Freeze time after device activation

[Matteo Pachera]

Hi,

I would like to control a vent using the FREEZE TIME option taking the relative time after the activation of a certain control and not the absolute time. 

In my simulation after a temperature device reaches a certain limit I need to decrease the flow rate through a vent after a certain delay and following a prescribed ramp.

&DEVC XYZ=0.000,0.000,0.100 ,ID=' fire_control', QUANTITY='TEMPERATURE', SETPOINT=300, INITIAL_STATE=F/

&DEVC XYZ=0.000,0.000,0.100, QUANTITY='TIME', NO_UPDATE_CTRL_ID='fire_control', ID='FREEZE TIME'/

&SURF ID='test', COLOR='RED', VEL=5.0, RAMP_Q='ramp'/

&RAMP ID=' ramp  ', T=0000, F=1.00000, DEVC_ID='FREEZE TIME'/

&RAMP ID=' ramp  ', T=0100, F=1.00000/

&RAMP ID=' ramp  ', T=0200, F=0.50000/

I see that using these lines the simulation takes the absolute time and not the relative time after the temperature reaches the setpoint. 

Is there a certain way to output the time when the temperature reaches the setpoint and possibly subtract it from TIME?

Thanks in advance,

Matteo

Is there 

[dr_jfloyd]

have two TIME devices where one freezes and the other doesn't.  Subtract one from the other.

[Matteo Pachera]

Thank you Jason for the answer, your solution works perfectly

[Matteo Pachera]

Dear Jason,

I have a new version of the problem which I can't fix with the same logic you proposed above. I would like to ramp up my volume flow until the activation of the control and later limit it to the half of the maximum reached value following a certain ramp.

I though about unfreezing first the time to run the first part of the ramp, later freeze it once the control activates and then unfreeze it once more for the second part of the ramp. However, this this solution will further increase my flow because the ramp will first go to 1 and later descend to half of the maximum value, whereas I would like to limit it to the maximum value it reaches and then drop it to the half of latter value.

Do you have any suggestion about how I could fix this?

Thanks in advance,

[dr_jfloyd]

You can do this by summing CUSTOM CTRL functions and having an x = y RAMP for the volume flow

the input to the volume flow ramp (T value = F value) is the sum of a linear ramp that goes from 0 to x and a linear ramp that goes from 0 to -x.

The 0 to x ramp uses the current freeze value you have now as the input.

The 0 to -x ramp would use as input the a second freeze time device - the first freeze time device. So until time is frozen on the first device this will be t = 0 and return 0.  When the first device freezes it will start returning negative values from the ramp. You would then have some set of control functions to determine when to free the second freeze time device. 

[Matteo Pachera]

Thank you Jason for the explanation I will try to implement it for my case.

[Matteo Pachera]

Dear Jason,

I think I understood your approach, but I'm struggling with the implementation in FDS. I understand that you want to create two ramps that I can later sum and move inside these two ramps controlling the time, but can I assign two ramps to the same surface? 

Thanks for your time,

Matteo

On Thursday, 3 February 2022 at 14:00:34 UTC+1 dr_jfloyd wrote:

[dr_jfloyd]

You have two RAMPs that generate the growth (positive value ramp) and the decay (negative value ramp).  Summing those gives the fraction of the SURF HRRPUA you want at that time.  Use that fraction as input to a third ramp that is y = x, i.e.:

&SURF ID='FIRE',HRRPUA=x,RAMP_Q='final HRR'..../

&RAMP ID='final HRR',T=0,F=0,CTRL_ID='id of final sum function'/

&RAMP ID='final HRR',T=1,F=1/

make the x value whatever it needs to be to make sure you fire can grow to the size it needs to reach.  

[Matteo Pachera]

Thank you for the explanation, I managed to combine the two ramps in the way you suggested.