Simple Example of Jet Fire

[Sam]

Hi everyone, I am new to FDS. I want to model a vertical jet fire due to leakage from a pipe or tank. By going through the previous posts relating to Jet Fire, it is mentioned that FDS is for low speed Ma<0.3 thermally driven flow. How can I model a jet fire then as speed will be sonic?  If somebody can share me any simple example so that i can kick start from there, that will be really helpful. 

Thanks. 

[Kevin]

If you are new to FDS, I suggest that you not try something like this. I suggest you create a similar case where the mass flow rate is the same and the orifice is large enough to keep the speed within range.

[Sam]

Dear Kevin, thanks for your reply. I made a geometry and specify a burner with HRRPUA. How to increase the speed of fire (Like the case will be in case of jet fire?)? Is it possible by using command Louver?. The orifice you are mentioning to make it large enough, is it about specifying the area of burner with the command OBST XB? 

Thats what model i made initially.

HEAD CHID='LNG_JET_FIRE', TITLE='jet'

&MESH ID='jet fire', IJK=40,40,12, XB=0.0,10.0,0.0,10.0,0.0,3.0/

&TIME TWFIN=10.0 /

&SURF ID='BURNER',

      COLOR='RED',

      HRRPUA=5.0E4/

&REAC ID='METHANE',

C=1.,H=4.,

SOOT_YIELD=0.1 / Fuel specification

&OBST XB=0.0,10.0,0.0,10.0,0.0,0.25, SURF_ID='INERT'/ Obstruction

&OBST XB=3.0,3.2,3.0,3.2,0.25,0.27, SURF_IDs='BURNER','INERT','INERT'/Burner

&VENT SURF_ID='OPEN', XB=0.0,10.0,0.0,0.0,0.0,3.0/ Vent01

&VENT SURF_ID='OPEN', XB=10.0,10.0,0.0,10.0,0.0,3.0/ Vent02

&VENT SURF_ID='OPEN', XB=0.0,10.0,10.0,10.0,0.0,50.0/ Vent03

&VENT SURF_ID='OPEN', XB=0.0,0.0,0.0,10.0,0.0,30.0/ Vent

&VENT SURF_ID='OPEN', XB=0.0,10.0,0.0,10.0,3.0,3.0/ Vent04

&DEVC XYZ=5.80,1.20,2.35,QUANTITY='THERMOCOUPLE',ID='T3'/

&SLCF QUANTITY='TEMPERATURE', PBZ=1.0/

&TAIL/

[Kevin]

I assume you know the fuel flow rate and the area of the orifice. If so, you need to specify the MLRPUA (Mass Loss Rate Per Unit Area) and OBST dimensions so that you get the equivalent fuel flow rate.

[Sam]

Dear Kevin, I specified the flow rate and area of orifice as follows.

Leakage of methane from orifice of diameter 0.676 m2 and flow rate is 4 Kg/s. After that, calculated the MLRPUA to be 60 (Mach Number is 0.02) and specify in the input file. I made a simple geometry of 10 m by 10 m by 5 m just to check whether it worked or not.

1. What should i mention the TMP_INNER?

2. Did i correctly modeled the jet fire?

3. What about modelling of higher flow rates having MLRPUA more than 400? Can we model it?

As my task is to model jet fire on offshore platforms where in case of pipe rupture, flow rate are quite high, more than 20 kg/s.

4. To reduce the simulation time, Can I use fine mesh near the leak and coarse mesh on rest of geometry?

Need your suggestion.

The input file is this

&HEAD CHID = 'jet fire', TITLE = '786'/ 

&TIME T_END = 5./

&MESH ID='Jet fire', RGB=233,155,99, IJK=60,70,35, XB=0.0,10.0,0.0,10.0,0.0,5.0/ Element Size =.16,.14,.14

HUMIDITY = 70.

RESTART = .FALSE.

BNDF_DEFAULT = .FALSE./

&REAC ID='METHANE',

      FUEL='REAC_FUEL',

      FORMULA='C1H4',

      SOOT_YIELD=0.02,

      HEAT_OF_COMBUSTION=5.0E4,

      IDEAL=TRUE/

&SURF ID='JET FIRE',  

      RGB=0,51,51,

      MLRPUA=60,

      RAMP_Q='JET FIRE_RAMP_Q',

      TMP_INNER=-25,/leakage from a pipe at 4 kg/s, leakage size 0.260 m diameter, Mach number= 0.02

     

&RAMP ID='JET FIRE_RAMP_Q', T=0.0, F=0.0/

&RAMP ID='JET FIRE_RAMP_Q', T=1.0, F=1.0/

&MATL ID = 'STEEL'

DENSITY = 8050

CONDUCTIVITY = 12

SPECIFIC_HEAT = .450/

&SURF ID='steel wall',

      MATL_ID = 'STEEL'

      COLOR='GRAY 80',

      THICKNESS=0.1/

&OBST XB=0.0,10.0,0.0,10.0,0.0,0.0, RGB=211,240,1, ALLOW_VENT=.FALSE., SURF_ID='steel wall'/ floor

&OBST XB=3.0,3.26,3.0,3.26,0.0,0.01, SURF_ID='JET FIRE'/ 

&VENT SURF_ID='OPEN', XB=0.0,80.0,0.0,0.0,0.0,50.0/ Vent01

&VENT SURF_ID='OPEN', XB=80.0,80.0,0.0,70.0,0.0,50.0/ Vent02

&VENT SURF_ID='OPEN', XB=0.0,80.0,70.0,70.0,0.0,50.0/ Vent03

&VENT SURF_ID='OPEN', XB=0.0,80.0,0.0,70.0,50.0,50.0/ Vent04

&VENT SURF_ID='OPEN', XB=0.0,0.0,0.0,70.0,0.0,50.0/ Vent

&BNDF QUANTITY='INCIDENT HEAT FLUX'/

&BNDF QUANTITY='WALL TEMPERATURE'/

&BNDF QUANTITY='BURNING RATE'/

[Kevin]

These are questions you need to answer. I am not responsible for doing your engineering work. I will answer specific questions about FDS, but you are now asking me how to model your specific scenario.

[Sam]

Dear Kevin, I understood.

 By question related to FDS is, Can higher flow rates having MLRPUA more than 500 (Mach Number more than 1) be modeled? 

[dr_jfloyd]

FDS cannot model compressible flow. If you have some kind of high pressure leak you would need to establish a boundary condition downstream of the leak that preserves momentum, mass, and energy. 

[Sam]

Dear Dr, thanks for your answer.