Estimation of Risk Factors of Human Ignition of Fires in ...
Ignition-to-Spread Transition of Electrical Wire fires
-
Upload
xinyan-huang -
Category
Documents
-
view
11 -
download
1
description
Transcript of Ignition-to-Spread Transition of Electrical Wire fires
![Page 1: Ignition-to-Spread Transition of Electrical Wire fires](https://reader034.fdocuments.in/reader034/viewer/2022042721/577cc1281a28aba711926a42/html5/thumbnails/1.jpg)
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 โ 34th international symposium on Combustion, Warsaw, Poland
![Page 2: Ignition-to-Spread Transition of Electrical Wire fires](https://reader034.fdocuments.in/reader034/viewer/2022042721/577cc1281a28aba711926a42/html5/thumbnails/2.jpg)
August 21, 2014 2
Outline
โข Motivation
โข Simplified Ignition-to-Spread Model
โข Experimental Setup
โข Experimental Results and Discussion
โข Conclusions and Future Work
2August 3, 2012 โ Ignition-to-Spread Transition of Externally Heated Electrical Wire
![Page 3: Ignition-to-Spread Transition of Electrical Wire fires](https://reader034.fdocuments.in/reader034/viewer/2022042721/577cc1281a28aba711926a42/html5/thumbnails/3.jpg)
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 โ Ignition-to-Spread Transition of Externally Heated Electrical Wire
[1] Y. Nakamura et al., Proc. Combust. Inst., 32 (2009), pp. 2559โ2566
![Page 4: Ignition-to-Spread Transition of Electrical Wire fires](https://reader034.fdocuments.in/reader034/viewer/2022042721/577cc1281a28aba711926a42/html5/thumbnails/4.jpg)
August 21, 2014 4
Outline
โข Motivation
โข Simplified Ignition-to-Spread Model
โข Experimental Setup
โข Experimental Results and Discussion
โข Conclusions and Future Work
4August 3, 2012 โ Ignition-to-Spread Transition of Externally Heated Electrical Wire
![Page 5: Ignition-to-Spread Transition of Electrical Wire fires](https://reader034.fdocuments.in/reader034/viewer/2022042721/577cc1281a28aba711926a42/html5/thumbnails/5.jpg)
August 21, 2014 5
Ignition Model at Flashpoint
5
Assumptions:
Uniform heat flux within ๐ฟ
Thermally thin: ฮด ~ 0.2 mm โช ๐ผ๐ก๐๐
Uniform cross-sectional temperature: ๐ต๐๐ โช ๐ต๐๐ < 0.3
๐๐ โช ๐๐ and ๐๐๐ ๐ โช ๐๐๐ ๐
Governing Equations
โ๐๐๐ด๐๐
๐๐ก= ๐ด๐๐๐
๐2๐
๐๐ฅ2+ ๐๐ ๐๐
โฒโฒ โ ๐๐๐๐ ๐ โฒโฒ 0 < ๐ฅ <
๐ฟ
2
โ๐๐๐ด๐๐
๐๐ก= ๐ด๐๐๐
๐2๐
๐๐ฅ2โ ๐๐โ ๐ โ ๐๐ ๐ฅ >
๐ฟ
2
August 3, 2012 โ Ignition-to-Spread Transition of Externally Heated Electrical Wire
where ๐๐ฟ/2โ = ๐๐ฟ/2+ ,๐๐
๐๐ฅ๐ฟ/2โ
=๐๐
๐๐ฅ๐ฟ/2+
,
๐๐
๐๐ฅ0
= 0, d ๐โ = ๐๐ , aafor ๐ก > 0.
Flashpoint: pyrolysis vapors achieve a
fuelโs lower flammability limit, defined
by a critical mass flux or temperature
(at ๐๐๐๐ฅ = ๐๐๐ โฅ ๐๐).
![Page 6: Ignition-to-Spread Transition of Electrical Wire fires](https://reader034.fdocuments.in/reader034/viewer/2022042721/577cc1281a28aba711926a42/html5/thumbnails/6.jpg)
August 21, 2014 6
Ignition Model at Flashpoint
6
Ignition time
๐ก๐๐ โ๐ ๐, ๐๐ , ๐ฟ, ๐ฟ
๐๐โฒโฒ๐
where ๐ denotes ignition properties.
๐ โ 1 (thermally thin)
๐ โ 2 (thermally thick)
Critical/minimum ignition heat flux:
๐๐ ๐๐ก = 0, ๐0~๐ฟ/2 = ๐๐๐
โ ๐๐,๐๐๐กโฒโฒ = ๐๐๐๐ ๐
โฒโฒ +๐๐๐ฟ
๐๐๐ โ ๐๐ โ๐๐๐๐
Increase with the conductance
(conductivity and core diameter)
Decrease with heating length (๐ฟ)
August 3, 2012 โ Ignition-to-Spread Transition of Externally Heated Electrical Wire
Governing Equations
โ๐๐๐ด๐๐
๐๐ก= ๐ด๐๐๐
๐2๐
๐๐ฅ2+ ๐๐ ๐๐
โฒโฒ โ ๐๐๐๐ ๐ โฒโฒ 0 < ๐ฅ <
๐ฟ
2
โ๐๐๐ด๐๐
๐๐ก= ๐ด๐๐๐
๐2๐
๐๐ฅ2โ ๐๐โ ๐ โ ๐๐ ๐ฅ >
๐ฟ
2
๐๐ฟ/2โ = ๐๐ฟ/2+ , ๐T/๐x ๐ฟ/2โ = ๐T/๐x ๐ฟ/2+ ,
๐T/๐x 0 = 0, d ๐โ = ๐๐ , aafor ๐ก > 0.
![Page 7: Ignition-to-Spread Transition of Electrical Wire fires](https://reader034.fdocuments.in/reader034/viewer/2022042721/577cc1281a28aba711926a42/html5/thumbnails/7.jpg)
August 21, 2014 7
To calculate the temperature profile, the heat-transfer equation during spread is
๐ด๐๐๐๐2๐
๐๐ฅ2+ ๐๐๐ด ๐๐๐
๐๐
๐๐ฅ= ๐๐
โฒ
๐๐ โฒ : heat transfer in radial direction
Spread Point
7
Flame spread rate (๐๐) and flame width
(๐๐) 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 โ Ignition-to-Spread Transition of Externally Heated Electrical Wire
Four Regions:
I. Unburned wire, ๐๐ ,1โฒ = ๐๐โ1 ๐ โ ๐๐
II. Boiling polymer within the flame
๐๐ ,2โฒ = ๐๐ ๐๐
โฒโฒ = ๐๐โ๐ ๐ โ ๐๐๐
III. The wire core exposed to the flame
๐๐ ,3โฒ = ๐๐ ๐๐
โฒโฒ โ ๐ ๐4 โ ๐๐4 โ โ๐ ๐ โ ๐๐
IV. The wire core exposed to atmosphere
๐๐ ,4โฒ = ๐๐โ4 ๐ โ ๐๐
![Page 8: Ignition-to-Spread Transition of Electrical Wire fires](https://reader034.fdocuments.in/reader034/viewer/2022042721/577cc1281a28aba711926a42/html5/thumbnails/8.jpg)
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 โ Ignition-to-Spread Transition of Externally Heated Electrical Wire
[2] J.L. Torero et al., Combust. Sci. Tech., 174 (2002), pp. 187โ203[3] Y. Nakamura et al., J. Therm. Sci. Tech., 3 (2008), pp. 430โ441
๐ป๐๐๐ ?โ temperature profile
Burning rate [2]
๐โฒโฒ =๐ฟ๐๐
๐๐๐๐ โ
๐๐ข โ ๐๐
๐๐๐๐ln 1 + ๐ต ,
where ๐ต =๐๐2,โ โ๐ป๐/๐ + ๐๐ ๐๐ โ ๐๐๐
๐ฟ๐ฃ โ ๐๐โฒโฒ/ ๐โฒโฒ
= 4 ~ 5
Additional heating source from wire core
๐๐โฒโฒ = โ๐ ๐2 โ ๐๐๐ , โ๐~
๐
๐ฟ
๐2 = ๐๐๐๐ฅ + ๐๐๐ /2, (linear)
Measuring ๐ฝ๐,๐๐ & ๐พ๐,๐๐ โ ๐ป๐ โ ๐ป๐๐๐
![Page 9: Ignition-to-Spread Transition of Electrical Wire fires](https://reader034.fdocuments.in/reader034/viewer/2022042721/577cc1281a28aba711926a42/html5/thumbnails/9.jpg)
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
๐ฅ๐ = ๐ป๐ ๐ โ ๐ป๐๐ โ ๐ฅ๐ ๐๐ฅ๐ป๐ ; ๐ฅ๐ก =๐ฅ๐
๐๐โฒโฒ๐๐๐ฟ/2
Enthalpy at spread point: ๐ป๐ ๐ = 0โโ๐๐๐ด ๐ ๐๐ฅ (blue line)
Enthalpy at flashpoint: ๐ป๐๐ = 0โโ๐๐๐ด ๐ ๐๐ฅ (red line)
Heat from flame: ๐ฅ๐ ๐๐ฅ๐ป๐
August 3, 2012 โ Ignition-to-Spread Transition of Externally Heated Electrical Wire
o For high-conductivity wire or short heating length, ๐ป๐ ๐ โ ๐ป๐๐ is large and a
long additional heating time is expected;
o Thick coating may produce more flame heating and reduce the additional heat.
![Page 10: Ignition-to-Spread Transition of Electrical Wire fires](https://reader034.fdocuments.in/reader034/viewer/2022042721/577cc1281a28aba711926a42/html5/thumbnails/10.jpg)
August 21, 2014 10
Outline
โข Motivation
โข Simplified Ignition-to-Spread Model
โข Experimental Setup
โข Experimental Results and Discussion
โข Conclusions and Future Work
10August 3, 2012 โ Ignition-to-Spread Transition of Externally Heated Electrical Wire
![Page 11: Ignition-to-Spread Transition of Electrical Wire fires](https://reader034.fdocuments.in/reader034/viewer/2022042721/577cc1281a28aba711926a42/html5/thumbnails/11.jpg)
August 21, 2014 11
Ignition Experiment
Chamber size: 365 mm (L) ร 260 mm (W) ร 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 ยฑ0.01 A) and compact PLC (accurate to ยฑ0.1 s);
Sony HDR-XR 500V video camera (30 fps);
Two pipe line: (1) from air/oxygen tank; (2) to vacuum pump.
11August 3, 2012 โ Ignition-to-Spread Transition of Externally Heated Electrical Wire
![Page 12: Ignition-to-Spread Transition of Electrical Wire fires](https://reader034.fdocuments.in/reader034/viewer/2022042721/577cc1281a28aba711926a42/html5/thumbnails/12.jpg)
August 21, 2014 12
Sample Wires & Test Conditions
12
Wire core: nichrome (NiCr) and copper (Cu) wires
(๐๐)๐๐๐ถ๐
โ (๐๐)๐ถ๐ข
โ (๐๐)๐๐ธ
, but ๐๐๐๐ถ๐ โถ ๐ ๐ถ๐ข โ 1 โถ 25
Polyethylene coating: ๐ฟ โค 0.3 mm (thermally thin), ๐๐ โช ๐๐
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 โ Ignition-to-Spread Transition of Externally Heated Electrical Wire
* ๐๐ โ 0.2 ๐/๐๐พ, ๐๐๐๐ถ๐ โ 16๐/๐๐พ, ๐๐ถ๐ข โ 400๐/๐๐พ
* >3500 runs in this study
![Page 13: Ignition-to-Spread Transition of Electrical Wire fires](https://reader034.fdocuments.in/reader034/viewer/2022042721/577cc1281a28aba711926a42/html5/thumbnails/13.jpg)
August 21, 2014 13
Outline
โข Motivation
โข Simplified Ignition-to-Spread Model
โข Experimental Setup
โข Experimental Results and Discussion
โข Conclusions and Future Work
13August 3, 2012 โ Ignition-to-Spread Transition of Externally Heated Electrical Wire
![Page 14: Ignition-to-Spread Transition of Electrical Wire fires](https://reader034.fdocuments.in/reader034/viewer/2022042721/577cc1281a28aba711926a42/html5/thumbnails/14.jpg)
August 21, 2014 14
Typical Ignition Process at Normal Atmosphere
14
NiCr-A wire: do = 1.0 mm, dc = 0.7 mm, ๐ฟ = 0.15 mm,
๐ผ = 10 ๐ด, coil heater (2 cm), ๐๐ = 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 โ Ignition-to-Spread Transition of Externally Heated Electrical Wire
As heating duration increases:
Blue premix flame (flashpoint) โ yellow diffusion flame (fire point) โ flame separate and spread to the edge of coil heater
โ spread out (spread point)
![Page 15: Ignition-to-Spread Transition of Electrical Wire fires](https://reader034.fdocuments.in/reader034/viewer/2022042721/577cc1281a28aba711926a42/html5/thumbnails/15.jpg)
August 21, 2014 15
Comparison at Flashpoint
15
I. At flashpoint, experimental results qualitatively agree with modeling results.
II. Index ๐ 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
๐ก๐๐ โ๐ ๐, ๐๐ , ๐ฟ, ๐ฟ
๐๐โฒโฒ๐
August 3, 2012 โ 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 โ
![Page 16: Ignition-to-Spread Transition of Electrical Wire fires](https://reader034.fdocuments.in/reader034/viewer/2022042721/577cc1281a28aba711926a42/html5/thumbnails/16.jpg)
August 21, 2014 16
Transition and Spread point
16
Estimating the experimental heat flux from previous
comparison by connecting ๐ผ, ๐ก๐๐ to ๐๐โฒโฒ, ๐ก๐๐ .
Assuming heating efficiency from the flame: ๐ = 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 โ Ignition-to-Spread Transition of Externally Heated Electrical Wire
I. Model qualitatively predicts the experimental trend, โ๐ก increases with increasing thermal conductance (Cu-A>Cu-B>NiCr-A>NiCr-B).
II. โt decreases as heat flux decrease because a longer heating duration increases ๐ป๐๐
III. In high-heat-flux experiment, the required heating duration is underestimated because the high-temperature coil continues to heat after power off.
Conductance โ
![Page 17: Ignition-to-Spread Transition of Electrical Wire fires](https://reader034.fdocuments.in/reader034/viewer/2022042721/577cc1281a28aba711926a42/html5/thumbnails/17.jpg)
August 21, 2014 17
Heating-Length Effect
17
I. Increasing the heating length increases the ๐ป๐๐ and ๐ฅ๐ ๐๐ฅ๐ป๐ , 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 โ Ignition-to-Spread Transition of Externally Heated Electrical Wire
๐ฅ๐ = ๐ป๐ ๐ โ ๐ป๐๐ โ ๐ฅ๐ ๐๐ฅ๐ป๐ ;
โ ๐ฅ๐ก =๐ฅ๐
๐๐โฒโฒ๐๐๐ฟ/2
๐๐,๐๐๐กโฒโฒ = ๐๐๐๐ ๐
โฒโฒ +๐๐๐ฟ
๐๐๐ โ ๐๐ โ๐๐๐๐
(Additional heat time)
Heating length โ
![Page 18: Ignition-to-Spread Transition of Electrical Wire fires](https://reader034.fdocuments.in/reader034/viewer/2022042721/577cc1281a28aba711926a42/html5/thumbnails/18.jpg)
August 21, 2014 18
Ignition and Spread at Reduced Pressures
18
๐ท๐ = ๐๐ kPa
Cu-B wire: ๐ผ = 11 ๐ด, heating time = 9.0 sec,
coil heater (2 cm)
๐ท๐ = ๐๐ kPa
Cu-B wire: ๐ผ = 11 ๐ด, heating time = 10.0 sec,
coil heater (2 cm), no steady-state spread
August 3, 2012 โ Ignition-to-Spread Transition of Externally Heated Electrical Wire
Blue, weak, and spherical flame during ignition
Flame spread faster as pressure decreases
![Page 19: Ignition-to-Spread Transition of Electrical Wire fires](https://reader034.fdocuments.in/reader034/viewer/2022042721/577cc1281a28aba711926a42/html5/thumbnails/19.jpg)
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 (โ๐), 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 โ Ignition-to-Spread Transition of Externally Heated Electrical Wire
![Page 20: Ignition-to-Spread Transition of Electrical Wire fires](https://reader034.fdocuments.in/reader034/viewer/2022042721/577cc1281a28aba711926a42/html5/thumbnails/20.jpg)
August 21, 2014 20
Oxygen-Concentration Effect
20
Heating time decreases as oxygen concentration increases (๐๐2 < 40%),
even under the low pressure environment, because a higher flaming temperature increases the heating efficiency (๐).
๐๐2 > 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 โ Ignition-to-Spread Transition of Externally Heated Electrical Wire
![Page 21: Ignition-to-Spread Transition of Electrical Wire fires](https://reader034.fdocuments.in/reader034/viewer/2022042721/577cc1281a28aba711926a42/html5/thumbnails/21.jpg)
August 21, 2014 21
Ignition Delay at High Oxygen Concentrations
21
Cu-B wire
o do = 0.8 mm, dc = 0.5 mm, ๐ฟ = 0.15 mm,
o ๐ฟ๐ถ๐ = ๐๐%, 1 atm
o coil heater (2 cm), ๐ผ = 11 ๐ด
Heating time = 4.1 sec
Flashpoint = 10.4 sec (~ 6 sec delay)
August 3, 2012 โ Ignition-to-Spread Transition of Externally Heated Electrical Wire
[4] O. Fujita et al., Proc. Combust. Inst., 33 (2011), pp. 2617โ2623
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.
![Page 22: Ignition-to-Spread Transition of Electrical Wire fires](https://reader034.fdocuments.in/reader034/viewer/2022042721/577cc1281a28aba711926a42/html5/thumbnails/22.jpg)
August 21, 2014 22
Outline
โข Motivation
โข Simplified Ignition-to-Spread Model
โข Experimental Setup
โข Experimental Results and Discussion
โข Conclusions and Future Work
22August 3, 2012 โ Ignition-to-Spread Transition of Externally Heated Electrical Wire
![Page 23: Ignition-to-Spread Transition of Electrical Wire fires](https://reader034.fdocuments.in/reader034/viewer/2022042721/577cc1281a28aba711926a42/html5/thumbnails/23.jpg)
August 21, 2014 23
Conclusions
23
โข A simplified ignition-to-spread model for thin electrical wires is developed, which qualitatively agrees with experimental results.
โข 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.
โข Ignition becomes difficult in reduced pressures because the heating source becomes weak.
โข 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 โ Ignition-to-Spread Transition of Externally Heated Electrical Wire
![Page 24: Ignition-to-Spread Transition of Electrical Wire fires](https://reader034.fdocuments.in/reader034/viewer/2022042721/577cc1281a28aba711926a42/html5/thumbnails/24.jpg)
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 (๐๐๐๐) 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 โ Ignition-to-Spread Transition of Externally Heated Electrical Wire
![Page 25: Ignition-to-Spread Transition of Electrical Wire fires](https://reader034.fdocuments.in/reader034/viewer/2022042721/577cc1281a28aba711926a42/html5/thumbnails/25.jpg)
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 โ Ignition-to-Spread Transition of Externally Heated Electrical Wire
![Page 26: Ignition-to-Spread Transition of Electrical Wire fires](https://reader034.fdocuments.in/reader034/viewer/2022042721/577cc1281a28aba711926a42/html5/thumbnails/26.jpg)
August 21, 2014 26
QUESTIONS?
Thanks for your attention!
Presented by Xinyan Huang
University of California, San Diego