1、4740 Wind Effects on Performance of Static Smoke Exhaust Systems: Horizontal Ceiling Vents W.K. Chow, Ph.D. Member ASHRAE ABSTRACT For static smoke exhaust systems, such as horizontal ceil- ing vents, buoyancy of the smoke layer is the driving force for smoke removal. Howevel; wind efSect should als
2、o be consid- ered, as the smoke layer interface height can be raised up or pulled down, depending on the conditions. Key equations on calculating the smoke exhaust rates and the required vent area will be reviavedjrst in this paper. Modifications of those equa- tions with wind eflects are discussed.
3、 An atrium is taken as an example to study the wind eflects under two scenarios: apre at the atriumfloor to give an misymmetric plume and afire at a shop adjacent to the atrium to give a balcony spill plume. INTRODUCTION Natural vents, also known as a static smoke exhaust system in some fire codes (
4、e.g., Fire Services Department 1994), are commonly installed in large atria for removing smoke in the Far East. A natural venting system is sometimes more preferred because of the difficulties in allocating spaces for the mechanical system. Further, the extraction rates might be so big that it is no
5、t easy to design a workable system. Cost is another concern. Most of them are horizontal ceiling vents installed at the roof, especially in those atria located at the central core of a building. The driving forces for natural venti- lation (e.g., Klote and Milke 1992) are stack effect due to tempera
6、ture differences between indoors and outdoors, wind- induced action, and buoyancy of smoke. In areas with low temperature difference between indoors and outdoors, stack effect is only significant in tall lift shafts or staircases (Hung and Chow 2001). Wind-induced air flow is a transient phenomenon
7、depending on the ambient condi- tions. Buoyancy of the hot smoke layer is rather strong in an J. Li atrium fire, especially during a later stage of the fire. There- fore, natural vent design was based on removing smoke by taking buoyancy as the driving force. But when sufficiently strong wind is blo
8、wing toward the atrium, positive or negative pressure might be induced at the windward and leeward sides. The ceiling vent might become an air intake point rather than an extract point (Marchant 1984; Kandola 1990; Than 1992; Ingason and Persson 1995; Poreh and Trebukov 2000), depending on the vent
9、pressure at that moment in comparing with the pressure distribution inside. Under extreme conditions, downward wind pressure might be even greater than the upward pressure induced by buoyancy. Therefore, wind pressure should be considered in calculating the smoke exhaust rate through the ceiling ven
10、t. In an atrium fire, smoke might be originated from a fire on the atrium floor or from a compartment adjacent to the atrium. An axisymmetric plume or a spill plume can be expected, respectively, for these two fire conditions WFPA 2000). Since the mass entrainment rate into the rising plume increase
11、d rapidly with the height of rise of the plume, a higher smoke layer height would result in a higher smoke exhaust rate and, hence, a cooler smoke layer. According to Morgan et al. (1999), the practical limitations to the use of the vent through the atrium are: maximum mass flow rate of 150-200 kg/s
12、 and or minimum smoke layer temperature of 20C above the vent. These limitations can be applicable to either a static ventila- tion system or a mechanical ventilation system. Based on the estimation of the mass entrainment rate of the balcony spill plumes, they suggested that one or another limit is
13、 usually reached when the height of rise above the fire room opening exceeds 8-12 m (e.g., Hansel1 and Morgan 1994, Morgan et W.K. Chow is chair professor of architectural science and fire engineering and director of the Research Centre for Fire Engineering, and J. Li is a Ph.D. student in the Depar
14、tment of Building Services Engineering, The Hong Kong Polytechnic University, Hong Kong. 02004 ASHRAE. 479 al. 19991). These limitations should be considered carefully in designing static ventilation systems for tall atria. In this paper, key equations in traditional flow models (eg, NFPA 2002, Hans
15、el1 and Morgan 1994, Klote and Milke 19921, Morgan et al. 19991) for sizing horizontal ceil- ing vents will be reviewed. The effect of wind action on smoke exhaust (Marchant 1984; Kandola 1990; Than 1992; Ingason and Persson 1995; Poreh and Trebukov 2000) for atria is then discussed. Both axisymmetr
16、ic plume (eg, Heskestad 1982, 20021) due to a fire at the atrium floor and balcony spill plume (eg, Law 1986, NFPA 2000, CIBSE 1997) due to a fire in a shop adjacent to the atrium are considered. - I. TRADITIONAL VENT FLOW MODEL DUE TO BUOYANCY A typical description of an elevated smoke layer in an
17、atrium with a natural vent is shown in Figure la. This is the physical basis of two-layer zone models and some design guides for smoke management systems. The ceiling jet is assumed to be completely immersed in the smoke layer in most of the zone models. Circulation within the layer is not considere
18、d, giving a stagnant environment at a uniform temperature. Mixing between the smoke layer and the cool air underneath is inhibited by the density difference and neglected in many simulations. Following the analysis of Rayleigh- Taylor instability (Sharp 1984), lighter fluid placed above a dense flui
19、d with an acceleration acting perpendicular toward their intersection plane will give a stable situation. It is assumed that the smoke layer is effectively stagnant and thick enough to give a length scale bigger than the linear dimension of the vent. Applying Bernoullis theorem between points A and
20、C (e.g., Ingason and Persson 1995) with pressure PA and Pc for an atrium with height Hin terms of the - A H Po ambient air density pa and temperature Ta, smoke density pg and temperature Tg, outlet velocity v, inlet velocity vA, smoke layer interface height Hg, and atmospheric pressures at the floor
21、 and ceiling levels Po andPoo, together with the continuity equation through discharge coefficients of the inlet and outlet vents Ci and Co, respectively, gives 1 12 Applying the ideal gas law and subtracting pgT, from both sides of the above equation gives the mass flow rate as r 1 Detailed derivat
22、ion of me is shown in Appendix A. Assuming A, A, the mass now through the ceiling vent can be calculated by Equations 2 and 3 are identical by taking A, in Equation 3 as an effective vent area A, defined by (eg, Cooper 2002) (4) (b) balcony spill plume Figure I Geometry of the problem. 480 ASHRAE Tr
23、ansactions: Research Figure 2 Efect of inlet area on the egective vent area. Ratios of AL to A, are plotted against the ratio ofAA to one of the plume models is mp = 0.071 QE3Hp/3 + O.OOlSQ, . (8) Balcony spillplume (e.g., CIBSE 1997, NFPA 2000, Klote and Milke 19921, Law 19861) in a shop adjacent t
24、o the atrium, as in Figure lb, with one plume model given by (9) 1 /3 m P = 0.36(Q#) (Hg-0.75hb), where W is the width of the balcony and hb is its height above the floor. There have been numerous arguments (Thomas 2000) on selecting suitable plume equations for the above. The details of deriving th
25、ose equations and validation by experiments or computational fluid dynamics can be found in the literature (Chow and Li 2001; Chow and Yin 2002) and will not be repeated in this paper. ASHRAE Transactions: Research 481 rior pressures on a building due to wind are related to the wind velocity by the
26、expression (10) v, = 2 P, - Pref = cwpav,/2 . In the above equation, P, is the pressure due to wind, Pref is the reference pressure that is taken as the static pressure of the undisturbed airflow, v, is the velocity of undisturbed free wind stream, and C, is the pressure coefficient. Values of C, ca
27、n be positive or negative, depending on the surface of the building, wind direction, surrounding buildings, and the local terrain structure. It can be measured fairly accurately in wind tunnel experiments with scale models. Apply Bernoullis theorem between points A and C under windy conditions (e.g.
28、, Ingason and Persson 1995), with subscript on quantities to denote those under wind action, - . (20) - 2 (Cw,- Cw)Pavw+2g(Pa- Pg)(H-H,) (1 1) 1 2 Po = P +-pav, A2 Applying the ideal gas law to v, , the mass flow rate from the outlet under the wind condition is given by 1112 r (21) Detailed derivati
29、on of me is also shown in Appendix A. For an air intake area much greater than the venting area, A, ” A, (12) 1 2 28 P, = -p vc +Poo Define the pressure difference across the inlet opening and roof vent as AP; = P, - POo (14) since 2 (16) Pav, 2 Po = CwA-+Po where C, is the outside pressure coeffici
30、ent at the inlet loca- tion and Cwc is the outside pressure coefficient at the vent location. Putting Equations 15,16, and 17 into Equations 1 I and 12 would give Define M,as the pressure difference across the roof vent due to buoyancy, and CPF is the internal pressure coefficient generated by buoya
31、ncy. L (23) 1 PaV, AP, = P,(T,- T,)(H - H 1- = c - g Tg pF 2 This gives For a roof vent operating under the “extraction” mode, C, - C, + CpF must be positive. If C, - C, 5 -epF, the effect due to wind action would be greater than that due to buoyancy. The net outflowing rate might be zero, or even c
32、ool air above the ceiling vent would flow down to the atrium. This must be avoided in designing static smoke exhaust systems. The critical wind speed Y, to give such failure conditions for a vent can be calculated by ASHRAE Transactions: Research or The performance of the natural vent will not be ef
33、fective when the wind speed exceeds this critical value. Installing a standby mechanical ventilation system might be considered if feasible. From the mass flow equation and assuming no changes in CwA, the formation of a negative pressure zone above the vent plane (i.e., Cw, O) would reduce the vent
34、efficiency. Similarly, calculations based on the above equation show that the system is more efficient when the inlet opening is located at the windward side (CWA O) in comparison to the leeward side (C, O). From the above analysis, negative Cw, or positive CwA would give a larger smoke extraction r
35、ate. But if the value is too large, fresh air under the smoke layer might also be extracted through the vent. A thicker smoke layer would result and more occupants would be exposed to smoke. This phenomenon is known as “plugholing“ (Spratt and Heselden 1974; Morgan and Gardner 1990) and the critical
36、 volume flow rate for avoiding this is The critical wind velocity can be derived as Ifthe wind speed exceeds this value, more vents should be provided to give higher efficiency. Putting in conservation of energy in the upper layer, with Tg expressed in terms of Q, m, , and Cp, ignoring the wind effe
37、ct on the air entrainment in the plume, and with mass conservation of the upper smoke layer, Equation 24 can be rewritten as The smoke layer interface height under the windy condi- tion is given as In the above equation, mP is given by the plume Equations 8 or 9, depending on the fire scenario. The
38、new smoke layer interface height Hi due to wind action can be calculated by a numerical method, as Equation 29 is quite complicated to get analytical expression. Wind effect on the vent flow can be investigated by comparing the results on Hg and Hg . CASE STUDIES Taking an atrium of height 20 m (66
39、ft) as an example, suppose the smoke layer interface height is 14 m (46 fi), the discharge coefficient of the vent is 0.6, the design fire size is 5 MW, the pressure coefficient CwA is 0.2, and C, is -0.6 for the intake vent at the leeward side and 0.4 for the intake vent at the windward side. Two s
40、cenarios are considered: Scenario A: Fire at the atrium floor. Under an axisym- metric plume with mp given by Equation 8, the required vent area is calculated to be 40 m2 (431 ft2). This is quite a large value for keeping smoke at higher levels. Scenario B: Fire in a shop adjacent to the atrium. Und
41、er a balcony spill plume, assuming that the height of the balcony is 5 m (1 6 fi) and the width of the plume ic 5 m (16 ft) as in the above example, the required vent area is 85 m2 (915 ft2) as estimated by Equation 7. The vent area is much bigger than that for scenario A because of the larger air e
42、ntrainment rate for a balcony spill plume. The variations ofthe smoke layer interface height and the vent capacity with the fire size for scenarios A and B with vent areas 40 m2 (431 ft2) and 85 m2 (915 fi2), respectively, are shown in Figures 3 and 4. The effects of wind speed Y, of 5 ms-* (16 fis-
43、) were included in these figures. The smoke layer interface heights were observed to be lower (giving thicker smoke layers) when Q increased, although the vent capacity increased as well. From these figures, a negative wind pressure zone (e.g., Cwc = -0.6) above the vent would give a faster smoke re
44、moval rate and, hence, keep the smoke layer at a higher level. Positive pressure (e.g., C, = 0.4) would reduce the exhaust rate of the smoke vent and so a thicker smoke layer would result. This point is clearly illustrated in Figures 5a and 5b in plotting Hi against H. Taking negative wind-induced p
45、ressure as an example, the variation of smoke layer interface height with wind speed is shown in Figure 6. The smoke layer interface height would be pulled up due to higher wind speeds at the leeward side. ASHRAE Transactions: Research 483 1.0 1 xu o .4 o .2 O .E 0.6 200 160 120 (a) smoke layer inte
46、rface height o I I I 1 O 5000 10000 15000 20000 Q kW (b) mass flow rate Figure 3 Variation ofjre size for scenario A. 484 ASHRAE Transactions: Research O8 lo No wind - _, 0.6 Wind with Cwcof 0.4 1 o2 O0 O 5000 1ouoo 15000 20000 Q (kW) (a) smoke layer interface height 1 I I 1 1 O 5000 1 O000 15000 20
47、000 Q kW) (b) mass flow rate Figure 4 Variation ofjre size for scenario B. ASHRAE Transactions: Research 485 0.6 / I I I 0.6 0.7 0.8 0.9 1 .o 0.45 0.50 0.55 0.60 0.65 0.70 0.75 0.80 0.85 0.90 O .60 O .55 O 50 0.45 H,/H (a) fire at atrium floor I I No wind v, = o Ids I 1. ,vw = 3 da, c, / 1 . v, = 3
48、da, C, = 0.4 V, = 5 ds, C, = 0.4 (b) fire in the shop adjacent to the atrium Figure 5 Effect of wind on smoke layer interface height. 486 ASHRAE Transactions: Research 0.6 - x r XM 0.4 - 0.2 - 0.0 Figure 6 Effects of wind speed with negative induced roofpressure for scenario A. I I I , CONCLUSION Bu
49、oyancy is the driving force for extracting smoke out of a horizontal ceiling vent. A traditional vent flow model was developed from Bernoullis theorem. The required vent area was estimated from the derived smoke exhaust rate driven by buoyancy, but for very tall atria, the smoke might be cooled down while moving up. Buoyancy of smoke will then be reduced to give lower extraction rate. Wind effects cannot be ignored in places with strong wind environment. In this paper, wind effect on a horizontal ceiling
