burn away time

[info]

Hi, I would like to know if it is correct to estimate the time needed to a cell to burnout as it follows: 

energy contained in the cell = E = (cell volume) * (matl density) * (heat of combustion)

power produced by the cell = P = HRRPUA * (cell surface) 

burn away time = E/P

I used this method to estimate the burn away time of a cell but I got a value larger (double) than the one I observed in smokeview.

Furthermore, I would expect that many cells that ignite at the same time, will disappear at the same time. Instead, they show different burn away times.

Could anyone please suggest where I'm wrong?

Thank you in advance

[Randy McDermott]

The heat release by a flame is not 100% efficient at feeding back heat to the solid.  Most of the heat is lost by radiation and convection away from the surface.  Further, the heat feedback is not uniform back to the solid, hence why cells do not burn away the same rate.

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[info]

Thank you, but cells seem to burn with the same burning rate (I refer to the burning rate boundary quantity)

[Randy McDermott]

Are you sure the colorbar is set to global min and max?

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[Randy McDermott]

Also, corner and edge cells have more surface area and may burn away faster.

[info]

Hi, yes the colorbar is set between global min and max. 

I compared the HRR with the number of cells showing a burning rate equal to  'REAC_FUEL Mass Flux' (that I  I found in the .out file), and these seems comparable if I suppose that just one face of each cell (the one in front of the heater) is burning.

I do not understand why cells burn-away so faster than I expected.

This is my file:

 &MESH ID='Mesh01-a-merged', IJK=52,30,24, XB=-2.2,3.0,-2.6,0.4,0.0,2.4/

&REAC ID='POLYURETHANE_REAC',

      FYI='SFPE Handbook, GM27',

      FUEL='REAC_FUEL',

      C=1.0,

      H=1.7,

      O=0.3,

      N=0.08,

      CRITICAL_FLAME_TEMPERATURE=1327.0,

      CO_YIELD=0.042,

      SOOT_YIELD=0.198/

&MATL ID='stuff',

      FYI='stuff',

      SPECIFIC_HEAT=1.0,

      CONDUCTIVITY=2.0,

      DENSITY=100.0/

&SURF ID='mysurface',

      RGB=146,202,166,

      HRRPUA=1000.0,

      IGNITION_TEMPERATURE=400.0,

      BURN_AWAY=.TRUE.,

      MATL_ID(1,1)='stuff',

      MATL_MASS_FRACTION(1,1)=1.0,

      THICKNESS(1)=0.1/

&SURF ID='heat',

      TMP_FRONT=1500.0/

&OBST ID='Obstruction', XB=-1.0,1.0,0.0,0.1,0.2,1.2, BULK_DENSITY=100.0, SURF_ID='mysurface'/ 

&OBST ID='Obstruction', XB=-0.2,0.2,-1.2,-1.1,0.0,0.6, SURF_ID='heat'/ 

&VENT ID='Mesh Vent: Mesh01 [YMIN]', SURF_ID='OPEN', XB=-2.2,3.0,-2.6,-2.6,0.0,2.4/ 

&VENT ID='floor', SURF_ID='INERT', XB=-2.2,3.0,-4.2,0.4,1.0E-3,1.0E-3/ 

&VENT ID='wall ', SURF_ID='OPEN', XB=-2.2,3.0,0.4,0.4,0.0,2.4/ 

&VENT ID='wall', SURF_ID='OPEN', XB=-2.2,-2.2,-4.2,0.4,0.0,2.4/ 

&VENT ID='wall', SURF_ID='OPEN', XB=3.0,3.0,-4.2,0.4,0.0,2.4/ 

&VENT ID='ceiling', SURF_ID='OPEN', XB=-2.2,3.0,-4.2,0.4,2.4,2.4/ 

&BNDF QUANTITY='BURNING RATE'/

&BNDF QUANTITY='GAS TEMPERATURE'/

&BNDF QUANTITY='RADIATIVE HEAT FLUX'/

&BNDF QUANTITY='WALL TEMPERATURE'/

Il giorno lunedì 27 gennaio 2020 10:54:23 UTC+1, info ha scritto:

[Randy McDermott]

Simplify your scenario until is it obvious what is happening.  Start with a single cell OBST without a fire, and apply an external flux with no convection (so you control the flux exactly).  Once you understand this, then start adding complexity little by little.

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[info]

Thank you Randy, I followed your advice and now this is clear to me. I have another question: how can I simulate the heating up (and then burn-away) of a solid thicker than 1 cell surrounded by heat sources? I would use the exposed backing to better simulate the heating up of many faces of the solid, but I'm afraid I cannot do it, if the solid is thicker than 1 cell. What could I do?

Thank you in advance

Il giorno lunedì 27 gennaio 2020 10:54:23 UTC+1, info ha scritto:

[Randy McDermott]

If it is thicker than one cell, it does not have to burn away.  Or you could try the 3D routines.

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[info]

Sorry, I do not understand why it should not burn away. The solid object disappears from the calculation cell by cell, so I supposed that the solid can start disappear from the hottest cells on the edges and then the inner cells starts to burn and diseapper too. Am I wrong?

Il giorno mercoledì 29 gennaio 2020 15:14:54 UTC+1, Randy McDermott ha scritto:

If it is thicker than one cell, it does not have to burn away.  Or you could try the 3D routines.

On Wed, Jan 29, 2020 at 9:12 AM info <infocan...@gmail.com> wrote:

Thank you Randy, I followed your advice and now this is clear to me. I have another question: how can I simulate the heating up (and then burn-away) of a solid thicker than 1 cell surrounded by heat sources? I would use the exposed backing to better simulate the heating up of many faces of the solid, but I'm afraid I cannot do it, if the solid is thicker than 1 cell. What could I do?

Thank you in advance

Il giorno lunedì 27 gennaio 2020 10:54:23 UTC+1, info ha scritto:

Hi, I would like to know if it is correct to estimate the time needed to a cell to burnout as it follows: 

energy contained in the cell = E = (cell volume) * (matl density) * (heat of combustion)

power produced by the cell = P = HRRPUA * (cell surface) 

burn away time = E/P

I used this method to estimate the burn away time of a cell but I got a value larger (double) than the one I observed in smokeview.

Furthermore, I would expect that many cells that ignite at the same time, will disappear at the same time. Instead, they show different burn away times.

Could anyone please suggest where I'm wrong?

Thank you in advance

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[Randy McDermott]

If you supply a THICKNESS for a SURF and then the cell burns away, the newly exposed surface will inherit the properties of the original surface.  This may not be the behavior you intend.

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[info]

Ok, I suppose the properties of the original surface are the thickness and the initial temperature.

Regarding the burn-away, if I set a bulk density the fuel mass is computed taking into account the cell volume and the bulk density. The SURF thickness is not considered?

Thank you very much

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[Randy McDermott]

The cell will burn away when MASS_FLUX*AREA*TIME = CELL_VOLUME*BULK_DENSITY.

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[Randy McDermott]

I should add, the THICKNESS enters into the problem affecting the MASS_FLUX, as it factors into the heat transfer solution and hence the rate of pyrolysis.

[info]

Ok. Could you please explain what does mean this sentence in the User Guide? "Note that without a BULK_DENSITY specified, the total amount of mass burned will depend upon the grid resolution. The use of the BULK_DENSITY parameter ensures a specific fuel mass per unit volume that is independent of the grid resolution." I do not understand which is the effect of grid resolution.

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[Randy McDermott]

It means that FDS will not adjust the density of the material if the specified OBST does not snap exactly to the grid.

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