DOES FDS CALCULATE THE TEMPERATURE OF GASES ON PYROLYSIS?

[MarioCruzM]

Hi everyone, I'm working on a model for the thermal degradation of ammonium nitrate. To represent this thermal degradation, I'm using the pyrolysis of the material. So far I've been able to represent the first degradation reaction that happens, but I've had problems with the second degradation reaction. It is important to say that the parameters chosen for the pyrolysis are from the same article (experiments). These are the two problems I've not been able to solve.

1. Although I'm able to measure the concentration of the residual species for the pyrolysis, these species don't seem to contribute any heat to the environment, which brings me to the title of this post: Does FDS calculate the temperature of the gases from pyrolysis, or does this only happen when there's combustion of these gases?.

2. The second problem is that I get a numerical instability when running this simulation. So far I've seen that there is a really fast change in pressure and velocity of the gases. The enthalpy for this reaction is around -36 KJ/mol, which converts to roughly 450 KJ/kg. This EA is more on the "high" exothermic reaction, but I do not understand why I get the residual species for around 5 seconds, and after that it seems to release all the energy. Is it not supposed to be that the energy from the exothermic reaction is released since the residual species appear?

This is the code I've been using, as you can see it is a simple scenario.

Thank you in advanced for any reply.

T1.fds
Generated by PyroSim 2025.1.0826
12 Dec 2025, 12:38:49 pm

&HEAD CHID='T1', TITLE='1'/
&TIME T_END=500.0, DT=0.5, WALL_INCREMENT=1/
&DUMP DT_RESTART=300.0, DT_SL3D=0.25/
&MISC TMPA=40.0/

&MESH ID='MESH-01', IJK=28,23,46, XB=-0.2,2.6,-0.6,1.7,-0.2,4.4/
&MESH ID='MESH-02', IJK=28,23,46, XB=-0.2,2.6,1.7,4.0,-0.2,4.4/
&MESH ID='MESH-03', IJK=28,23,46, XB=2.6,5.4,-0.6,1.7,-0.2,4.4/
&MESH ID='MESH-04', IJK=28,23,46, XB=2.6,5.4,1.7,4.0,-0.2,4.4/
&MESH ID='MESH-05', IJK=28,23,46, XB=5.4,8.2,-0.6,1.7,-0.2,4.4/
&MESH ID='MESH-06', IJK=28,23,46, XB=5.4,8.2,1.7,4.0,-0.2,4.4/

&SPEC ID='NITROUS OXIDE'/
&SPEC ID='WATER VAPORp'/

&DEVC ID='THCP', QUANTITY='THERMOCOUPLE', XYZ=3.7,1.7,0.3/
&DEVC ID='THCP01', QUANTITY='THERMOCOUPLE', XYZ=3.7,1.7,0.7/
&DEVC ID='THCP02', QUANTITY='THERMOCOUPLE', XYZ=3.7,1.7,2.9/

&MATL ID='AN02',
FYI='Nitrato de Amonio',
SPECIFIC_HEAT=1.76,
CONDUCTIVITY=0.238,
DENSITY=1725.0,
N_REACTIONS=1,
HEAT_OF_REACTION=-449.7,
SPEC_ID(1,1)='WATER VAPORp',
SPEC_ID(2,1)='NITROUS OXIDE',
NU_SPEC(1,1)=0.5,
NU_SPEC(2,1)=0.5,
REFERENCE_TEMPERATURE=326.0,
HEATING_RATE=20.0,
PYROLYSIS_RANGE=60.0/

&SURF ID='HEAT FLUX',
RGB=255,0,1,
NET_HEAT_FLUX=3000.0/
&SURF ID='AN',
RGB=146,202,166,
TMP_INNER=40.0,
MATL_ID(1,1)='AN02',
MATL_MASS_FRACTION(1,1)=1.0,
THICKNESS(1)=0.2,
MATL_ID=AN02/

&OBST ID='Obstruction', XB=3.6,3.8,1.6,1.8,0.0,0.2, SURF_IDS='HEAT FLUX','INERT','INERT'/
&OBST ID='Obstruction', XB=2.8,4.6,0.8,2.6,0.8,2.4, SURF_ID='AN'/
&OBST ID='Obstruction', XB=-0.2,2.6,-0.6,1.7,-0.2,0.0/
&OBST ID='Obstruction', XB=-0.2,2.6,1.7,4.0,-0.2,0.0/
&OBST ID='Obstruction', XB=2.6,5.4,-0.6,1.7,-0.2,0.0/
&OBST ID='Obstruction', XB=2.6,5.4,1.7,4.0,-0.2,0.0/
&OBST ID='Obstruction', XB=5.4,8.2,-0.6,1.7,-0.2,0.0/
&OBST ID='Obstruction', XB=5.4,8.2,1.7,4.0,-0.2,0.0/

&VENT ID='Mesh Vent: MESH-06 [XMAX]', SURF_ID='OPEN', XB=8.2,8.2,1.7,4.0,-0.2,4.4/
&VENT ID='Mesh Vent: MESH-06 [YMAX]', SURF_ID='OPEN', XB=5.4,8.2,4.0,4.0,-0.2,4.4/
&VENT ID='Mesh Vent: MESH-06 [ZMAX]', SURF_ID='OPEN', XB=5.4,8.2,1.7,4.0,4.4,4.4/
&VENT ID='Mesh Vent: MESH-06 [ZMIN]', SURF_ID='OPEN', XB=5.4,8.2,1.7,4.0,-0.2,-0.2/
&VENT ID='Mesh Vent: MESH-05 [XMAX]', SURF_ID='OPEN', XB=8.2,8.2,-0.6,1.7,-0.2,4.4/
&VENT ID='Mesh Vent: MESH-05 [YMIN]', SURF_ID='OPEN', XB=5.4,8.2,-0.6,-0.6,-0.2,4.4/
&VENT ID='Mesh Vent: MESH-05 [ZMAX]', SURF_ID='OPEN', XB=5.4,8.2,-0.6,1.7,4.4,4.4/
&VENT ID='Mesh Vent: MESH-05 [ZMIN]', SURF_ID='OPEN', XB=5.4,8.2,-0.6,1.7,-0.2,-0.2/
&VENT ID='Mesh Vent: MESH-04 [YMAX]', SURF_ID='OPEN', XB=2.6,5.4,4.0,4.0,-0.2,4.4/
&VENT ID='Mesh Vent: MESH-04 [ZMAX]', SURF_ID='OPEN', XB=2.6,5.4,1.7,4.0,4.4,4.4/
&VENT ID='Mesh Vent: MESH-04 [ZMIN]', SURF_ID='OPEN', XB=2.6,5.4,1.7,4.0,-0.2,-0.2/
&VENT ID='Mesh Vent: MESH-03 [YMIN]', SURF_ID='OPEN', XB=2.6,5.4,-0.6,-0.6,-0.2,4.4/
&VENT ID='Mesh Vent: MESH-03 [ZMAX]', SURF_ID='OPEN', XB=2.6,5.4,-0.6,1.7,4.4,4.4/
&VENT ID='Mesh Vent: MESH-03 [ZMIN]', SURF_ID='OPEN', XB=2.6,5.4,-0.6,1.7,-0.2,-0.2/
&VENT ID='Mesh Vent: MESH-02 [XMIN]', SURF_ID='OPEN', XB=-0.2,-0.2,1.7,4.0,-0.2,4.4/
&VENT ID='Mesh Vent: MESH-02 [YMAX]', SURF_ID='OPEN', XB=-0.2,2.6,4.0,4.0,-0.2,4.4/
&VENT ID='Mesh Vent: MESH-02 [ZMAX]', SURF_ID='OPEN', XB=-0.2,2.6,1.7,4.0,4.4,4.4/
&VENT ID='Mesh Vent: MESH-02 [ZMIN]', SURF_ID='OPEN', XB=-0.2,2.6,1.7,4.0,-0.2,-0.2/
&VENT ID='Mesh Vent: MESH-01 [XMIN]', SURF_ID='OPEN', XB=-0.2,-0.2,-0.6,1.7,-0.2,4.4/
&VENT ID='Mesh Vent: MESH-01 [YMIN]', SURF_ID='OPEN', XB=-0.2,2.6,-0.6,-0.6,-0.2,4.4/
&VENT ID='Mesh Vent: MESH-01 [ZMAX]', SURF_ID='OPEN', XB=-0.2,2.6,-0.6,1.7,4.4,4.4/
&VENT ID='Mesh Vent: MESH-01 [ZMIN]', SURF_ID='OPEN', XB=-0.2,2.6,-0.6,1.7,-0.2,-0.2/

&BNDF QUANTITY='BACK WALL TEMPERATURE'/
&BNDF QUANTITY='WALL TEMPERATURE'/

&SLCF QUANTITY='VOLUME FRACTION', SPEC_ID='NITROUS OXIDE', ID='', XB=-0.2,8.2,-0.6,4.0,-0.2,4.4/
&SLCF QUANTITY='VOLUME FRACTION', SPEC_ID='NITROGEN', ID='', XB=-0.2,8.2,-0.6,4.0,-0.2,4.4/
&SLCF QUANTITY='VOLUME FRACTION', SPEC_ID='WATER VAPORp', ID='', XB=-0.2,8.2,-0.6,4.0,-0.2,4.4/
&SLCF QUANTITY='TEMPERATURE', ID='', XB=-0.2,8.2,-0.6,4.0,-0.2,4.4/
&SLCF QUANTITY='VELOCITY', ID='', XB=-0.2,8.2,-0.6,4.0,-0.2,4.4/
&SLCF QUANTITY='PRESSURE ZONE', ID='', XB=-0.2,8.2,-0.6,4.0,-0.2,4.4/
&SLCF QUANTITY='PRESSURE', ID='', XB=-0.2,8.2,-0.6,4.0,-0.2,4.4/

&TAIL /

[drjfloyd]

First question: see App P in the Technical Reference Guide.

Second quesiton: the solid phase reaction happens inside the solid phase. The energy is added to remaining solid raising its temperature.

Running with the current source, if I look at the temperature slice, I see gas temperatures above 300 below the block as it begins to decompose. This matches well with the reference temperature for the surf. Seems to me that hot gas is entering the domain as expected.

You have WATER VAPORp which is an ionized version of water vapor. This is a species that would typically only be used in highly detailed gas phase chemistry. Did you mean WATER VAPOR?

Suggest not setting DT on TIME. Generally best to let FDS set the first time step and only set DT if that initial step is too large.

Your solid phase reaction is not quite balanced. Ammonium nitrate is NH_4NO_3. This decomposes into N_2O + 2 H_2O, This is not a 50 / 50 split in mass.

Running your case to see if an instability happens with the current source.

[drjfloyd]

I had an instability about 67 s into the simulation. I added BNDF of MASS FLUX WALL. I see cells that briefly spike to some large mass flux values (multiple kg/s). With the reaction being exothermic, I suspect that some wall cell gets warm enough to drive the reaction rate high resulting in the reaction running away resulting in a surface velocity that violates the low Mach assumption in FDS.