Reducing Exhaust Quantities for Atrium Smoke Control
Duncan A. Phillips, PhD, PEng Ray Sinclair, PhDAssociate Member ASHRAE Member ASHRAE
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ABSTRACT
The objective of this paper is to demonstrate the value ofusing performance-based design techniques in the design ofatrium smoke management systems. This approach can reduceproposed smoke exhaust flow rates frequently by more than afactor of two of the initial code-calculated values. The reduc-tion in exhaust flow rates can save money in terms of reducingthe size of the fans and related emergency power systems. Thereduction in exhaust flow rates leads to a reduction in makeupair quantities. For some projects these cost reductions areessential to keep the atrium feature in the building design.Additionally, the reduction in makeup air quantities maintainsthe architectural integrity of the design.
The three smoke management design approachesdescribed in the paper are: zoning the atrium, timed rate ofdescent, and strategic makeup air distribution. The paperprovides advice and caution when using these approaches. Itincludes discussion on the potentially adverse impact of poormakeup air distribution and problems caused by relativelywarm or hot makeup air. CFD modeling is demonstrated to bea valuable design tool. The need for calibration of the CFDmethodology to ensure realistic prediction of plume entrain-ment is highlighted.
Three atrium case studies are presented. In each case, theexhaust quantities required from standard code calculationsresulted in exhaust and corresponding makeup airflow ratesthat were not feasible given the atrium architecture. Solutionalternatives are presented, and CFD computer modeling isused to prove performance.
INTRODUCTION
While the inclusion of an atrium within a building canenhance the environment, the architectural complexity of anatrium can cause design challenges for the smoke manage-ment system. For example, Figure 1 presents an atrium inwhich there is a central atrium of five levels and two smallerconnected spaces with approximately three and four levelswithin them. The main atrium has occupied zones on upperlevels that would require people to exit through the atrium toegress the building. In addition, the combination of a bridge,overhangs, and open passages results in a complicated flowpath for the smoke should a fire occur.
Figure 1 Complicated atrium with four smokemanagement systems.
Duncan Phillips is an associate and senior specialist and Ray Sinclair is a principal and project director at Rowan Williams Davies and IrwinInc., Guelph, Ontario, Canada.
Complicated architecture can cause unwanted mixing ofroom air into the rising smoke plume. This occurs for firesunder balconies or pedestrian bridges within the atrium.Sinclair and Phillips (2004) describe the complications foundin typical atria and the physics that leads to increases in smokeproduction. The increased smoke production rates caused bythe mixing leads to a need for larger exhaust flow rates. In mostcases, increasing the size of the exhaust fans is not difficult.Unfortunately, the corresponding increase in makeup air thatis required can be very difficult to introduce into the atrium andmay result in an impractical atrium smoke managementsystem.
Atrium smoke management systems are receiving asignificant level of attention in engineering trade magazines.Articles such as those by Duda (2004), Green (2003), Klote(2000), and Sinclair and Phillips (2004) demonstrate the inter-est in articles with discussions of design practices.
This paper presents three design approaches that can leadto a reduction in the exhaust flow rate of an atrium smokemanagement system. These three concepts are:
• Zoning the atrium—whereby the atrium is divided intosmaller spaces with lower ceiling height by deployableor permanent smoke barriers.
• Timed rate of descent—where the rate of descent of thesmoke layer is slowed to permit adequate time for exit-ing from the atrium.
• Strategic makeup air distribution—a means of protect-ing egress routes with makeup air.
Three examples of real atria are discussed to demonstratethe concepts described. These examples highlight the value ofa performance-based design. In each case the atrium architec-ture is described and assessed using code-based calculationtechniques. The same atria are then reassessed using the tech-niques described, which leads to a potential reduction inexhaust. Finally CFD (computational fluid dynamics) simu-lations are presented that predict the level of performance. Anoutline of the CFD modeling methodology and validation ofthe modeling approach are presented in appendices.
Through the above design process, the atrium smokemanagement systems are shown to have acceptable levels ofperformance that meet the design goals. They are practical inthe quantities of exhaust and makeup air, and capital costs arereduced.
BACKGROUND
A variety of calculation tools are available to designers ofatrium smoke management systems. The tools that have beendeveloped over the years have varying degrees of successpredicting this movement of smoke. In addition, the experi-ence of the user of these tools can greatly affect the validity ofthe results. Indeed, as will be shown below, the improper useof some tools can lead to poor design.
Design Tools: Code Equations
In the initial design of an atrium smoke managementsystem, two equations are commonly used: the axisymmetricand balcony-spill plume equations. These equations,commonly referred to as “code equations,” are reproduced intheir SI form in Equations 1 and 2 below. They are presentedin many building codes (e.g., ICC [2003]), guidelines (e.g.,NFPA-92b [NFPA 2000]), and technical articles (e.g., Kloteand Evans [2004]). Klote (2000) presents a short discussion oftheir history.
(1)
where is the mass flow rate (kg/s) in the smoke plume atheight z (m) for a fire with a convective heat output of Qc(kW). Qc is typically 70% of the full fire heat output: theremainder is radiated to the surroundings (e.g., walls). Equa-tion 1 is valid for heights (z) greater than the flame limitingheight.
(2)
where is the mass flow rate (kg/s) in the smoke plume atheight Zb (m) above a balcony at height H (m) above the firewith a total heat output of Q (kW). The approximate spillwidth is W (m).
These equations were generated empirically from datasets (primarily laboratory scale): curve-fitting techniqueswere applied to the data and the most reasonable matches usedto represent the data. The experiments used to generate data ofan axisymmetric plume were taken in a relatively large openspace, while the data representing flow conditions arisingfrom a balcony spill plume were taken in a 1/10th scale model.The applicability of the equations is therefore limited to theexperimental conditions under which the data set was gener-ated. Extrapolation outside the conditions in which the exper-imental data were collected is possible, but the practitionermust be experienced in the dynamics of smoke movement tobe able to do this.
The code equations were developed for specific environ-ments that are not necessarily found in all atria. They predictplume growth in ideal conditions where there is no back-ground air movement. In real atrium situations, the architec-ture and makeup air often induce background air movement.In some cases, background air speeds of less than 0.89 m/s(175 fpm) (less than the 1.02 m/s [200 fpm] threshold typicallyquoted as an upper limit) have been seen in CFD model simu-lations (see Appendix A for a discussion of methodology) toadversely affect smoke plumes. One effect is that the plumecan be bent over from the expected vertical flow and have adiagonal trajectory. Another effect is that the increased turbu-lence of the background airflow causes greater entrainmentinto the smoke plume. If the exhaust system is not sized tohandle the greater volume flow rate of smoke, both effects cancause impacts on upper occupied levels as well as a faster timerate of fill of smoke within the upper levels.
m· 0.071Qc
1 3⁄z5 3⁄
0.0018Qc
+=
m·
m· 0.36 QW2
( )1 3⁄
Zb
0.25H+( )×=
m·
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The influence of background air movement on smokeplumes is not addressed in the code equations. To overcomethe negative effect of adverse mixing, the total exhaust rate forthe atrium often needs to be as much as 40% greater than thatcalculated by Equation 1 for an axisymmetric plume fire event.Thus, use of the axisymmetric equation alone can lead toundersized atrium smoke management systems. See Appen-dix B for a discussion of validating computer simulations.
For balcony-spill plume events, there is an over-predic-tion of required exhaust rates by the code equation, which iswidely acknowledged by specialists in the field. This overpre-diction is often more than 30% to 40% (see discussion of cali-bration, Appendix B). Of the difficulties in using the equation,the uncertainty in selecting the appropriate width of the spillplume is significant. This factor has a tremendous effect on thepredicted smoke exhaust rate.
This leads to the problem facing the designers of compli-cated atria. The code equations are not generally accuratepredictors of required exhaust rates and they can result in esti-mated exhaust flow rates that can be either significantly under-or overpredicted. In many cases, however, the code equationsare all that the design professionals have available to them.Current efforts by researchers, for example, IRC (2003) andHarrison (2004), intend to improve the accuracy of the equa-tion predictions.
Recent revisions of the IBC (ICC 2004) have led to theremoval of the balcony spill plume equation from the code.While the equation appears to have been overly conservativefor the purposes of design, it did force the designer to consideralternative fire scenarios and provide a reference point forinitial calculations. The removal leads to a lack of consistentdesign guidance to the design community as the equationappears in guidelines such as NFPA 92B (NFPA 2000) but notthe amended version of the IBC (ICC 2004).
Design Tools: CFD Modeling
The use of computational fluid dynamics (CFD) model-ing is now quite popular. The quantity of recent articles inforums such as ASHRAE Transactions is witness to this, e.g.,Zhang and Roby (2003), Jiang et al. (2003), and Tan andMusser (2004).
CFD modeling is a process by which a volume, such as anatrium, is split into a very large number of smaller volumescalled cells. The standard equations of fluid flow (the NavierStokes equations) as well as equations addressing conserva-tion of energy, the effects of turbulence, and the transport ofcontaminants are applied to each of these cells individually.CFD fundamentals are discussed in many references includingPatankar (1980), Chen and Srebric (2001), and Jiang et al.(2003), for example.
A variety of commercial and research CFD softwarepackages are capable of simulating fire scenarios in buildings.Most of these are based on a traditional approach to modelingturbulence called Reynolds averaged Navier Stokes(RANS)—see Jiang et al. (2003). A newer CFD approach that
uses an alternative representation of turbulence is gainingprominence: the large eddy simulation (LES) formulation forturbulence overcomes some of the deficiencies associatedwith using RANS-based modeling—see McGrattan et al.(1999). However, LES modeling also has its own limitationsthat are generally not considered by new practitioners. Forexample, the size of the computational cells by definitionfilters the scales of turbulence simulated; the prediction ofconditions at walls is heavily dependent on the grid size nearthe wall; and the subgrid scale model has limitations based onits assumptions. Jiang et al. (2003) recommend a differenttreatment of the subgrid scales than is commonly used in LESmodeling in order to improve the quality of the predictions.
These modeling tools are becoming increasingly accessi-ble to users (one LES modeling tool is available for free down-load). The setup of problems or scenarios is becoming easier,and the visualization of the smoke movement about the spaceis making the results appear intuitive. It is important to notethat while CFD modeling is a very powerful design tool andcan generate useful visualization aids, it can lead to incorrectanswers that may not be obvious to the user.
No matter which CFD technique is used, there is a needto validate the methodology applied. In particular, it is impor-tant to be sure that the plume entrainment rate is correctlypredicted. Inexperienced users can make simple mistakes thattypically have the CFD model underpredict the plume entrain-ment. This can result in predicted smoke layer heights thatshow as a pass, when in reality smoke impacts may be moresevere and potentially affect exiting from the building.Common issues overlooked by some CFD users are:
• appropriateness of the soot generation rate for thedesign fire;
• suitable prediction of visibility in egress routes;• incorporation of activation of detection systems and
consideration of effects of: sprinklers and deployablesmoke barriers;
• incorporation of potentially adverse effects that the nor-mal ventilation system can have prior to detection andshutdown of the HVAC systems;
• lighting systems, signage;• sprinklers;• effects of exhaust reentrainment into makeup air intakes;• effects of temperature of makeup air; and• wind effects on building facades.
The significance of each of the above issues changes fromone building to the next. For instance, hot makeup air enteringa building at a temperature that is 15°C to 20°C (27°F to 36°F)warmer than the internal air temperature can cause a severemixing problem within the stratified smoke layer. Warmmakeup air rises, causing turbulence. This added mixing andthe additional airflow in the upper part of the atrium candisplace and mix smoke down into occupied zones that may bepart of exit pathways. This problem can occur rapidly. It does,however, subside with time as the interior environmentapproaches the makeup air temperature, thereby reducing thebuoyancy effects on the inflowing makeup air.
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ATRIUM SMOKE MANAGEMENTDESIGN APPROACHES
Three design approaches are described in this section. Anexample of how each has been implemented is also presented.
Zoning The Atrium
The purpose of zoning an atrium for the purposes ofsmoke management design is to divide a complex atrium intosmaller regions. Zoning can have a number of benefits:
• it creates a region that can serve as a storage volume atthe top of the space;
• it provides a means by which the smoke can beexhausted locally;
• it reduces the plume height of rise;• it can eliminate a balcony spill scenario; and • it segregates occupied zones at the top of the atrium
from the smoke affected region.
Segregating an atrium into smaller spaces can be achievedby adjusting the architectural features of the atrium and byincluding (additional) glazing (sprinkler-protected if neces-sary) or doors. It can also be achieved by using deployablesmoke barriers. The atrium in Figure 1 was segregated intofour different smoke exhaust zones. Two of these zones wereseparated by deployable curtains, as shown by the surfacesnoted in the figure. Smoke from fires located in the lowerregions needed to be exhausted as close to the fire as possible.If the smoke from these fire locations was permitted to spillinto the taller atrium region, the smoke layer would inundatethe upper occupied zones. Additional exhaust capacity at theupper ceiling level would not reduce the severity of the smokeimpacts.
The advantage of segregating or zoning an atrium is thatit leads to reductions in total exhaust air quantities. While thiscontributes a cost savings for the project, reductions in exhaustquantities also leads to reductions in makeup air quantities. Areduction in makeup air requirements from an initial code-calculated exhaust flow rate of 270 m3/s (570,000 cfm) for thebuilding in Figure 1 to a design maximum value of 118 m3/s(250,000 cfm) can make the atrium viable within a building.
In some cases, such as the atrium in Figure 1, the processof zoning an atrium permits the design team to use makeup airdelivered into any one particular zone to meet the makeup airrequirements of other zones, too, assuming independent firescenarios. Additionally, it is possible to use the same mechan-ical equipment to exhaust different zones depending on the firescenarios. Once again, this is a potential capital cost reduction.
Timed Rate of Descent
This design approach is one in which the atrium is permit-ted to fill with smoke. The rationale for the approach isdescribed in NFPA 92B (NFPA 2000) section 2.4.1. The rateat which the atrium fills with smoke is compared to the timethat it would take occupants to exit the area. Provided that the
descent of the smoke layer is slow enough such that occupantsare able to exit, the approach provides an alternative criterionby which to judge the performance of a smoke managementsystem.
NFPA 92B describes two variations of the timed rate ofdescent approach: with and without smoke exhaust in place.The atria that are the best candidates for a timed rate of descentapproach to smoke management need to have the followingcharacteristics:
• A vaulted reservoir above the highest region with poten-tial occupancy. This reservoir can act to store smoke anddelay the start of the descent of the smoke layer throughthe occupied zones.
• Limited permanent or large occupancy spaces directlyconnected to the atrium at the upper zones.
• Multiple exit routes from the atrium into adjacent pro-tected zones1 minimize the number of deadend pas-sages. It is important that occupants of an atrium havealternative exit routes from that atrium into protectedzones should it start to fill with smoke.
• No occupied spaces from which occupants need to passthrough the atrium in order to egress the building. It iscommon for an atrium to serve as a central connectioncore between different parts of a building complex. Insome circumstances, there are rooms directly connectedto the atrium only. These might be conference or readingrooms or toilets. In these scenarios, if occupants mustpass through the atrium in order to egress the building,they might be confronted with untenable smoke-filledpassages within the atrium.
Strategic Makeup air Distribution
In many cases, despite the proper implementation ofexhaust and widely distributed makeup air, there are circum-stances in which smoke from certain fire scenarios will inev-itably penetrate into a region that needs to remain clear. Thiscan occur for a fire located in close proximity to an overhang-ing walkway above. While the location of the fire might notcause a balcony spill plume, the rising smoke can strike theunderside of the walkway ceiling and cause tenability reduc-tions in the walkway (see Figure 2).
In these circumstances, makeup air can be provided tomitigate the severity of the smoke impact in the followingways:
• Diluting the smoke that penetrates into the region ofconcern such that the smoke, at reduced concentrations,is tenable. This approach reduces the severity of theimpact—it does not prevent it.
1. The use of the term “protected zone” here refers to a region in thebuilding, adequately separated from the atrium, within whichtenability is maintained for a sufficiently long period of time suchthat occupants are able to safely evacuate the building and fireresponders are able to assist those with mobility difficulties.
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• Strategic makeup air may even be used to ensure thatthe main egress point remains relatively clear of smokedespite the surrounding region being smoke affected tothe point of nontenability. This can require significantflow rates and suitable air velocities to ensure an effec-tive sweep across the target occupied area.
• Preventing the smoke, at concentrations that wouldcause a degradation of tenability, from entering theregion. This requires that the makeup air entering theregion of concern be sufficiently well distributed that itcauses a unidirectional flow out of the zone. If they arekey egress paths, corridors can generally be protected inthis manner. NFPA 92B (NFPA 2000) Section 3.13describes a calculation methodology for a corridor-likescenario.
EXAMPLE ATRIA
Table 1 summarizes fire scenarios defined by the archi-tectural features of the example atria. The atria range from anarrow five-story atrium with minimal opportunities for theintroduction of makeup air to a seven-story, small-footprintatrium that shifts in plan as it rises so that someone on thelowest level cannot see the ceiling at the top of the atrium.
Example Atrium #1: Zoning Atrium to CreateTwo Distinct Smoke Management Zones
The atrium shown in Figure 3 can be thought of as a five-level space connected across its two upper floors to a four-level space. Figure 4 shows the smoke penetration within theatrium for a smoke management system configured with asingle exhaust flow at the top of the space. Clearly, the smoke
management design does not achieve an appropriate level ofperformance.
Table 1 describes the heights and corresponding flowrates that are calculated using the code equations. The esti-mated exhaust flow rate of 343.6 m3/s (728,000 cfm) is basedon a height of rise from the bottom level to 1.8 m (6 ft) abovethe upper floor. The configuration of the atrium is such thatwith this level of exhaust, and even greater quantities, condi-tions in the upper levels of the space will not be tenable. Thisis because the turbulence generated as the smoke spills fromthe lower to upper space significantly increases mixing.
This particular atrium can be thought of as two distinctatria that are connected at two levels across walkways orbridges. For the scenario presented in Figure 4, smoke from afire in the lower atrium must flow across the bridges in orderto reach the exhaust at the top of the upper atrium. Segregatingthe two parts of the atrium and exhausting the smoke from afire in the lower region before it spills to above has a numberof advantages:
• The barriers at the third and fourth level prevent smokefrom spilling to above, eliminating the balcony spill sce-nario. These barriers are shown in Figure 3.
• The implementation of deployable barriers at the thirdand fourth levels creates a smoke reservoir above thelower part of the atrium.
• The target smoke layer height can be lowered to 1.8 m(6 ft) above the second level. This results in a revisedexhaust capacity requirement of 56.6 m3/s (120,000cfm).
Figure 5 shows the CFD simulation results of the samefire scenario as presented in Figure 4 with the revised smokemanagement strategy as described. Figure 5 clearly shows thecontainment of smoke in the lower atrium zone. The smokelayer is above the criterion height identified by the highlightedhorizontal strip around the atrium, and no smoke penetratesinto the corridors connected to the lower atrium. The exhaustrequired for this smoke management system is 56.6 m3/s(120,000 cfm).
The results of the CFD simulation of the base smokemanagement system design, shown in Figure 4, also highlightthat the 6th level cannot remain open to the atrium below.Capping the skylight provides positive protection to the 6thlevel while permitting light to enter the space below. In orderto protect the 5th-level occupied regions from smoke runningalong the ceiling, the 5th level must be protected with a phys-ical barrier. It is not possible to protect the 5th level usingexhaust alone. With these changes, the required exhaust flowrate is approximately 47.2 m3/s (100,000 cfm) with the targetsmoke layer height at approximately 5.9 m (19 ft).
Figure 6 shows the CFD simulation results of a firescenario postulated to occur in the upper atrium. The fire islocated on the third level and the criterion line is 5.9 m (19 ft)above the fire, corresponding to +1.83 m (6 ft) above the fourthlevel. The exhaust quantity used is 47.2 m3/s (100,000 cfm).
Figure 2 Local impacts of smoke on balcony from axi-symmetric plume.
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The makeup air enters the atrium through the third- and fourth-level corridors, as well as through the opening to the loweratrium. The smoke management system is configured so thatdetection of smoke in the upper level only deploys the smokebarrier at the fourth level: the third level remains open. Thispermits the upper atrium to use some of the makeup air capac-ity that is available to the lower atrium. Needless to say, thedesign precludes a fire being detected in both atria spaces atthe same time.
Of note for this upper atrium fire scenario is that the codeequation calculated exhaust flow rate for a 2,100 kW(2,000 Btu/s) fire generating an axisymmetric plume, with a70% convective component and a height of rise of 5.9 m(19 ft), which is approximately 19.8 m3/s (42,000 cfm). Thereason that the exhaust flow rate in the upper atrium is largerthan the code equation might require is that the makeup air thatenters the space from the surrounding corridors and from thelower atrium (see Figure 6) disturbs the rising smoke plume,causing mixing of ambient air into it. The additional exhaustcapacity is required to manage the additional smoke generatedas a result of the mixing.
In addition to sharing makeup air capacity, the two atriaalso share the mechanical exhaust. Dampers in the ceilingabove the upper atrium are configured to direct the exhaust toeither the ceiling of the 5th level for an upper atrium fire eventor to the ceiling of the 4th level for a lower atrium fire event.Sharing exhaust fan capacity reduces the overall capital cost.
Example Atrium #2: Timed Rate of Descent
Figure 7 presents the configuration of a relatively smallatrium. The space is 31.6 m (104 ft) tall and the area of theopening from floor to floor is 155.6 m2 (1675 ft2). The atriumis taller that it is long by a factor of two. The atrium has fivelevels connected to it and has no permanent occupancyplanned. It serves as a circulation core. There are at least twoexit paths from the atrium on each level, and the traveldistances to get to the farthest of the two exits are approxi-mately 25 m (82 ft). One of the features of the atrium is thatan individual is able to see the main core of the atrium from anylocation in the atrium. This means that rising smoke would bevisible immediately, which leads to an almost immediate deci-sion to exit. Finally, the atrium has a relatively large (8.7 m,
Table 1. Characteristic Dimensions and Exhaust Flow Rates of Example Atria
Building Fire Scenario Characteristics of Smoke Rise
Exhaust Flow Based on Code
Equationm3 s-1 (cfm)
Final Exhaust Flow Rate Using Design Technique
m3 s-1 (cfm)
Zoning the Atrium
University Laboratory Building: BOCA jurisdic-tion, see Figure 3
2.0 MW fire at bottom of building with smoke bumping off a ceiling and spilling into a sec-ond atrium region—see Figure 4.
HSR = 25.7 m (84.25 ft)Wb = 18.9 m (62 ft)Hb = 19.2 m (63 ft)
343.6 (728,000) • 56.6 (120,000)for lower zone
• 47.2 (100,000)for upper zone
Timed Rate of Descent
University Laboratory Building: BOCA jurisdic-tion,see Figure 7
Axisymmetric 2,100 MW (2,000 Btu/s) fire
HSR = 21 m (68.8 ft) 115.8 (245,500) 47.2 (100,000) (32 ft high smoke layer)
Balcony spill plume, 2d floor scenario,2,100 MW (2,000 Btu/s) fire
HSR = 14.9 m (48.8 ft)Wb = 13 m (42 ft)Hb = 2.7 m (9 ft)
283.1 (600,000) 47.2 (100,000) (EL+38 ft smoke layer)
Strategic Placement of Makeup Air
Public Fitness Center in an IBC 2000 jurisdiction,see Figure 9
5.25 MW fire, balcony spill plume through a small opening to floor above.
HSR = 13.2 m (43 ft)Wb = 13 m (42 ft)Hb = 7.9 m (26 ft)
209.1 (443,000) 85. 9 (182,000)This exhaust flow rate includes additional capac-ity to accommodate the strategic makeup air located in the smoke layer.
Notes:HSR = height of smoke rise to underside of target smoke layer height.Wb = balcony spill plume widthHb = height of balcony
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28.5 ft), tall vaulted ceiling. This volume provides a storagecapacity for the smoke generated during the early stages of thefire.
Using the balcony spill plume equation provided in codes,an exhaust flow rate of 283 m3/s (600,000 cfm) would berequired. This calculation is based on a 2 MW (1,900 Btu/s)fire, with a balcony spill width of approximately 13 m (42 ft),2.7 m (9 ft) above the fire, and a required smoke layer heightof 14.9 m (48.8 ft), also above the fire. Conducting a calcula-tion using the code equations for a 2 MW fire located on thefirst level that generates an axisymmetric plume results in anestimated exhaust flow rate of 116 m3/s (246,000). Thisassumes a 21.0 m (69 ft) smoke plume height or rise. With the
endorsement of the local authority having jurisdiction, asmoke management system was configured for this atrium thatuses the timed rate of descent approach to manage the smoke.An exhaust capacity of 47.2 m3/s (100,000 cfm) was selectedto provide the required control over the smoke layer height.
Figures 8a, 8b, and 8c present the smoke penetration atvarious points in time after the start of the growth of a 2 MW(1,900 Btu/s) fire, postulated to generate a balcony spill plumefrom the second floor:
• Figure 8a: 30 seconds (at approximately the detectiontime),
• Figure 8b: 5 seconds (immediately after the emergencysmoke management system has activated), and
Figure 3 Overall geometry of tandem atrium.
Figure 5 Tandem atrium: zoned atrium smokemanagement system design for lower atrium firescenario.
Figure 4 Tandem atrium: initial smoke managementsystem design.
Figure 6 Tandem atrium: zoned atrium smokemanagement system design for upper atrium firescenario.
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• Figure 8c: 140 seconds (when smoke impact in theoccupied zone of the fifth level has started).
Figure 8c highlights that, for this fire scenario, a reason-able portion of the upper level corridors still allow satisfactoryvisibility for exiting. This persists until after 150 seconds intothe fire event, which is about a factor of 5 more than the exitingtime required for people initially in the atrium to exit to eitherend of each balcony walkway. This was deemed to be a satis-factory design by all parties concerned.
It was acknowledged that as more time passed, starting atapproximately 170 seconds (not shown) after the start of thegrowth of the fire, smoke impacts on the upper levels weresignificant. However, at this time, no one is expected to be onthese walkways of the atrium.
Example Atrium #3: Strategic Makeup Air
Figure 9 presents a partial view of an atrium space. Theview shows the third level of a building, used as an exercisefacility, and an opening in the floor slab to a lobby below. Theopening to below could not be closed off—the programmingof the space required that it be open during normal operationsof the building, and the infrastructure to close the openingduring an emergency was deemed to be too awkward andexpensive to implement. One of the two emergency exits isshown in Figure 9 to be immediately adjacent to the openingto below. The ceiling above the opening was vaulted, whichprovided for a logical location to exhaust the smoke. Unfor-tunately, in order to draw smoke from the space below throughto the exhaust, the smoke must pass through the narrow open-ing and in the process of doing so would mix excessively.Early CFD simulations showed that it was not possible toprevent the entire region from being flooded with smoke rela-tively early after the start of the growth of the fire.
Figure 7 Overall geometry of small atrium.
Figure 8 Small atrium transient smoke penetration withtimed rate of descent of smoke layer: (a)conditions at 30 seconds, (b) conditions at 85seconds, (c) conditions at 140 seconds.
(a)
(b)
(c)
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The calculations of exhaust flow rates using the codeprovided equations for the plume spilling into the fitness roomthat resulted in an estimated flow rate of 209.1 m3/s (443,000cfm). These calculations assume a required smoke layerheight of 13.2 m (43 ft), a spill width of 13 m (42 ft), and abalcony height above the 5.25 MW (5,000 Btu/s) fire of 7.9 m(26 ft). This calculated flow rate was not only impractical forimplementation within the atrium, but it would also not resultin a stratified smoke layer height 10 ft above the fitness levelfloor.
The fitness center itself was relatively open, meaning thatan occupant would be able to see smoke spilling from below.In addition, all locations in the space have a direct line of sightto both exit doors. Finally, the travel times to get to an exit wereless than a minute. These features were considered to be suffi-cient to consider a performance-based system. An alternatesmoke management system design was implemented in whichthe base exhaust capacity was specified to be able to managethe smoke spilling from below and maintain the smoke layerat least 10 ft above the occupied level below. An exhaust flowrate of 70.8 m3/s (150,000 cfm) was required. This preventedsmoke from dropping into the occupied levels below.
Tests using CFD to evaluate various means of protectingthe exits in the fitness center were conducted, and the config-uration of grilles around the door as shown in Figure 9 wasselected to be the most viable. It required 9.4 m3/s (20,000cfm) of air at a velocity of approximately 2.0 m/s (400 fpm).The makeup air introduced through these grilles enters thebuilding in a zone anticipated to be above the equilibriumsmoke layer height. Therefore, once this air travels away fromthe door, it will become smoke laden. The exhaust fan capacityneeded to be increased by approximately the same volume ofair as was introduced through the grilles into the smoke layer.In addition to the exit discussed here, there was a second exiton the opposite side of the opening (not shown). This exit also
required makeup air protection, although the air volumerequirements were only 5.7 m3/s (12,000 cfm).
Figure 10 shows the level of smoke penetration 150seconds after the start of the growth of the fire in the spacebelow. The final design for the atrium smoke managementsystem provided positive protection for the exits out of theupper space for at least 150 seconds following the start of thegrowth of the fire. The final exhaust flow rate was 85.9 m3/s(182,000 cfm), which is 15.1 m3/s (32,000 cfm) over the basequantity of exhaust required for the occupied zone below thefitness center.
CONCLUSIONS
The preceding discussion and examples of performance-based atrium smoke management design have demonstratedhow smoke exhaust flow rates can be reduced to manageablelevels without compromising the safety of a building. In addi-tion to providing more practical designs, the approachesdiscussed have also enabled the architects to achieve theirdesign goals without the fear that the smoke managementsystem would be too large to allow the atrium to remain in theproject.
This paper points out that it is important that users of thedesign tools available be aware of the limitations of their appli-cability. The case studies demonstrate that performance-baseddesign approaches can lead to smoke management systemsthat meet the requirements of the codes with lower exhaustflow rates. CFD modeling is highlighted as a powerful designtoo when the methodology is properly calibrated.
ACKNOWLEDGMENTS
The authors are grateful to the following people at RowanWilliams Davies and Irwin Inc. for their contributions on vari-ous aspects of this work: Vincent Tang, Mingyang Jia, andTara Bridger. We would also like to express our thanks to thereviewers for their positive suggestions for the improvement tothis paper.
Figure 9 Overall geometry of atrium with opening to belowfor strategic make-up air.
Figure 10 Strategic make-up air protecting a zone oftenability.
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REFERENCES
Duda, S. 2004. Atria smoke exhaust: 3 approaches toreplacement air delivery. ASHRAE Journal, June, pages20-26.
Chen, Q., and J. Srebric. 2001. Simplified diffuser boundaryconditions for numerical room airflow model: Finalreport of RP-1009. Atlanta: American Society of Heat-ing, Refrigerating and Air-Conditioning Engineers.
Evans, D., and J. Klote. 2004. Smoke Control Provisions ofthe 2000 IBC: Interpretation and Applications Guide.Country Club Hills, IN: International Code Council(ICC) Inc.
Green, J.D. 2003. Correct application of smoke controlcodes. HPAC Engineering, May, pp. 51-55.
Harrison, R. 2004. Smoke Control in Atrium Buildings: AStudy of the Thermal Spill Plume. Thesis in Fulfillmentof the Master of Engineering in Fire Engineering at theUniversity of Canterbury, NZ.
ICC. 2003. International Building Code. Falls Church, VA:International Code Council (ICC) Inc.
ICC. 2004. 2004 Supplement to the International Codes.Falls Church, VA: International Code Council (ICC)Inc.
IRC. 2003. ASHRAE/IRC joint research project will investi-gate atrium smoke-management design to deal with bal-cony spill plumes. Construction Innovation 8(3).Ottawa, ON: Institute for Research in Buildings.
Jiang, Y., M. Su, and Q. Chen. 2003. Using large eddy simu-lation to study airflows in and around buildings.ASHRAE Transactions 109(2):517-526.
Klote, J.H. 2000. An overview of atrium smoke manage-ment. Fire Protection Engineering, No. 7, pp. 24-34.
Klote, J.H., and J.A. Milke. 2002. Principals of Smoke Man-agement. Atlanta: American Society of Heating, Refrig-erating and Air-Conditioning Engineers.
Klote, J.H., and D.H. Evans. 2004. Smoke control and theInternational Building Code. ASHRAE Transactions110(1).
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APPENDIX A
The CFD modeling results presented in this paper haveused a standard CFD modeling methodology within acommercially available RANS-based code. The purpose ofthis appendix is to briefly lay out the features of this method-ology. Appendix B describes the calibration of the methodol-ogy for representing the fire and resultant smoke plume.
Representation of the Fire
The fire has been modeled using a volumetric heat sourceequivalent to the convective fire component. In large spaces,the radiative component of the fire heat output is less impor-tant. The fuel used is a flexible polyurethane foam (GM27),which has a soot production rate of 20% of the fuel consumed.This is a relatively dirty fuel and is used as a conservativelyworst case in the assessment of tenable visibility levels in exitpathways.
Smoke Plots
The plots of smoke presented in the paper show the extentof the smoke plume that would cause visibility to drop toapproximately 7.62 m (25 ft). There are a variety of equationsavailable for calculating visibility based on soot concentra-tion. They rely on information about the soot extinction coef-ficient and in some cases the irritability of the smoke on theeye. See Klote and Milke (2002) for a discussion on calculat-ing visibility.
Activation of Smoke Management System
For a transient (time varying) CFD simulation, the acti-vation of the smoke management system can be eitherprescribed or predicted by the CFD modeling. The activationtime in a steady-state CFD simulation is not relevant. Thedelay in fully activating the smoke management system iscomposed of two periods: the time before the fire is detected(by either an individual, a smoke or heat detector, or sprinkler)and the lag while components of the system ramp up (fansturning on, dampers switching setpoints, and smoke barriersdeploying). The longer the delay activating the smokemanagement system, the more smoke is generated by thedesign fire prior to the system starting.
In some cases, the design team is seeking to achieve a totalactivation time that is either mandated by the relevant code ora design target in order to achieve a specific level of perfor-mance. In these cases, the activation time in the transient CFDsimulation is prescribed. In other cases, the CFD model can beused to predict the detection time (see below) and an appro-priate ramp-up lag imposed before the smoke managementsystem is fully engaged.
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In general, fans, doors, dampers, and curtains graduallychange from their normal operating mode to an emergencymode. For the transient simulations presented here (AtriumCases #2, #3), the activation of the system was instantaneousat the end point of the total activation time (e.g., following thedetection and ramp-up lag).
The steady-state simulations of the zoning strategy didnot require detection or activation of a curtain. This aspect ofperformance was discussed with the design team.
Smoke/Fire Detection
A number of detector technologies are used to detect thepresence of a fire. Two common ones are beam and attenua-tion-based spot smoke detectors. These rely on the detectionof a decrease in intensity of a light source. In the case of a beamdetector, the light source can be many meters away. Themodeling of the beam detector requires the user to integratelight attenuation along a path defined by the beam detector’smounting position. This is then compared to the attenuation atwhich the detector is set to signal an alarm. Modeling a spotdetector in CFD requires the user to monitor the soot concen-tration at the detector location and convert the reading into alocal attenuation. Klote and Milke (2002) provide equationssufficient for this task.
The point at which the integrated or local attenuationpasses the threshold of the detector identifies the ramp-up lagstart time. It is good design practice to wait for two detectorsto reach activation attenuation levels prior to starting thecountdown to full activation. In addition, if the simulationswould normally use a conservatively high soot yield rate, asdescribed above, it is also good practice to recalculate theattenuation at the detectors based on a cleaner burning fuel.This will result in a delayed detection of the smoke by thesmoke management system, which is, in general, a conserva-tive estimate of the potential delay in detection.
In the case of the transient simulation presented here forthe timed rate of descent design strategy (Atrium Case #2), theactivation time was prescribed at 84 seconds by the design
team, which was estimated by an independent analysis. Thisrepresented a worst-case detection time of 60 seconds and aramp-up lag of 24 seconds for the fans. It turns out that thetemperature-based detection would have started the ramp-upat approximately 30 seconds.
APPENDIX B
The CFD modeling strategies employed have been cali-brated against published data and standardized so that themethodology behaves similarly from one atrium to the next.The technique described in Appendix A to model the fire andsoot generation has been compared to Equation 1 for an openindoor environment with ideal makeup air. The ideal makeupair was implemented as a low-turbulence flow of air spreadacross the floor of the space with a uniform velocity of approx-imately 0.2 m/s (40 fpm). This represents a makeup air strat-egy that imposes minimal mixing on the smoke plume.Comparison of the smoke transported within the plume as afunction of height above the fire between the CFD modelingand Equation 1 is found to be satisfactory.
When the same open indoor environment has a makeupair strategy implemented that is representative of one thatmight be found in real atrium, the smoke layer drops unless thetotal exhaust flow rate is increased. This is because of theincreased mixing caused by the makeup air interaction on theplume. The increase in required exhaust at the top of thecomputational domain can be as high as 40% with relativelypoor (high velocity) makeup air distribution. Increases of atleast 10% are not uncommon though because most practicalimplementations of makeup air distributions in real atriumdesigns balance smoke management system performance witharchitectural and mechanical design constraints.
The CFD modeling methodology described above hasalso been applied to a balcony spill plume scenario andcomparisons made to Equation 2. In that case, the upward flowof smoke is overpredicted by the balcony spill plume equation.This varies depending on the parameters, such as spill plumewidth, implemented in the code equation.
