3. Introduction
Aviation, Automobile, Industrial machinery fires
may be majorly attributed to leakage of
flammable fluids onto hot surfaces, causing it to
ignite
Most of the designs use the minimum Auto
Ignition Temperature (AIT) provided by ASTM to
prevent such hot surface ignition accidents
However it is experimentally found that the
actual Minimum Hot Surface Ignition
temperature of fuels is much greater than the
Minimum AIT (Reasons discussed later)
More data is required to better understand the
flow mechanism governing this type of ignition
to prevent such fires
4. Background
Auto Ignition Temperature (AIT): A fluid's AIT is that temperature at which its
vapors will ignite in air at atmospheric pressure without an external source
of ignition. [1][2][3][4][9]
Thermal Ignition: Thermal Ignition of a flammable mixture of fuel and air
occurs when the heat release from chemical reactions exceeds the heat
loss such that the mixture becomes self-heating [1]. In other words it is the
process of flame initiation [9]
Flames, in most cases, are defined as highly exothermic reactions between
fuel vapors (or gases) and an oxidant (in this case, oxygen) resulting in
both the rapid generation of combustion products at relatively high
temperatures (generally above about 11000C) and the emission of light. [9]
5. Background
Hot Surface Ignition Temperature (HSIT): It is the lowest temperature at
which a specified hot surface will ignite a droplet of fuel that falls upon it. It
is an auto ignition related phenomena [7]
HSIT and AIT are not always the same. In fact as it turns out from
experiments, HSIT is greater than AIT (in some cases more than 1000C
greater)
HSIT is greater than AIT because of the uncontrolled loss of vapor and heat
after the droplet hits the surface [7]
Because reaction rates are strongly temperature dependent, once self
heating occurs the reaction rate rapidly increases and a flame front
propagates away from the point of ignition [1]
6. Background
The flash point and the minimum auto ignition temperature are well
defined properties that can be measured with ASTM standards
Unlike them, the temperature at which hot surface ignition occurs is not a
fundamental fluid property and is strongly coupled with numerous factors
[1]
Because of this coupling the temperature required for ignition can vary
widely
7. Background
Hot surface ignition is further complicated because liquid fuel is often
dispensed directly onto the hot surface, which makes the flow non uniform
and turbulent in most cases
Since a portion of the fuel must be evaporated and come into contact
with the hot surface such that sufficient heat transfer can occur, ignition is
directly coupled with these complex phenomenon
8. Mechanism of Ignition
To understand the mechanism of ignition due to a hot surface, we can
divide the study into two types of different modes which work together to
lead to an ignition,
i.e.:
Boiling modes
Ignition modes
9. Boiling Modes
From the tests performed in by Bennett and Ballal (2003), three different
boiling modes are observed for a combustible fuel over a hot surface:
Nucleate Boiling
Transition Boiling
Film Boiling
10. Nucleate Boiling
Features observed during Nucleate boiling:
Intensive bubbling of the liquid pool on the plate surface
Slow expansion due to no vapor barrier to reduce friction
Heavy vapor formation
Frequently loud sizzling sounds to accompany the rapid vaporization
This mode occasionally features a rapid explosion (disintegration) of pools and
globules that go in all directions
Such intense activities are observed at the highest temperatures and is defined
by some as the “maximum evaporation mode”
11. Transition Boiling
Observed at plate temperatures above nucleate boiling
A hybrid scenario which exhibits quicker liquid pool movement due to the
formation of some vapor underneath
Also an establishment of curvature of the pool leading edge due to
surface tension is observed
This mode has reduced nucleate bubbles in the leading edge perimeter,
pool center, or dispersed when compared to nucleate boiling
12. Film Boiling
This type of boiling is observed at temperatures above transition boiling
This mode exhibits complete lifting of the pool off the heated plate surface
Which facilitates in rapid transfer of liquid to the plate edge and little to no
nucleate bubbles
Also this mode often exhibits breakup of the pool into smaller globules due
to surface tension of the liquid
13. Boiling modes vs. plate temperature
Here is shown a comparison
of the boiling modes vs. plate
temperature for three fuels,
viz. Hexadecane, heptane
and kerosene
[5]
14. Ignition Modes
The ignition modes are classified into three types by Bennett and Ballal
(2003)
They are;
Hood Fires (unique to the constraints of fume hood apparatus detailed
ref [5] )
Gutter Fires
Airborne Fires
15. Hood Fires
This mode is commonly observed at the lowest plate temperatures
In this mode, the fuel does not ignite until it reaches the plate perimeter
However, ingestion of fresh air into the vapor (e.g. By a fan, etc.) initiates a
fire event
Considered as “oxygen-poor” condition
Near the plate surface the fuel and air is hot enough to ignite but does not
entrain enough oxygen
At higher elevation above the plate, lower temperatures prevent ignition
Oxygen is entrained when a fan, etc. is used and thus ignition occurs
16. Gutter Fires
In these types of fires, the fuel does not ignite at the highest plate
temperatures until it is collected in the gutter (a relief zone provided for the
fuel to accumulate after being spread on the hot plate) after spilling over
May be considered a “temperature poor” condition
Occurs due to extended period of time during which the fuel is collected
in the gutter and due to a greater contact area which facilitate ignition
17. Airborne Fires
The kernel located at a substantial distance above the plate is ignited in
this type
This kernel quickly spreads downward to engulf the entire region with fire
This type of fire is not apparatus dependent and is influenced by external
environment
Five fundamental parameters are related to the ignition phenomena:
fuel vapor concentration profile, temperature profile, convection
velocities, AIT and ignition delay
18. Airborne fires
The figure below shows a time-lapse of video frames of one such airborne ignition;
[5]
An additional special category of airborne fires is observed for some fuels
like heptane and dodecane, in which instantaneous ignition occurred
upon contact with the heated plate
This mode was only observed at the highest plate temperatures in only a
few experiments
19. 1-D Model for Airborne ignition event
As fuel begins to spread across the plate,
vapor concentration profile exhibits a steep
initial concentration gradient in region of
little upward diffusion of fuel vapor in air
Continued vapor diffusion, buoyancy and
mixing with surrounding air spread the vapor
concentration limits for ignition over wide
elevation range
This dense heated mixture also raises the bulk
vapor/air mixture temperature above the
plate further supporting favorable ignition
conditions
[5]
20. Experimental Techniques
ASTM E659:
In this method, approximately 100µl of
fuel is inserted in to a uniformly
preheated 500ml glass flask at
containing air at a predetermined
temperature as shown in figure 1.
As the liquid enters, it evaporates and
mixes with the surrounding air.
[1]
21. Experimental Techniques
This mixture is then observed for 10 minutes or until auto ignition occurs.
Temperature at which Auto ignition can occur is dependent on fuel
mole fraction
Hence a series of test must be performed in which volume of the fuel
injected is varied until minimum auto ignition temperature is observed
22. Experimental Techniques
This is at a total pressure of 1atm
As evident, the forced ignition regime is
bounded by the saturated liquid curve and
upper and lower flammability limits
[1]
To the left is the relationship between
temperature and fuel vapor pressure for both
forced and auto ignition regimes
Along the saturated curve are saturated fuel
and air mixtures, whereas mixtures to the right
of this line are unsaturated
23. Experimental Techniques
Also shown in figure 2 are flash point and minimum auto ignition
temperature as measured by ASTM tests
The upper and lower flammability limits are based on an ideal ignition
device such as a pilot flame or a high-energy electric spark
If a less ideal source is used (low-energy electric spark) the forced ignition
regime contracts relative to the flammability limits
[1]
Similarly the true boundary between the forced and auto ignition regions is
also based on an ideal ignition device
This boundary also contracts and shifts towards higher temperatures when
less ideal ignition device, such as an exposed hot surface, is used
24. Experimental Techniques
Note: Since non ideal ignition devices generally involve a temperature
gradient from the hot surface responsible for ignition, the temperature
shown in figure 2 is the surface temperature
The device is essentially isothermal, since the surface of the glass flask and
the mixture of fuel and air are at approximately the same temperature
Conditions which affect the Minimum Hot surface ignition temperature
(MHSIT) and the shift in auto ignition boundary include heat loss, exposure
time and the interaction between liquid fuels and the hot surface
27. Experimental Techniques
Experimental setup 2:
This experimental setup is from ref[1] in
which the results are turn out to be
interesting
The test surface consisted of an electrically
heated stainless steel plate surrounded on
three sides by draft shields as shown in the
figure
Insulation is provided to produce a
relatively uniform temperature profile with
the minimum temperature at the center
[1]
28. Experimental Techniques
Due to thermal stresses developed within
the plate a slightly concave shape was
developed when heated
This kept the liquid drops boiling on the
surface near the center
[1]
29. Probabilistic nature of Ignition
The ignition data shows an
interesting trend that the
ignition is probabilistic in nature
The curve on the right is for
gasoline
Here, 1 is assigned to the test
which resulted in ignition and 0
to the one’s which did not
From logistic regression curve fit
to the data, we have;
[1]
30. Probabilistic nature of Ignition
Here is a table for the
regression coefficients that
are used in the formula to
calculate the ignition
probability
[1]
31. Probabilistic nature of Ignition
Here instead of a sharp
demarcation point for a well
defined ignition
temperature, we have a
broad range over which
ignition becomes more likely
Data sensitive to test
conditions, including liquid
injection location, air
velocity, fluid injection
methods (sprays/ streams)
with the mass flow rate for
each case
[1]
32. Probabilistic nature of Ignition
The data is collected from
different references is shown
Here the present study is ref [1]
described earlier
This is a range of temperature
from the lowest temperature
which resulted in ignition to
the highest temperature which
did not produce ignition
Criteria for lowest ignition
temperature is that the air
velocity is less than or equal to
the velocity that produced
lowest ignition temperature
33. Factors affecting Hot surface ignition
Heat Loss: Heat loss from a reaction zone near the hot surface is generally
greater than in the ASTM E659 test [1]
Because the hot surfaces are often exposed, forced airflow or even
natural convection moves the flammable vapor over the surface and
limits the time it is exposed to a high temperature (exposure time) [1]
This reduction in exposure time increases the temperature necessary to
achieve ignition
Also air flow increases the heat loss, hence raising the MHSIT [1]
34. Factors affecting Hot surface ignition
Fuel Flow Rates: Lower fuel flow rates results in greater ignition delays, while
as the fuel flow rates is increased the ignition delay is significantly
shortened [5]
Volatility of fuels:
Higher the volatility of the fuel the more difficult it is to ignite under the same test
conditions. [6]
Because the combustible vapor/air mixtures in high volatility fuels is well away
from the thermal source and hence difficult to ignite
Hence from a design safety point of view the more volatile the fuel the less
chances of it being ignited by a hot surface
35. Factors affecting Hot surface ignition
Convection Velocities:
The evaporated fuel rises above the hot plate in a plume. This plum entrains the
surrounding air and forms a combustible mixture.
The convection velocity of this mixture arises due to combined effects of
pressure gradients and buoyancy.
Higher fuel flow rates produces higher convection velocities (which are
however significantly lower than laminar burning velocities). [5]
Hence the flame kernel propagates (flashback) upstream to the plate surface
and consumes all of the reactants [5]
36. Factors affecting Hot surface ignition
Air Speed: Air speeds govern the mixing of the fuel with air and greater air
speeds result in greater heat loss, hence higher temperature for hot
surface ignition
Anti Misting Additives: Addition of anti-misting additives to a fuel reduces
the minimum temperature at which it ignites on a hot surface
This can also be used to compare two anti-misting additives, by comparing the
drop in the MHSIT of the same fuel using two additives
Surface Material: Different materials affect the behavior of fuels to ignite
on the hot surface
37. Factors affecting Hot surface ignition
Below is a comparison for Minimum surface ignition temperatures vs. velocity for two surface
materials (viz. Stainless steel and Titanium)
[6]
[6]
38. Factors affecting Hot surface ignition
Ambient Pressure: As the total ambient pressure increases the MHSIT
decreases [10]
A table provided by Bennett and Ballal (2003) compares the affect of
various physical properties and factors affecting the evaporation lifetime,
ignition delay and surface ignition temperature
This list also includes the observed and predicted behaviors
40. Computational Results
Two simulations were performed using ANSYS
Workbench 14.0. First for the 2-D case and then for the
3-D case
The test conditions were same for both:
Circular Hot plate of 12 inch diameter, with a temperature
of 2000C
Free stream velocity of 0.1m/s and ambient temperature
of 200C
Hence the mesh for the simulation was created after
calculating the height of the thermal and
hydrodynamic boundary layer and creating a cell size
less than the smallest (in this case hydrodynamic)
boundary layer in order to capture its effects
Fig. Mesh
42. Computational Results
For the three dimensional case, the test conditions were kept the same, except that the flow
now comes in from all sides for a circular hot plate with a cylindrical domain
43. Computational Results
For both 2-D and 3-D, the temperature variation with the vertical direction was with a sudden
drop in the region close to the hot plate followed by a gradual decrease as shown below;
2-D
3-D
44. Computational Results
The temperature profile obtained from the
simulation is in good agreement with that obtained
in other literature [12][13]
Within the initial region of rapid decrease in
temperature the flow is developing along the plate
surface in a radial direction towards the center of
the plate, whereupon the flow collides and forms
the turbulent plume indicated by the image below
[12]
[12]
[12]
45. Summary
Because of the dependence of the MHSIT on various physical, chemical and
experimental factors, Hot Surface data cannot be easily extrapolated to
different conditions and general rules of thumb based on the minimum auto
ignition temperature can be very inaccurate
Ignition of a fuel on a hot surface is highly probabilistic in nature, hence should
be handled statistically using linear regression
Three boiling modes (Nucleate, Transition and Film) were observed for a liquid
pool over a hot surface
Three ignition modes (Hood, Gutter and Airborne) were observed
With simple air flows over a heated surface, a more volatile fluid applied to the
surface was more difficult to ignite (it required a higher local surface
temperature) than a much less volatile combustible liquid
46. Summary
Local air velocity is a very important factor in hot surface ignition. With a very
high local air velocity hot surface ignition is almost impossible
Additives to improve a feature of the fuel (anti-mist, etc.) must be carefully
investigated because of their potential to deteriorate the ability of a fuel to
resist ignition on a heated surface
From the computational data, two distinct regions near the hot surface are
obtained, first closer to the surface where the temperature drop is rapid and
second farther away where the temperature drop is gradual
One of the potential remedy to prevent hot surface fires is to use a pattern of
micro-cavities, sized to prevent fluid seepage, on the exterior of the heated
surface. This configuration is expected to reduce the heat transfer from surface
to liquid due to reduced direct contact, inhibit the formation of superheated
vapor films, and mitigate ignition.
47. References
1.
J. D. Colwell and A. Reza, “Hot surface ignition of automotive and aviation fluids,” Fire Technology, vol. 41, pp. 105–123,
2005.
2.
S. Davis, S. Kelly, and V. Somandepalli, “Hot surface ignition of performance fuels,” Fire Technology, vol. 46, pp. 363–374,
2010.
3.
Johnson, A.M. and Moussa, N.A., “Hot Surface Ignition Tests of Aircraft Fluids”, Final Report for period May 1987 to May
1988, Aero Propulsion Laboratory, Air Force Wright Aeronautical Laboratory, Wright-Patterson Air Force Base, Ohio,
November 1988, AFWAL-TR- 88-2101.
4.
J.M. Bennett, “Ignition of Combustible Fluids by Heated Surfaces,” Process Safety Progress, vol. 20, no. 1, 2001, pp. 29–36
5.
J.M. Bennett and D.R. Ballal, “Ignition of Combustible Fluids by Heated Surfaces,”AIAAPaper 2003-18, 2003.
6.
D.J. Myronuk, “Dynamic, Hot Surface Ignition of Aircraft Fuels and Hydraulic Fluids,” Report No. AFAPL-TR-79-2095, WrightPatterson Air Force Base, OH, 1980
7.
A. Strasser, N.C. Waters, and J.M. Kuchta, “Ignition of Aircraft Fluids by Hot Surfaces Under Dynamic Conditions,” Bureau
of Mines PMSRC Report No. 4162, Report No. AFAPL-TR-71- 86, Wright-Patterson Air Force Base, OH, 1971.
48. References
8.
D. Drysdale, An Introduction to Fire Dynamics, 2nd ed., Chichester: John Wiley & Sons, 2002
9.
Wilbur A. Affens et al, “Ignition studies part VII. The determination of auto ignition temperatures of hydrocarbon fuels”,
Naval Research Laboratory, Washington D. C., 1974
10.
Technical Report, “Summary of Auto Ignition properties of jet fuels and other aircraft combustible materials”, U.S. Bureau of
Mines, Pittsburg mining and safety research center, 1975
11.
“Auto Ignition temperatures for combustible fuels”, Wikipedia.org
12.
Toy N, Nenmeni V R, Bai X, Disimile P J, “Surface Ignition on a heated horizontal flat plate”, 4th international aircraft fire and
cabin safety research conference
13.
S.K. Menon, P.A. Boettcher, B. Ventura, J.E. Shepherd, G. Blanquart, “Modeling Hot surface ignition of hydrocarbon air
mixtures”, 7th US National Technical Meeting of the Combustion Institute, Georgia Institute of Technology, Atlanta, GA
March 20-23, 2011
14.
Park H., “HOT SURFACE IGNITION TEMPERATURE OF DUST LAYERS WITH AND WITHOUT COMBUSTIBLE ADDITIVES”, Masters
thesis, Worcester Polytechnic Institute, 2006
