl Etherdge D., M. Sandberg, Buldng ventlatn Thery and Measurements, Jhn Wley & sns, Chchester, 1996, pp , Schaeln, A., Van der Maas J., Mser, A., Smulatn f ar flw thrugh large penngs n buldngs, ASHRAE Transactns, part, 199. ACKNOWLEDGMENTS The research actvty presented n ths wrk has been develped n the frame f Indr Envrnment Engneerng a prject c-funded by the Italan Mnstry f Unversty and Reasearch (MURST). The partcpatn f the authrs t IEA Annex 35 s supprted by ENEA Ente Naznale Energe Alternatve Italy. 11