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Aerodynamic efficiency of smoke ventilators in light streets and shed-type roofs
Journal of Wind Engineering and bMustrial Aerodynamics. 45 (1993) 341 353 Elsevier Science Publishers
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Aerodynamic efficiency of smoke ventilators in light streets and shed-type roofs H.J. G e r h a r d t a n d C. K r a m e r I.F.I. Institut fiir Industrieaerodynamik GmbH, Institut an der Fachhochschulc Aachetz, VVelkenrather Strafle 120, W-31OOAachen. Germany
Summary Low-rise industrial buildings in continental Europe have usually no or very little window area in the sidewalls. To provide the necessary daylight, translucent surfaces are fitted in the roof. Well known examples are shed roofs or curved and shed-type light streets in fiat roofs. For economic reasons smoke ventilators are then integrated into the light surfaces. This paper gives typical examples of smoke ventilators installed in shed roofs and in curved or shed-type light streets. The measureumnt of the aerodynamic free areas on full scale apparatus is not possible due to the large dimensions of the relevant roof surfaces. Therefore, tests have to be conducted in model scale. The relevant similarity considerations for such model tests are discussed and the applicability of model scale tests is demonstrated. Finally. the most important parameters influencing the aerodynamic efficiency of typical ventilator installations in shed-roofs and curved or shed-type light streets are described for the cases without and with side wind.
1. I n t r o d u c t i o n T h e d e s i g n r u l e s for s m o k e v e n t i l a t i o n s y s t e m s a r e g i v e n for t h e F e d e r a l R e p u b l i c o f G e r m a n y i n D I N 18232. P a r t 3 o f t h i s s t a n d a r d p r e s e n t s t h e guidelines for the testing of smoke ventilators. The determination of the a e r o d y n a m i c f r e e a r e a Aeff a s g i v e n in t h e s t a n d a r d is o n l y v a l i d for s m o k e v e n t i l a t o r s in f l a t r o o f s o r r o o f s w i t h s m a l l i n c l i n a t i o n a n g l e ( 0 < 2 5 ) . In p a r t i c u l a r , s e c t i o n 3.7 o f D I N 18 232 p a r t 3 s t a t e s t h a t " v e n t i l a t o r s o f s p e c i a l d e s i g n s u i t a b l e , for e x a m p l e , f o r i n s t a l l i n g e x c l u s i v e l y in r o o f a r e a s w i t h a p i t c h e x c e e d i n g 2 5 , s h a l l be t e s t e d on t h e l i n e s o f t h i s s t a n d a r d " . In i n d u s t r i a l b u i l d i n g s w i t h s h e d roofs, t h e s m o k e v e n t i l a t o r s a r e p l a c e d in t h e r e l a t i v e ly s t e e p l y i n c l i n e d s h e d s u r f a c e s , in i n d u s t r i a l b u i l d i n g s w i t h flat roofs, t h e y are commonly placed in curved light streets or shed-type light streets.
Correspondence to: H.J. Gerhardt, I.F.I. Institut ffir Industrieaerodynamik GmbH, Institut an der Faehhochschule Aachen, Welkenrather Stral3e 120, W-5100 Aachen, Germany. 0167-6105/93/$06.00 4 ~, 1993 Elsevier Science Publishers B.V. All rights reserved.
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H.J. (;erhardt, ('. Kram('r/Aerodynamic c[]iciency of smoke t (mtilator,~
W h e n placing the smoke v e n t i l a t o r s in shed rooi~, curved light streei~ or shed-type light streets, t h e i r a e r o d y n a m i c efficiency will be greatly influenced by the flow field a r o u n d the individual roof surfaces. The flow field ar~)und a shed-type light s t r e e t for example has to be superimposed on the building flow field. The element flow field may lead to an additional flow a c c e l e r a t i o n with a s u b s e q u e n t pressure decrease or to a s t a g n a t i n g flow with a subseqm, nt pressure increase in the vicinity of the smoke ventilators. Due to the ]~t'ge a r c h i t e c t u r a l wu'iety it is not possible to giw~ g e n e r a l l y wdid inf'orma,~ion c o n c e r n i n g the influence of the p a r t i c u l a r i n s t a l l a t i o n of a smoke v e n t i l a t o r on its a e r o d y n a m i c efficiency. Thus, the a e r o d y n a m i c free area has to be determined e x p e r i m e n t a l l y for each individual type of installation. This individual, building-related test has been r e q u e s t e d by the working group "Testin~ (,{' smoke v e n t i l a t o r s of DIN 18 232 part 3 (Arbeitskreis " P r i i f u n g e n f'/ir I~auchab. z/ige (APR)"). The i n t e r f e r e n c e of the flow fiehl a r o u n d two subsequent shedtype light streets may lead to a s t a g n a t i n g flow at the d o w n s t r e a m light s;t ~'e(>t and t h u s to a considerable r e d u c t i o n in the a e r o d y n a m i c efficiency of a smoke v e n t i l a t o r placed t h e r e as compared to the same smoke v e n t i l a t o r placed i i~ an isolated shed-type light street. After reviewing typical installations of smoke ventilators in shed roofs and in curved and shed-type light streets the a e r o d y n a m i c free areas determined in model scale investigations will be presented for some cases of practical importance.
2. Typical ventilator installations 2.1. L i g h t streets
Curved strip-like r o o f areas consisting of cylindrical sections of' t r a n s p a r e n t m a t e r i a l like plexiglass or p o l y c a r b o n a t e , are c o m m o n l y called light streets. T h e y are often placed in flat roofs of industrial buildings to allow sufficient d a y l i g h t to e n t e r the building w i t h o u t the need for windows. The g e o m e t r y may be described by the width B , the radius of c u r v a t u r e R and the m a x i m u m height S, see Fig. 1. The m a x i m u m h e i g h t is usually in the r a n g e of 1/6 to 1/2 of the width B. The width may be up to 7.5 m; for a m a x i m u m r e l a t i v e h e i g h t 1/5 this would lea(! to S - 1.5 m. F i g u r e 2 gives some typical smoke v e n t i l a t o r a r r a n g e m e n t s in light streets. For small spans (B~<2.5 m) a light s t r e e t e l e m e n t e x t e n d i n g over the whole width is n o r m a l l y opened for v e n t i n g purposes, see Fig. 2a. F o r l a r g e r spans the two a r r a n g e m e n t s shown in Figs. 2b and 2c may be used. For the arrangements a c c o r d i n g to Figs. 2a and 2b single flaps as s h o w n or double flap s may be used. F i g u r e 3 gives a n o t h e r typical v e n t i l a t o r a r r a n g e m e n t in light streets. Again, single or double flaps are possible. F u r t h e r m o r e , u n s y m m e t r i c a l light s t r e e t geometries are in use. 2.2. S h e d roofs
Shed roofs consist of r o o f elements of identical t r i a n g u l a r shape e x t e n d i n g parallel to each other, see Fig. 4. S y m m e t r i c a l s h e d roofs with r o o f slopes
H.J. Gerhardt, C. Kramer,'Aerod)'namic e/]icienc)' o/ smoke ventilators
"kR J
B
Fig. 1. Typical shapes oflightstreets having different relative heights S/B.
if-
(c)
Fig. '2. Commonly used smoke ventilator arrangements for lightstreets.
34;{
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H.J. Gerhardt, C. Kramer/Aerodynamic e~iciency of smoke ventilators
\
•
//
,y -//
, ,/
, ~,
,,-~-,~\~ % "\ ~.
.
.
/
1 I .....
E Fig. 3. Typical arrangement of smoke ventilators in lightstreets with straight sideparts.
Fig. 4. Commonly used shed-roofs.
15 '/15 '; 3 0 / 3 0 :; 45'/45':' and 60'/60' and unsymmetrical arrangements with roof slopes 6ff/30 '; 9 0 / 3 0 and 90"y15 <~are common. Louvre=type smoke ventilators, single flap (see Fig. 5) or double flap (see Fig. 6) arrangements are normally used.
H.J. Gerhardt, C. Kramer/Aerodynamic efficiency of smoke ventilators
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Fig. 5. Single-flap smoke ventilators in shed-roofs or shed-type lightstreets.
Fig. 6. Double-flap ventilators for shed-roofs and shed-type |ightstreets.
2.3. Shed-type light streets A typical a r r a n g e m e n t of shed-type light streets in a flat r o o f is s h o w n in Fig. 7. The s m o k e v e n t i l a t o r s s h o w n in the p h o t o g r a p h are in the v e n t i l a t i o n position. The wind flow a b o u t the shed-type light s t r e e t s is influenced by the h e i g h t H a b o v e the flat r o o f surface, the width B, the i n c l i n a t i o n a n g l e 6) and the s p a c i n g T o f t w o n e i g h b o u r i n g light streets. M o r e i n f o r m a t i o n will be given in Section 3.3. Typical spacings ( = d i s t a n c e b e t w e e n two light s t r e e t axes) are in the r a n g e of T = 20 m for width B = 4 m and h e i g h t H = 2.5 m. S m o k e v e n t i l a t o r s for use in shed roofs m a y also be installed in shed-type light streets. T h e y m a y differ, h o w e v e r , in the a r r a n g e m e n t and size of the wind deflectors n e c e s s a r y to e n s u r e the n e c e s s a r y i n s e n s i t i v i t y to side wind effects.
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H.J. Gerhardt, C. Kramer/Aerodynamic e/flciency of smoke ventilator,~"
Fig. 7. View of shed-type lightstreets m a ttat roof with smoke ventilators.
3. Determination of the aerodynamic efficiency E x p e r i m e n t a l i n v e s t i g a t i o n s to d e t e r m i n e the a e r o d y n a m i c free a r e a of s m o k e v e n t i l a t o r s installed in shed-roofs and in c u r v e d or shed-type light s t r e e t s may, w i t h o u t u n d u e e x p e n d i t u r e , o n l y be Conducted in model scale. R o o f a r e a s with v e n t i l a t o r s to be c o n s i d e r e d {'or t e s t i n g h a v e typical dimensions of h e i g h t H = 1.5 m t h r o u g h 2.5 m - for l a r g e s p a n s of l i g h t s t r e e t s even m u c h m o r e and v e n t i l a t o r d i m e n s i o n s in the d i r e c t i o n of the light s t r e e t axis of up to a b o u t 2.5 m. T o e n s u r e a r e a l i s t i c flow s i t u a t i o n for the exiting flow n e a r the v e n t i l a t o r , l i g h t s t r e e t s u r f a c e s of a b o u t equal d i m e n s i o n as the s m o k e v e n t i l a t o r d i m e n s i o n in d i r e c t i o n of the s t r e e t axis h a v e to be p r o v i d e d at b o t h sides of the opening. Thus, t y p i c a l d i m e n s i o n s for the t e s t s p e c i m e n are 7 . 5 m l e n g t h and 2 . 5 m height. R e s t r i c t i n g the test s e c t i o n b l o c k a g e r a t i o c = Ave.ti~tor/At¢~, ~¢~tio,to a b o u t 15% for open, free jet t e s t s e c t i o n s a n d to a b o u t 5% for closed t e s t sections, the n e c e s s a r y cross s e c t i o n a r e a s of the test s e c t i o n a r e A,e~, ~¢¢tio.= 125 m 2 for o p e n and 375 m 2 for closed t e s t sections: To c o n d u c t full scale wind t u n n e l m e a s u r e m e n t s on s m o k e v e n t i l a t o r s i n s t a l l e d in shed-type light streets the n e c e s s a r y t e s t s e c t i o n l e n g t h w o u l d be a b o u t 20 m or more. It is o b v i o u s t h a t t e s t i n g full scale s m o k e v e n t i l a t o r s i n s t a l l e d :in l i g h t s t r e e t s or shed roofs is not feasible. T h e r e f o r e , a e r o d y n a m i c t e s t i n g of such s m o k e v e n t i l a t o r s is only c o n d u c t e d in model scale.
H..I. Gerhardt, C. Kramer/Aerodynamic e[]iciency of smoke ventilators
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3.I. Similarity considerations The discharge coefficient c,o without consideration of side wind effects depends mainly on the ventilator geometry and the Reynolds number. The Reynolds number for the test conditions specified in the German standard lies in the range of 2 × 105 through 4 × 105 for typical ventilator dimensions. Here, the Reynolds number is based on the hydraulic diameter of the ventilatot and on the mean exit velocity vM-2.86 m/s corresponding to the standard condition tbr the pressure differential between settling chamber and ambient Ap=5 Pa. According to the detailed studies of Pepping [1] on rect angul ar openings m a large wall the discharge coefficient depends only mildly on the Reynolds number in this Reynolds-number range. Thus, the c,.-value of common ventilators is influenced mainly by the upstand height parameter. The upstand height parameter 5 is the ratio of the upstand height or the height, of the flow channel given by the louw:es of louvre type ventilators related to the hydraulic diameter. The influence of the upstand height parameter has been studied extensively by Kramer and Gerhardt [2] on full scale ventilators and by Meier [3] on scale models. The relatively small Reynolds number sensitivity is due to the sharp edged construction of commonly used ventilators. The discharge coefficient with side wind influence is mainly determined by the external pressure field around the ventilator. For ventilators with sharp edged upstand, frame of the flap and wind deflectors, this flow field is mainly due to the flow separation at the sharp edges. Reynolds-number effects may only occur for well curved ventilator designs. The characteristic velocity for calculating the Reynolds number is the velocity of the undisturbed side wind flow U,. For full scale testing the side wind velocity U, = 10 m/s is specified in DIN 18 232 part 3. For model scale testing Reynolds similarity may he achieved by increasing the side wind velocity. Meier [4] showed that even tbr ridge ventilators with curved external surfaces the external pressure distribution is independent of the Reynolds number within the accuracy of measurement. The variation in external pressure distribution due to the turbulence of the oncoming flow is equally negligible. In summary, model scale tests on smoke ventilators with scales of about l :10 through 1:3 will lead to discharge coefficients which may be directly applied to full scale ventilators.
3.2. Discharge coefficient without side w i n d The discharge coefficients fbr smoke ventilators in light streets of relatively small width, see Fig. 2a, are usually in the range of e,.o=0.7 through 0.75. Placing ventilators in the ridge area of curved light streets, see Fig. 2b, will lead to flow situations where the aerodynamic efficiency depends on the curved geometry and on the relative position of the ventilator in the curved light street. The relative position may be charaeterised by the relative step height Ah/Ab, see Fig. 8. It presents as an example results of a model study. The
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H.J. Gerhardt, C. Kramer/Aerodynamic e/fieiency of smoke ventilators
discharge coefficient is plotted versus the relative step height Ah/Ab. For unsymmetrical ventilator arrangements in light streets of large span, see Fig. 2c, the discharge coefficient C,o depends also on the relative position of the ventilator. However, the variation of the C,o-values are much smaller than ~br ventilator arrangements symmetrical to the ridge. Louvre-type ventilators or ventilators with flat flaps are usually ust~d for installation in shed roofs and shed-type roofs. For both ventilator types the discharge coefficient without side wind depends mainly on the opening angle and in a much smaller measure on the roof surface inclination ang!e 6). Figure 9 gives as a typical example results of measurements for a single flap v e n tilato r installed in a 30 /30:-shed-type light street, Fig. 10 the equivalent c~o-plot for a louvre-type ventilator. Both plots exhibit a so-called saturation behaviour, i.e. the c,,,-values approach asymptotically a limiting value. Within the accu r acy of measurement the limit is reached for opening angles ~-= 75:. Larger opening angles are therefore not necessary.
c[vO] 0,8 i ---~ . . . . . . . . . . . . . . . . . . . . i
o
i
0,75
:
i
i
:
! 0,7
i
]
0,65
.............
: ..........
i
~. . . . . . . . . .
0,6 0
2
4 Z~h/A
:
~
L
8
10
........
12
b
Fig. 8. Discharge coefficient without sidewind C,:ofor smoke ventilators in lightstreets of varying street width.
It.J. Gerhardt, C. Kramer/'Aerodynan~ic effi(ieney of smoke veutilators
349
c[vO] 0,7 -
0,65 i
I
i
0,6
T-
i-'
[
!
J
: i
I 0,55
0,5
!
,
80
90
0,45
30
40
50
60
70
1O0
alpha
Fig. 9. Discharge coefficient without sidewind c , for single-flap ventilators in a shed-type lightstreet versus flap opening angle.
3.3. Discharge coefficient with side wind To obtain a sufficient insensitivity concerning side wind effects, the smoke ventilators have usually to be provided with wind deflectors. The efficiency of wind deflectors is discussed extensively in [2]. The necessary height of the wind deflectors for smoke ventilators installed in light streets of relatively small span (B~<2.5 m) depends on the ventilator size and on the opening angle• Figure 2 shows typical deflector arrangements. For smoke ventilators in shed roofs the critical wind direction leading to the smallest discharge coefficient is the wind flow parallel to the ridge. Therefore, wind deflectors at the ventilator sides perpendicular to the ridge are sufficient to ensure the necessary side wind insensitivity, see Figs. 5 and 6. The necessary height of the wind deflectors depends on the length of the opening parallel to the ridge. A deflector height of 10% of the opening length is sufficient for typical single-flap ventilators with an aerodynamic efficiency corresponding to C~w= 0•65•
H.J. Gerhardt, C. Kramer/Aerodynamic efficiency of smoke ventih~tors
350 c[vO] 0,7
0,65
. . . .
0,6
~
)i
.
0,55 . . . . . .
]
i!
]
o,5
--t---
i
I I
0,4 30
40
50
60
70
80
90
1O0
alpha
Fig. 10. Discharge coefficientwithout sidewind c for a louvre-type ventilator in a shed-type lightstreet versus opening angle. The optimisation of wind deflectors for ventilators in shed-type light streets is much more difficult. For ventilators in shed-type light streets with small inclination angles, e.g. 1 5 / 1 5 the necessary deflector arrangements are simliar to the ones used on lightdome-type smoke ventilators installed in fiat roofs, see Fig. 6, For larger inclination angles of shed-type light streets, e.g. 45/45 , the necessary wind deflectors are similar to the ones used in shed roofs. However, the interference of consecutive shed-type light streets has to be taken into account. Figure 11 shows schematically the flow situation. For critical distances of the light streets the flow separating at the ridge of the upstream light street will reattach at the ventilator position of the downstream light street. Rather t h a n venting smokes fresh air will now flow into the smoke layer underneath the roof. The main influence parameters of the discharge coefficient with side wind for the arrangement shown in Fig. 11 are c~w=f(T/B; H/B; hwLw/B; inclination angle ~; ventilator geometry). The determination of the critical spacing Tk of shed,type l i g h t streets may be based on a fundamental study of the flow field around block-type building
H.J. Cerhardt, C. Kramer/Aerodynanfic efficiency of smoke ventilators
351
i' i
°
I
V V
""2, WLW
.- "
',
/
5!1
~/=
il
/' Dachfl&che i,
Fig. 1 I. W i n d flow a r o u n d s h e d - t y p e light-streets with smoke ventilators.
models. The distance of the reattachment point of a flow separating at the leading edge of the roof surface may be taken as the equivalent of the critical spacing Tk. The applicability of the results of a fundamental study by Gowda. Gerhardt and Kramer [5] to the problem of the aerodynamic efficiency of smoke ventilators in shed-type roof surfaces has already been discussed in [6]. The evaluation of the results presented in [5], using the nomenclature according to Fig. 11 leads to the critical relative spacing Tk/B as function of the relative height of the shed-type light streets H/B, see Fig. 12. Included in this diagram
352
H.J. Gerhardt, C. Kramer/ Aerodynamic efficiency of smoke ventilators
T[kl/B 4
/
/..-
c[vw]>,c[vo]
/
f
/
/-
/
/
/
c[vw] < c[v0]
2t
0 -
o
0,2
0,4
0,6
0,8
1
1,2
H/B
Fig. 12. Critical relative spacing T~./B versus relative height of shed-type lightstreet h/b.
are the a v a i l a b l e results of model studies into the a e r o d y n a m i c efficiency of smoke v e n t i l a t o r s in shed-type light streets. T h e model scale tests on v e n t i l a t o r s a g r e e v e r y well with the results of the f u n d a m e n t a l study. A sufficient side wind i n s e n s i t i v i t y of c o m m o n v e n t i l a t o r s m a y be o b t a i n e d for r e l a t i v e spacings l a r g e r t h a n the critical spacing. F o r v e r y small spacings a sufficient side wind i n s e n s i t i v i t y is possible, too. H o w e v e r , the l o w e r limit of the critical r a n g e of spacings depends far more on the shape of the v e n t i l a t o r and on the shape of the wind deflectors t h a n the u p p e r b o u n d of the critical range.
References [1] E. Pepping, Die Durchflu6zahl des Rechteckschlitzes in einer sehr groBen Wan d, Forschungsbericht des Wirtschafts- und Verkehrsministeriums Nordrhein-Westfalen Nr. 330, Westdeutscher Verlag, K61n und Opladen (1957). [2] C. Kramer und H.J. Gerhardt, Aerodynamische RA-Optimierung, Industriebau 5/85, 358-362. [3] H.U. Meier, Ober die Wirkungsweise, Auslegungsprobteme und Prfifungen von Rauchund W~irmeabzfigen (RWA), VFDB-Zeitschrift 2/76, 50-55.
tt.J. Gerhardt, C. Kramer/Aerodynamic efficiency of smoke ventilators
353
[4] H.U. Meier, D. Baumgarten, D. Htibner and A. Landhatiser, The measurement of wind loads on roof ventilators, J. Wind Eng. Ind. Aerodyn., 11 (1983) 261 271. [5] B.H.L. Gowda, H.J. Gerhardt and C. Kramer, Surface flow field around three-dimensional bluff bodies, J. Wind Eng. Ind. Aerodyn., 11 (1983) 405 420. [61 C. Kramer and H.J. Gerhardt, Wind effects on heat and smoke control of industrial buildings in case of a fire, J. Wind Eng. Ind. Aerodyn., 36 (1990) 499 508.
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Aerodynamic efficiency of smoke ventilators in light streets and shed-type roofs H.J. G e r h a r d t a n d C. K r a m e r I.F.I. Institut fiir Industrieaerodynamik GmbH, Institut an der Fachhochschulc Aachetz, VVelkenrather Strafle 120, W-31OOAachen. Germany
Summary Low-rise industrial buildings in continental Europe have usually no or very little window area in the sidewalls. To provide the necessary daylight, translucent surfaces are fitted in the roof. Well known examples are shed roofs or curved and shed-type light streets in fiat roofs. For economic reasons smoke ventilators are then integrated into the light surfaces. This paper gives typical examples of smoke ventilators installed in shed roofs and in curved or shed-type light streets. The measureumnt of the aerodynamic free areas on full scale apparatus is not possible due to the large dimensions of the relevant roof surfaces. Therefore, tests have to be conducted in model scale. The relevant similarity considerations for such model tests are discussed and the applicability of model scale tests is demonstrated. Finally. the most important parameters influencing the aerodynamic efficiency of typical ventilator installations in shed-roofs and curved or shed-type light streets are described for the cases without and with side wind.
1. I n t r o d u c t i o n T h e d e s i g n r u l e s for s m o k e v e n t i l a t i o n s y s t e m s a r e g i v e n for t h e F e d e r a l R e p u b l i c o f G e r m a n y i n D I N 18232. P a r t 3 o f t h i s s t a n d a r d p r e s e n t s t h e guidelines for the testing of smoke ventilators. The determination of the a e r o d y n a m i c f r e e a r e a Aeff a s g i v e n in t h e s t a n d a r d is o n l y v a l i d for s m o k e v e n t i l a t o r s in f l a t r o o f s o r r o o f s w i t h s m a l l i n c l i n a t i o n a n g l e ( 0 < 2 5 ) . In p a r t i c u l a r , s e c t i o n 3.7 o f D I N 18 232 p a r t 3 s t a t e s t h a t " v e n t i l a t o r s o f s p e c i a l d e s i g n s u i t a b l e , for e x a m p l e , f o r i n s t a l l i n g e x c l u s i v e l y in r o o f a r e a s w i t h a p i t c h e x c e e d i n g 2 5 , s h a l l be t e s t e d on t h e l i n e s o f t h i s s t a n d a r d " . In i n d u s t r i a l b u i l d i n g s w i t h s h e d roofs, t h e s m o k e v e n t i l a t o r s a r e p l a c e d in t h e r e l a t i v e ly s t e e p l y i n c l i n e d s h e d s u r f a c e s , in i n d u s t r i a l b u i l d i n g s w i t h flat roofs, t h e y are commonly placed in curved light streets or shed-type light streets.
Correspondence to: H.J. Gerhardt, I.F.I. Institut ffir Industrieaerodynamik GmbH, Institut an der Faehhochschule Aachen, Welkenrather Stral3e 120, W-5100 Aachen, Germany. 0167-6105/93/$06.00 4 ~, 1993 Elsevier Science Publishers B.V. All rights reserved.
342
H.J. (;erhardt, ('. Kram('r/Aerodynamic c[]iciency of smoke t (mtilator,~
W h e n placing the smoke v e n t i l a t o r s in shed rooi~, curved light streei~ or shed-type light streets, t h e i r a e r o d y n a m i c efficiency will be greatly influenced by the flow field a r o u n d the individual roof surfaces. The flow field ar~)und a shed-type light s t r e e t for example has to be superimposed on the building flow field. The element flow field may lead to an additional flow a c c e l e r a t i o n with a s u b s e q u e n t pressure decrease or to a s t a g n a t i n g flow with a subseqm, nt pressure increase in the vicinity of the smoke ventilators. Due to the ]~t'ge a r c h i t e c t u r a l wu'iety it is not possible to giw~ g e n e r a l l y wdid inf'orma,~ion c o n c e r n i n g the influence of the p a r t i c u l a r i n s t a l l a t i o n of a smoke v e n t i l a t o r on its a e r o d y n a m i c efficiency. Thus, the a e r o d y n a m i c free area has to be determined e x p e r i m e n t a l l y for each individual type of installation. This individual, building-related test has been r e q u e s t e d by the working group "Testin~ (,{' smoke v e n t i l a t o r s of DIN 18 232 part 3 (Arbeitskreis " P r i i f u n g e n f'/ir I~auchab. z/ige (APR)"). The i n t e r f e r e n c e of the flow fiehl a r o u n d two subsequent shedtype light streets may lead to a s t a g n a t i n g flow at the d o w n s t r e a m light s;t ~'e(>t and t h u s to a considerable r e d u c t i o n in the a e r o d y n a m i c efficiency of a smoke v e n t i l a t o r placed t h e r e as compared to the same smoke v e n t i l a t o r placed i i~ an isolated shed-type light street. After reviewing typical installations of smoke ventilators in shed roofs and in curved and shed-type light streets the a e r o d y n a m i c free areas determined in model scale investigations will be presented for some cases of practical importance.
2. Typical ventilator installations 2.1. L i g h t streets
Curved strip-like r o o f areas consisting of cylindrical sections of' t r a n s p a r e n t m a t e r i a l like plexiglass or p o l y c a r b o n a t e , are c o m m o n l y called light streets. T h e y are often placed in flat roofs of industrial buildings to allow sufficient d a y l i g h t to e n t e r the building w i t h o u t the need for windows. The g e o m e t r y may be described by the width B , the radius of c u r v a t u r e R and the m a x i m u m height S, see Fig. 1. The m a x i m u m h e i g h t is usually in the r a n g e of 1/6 to 1/2 of the width B. The width may be up to 7.5 m; for a m a x i m u m r e l a t i v e h e i g h t 1/5 this would lea(! to S - 1.5 m. F i g u r e 2 gives some typical smoke v e n t i l a t o r a r r a n g e m e n t s in light streets. For small spans (B~<2.5 m) a light s t r e e t e l e m e n t e x t e n d i n g over the whole width is n o r m a l l y opened for v e n t i n g purposes, see Fig. 2a. F o r l a r g e r spans the two a r r a n g e m e n t s shown in Figs. 2b and 2c may be used. For the arrangements a c c o r d i n g to Figs. 2a and 2b single flaps as s h o w n or double flap s may be used. F i g u r e 3 gives a n o t h e r typical v e n t i l a t o r a r r a n g e m e n t in light streets. Again, single or double flaps are possible. F u r t h e r m o r e , u n s y m m e t r i c a l light s t r e e t geometries are in use. 2.2. S h e d roofs
Shed roofs consist of r o o f elements of identical t r i a n g u l a r shape e x t e n d i n g parallel to each other, see Fig. 4. S y m m e t r i c a l s h e d roofs with r o o f slopes
H.J. Gerhardt, C. Kramer,'Aerod)'namic e/]icienc)' o/ smoke ventilators
"kR J
B
Fig. 1. Typical shapes oflightstreets having different relative heights S/B.
if-
(c)
Fig. '2. Commonly used smoke ventilator arrangements for lightstreets.
34;{
344
H.J. Gerhardt, C. Kramer/Aerodynamic e~iciency of smoke ventilators
\
•
//
,y -//
, ,/
, ~,
,,-~-,~\~ % "\ ~.
.
.
/
1 I .....
E Fig. 3. Typical arrangement of smoke ventilators in lightstreets with straight sideparts.
Fig. 4. Commonly used shed-roofs.
15 '/15 '; 3 0 / 3 0 :; 45'/45':' and 60'/60' and unsymmetrical arrangements with roof slopes 6ff/30 '; 9 0 / 3 0 and 90"y15 <~are common. Louvre=type smoke ventilators, single flap (see Fig. 5) or double flap (see Fig. 6) arrangements are normally used.
H.J. Gerhardt, C. Kramer/Aerodynamic efficiency of smoke ventilators
345
Fig. 5. Single-flap smoke ventilators in shed-roofs or shed-type lightstreets.
Fig. 6. Double-flap ventilators for shed-roofs and shed-type |ightstreets.
2.3. Shed-type light streets A typical a r r a n g e m e n t of shed-type light streets in a flat r o o f is s h o w n in Fig. 7. The s m o k e v e n t i l a t o r s s h o w n in the p h o t o g r a p h are in the v e n t i l a t i o n position. The wind flow a b o u t the shed-type light s t r e e t s is influenced by the h e i g h t H a b o v e the flat r o o f surface, the width B, the i n c l i n a t i o n a n g l e 6) and the s p a c i n g T o f t w o n e i g h b o u r i n g light streets. M o r e i n f o r m a t i o n will be given in Section 3.3. Typical spacings ( = d i s t a n c e b e t w e e n two light s t r e e t axes) are in the r a n g e of T = 20 m for width B = 4 m and h e i g h t H = 2.5 m. S m o k e v e n t i l a t o r s for use in shed roofs m a y also be installed in shed-type light streets. T h e y m a y differ, h o w e v e r , in the a r r a n g e m e n t and size of the wind deflectors n e c e s s a r y to e n s u r e the n e c e s s a r y i n s e n s i t i v i t y to side wind effects.
'546
H.J. Gerhardt, C. Kramer/Aerodynamic e/flciency of smoke ventilator,~"
Fig. 7. View of shed-type lightstreets m a ttat roof with smoke ventilators.
3. Determination of the aerodynamic efficiency E x p e r i m e n t a l i n v e s t i g a t i o n s to d e t e r m i n e the a e r o d y n a m i c free a r e a of s m o k e v e n t i l a t o r s installed in shed-roofs and in c u r v e d or shed-type light s t r e e t s may, w i t h o u t u n d u e e x p e n d i t u r e , o n l y be Conducted in model scale. R o o f a r e a s with v e n t i l a t o r s to be c o n s i d e r e d {'or t e s t i n g h a v e typical dimensions of h e i g h t H = 1.5 m t h r o u g h 2.5 m - for l a r g e s p a n s of l i g h t s t r e e t s even m u c h m o r e and v e n t i l a t o r d i m e n s i o n s in the d i r e c t i o n of the light s t r e e t axis of up to a b o u t 2.5 m. T o e n s u r e a r e a l i s t i c flow s i t u a t i o n for the exiting flow n e a r the v e n t i l a t o r , l i g h t s t r e e t s u r f a c e s of a b o u t equal d i m e n s i o n as the s m o k e v e n t i l a t o r d i m e n s i o n in d i r e c t i o n of the s t r e e t axis h a v e to be p r o v i d e d at b o t h sides of the opening. Thus, t y p i c a l d i m e n s i o n s for the t e s t s p e c i m e n are 7 . 5 m l e n g t h and 2 . 5 m height. R e s t r i c t i n g the test s e c t i o n b l o c k a g e r a t i o c = Ave.ti~tor/At¢~, ~¢~tio,to a b o u t 15% for open, free jet t e s t s e c t i o n s a n d to a b o u t 5% for closed t e s t sections, the n e c e s s a r y cross s e c t i o n a r e a s of the test s e c t i o n a r e A,e~, ~¢¢tio.= 125 m 2 for o p e n and 375 m 2 for closed t e s t sections: To c o n d u c t full scale wind t u n n e l m e a s u r e m e n t s on s m o k e v e n t i l a t o r s i n s t a l l e d in shed-type light streets the n e c e s s a r y t e s t s e c t i o n l e n g t h w o u l d be a b o u t 20 m or more. It is o b v i o u s t h a t t e s t i n g full scale s m o k e v e n t i l a t o r s i n s t a l l e d :in l i g h t s t r e e t s or shed roofs is not feasible. T h e r e f o r e , a e r o d y n a m i c t e s t i n g of such s m o k e v e n t i l a t o r s is only c o n d u c t e d in model scale.
H..I. Gerhardt, C. Kramer/Aerodynamic e[]iciency of smoke ventilators
:~47
3.I. Similarity considerations The discharge coefficient c,o without consideration of side wind effects depends mainly on the ventilator geometry and the Reynolds number. The Reynolds number for the test conditions specified in the German standard lies in the range of 2 × 105 through 4 × 105 for typical ventilator dimensions. Here, the Reynolds number is based on the hydraulic diameter of the ventilatot and on the mean exit velocity vM-2.86 m/s corresponding to the standard condition tbr the pressure differential between settling chamber and ambient Ap=5 Pa. According to the detailed studies of Pepping [1] on rect angul ar openings m a large wall the discharge coefficient depends only mildly on the Reynolds number in this Reynolds-number range. Thus, the c,.-value of common ventilators is influenced mainly by the upstand height parameter. The upstand height parameter 5 is the ratio of the upstand height or the height, of the flow channel given by the louw:es of louvre type ventilators related to the hydraulic diameter. The influence of the upstand height parameter has been studied extensively by Kramer and Gerhardt [2] on full scale ventilators and by Meier [3] on scale models. The relatively small Reynolds number sensitivity is due to the sharp edged construction of commonly used ventilators. The discharge coefficient with side wind influence is mainly determined by the external pressure field around the ventilator. For ventilators with sharp edged upstand, frame of the flap and wind deflectors, this flow field is mainly due to the flow separation at the sharp edges. Reynolds-number effects may only occur for well curved ventilator designs. The characteristic velocity for calculating the Reynolds number is the velocity of the undisturbed side wind flow U,. For full scale testing the side wind velocity U, = 10 m/s is specified in DIN 18 232 part 3. For model scale testing Reynolds similarity may he achieved by increasing the side wind velocity. Meier [4] showed that even tbr ridge ventilators with curved external surfaces the external pressure distribution is independent of the Reynolds number within the accuracy of measurement. The variation in external pressure distribution due to the turbulence of the oncoming flow is equally negligible. In summary, model scale tests on smoke ventilators with scales of about l :10 through 1:3 will lead to discharge coefficients which may be directly applied to full scale ventilators.
3.2. Discharge coefficient without side w i n d The discharge coefficients fbr smoke ventilators in light streets of relatively small width, see Fig. 2a, are usually in the range of e,.o=0.7 through 0.75. Placing ventilators in the ridge area of curved light streets, see Fig. 2b, will lead to flow situations where the aerodynamic efficiency depends on the curved geometry and on the relative position of the ventilator in the curved light street. The relative position may be charaeterised by the relative step height Ah/Ab, see Fig. 8. It presents as an example results of a model study. The
348
H.J. Gerhardt, C. Kramer/Aerodynamic e/fieiency of smoke ventilators
discharge coefficient is plotted versus the relative step height Ah/Ab. For unsymmetrical ventilator arrangements in light streets of large span, see Fig. 2c, the discharge coefficient C,o depends also on the relative position of the ventilator. However, the variation of the C,o-values are much smaller than ~br ventilator arrangements symmetrical to the ridge. Louvre-type ventilators or ventilators with flat flaps are usually ust~d for installation in shed roofs and shed-type roofs. For both ventilator types the discharge coefficient without side wind depends mainly on the opening angle and in a much smaller measure on the roof surface inclination ang!e 6). Figure 9 gives as a typical example results of measurements for a single flap v e n tilato r installed in a 30 /30:-shed-type light street, Fig. 10 the equivalent c~o-plot for a louvre-type ventilator. Both plots exhibit a so-called saturation behaviour, i.e. the c,,,-values approach asymptotically a limiting value. Within the accu r acy of measurement the limit is reached for opening angles ~-= 75:. Larger opening angles are therefore not necessary.
c[vO] 0,8 i ---~ . . . . . . . . . . . . . . . . . . . . i
o
i
0,75
:
i
i
:
! 0,7
i
]
0,65
.............
: ..........
i
~. . . . . . . . . .
0,6 0
2
4 Z~h/A
:
~
L
8
10
........
12
b
Fig. 8. Discharge coefficient without sidewind C,:ofor smoke ventilators in lightstreets of varying street width.
It.J. Gerhardt, C. Kramer/'Aerodynan~ic effi(ieney of smoke veutilators
349
c[vO] 0,7 -
0,65 i
I
i
0,6
T-
i-'
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!
J
: i
I 0,55
0,5
!
,
80
90
0,45
30
40
50
60
70
1O0
alpha
Fig. 9. Discharge coefficient without sidewind c , for single-flap ventilators in a shed-type lightstreet versus flap opening angle.
3.3. Discharge coefficient with side wind To obtain a sufficient insensitivity concerning side wind effects, the smoke ventilators have usually to be provided with wind deflectors. The efficiency of wind deflectors is discussed extensively in [2]. The necessary height of the wind deflectors for smoke ventilators installed in light streets of relatively small span (B~<2.5 m) depends on the ventilator size and on the opening angle• Figure 2 shows typical deflector arrangements. For smoke ventilators in shed roofs the critical wind direction leading to the smallest discharge coefficient is the wind flow parallel to the ridge. Therefore, wind deflectors at the ventilator sides perpendicular to the ridge are sufficient to ensure the necessary side wind insensitivity, see Figs. 5 and 6. The necessary height of the wind deflectors depends on the length of the opening parallel to the ridge. A deflector height of 10% of the opening length is sufficient for typical single-flap ventilators with an aerodynamic efficiency corresponding to C~w= 0•65•
H.J. Gerhardt, C. Kramer/Aerodynamic efficiency of smoke ventih~tors
350 c[vO] 0,7
0,65
. . . .
0,6
~
)i
.
0,55 . . . . . .
]
i!
]
o,5
--t---
i
I I
0,4 30
40
50
60
70
80
90
1O0
alpha
Fig. 10. Discharge coefficientwithout sidewind c for a louvre-type ventilator in a shed-type lightstreet versus opening angle. The optimisation of wind deflectors for ventilators in shed-type light streets is much more difficult. For ventilators in shed-type light streets with small inclination angles, e.g. 1 5 / 1 5 the necessary deflector arrangements are simliar to the ones used on lightdome-type smoke ventilators installed in fiat roofs, see Fig. 6, For larger inclination angles of shed-type light streets, e.g. 45/45 , the necessary wind deflectors are similar to the ones used in shed roofs. However, the interference of consecutive shed-type light streets has to be taken into account. Figure 11 shows schematically the flow situation. For critical distances of the light streets the flow separating at the ridge of the upstream light street will reattach at the ventilator position of the downstream light street. Rather t h a n venting smokes fresh air will now flow into the smoke layer underneath the roof. The main influence parameters of the discharge coefficient with side wind for the arrangement shown in Fig. 11 are c~w=f(T/B; H/B; hwLw/B; inclination angle ~; ventilator geometry). The determination of the critical spacing Tk of shed,type l i g h t streets may be based on a fundamental study of the flow field around block-type building
H.J. Cerhardt, C. Kramer/Aerodynanfic efficiency of smoke ventilators
351
i' i
°
I
V V
""2, WLW
.- "
',
/
5!1
~/=
il
/' Dachfl&che i,
Fig. 1 I. W i n d flow a r o u n d s h e d - t y p e light-streets with smoke ventilators.
models. The distance of the reattachment point of a flow separating at the leading edge of the roof surface may be taken as the equivalent of the critical spacing Tk. The applicability of the results of a fundamental study by Gowda. Gerhardt and Kramer [5] to the problem of the aerodynamic efficiency of smoke ventilators in shed-type roof surfaces has already been discussed in [6]. The evaluation of the results presented in [5], using the nomenclature according to Fig. 11 leads to the critical relative spacing Tk/B as function of the relative height of the shed-type light streets H/B, see Fig. 12. Included in this diagram
352
H.J. Gerhardt, C. Kramer/ Aerodynamic efficiency of smoke ventilators
T[kl/B 4
/
/..-
c[vw]>,c[vo]
/
f
/
/-
/
/
/
c[vw] < c[v0]
2t
0 -
o
0,2
0,4
0,6
0,8
1
1,2
H/B
Fig. 12. Critical relative spacing T~./B versus relative height of shed-type lightstreet h/b.
are the a v a i l a b l e results of model studies into the a e r o d y n a m i c efficiency of smoke v e n t i l a t o r s in shed-type light streets. T h e model scale tests on v e n t i l a t o r s a g r e e v e r y well with the results of the f u n d a m e n t a l study. A sufficient side wind i n s e n s i t i v i t y of c o m m o n v e n t i l a t o r s m a y be o b t a i n e d for r e l a t i v e spacings l a r g e r t h a n the critical spacing. F o r v e r y small spacings a sufficient side wind i n s e n s i t i v i t y is possible, too. H o w e v e r , the l o w e r limit of the critical r a n g e of spacings depends far more on the shape of the v e n t i l a t o r and on the shape of the wind deflectors t h a n the u p p e r b o u n d of the critical range.
References [1] E. Pepping, Die Durchflu6zahl des Rechteckschlitzes in einer sehr groBen Wan d, Forschungsbericht des Wirtschafts- und Verkehrsministeriums Nordrhein-Westfalen Nr. 330, Westdeutscher Verlag, K61n und Opladen (1957). [2] C. Kramer und H.J. Gerhardt, Aerodynamische RA-Optimierung, Industriebau 5/85, 358-362. [3] H.U. Meier, Ober die Wirkungsweise, Auslegungsprobteme und Prfifungen von Rauchund W~irmeabzfigen (RWA), VFDB-Zeitschrift 2/76, 50-55.
tt.J. Gerhardt, C. Kramer/Aerodynamic efficiency of smoke ventilators
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[4] H.U. Meier, D. Baumgarten, D. Htibner and A. Landhatiser, The measurement of wind loads on roof ventilators, J. Wind Eng. Ind. Aerodyn., 11 (1983) 261 271. [5] B.H.L. Gowda, H.J. Gerhardt and C. Kramer, Surface flow field around three-dimensional bluff bodies, J. Wind Eng. Ind. Aerodyn., 11 (1983) 405 420. [61 C. Kramer and H.J. Gerhardt, Wind effects on heat and smoke control of industrial buildings in case of a fire, J. Wind Eng. Ind. Aerodyn., 36 (1990) 499 508.