Hot surface ignition of combustible fuels

Hot Surface Ignition of

Combustible Fuels BY: DHAVAL SHIYANI

AVIATION FIRE DYNAMICS

Outline

Introduction

Background

Mechanism of Ignition

Experimental Techniques

Factors affecting Hot Surface Ignition

Computational Results

Summary

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

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]

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]

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

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

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

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

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”

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

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

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]

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

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

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

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

Airborne fires

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

The figure below shows a time-lapse of video frames of one such airborne ignition;

[5]

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]


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]


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


To the left is the relationship between

temperature and fuel vapor pressure for both

forced and auto ignition regimes

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

Along the saturated curve are saturated fuel

and air mixtures, whereas mixtures to the right

of this line are unsaturated

[1]


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

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

[1]


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

ASTM AIT for some fuels

[11]

AIT for fuels

[10]

[10]


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]


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]

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]


Here is a table for the

regression coefficients that

are used in the formula to

calculate the ignition

probability

[1]


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]


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

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]


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


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]


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


Below is a comparison for Minimum surface ignition temperatures vs. velocity for two surface

materials (viz. Stainless steel and Titanium)

[6] [6]


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


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


Fig. Velocity Vectors Fig. Temperature profile


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


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


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]

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

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.

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, Wright-Patterson 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.

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

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