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Physical Scaling for Performance Evaluation
of Water Mist Systems in Enclosures
Hong-Zeng (Bert) Yu, FM Global Research
Jonathan Carpenter, FM Approvals
Fire Sprinkler International
May 20-21, 2014
London, United Kingdom
Outline
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Motivation
Review of the scaling laws
Scaling law validations
A test case for scaling up water mist protection
Approval process of FM Approval
Standard 5560 using scaling approach
• Summary
Why scale water mist fire suppression?
• To reduce the development cost of water mist
fire protection systems in enclosures.
– Currently, full-scale testing is required for system development
and listing.
– Often times it is a trial-and-error process
– Full-scale fire testing costs are often prohibitive for niche
applications.
• Goal: To replicate water mist fire protection
with scaled-down facilities.
Froude-Based Physical Scaling Principles
In geometrically similar control volumes,
Conserve:
1) Froude number of gas flow
momentum force
ρu 2
Fr 

buoyancy force (ρ   ρ)gL
2) Momentum transfer characteristics between
water droplets and gas flow.
3) Droplet vaporization characteristics.
4) Scalar quantities in the control volumes.
Froude-Based Scaling Relationships
Re d 
 
d ud  ug
νg
Scaling Parameters
Drag Coefficient (Cd)
Scale Ratio S=L2/L1
Temperature, Concentration
Nozzle Spray Angle
Time
Nozzle Spacing
Nozzle Operating Pressure
Ventilation Rate
Fire Convective Heat
Release Rate
Any Red*
Cd ~Red -x
S1
S0
S0
S1/2
S1
S1
S5/2
Red 1
Cd ~ Red -1
S1
S0
S0
S1/2
S1
S1
S5/2
S5/2
S5/2
Total Water Discharge Rate
S5/2
S5/2
S(2-x)/(2+2x)
S1/4
Droplet Diameter
* Fire Safety Journal, 2012.
A Scaling Illustration for Red <1
Full Scale
20 x 20 x 10 m high Enclosure
4 x 4 m Door Opening
Fire size
(S2.5)
Water mist discharge rate
6000 kW
1/4 Scale
5 x 5 x 2.5 m high Enclosure
1 x 1 m Door Opening
188 kW
100 liters/min
3.1 liters/min
90 microns
64 microns
30 s
15 s
(S2.5)
Droplet size
(S0.25)
Water mist discharge time
(S0.5)
Validations of Scaling Relationships
• Droplet size dependence on Red (Scale ratio = 9 :1)
(Fire Safety Journal, 2008)
• Spray cooling in open space (Scale ratio = 9:1)
(Fire Safety Journal, 2008)
• Fire radiation effect (Scale ratio = 9:1)
(Fire Safety Journal, 2012)
• Fire suppression and extinguishment for different fuel
types in enclosures (Scale ratio = 3:1)
– Gas fires (propane)
(9th IAFSS Symposium, 2008)
– Pool fires (heptane)
(10th IAFSS Symposium, 2011)
– Solid combustible fires (wood cribs)
(11th IAFSS Symposium, 2014)
Pool Fire Suppression in Enclosures
Scale-3 Enclosure: 3.66x3.66x3.66 m
Scale-1 Enclosure: 1.22x1.22x1.22 m
Door Opening: 0.30x0.61 m high
0.61x0.61 m high
Door Opening: 0.91x1.83 m high
1.83x1.83 m high
Scaling Pool Fires in Open Space -1
Scale-1 Heptane Pool Fires:
Smaller fire: 0.23-m diameter pool  25 kW
Larger fire: 0.32-m diameter pool  58 kW
Scale-3 Heptane Pool Fires ( Scale-1 Rate x 32.5):
0.55-m pool  390 kW
0.74-m pool  900 kW
Test Conditions
Conditions
Scale 1
Scale 3
Enclosure Size (S)
1.22 x 1.22x1.22 m
3.66x3.66x3.66 m
Door Opening (S)
0.30x0.61 m
0.61x0.61 m
0.91x1.83 m
1.83x1.83 m
Fire Heat Release Rate
(S2.5)
25 kW
58 kW
390 kW
900 kW
Nozzle Matrix (S0)
3x3
3x3
Nozzle Spacing (S)
0.30x0.30 m
0.91x0.91 m
Pressure (S)
13.8 bar
41.4 bar
Total Discharge Rate (S2.5)
1.65 lpm
25.0 lpm
Volume-Median Droplet
Size (S0.25 )
62 micron
90 micron
Spray angle (S0)
60°
60°
Fire Locations and Measurements
• Fire heat release rate
• Floor-level oxygen
concentrations
Scalability Evaluation for Pool Fire Suppression
Replicate
• Fire extinguishability.
• Fire heat release rate with time.
• Oxygen concentrations.
Ignitable Liquid Cut-Off Room
Challenges: 1) low flash point liquids.
2) large door openings.
3) high ceilings.
4) ceiling nozzles only.
5) obstructed fires.
Ignitable Liquid Cut-Off Room Protection*
Targeted cut-off room and fire hazard:
1) Cut-off room size: 7.47x7.47x7.47 m high (6 times the Scale-1 enclosure)
2) Door opening: 3.66x3.66 m high (6 times the Scale-1 enclosure’s larger opening)
3) Heptane storage tank: 1.83 m diameter x 2.74 m high, 0.91 m above floor.
4) Fire scenario: Cascading heptane spill from the top of tank at 38 liters/min .
(~ 8 MW > 5.1 MW = 58 kW of Scale-1 fire x 62.5)
Based on the water mist protection for the Scale-1 enclosure, the water mist
requirement for the cut-off room is:
Scale
Nozzle
Spacing
(m)
Pressure
(bar)
Spray Angle
(deg)
Volume-Median
Droplet Size
(μm)
Total discharge Rate
(liter/min)
6
1.87
83
60
97
146
* Journal of Fire Protection Engineering, 2013.
Key Components for the Scaling Approach
Three key components:
 Nozzle spray characterization (droplet size and
distribution, spray discharge rate and momentum,
spray angle and flux distribution pattern, etc.)
 Well mixed model – a predictive tool to assess the
total water mist discharge rate and droplet size for
full-scale protection, for screening purpose.
 Scaling methodology to scale down the large-scale
fire tests.
Approval Process for the Scaling Approach
After implementing the scaling approach to FM Approval 5560 – Water
Mist Systems:
• Manufacturer applies for FM Approval with parameters for fullscale protection and supplies scaled-down nozzles to FM
Approvals.
• FM Approvals assesses the adequacy of the intended full-scale
protection parameters with the well-mixed model using data from
the scaled-down nozzle characterization activity. This is a
screening approach to identify a potential solution that works.
• Conduct testing in the scaled-down mockup with the scaled-down
protection parameters and nozzles to ensure fire extinguishment
results.
• Manufacturer submits the full-scale nozzle for spray
characterization to ensure meeting the scaled-up requirement.
• Approval granted for full-scale protection by scaling up the
scaled-down test results according to the scaling relationships.
Current Approach – Full Scale Testing
New Approach – Reduced Scale Testing
Benefits to Industry
• Potential reduction in approval cost and time
• Potential reduction in internal cost to the manufacturer
could be significant due to the screening test availability.
• Potential reduction in required number of reduced-scale
tests in the future (after gaining some experience)
• Ultimately increase the number of certified applications!
Summary
• The Froude-based scaling laws have been validated with
laboratory experiments for the effects of droplet size, fire
radiation, enclosure and different types of combustibles.
• The protection of an ignitable liquid cut-off room has been
successfully developed by scaling up the known protection
requirements for smaller enclosures.
• The approval process based on the scaling approach is to
be implemented in FM Approval Standard 5560 – Water Mist
Systems, to help reduce the system development and
approval cost and increase applications.
Questions ?