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AS/NZS 1668.1:2015 – IS IT GOOD ENOUGH FOR PERFORMANCE-BASED SMOKE EXHAUST SYSTEM DESIGN?

M.C. HuiSenior Fire Engineer ConsultantBCA Logic Pty Ltd


WHY DO WE NEED TO PROVIDE A SMOKE EXHAUST SYSTEM?

• BCA Deemed-to-Satisfy Provisions ask for it (optional).• Enhance life safety – to delay the time of onset of untenable

conditions.• Contents and property protection – to reduce damage of contents

and building fabric by heat and soot.


WHAT DO SMOKE EXHAUST SYSTEMS EXHAUST – A DUMB QUESTION?


TYPES OF FIRE PLUMES

Point source axisymmetric plume



Finite area axisymmetric plume Wall plume



Corner plume Line plume



Window plume Balcony spill plume


AS/NZS 1668.1:2015 SECTION 9 – HOT LAYER SMOKE CONTROL SYSTEMSHot layer smoke control systems shall comply with this Section and the relevant requirements of Sections 2, 3 and 4.The arrangement of hot layer smoke control systems shall comply with the following:a) Smoke shall be collected in a hot layer under and within bounding

construction.b) The collected smoke shall discharge directly to atmosphere.c) Uncontaminated make-up air shall be introduced at a level below the hot

layer.d) Central plant systems shall not use an atrium as part of a return air path

in fire mode.


AS/NZS 1668.1:2015 SECTION 9 – HOT LAYER SMOKE CONTROL SYSTEMS9.4.3 Exhaust air capacityThe required exhaust airflow rate shall be determined by the following:

(a) General cases – Figure 9.2(a); or(b) Atrium – Figure 9.2(b).

NOTE: Exhaust air capacity may also be calculated using the following equation:EAC = 2.6864Q + 0.1513*FP*SLH1.5

EAC = exhaust air capacity (m³/s)Q = fire size (refer Table 9.1)FP = fire perimeter, in metres (refer Table 9.1)SLH = height to underside of smoke layer, in metres


AS/NZS 1668.1:2015 SECTION 9 – HOT LAYER SMOKE CONTROL SYSTEMS

Figure 9.2(a) in AS/NZS 1668.1:2015 (identical to Figure 2 in Spec E2.2b of BCA)

Figure 9.2(b) in AS/NZS 1668.1:2015 (technically identical to Figure 3.4 in Spec G3.8 of BCA)


Table 9.1 Fire Size and Perimeter Data for Various Building Applications

AS/NZS 1668.1:2015 SECTION 9 – HOT LAYER SMOKE CONTROL SYSTEMS

The technical contents of Table 9.1 are exactly the same as that in Spec E2.2b & Spec G3.8 of BCA except the fire perimeter.


PLUME THEORIESLARGE FIRE PLUME (THOMAS ET AL. 1963)


PLUME THEORIESLARGE FIRE PLUME (NFPA 92 - 2012)

First derived in conjunction with NFPA204-1982 “Standard for Smoke and Heat Venting”.Limiting flame height z1:zl = 0.166Qc

2/5 (m)If the distance between base of fire and bottom of smoke layer z is ≤ z1:M = 0.032Qc

3/5z (kg/s)


PLUME THEORIESPOINT SOURCE AXISYMMETRIC (STRONG) PLUME

Thomas et al. (1963) based on Yokoi (1960) and Yih (1952):

M = 0.153ρc[Qcg/(ρcCpTo)]⅓z5/3

SI unit (Heskastad 1982) in NFPA92B (1991):

M = 0.071Qc⅓z5/3 + 0.0018Qc (kg/s) when z > zl

zl = 0.166Qc2/5 (m)


Total HRR (MW) 1.5 5 10 15Smoke exhaust rate (m³ s-1)(UK large fire plume) 18.6 41.1 61.4 81.8

Smoke exhaust rate (m³ s-1)(US large fire plume) 17.9 39.5 58.9 78.4

Smoke exhaust rate (m³ s-1)(point source axisymmetric strong plume)

18.5 36.2 57.0 76.2

Smoke exhaust rate (m³ s-1)(AS/NZS 1668.1:2015) 19.1 43.5 67.0 90.4

SMOKE EXHAUST RATES BY JOINT STANDARD AND PLUME THEORIES


HOT LAYER TEMPERATURES FROM PLUME THEORIES

Total HRR (MW) 1.5 5 10 15

Smoke layer temperature (°C)(UK large fire plume) 37 76 132 189

Smoke layer temperature (°C)(US large fire plume) 98 146 186 215

Smoke layer temperature (°C)(point source axisymmetric strong plume) 78 134 181 215


NUMERICAL EXPERIMENTS:ZONE MODELLING – SETUP (CFAST 7.2.1)


NUMERICAL EXPERIMENTS:ZONE MODELLING – RESULTS (CFAST 7.2.1)

0

2000

4000

6000

8000

10000

12000

14000

16000

0 200 400 600 800 1000 1200 1400 1600 1800

HR

R (

kW

)

Time (s)

HRR-1.5 HRR-5

HRR-10 HRR-15

5.0

5.5

6.0

6.5

7.0

7.5

8.0

8.5

9.0

0 200 400 600 800 1000 1200 1400 1600 1800

He

igh

t (m

)

Time (s)

HGT-1.5 HGT-5

HGT-10 HGT-15

Design0

50

100

150

200

250

300

0 200 400 600 800 1000 1200 1400 1600 1800

Tem

pe

ratu

re (

°C)

Time (s)

TEMP-1.5 TEMP-5

TEMP-10 TEMP-15


NUMERICAL EXPERIMENTS:ZONE MODELLING – RESULTS (CFAST 7.2.1)

Additional trial and error CFAST simulations indicate:


NUMERICAL EXPERIMENTS:FIELD MODELLING – SETUP (FDS 6.5.3)


NUMERICAL EXPERIMENTS:FIELD MODELLING – INPUT (FDS 6.5.3)

• Fuel – 50% wood and 50% flexible PU foam.• Soot yield one-third of that reported in the SFPE Handbook as FDS over-

predicts smoke concentration 2-5 times.• Fire bed perimeter per AS/NZS 1668.1:2015 – fire intensity of 667, 556, 625

and 600 kW/m².• Simulation duration approx. 10 minutes after fire reached peak HRR.• Grid cell size for fire mesh to capture entire plume – checked D*/δx for

optimum.• Fire mesh should be 0.05D* to 0.1D*, other meshes should not exceed 0.5D*.• Domain extension – one hydraulic diameter of make-up air inlet opening.


NUMERICAL EXPERIMENTS:FIELD MODELLING – GRID INDEPENDENCE

Fire total heat release rate (MW)

Fire region grid cell size

(cm)

Bounding meshes grid

size (cm)

Simulation run scenario

ID

Total smoke exhaust

outlet area (m2)

Total make-up air inlet area (m2)

Total number of cells

CPU time (hours)

1.5

0.1 x 0.1 x 0.1 0.4 x 0.4 x 0.4 1.5-100 5.12(1.6 m x

0.8 m for each of 4 outlets)

15.36(2.4 m x

1.6 m for each of 4

inlets)

587,288 79.9

0.2 x 0.2 x 0.2 0.4 x 0.4 x 0.4 1.5-200 257,560 10.7

5

0.1 x 0.1 x 0.1 0.4 x 0.4 x 0.4 5-100 10.24(1.6 m x

1.6 m for each of 4 outlets)

33.28(5.2 m x

1.6 m for each of 4

inlets)

795,800 125.1

0.2 x 0.2 x 0.2 0.4 x 0.4 x 0.4 5-200 283,800 23.5

10

0.15 x 0.15 x 0.15

0.6 x 0.6 x 0.6 10-150 14.4(3 m x 1.2 m for each of 4

outlets)

30.24(4.2 m x

1.8 m for each of 4

inlets)

292,336 40.3

0.3 x 0.3 x 0.3 0.6 x 0.6 x 0.6 10-300 90,736 18.4

15

0.15 x 0.15 x 0.15

0.6 x 0.6 x 0.6 15-150 23.04(2.4 m x 2.4 m for each of 4 outlets)

43.2(6 m x 1.8 m for each of 4

inlets)

356,848 92.3

0.3 x 0.3 x 0.3 0.6 x 0.6 x 0.6 15-300 97,904 6.3


NUMERICAL EXPERIMENTS:FIELD MODELLING – GRID INDEPENDENCE


NUMERICAL EXPERIMENTS:FIELD MODELLING – RESULTS (FDS 6.5.3)


NUMERICAL EXPERIMENTS:WHAT DO THE RESULTS MEAN?

Fundamental principle �̇�𝐌𝐨𝐨𝐨𝐨𝐨𝐨

�̇�𝐌𝐞𝐞𝐞𝐞𝐨𝐨

�̇�𝐌𝐨𝐨𝐨𝐨𝐨𝐨 �̇�𝐌𝐞𝐞𝐞𝐞𝐨𝐨=

�̇�𝐌𝐢𝐢𝐞𝐞

= �̇�𝐌𝐢𝐢𝐞𝐞



Experimental errors associated with derivation of the plume formulas – typically 20%.



• CFAST is well know on overpredicting hot layer temperature – in some experiments deviation was 10 – 50% (NUREG 33%).

• FDS can also overpredict upper layer temperature – error up to 21%.



• CFAST can overpredict smoke layer interface – due to mass flow rate across the interface not being accounted for in the model which could be up to 30% of exhaust rate.

• FDS was found in literature to overpredict smoke filling rate (12% to 40% error) in the earlier stage of fire development, followed by underprediction in the later stage (35% to 42% error).



• Deduction: Having the capability to provide detailed spatial and temporal information on various parameters in an enclosure (CFD) does NOT necessarily equate to having higher accuracy and reliability in the simulation results, when compared to simpler techniques (zone modelling and empirical equations).


CONCLUSION

• AS/NZS 1668.1:2015 Section 9 reviewed: Not clear which plume type is intended to be covered by the prescribed high precision equation, no information on the technical basis of the proposed methodology, and no guidance on how to estimate hot layer temperature. Needs significant revision to be useful.

• Numerical experiments by CFAST (V7.2.1) – smoke filling rates unpredicted by 11 to 13%.

• Numerical experiments by FDS (V6.5.3) – smoke filling rates overpredicted by 60%.

• Additional research required to examine the predictive capabilities of FDS in mechanical exhaust scenarios.


QUESTIONS?

THANK YOU

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