How to start multiple fire with different condition

[xovennn]

Hello all,
I was wondering if it is possible to ignite multiple things using a minimal net heat flux.
Lets say I have two 1m3 PVC box. I set a fire ramp at 0s for the first.
And for the second box I want it to ignite if its surface temperature exceeds 50°c AND the net heat flux at its surface facing the first box also exceeds 5kW/m2.
Is there a way to ask for both condition ?
I really want to set a minimum net heat flux for ignition

[mcgratta]

You can set an ignition temperature, but not a minimum flux. You can fake it by using devices and control functions, but I cannot say exactly how at the moment. Why do you want to do this? If you are able to set some sort of threshold heat flux, you might find that things ignite too readily because you may have an instantaneously high value of heat flux.

[xovennn]

Hi,
I think you could just use IGNITION TEMPERATURE = 30 instead of setting up ctrl command like you do.
Specially if you want only a temperature threshold it will be way easier.

IGNITION TEMPERATURE must be set in the &SURF as the MATL_ID, THICKNESS,...you will also have to set up HRRPUA and a RAMP_Q for basic fire also in the &SURF.
It should look like this :
&SURF ID ='...'
MATL_ID
THICKNESS
HRRPUA
IGNITION_TEMPERATURE
RAMP_Q
.../

[joelkohyh]

Hi, I think you could just use IGNITION TEMPERATURE = 30 instead of setting up ctrl command like you do. Specially if you want only a temperature threshold it will be way easier.

IGNITION TEMPERATURE must be set in the &SURF as the MATL_ID, THICKNESS,...you will also have to set up HRRPUA and a RAMP_Q for basic fire also in the &SURF. It should look like this : &SURF ID ='...' MATL_ID THICKNESS HRRPUA IGNITION_TEMPERATURE RAMP_Q .../

Yup, I was considering this method which is much more straightforward. However, I am unsure how the burning area on my obstacle is calculated (to obtain HRRPUA from HRR and vice versa) - are all 6 faces included, or are faces "covered" by another obstacle or contact with the computational domain excluded? By implementing the CTRL command, I can directly control which surfaces are burning or INERT using the SURF_ID6 command.

@mcgratta Are you able to advise me on this, or refer me to the section in the FDS User Guide that discusses the above? Thank you both for your help :-)

[drjfloyd]

IGNITION_TEMPERATURE operates wall cell-by-wall cell. It does not cause the entire OBST or VENT to ignite.

[joelkohyh]

IGNITION_TEMPERATURE operates wall cell-by-wall cell. It does not cause the entire OBST or VENT to ignite.

Hi Jason, I saw your comment on another thread about how IGNITION_TEMPERATURE is evaluated on a cell-by-cell basis. I would infer that as long as the (cell) surface is defined within the &OBST and reaches the IGNITION_TEMPERATURE, it will burn according to the prescribed HRRPUA, regardless if the &OBST is blocked by another obstacle or not. Is this right? If that is the case, how would I calculate the area to obtain my HRRPUA from HRR? Thank you.

[drjfloyd]

Do you need to obtain a specific secondary fire size? If that is the case IGNITION_TEMPERATURE is probably not the approach to use. You will not know a priori how much of a VENT or OBST will reach the ignition temperature. You could come up with a set of control functions that integrates the area of wall cells that have a non-zero burning rate and use that to set a RAMP for the HRRPUA (there is an example in the current nightly build of this); however, if only one wall cell ignites, then you could wind up with a very high momentum jet flame.

If you don't need to obtain a specific secondary fire size and are happy to let as many wall cells ignite as reach the temperature, then you will need to decide what makes sense for HRRPUA. You could take the full real world fuel package peak heat release rate and divide that by the area realized in FDS. Of course it might be that the real world fuel package reaches its peak without the full area of the fuel package involved and maybe you want to account for that somehow. I can't tell you what to do for this. It is a decision you need to make based upon what your specific objectives are for using FDS.