Why does the temperature drop instead when the rate of heat release is maximised?

[hahaha666123-arch]

Hi all, I'm a beginner in FDS and I'm simulating a scenario where an object (with a heat release rate I specify) burns, but when the heat release rate reaches its maximum, the temperature of the probe point arranged above the fire source drops instead? I don't know what the reason is? I hope you can give me some suggestions, thanks!
Here's my FDS code：

&HEAD CHID='test'/

&TIME T_END=650/

&MISC PARTICLE_CFL=T,SIMULATION_MODE='LES'/

&MESH ID='MESH01', IJK=37,36,25, XB=0.0,0.074,0.0,0.072,0.0,0.052,MULT_ID='M'/

&MULT ID='M',DZ=0.052,K_UPPER=11/

&SLCF QUANTITY='TEMPERATURE',CELL_CENTERED=T,PBY=0.036/

&SLCF QUANTITY='VELOCITY',CELL_CENTERED=T,PBY=0.036/

&SPEC ID='METHANE', LUMPED_COMPONENT_ONLY=.TRUE./

&SPEC ID='ETHYLENE', LUMPED_COMPONENT_ONLY=.TRUE./

&SPEC ID='HYDROGEN', LUMPED_COMPONENT_ONLY=.TRUE./

&SPEC ID='VENTED GASES',
SPEC_ID(1)='ETHYLENE',
SPEC_ID(2)='METHANE',
SPEC_ID(3)='CARBON MONOXIDE',
SPEC_ID(4)='CARBON DIOXIDE',
SPEC_ID(5)='HYDROGEN',
VOLUME_FRACTION(1)=8.2,
VOLUME_FRACTION(2)=6.8,
VOLUME_FRACTION(3)=13,
VOLUME_FRACTION(4)=41.2,
VOLUME_FRACTION(5)=30.8/

&REAC ID='Reaction1',
FUEL='VENTED GASES'/

&MATL ID='CONCRETE',
SPECIFIC_HEAT=1.04,
CONDUCTIVITY=1.8,
DENSITY=2280.0/

&MATL ID='STEEL',
SPECIFIC_HEAT=0.46,
CONDUCTIVITY=45.8,
DENSITY=7850.0,
EMISSIVITY=0.95/

&SURF ID='Beton',
RGB=212,172,174,
TMP_INNER= 20.0,
MATL_ID(1,1)='CONCRETE',
MATL_MASS_FRACTION(1,1)=1.0,
THICKNESS(1)=0.2/

&SURF ID='Steel',
RGB=152,160,160,
TMP_INNER=20.0,
MATL_ID(1,1)='STEEL',
MATL_MASS_FRACTION(1,1)=1.0,
THICKNESS(1)=3.0E-4/

&SURF ID='Burner',
COLOR='RED',
TMP_FRONT=300.0,HRRPUA= 3.73E4,
RAMP_Q='Burner_RAMP_Q'/

&RAMP ID='Burner_RAMP_Q', T=0.0, F=0.0/

&RAMP ID='Burner_RAMP_Q', T=1, F=0/

&RAMP ID='Burner_RAMP_Q', T=39, F=0/

&RAMP ID='Burner_RAMP_Q', T=78, F=0/

&RAMP ID='Burner_RAMP_Q', T=116, F=0/

&RAMP ID='Burner_RAMP_Q', T=155, F=0/

&RAMP ID='Burner_RAMP_Q', T=199, F=0/

&RAMP ID='Burner_RAMP_Q', T=243, F=0/

&RAMP ID='Burner_RAMP_Q', T=281, F=0/

&RAMP ID='Burner_RAMP_Q', T=316, F=0/

&RAMP ID='Burner_RAMP_Q', T=323, F=0.03/

&RAMP ID='Burner_RAMP_Q', T=325, F=0.1/

&RAMP ID='Burner_RAMP_Q', T=328, F=0.2/

&RAMP ID='Burner_RAMP_Q', T=329, F=0.27/

&RAMP ID='Burner_RAMP_Q', T=330, F=0.27/

&RAMP ID='Burner_RAMP_Q', T=332, F=0.47/

&RAMP ID='Burner_RAMP_Q', T=333, F=0.59/

&RAMP ID='Burner_RAMP_Q', T=334, F=0.68/

&RAMP ID='Burner_RAMP_Q', T=336, F=0.76/

&RAMP ID='Burner_RAMP_Q', T=341, F=0.92/

&RAMP ID='Burner_RAMP_Q', T=344, F=1/

&RAMP ID='Burner_RAMP_Q', T=350, F=0.93/

&RAMP ID='Burner_RAMP_Q', T=352, F=0.85/

&RAMP ID='Burner_RAMP_Q', T=354, F=0.77/

&RAMP ID='Burner_RAMP_Q', T=356, F=0.7/

&RAMP ID='Burner_RAMP_Q', T=359, F=0.6/

&RAMP ID='Burner_RAMP_Q', T=362, F=0.54/

&RAMP ID='Burner_RAMP_Q', T=367, F=0.4/

&RAMP ID='Burner_RAMP_Q', T=371, F=0.32/

&RAMP ID='Burner_RAMP_Q', T=375, F=0.26/

&RAMP ID='Burner_RAMP_Q', T=382, F=0.2/

&RAMP ID='Burner_RAMP_Q', T=390, F=0.15/

&RAMP ID='Burner_RAMP_Q', T=398, F=0.11/

&RAMP ID='Burner_RAMP_Q', T=405, F=0.08/

&RAMP ID='Burner_RAMP_Q', T=414, F=0.06/

&RAMP ID='Burner_RAMP_Q', T=421, F=0.04/

&RAMP ID='Burner_RAMP_Q', T=434, F=0.01/

&RAMP ID='Burner_RAMP_Q', T=500, F=0/

&RAMP ID='Burner_RAMP_Q', T=560, F=0/

&RAMP ID='Burner_RAMP_Q', T=600, F=0/

&RAMP ID='Burner_RAMP_Q', T=650, F=0/

&OBST ID='Obstruction', XB=0.028,0.046,0.032,0.038,0.0,0.064, THICKEN=.TRUE., SURF_ID='Steel'/

&OBST ID='Obstruction', XB=0.03,0.044,0.038,0.042,0.0,0.064, THICKEN=.TRUE., SURF_ID='Steel'/

&OBST ID='Obstruction', XB=0.032,0.042,0.042,0.044,0.0,0.064, THICKEN=.TRUE., SURF_ID='Steel'/

&OBST ID='Obstruction', XB=0.03,0.044,0.028,0.032,0.0,0.064, THICKEN=.TRUE., SURF_ID='Steel'/

&OBST ID='Obstruction', XB=0.032,0.042,0.026,0.028,0.0,0.064, THICKEN=.TRUE., SURF_ID='Steel'/

&OBST ID='Obstruction', XB=0.03,0.044,0.028,0.042,0.064,0.066, THICKEN=.TRUE., SURF_ID='Steel'/

&VENT ID='Vent', SURF_ID='OPEN', MB= XMIN /

&VENT ID='Vent07', SURF_ID='OPEN', MB= XMAX /

&VENT ID='Vent08', SURF_ID='OPEN', MB= YMIN /

&VENT ID='Vent09', SURF_ID='OPEN', MB= YMAX /

&VENT ID='Vent10', SURF_ID='OPEN', MB= ZMAX /

&VENT ID='Vent100', SURF_ID='Beton', XB=0.0,0.074,0.0,0.072,0.0,0.0/

&VENT ID='Burner', SURF_ID='Burner', XB=0.032,0.042,0.03,0.04,0.066,0.066/

&DEVC ID= 'T-1' QUANTITY = 'TEMPERATURE' XYZ= 0.042 0.04 0.07/
&DEVC ID= 'T-2' QUANTITY = 'TEMPERATURE' XYZ= 0.042 0.04 0.08/
&DEVC ID= 'T-3' QUANTITY = 'TEMPERATURE' XYZ= 0.042 0.04 0.09/
&DEVC ID= 'T-4' QUANTITY = 'TEMPERATURE' XYZ= 0.042 0.04 0.10/
&DEVC ID= 'T-5' QUANTITY = 'TEMPERATURE' XYZ= 0.042 0.04 0.11/
&DEVC ID= 'T-6' QUANTITY = 'TEMPERATURE' XYZ= 0.042 0.04 0.12/
&TAIL/

This is the temperature profile at the location where I set the probe point

[Er9y714]

HRR curve also decrease similar to temperature output. Since there is not heat output, temperatures are likely to drop.

What do you see in results in smokeview (or Results) in temperature and flame outputs at that point you expect the temperatures to be higher?

[hahaha666123-arch]

Thank you for your reply, my expectation of the output probe point temperature should be in line with the trend of the heat release rate curve, i.e. the temperature should not be maxed out when the heat release rate is not maxed out, but should keep increasing until the temperature is maxed out when the heat release rate is maxed out. Thanks again for your reply and suggestions!

[Vojta-Salek]

I strongly suggest trying a much less computationally demanding scenario. Do you really need to simulate more than 300 seconds without any heat source and 33k+ cells to check if a simple burner is working well?

The temperatures correspond roughly to your 'Burner_RAMP_Q' which defines the amount of fuel produced. However, the fuel produced does not necessarily mean that it is always burned at the exact moment of fuel production. You can for example display concentration profiles or mixture fraction to see where and when your fuel burns and causes temperature increase/decrease.

[hahaha666123-arch]

Thank you for your reply and suggestion, I will try to change some settings as you said!