Computational Fluid Dynamics and Building Energy Performance Simulation Nielsen, Peter Vilhelm; Tryggvason, Tryggvi

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1 Aalborg Universitet Computational Fluid Dynamis and Building Energy Performane Simulation Nielsen, Peter Vilhelm; Tryggvason, Tryggvi Publiation date: 1998 Doument Version Publisher's PDF, also known as Version of reord Link to publiation from Aalborg University Citation for published version (APA): Nielsen, P. V., & Tryggvason, T. (1998). Computational Fluid Dynamis and Building Energy Performane Simulation. Aalborg: Dept. of Building Tehnology and Strutural Engineering. Indoor Environmental Engineering, No. 88, Vol.. R9832 General rights Copyright and moral rights for the publiations made aessible in the publi portal are retained by the authors and/or other opyright owners and it is a ondition of aessing publiations that users reognise and abide by the legal requirements assoiated with these rights.? Users may download and print one opy of any publiation from the publi portal for the purpose of private study or researh.? You may not further distribute the material or use it for any profit-making ativity or ommerial gain? You may freely distribute the URL identifying the publiation in the publi portal? Take down poliy If you believe that this doument breahes opyright please ontat us at vbn@aub.aau.dk providing details, and we will remove aess to the work immediately and investigate your laim. Downloaded from vbn.aau.dk on: oktober 19, 218

2 Computational Fluid Dynamis and Building Energy Performane Simulation P V Nielsen. T Tryggvason ~ l l I - CO CO z, Q.).. m... Ol ;:: Ol w u r._ :;: w._ ' E u ~._ L x U5 o::i m f.- z w > 2 :: 4-- (/) Ol ' (.)._.. E. :.::. (.) if) (/) E :: I'- r- ' E.,--.,-- ::1. ;:: i_ o_.~...-- '- <( g o::i m (.)...--._ 4-- ' s: u (f)

3 The Indoor Environmental Engineering papers are issued for early dissemination of researh results from the Indoor Environmental Engineering Group at the Department of Building Tehnology and Strutural Engineering, Aalborg University. These papers are generally submitted to sientifi meetings, onferenes or journals and should therefore not be widely distributed. Whenever possible, referene should be given to the final publiations (proeedings, journals, et.) and not to the Indoor Environmental Engineering papers. Printed at Aalborg University

4 Computational Fluid Dynamis and Building Energy Performane Simulation P V Nielsen, T Tryggvason

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6 COMPUTATIONAL FLUID DYNAMICS AND BUILDING ENERGY PERFORMANCE SIMULATION Peter V. Nielsen 1, Tryggvi Tryggvason 2 1 Aalborg University, Aalborg, Denmark 2 pus ehf, Akureyri, Ieland ABSTRACT An interonnetion between a building energy performane simulation program and a Computational Fluid Dynamis program (CFD) for room air distribution will be introdued for improvement of tfie preditions of both the energy onsumption and the indoor environment. The building energy performane simulation program requires a detailed desription of the energy flow in the air movement whih an be obtained by a CFD program. The paper desribes an energy onsumption alulation in a large building, where the building energy simulation program is modified by CFD preditions of the flow between three zones onneted by open areas with pressure and buoyany driven air flow. The two programs are interonneted in an iterative proedure. The paper shows also an evaluation of the air quality in the main area of the buildings based on CFD preditions. It is shown that an interonnetion between a CFD program and a building energy performane simulation program will improve both the energy onsumption data and the predition of thermal omfort and air quality in a seleted area of the building. KEYWORDS Air flow pattern, Air quality, CFD, Energy, Ventilation effiieny, Building energy performane simulation. INTRODUCTION An interonnetion between a building energy performane simulation program and a CFD program for room air distribution will improve the preditions of both the energy onsumption and the indoor environment. It is neessary to have a desription of the boundary onditions as e.g. surfae temperature or heat transmission when the air distribution is predited in a room. This is espeially neessary in onstrutions where radiation and free onvetion are important. These boundary onditions an be obtained by a building energy performane simulation program whih takes aount of radiation, ondution in the struture and transient responses of the building. I - _J I I L = ::: - -!l T1 "" /!l TT - - = A ;: B I I I r Figure I Priniples for Computational Fluid Dynamis (A), and building energy performane simulation (B). Figures I A and I B show the harateristis of a CFD program and a building energy performane simulation program. The CFD program predits the flow in a room by solving all the flow equations,

7 inluding the energy transport equation, in a grid overing the volume of the room, see Figure la. The solution domain is surrounded by boundary onditions suh as air terminal devies, return openings, surfae temperatures or energy flow at the surfaes. The building energy performane simulation program, see Figure 1B, often desribes the energy flow in the room air movement by a single grid point. The energy flow in the walls, the floor and the eiling is desribed by a grid number suffiient to predit the detailed dynami energy flow and the onsumption of the whole building during a period with a given indoor and outdoor load. Figures ~ 1 A and 1 B indiate the improvements whih an be obtained by an interonnetion between the two programs. The building energy performane simulation program is able to predit the temperature distribution, or energy flow, through all the surfaes in the building, and the values found are used as boundary onditions for the CFD program. The CFD program is, on the other hand, able to predit the air flow in the rooms of the building and thereby give an improved desription of the energy flow between the surfaes of the building. The method an be strutured in different ways. A building energy performane simulation program an be onneted to a separate CFD program where the CFD program makes the predition of the energy flow in seleted situation. It is also possible to work with a CFD program whih is extended to inlude the possibility of finding a ombined solution of radiation, ondution and thermal storage, in parallel with the CFD solution of the flowfield. This model is often alled a onjugate heat transfer model. Another possibility is to extend a large building energy performane simulation program with a CFD ode to be used in seleted room. Some of the earliest publiations based on the ombined use of CFD and building energy performane simulation were given by Chen ( 1988) and Homes et al. ( 199). New examples of onjugate heat transfer models are given by Maser et al. (1995), Kato et al. (1995) and Shild (1997). One of the main reasons for the use of ombined models or a onjugate heat transfer model is, as mentioned earlier, to obtain a good desription of the energy flow at surfaes and between rooms in different situations. This paper desribes an energy and indoor air quality study of the Central Library of North Jutland. Combined programs are used, espeially to predit the energy flow in a spae divided into several rooms with open onnetions and, furthermore, to study the indoor air quality in the main hall. Building energy performane simulation The Central Library of North Jutland onsists of 17 rooms ventilated partly by a CA V system partly by a V A V system. Figure 2 Library hall and openings to two other lending departments. Most of the rooms are heated by radiators, but the main hall is heated by the air onditioning system. The main library hall is onneted with 2

8 (: ~ ~~ l :----~: ~ Figure 3 The library hall, Zl, and two other zones, Z2 and Z3, with open onnetions to eah other. large openings to two other rooms as shown in Figure 2. Figure 3 shows the library hall as zone 1 (Zl), and the two other rooms as zone 2 (Z2) and zone 3 (Z3). The figure shows a vertial onnetion between zone 2 and zone 3, horizontal onnetions between zone I and zone 2 and between zone I and zone 3 as well. All three zones are equipped with supply and return openings for individual air distribution in the different spaes. Pressure and temperature differenes indue an air movement between the three zones, and this movement will in ertain situations be supported by thermal flow from radiators loated in zone 2 below the vertial openings to zone 3. The energy onsumption of the building is alulated by a building energy performane program alled tsbi3, developed by the Danish Building Researh Institute. Benhmark tests of this program and of several other programs are given by Lamas et al. (1994). Rooms with idential load profiles, idential heating and ventilation priniple and idential orientation of windows with equal window to floor ratio are grouped in a single zone, and 17 rooms are simulated as a total of 27 zones. The simulation program is not able to predit the flow between the zones Zl, Z2 and Z3 as a funtion of the pressure and temperature distribution in the zones, and it is assumed, as an initial guess, that the energy flow between the zones is low and therefore unimportant. Figure 4 shows the development of temperatures in the three zones during a week in June (week 23) when the building is loaded aording to the Danish referene year. The library hall, zone 1, is heated by solar radiation through skylights, and it will in ertain periods obtain a temperature level whih is 8 C above the temperature in the surrounding zones. It is obvious that this temperature differene will indue an energy flow between the zones, and it is therefore neessary to extend the tsbi3 program to inlude information on the air movement in the open onnetions in the building. T.[OC] 4,---,---, , Zl ZJ Z2 Figure 4 The temperature development in the zones 1, 2 and 3 during week 23. It is assumed that there is no air exhange between the zones. Building energy performane simulation ombined with Computational Fluid Dynamis This hapter shows the possibilities whih an be obtained by making a ombined simulation with tsbi3 and the CFD program FLOVENT. Figure 5 Three-dimensional CFD model for predition of air exhange between the zones Z I, Z2 and Z3. 3

9 It is very time-onsuming to run a CFD program, and it is therefore neessary to study the possibilities of simplified preditions of the energy flow. In some situations it is possible to obtain good results in a twodimensional geometry, see e.g. Nielsen ( 1995), but in this ase it is both neessary to preserve the three-dimensional effet of the flow around the openings and to work with the atual vertial dimensions. Figure 5 shows the simplified three-dimensional geometry whih is seleted for the predition of the energy flow. The geometry represents a ompromise with orret geometry around the openings. The ratio between the total flow area and the floor area of the three zones is the same in the CFD predition and in full sale in the building. Air supply and return openings are only defined in a oarse grid suffiient for energy flow preditions but insuffiient for a detailed analysis of the air veloity distribution in the oupied zone. The CFD simulation is made on the fourth day of week 23 at four o'lok in the afternoon. The boundary onditions for the energy flow are obtained by the tsbi3 program and they are based on given values of the heat transfer oeffiient. The boundary onditions ould also be given as a temperature distribution, but is well known that it is diffiult to make an aurate predition of the heat transfer oeffiient with the wall funtions in a CFD program, see Chen ( 1992) and Nielsen (1998). It is neessary to obtain the final solution by an iteration between tsbi3 and the CFD program, beause a hange in the air flow between the three zones will hange the temperature level and ause a hange in the energy flow in the surrounding surfaes. Figure 6 shows the stages in this iteration. The initial tsbi3 preditions are made without air exhange between the zones Z1, Z2 and Z3. The orresponding CFD preditions give an air exhange whih is introdued into the tsbi3 program. The new energy flow at the surfaes is introdued into the CFD program resulting in a new air exhange between the three zones. The iterations ontinue till the hange in the air flows is below 5 o/o ompared with the last values. Different situations during the referene year are seleted for this analysis, and similarity between the preditions from the tsbi3 program and the CFD program has been obtained after 3-6 iterations. Seletion of time for CFD-sim ulation tsbi3-simulation with the last found values of internal air exhange Calulation of boundary values for the CFD-model CFD-simulation of air exhange Final solutions Error >5% Figure 6 Flow hart whih shows the iterations between the building energy performane simulation program and the Computational Fluid Dynamis program. Figure 7 shows the air distribution found by the CFD program. The highest temperature is found in zone I and the lowest in zone 2. The air is flowing from zone 1 into zone 3 and further into zone 2. The flow redues the temperature in the library hall, zone 1, and inreases the level in zone 3. Figure 8 shows the development of the temperatures in the three zones during week 23 if the air exhange is onsidered. The air flow between the zones auses a derease in the temperature differene between the three zones as well as a derease in the temperature level in the library hall. The highest temperature differene between any of the zones is only 3 C ompared with 8 C found without air exhange between the zones, see Figure 4. It 4

10 Contour ~~ture y.. 6.5E-1 /,...,,, : / /, I\ '. ' ' -- I\ ' ,, Ref Vetor 1.5 > y 6.5B-1 Figure 7 CFD preditions of temperature distribution and air movement between the zones 1, 2 and 3 on the fourth day of week 23 at four in the afternoon. T ["C] 4 3 j'j N 1, /~ J\.l lrj 11/ /dl\ fir' 1~. " l ~ l~f ~ " ~ u ~ Jl H g 2 "' 2 6 (\!\ Zl I(' Z3 {\ t" /" Z2 Figure 8 The temperature development in the zones 1,2 and 3 during week 23. The air exhange between the zones is obtained by CFD preditions. must be assumed that the energy onsumption alulations of the highest quality are obtained by onsidering the air exhange between the different open zones in the building. Figure 7 shows that the vertial temperature gradient in the rooms is small. Larger gradients an be obtained in other situations where it will be diffiult to obtain similarity between preditions made by the two programs. The building energy performane simulation program is only able I/ I~ to ope with situations without vertial gradients beause the room air volume is desribed by one grid point, see Figure 1 B. A zonal model with more vertial zones m a room will be neessary in this situation. CFD predition of air distribution and ventilation effiieny This hapter desribes an evaluation of the air quality in the library hall based on CFD preditions. The predition is made in a geometry whih orresponds to the real layout of the library hall, inluding bookases in the oupied zone but without the zones 2 and zone 3. The effet of these zones is introdued by flow rates and temperatures through vertial surfaes in the hall. One of the important aspets is the simulation of air terminal devies whih onsist of irular openings with a diameter of 16 m. Figure 2 shows the supply opening in the wall above the oupied zone and the return opening just below the eiling lose to the skylights. A speial set of preditions, with fous on the flow from a supply opening, has been made to study the neessary number of grid points in the veloity deay zone from an opening. It was possible to obtain a suffiient desription of both supply openings, bookases and the flow in the oupied zone by 114 grid 5

11 Figure 9 Air movement in two horizontal setions of the library hall. The upper figure shows the preditions at a height of 7 m, and the lowest figure shows the preditions at floor level (.25 m). points. The boundary onditions for the energy flow and the flow in onnetion with the zones 2 and 3 are given from the ombined preditions mentioned in the last hapter. Indoor air quality problems are observed during the winter beause the library hall is heated by the ventilation system and the supply air seems to bypass the oupied zone due to the buoyany effet. Figure 9 shows the air distribution in the hall on the 8th of January at eleven o'lok in the morning. Two horizontal setions are shown, one setion just below the eiling and one setion 25 m above the floor. It is obvious that the heated jets (4.56 m 3 /s, 32.5 C) move upwards and generate a radial flow below the eiling. Some air will move down in the orners of the hall beause three of the orners are without supply openings. The flow in the top right-hand orner of the figure is the flow from zone 2, whih in this situation will supply air to the oupied zone (2.7 m 3 /s, l8.6 C). The flow in the bottom right-hand orner of the figure is partly old downdraught from a glazed area in this orner of the hall, and partly indued flow from the upper part of the building. Figure 9 shows that the veloity level is very low in the main part of the oupied zone, whih orresponds to a low ventilation effiieny in this area of the hall. The onentration distribution has been predited for a situation where the low surfaes in the hall are the soures. The maximum onentration in the oupied zone, P/R, has the level of 2.5 whih orresponds to a loal ventilation index, P, of.49 ( P is the onentration in the air and R is the mean onentration in the return openings). The average onentration in the oupied zone, ",JR, is 1.3 orresponding to a mean ventilation effiieny of.77. This is not a high value, and it must be onsidered that reirulation in the ventilation system will further derease the level of fresh air in the oupied zone. 6

12 CONCLUSIONS An interonnetion between a building energy performane simulation program and a CFD program for room air distribution is introdued, and it an be onluded that improvement of the preditions of both the energy onsumption and the indoor environment an be ahieved. The building energy performane simulation program in a large building is modified by CFD preditions of the flow between three zones onneted by open areas with pressure and buoyany driven air flow. The two programs are interonneted in an iterative proedure. It is shown that the int-eronnetion between the CFD program and the building energy performane simulation program will improve the alulation of dynami temperature development in a room as well as the predition of thermal omfort in a hall taking the flow from neighbouring zones into onsideration. REFERENCES Chen, Q. (1988) Indoor Airflow, Air Quality and Energy Consumption of Buildings. Ph.D. Thesis, Delft University of Tehnology, the Netherlands. Bloomfield, D. (1994) Empirial Validation of Thermal Building Simulation Programs using Test Room Data. Vol. I : Final Report, lea BCS, Annex 21 & lea SHC Task 12. Nielsen, P.V. (1995) Airflow in a World Exposition Pavilion Studied by Sale-Model Experiments and Computational Fluid Dynamis. ASHRAE Transations, Vol. 11, Pt.2, pp Nielsen, P.V. ( 1998) The Seletion of Turbulene Models for Predition of Room Airflow. ASHRAE Transations, Vol. I 4, Pt. 1. Moser, A., Off, F., Shalin, A. and Yuan, X. (1995) Numerial Modeling of Heat Transfer by Radiation and Convetion in an Atrium with Thermal Inertia. ASHRAE Transations, Vol. 11, Part 2, pp Shild, P. ( 1997) Aurate Predition of Indoor Climate in Glazed Enlosures. Ph.D. Thesis, Dep. of Refrigeration and Air Conditioning, Norwegian University of Siene and Tehnology (NTNU), Norway. Chen, Q. and Jiang, Z. ( 1992) Signifiant Questions in Prediting Room Air Motion. ASHRAE Transations, Vol. 98, Pt. I, pp Holmes, M.J., Lan, J.K-W., Ruddik, K.G. and Whittle, G.E. (199) Computation of Condution, Convetion and Radiation in the Perimeter Zone of an Offie Spae. Proeedings ROOMVENT '9, Oslo, Norway. Kato, S., Murakami, S., Shoya, S., Hanyu, F. and Zeng, J. ( 1995) CFD Analysis of Flow and Temperature Fields in Atrium with Ceiling Height of 13 m. ASHRAE Transations, Vol. I I, Part 2, pp Lamas, K., Eppel, H., Martin, C. and 7

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14 RECENT PAPERS ON INDOOR ENVIRONMENTAL ENGINEERING PAPER NO. 79 : P.V. Nielsen: Design of Loal Ventilation by Full-Sale and Sale Modelling Tehniques. ISSN R9743. PAPER NO. 8 : P. Heiselberg, K. Svidt, H. Kragh : Appliation of CFO in Investigation of Ventilation Strategies for Improvement of Working Environment in a Waste Inineration Plant. ISSN R PAPER NO. 81 : P. Heiselberg, C. Topp : Removal of Airborne Contaminants from a Swtae Tank by a Push-Pull System. ISSN R9746. PAPER NO. 82 : P. Heiselberg : Simplified Method fo r Room Air Distribution Design. ISSN R9747. PAPER NO. 83 : t.. Davidson, P.V Nielsen: A Study of Laminar Bakward-Faing Step Flow. ISSN R982. PAPER NO. 84 : P.V. Nielsen: Aililow in a World Exposition Pavilion Studied by Sale-Model Experiments and Computational Fluid Dynamis. ISSN R9825. PAPER NO. 85 : P.V. Nielsen: Stratified Flow in a Room with Displaement Ventilation and Wall-Mounted Air Terminal Devies. ISSN R9826. PAPER NO. 86 : P.V. Nielsen: The Seletion of Turbulene Models for Predition of Room Aililow. ISSN R9828. PAPER NO. 87 : K. Svidt, G. Zhang, B. Bjerg : CFO Simulation of Air Velo ity Distribution in Oupied Livestok Buildings. ISSN R9831. PAPER NO. 88 : P. V. Nielsen, T. Tryggvason : Computational Fluid Dynamis and Building Energy Peliormane Simulation. ISSN R9832. PAPER NO. 89 : K. Svidt, B. Bjerg, S. Morsing. G. Zh ang: Modelling of Air Flow th ro ugh a Slatted Floor by CFO. ISSN R9833. PAPER NO. 9 : J.R. Nielsen, P.V. Nielsen, K. Svidt: Th e Influ ene of Furniture on Air Veloity in a Room -An Isothermal Case. ISSN R9843. PAPER NO. 91 : P. Lengweiler, J.S. Stre>m, H. Takai, P. Ravn, P.V. Nielsen, A. Maser: Oust Load on Suliaes in Animal Buildings: An Experimental Measuring Method. ISSN R9844. PAPER NO. 92 : P. Lengweiler, P.V. Nielsen, A. Maser, P. Heiselberg, H. Taka i: Deposition and Resuspension of Pat1iles: Whih Parameters are lmpot1ant? ISSN R9845. PAPER NO. 93 : C. Topp, P.V. Nielsen, P. Heiselberg, L.E. Sparks, E.M. Howard, M. Mason : Experiments on Evaporative Emissions in Ventilated Rooms. ISSN R9835. PAPER NO. 94 : L. Davidson, P.V. Nielsen : A Study of Low-Reynolds Number Effets in Bakward-Faing Step Flow using Large Eddy Simulations. ISSN R9834. Complete lis t of papers:

15 ISSN R9832 Dept. of Building Tehnology and Strutural Engineering Aalborg University, Deember 1998 Sohngaardsholmsvej 57, DK-9 Aalborg, Denmark Phone Fax :