Thermal Performance Evaluation of Domed Roofs

Transcription

1 hermal Perormance Evaluation o Domed Roos hmadreza K. Faghih 1,*, Mehdi N. Bahadori 1 Yazd University, Yazd, Iran Shari University o echnology, ehran, Iran * orresponding author. el: , Fax: , aghih@yazduni.ac.ir bstract: Domed roos (DRs have been used in Iran and many other countries to cover large buildings such as mosques, shrines, churches, schools, etc. hey have been also employed in other buildings like bazaars or market places in Iran due to their avorable thermal perormance. he aim o this research is to study about DRs thermal perormance in order to determine how they can be helpul in reducing the maximum air temperature o inside buildings during the warm seasons considering all parameters like air low around them, solar radiation, radiation heat transer with the sky and the ground as well as some openings on the building. he results o the study show that the thermal perormance o the investigated DR is better than the building with lat roo (FR, particularly when the dome is covered with glazed tiles. In addition to their aesthetic values, domes covered with glazed tiles have thermal beneits o keeping the inside air o these buildings relatively cool during the summer. Moreover, openings cause passive air low inside building, which is helpul or human comort. Keywords: domed roo, thermal perormance, air low, solar radiation, numerical simulation, thermal network Nomenclature Speciic heat... J.kg -1 K -1 l Depth o ground... m D Discharge coeicient... m Inside building air mass... kg p Pressure coeicient... m ir mass low rate... kg.s -1 D Wall or roo thickness... m q Heat transer rate... w.m - F View actor... t ime... s H,h Height... m β Slope... R low resistance... kg.m -4.s -1 Surace emittance... emperature... s Sky emissivity... V Volumetric air low rate... m 3.s -1 ρ Density... kg.m -3 V Velocity... m.s -1 σ Stean-Boltzmann constant... w.m -.k Subscript R Roo... i Inner surace, Inside building, i opening W Wall, roo... j j opening... H Height... m Maximum... a ir... n Minimum, Natural, North... abs bsorbed... o Outer surace, mbient... c onvection... r Radiation... dp Dew point... rg Radiation with ground... e East... rs Radiation with sky... Floor... s bsorbed solar radiation, Sky, South... g Ground... w West Introduction DRs have traditionally been used throughout the world to cover large areas. Solar energy absorbed by a DR causes its temperature to rise above the ambient air temperature. Wind blowing over the dome increases the convection heat transer to the ambient air. Furthermore, the heat loss rom the roo is increased by thermal radiation to the sky. he rest o the heat absorbed by the dome is conducted through the dome material, and is inally transerred to the 1946

2 inside air by convection, and to the interior walls by radiation. he geometry o these roos causes the wind velocity to increase over them, resulting in an increase in the convection heat transer coeicient. Furthermore, the heat transer rom these roos is increased by the act that their areas are greater than the comparable lat ones. In addition to their structural applications, DRs have been employed in Iran or natural ventilation and passive cooling o buildings. cross section o a typical DR employed in such applications is shown in Fig. 1. Fig. 1. ross section o a typical DR, with air circulation in and over it [1] In 1978, Bahadori introduced the important role o DRs in providing cold water in cistern as well as indoors air condition in warm and dry areas in Iran [1]. Konya [] showed that the temperature o domed roo buildings (DRBs is lower compared to lat ones. Mainstone [3] proessed that the main reason o lower temperature o inside DRBs in comparison with FRs is the higher ground and sky relected radiation heat loss. Olgyay [4] guessed that lower indoor air temperature in DRBs is due to the lower absorbed solar radiation in comparison with FRs. ang [5] showed greater solar radiation and heat transer through DRs and reused what Bahadori [1] had been asserted beore. he aim o this research is to study about DRs thermal perormance considering all parameters like air low, solar radiation, radiation heat transer with the sky and the ground as well as some openings on the building by using the thermal network method. onstant wind velocity and direction is assumed, and the I during a day is the comparison index. We investigated air low over domed and FRBs numerically [6] and experimentally [7] to ind proper data or our own study. In these studies we consider the three-dimensional model o the dome o the School o heology (the reerence dome shown in Fig. which is located in Yazd, Iran (a city with very high solar radiation with a speciied boundary layer air low. For this study, the absorbed solar radiation reported by the authors in [8] or the reerence dome and the corresponding lat one with the same base area is used. In [8] it is ound that DRs receive more solar radiation in comparison with lat ones. Fig.. Model o the reerence DR. 1Bbsorbed Heat In Eq. (1, solar radiation ( Q in each hour is assumed to be constant and is equal to its exact s amount in the middle o the speciied hour. he convection heat transer between the ambient air and the outer surace o the building is deined in Eq. ( [9]. Q Q Q Q Q (1 abs s c rs rg 1947

3 Q c ( h ( 0 W o n 0.89 [.38V ] h h (3 W V W is the air low velocity near the wall which is deined by the authors numerically and experimentally in [6, 7]. Boundary layer air velocity proile near the ground deined in Eq. (4 [10] is assumed in those studies. V 400 is the wind velocity at height 400 m. h n in Eq. (3 is the natural convection coeicient deined in Eq. (5 and (6 or upward and downward heat transer respectively [11]. 0.8 V h h (4 V h n osβ (5 3 h n osβ (6 β is the slope o each surace and is the temperature dierence between the ambient and each surace in the speciied time. he ambient air temperature is deined using Eq. (7 [1]. m n m n t 15 ( (. os(180( (7 1 he radiation heat transer between the outer surace o the building and the sky and the ground is deined by Eq. (8 and (11. Note that is assumed to be 0.85, s is the sky temperature deined in Eq. (9 [13] and g is the ground temperature which is similar to the ambient temperature at each time. Q rs osβ σ ( S W ( (8 0.5 ( (9 S S (10 S dp Q rg σ ( os ( 4 4 β g W (11 3. Boundary onditions he outer surace: his surace receives absorbed heat transer which is discussed in Section. he absorbed coeicient o the ordinary material and the glazed tile are assumed to be 0.8 and 0.4 respectively. he FR and the wall are covered with the ordinary material, but both the ordinary material and the glazed tile are considered to cover the DR. wall thickness: onduction heat transer parameters are k1.4 w.m -1.k -1, 880 J.kg -1.K -1, and ρ

4 kg.m -3. he inner surace and the loor: For simpliication, constant convection heat transer coeicient (3 w.m -.k -1 is considered. he wall base: assumed to be insulated. 4. Initial onditions he initial temperature o the walls, roo, loor, and inside air are equal to the ambient air temperature at initial time, which is 6.M. lumped system is considered or the I and changing o this parameter can be determined by Eq. (1. t is time step, m is the inside building air mass, and v is assumed to be 717 J.kg -1.K -1. Deviations o the I during a speciied day (6 ugust in the city o Yazd, in the central desert region o Iran (31.54 N; E; maximum air temperature: 37.9 o ; minimum air temperature: 1.7 o ; dew point: 8 o, or the domed and the FRBs are compared. ai hi ( Wi ai t m v (1 5. hermal Network cylinder with the height o 3m and diameter o 6m is assumed to be a building or both models (Fig.. he height o the dome and the wall thickness in both models are 3m and 15cm respectively. In the air low study o these models V m/s, thereore according to Eq. (4 the air low velocity on top o the DR (6 m is 15 m/s (Re In this method, the geometry o the building has been simpliied to take the radiation heat transer between inner suraces o the building into account. Fig. 3 shows the simpliied geometry. ll view actors can be determined in this simpliied geometry. In this geometry, the heights are similar to the actual model, but the dome is assumed as a triangular pyramid. Each surace o this pyramid aces to the north, south, east or west. o have similar base area, the base width and length are assumed to be 5.3m. Fig. 3. he simpliied model o the DRB used in the thermal network method 5.1. Heat ranser Equations he thermal network o the DRB used in this study consists o 18 nodes. 8 nodes are located on the outer suraces (4 nodes on the roo suraces and 4 nodes on the wall suraces, 8 nodes are located on the inner suraces, 1 node representing the inside building air and 1 node representing the loor. here are 1 nodes in the thermal network o the FR. Fig. 4 shows the thermal network o the southern roo in the simpliied DRB. s shown in Fig. 4, the inner surace o the southern roo has radiation heat transer with the three other inner suraces o the roo (north, east and west as well as our inner suraces o the wall. here is a convection heat transer with the inside building air as well. Eq. (13 and (14 show the energy balance o the inner and outer nodes o the southern roo. 1949

5 Fig. 4. he thermal network o the southern roo in the simpliied DRB Rso ( qs q c q rs q rg k ρw D D t Rso Rso (13 k Rso D Rniσ ( Rni 1 F Rni 4 Rni Rni 4 Rni σ (... 1 F 4 4 h c, ai ( ai ρ W D t (14 In Eq. (13, q s is the averaged absorbed solar radiation o the quarter o the dome ace to the south. q c is the convection heat transer which is calculated based on Eq. ( and (3. ir low velocity is the average air velocity near the quarter o the dome ace to the south. Energy balance equations or all nodes are derived similarly. Emittance o all suraces is assumed to be 0.85 in this study. Fig. 5 shows the thermal network o the loor. s shown in Fig. 5, there is a conduction heat transer between the loor and the depth o the ground (l 5m [14] with the annually averaged temperature ( o the under study area (Yazd which is reported 18 o. Eq. (15 shows the energy balance o the loor node. Fig. 5. he thermal network o the loor k l 4 4 σ ( l (15... hc, ai ( ai ρ W 1 t F W In this study ρ 1300 kg.m -3, k 0.75 w.m -1.K -1 and 970 J.kg -1.K -1. Eq. (16 shows the energy balance o the inside building air. In this Eq. m is the air mass low rate (kg.s -1 which is equal to zero, when there is no opening on the building. Energy balance equations deine all temperatures or next time steps. his method is not sensitive to the time step. We consider the time step o 15 seconds or both the domed and the FRBs. he simulation starts rom 6.M. and it continues until we get 0.1 o dierence between the I or the speciied day (6 ugust and a day ater that at 6.M. 1950

6 ai ai hc ai ( ai hc ai ( ai... m p ( ai ρav (16,, v t 5.. hermal Network Results Fig. 6 shows the sky, the ambient, and the I or the DRB covered with the ordinary material, when the wind direction is south. here are similar igures or other cases, but they are not presented here or the sake o brevity. able 1 compares the maximum I. he reerence DRB has better thermal perormance in comparison with the FRB with similar conditions. It can be also seen that using glazed tiles improves DRB thermal perormance. Wind direction does not have considerable aect on the thermal perormance o buildings. emperature Inside ir mbient Sky ime o Day Fig. 6. he sky, the ambient, and the I or the DRB covered with the ordinary material able 1. Maximum air temperature, using the numerical simulation method ime Max. emperature Roo bsorb oeicient ype o Roo Wind Direction 16: Flat 17: Dome South 17: Dome Fig. 7 shows the deviation o all heat transer to the outer surace o the DRB covered with the ordinary material, when the wind direction is south. onvection heat transer between the outer surace o the building and the ambient air is always negative except rom 4.5 to 7.M., which shows that most o the time the walls and the roo are warmer than the ambient. q (kw Qsolar Qground Qsky Qconvection ime o Day Fig. 7. he deviation o all kinds o heat transer to the outer surace o the DRB BOpenings o investigate the eect o openings, we consider the model shown in Fig. 8. his Fig. depicts the locations and dimensions o two openings on the wall and one hole on its apex. One opening aces to the wind low (windward and the second one is behind (leeward. In the ollowing, we study two scenarios which are "no wind low" and "with wind low". 1951

7 Fig. 8. he locations and dimensions o two openings on the wall and one hole on its apex No Wind Flow ir temperature dierence causes air low in this condition. ir low rate can be deined by Eq. (17 or (18 [11]. D in these equations is opening discharge coeicient [11]. onsidering Eq. (16, as well as the air low rate we can deine the I in the next time step ( ai. he convection heat transer coeicient between the inner surace o the building and the I is assumed to be always 4 w.m -.K -1. ( ai D g H ai V ( ai D g H V D ai > ai (17 < ai ( (19 he results show that the maximum I in this scenario is 4.95 which is at 16:7. In the same situation when the wind direction was south in section 5., the maximum I was his means that without wind low, openings cannot be helpul. Furthermore, we see that the convection heat transer reduces in this situation. It is due to the reduction in the air low velocity near the building's roo and wall. Increasing in radiation heat transer with the sky is sensible because the temperature dierence between walls and the sky increases With Wind Flow In the wind low scenario, both air temperature dierence and air pressure dierence between openings cause air low inside the building. o deine the air low caused by pressure dierence, low network method is used in this study. In this method, air volumetric low rate rom each opening (j is deined by Eq. (0. Where P i is the inside air pressure and P j is the air pressure on the related opening. P j can be determined by Eq. (1. pj is deined in previous surveys [6, 7]. p is assumed to be -1.1, 1, and 0.3 or the hole on apex, windward opening, and leeward opening respectively. In this study all openings discharge coeicients are assumed to be he inside air pressure coeicient ( pi is deined by using Eq. (1 with i as its index. ombining Eq. (0 to (, the air low rate o the opening can be ound by Eq. (3. P P j i j (0 Rij V P R j ij pj 1 ρ V 0 (1 1 ρ ( Pj Pi ( j dj jdjv pj pi pj pi V (3 j 0 195

8 Using Eq. (1 with i as its index and (3 and try and error method, pi and V j can be determined. For the wind velocity proile similar to (4, the network low method leads to the value o 1.8 m 3 /s. We use the reerence velocity o 15 m/s. his amount is assumed to be constant during the day. he convection heat transer coeicient between the inner surace o the building and the inside building air is assumed to be constant and equal to 6 w.m -.K -1. his coeicient is the greatest one in comparison with the other cases (3 and 4 w.m -.K -1 due to the higher air low velocity near the inner surace o the building. Maximum air temperature in this condition is occurred at 15:09. In similar conditions with no openings, maximum air temperature was So again in the wind low condition the thermal perormance o the building with no opening is better, however the air low inside the building caused by openings can be helpul or human comort on warm days. 6. onclusions Based on the analysis carried out the ollowing conclusion can be made: hermal perormance o the DRB under investigation is better than the building with FR on warm days, particularly when the dome is covered with glazed tiles. Wind low direction is not an important parameter in decreasing room temperature o the DRB o the speciied cases. For no wind low condition, the FRB perorms better than the DR one. he passive air low inside the DRB caused by openings can be helpul or human comort on warm days. Reerences [1] M. N. Bahadori, Passive ooling Systems in Iranian rchitecture, Scientiic merican, Vol. 38, 1978, pp []. Konya, Design Primer or Hot limate, the rchitectural Press, 1980, pp [3] R. J. Mainstone, Developments in Structural Form, M.L.. Press, 1983, pp [4] V. Olgyay, Design with limate, Princeton University Press, Princeton, 1973, p. 7. [5] R. ang, I. Meir, Y. Etzion, hermal Behavior o Buildings with urved Roos as ompared with Flat Roos, Solar Energy, Vol. 74, 003, pp [6]. K. Faghih, M. N. Bahadori, hree Dimensional Numerical Investigation o ir Flow over Domed Roos, J Wind Eng Ind erod, Vol. 98, 010, pp [7]. K. Faghih, M. N. Bahadori, Experimental Investigation o ir Flow over Domed Roos, Iranian J o Science and echnology; Engineering, Vol. 33, 009, pp [8]. K. Faghih, M. N. Bahadori, Solar Radiation on Domed Roos, Energy and Buildings, Vol. 41 (B3, 009, pp [9] M. Yazdanian, J. Klems, Measurement o the Exterior onvective Film oeicient or Windows in Low-rise Buildings, SHRE ransactions 100 (1, 1994, pp [10]. D. Penwarden, Wise, Wind Environment around Buildings, Bldg. Res. Estab., [11] SHRE, SHRE Handbook-Fundamentals, merican Society o Heating, Rerigerating, and ir-conditioning Engineers, Inc., tlanta, [1] M. N. Bahadori, hamberlain, Simpliication o Weather Data to Evaluate Daily and Monthly Energy Needs o Residential Buildings, Sol Energy, Vol. 36, 1986, pp [13] Berdahl, Fromberg, the hermal Radiance o lear Skies, Solar Energy, Vol. 9, p. 99. [14] E. hmadi, Heat ranser nalysis in Ground, M.S. hesis (in Persian, School o Mechanical Engineering, Shari University o echnology,