Ember ignition enhancement — heat transfer coupling and logistic model

[mcgratta]

Discussed in #16071

^{Originally posted by ghaffari-eng March 18, 2026}

Summary

Extended the existing EMBER_IGNITION_MODEL to support heat transfer from landed embers to solid surfaces and added an alternative logistic regression ignition model. All changes are backward compatible — defaults produce identical results to the current code.

Branch with changes: https://github.com/ghaffari-eng/fds/tree/ember-ignition
Diff: master...ghaffari-eng:fds:ember-ignition

What changed

5 source files modified: type.f90, read.f90, part.f90, wall.f90, func.f90

1. Ember-to-surface heat transfer: When a landed ember has EMBER_HEAT_TRANSFER=.TRUE. on the receiving SURF, it deposits a heat flux (radiative + convective) onto the wall cell based on ember temperature and contact area. This feeds into Q_NET_F in SOLID_HEAT_TRANSFER, so the existing 1D heat conduction solver and TMP_IGN ignition check handle the rest. Multiple embers on the same cell accumulate.
2. Logistic ignition model: EMBER_IGNITION_TYPE=1 replaces the power-based normal CDF with a logistic regression that accounts for fuel moisture content, wind speed, ember mass, and ember temperature. Coefficients are user-configurable. The original model is untouched at EMBER_IGNITION_TYPE=0 (default).
3. Heat-transfer-based ignition: EMBER_IGNITION_TYPE=2 skips the probabilistic check entirely — ignition is determined by the solid phase solver when surface temperature reaches TMP_IGN from the deposited ember heat flux.

New SURF parameters

Parameter | Default| Description
EMBER_HEAT_TRANSFER |.FALSE.| Enable ember heat flux to surface
EMBER_CONTACT_FRACTION | 0.5 | Fraction of ember area in contact
EMBER_CONTACT_HTC | 25.0 | Contact HTC (W/m2/K)
EMBER_EMISSIVITY | 0.9 | Ember emissivity
EMBER_IGNITION_TYPE | 0 | 0=current CDF, 1=logistic, 2=HT-based
EMBER_IGNITION_FMC | 10.0 | Fuel moisture content (%)
EMBER_IGNITION_COEFFS(5) | 9.98 | Logistic regression coefficients

Test results

Two verification cases in Verification/WUI/ — a 700°C ember dropped onto a wood surface:

Case EMBER_HEAT_TRANSFER Max floor temp (°C)
ember_ignition_ht .TRUE. 35.76
ember_ignition_ht_off .FALSE. (default) 31.47

Looking for feedback on the approach, particularly whether adding Q_EMBER to QDXKF in the implicit solver is the right integration point, and whether this aligns with the intended direction for ember physics.

[mcgratta]

I remember now that some of the ember impact machinery in FDS was added by me. I vaguely remember adding it but have never used it. I'll take a look at your verification case. I need to remind myself about the current capabilities before considering any changes or additions.

[mcgratta]

One thing I can do now is add conductive heat transfer between a hot particle and a solid surface. This need not be ember-specific. In fact, I'd like to incorporate basic radiative, convective and conductive heat transfer without invoking the word "ember" specifically. In this way, the code can be used for other applications.

As for ember-specific things, I need an explanation or reference for this chunk of code below. It seems as if these various ignition enhancers are being added together in some way to make ignition more likely. We hesitate to add this kind of logic into FDS unless there is a very strong consensus in the community for it. Even the current ember ignition model is a crude attempt to have some way to ignite a level-set surface cell. I don't want to add more complexity unless there is some strong support for it.

CASE (1) IGNITION_TYPE_SELECT

   ! Logistic regression ignition model

   WIND_SPEED = SQRT(M%U(BC%IIG,BC%JJG,BC%KKG)**2 + &
                     M%V(BC%IIG,BC%JJG,BC%KKG)**2 + &
                     M%W(BC%IIG,BC%JJG,BC%KKG)**2)
   EMBER_MASS_G = LP%MASS*1000._EB
   EMBER_TMP_C  = B1_LP%TMP_F - TMPA
   LOGIT = SF%EMBER_IGNITION_COEFFS(1) + &
           SF%EMBER_IGNITION_COEFFS(2)*SF%EMBER_IGNITION_FMC + &
           SF%EMBER_IGNITION_COEFFS(3)*WIND_SPEED + &
           SF%EMBER_IGNITION_COEFFS(4)*EMBER_MASS_G + &
           SF%EMBER_IGNITION_COEFFS(5)*EMBER_TMP_C
   F_N = 1._EB/(1._EB + EXP(-LOGIT))
   ! Adjust for weighting factor
   F_N = 1._EB - (1._EB - F_N)**LP%PWT```

[mcgratta]

Here's another issue:

EMBER_CONTACT_FRACTION | 0.5 | Fraction of ember area in contact
EMBER_CONTACT_HTC | 25.0 | Contact HTC (W/m2/K)

I have been reading up on thermal contact resistance vis a vis an ember and the ground. It seems that the default value for the contact heat transfer coefficient is very low and the area fraction is very high. The product of the two may be appropriate, but I am hesitant to add something like this unless we can provide some reasonable estimates for both. For example, one might use one of the values and not the other, leading to something that is wildly inaccurate. Do you have a reference for your chosen default values?