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Ignition-to-Spread Transition of Electrical Wire fires
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Transcript of Ignition-to-Spread Transition of Electrical Wire fires
August 21, 2014 1
Ignition-to-Spread Transition of Externally
Heated Electrical Wire
Xinyan Huang and Forman A. Williams
University of California, San Diego
Yuji Nakamura
Hokkaido University, Japan
August 3, 2012 β 34th international symposium on Combustion, Warsaw, Poland
August 21, 2014 2
Outline
β’ Motivation
β’ Simplified Ignition-to-Spread Model
β’ Experimental Setup
β’ Experimental Results and Discussion
β’ Conclusions and Future Work
2August 3, 2012 β Ignition-to-Spread Transition of Externally Heated Electrical Wire
August 21, 2014 3
Motivation
In 2010, electrical fires accounted for 28,600 incidents and $ 1.1 billion in property losses, 53% of which involved electrical wiring.
Electrical wire fires account for 42% of total fire cases in Nuclear Power Plants. Also, fire scenarios for wire fires in sub-atmospheric pressure and oxygen-enriched (space) applications should be determined.
Previous study showed flames spread faster in a higher-conductivity wire and the spread rate increases as pressure decreases [1]. But no systematic experimental study or theory for wire ignition by externally heating exists.
How do the thermal conductivity, dimension of wire and atmospheric conditions (pressure and oxygen concentration) affect ignition and the consequent transition to spread?
3August 3, 2012 β Ignition-to-Spread Transition of Externally Heated Electrical Wire
[1] Y. Nakamura et al., Proc. Combust. Inst., 32 (2009), pp. 2559β2566
August 21, 2014 4
Outline
β’ Motivation
β’ Simplified Ignition-to-Spread Model
β’ Experimental Setup
β’ Experimental Results and Discussion
β’ Conclusions and Future Work
4August 3, 2012 β Ignition-to-Spread Transition of Externally Heated Electrical Wire
August 21, 2014 5
Ignition Model at Flashpoint
5
Assumptions:
Uniform heat flux within πΏ
Thermally thin: Ξ΄ ~ 0.2 mm βͺ πΌπ‘ππ
Uniform cross-sectional temperature: π΅ππ βͺ π΅ππ < 0.3
ππ βͺ ππ and πππ π βͺ πππ π
Governing Equations
βπππ΄ππ
ππ‘= π΄πππ
π2π
ππ₯2+ ππ ππ
β²β² β ππππ π β²β² 0 < π₯ <
πΏ
2
βπππ΄ππ
ππ‘= π΄πππ
π2π
ππ₯2β ππβ π β ππ π₯ >
πΏ
2
August 3, 2012 β Ignition-to-Spread Transition of Externally Heated Electrical Wire
where ππΏ/2β = ππΏ/2+ ,ππ
ππ₯πΏ/2β
=ππ
ππ₯πΏ/2+
,
ππ
ππ₯0
= 0, d πβ = ππ , aafor π‘ > 0.
Flashpoint: pyrolysis vapors achieve a
fuelβs lower flammability limit, defined
by a critical mass flux or temperature
(at ππππ₯ = πππ β₯ ππ).
August 21, 2014 6
Ignition Model at Flashpoint
6
Ignition time
π‘ππ βπ π, ππ , πΏ, πΏ
ππβ²β²π
where π denotes ignition properties.
π β 1 (thermally thin)
π β 2 (thermally thick)
Critical/minimum ignition heat flux:
ππ ππ‘ = 0, π0~πΏ/2 = πππ
β ππ,πππ‘β²β² = ππππ π
β²β² +πππΏ
πππ β ππ βππππ
Increase with the conductance
(conductivity and core diameter)
Decrease with heating length (πΏ)
August 3, 2012 β Ignition-to-Spread Transition of Externally Heated Electrical Wire
Governing Equations
βπππ΄ππ
ππ‘= π΄πππ
π2π
ππ₯2+ ππ ππ
β²β² β ππππ π β²β² 0 < π₯ <
πΏ
2
βπππ΄ππ
ππ‘= π΄πππ
π2π
ππ₯2β ππβ π β ππ π₯ >
πΏ
2
ππΏ/2β = ππΏ/2+ , πT/πx πΏ/2β = πT/πx πΏ/2+ ,
πT/πx 0 = 0, d πβ = ππ , aafor π‘ > 0.
August 21, 2014 7
To calculate the temperature profile, the heat-transfer equation during spread is
π΄ππππ2π
ππ₯2+ πππ΄ πππ
ππ
ππ₯= ππ
β²
ππ β² : heat transfer in radial direction
Spread Point
7
Flame spread rate (ππ) and flame width
(ππ) are the eigenvalue of the system,
which quantify the wire conductance.
To sustain the flame spread, the wire temperature profile should be higher than that at steady-state flame-spread .
August 3, 2012 β Ignition-to-Spread Transition of Externally Heated Electrical Wire
Four Regions:
I. Unburned wire, ππ ,1β² = ππβ1 π β ππ
II. Boiling polymer within the flame
ππ ,2β² = ππ ππ
β²β² = ππβπ π β πππ
III. The wire core exposed to the flame
ππ ,3β² = ππ ππ
β²β² β π π4 β ππ4 β βπ π β ππ
IV. The wire core exposed to atmosphere
ππ ,4β² = ππβ4 π β ππ
August 21, 2014 8
Spread Point
8
* Alternatively, the temperature profile can be measured by a fixed thermocouple [3], and the result agrees with the current semi-analytical estimation.
August 3, 2012 β Ignition-to-Spread Transition of Externally Heated Electrical Wire
[2] J.L. Torero et al., Combust. Sci. Tech., 174 (2002), pp. 187β203[3] Y. Nakamura et al., J. Therm. Sci. Tech., 3 (2008), pp. 430β441
π»πππ ?β temperature profile
Burning rate [2]
πβ²β² =πΏππ
ππππ β
ππ’ β ππ
ππππln 1 + π΅ ,
where π΅ =ππ2,β βπ»π/π + ππ ππ β πππ
πΏπ£ β ππβ²β²/ πβ²β²
= 4 ~ 5
Additional heating source from wire core
ππβ²β² = βπ π2 β πππ , βπ~
π
πΏ
π2 = ππππ₯ + πππ /2, (linear)
Measuring π½π,ππ & πΎπ,ππ β π»π β π»πππ
August 21, 2014 9
Ignition-to-Spread Transition
9
Transition from flashpoint to spread point
Flame is weak during the transition
Large conductive heat losses along the metal core may quench the flame
Additional heating may be required
Additional heat & heating duration
π₯π = π»π π β π»ππ β π₯π ππ₯π»π ; π₯π‘ =π₯π
ππβ²β²πππΏ/2
Enthalpy at spread point: π»π π = 0ββπππ΄ π ππ₯ (blue line)
Enthalpy at flashpoint: π»ππ = 0ββπππ΄ π ππ₯ (red line)
Heat from flame: π₯π ππ₯π»π
August 3, 2012 β Ignition-to-Spread Transition of Externally Heated Electrical Wire
o For high-conductivity wire or short heating length, π»π π β π»ππ is large and a
long additional heating time is expected;
o Thick coating may produce more flame heating and reduce the additional heat.
August 21, 2014 10
Outline
β’ Motivation
β’ Simplified Ignition-to-Spread Model
β’ Experimental Setup
β’ Experimental Results and Discussion
β’ Conclusions and Future Work
10August 3, 2012 β Ignition-to-Spread Transition of Externally Heated Electrical Wire
August 21, 2014 11
Ignition Experiment
Chamber size: 365 mm (L) Γ 260 mm (W) Γ 180 mm (H);
Coil heater: d = 5 mm, L = 1.2 cm, 2.0 cm (wound 10 times), and 3.0 cm;
Change the coil heater power to provide different external heat flux and heating time by regulated DC power (accurate to Β±0.01 A) and compact PLC (accurate to Β±0.1 s);
Sony HDR-XR 500V video camera (30 fps);
Two pipe line: (1) from air/oxygen tank; (2) to vacuum pump.
11August 3, 2012 β Ignition-to-Spread Transition of Externally Heated Electrical Wire
August 21, 2014 12
Sample Wires & Test Conditions
12
Wire core: nichrome (NiCr) and copper (Cu) wires
(ππ)πππΆπ
β (ππ)πΆπ’
β (ππ)ππΈ
, but ππππΆπ βΆ π πΆπ’ β 1 βΆ 25
Polyethylene coating: πΏ β€ 0.3 mm (thermally thin), ππ βͺ ππ
At least 5 repeated tests at each case
Ignition curves are plotted along the 50% chance condition (e.g. 3 times flash/spread and 3 time not flash/spread).
Configurations of thin wires used in this study
August 3, 2012 β Ignition-to-Spread Transition of Externally Heated Electrical Wire
* ππ β 0.2 π/ππΎ, ππππΆπ β 16π/ππΎ, ππΆπ’ β 400π/ππΎ
* >3500 runs in this study
August 21, 2014 13
Outline
β’ Motivation
β’ Simplified Ignition-to-Spread Model
β’ Experimental Setup
β’ Experimental Results and Discussion
β’ Conclusions and Future Work
13August 3, 2012 β Ignition-to-Spread Transition of Externally Heated Electrical Wire
August 21, 2014 14
Typical Ignition Process at Normal Atmosphere
14
NiCr-A wire: do = 1.0 mm, dc = 0.7 mm, πΏ = 0.15 mm,
πΌ = 10 π΄, coil heater (2 cm), ππ = 1 atm
Transition (no-spread)
heating time = 7.4 sec
Flashpoint
heating time = 6.9 sec
Spread point
heating time = 7.5 sec
August 3, 2012 β Ignition-to-Spread Transition of Externally Heated Electrical Wire
As heating duration increases:
Blue premix flame (flashpoint) β yellow diffusion flame (fire point) β flame separate and spread to the edge of coil heater
β spread out (spread point)
August 21, 2014 15
Comparison at Flashpoint
15
I. At flashpoint, experimental results qualitatively agree with modeling results.
II. Index π increases with increasing wire conductance, and therefore does not confirm ideality.
III. a wire with a larger conductivity and a larger core diameter, a longer heating duration is required, and critical heat flux or electrical current is larger.
Ignition time by fitting modeling results
π‘ππ βπ π, ππ , πΏ, πΏ
ππβ²β²π
August 3, 2012 β Ignition-to-Spread Transition of Externally Heated Electrical Wire
* Note that it is impossible to calculate the exact heat flux from the coil heater.
Conductance β
August 21, 2014 16
Transition and Spread point
16
Estimating the experimental heat flux from previous
comparison by connecting πΌ, π‘ππ to ππβ²β², π‘ππ .
Assuming heating efficiency from the flame: π = 5%, and calculate the required heating duration.
Experimental observation:
I. For large-conductance wires, additional heating duration is required in experiments.
II. For low-conductance wires, once flash, flame can spread out.
August 3, 2012 β Ignition-to-Spread Transition of Externally Heated Electrical Wire
I. Model qualitatively predicts the experimental trend, βπ‘ increases with increasing thermal conductance (Cu-A>Cu-B>NiCr-A>NiCr-B).
II. βt decreases as heat flux decrease because a longer heating duration increases π»ππ
III. In high-heat-flux experiment, the required heating duration is underestimated because the high-temperature coil continues to heat after power off.
Conductance β
August 21, 2014 17
Heating-Length Effect
17
I. Increasing the heating length increases the π»ππ and π₯π ππ₯π»π , which may
converge the spread point to the flashpoint.
II. A shorter heating zone requires more heating duration to ensure spread.
III. Critical heat flux decreases with increasing heating length.
August 3, 2012 β Ignition-to-Spread Transition of Externally Heated Electrical Wire
π₯π = π»π π β π»ππ β π₯π ππ₯π»π ;
β π₯π‘ =π₯π
ππβ²β²πππΏ/2
ππ,πππ‘β²β² = ππππ π
β²β² +πππΏ
πππ β ππ βππππ
(Additional heat time)
Heating length β
August 21, 2014 18
Ignition and Spread at Reduced Pressures
18
π·π = ππ kPa
Cu-B wire: πΌ = 11 π΄, heating time = 9.0 sec,
coil heater (2 cm)
π·π = ππ kPa
Cu-B wire: πΌ = 11 π΄, heating time = 10.0 sec,
coil heater (2 cm), no steady-state spread
August 3, 2012 β Ignition-to-Spread Transition of Externally Heated Electrical Wire
Blue, weak, and spherical flame during ignition
Flame spread faster as pressure decreases
August 21, 2014 19
Pressure Effect
19
Modeling results at flashpiont
Experimental results
o Simulation results show that reducing the ambient pressure reduces the convective cooling (βπ), resulting in a short heating time at flashpoint.
o Dash line indicates the limiting condition of no gravity.
o In general, pressure effect should be small.
In low-pressure experiments, the heating time at flashpoint increases with decreasing pressure because the convective heating (from both coil and flame) decreases remarkably.
The same reason for a longer the heating time at spread point.
Thus, when consider the ignition difficulty in low pressures, how the pressure affects the heating source should not be neglected.
August 3, 2012 β Ignition-to-Spread Transition of Externally Heated Electrical Wire
August 21, 2014 20
Oxygen-Concentration Effect
20
Heating time decreases as oxygen concentration increases (ππ2 < 40%),
even under the low pressure environment, because a higher flaming temperature increases the heating efficiency (π).
ππ2 > 40%, spread point always
converges to the flashpoint, indicating a different mechanism to control the spread point.
Ignition model failsIgnition model works
August 3, 2012 β Ignition-to-Spread Transition of Externally Heated Electrical Wire
August 21, 2014 21
Ignition Delay at High Oxygen Concentrations
21
Cu-B wire
o do = 0.8 mm, dc = 0.5 mm, πΏ = 0.15 mm,
o πΏπΆπ = ππ%, 1 atm
o coil heater (2 cm), πΌ = 11 π΄
Heating time = 4.1 sec
Flashpoint = 10.4 sec (~ 6 sec delay)
August 3, 2012 β Ignition-to-Spread Transition of Externally Heated Electrical Wire
[4] O. Fujita et al., Proc. Combust. Inst., 33 (2011), pp. 2617β2623
Ignition delay is observed that ignition occurs several seconds after the end of heating, which is also observed in the overloading ignition (microgravity) experiments [4].
In high oxygen concentrations, this heat-transfer based ignition model is no longer appropriate.
Both mixing and chemical kinetics in gas phase should be considered.
August 21, 2014 22
Outline
β’ Motivation
β’ Simplified Ignition-to-Spread Model
β’ Experimental Setup
β’ Experimental Results and Discussion
β’ Conclusions and Future Work
22August 3, 2012 β Ignition-to-Spread Transition of Externally Heated Electrical Wire
August 21, 2014 23
Conclusions
23
β’ A simplified ignition-to-spread model for thin electrical wires is developed, which qualitatively agrees with experimental results.
β’ For a higher-conductance wire, a longer heating duration is required to achieve both flashpoint and spread point, and the weak flame is easier to be quenched during the transition.
β’ Ignition becomes difficult in reduced pressures because the heating source becomes weak.
β’ In high oxygen concentration, flashpoint converges to the spread point and ignition delay occurs, which cannot be included in this heat-transfer based ignition model.
August 3, 2012 β Ignition-to-Spread Transition of Externally Heated Electrical Wire
August 21, 2014 24
Future Work
24
Future work can be focused on
1) looking for the critical/minimum coating thickness for ignition/flame-spread;
2) Quantify the effective conductivity (ππππ) of wire by
including the conductance of wire core to calculate both ignition time and spread-rate by classical theories;
3) For some coating materials, a different transition to smoldering ignition;
4) evaluating the applicability of the theory to other more widely used wires.
August 3, 2012 β Ignition-to-Spread Transition of Externally Heated Electrical Wire
August 21, 2014 25
Acknowledgements
25
Hokkaido University for offering this internship opportunity
Gao Jian, Junya Iwakami and Yangkyun Kim (Hokkaido Univ.) for their help to my experiments
Prof. Michael Gollner (Maryland), Prof. Kal Seshadri, and Ulrich Nieman (UCSD) for valuable discussions
Financial support for this research provided by JSPS (Grants-in-aid for Young Scientists: #21681022) and the Japan Nuclear Energy Safety Organization (JNES).
August 3, 2012 β Ignition-to-Spread Transition of Externally Heated Electrical Wire
August 21, 2014 26
QUESTIONS?
Thanks for your attention!
Presented by Xinyan Huang
University of California, San Diego