Characteristics of Air Flow through Windows Heiselberg, Per Kvols; Dam, Henrik; Sørensen, Lars C. ; Nielsen, Peter Vilhelm; Svidt, Kjeld

Transcription

1 Aalborg Universitet Charateristis of Air Flow through Windows Heiselberg, Per Kvols; Dam, Henrik; Sørensen, Lars C. ; Nielsen, Peter Vilhelm; Svidt, Kjeld Publiation date: 1999 Doument Version Publisher's PDF, also known as Version of reord Link to publiation from Aalborg University Citation for published version (APA): Heiselberg, P., Dam, H., Sørensen, L. C., Nielsen, P. V., & Svidt, K. (1999). Charateristis of Air Flow through Windows. Aalborg: Dept. of Building Tehnology and Strutural Engineering, Aalborg University. Indoor Environmental Engineering, No. 13, Vol.. R994 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 15, 218

2 Charateristis of~air Flow Thr9ugh Windows P Heiselberg, H. Dam, L. C. Srensen, P V Nielsen and K. Svidt CV) ~ z ~ Q).. u o_ Ol a> ID Ol w m ID r- : : '- :; : w I- u : E ri ::J '-- 9" ll E ::J ~ '- m LL ID ID > _o m O'l ~ O'l I O'l ~ I- I- ID _o 2 ~ :;:::; E ID ID UJ. > ID I- (/) u::: u ID _o. ~ ~ :5 m u I I- u UJ u ::J ID m <( ID I- m ID >- en ::J : ID u u I- Q_ z en

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 ataalborg Univers ity

4 Charateristis of Air Flow Through Windows... P Heiselberg, H. Dam, L. C. Srensen, P. V. Nielsen and K. Svidt

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6 CHARACTERISTICS OF AIR FLOW THROUGH WINDOWS Per Heiselberg, Henrik Dam, Lars C. Sarensen, Peter V. Nielsen, Kjeld Svidt Indoor Environmental Engineering Aalborg University Sohngaardsholmsvej 57, DK-9 Aalborg, Denmark Fax: ABSTRACT This paper desribes the first results of a series of laboratory investigations that is performed to haraterise three different window types. The results show the air flow onditions for different ventilation strategies and temperature differenes. For one of the windows values of the disharge oeffiient are shown for both isothermal and non-isothermal flow onditions and the thermal omfort onditions are evaluated by measurements of veloity and temperature levels in the air flow in the oupied zone. lt is demonstrated that different window types have quite different harateristis. A ombinatiq,n of different window types in the same natural ventilation design an by using their strong sides improve both ventilation apaity, thermal omfort and IAQ. INTRODUCTION In natural ventilation systems fresh air is often provided through opening of windows. There is a wide range of possibilities with regard to seletion of window type, see figure 1, size and loation. However, the knowledge of the performane of individual windows is rather limited and is based on theoretial assumptions on the main driving fores, effetive areas and air flow within rooms. lt is only possible in window design for natural ventilation to give rough estimates of the thermal omfort, the draught risks and the IAQ levels that an be expeted. Some window types are regarded as better than others, but this is only based on qualitative measures and the differenes and limitations in the appliation of individual window types annot be quantified. Figure 1. Examples of different window types.

7 Therefore, there is ertainly a need for quantitative information on window performane that an improve the window design methods to a level, where they an math the design methods of air inlets in mehanial ventilation. This paper desribes the first results of a series of laboratory investigations that is performed to haraterise three different window types. The results show the air flow onditions for different ventilation strategies and temperature differenes. For one of the windows values of the disharge oeffiient are shown for both isothermal and non-isothermal flow onditions and the thermal omfort onditions are evaluated by measurements of veloity and temperature levels in the air flow in the oupied zone. 2 DESCRIPTION OF LABORATORY SET-UP The investigations is performed in a laboratory test room with the size of LengthxWidthxHeight = 8m x 6m x 3m, see figure 2a. The room is divided into two separate rooms by an insulated wall, see figure 2b. The small room an be ooled to a temperature of about oo ~while the large room an be kept at normal room temperature. Three different window types have been plaed in the insulated wall, see figure 3a. Window type 1 is a ombined side/bottom hung window that is plaed lose to the oupied zone. Window type 2 is a narrow window that is plaed high in the room ana has been used both as a top and bottom hung window. Window type 3 is a horizontal pivot window plaed lose to the oupied zone, see figure 3b-d. A 8 J.::;. ~r:m ::Sr.*-1p"::.'1d>.~ t{:(: r. 1:1:: :: : :,;~k~f~rq ~~ r :l!n:-1:;..:'!-imr.t u.~~~ 1{:(: m :~.:: : : ~ J~ ~11:nJ l C. ~ N : ) t'fif:lp:,:~ ~ l: Figure 2. A) Sketh of laboratory test room. B) Sketh of insulated wall onstrution. A I : I I 2

8 8 D Ceiling A 8 f \ f/ r Outside Inside Outs ide Ins ide ulside Inside Figure 3. A) Sketh of window loation in insulated wall. B) Photo of window type 1, C) Photo of window type 2 and D) Photo of window type 3 (top window) with indiation of the three onfigurations used in the investigation. 3 AIR FLOW THROUGH WINDOWS The air flow through a window depends on the hosen natural ventilation strategy, see figure 4. Single sided ventilation relies on openings being on only one side of the ventilated enlosure. A lose approximation is a ellular building with opening windows on one side and losed internal doors on the other side. With a single opening in the room the main driving fore for natural ventilation in winter is the thermal stak effet, where the air will flow into the room in the bottom half of the window and out of the room in top half of the window. The main driving fore in summer will be the wind turbulene. Compared with other strategies, lower ventilation rates are generated. Stak indued flows inrease with the vertial 3

9 separation of the openings. Window type 2, with the main opening area divided between the top and the bottom of the window, is therefore more effetive than types 1 and 3, where the main opening area is onentrated either in the top or the bottom of the window. In ross- and stak-ventilation there are ventilation openings on both sides of a spae. Air flow from one side of the building to the other and leaves through another window or door. Cross ventilation is usually wind driven while stak ventilation is thermal (and wind) driven. With suh ventilation strategies there will only be an inflow of air through the window and the pressure differene will be muh higher. The apaity of the opening will not depend on the distribution of the opening area, but only on the total area. Cross/Stak Ventilation H H O"";do u Room z 1t p - Sin le-sided Ventilation H H ---I Outside Room ----'N'-----'-..P -. H, H, Neutralplane Figure 4. Air flow through a window with a single sided and a ross ventilation strategy, respetively 4 AI R FLOW INSIDE ROOM The air flow in the room was investigated by smoke tests for both a single-sided and a ross/stak ventilation strategy for all three window types. For a single sided ventilation strategy air flow through window type 1 and 2 was supplied diretly to the oupied zone and dependent on temperature differene and window opening area the air reahed the floor from,5-1.5 m from the window, see figure 5. The air flow along the floor ould be haraterised as stratified flow. Even very small opening angles resulted in large air flows and high veloity levels in the oupied zone. PK p..p 4

10 Figure 5. Air flow through window type 2 with single sided ventilation and a temperature differene of 2 C. For a single sided ventilation strategy the air flow through window type 3 was almost idential for all three onfigurations on figure 3. At small opening angles only a very small amount of air entered the room at low veloity. With inreasing opening angles the air flow and veloity level inreased. In all ases the air flow was downwards along the wall and at large opening angles the air reahed the floor and turned into the oupied zone as stratified air flow along the floor, see figure 6. F;;;;::;.q(l:;<!~<l;.l' ~. I i I Figure 6. Air flow through window type 3 with single sided ventilation and a temperature differene of 2 C. For a single sided ventilation strategy window type 3 is the best hoie in winter beause the air is supplied outside the oupied zone and an be ontrolled by hanging the opening angle. Window type 1 and 2 is not a good hoie as the air is supplied diretly to the oupied zone and is diffiult to ontrol beause the amount of air and the veloity levels inrease very rapidly with inreasing opening angles. In summer with small temperature differenes window type 3 will not be able to supply enough air to the room, but will have to be ombined with window type 1 or 2. 5

11 For a ross- or stak-ventilation strategy the available pressure differene aross the openings is generally muh higher. For window type 1 and 2 the air flow into the room ated as a thermal jet that reahed the floor in a ertain distane dependent on temperature differene, pressure differene and opening angle. The problems under winter onditions with high air veloities and with a proper ontrol of the air flow inreased. The air flow onditions for window type 3 showed large differenes for the three onfigurations. Generally the air flow ated as a thermal jet. For both a bottom hung window opening in and a top hung window opening out the air flow ated as a thermal wall (eiling) jet. However, the distane from the wall where the jet separated from the eiling was larger for the bottom hung window, resulting in lower veloities in the oupied zone. For the top hung window opening in the air flow ated as a free thermal jet and reahed quikly the oupied zone resulting in very high air veloities, see figure 7. A 8 Figure 7. Air flow through window type 3 with ross- or stakventilation and a temperature differene of 2 C. A) Bottom hung, opening in. B) top hung, opening in. For a ross- or stakventilation strategy window type 3 in a bottom hung onfiguration is the best hoie in winter beause the air travels the largest distane before it reahes the oupied zone and the veloity levels therefore will be the lowest. Window type 1 and 2 is not a good hoie as the air is supplied diretly to the oupied zone at very high veloities and is very diffiult to ontrol beause the amount of air and the veloity levels inrease very rapidly with inreasing opening angles. 5 WINDOW AIR FLOW CAPACITY The air flow through a window an be estimated by equation (1) (1) where Q is volume flow rate (m 3 /s) d is disharge oeffiient (-) A is geometrial window opening area ilp is pressure differene aross the window (pa) p is density of air (kg/m 3 ) 6

12 The disharge oeffiient is a harateristi parameter for a speifi window and takes both the ontration and the frition loss in the window opening into aount. The size of the oeffiient is only known for very simple opening types. For windows, whih have a very ompliated geometrial struture, the size of the oeffiient is unknown and its dependene on parameters like for example opening area, veloity level (pressure differene) and temperature differene is not known either. Preliminary measurements on window type 1 with a side hung opening shows some interesting harateristis of the disharge oeffiient. The estimation of the geometrial opening area of the window is very diffiult beause of the ompliated geometry and the unertainty is espeially high at small opening angles. The absolute value of the disharge oeffiient is therefore unertain espeially at small opening angles and measured values above 1 must primarily be aused by inorret estimation of the opening area. Figure 8 shows the disharge oeffiient as a funtion of the pressure differene aross the opening for different opening areas, Figure 9 as a funtion of the opening area of the window and Figure 1 as a funtion of a redued Arhimedes number (ilt/ 2 ). 1,2 ' : CD '5,8 ~ a () CD,6 e>,4 CO. trl,2 Ci ~ Opening Area (m2) -! +,4..1.&.,18 f eo,28 xo.12 +o,84 l ~ Pressure Differene (pa) Figure 8. Disharge Coeffiient, Cd, for window type 1 (side hung) as a funtion of pressure and for different opening areas. 1,2, ! z! ~.. a ~, L----"" *------; 8, :_: ---; CD Cl Cii, i. iil, ; Ci o+----r---~r----r----r----~,2,4,6,8 Opening Area (m2) Figure 9. Disharge Coeffiient, Cd, for window type 1 (side hung) as a funtion of the opening area. 7

13 -1,2 -, ~ !.- 1 Q) ~,8 -t------"''mr-<~>~r ! Q),6 -t _ ! ~, Opening Area (m2) ell t5 o, e o,o4 &,1 {/) O+---,---~----r----~----~ Redued Arhimedes number (Ks2/m6) Figure 1. Disharge Coeffiient, Cd, for window type 1 (side hung) as a funtion of a redued Arhimedes number. The measurement results shows that the in an isothermal ase the disharge oeffiient is independent of the pressure differene aross the window, but dependent on the opening area. In a situation with both a temperature and a pressure differene aross the opening the disharge oeffiient an be desribed as a funtion of the Arhimedes number and the opening area. So, the use of a onstant value for the disharge oeffiient independent of opening area, temperature- and pressure differene an lead to serious errors in the predition of air flow apaity of window openings. 6 AIR VELOCITIES IN THE OCCUPIED ZONE The air flow from window type 1 with side hung opening will at as a thermal jet. The distane from the wall, where the air jet will reah the floor will be dependent on the pressure differene (air flow rate) and the temperature differene. Figure 11 shows that the maximum veloity in the air flow along the floor also will be dependent on the air flow rate and temperature differene. The veloity level inreases with inreasing air flow rate and inreasing temperature differene, but dereases with inreasing distane to the wall. This is a very typial result for stratified flow onditions. Preliminary analysis have showed that it is possible to develop an equation system that an be used to predit the veloity level in the oupied zone as a funtion of opening area, pressure differene and temperature differene. This an be used to predit the omfort performane of window openings and estimate the limitations of a speifi window type. In this way the design of window openings for natural ventilation beomes not only a question of providing the neessary opening area to ensure satisfatory apaity but also a question of seleting the optimum window type for thermal omfort. 8

14 ,5-,----, ==-~~ j,4 --E,35 -,3 o,25 Q),2 > I.-,15 <, m3/h 6,7 C '"""""-=--~~...,_,------l -fli-253 m3/h 6,9 C ~~~~~---l --*-15 m3/h 5,7 C -e-151 m3/h 8,5C, m3/h 4,5 C, Distane from wall (m) Figure 11. Veloity level in air flow along the floor from window type 1 {side hung) as a funtion of distane to wall, air flow rate and temperature differene. 7 CONCLUSIONS The results have been promising and work will ontinue by investigation of the performane of other window types, espeially those loated at high levels in the room. The results showed that the disharge oeffiient for a window opening varies onsiderably with opening area and temperature differene and that the use of a onstant value an lead to serious errors in the predition of air flow apaity. lt should be possible to develop equations systems to predit thermal omfort in the oupied zone beause of air flow from window openings, whih is very important for the seletion of optimum window types. A further next step ould be to investigate the dynamis of window opening and the performane of different ontrol systems in ontrolling the air flow through the windows. But as long as we do not know the harateristis of the window, it is very diffiult to define the requirements to the ontrol system. 8 ACKNWLEDGEMENT This work is a part of the o-operative work in lea ECB&CS Annex 35, Hybrid Ventilation in New and Retrofitted Offie Buildings, and has been supported by the Danish Energy Ageny (Energiforskningsprogram, EFP). 9

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16 RECENT PAPERS ON INDOOR ENVIRONMENTAL ENGINEERING 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 : CFD Simulation of Air Veloity Distribution in Oupied Livestok Buildings. ISSN R9831. PAPER NO. 88: P. V. Nielsen, T. Tryggvason: Computational Fluid Dynamis and Building Energy Pe1iormane Simulation. ISSN R9832. PAPER NO. 89: K. Svidt, B. Bjerg, S. Morsing, G. Zhang : Modelling of Air Flow through a Slatted Floor by CFD. ISSN R9833. PAPER NO. 9: J.R. Nielsen, P.V. Nielsen, K. Svidt: The Influene of Furniture on Air Veloity in a Room- An Isothermal Case. ISSN R9843. PAPER NO. 91 : P. Lengweiler, J.S. Stmm, H. Takai, P. Ravn, P.V. Nielsen, A. Moser: Dust Load on Swiaes in Animal Buildings: An Experimental Measuring Method. ISSN R9844. ~ PAPER NO. 92: P. Lengweiler, P.V. Nielsen, A. Moser, P. Heiselberg, H. Takai : Deposition and Resuspension ofpa11iles: Whih Pararneters are Important? 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. PAPER NO. 95 : A. Nielsen : VRML Programs for Room Ventilation Appliations. ISSN R9846. PAPER NO. 96: E. Bj rn, P.V. Nielsen : CFD Simulations of Contaminant Tran spo/1 between Two Breathing Persons. ISSN R989. PAPER NO. 97: C. Topp, P.V. Nielsen, P. Heiselberg: Modelling Emission from Building Materials with Computational Fluid Dynamis.. ISSN R9915. PAPER NO. 98: H. Brohus: Influene of a Cooled Ceiling on Indoor Air Quality in a Displaement Ventilated Room Examined by Means of Computational Fluid Dynamis. ISSN R992. PAPER NO. 99: P. Lengweiler. A. Moser, P.V. Nielsen: New Funtions to Model Measured Deposition and Resuspension Rates of Pat1iles. ISSN R9924. PAPER NO. 1: P. Lengweiler, A. Moser, P.V. Nielsen, H. Takai : Modelling Measured Deposition and Resuspension Rates of Pa11ile s. in Animal Buildings. ISSN R9925. PAPER NO. 11 : P.V. Nielsen: The lmpol1ane of a Thermal Manikin as Soure and Obstale in Full-Sale Experiments. ISSN R9932. PAPER NO. 12: P. Heiselberg: Outline of Hybvent. ISSN R9939. PAPER NO. 13: P. Heiselberg, H. Dam, L.C. Srensen, P.V. Nielsen, K. Svidt: Charateristis of Air Flow Through Windo ws. ISSN R994. Complete list of papers:

17 ISSN R994 Dept. of Building Tehnology and Strutural Engineering Aalborg University, Deember 1999 Sohngaardsholmsvej 57, DK-9 Aalborg, Deri111ark Phone: Fa x dk