Mass Flux vs Velocity.

[Seto]

Hi all FDS users,

I have a problem regarding my release case. As I have mentioned in a
previous question, I am using FDS to model dispersion of an exhaust
release. Recently, I compared the result from an FDS simulation with
another commercial dispersion software (PHAST) and found the result
significantly different.

The case I modeled was for a vertical vent releasing exhaust from a
generator. The hot exhaust gas composed mainly of nitrogen and carbon
dioxide. I specified the composition by specifying a mass_flux. I
executed the model for 30 seconds. I measured the distance of the
thermal plume and compared it to the PHAST result. I found that for a
certain temperature contour, the PHAST plume is generally higher than
the FDS. I thought this may be because of the velocity of the release,
which I did not specify on FDS because I specified the mass flux.
So, I tried specifying the velocity instead of the mass flux on the
FDS using the velocity I got from PHAST and the result seems more
similar.

This got me wondering how FDS calculates the velocity of the vent if
we are only specifying the mass flux. I always assumed that FDS uses
the gas density at atmospheric pressure to change the mass flux rate
into volume flow. Then divide the volume flow by the area of the vent
to get the velocity. Is this assumption correct? And if it is, then
can I specify the mass flow AND the velocity? This could also mean
specifying the release pressure, right?

The following is my input file:

&MESH IJK=210,40,100, XB=0,21,0,4,0,10 /

&TIME T_END=30. /

&DUMP DT_PL3D=1.E10, MASS_FILE=.TRUE. /

&MISC RADIATION=.FALSE., LAPSE_RATE=0.04, TMPA=30., U0 = 2. /

&SURF ID='WIND',VEL=-2.0, TMP_FRONT = 30., PROFILE='ATMOSPHERIC', Z0 =
10., PLE=0.52 /
&VENT MB='XMIN' ,SURF_ID='WIND' /
&VENT MB='XMAX' ,SURF_ID='OPEN' /
&VENT MB='YMIN' ,SURF_ID='OPEN' /
&VENT MB='YMAX' ,SURF_ID='OPEN' /
&VENT MB='ZMIN' ,SURF_ID='OPEN' /
&VENT MB='ZMAX' ,SURF_ID='OPEN' /

&SPEC ID='CARBON DIOXIDE' /
&SPEC ID='OXIGEN' /
&SPEC ID='NITROGEN' /
&SPEC ID='WATER VAPOR' /
&SPEC ID='NITROGEN DIOXIDE', MW=46. /
&SPEC ID='CARBON MONOXIDE' /
&SPEC ID='N-DECANE' /

&SURF ID='adiabatic wall', ADIABATIC=.TRUE./
&SURF ID='release', MASS_FLUX(1)=0.1742, MASS_FLUX(2)=0.07719,
MASS_FLUX(3)=0.752, MASS_FLUX(4)=0.0195, MASS_FLUX(5)=0.001,
MASS_FLUX(6)=0.0001, MASS_FLUX(7)=0.00001, TMP_FRONT=373. /

&SURF ID='exhaust pipe', RGB = 100,200,200 /
&OBST XB=2,2.3, 1.95,2.05, 0,2.95, SURF_ID='exhaust pipe'/
&VENT XB=2,2.3, 1.95,2.05, 2.95,2.95, SURF_ID='release',
COLOR='RED' /


I would appreciate any help with this.

thank you very much and please let me know if I am being unclear.

Seto

[dr_jfloyd]

FDS uses the wall temperature of the vent and the local pressure to compute the density at the vent.  Then velocity computed using the actual area of the vent.

PHAST is a Gaussian plume model.  I would not necessarily expect the two to match.  That being said, FDS is a LES code and computing the correct entrainment for the plume will require resolving the plume.  At your grid resolution, your plume source is only one grid cell wide. 

[Seto]

Hi dr_jfloyd,

Thank you very much for the information. I will re-try my simulation
with a bigger vent size and see if the results are more similar. My
FDS and PHAST result differ in the plume height. Around 3m difference,
the PHAST result is around 15 m while the FDS is 11.5 m.

Regarding the density and velocity, is there a way to check the
density or velocity of the gas exiting the vent? Maybe in the out
file? This would help me check if I am specifying the correct
flowrate.

Also, I noticed in the manual that there is a way to specify the
pressure at a vent but only for open vents. Can I do the same to my
vent? Then I can specify the pressure that FDS is using to calculate
my velocity. Is this correct?

Thank you again and regards,

Seto

Sorry if I'm unclear. English is not my first language.

[dr_jfloyd]

Read the User's manual on how to get outputs of density and velocity.  It won't be exactly at the vent but you can get it in the first grid cell next to the vent.

The pressure boundary condition is really mean for wind velocity conditions and is only applicable for an OPEN boundary

[Nikolai Ortiz]

Hi..

I think this could be the place to make my question.

I'm writing a species discharge of NAFTHA.

My Inititial Conditions are to be taken from PHAST.

A discharge velocity of 52.49 m/s

and a release area of: 0.025 m2

So i'm getting a little confuse about the way to calculate the mass_flux.

the NAFTHA property that I got from FDS are:

NAFTHA                         

   Molecular Weight (g/mol)            80.28420

   Ambient Density (kg/m^3)            3.338

   Initial Mass Fraction               0.000

   Enthalpy of Formation (J/kg)       -2.12E+06

   Sub Species                    Mass Fraction     Mole Fraction

   N-PENTANE                      4.044003E-01      4.500000E-01

   N-HEXANE                       2.576109E-01      2.400000E-01

   N-HEPTANE                      1.123281E-01      9.000000E-02

   N-OCTANE                       7.114010E-02      5.000000E-02

   N-DECANE                       5.316676E-02      3.000000E-02

   BUTANE                         1.013538E-01      1.400000E-01

 

   Viscosity (kg/m/s)   Ambient (293 K):  1.80E-05

                                  500 K:  2.72E-05

                                 1000 K:  4.46E-05

                                 1500 K:  5.87E-05

   Therm. Cond. (W/m/K) Ambient (293 K):  1.38E-02

                                  500 K:  2.08E-02

                                 1000 K:  3.41E-02

                                 1500 K:  4.49E-02

   Spec. Heat (J/kg/K)  Ambient (293 K):  3.62E+02

                                  500 K:  3.62E+02

                                 1000 K:  3.62E+02

                                 1500 K:  3.62E+02

   Diff. Coeff. (m^2/s) Ambient (293 K):  1.22E-05

                                  500 K:  3.11E-05

                                 1000 K:  1.01E-04

                                 1500 K:  1.98E-04

My density entry of PHAST is 634.99 kg/m3

so...

¿To get the mass_flux (kg/sm2) I have to use the Ambient Density  of the specie given by FDS or the Density calculated by PHAST?

Is a big deal because if I use FDS Ambient Density I got: 175.21162 kg/sm2

and if I use the density calculated by PHAST I get 33330.6251 kg/sm2.

So, if I fail the calculation, I will lead my simulation to a numerically instability.

and also my first goal is get the same Release Rate (kg/s) in FDS and PHAST

I will appreciate some advice.

Thanks a lot.

[dr_jfloyd]

Mass_flux × VENT area = release rate. You know you release rate, VENT area is the area of your VENT injecting the made. Solve for mass_flux