Surface with given HRRPUA affected by fuel type and surface material property

[Yanbin Xu]

Hi all!

I'm on this simulation of fuel tanker semi trailer fire, which I intended to simplify by setting up a surface with given HRRPUA (2500kW/m2 as suggested SPFE Handbook), problem is, as I am using FDS 6.0, a REAC line must be defined, so I define a REAC line. And I thought neither the fuel type nor the surface material property should have no actual influence on my 'fire' and the 'fire' is characterised uniquely by the HRRPUA and its area. 

And here is the input lines:

&REAC SOOT_YIELD=0.018,FUEL='PROPANE'/

&SURF ID='FUEL_TANKER'

      COLOR = 'RED'

      MATL_ID(1,:) = 'MAT_A'

      BACKING = 'INSULATED'

      HRRPUA = 2500.

      RAMP_Q = 'HRR1'

      THICKNESS=0.01 /

&RAMP ID='HRR1', T=0.,F=0 /

&RAMP ID='HRR1', T=20.,F=1 /

&RAMP ID='HRR1', T=400.,F=1 /

However, when I firstly change the fuel type among 'propane', 'hydrogen', 'N-OCTANE', the adiabatic surface temperature I get at the same point could have a difference of 40-80°C while the peak temperature is 1000°C, is this normal, or does the fuel type here do affect the result? In this case, what role is the REAC of a certain fuel type playing here, knowing that I already have a surface releasing heat of 2500kW/m2 ? 

Secondly, I could make this model run without defining the MAT_A, and again, the result could be 100°-200°C different, so the material properties and thickness etc. of the surface have also influence? If so, could anyone give me suggestions on how to define a  reasonable surface for a fuel tanker fire?

Many thanks!

Yanbin

[dr_jfloyd]

Why do you expect that you would get the exact same fire and same heat feedback from different fuels and surfaces?  They require different masses of oxygen per mass of fuel.  They produce different products which will change the radiation absorptivity. The initial conditions for the plume will be influenced by the temperature at which the fuel leaves the surface. 

[许炎彬]

Hi,

Thanks for your reply! 

Only that I initially wanted to simplify the fire by setting up a surface emitting specified heat, without defining a reaction. I thought my surface is uniquely characterized by the given HRRPUA.

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[许炎彬]

Hi,

Didn't finish...

Well I now know that fuel type and surface material property do influence. My question shall be:

As the REAC line defines the fuel type, which is supposed to be the thing that burns on my defined surface, do I still need to add the MAT_A below?

Thanks a lot!

Yanbin

On 2014年12月22日, at 21:06, dr_jfloyd <drjf...@gmail.com> wrote:

--

[dr_jfloyd]

HRRPUA defines a fuel mass flux and not a heat flux (hence why it requires a REAC).  If you want to specify a fixed heat flux without a fire, use the inputs for a fixed heat flux.  However, from the point of view of the fluid dynamics - releasing a constant X kW of energy from a flat surface is not the same as releasing the same average x kW as a time-varying 3D energy release (e.g. a fire)

[dr_jfloyd]

HRRPUA only defines the species boundary condition.  It does not set any thermal boundary condition.  It is up to you to decide if your simulation requires setting something other than the default thermal boundary condition.

[许炎彬]

Thanks again, very helpful! I guess I'll re-read the user guide and see what comes next:)

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[TimoK]

The surface properties of your burner surface (where HRRPUA is coming from)
do influence the calculation. One type of influence is radiation between
different surfaces in the simulation. The burner surface and other
surfaces (and the gas phase also) exchange energy by radiative heat
transfer. For a burning liquid, the surface temperature should be close
to the boiling point, i.e, not very hot. So if you use default thermal
model (is it INERT?), you do not miss much radiative heating from
the "not so hot" liquid surface. Well, you could specify your
TMP_FRONT as the boiling temperature of your liquid fuel.

For a sand burner things are not so simple. The top of the sand
heats up due to the radiation from the flame and could be quite
hot. And this hot surface heats up neighbouring surfaces and
this might be important, if you burner is next to your specimen
like you have, e.g., in the European SBI test (a burner in the corner),
or a line burner next to a wall specimen. Here you should have
the top of the sand hot, but the fuel gas is not so hot, it is coming
from a cold (ambient) pressure bottle. So, in this case you should
think what you want to study. Are things close to the burner
important or not?

TimoK

[Yanbin Xu]

Hi TimoK,

Thanks for reply!

I wish to study the bridge behaviour subjected to fuel tanker semi trailer fire. I have mainly two fire sources, the gasoline burning inside the tanker (with given HRRPUA 2500kW/m2), and the spilled gasoline burning on the road (with HRRPUA 500). I would use 90# with formula as C7.9H17.8 for the gasoline. What I want to obtain from FDS, is the AST(adiabatic surface temperature) contouring the bridge's girders, with a clearance of 5.5m. So only burning liquid, no other pyrolysis process in my simulation.

I'd like to put the burning surface of the tanker gasoline at 1m's height (the depth of the tanker fuel is also 1m), and 0m for the spill gasoline. And the input lines now are:

&REAC ID                 = 'GASOLINE'

      C=7.9

      H=17.8

      SOOT_YIELD =0.018

      HEAT_OF_COMBUSTION =47000 /

&MATL ID = 'GASOLINE'

      EMISSIVITY = 1.0

      CONDUCTIVITY =0.1 

      SPECIFIC_HEAT = 1.0

      DENSITY = 750.0

      HEAT_OF_REACTION=348.9

      BOILING_TEMPERATURE=85

      ABSORPTION_COEFFICIENT=2700/

&SURF ID='FUEL_TANKER'

      COLOR = 'RED'

      MATL_ID(1,:) = 'GASOLINE'

      BACKING = 'INSULATED'

      HRRPUA = 2500.

      RAMP_Q = 'HRR1'

      THICKNESS=1.00 /

&RAMP ID='HRR1', T=0.,F=0 /

&RAMP ID='HRR1', T=20.,F=1 /

&RAMP ID='HRR1', T=400.,F=1 /

&SURF ID='FUEL_SPILL'

      COLOR = 'TOMATO 2'

      MATL_ID(1,:) = 'GASOLINE'

      BACKING = 'INSULATED'

      HRRPUA = 500.

      RAMP_Q = 'HRR2'

      THICKNESS=0.01 /

&RAMP ID='HRR2', T=0.,F=0 /

&RAMP ID='HRR2', T=20.,F=1 /

&RAMP ID='HRR2', T=400.,F=1 /

Two additional questions are:

I have no SPEC_ID in MATL line, it seems that I don't need it here, right?

I didn't get a value for absorption coefficient of gasoline, 2700 is that of methane, anyone has the value?

Thanks!

Yanbin

[TimoK]

See the FDS Guide, Chapter "8.5.7 Liquid Fuels".

"The inclusion of BOILING_TEMPERATURE on the MATL line tells FDS to use its liquid pyrolysis model."

So, you should not have BOILING_TEMPERATURE (matl namelist) and HRRPUA (surf namelist that has
a matl_id that has boiling_temperature) defined for the same surface. HRRPUA is a simple gas burner
type boundary condition for a fire and BOILING_TEMPERATURE sets up a liquid pyrolysis model. These
are different and can not be used at the same surface.

If you want to use 2500 kW/m2 (and 500 kW/m2) as your fire size per unit area, then you should
take the boiling_temperature away. But then the liquid surface could heat up too much, because
it is not loosing energy by vaporization. So, the surface of the liquid OBST is going to bee too hot.
Well, you could just set TMP_FRONT=85, so that the surface will be at the boiling temperature.

The other way is take the HRRPUA and RAMP_Q away from the SURF namelist and let the
MATL namelist with boiling_temperature and other liquid fuel pyrolysis model parameters predict
the actual HRR. But here you need correct parameters so that you get reasonable HRR. You need
to do some testing for your liquid model. And you should read the liquid fuel model chapters in the
manuals. For example:

"Obviously, the convection of the liquid is important, but is not considered in the model."

"Liquids do not just absorb radiation at the surface, but rather over a thin layer near the
surface. Its effect on the burning rate is significant."

TimoK

[Yanbin Xu]

Thank you Timok, it has been very nice of you and it's very helpful!

I think the first way you suggested suits well my study, as it's simple and also the constant 2500kW/m2 is a pretty credible hypothesis. 

The second way is worthy having a try, it will be good if I could get an equivalent fire source as the first one.

Reading and working on.

Yanbin