burn out

[Станислав Шахов]

Good day! Tell! Your guide has a burnout time formula. This time = density x thickness x heat of combustion / rate of heat release per unit area. In turn, HHRPUA = heat of combustion х mass loss rate HHRPUA = J | kg x kg (m2 x s), right? So if i want to count, how much time will take burning for example wood material? i do next. For example

Density = 1 kg \ m3

Heat of combustion = 13.8 kJ / kg

Thickness = 0.25 m.

Mass loss rate = 0.019 kg / m2xs.

I count all it and got: 

When i did modeling, time burnout with the same parameters was 10,5 sec.

Tell me where i do mistake? 

why the material properties do not affect the burn-out time?

I tried to change parameters such as heat of combustion in reaction,  no changes for time burnout. I tried to change thermal properties of material - no changes  for burnout time. Please tell where am i wrong?  It's difficult to me explain all these in English, but i hope u will understand  my problem. Thank a lot. 

[dr_jfloyd]

Without knowing your inputs, we cannot tell you what you have done wrong.

[Станислав Шахов]

четверг, 25 ноября 2021 г. в 18:07:09 UTC+2, dr_jfloyd:

[dr_jfloyd]

The burnout time of an OBST made up of multiple cells with more than one exposed face is not as you have calculated.  MLRPUA does not start at its full value it takes 1 s to ramp up.

From 11.5.8 Fuel Burnout:

"A parameter called BULK_DENSITY (kg/m3) can be applied to the OBST rather than the SURF line. This parameter is used to determine the combustible mass of the solid object. The calculation uses the user-specified object dimensions, not those of the mesh-adjusted object. This parameter over-rides all other parameters from which a combustible mass would be calculated"

Since you used BULK_DENSITY this determines the mass present in each cell occupied by the OBST and not any other input parameter. Each cell looses mass independently of the other cells. MLRPUA applies to all exposed faces of a cell the SURF is applied. The corner cells have 3 faces exposed. The edge cells have 2 faces exposed. The remaining cells on the top have once face exposed. At the start the corner cells will be loosing mass three times as fast as the center top cells so the corner cells will disappear first. Once that happens the edge cells go from 2 to 3 exposed faces. Those edge cells will have lost 2/3 of their mass by the time the corners disappears so they will only take 1/3 longer to disappear now that there are three exposed faces. When the original edge cells disappear, the center face cells go from 1 to 3 exposed faces.