Heat Flux in Fire Tests
Javier O. Trevino
Sr. Project Engineer
Outline
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Basics of Heat (Conductive, Convective, Radiation)
Convective Heat Transfer in Fire Tests
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Fire Basics (Hc, Fuel Flow, Temperature, Heat Release Rate)
Burner Types (diffuse, pre-mix)
Examples (Bunsen, FT4, P1717, UL1709)
Flame Impingement vs Absorbed Heat
Heat Flux vs Heat Release Rate
Radiation Heat Transfer in Fire Tests
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Furnaces Types (Open vs Enclosed)
Temperature Sensors (Open TCs, Closed TCs, Plate TCs, DFT
Heat Flux Sensors (Gardon, Schmidt Boelter, Water Calorimater, Slug,
DFT)
Flame Impingement or Radiation Impingement ??
Temperature and Heat Flux – Relationship??
Basics of Heat Transfer
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Conductive
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Convective
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Flow of Heat from Hot to Cold in Solids, Liquids
R-Value (Thermal Conductivity)
Flow of Heat from Fluid or Gas to Solid
Heat Transfer Coeff (Absorption of Heat)
Radiation
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IR Radiation Absorption of Solids, Liquids
Emissivity (Absorption of Heat)
Convective Heat Transfer in Fires
„ Fire Basics
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Hc (Heat of Combustion)
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Fuel Flow Æ Heat Flow
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Fuel Flow (kg/sec Æ lb/sec Æ cfm Æ LPM)
Temperature
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Gas – Common Hydrocarbon Fuels about 50 MJ/kg Æ BTU/lb
Yellow Flame = 1400 F to 1800 F (65% Convective, 35% Radiative)
Blue Flame = 1900F to 2200 F (80% Convective, 20% Radiative)
Heat Release Rate (Watts, BTU/min)
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Conversion (kg/sec Æ MJ/sec Æ kW Æ BTU/min)
So…. Gas Flow Rate = Heat Flow Rate
Convective Heat Transfer in Fires
„ Burner Types
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Diffuse
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Fuel mixes with outside air then ignites
Yellow Flame = 1400 F to 1800 F
Pre-Mix
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Fuel mixes with air in mixing chamber then ignites
Blue Flame = 1900F to 2200 F
Convective Heat Transfer in Fires
„ Examples of Burners
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Bunsen (Methane)
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FT4/Mod 1709
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Pre-mix, 10 to 20 kW, 2000 F
P1717
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Pre-mix, 3 kW, 1400 to 2200 F (Air Intake Dependent)
Pre-mix, 50 to 80 kW, 2000 F
UL 1709 (Furnace Volume Dependent)
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Pre-mix, 200 kW per burner x 12, 2000 F
Convective Heat Transfer in Fires
„ Flame Impingement vs Absorbed Heat
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Fire Hits Specimen
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% Absorbed (Convection, Radiation)
% Reflected (Emissivity, Heat Transfer Coeff, )
% Misses (Depends on Fire Size)
Burner VS Furnace (Cable Fire Test)
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Both are 2000 F.
Large % of burner flame not absorbed
Cable “ENGULFED” in 2000 F Environment
Cable keeps absorbing until equilibrium
Convective Heat Transfer in Fires
„ Heat Flux (Convective)
vs Heat Release Rate (HRR)
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Example: Sand Diffusion Burner
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1 ft x 1 ft sand burner
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100 kW Fuel Flow Rate
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Convective Heat Flux = 100 kW/1/10 sq m = 1000 kW/sq m ???
„ Specify HRR rather than Heat Flux in burner tests.
Radiation Heat Transfer in Fires
„ Furnace Types
„ Open vs Enclosed Furnaces
„ UL 1709 Æ Closed Furnace Æ Large Volume Æ No
direct Flame Impingement Æ Radiation dominant mode
of heat transfer
„ Mod. UL 1709 (precursor to P1717) Æ Open Furnace Æ
Small Volume Æ Direct Flame Impingement Æ
Convective dominant mode of heat transfer
„ P1717 Æ Semi-Closed Furnace Æ Small Volume
ÆDirect Flame Impingement Æ 50/50
Convective/Radiation heat transfer mode
Radiation Heat Transfer in Fire
Tests
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Temperature Sensors
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Open Thermocouples: Bare Thermocouple Wire
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Closed Thermocouples: Inconel or Steel Sheathed
Thermocouple Wire
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Plate Thermometer: Thermocouple welded to thin stainless
plate
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DFT (Directional Flame Thermometer): Two plate TCs
sandwiching a layer of Cerablanket Insulation
Radiation Heat Transfer in Fire
Tests
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Heat Flux Sensors
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Gardon Gauge: Electronic Plate Thermocouple (Plate-Wire Thermocouple).
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Schmidt Boelter Gauge: Electronic Thermopile (Multiple TCs connected
together). Linear mV Signal
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Water Calorimeter: Water Flowing through steel pipe or other shape. Calculated
based on water temperature, flow rate, absorption of steel
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Slug Calorimeter: Chunk of steel with TC’s inside. Calculated based on steel
properties and rate of temperature rise.
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DFT: Reverse Code Calculation
Linear mV signal
Radiation Heat Transfer in Fire
Tests
Flame Impingement
or
Radiation Impingement ??
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In UL 1709 Furnace
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1st few minutes –
Burners fill Furnace with Hot Exhaust
Æ Flame Dominated Volume
After 5 minutes –
2000 F Hot Gas Becomes Transparent,
Walls of Furnace Glow Red Hot
Æ Radiation Dominated Volume
Radiation Heat Transfer in Fire
Tests
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Temperature and Heat Flux – Relationship??
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In UL 1709 Furnace – Determined by Calibration
Calibration of OPL's Horizontal Furnace
i.a.w. MIL-PRF-32161
2300
2200
Flux VS Temp
Linear (Flux VS Temp)
2100
y = 3.4996x + 1286.3
2000
2
o
188 kW/m = 1944 F
2
204 kW/m = 2000oF
220 kW/m2 = 2056oF
1900
1800
1700
140
150
160
170
180
190
200
210
220
230
2
Furnace Heat Flux (kW/m )
240
250
260
270
Radiation Heat Transfer in Fire
Tests
Temperature and Heat Flux – Relationship??
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……….Followed by Proof Test
Verification of OPL's Horizontal Furnace
i.a.w. MIL-PRF-32161
Test Conducted on 6/14/05
250
3000
2500
200
2000
150
Furnace Temp
Furnace Flux
Upper Heat Flux Limit
Lower Heat Flux Limit
1500
100
1000
50
500
0
0
0
5
10
15
Time (min)
20
25
30
Heat Flux (kW/m2)
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Radiation Heat Transfer in Fire
Tests
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Temperature and Heat Flux – Relationship??
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In P1717 Furnace – Determined by Calibration
Heat Flux VS Temperature
Calib 250/9 mV
250
Flux (kW/sq m)
200
150
Flux w 10mV Calib (kW/sq m)
100
50
0
0
500
1000
1500
Temperature (deg F)
2000
2500