Modelling flame blow-off by airflow

[MeloQuan]

Hello team,

I'm trying to realize the blow-off of methanol pool fire by airflow. I chose the "extinction 2" as extinction model. Cell size is 0.5 mm. CFT and AIT were set to 1257 ℃ and 200 ℃, respectively.

However, the pool cannot be ignited. When I remove AIT, it can be ignited but cannot be blown out.

The main part of the code is as follows:

&REAC FUEL='METHANOL'
RADIATIVE_FRACTION=0.22
N_SIMPLE_CHEMISTRY_REACTIONS=2
FUEL_C_TO_CO_FRACTION=1.0
HEAT_OF_COMBUSTION = 19000.
CRITICAL_FLAME_TEMPERATURE=1257
AUTO_IGNITION_TEMPERATURE=200/

&COMB EXTINCTION_MODEL='EXTINCTION 2'/

&MATL ID = 'METHANOL LIQUID'
EMISSIVITY = 1.
NU_SPEC = 1.
SPEC_ID = 'METHANOL'
HEAT_OF_REACTION = 1165
CONDUCTIVITY = 0.20
SPECIFIC_HEAT = 2.48
DENSITY = 796
ABSORPTION_COEFFICIENT = 1500
BOILING_TEMPERATURE = 64.65 /
&MATL ID='STEEL', CONDUCTIVITY=54, DENSITY=7800, SPECIFIC_HEAT=0.465, EMISSIVITY=0.8 /

&SURF ID='PAN', COLOR='GRAY 50', MATL_ID='STEEL', THICKNESS=0.003/
&SURF ID='POOL', COLOR='LIGHT BLUE', MATL_ID='METHANOL LIQUID', THICKNESS=0.1, EXTERNAL_FLUX=20, RAMP_EF='rampef' /
&RAMP ID='rampef', T= 0., F=1. /
&RAMP ID='rampef', T=15., F=1. /
&RAMP ID='rampef', T=16., F=0. /

&SURF ID='SUPPLY',
RGB=26,204,26,
VEL=-5.0,
RAMP_V='SUPPLY_RAMP_V'/
&RAMP ID='SUPPLY_RAMP_V', T=0.0, F=0.0/
&RAMP ID='SUPPLY_RAMP_V', T=35, F=0.0/
&RAMP ID='SUPPLY_RAMP_V', T=36, F=1.0/
&RAMP ID='SUPPLY_RAMP_V', T=45, F=1.0/
&RAMP ID='SUPPLY_RAMP_V', T=46, F=0.0/

&VENT MB='ZMIN', SURF_ID='OPEN' /
&VENT MB='ZMAX', SURF_ID='OPEN' /
&VENT MB='YMIN', SURF_ID='OPEN' /
&VENT MB='YMAX', SURF_ID='OPEN' /
&VENT MB='XMIN', SURF_ID='OPEN' /
&VENT ID='XMAX', SURF_ID='OPEN', XB=0.1,0.1,-0.1,0.1,0.15,0.35/
&VENT ID='Vent', SURF_ID='SUPPLY', XB=0.1,0.1,-0.1,0.1,-0.05,0.15, IOR=-1/

[Er9y714]

See User Guide 13.1.7 Piloted Ignition

[MeloQuan]

Thanks for reply. Now I try to use the spark ignition, but I still can't ignite the pool. Could you please help me check it again? Thanks a lot!

&REAC FUEL='METHANOL'
RADIATIVE_FRACTION=0.22
N_SIMPLE_CHEMISTRY_REACTIONS=2
FUEL_C_TO_CO_FRACTION=1.0
HEAT_OF_COMBUSTION = 19000.
CRITICAL_FLAME_TEMPERATURE=1257
AUTO_IGNITION_TEMPERATURE=464/
&COMB EXTINCTION_MODEL='EXTINCTION 2'/
&MATL ID='STEEL', CONDUCTIVITY=54, DENSITY=7800, SPECIFIC_HEAT=0.465, EMISSIVITY=0.8 /
&MATL ID = 'METHANOL LIQUID'
EMISSIVITY = 1.
NU_SPEC = 1.
SPEC_ID = 'METHANOL'
HEAT_OF_REACTION = 1165
CONDUCTIVITY = 0.20
SPECIFIC_HEAT = 2.48
DENSITY = 796
ABSORPTION_COEFFICIENT = 1500
BOILING_TEMPERATURE = 64.65 /
&PROP ID='spark', SPARK=T /
&DEVC ID='spark-1', XYZ=0.002,0.002,0.012, QUANTITY='TIME', SETPOINT=5, PROP_ID='spark' /

&SURF ID='PAN', COLOR='GRAY 50', MATL_ID='STEEL', THICKNESS=0.003/
&SURF ID='POOL', COLOR='LIGHT BLUE', MATL_ID='METHANOL LIQUID', THICKNESS=0.1/

[drjfloyd]

If you are predicting heat release rates you are doing a research application. You need to ensure you have sufficient outputs for you to determine what is happening.

Do you have any vapor being generated at all?
If you do, does that vapor generation rate seem reasonable given the pool temperature?
If you have vapor being generated, do you have any combustion happening?
If you have combustion happening how much and is it raising the gas temperatures in adjacent gas cells enough for them to combust (your spark is only at one point)?
What heat feed back is that giving to your pool?

[MeloQuan]

Thank you so much for the reply.
I want to use wind to blow off a flame. As I attached above, I introduced the extinction model 2 with CFT and AIT.
When I remove AIT, the methanol pool can be ignited by an external flux set on the pool surface. It can produce a good pool fire. And a wind can blow out the flame, even though it is not correct (when the temperature decreases to room temperature, the HRR is still high)
However, when I set AIT to 464 ℃, whether I use spark or external flux, it cannot be ignited, so I think the problem may be with AIT?
Or if I need to set more spark points?

I'm sorry to bother you, but I'm new to using FDS and having some problems. Thanks for your time again!

[MeloQuan]

sorry, I just found that if I set external flux of pool surface extra large, it could be ignited.
So how can I adjust the power of a spark? I can not find any information in User's guide 13.1.7

[mcgratta]

Keep in mind that both extinction models in FDS neglect the effect of high strain rate. The extinction method is purely thermodynamic. You may not achieve "blow out" because that would require some accounting for high strain extinction.

[MeloQuan]

Thanks for reply.

At first, we used a finite rate combustion, then the pool can be ignited by a hot particle and can be blown off by wind. But it requires a lot of computing resources and time. So now we try to use fast reaction and extinction model 2 to realize the blow off process.

I think theoretically wind can decrease temperature and fuel gas below the pool surface, then cause blow-off. Even if FDS neglect the effect of high strain rate, I think it can still achieve blow-off.

Now the other problem is that I use evaporation model to produce fuel gas, but it cannot be ignited after I add AIT and spark ignition according to User guide 13.1.6-13.1.7

[mcgratta]

Look at HRRPUV on the INIT line.