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 • • • • • 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 ?