Description of Combustion Predicted in an FDS n-Heptane Liquid Pool-Fire Model Without an Explicit Ignition Source

[jianyiping730-cyber]

When using PyroSim to establish an n-heptane liquid pool-fire model, an n-heptane liquid-fuel surface and a gas-phase combustion reaction were defined, but no explicit ignition source—such as an open flame, heat source, or electric spark—was specified. Nevertheless, the simulation predicted that a flame appeared after a certain period and that sustained combustion subsequently developed.
The main input parameters are shown below:
&REAC ID='Reaction1',
FUEL='N-HEPTANE',
CO_YIELD=0.01,
SOOT_YIELD=0.037/

&MATL ID='liquid',
SPECIFIC_HEAT=2.24,
CONDUCTIVITY=0.14,
DENSITY=675.0,
ABSORPTION_COEFFICIENT=333.0,
HEAT_OF_REACTION=317.0,
SPEC_ID(1,1)='N-HEPTANE',
NU_SPEC(1,1)=1.0,
BOILING_TEMPERATURE=98.5/

&SURF ID='pool',
MATL_ID(1,1)='liquid',
MATL_MASS_FRACTION(1,1)=1.0,
THICKNESS(1)=0.02/

&OBST ID='liquid-pool',
XB=-0.5,0.5,-0.5,0.5,0.0,0.35,
SURF_IDS='pool','steel','steel'/
To examine the influence of ambient temperature on this phenomenon, simulations were conducted at ambient temperatures of 20 °C, 0 °C, -4 °C, and -20 °C. The Smokeview results show that the lower the ambient temperature, the later the flame appeared. At 20 °C, a visible flame appeared relatively early and developed into a well-established flame . At 0 °C, flame appearance was delayed . At -4 °C, the delay increased further. At -20 °C, a very long period was required before a clearly visible flame developed .
Comparison of the -4 °C and -20 °C cases suggests that, as the ambient temperature decreases, n-heptane evaporation becomes weaker and the formation of a flammable vapor layer above the liquid surface becomes slower, resulting in a later onset of visible flaming in Smokeview. However, even at -20 °C—well below the flash point of n-heptane and far below its autoignition temperature—the model still predicted combustion after a sufficiently long period. This raises the following question: in the default mixture-fraction combustion model, is vaporized liquid fuel allowed to burn once it mixes with oxygen, even when no explicit ignition source has been defined? In other words, is there an implicit or default ignition mechanism in FDS?
We would therefore like to ask FDS developers and users why the FDS liquid-fuel model predicts combustion in the absence of an explicit ignition source. Is this an intended feature of the default FDS combustion model, or does it indicate a problem in the model setup? In addition, to represent realistic ignition or autoignition conditions more accurately, should the actual autoignition temperature of n-heptane be specified explicitly, or should a separate ignition source be included?

FDS_n-heptane_no_explicit_ignition_source_issue_English.docx

[luoquan066-prog]

upupupup

[drjfloyd]

Please read the FDS documentation with a focus on Chapter 5 of the Technical Reference Guide and Chapter 13 of the User's Guide.

[jianyiping730-cyber]

Thank you. I'll check these documents now. If there is anything I don't understand later, I hope I can ask you!