EFFECTS OF WALL ORIENTATION AND THERMAL INSULATION ON TIME LAG AND DECREMENT FACTOR

Size: px

Start display at page:

Download "EFFECTS OF WALL ORIENTATION AND THERMAL INSULATION ON TIME LAG AND DECREMENT FACTOR"

Felix Warren
5 years ago
Views:

1 HEFAT212 9 th Internatinal Cnference n Heat Transfer, Fluid Mechanics and Thermdynamics July 212 Malta EFFECTS OF WA ORIENTATION AND THERMA INSUATION ON TIME AG AND DECREMENT FACTOR Ozel M.* and Ozel C. *Authr fr crrespndence Department f Mechanical Engineering, University f Firat, Elazığ, 23279, Turkey, mzel@firat.edu.tr ABSTRACT decrement factr is investigated numerically using an implicit finite difference methd under steady peridic cnditins. The investigatin is carried ut fr three different insulatin materials in the climatic cnditins f Istanbul, Turkey. Fr this purpse, the utside surface f the wall is expsed t peridic slar radiatin and utdr envirnmental temperature. The inside surface is expsed t rm air maintained at cnstant indr design temperature. The insulatin is placed at utside f wall. It is seen that as expected, as the insulatin thickness increases, decrement factr decreases while time lag increases. Results shw that wall rientatin has a great effect n time lag while it has a small effect n decrement factr. It is seen that maximum time lag and minimum decrement factr are btained in an east riented wall. INTRODUCTION Building energy requirement fr the winter and summer seasns cnstitutes lss r gain thrugh windws, walls, rfs, infiltratin and equipment. In many buildings, walls and rfs have very significant fractin f heat gains fr the heating r cling lads. Accurate determinatin f the heat gain thrugh the walls and rfs f a building is very imprtant in rder t select a suitable air cnditining system fr the efficient utilizatin f energy [1]. Climatisatin f buildings culd be managed by passive and active slar systems. Active systems need heat transfer and stre fluids, cntrl and transfer elements in rder t cllect and stre slar energy [2]. In passive systems, building walls are used as thermal strage elements, and energy stred in the walls during the day can be used fr heating the building at night. Further, a decrease in the indr temperatures can be reduced when the heating system is switched ff [3, 4]. There are tw parameters t evaluate the thermal perfrmance f the wall, which are the time lag and the decrement factr, respectively. The time it takes fr the temperature wave t prpagate frm the uter surface t the inner surface is named as time lag and the decreasing rati f its temperature amplitude is named as decrement factr. In general, if the time lag f the wall is high and the decrement factr f the wall is lw, the fluctuatin f the indr air temperature will be small, and the thermal cmfrt f the rm will be enhanced [5]. NOMENCATURE a [-] slar absrptivity f utdr surface f wall c [J/kg K] specific heat h [W/m 2 K] heat transfer cefficient I T [W/m 2 ] incident ttal slar radiatin fr vertical surfaces I b [W/m 2 ] beam slar radiatins n the hrizntal surface I d [W/m 2 ] diffuse slar radiatins n the hrizntal surface I [W/m 2 ] ttal slar radiatins n the hrizntal surface k [W/m K] thermal cnductivity q i [W/m 2 ] heat flux at indr surface f the wall t [s] time T [ C] temperature Special characters α [m 2 /s] thermal diffusivity δ [deg.] declinatin angle φ [deg.] latitude Ф [h] time lag F [-] decrement factr γ [deg.] surface azimuth angle ω [deg.] hur angle ρ [kg/m 3 ] density Subscripts i max min x x inside ayer number maximum minimum utside indr surface utdr surface Asan [6] investigated the ptimum insulatin psitin fr six different cnfiguratins frm maximum time lag and minimum decrement factr pint f view. Ozel and Pihtili [7] investigated ptimum lcatin and distributin f insulatin layers pint f view maximum time lag and minimum decrement factr fr 68

2 varius wall rientatins. Yumrutaş et al. [8] develped a theretical methdlgy t find ttal equivalent temperature difference values based n time lag and decrement factr. Fr this purpse, ne-dimensinal transient heat transfer prblem fr multilayer flat rfs and walls f buildings was slved by cmplex finite Furier transfrm (CFFT) technique. Kntelen and Bikas [9] investigated the effect f utdr absrptin cefficient f an paque wall n time lag, decrement factr and temperature variatins by emplying a dynamic thermalnetwrk mdel. Al-Sanea et al. [1] investigated effects f thermal mass n transmissin lads, energy strage rate, dynamic thermal resistance, time lag, and decrement factr in building walls fr same nminal resistance value. In anther study, the effect f wall rientatin and exterir surface slar absrptivity n time lag and decrement factr fr several insulated wall cnfiguratins was investigated by Kntlen and Eumrfpulu [11]. In literature, studies dealing with effect f wall rientatin n them are in a limited number althugh there are many studies related t time lag and decrement factr. In this study, effect f wall rientatin and thermal insulatin n time lag and decrement factr is investigated numerically fr three different insulatin materials under climatic cnditins f Istanbul, Turkey. The results are cmpared with similar studies under different climatic cnditins and different insulatin materials MATHEMATICA FORMUATION AND CACUATION PROCEDURE A cmpsite wall structure cnsisting f 2 cm external plaster, insulatin material, 2 cm brick blck and 2 cm internal plaster is shwn schematically in Figure 1. The utside surface f the wall is expsed t peridic slar radiatin and utdr envirnmental temperature while the inside surface is expsed t rm air maintained at cnstant indr design temperature. q (t) Figure 1 Cmpsite wall structure The transient ne-dimensinal heat cnductin equatin thrugh the cmpsite wall may be written as: 2 T x 2 I T Outdr T (t), h 2 cm Plaster 1 α T t i Insulatin x 2 cm Cncrete 2 cm Plaster Indr T i, h i q i (t) (1) where α ( k /( ρ c ) is the thermal diffusivity f the th layer. ρ, c, k and T are the density, the specific heat, the thermal cnductivity and the temperature f the th layer, respectively. As initial cnditin, an arbitrary unifrm temperature field is assumed. The bundary cnditins at the utdr and indr wall surfaces are as fllws, respectively: k p T x x h ( Te( t) Tx ) (2) T hi ( Tx Ti ) x x (3) k p where h and h i are the cmbined (cnvective and radiative) heat-transfer cefficients at the utdr and the indr wall surfaces, respectively. k p is thermal cnductivity f plaster. T i is the indr air temperature. T e is the sl-air temperature including the effect f slar radiatin n the utdr temperatures and is expressed as fllws [12]: a IT T e T + (4) h where T is the utdr air temperature. I T and a dente the ttal slar radiatin and slar absrbtivity f the utdr wall surface, respectively. The ttal slar radiatin (I T ) fr vertical surfaces is calculated as: I T Rb I b + ( I d + I ρ g ) / 2 (5) Where I b, I d and I are beam, diffuse and ttal slar radiatins n the hrizntal surface. ρ g is grund reflectance. The gemetric factr R b is the rati f beam radiatin n the tilted surface t that n a hrizntal surface at any time and fr vertical walls surfaces is calculated as: R b cs δ sin φcs γ cs ω + cs δ sin γ sin ω sin δ cs φcs γ cs φ cs δ cs ω + sin φ sin δ (6) where δ, φ, ω and γ are declinatin angle, latitude angle, hur angle and surface azimuth angle, respectively. γ is zer fr an inclined plane facing suth. It is taken as negative frm the suth t the east, and the nrth, and psitive frm suth t west, and the nrth, i.e. 18 < γ < +18. Detailed calculatin prcedures are given by Duffie and Beckman [13]. The transient heat cnductin prblem has been previusly slved by emplying implicit finite-difference methd, and detailed calculatin prcedures are given in reference [7]. The numerical slutin is carried thrugh a number f cycles until a steady peridic state is fully btained. The instantaneus transmissin lad is calculated frm wall inner surface temperature as: 681

3 qi hi( Tx Ti) (7) The time lag is defined as the time that sinusidal temperature wave reaches frm utdr surface f wall t indr. On the ther hand, the decrement factr is defined as reductin rati in amplitude f the temperature wave at the indr surface cmpared t the utside surface. The time lag and decrement factr are cmputed using the fllwing relatins [7]: Φ t t (8) Tx (max) Tx (max) Tx (max) Tx (min) f (9) T (max) T (min) x x Where t Tx and (max) t represent the time that indr T x (max) surface temperatures and utdr surface temperatures are being maximum, respectively. (max), (min) T x T x and T x (max), T x (min) are maximum and minimum temperatures n the indr and utdr surfaces f wall, respectively. RESUTS AND DISCUSSION decrement factr is investigated by using an implicit finite difference methd fr climatic cnditins f Istanbul, Turkey. The investigatin is carried ut fr three different insulatin materials: Extruded plystyrene (XPS), expanded plystyrene (EPS) and glass wl. The thickness f insulatin is increased frm.5 cm t 1 cm. Thermal prperties f materials used in the wall structure are given in Table 1 [1,8]. Calculatins are made fr July 21 and, the averages f the hurly utdr air temperatures recrded in meterlgical data ver the years are used in the calculatins [14]. The indr air temperature is taken t be 23 C. The slar absrptivity f paque wall is selected t be equal t.8 fr dark-clred surfaces, and the cmbined heat-transfer cefficients at the indr and the utdr wall surfaces are taken t be 9 and 22 W/m 2 K, respectively [15]. Figure 2 shws variatin f incident slar radiatin fr all wall rientatins in summer. The value f the slar reflectance f grund is taken as.2. Figure 2 Daily variatin f incident slar radiatin fr July 21 in Istanbul Table 1 Thermal prperties f materials Material Thermal Density Specific Heat cnductivity heat capacity k (W/m K) r (kg/m 3 ) c (J/kg K) r.c (kj/ m 3 K) Plaster Cncrete XPS EPS Glass wl Figure 3 Variatin f time lag and decrement factr versus insulatin thickness f Extruded plystyrene insulated wall fr the all wall rientatins in July

4 Figure 5 Variatin f time lag and decrement factr versus insulatin thickness f Glass wl insulated wall fr the all wall rientatins in July 21 Figure 4 Variatin f time lag and decrement factr versus insulatin thickness f Expanded plystyrene insulated wall fr the all wall rientatins in July 21 Figure 6 Variatin f time lag and decrement factr versus insulatin thickness f a suth riented wall fr three different insulatin materials in July

5 Figure 3 and shws variatin f time lag and decrement factr versus insulatin thickness f extruded plystyrene insulated wall fr the all wall rientatins in July 21. The variatins f time lag and decrement factr fr expanded plystyrene and glass wl are shwn in Figure 4 and 5, respectively. It is seen that fr three different insulatin materials, maximum time lag are btained fr east riented wall while minimum time lag are btained fr west riented wall. This cnclusin can be supprted by incident slar radiatin fr the all wall rientatins in Figure 2. Because f slar radiatin, maximum temperature n the utside surface is reached earlier fr east wall while it is reached later fr west wall cmpared t the ther rientatins. The results shw that wall rientatin has a prfund effect n the time lag while it has a small effect n the decrement factr. Fr example, fr 4 cm XPS insulated wall, values f time lag f suth, nrth, east and west riented walls are btained as 7.11, 7.98, 8.92 and 5.13 h, respectively. On the ther hand, values f decrement factr t these rientatins are btained as.149,.155,.141 and.141. Besides, it is seen that as the insulatin thickness increases, the decrement factr decreases while time lag increases. The similar cnclusins were btained by using different climatic cnditins and insulatin materials [16-18]. Figure 6 and shws variatin f time lag and decrement factr versus insulatin thickness f a suth riented wall fr three different insulatin materials in July 21. It is seen that maximum time lag and minimum decrement factr ccur fr XPS insulatin material. It is als seen that decrement factrs btained fr expanded plystyrene (EPS) and glass wl insulatin materials which have the same thermal cnductivity are the same. The insulatin material which has the highest heat capacity and the lwest thermal cnductivity gives maximum time lag. On the ther hand, the insulatin material which has lwest thermal cnductivity gives minimum decrement factr. These results are in harmny with thse btained by references [17-19] under different climatic cnditins and different insulatin materials. CONCUSION decrement factr is investigated numerically fr three different insulatin materials under climatic cnditins f Istanbul, Turkey. Results shw that wall rientatin has a great effect n time lag while it has a small effect n decrement factr. It is seen that maximum time lag and minimum decrement factr are btained in an east riented wall. [4] Asan H, Sancaktar Y.S., Effects f wall s thickness and psitin n time lag and decrement factr, Energy and Buildings, Vl. 28, 1998, pp [5] Jin X., ZHANG X., Ca Y., Wang G., Thermal perfrmance evaluatin f the wall using heat flux time lag and decrement factr, Energy and Buildings, Vl. 47, 212, pp [6] Asan H., Investigatin f wall s ptimum insulatin psitin frm maximum time lag and minimum decrement factr pint f view, Energy and Buildings, Vl. 32, 2, pp [7] Ozel M., Pihtili K., Optimum lcatin and distributin f insulatin layers n building walls with varius rientatins, Building and Envirnment, Vl. 42, 27, pp [8] Yumrutaş R., Kaşka Ö., Yıldırım E., Estimatin f ttal equivalent temperature difference values fr multilayer walls and flat rfs by using peridic slutin, Building and Envirnment, Vl. 42, 27, pp [9] Kntlen K.J., Bikas D.K., The effect f suth wall s utdr absrptin cefficient n time lag, decrement factr and temperature variatins, Energy and Buildings, Vl , pp [1] Al-Sanea S.A., Zedan M.F., Al-Hussain S.N., Effect f thermal mass n perfrmance f insulated building walls and the cncept f energy savings ptential, Applied Energy, Vl. 89, 212, pp [11] Kntlen K.J., Eumrfpulu E.A., The influence f wall rientatin and exterir surface slar absrptivity n time lag and decrement factr in the Greek regin, Renewable Energy, Vl. 33, 28, pp [12] Threlkeld J.., Thermal envirnmental engineering. Englewd Cliffs,NJ: Prentice-Hall; [13] Duffie J.A., Beckman WA. Slar engineering f thermal prcesses. Jhn Wiley and Sns, inc., New Yrk: [14] Recrds fr weather data, State Meterlgical Statin, Turkey, [15] Kaşka Ö., Yumrutaş R., Cmparisn f experimental and theretical results fr the transient heat flw thrugh multilayer walls and flat rfs, Energy, Vl. 33, 28, pp [16] Al-Sanea S.A., Zedan M.F., Optimum insulatin thickness fr building walls in a ht-dry climate, Int. J. Ambient Energy Vl. 23, 22, pp [17] Ozel M., Pihtili K., Investigatin f the effect n time lag and decrement factr f insulatin thickness n the walls f varius rientatin (in Turkish), Firat University Jurnal f science and engineering, Vl. 17, 25, pp [18] M. Ozel, K.Pihtili, Duvar yönünün faz kayması ve sönüm ranına etkisi (in Turkish), Yalıtım dergisi (Jurnal f islatin) Vl. 67, 27, pp [19] Asan H., Numerical cmputatin f time lags and decrement factrs fr different building materials, Energy and Buildings, Vl. 41, 26, pp REFERENCES [1] Kaşka Ö., Yumrutaş R., Arpa O., Theretical and experimental investigatin f ttal equivalent temperature difference (TETD) values fr building walls and flat rfs in Turkey, Applied Energy, Vl. 86, 29, pp [2] Ulgen K., Experimental and theretical investigatin f effects f wall s thermphysical prperties n time lag and decrement factr, Energy and Buildings, Vl. 34, 22, pp [3] Asan H, Sancaktar Y.S., Effects f wall s thermphysical prperties n time lag and decrement factr, Energy and Buildings, Vl. 28, 1998, pp

CANKAYA UNIVERSITY FACULTY OF ENGINEERING MECHANICAL ENGINEERING DEPARTMENT ME 313 HEAT TRANSFER

CANKAYA UNIVERSITY FACULTY OF ENGINEERING MECHANICAL ENGINEERING DEPARTMENT ME 313 HEAT TRANSFER CANKAYA UNIVERSITY FACUTY OF ENGINEERING MECHANICA ENGINEERING DEPARTMENT ME 313 HEAT TRANSFER CHAPTER-3 EXAMPES 1) Cnsider a slab f thicness as illustrated in figure belw. A fluid at temperature T 1 with