Proceedings: Building Simulaion 007 SUDY ON HE AIR MOVEMEN CHARACER IN SOLAR WALL SYSEM Y Li, X Duanmu, Y Sun, J Li and H Jia College of Archiecure and Civil Engineering, Beijing Universiy of echnology, Beijing, 1000, China ABSRAC A mahemaical model for simulaing airflow in solar channel of he insulaed rombe solar sysem is proposed. I is assumed he glazing is isohermal and he solar hea absorbed by he is ransferred o he air in he channel wih a consan flux by naural convecion. he mass, momenum and energy conservaion equaions are discreized and solved using he finie difference conrol volume mehod. An experimenal sudy of solar chimney was used o validae he proposed mahemaical model. he differences beween he prediced resuls of airflow rae in solar and hose of measuremen daa are less han 3.0% when he widh of solar is 0.m and emperaure is lower han 50. When he is 0.3m wide, heses differences are lower han 5.0% is he emperaure less han 50. Flow and emperaure fields are produced and he resuls are presened in erms of emperaure and velociy disribuion in various pars of sysem. he resuls show ha he solar hea gain and channel widh are wo imporan parameers affecing he air flow paern and hea ransfer. Furher experimenal wor is needed o refine he model. KEYWORDS Solar, Building, Naural convecion, Modeling INRODUCION wo ways where solar energy is common used for buildings are solar chimney and solar. A solar chimney is one way in which one or more s of verical chimney are made ransparen by providing glazed s. Solar chimney is designed o provide venilaion o he building during he day and is locaed on he op of building. I plays an imporan role in providing a hermally suiable environmen for human comfor in under-developed counries by providing naural venilaion in dwellings. Solar chimney is similar o he classical rombe solar concep (Ormison e al 1987),. he disinc beween hem is ha in solar chimney is assumed o have negligible mass while he rombe has a massive hermal bul ha absorbs solar energy and recirculaes warm air for passive heaing of he building. here has been exensive use of solar energy for heaing buildings by means of sorage s since he wors of rombe were published. A comparaively sudy of four differen configuraions of solar was finished and repored heir main advanages and disadvanages (Zalewsi e al. 00). he sandard rombe has he drawbac of low hermal resisance which leads o significan losses a nigh-ime of during periods wih o sun. A composie solar, or a rombe-michel concep, an insulaed rombe ec. have been pu forward in order o avoid he shorcoming of rombe (Zriem and Bilgen, 1987) here are wo major caegories in analyzing he solar sysem. Firs, he naural convecion beween wo parallel plaes was concerned when simulaing he solar chimney; Second, he hea ransfer in he enire sysem including he solar collecor and he adjacen room was simulaed. A.66m high rombe wih air gaps varying from 0.10 o 0.35m were sudied by flow visualizaion echnology (Abarzaden 198). I was concluded ha he mode of hea ransfer resembled ha of urbulen free convecion beween wo single plaes and suggesed ha 0.5 m was an opimum gap. Experimenal ess were carried ou on a 1:1 small-scale model of he prooype of a solar. he experimenal resuls were hen used o validae a wo-dimensional laminar flow simulaion model. he hermal resiance newors mehods and empirical formula are used in his sudy. bu i was envisaged ha flow predicions could be improved upon by aing accoun of urbulence and hree-dimensional ecs as well as employing appropriae boundary condiions (Ong 003). he objecive of he presen sudy is o apply he echnique of compuaional fluid dynamics (CFD) o simulaing air flow and hea ransfer in he solar channel of solar sysem. A CFD program was firs validaed agains experimenal daa for an isohermally heaed chimney. Numerical invesigaion was hen carried ou ino he performance of he glazed solar channel. Effecs of geomeric parameers such as he channel widh, solar hea gain are examined. MAHEMAICAL MODEL he Schemaic diagram of an insulaed sysem solar sysem considered in he presen paper is illusraed in Figure 1(Zalewsi e al. 00). he solar comprises a ransparen ouer cover i.e. glazing, a sorage, a venilaed air layer, and - 97 -
Proceedings: Building Simulaion 007 finally an insulaion layer. wo vens have been drilled in massive s. An insulaion layer is fi on he bac of massive in order o increase he hermal resisance of he rombe. his insulaion layer blocs off virually all he supply in summerime. I is herefore no longer essenial o fi a solar shield. his solar wors as follows: he sorage absorbs par of he solar energy. Solar energy heas up he air inside he chimney. As a resul of he difference in air densiy beween he op and boom of he chimney a naural convecion airflow is hermally induced. In his solar sysem, nearly all he energy are ransferred o he air by convecion. Moreover, he energy colleced could be rapidly direced ino he room by he air layer (shor ime lag). When blocing off he circulaion of he air, he supply is also sopped, hereby overheaing could be avoided. radiaion is no aen as source erm. he ec is considered as giving he hea flux of surface. able 1: ranspor equaions for variable Φ in he flow field Figure 1. Equaion Φ ΓΦ Coninuiy 1 0 0 U Momenum u S Φ p u + v + + z w v Momenum v p + v + + + g ρ ρ ) ( 0 u z w w Momenum w p u + z z v + + z z w z Figure1. Schemaic diagram of insulaed rombe o sudy he naural convecion in solar chimney, he flow is assumed o be seady, urbulence and hreedimensional. he Boussinesq approximaion is used o accoun for he densiy variaion. Applicaions of CFD, based on solving a se of hree-dimensional equaions derived from conservaion laws on mass, momenum and energy are found o be suied for simulaing naural convecion and venilaion. he predicion of airflow in he presen solar channel is based on he soluion of general ranspor equaion (Pananar 1980): r div( V Φ ) = div Γ grad( Φ ) + S (1) [ ] ρ Φ Φ where Φ denoes he dependen variable, ρis he air densiy, ΓΦ is he diffusion coicien for variable Φ and SΦ is source erms for variable Φ. ΓΦ and S Φ are given in able 1(ao 001). As for energy equaion, he solar emperaure Kineic energy Dissipaion rae Pr + σ σ 0 G-ρ = + = + ρc / σ ( C1G C ρ ) C = 0.09, C 1 = 1. 44, C = 1. 9, σ =1. 0, σ = 1.3, σ = 0. 7 p he generaion erm G is defined as follows: G = G + G B u = v w u v + + + + u w v w + + + + + gβ z z σ () - 98 -
Proceedings: Building Simulaion 007 he firs erm on he righ side is shear producion. Since buoyancy would play an imporan role in he rising airflow, he producion of urbulence due o buoyancy and he ec of hermal sraificaion on he urbulence dissipaion rae are included by he second erm on he righ side. β is he coicien of hermal expansion he finie volume mehod is used o solve he imeaveraged Navier-Soe equaions wih a non-uniform newor. he diffusion erms and oher gradiens are discreized using he second-order cenral difference approximaion. he firs-order upwind scheme is employed for he convecive erms. Sandard funcions are used for he enclosure s. Boundary condiions In order o saisfy overall conservaion of mass, he velociy componens are exrapolaed from upsream nodal poins and hen adjused o he desired airflow raes. A he oule, a consan mass flux is applied. A he inle air emperaure is assumed equal o he room air emperaure and assumed consan. Warm air leaves he channel a he exi and flows bac room. emperaure a he surfaces of glass is assumed o eep isohermal. Resisance o flow due o fricion along he surfaces is assumed negligible. he hea flux on he surface of massive is assumed consan. Non-slip condiion is used for surface velociies. reasonable simulaing ime consumpion and can ge good accuracy resul. he soluion domain is discreized by using a non-uniform mesh wih smaller grid spacing near he s and larger spacing in he inerior, which allows he hydrodynamic and hermal boundary layer o be resolved wihou an excess of nodes. RESULS AND DISCUSSIONS Figure 3. Comparison beween he prediced resuls of mass flow rae and experimenal daa Validaion of program is performed by comparing he prediced resuls wih experimenal daa for naural convecion in a solar chimney (Bouchair 1994). he chimney was m high and of variable widh. All he surface were isohermally heaed by elecrical heaer o emperaures from 30 o 60. he inle air emperaure was conrolled a 0. Paerns of air movemen were measured wih he use of smoe. Deail of he experimenal measuremen and resuls were given by Bouchair(1994). In he experimenal sudy of solar chimney, he air flow rae hrough a solar chimney is given by Q = C d A ΔP ρ (3) Figure. he configuraion of solar channel Figure shows he configuraion of solar channel in he insulaed rombe solar sysem. I is.4 m all and of variable widh H. he lengh in z direcion is 1.0 m. he inle and oule openings are 0. m high and 1.0 m wide. he glazing is assumed o face souhward. he res s are insulaed bricwor. he calculaed region is divided ino 30, 40 and 0 compuaion cells along x, y and z direcion respecively. he grid independen for CFD are carried ou and i is found his grid sysem is Where Q is volume flow rae (m 3 /s), C d is he discharge coicien. A is he inle opening area (m ), and ΔP is he driving pressure due o buoyancy and wind ecs. he measuremen daa of airflow rae in he condiions where wind ec is no considered are used o validae he proposed model. he prediced and measured mass flow raes per uni lengh of he chimney for channels wih widh of 0. and 0.3 m are shown in Figure 3. he inle heigh of solar chimney is 0.1m. he differences beween he prediced resuls of airflow rae in solar and hose of measuremen daa are less han 3.0% when he widh of solar is 0.m and emperaure is lower han 50. When he is 0.3m wide, - 99 -
Proceedings: Building Simulaion 007 heses differences are lower han 5.0% is he emperaure less han 50. his gives confidence in using he compuer code o sudy he airflow and hea ransfer in solar caviies in his sudy. Since he inle air velociy is given in his sudy, i is imporan o calculae he quaniy of hea ransferred o airflow in channel. he useful hea ransfer o moving air sream can be wrien as (Ong 003): ( ) mc f f f, i q& = & (4) γhl where q& is hea flux ransferred o moving air sream. γ is a consan in mean emperaure approximaion, Hirunlabh e al oo a value of γ = 0.75 form heir experimenal observaion, m& is mass flow rae, c f is air specific hea, L is secion lengh a z direcion and is 1 m. f is mean air emperaure which can be approximaed from experimenal observaion. γ + ( 1 γ ) f f, i f, o where f, i and f, o = (5) are emperaures of air a inle and oule respecively. Figure 4. Quaniy of hea ransfer o air in channel In his paper, he inle air emperaure is 0. he glazing inner surface emperaure is 5. he inle air velociy, u, is 0. m/s. he solar hea gain is calculaed from he mean oal solar irradiance and mean solar gain facor. For example, he mean solar irradiance on a verical souh surface is 400 W/m, as for a verical wih double glazing whose mean solar gain facor is 0.64 (Hirunlabh e al 1999), he corresponding solar hea gain is 56 W/m. For he, only glazing facing side has a consan hea flux he oher side is aen as adiabaic. wo channels wih widh of 0. and 0.3 m are sudied. For he same channel, wo condiions where q& are 80 and 140 W/m are calculaed respecively. he radiaion beween he bac and glass is considered as he six-flux hermal radiaion model is seleced in CFD program. he quaniies of hea ransfer o moving air in he channel under differen condiions are illusraed in Figure 4. I can be seen ha hea ransfer quaniy is 0.475 W for q& = 80 W/m while i is 0.30 W for q& = 140 W/m when he channel widh is 0. m. he higher he massive hea flux, more hea would be ransferred o moving air. A low hea flux, more hea ransfer quaniy would be reduced for he same decrease degree of hea flux for he same channel condiions. he large reducion for lower hea flux is parly due o he flow reversal near he glazing. he ec of channel widh on hea ransfer quaniy o air is also illusraed in Figure 4. I can be seen for he same hea flux, he hea ransfer quaniy in narrow channel is larger han in channel of larger widh. he reason is ha he air mass flux is consan and he air velociy in narrow channel is higher. he lower velociy would resul in a lower hea ransfer coicien for he convecion on massive surface. However, here should be an opimum channel widh because he very narrow channel could lead o he plug flow where he mass flow rae m& in Eq.4 is very small and i would influence he hea ransfer. Figures 5 shows he flow paern in wo solar channels. For he 0.3 m wide solar channel i can be seen clearly ha here is circulae flow a he op of solar channel his means here is a larger air rising velociy in his condiion. As for he channel of 0. m wide, here is a higher velociy a he cross secion compared wih ha of 0.3 m wide channel wih he same inle air velociy. he reason is ha a larger airflow velociy is needed for passing he same mass flow rare wih a narrower passage. he emperaure field in wo channels wih 80 W/m hea flux are illusraed in Figure 6. I can be seen ha emperaure of air near massive is higher han hose of air in he channel cenre and ha near glazing. he highes air emperaure for 0.3m wide channel is 40 while he value is 44 in he 0. wide channel. his is because he higher airflow velociy leads o a lager quaniy of hea ransfer and hereby a higher emperaure near he massive surface. - 930 -
Proceedings: Building Simulaion 007 ACKNOWLEDGEMENS his paper is suppored by Funding Projec for Academic Human Resources Developmen in Insiuions of Higher Learning Under he Jurisdicion of Beijing Municipaliy (a) H = 0.3 m (b) H = 0. m Figure 5 Airflow paern in solar channels REFERENCES Abarzaden A, Charers WW & Lesslie DA. 198. hermocirculaion characerisics of a rombe Wall passive es cell, Solar Energy. 8(6):461-468. Bouchair, A, 1994. Solar chimney for promoing cooling venilaion in souh Algeria, Building Service Engineering Research and echnology, 15(1):81-93. Hirunlabh, J, Kongduang, W, Nampraai, J & Khedari J. 1999. Sudy of Naural Venilaion of House by a Meallic Solar Wall Under ropic Climae. Renewable Energy. 18:109-119.. Ong, KS. 003. A mahemaical model of a solar chimney, Renewable Energy, l3: 1047-1060. Ormison, S.J, Raihby, D.D & Hollands K.G. 1986. Numerical Predicion of Naural Convecion in rombe Wall Sysem. Inernaional Journal of Hea and Mass ransfer. 9:869-877. Pananar SV. 1980, Numerical Hea ransfer and Fluid Flow, Hemisphere McGraw-Hill. ao WQ. 001. Numerical Hea ransfer, nd Ediion, Xi an Jiaoong Universiy Press. Zalewsi L, Lassue S, Duhoi B & Buez M. 00. Sudy of Solar Walls-Validaing a Simulaion Model, Building and Environmen, 37():109-11. Zriem Z, Bilgen E. 1987. heoreical Sudy of a Composie rombe-michanel Wall Solar Collecor Sysem, Solar Energy, 39(6):409-419. (a) H = 0.3 m (b) H = 0. m Figure 6. emperaure field in solar channels CONCLUSION A mahemaical model for solar chimney is proposed and an experimenal sudy of solar chimney is used o validae he proposed mahemaical model. A good agreemen is found beween he prediced resuls of airflow rae and hose of measuremen in he differen condiions. he prediced hea ransfer rae increases wih channel and massive surface hea flux. he performance of a glazed solar chimney is influenced by he channel widh as well as solar hea gains. he presen sudy has shown ha he compuer program developed can be used for he predicion of buoyan airflow in solar channel. - 931 -