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Smoke ventilation in common areas of single stair residential
buildings
Some on-going issues and analysis Stewart Miles
International Fire Consultants
The study and results reported are part on-going, personal academic work, and do not necessarily represent the views of any organisation
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Smoke control in common areas
• Compartmentation is most important element
• Suppression may be significant
• Ventilation generally also required• Can assist means of escape
• Can assist fire service operations
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Smoke ventilation in single stair buildings
• To help protect stair enclosure• Limit smoke ingress before fire service arrival and during firefighting
• Where there are ‘extended’ corridor travel distances • Purge smoke that enters corridor prior to fire service arrival
• Reduce severity of conditions in corridor during firefighting
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Current ventilation practice
• Natural ventilation from corridor; or
• Mechanical extract ventilation from corridor• Sometimes augmented with mechanical supply too
• Or sometimes open vents for inlet air
• Open vent at top of stair enclosure (with either of above)
• Air supply / pressurisation in stair enclosure not common
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Some concerns
• Fire sizes considered are generally restricted to fuel-bed limited condition• i.e. contained fuel area and associated heat release rate
• But, can post-flashover fires actually be more severe?• (and take smoke ventilation system beyond designed capability?)
• Can too much depressurisation in the corridor lead to hazards that are not generally considered?• e.g. smoke drawn under door threshold
• Propensity for ‘backdraught’ conditions during firefighting?
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Building geometry modelled
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Apartment and common corridor
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Smoke ventilation methods analysed
• External wall AOV in corridor• (plus top of stair AOV)
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Smoke ventilation methods analysed
• Natural smoke shaft to corridor• (plus top of stair AOV)
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Smoke ventilation methods analysed
• Mechanical extract from corridor• (plus top of stair AOV)
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Smoke ventilation methods analysed
• Mechanical ‘push/pull’ in corridor• supply & extract in corridor
• (plus top of stair AOV)
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Smoke ventilation methods analysed
• Mechanical supply to stair• Fresh air into stair
• Pressure-relief damper
• (top of stair AOV omitted)
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Smoke ventilation methods analysed
• Mechanical supply to stair, plus either:
• External wall AOV in corridor, or
• Mechanical ‘push/pull’ in corridor
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Post-flashover fire scenario
• Steady-state condition
• Fire distributed across whole floor
• Assume:• Hot gas layer at 800°C & emissivity ~1
• Fuel heat of gasification = 4 000 kJ kg-1
• (ranges from 1 000 to 5000 kJ kg-1 for most fuels)
• Heat of combustion = 20 000 kJ kg-1
• Then• Energy (heat) release rate associated with fuel pyrolysis = 375 kW m-2
• (would be higher if lower heat of gasification adopted)
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Post-flashover fire scenario
• Fire spread across entire floor area in room of origin• 24m2 → 9 MW heat release
• (assuming all fuel combusts)
• Window/vent 3m by 1.4m• From empirical correlation ~7.5MW
heat release rate can be supported within room• (excess fuel can combust in flames
outside window)
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Illustration of fire from CFD model
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Illustration of fire from CFD model
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Comparison with a real fire
• Flame and smoke dynamics similar to the CFD model
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Illustration of fire from CFD model – apartment door open
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Modelling using Fire Dynamics Simulator
• 10cm mesh throughout
• Eddy dissipation combustion• Heat combustion 20 000 kJ kg-1
• 10% soot yield
• ‘Finite volume’ ray-tracing radiation model• 100 rays
• Grey gas absorption sub-model
• 0.35 radiative fraction (from combustion process)
• HVAC sub-model• For mechanical supply & extract boundary conditions
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Simulations (scenarios) at fire service intervention stage
• Steady (post-flashover) fire source
• Ventilation system running
• Apartment door open
• Stair door (on fire floor) open to 20° angle
• Stair bottom (entrance) door open
• Simulations conducted for two to three minutes or until quasi-steady state reached
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Simulations (scenarios) prior to fire service arrival
• Steady (post-flashover) fire source
• Ventilation system running
• Apartment door closed• 3mm threshold gap modelled using FDS ‘leak vent’ capability
• Stair door (on fire floor) closed • Unless ‘pulled-open’ as part of ventilation operation
• Stair bottom (entrance) door open
• Simulations conducted for two to three minutes or until quasi-steady state reached
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Smoke transport at firefighting stage
• No corridor ventilation
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Smoke transport at firefighting stage
• ADB corridor AOV• ~1m2 aerodynamic free area
• (equivalent to 1.5m2 unobstructed rectangular opening)
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Smoke transport at firefighting stage
• ADB natural smoke shaft
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Smoke transport at firefighting stage
• Mechanical extract at 2 m3s-1
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Smoke transport at firefighting stage
• Mechanical extract at 4 m3s-1
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Smoke transport at firefighting stage
• Mechanical extract at 6 m3s-1
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Smoke transport at firefighting stage
• Mechanical ‘push/pull’ at 2 m3s-1
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Smoke transport at firefighting stage
• Stair supply at 2 m3s-1
• No vent in corridor
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Smoke transport at firefighting stage
• Stair supply at 2 m3s-1
• Wall vent in corridor
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Smoke transport at firefighting stage
• Stair supply at 4 m3s-1
• No vent in corridor
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Smoke transport at firefighting stage
• Stair supply at 4 m3s-1
• Wall vent in corridor
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Smoke transport at firefighting stage
• Mechanical ‘push/pull’ at 2 m3s-1
• Stair supply at 2 m3s-1
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Smoke transport at firefighting stage
• Mechanical ‘push/pull’ at 2 m3s-1
• Stair supply at 4 m3s-1
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Smoke transport prior to fire service arrival
• Mechanical ‘push/pull’ at 2 m3s-1
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Smoke transport prior to fire service arrival
• Low level mechanical extract • Air leakage into corridor
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Smoke transport prior to fire service arrival
• Mechanical extract at 2 m3s-1
• ‘Reverse-hung’ stair door part-opening at ~10Pa
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To conclude
• Various issues relating to smoke ventilation in residential buildings warrants further investigation
• Impact of post-flashover apartment fires
• Potential for smoke and fie gases to be drawn into the corridor
• Possible benefit of supplying air mechanically into the stair is being missed
• For ‘extended corridors’ a combination of mechanical corridor smoke clearance (e.g. ‘push/pull’) and mechanical air supply into the stair may prove productive
• Work reported is very much ‘work in progress’ open for discussion
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Thank you for your attention
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