Measuring Radiated Thermal Output from Pyrotechnics …ukelg.ps.ic.ac.uk/50MW.pdf · Measuring...
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Measuring Radiated ThermalOutput from Pyrotechnics and
PropellantsPropellants
Mike WilliamsDepartment of Engineering and Applied Science
Cranfield University at the Defence Collegeof Management and Technology
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Topics
• Use of Surface Mounted Thermocoupleheat flux gauges.
• Short and long duration heat flux.
• Estimating Fireball Surface Temperature.• Estimating Fireball Surface Temperature.
• Estimating percentage of possible heatthat is radiated.
• Blast wave perturbations.
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Measuring Heat Flux with SurfaceMounted Thermocouples
“Fast” response gauges are very thin (5 micron), butt-joined, thermocouples bonded flat to a ceramicsurface (we use MACOR™).
No cooling is required. They are rugged and can beplaced close to thermal event.
They can be calibrated for heating due to radiationbut also report convection.
Broad band absorption – calibrated against 3 kWradiant heat source with steel surface.
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Radiation Flux Gauges
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Our Work
• MTV work based on paper to be published inPropellants, Explosives and Pyrotechnics(Wiley VCH). Work for Wallop DefenceSystems.Systems.
– MTV is a flare composition Magnesium/Teflon/Viton.
• Propellant work for Roxel.
• Ignition composition work for BAE Systems.
• Other work sponsored in-house.
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Surface Temperatures of Gauges
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Calculating Flux
• Method of S.V. Patankar.
– Heat flow into ceramic modelled usingthermal conductivity, density and heatthermal conductivity, density and heatcapacity. Temperature at depth assumedconstant but temperature gradientadapted in “slices” as heat flowprogresses
– Calculation only valid for short durationsof heat flow (a few seconds).
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Sensitivity Parameters
• Sensitivity Depends on:-
– Absorptivity of surface
– Thermal Conductivity, Density and Heat capacityof Substrateof Substrate
• Calculation runs in “Basic” program. Inessence
– dT/dt produces flux (Q)
– ∫Q . dt over duration of flux produces dose
• Flux lines can be erratic, dose usually smooth
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Heat Flux
Heat Flux, Pyrotechnic 24 kg, COTEC, 2nd June 2008.
2.50E+05
3.00E+05
3.50E+05
4.00E+05
Heat Flux,(W/sq.m)
6m (red)
9m (green)
0.00E+00
5.00E+04
1.00E+05
1.50E+05
2.00E+05
2.50E+05
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2
Time (s)
9m (green)
12m (blue)
Analysis by DASSR, Cranfield University.
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Heat Dose - medium fastcomposition
Heat Dose, Pyrotechnic, 24kg, COTEC, June 2nd 2008.
1.00E+05
1.25E+05
1.50E+05
Heat Dose(J/m2) 6m, (red)
9m (green)
0.00E+00
2.50E+04
5.00E+04
7.50E+04
0 0.5 1 1.5 2 2.5 3
Time (s)
9m (green)
12m (blue)
Analysis by DASSR, Cranfield University.
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Fast Burning Composition
Heat Flux - July 2008, - 3kg Igniter
1.00E+06
1.20E+06
1.40E+06
1.60E+06
1.80E+06
Heat Flux(W/sq.m)
2m (Red)
2m (Blue)
4m (Green)
6m (stripe)
0.00E+00
2.00E+05
4.00E+05
6.00E+05
8.00E+05
1.00E+06
0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4
Time (s)
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Dose from Burning Composition
Heat Dose - July 2008, - 3kg Igniter
6.00E+04
7.00E+04
8.00E+04
9.00E+04
1.00E+05
Heat Dose (J/sq.m)2m (Red)
2m (Blue)
4m (Green)
6m (stripe)
0.00E+00
1.00E+04
2.00E+04
3.00E+04
4.00E+04
5.00E+04
0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4
Time (s)
6m (stripe)
2nd Degree Burn
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Propellant – Three Speeds
Propellant Heat Flux with time - 25 kg at 4 m.
1.00E+05
1.20E+05
1.40E+05
Heat Flux
(W/sq.m)
Fast
Medium
0.00E+00
2.00E+04
4.00E+04
6.00E+04
8.00E+04
0 1 2 3 4 5 6 7 8 9 10
Time (sec)
Slow
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Propellant – Doses
Propellant Heat Dose with time, 25 kg at 4 m
1.20E+05
1.40E+05
1.60E+05
1.80E+05
Heat Dose
(J/sq.m) Thanks toROXEL/BAE
0.00E+00
2.00E+04
4.00E+04
6.00E+04
8.00E+04
1.00E+05
0 1 2 3 4 5 6 7 8 9 10
Time (sec)
Fast
Medium
Slow
2nd Degree Burn
ROXEL/BAE
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Fuel Fire Test
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IM Fuel fire test
Heat Flux
10000
15000
Heat Flux (W/sq.m) Bonfire Test, COTEC, Dec 21st 2001
-5000
0
5000
0 500 1000 1500 2000 2500 3000
Time (sec)
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Effect of 25g Explosive on Fire
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Convection
• Not easy to predict.
• Can more than double heat dose.
• Need to Consider:-– Reynolds number– Reynolds number
– Prandtl number
– Expansion number
– Four variables determined by experiment!• See Lawton and Klingenberg, “Transient
Temperature Measurement in Engineering”
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Flux with Convection
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Dose with Convection
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Peak Flux – MTV Composition
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Heat Transfer from Optically thickFlames and Hot Surfaces
M = εT . κ. T4
M = Rate of Heat transfer (Watts m-2)εT = Temperature dependant emissivity (a number ≤1) κ = Stefan Boltzmann constant = 5.67 x 10-8 W m-2 K-4
T is Emitting Surface Temperature (in Kelvin).T is Emitting Surface Temperature (in Kelvin).Using εT = 0.85 and assuming maximum flux fills fieldof view of gauges
Maximum M Surface Temperature
Propellant 0.12 MW m-2 1260 K
MTV 0.5 MW m-2 1800 K
Igniter 1.7 MW m-2 2440 K
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Effect of Distance and ViewFactor
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Simple View Factor
For Infinitely long coaxial cylinders. F1-2 =1
All of the heat emerging from the inner cylindermust pass through the outer one. ConcentricSpheres are the same – assumption of pointsource model.
Effect of distance = 1/d2
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Point Source Model
Assumes all radiated heat comes from a fixed point.
Assume view factor = 1.
Work back from measured J m-2 to total Joules usingsurface area of a sphere at the distance ofsurface area of a sphere at the distance ofmeasurement.
This will give an output in J kg-1 that can be comparedwith thermochemistry of material to estimate aneffective emissivity.
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Heat DoseHeat Dose from Pyrotechnic
6.00E+03
7.00E+03
8.00E+03
9.00E+03
1.00E+04
Heat Dose (J/sq.m)
2.5m NW
0.00E+00
1.00E+03
2.00E+03
3.00E+03
4.00E+03
5.00E+03
7 8 9 10 11 12 13
Time (Sec)
2.5m NW
2.5m SE
5m SE
10m SE
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Effect of Distance – PolythenePyrotechnic
Distance Heat Dose Total Heat(m) (J m-2) MJ kg-1
2.5 8400 1.64
5 1950 1.535 1950 1.53
10 430 1.35
Dose = K/(d 2.14)
The fact that the exponent in the distanceterm is >2 is probably due to atmosphericattenuation as the fireball was stable.
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Fireballs• Most fireballs are not well behaved – tending to be
buoyant. View factor is dynamic as fireball rises.
• Assael and Kakosimos (Fires, Explosions andToxic Gas Dispersions, CRC Press, 2010) have aformula linkingformula linking
Height/radius Effect of distance
0.5 1/d2.4
1.0 1/d1.98
5.0 1/d1.19
10.0 1/d1.08
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Rising Fireball Effects?
Peak flux seen 0.1 slater at 12m than
6m.
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Effect of Amount and DistanceMTV Flare
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Effect of Amount on View Factor
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Effect of amount on FireballDuration as Radiant Source
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Bigger Fireballs – 6m data
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Output and Effective Emisivity
• Pyrotechnic 1600 kJ/kg 20%
• Fuel Fire Test 8000 kJ/kg 20%
• Propellant 1500 kJ/kg 30%
• MTV Flare 6000 kJ/kg 30%• MTV Flare 6000 kJ/kg 30%
E-C Koch (Metal Fluorocarbon Based EnergeticMaterials, Wiley-VCH, 2012) calculated MTVeffective emissivity as 23% from measuredradiation between 1.8 and 4.8 microns. Kochalso measured the surface temperature of thefireball as 1940 K based on 1.6 to 1.7 microns.
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Blast Wave Effects
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Speed of Perturbation Effect
Distance(m)
Time ofArrival
OverallSpeed (m s-1)
SpeedbetweenPoints (m s-1)Points (m s
6 0.0086 698 750
9 0.0158 570 417
12 0.0234 513 395
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Conclusions• Surface Mounted Thermocouple Heat Flux gauges
can be used to calculate radiated heat flux and dosefrom relatively rapid thermal events.
• They can be used to estimate fireball surfacetemperatures.temperatures.
• They can be used to estimate effective emissivity forcompositions – which can then be used in hazardcalculations.
• They respond to but do not quantify convection.
• All thermocouples respond to fast pressurefluctuations.