Strange Result of SOOT

[godisreal]

Excuse me. I run an example as below. However, the smoke does not enter the small room top right.
I also run another example where soot reaches the wall and reflects back like air flow. So may I ask what problem it is?

&HEAD CHID='soot_upload2019',TITLE='soot_problem' /
&MESH IJK=39,24,3, XB=-0.6,30.6, -0.6,18.6, 0.0,3.0 /
&TIME T_END = 220.0, DT=0.2 /

&MISC SURF_DEFAULT = 'WALL',
      RADIATION=.FALSE.
	EVAC_SURF_DEFAULT = 'EVAC_WALL',  
	EVACUATION_DRILL=.false.  
	EVACUATION_MC_MODE=.false. /

&DUMP SMOKE3D=.TRUE.,
      NFRAMES=2000,
      DT_PART=0.5,
      DT_HRR=1.0,
      DT_SLCF=1.0,
      DT_BNDF=5.0,
      DT_PL3D=1000.0,
      DT_ISOF=5.0 /

&REAC ID         = 'POLYURETHANE'
      FYI        = 'C_6.3 H_7.1 N O_2.1, NFPA Handbook, Babrauskas'
      SOOT_YIELD = 0.10
      N          = 1.0
      C          = 6.3
      H          = 7.1
      O          = 2.1  /

&SURF ID='BURNER', HRRPUA=1000., COLOR='RASPBERRY' /

&MATL ID            = 'GYPSUM PLASTER'
      FYI           = 'Quintiere, Fire Behavior'
      CONDUCTIVITY  = 0.48
      SPECIFIC_HEAT = 0.84
      DENSITY       = 1440. /

&SURF ID             = 'WALL'
      RGB            = 160,160,160
      MATL_ID        = 'GYPSUM PLASTER'
      THICKNESS      = 0.012 /

 Boundary condition for the evacuation flow fields:
&SURF ID = 'OUTFLOW', VEL = +0.000001, TAU_V=0.1 /

&SURF ID    = 'EVAC_WALL'
      RGB   = 200,0,200 / or COLOR
	  
=======================================================
============= FIRE FDS GEOMETRY STARTS ================
=======================================================

Basic Outlines
&OBST XB = 0.0, 20.0, 17.9,18.0, 0.0,3.0, SURF_ID='WALL' / 
&OBST XB = 0.0, 20.0, 0.0, 0.2, 0.0,3.0, SURF_ID='WALL' /
&OBST XB = 0.0, 0.1,  0.0,18.0, 0.0,3.0, SURF_ID='WALL' /
&OBST XB = 20.0,20.1, 16.5,18.0, 0.0,3.0, SURF_ID='WALL' / 

Right Lounge Area
&OBST XB = 20.0,30.0,  0.0,0.2,  0.0,3.0, SURF_ID='WALL' / 
&OBST XB = 20.0,27.0, 15.9,16.0, 0.0,3.0, SURF_ID='WALL' /
&OBST XB = 27.0,30.0, 17.9,18.0, 0.0,3.0, SURF_ID='WALL' /
&OBST XB = 29.9,30.0,  0.0,18.0, 0.0,3.0, SURF_ID='WALL' /
&OBST XB = 27.0,27.1,  9.0,18.0, 0.0,3.0, SURF_ID='WALL' /

&OBST XB = 27.0,30.0, 9.0,9.1, 0.0,3.0, SURF_ID='WALL' /
&OBST XB = 27.0,30.0, 13.5,13.6, 0.0,3.0, SURF_ID='WALL' /

&OBST XB = 20.0,20.1, 2.0,14.5, 0.0,3.0, SURF_ID='WALL' / 

Doors and Exits in Right Lounge Area
&HOLE XB= 26.8,27.2, 14.5,15.5, 0.0,2.0 /
&HOLE XB= 26.8,27.2, 11,12, 0.0,2.0 /

Left Lounge Area
&OBST XB = 16.0, 20.0, 6.0, 6.2, 0.0,3.0, SURF_ID='WALL' /
&OBST XB = 6.0, 14.0, 6.0, 6.2, 0.0,3.0, SURF_ID='WALL' /
&OBST XB = 0.0, 4.0, 6.0, 6.2, 0.0,3.0, SURF_ID='WALL' /
OBST XB = 10.0, 10.2, 0.0, 6.0, 0.0,3.0, SURF_ID='WALL' /


The fire as a burner.
&OBST XB= 28.50, 29.50, 10.00, 11.00, 0.00, 0.60, SURF_ID='INERT' /
&VENT XB= 28.50, 29.50, 10.00, 11.00, 0.60, 0.60, SURF_ID='BURNER' /

&VENT MB='YMIN',SURF_ID='OPEN' / 
&VENT MB='YMAX',SURF_ID='OPEN' / 

[godisreal]

I also doubt if hot smoke cools down in FDS when it travel and mix with the cold air, especialy in a large-scale space like buildings.
When I run your example like simple_test.fds, it looks very good as smoke spread from ceiling to floor. But when I ran some other examples for a large-scale space, the smoke still spread very fast even if it is far away from the burner, and the simulation result looks a little strange and unrealistic.

Now I understand the problem. The key issue is you assume that soot and gas are the same in motion equation. This may be a good approximation if soot and gas are close enough to the burner. The hot gas carries small soot particle and they move together. However, if the gas has moved far from burner and has cooled down, such approximation is problematic. If soot particles follow Netwonian dynamics, is there any easy method to reduce the soot speed so that it spreads slower than gas?

If there is no burner or heat source, I suppose that soot movement should follow elementary diffusion equation. Comments are much welcome.

https://en.wikipedia.org/wiki/Diffusion_equation
https://cn.bing.com/search?q=diffusion+equation&go=Submit&qs=n&form=QBLH&sp=-1&pq=fortran+90+basics+charles+anthony&sc=0-33&sk=&cvid=246F068B4B6F4352883D3A66A7D043D8

https://www.life.illinois.edu/crofts/bioph354/diffusion1.html (Useful Tips)
Diffusion – useful equations
Diffusion coefficient, D
D = (1/f)kT
f - frictional coefficient
k, T, - Boltzman constant, absolute temperature
f = 6p h r
h - viscosity
r - radius of sphere
The value for f calculated for a sphere is a minimal value; asymmetric shape of molecule or non-elastic interaction with solvent (e.g. hydration) will increase f.
Diffusion coefficient depends on size and shape of molecule, interaction with solvent and viscosity of solvent.

Diffusion over a distance
The relationship below is generally valid:
= qiDt
- mean-square displacement
(x is the mean distance from the starting point that a molecule will have diffused in time, t)
qi - numerical constant which depends on dimensionality: qi = 2, 4, or 6, for 1, 2, or 3 dimensional diffusion.
D - diffusion coefficient (usual units are cm2 s-1).
t - time.