HRRPUA and burning rate in _hrr.csv

[jxh...@163.com]

question 1:
To specify a source of 1kW, the area of the source is 1m2, then the
HRRPUA is set to be 1kW/1m2 in the *.fds file. But due to the grid
resolution, the source is diaplayed in SMOKEVIEW with an area of 2m2,
would the source in calculation of FDS be changed to 1kW/m2*2m2 =
2kW ?

question 2
It is known that the burning of solid is converted to the fuel gas
with equivalent heat of combustion in mixture fraction model. The
burning rate of the fuel gas is the effective burning rate of
volatiles of the solid. Which one is shown in the *_hrr.csv file, the
generation rate of combustible volatiles of the solid or the effective
burning rate ?

regrads.

[Emanuele Gissi]

question 1:
FDS will adjust the HRRPUA if the obstruction or vent dimensions are
changed to conform to the numerical grid.
Your source will stay 1 kW.
This is indirectly stated in User's guide special topic Making Fuels
Disappear (BURN_AWAY) at page 74, point 4 of the bulleted list.
(I am sure there was a better paragraph on the topic, but I cannot
find it now.)

question 2:
I am rather confident that BURN_RATE from the *_hrr.csv file is the
"effective" total mass loss rate of fuel. The mass loss rate of fuel
gases is automatically adjusted so that the effective mass loss rate
multiplied by the single, global, gas phase heat of combustion
produces the expected heat release rate.

I would kindly ask for a confirmation from the developers for this
second reply.

Emanuele

[jxh...@163.com]

Thanks.
I agree with you about the second reply. If the effective mass loss
rate of fuel is recorded in hrr.csv file, it would be expected that
the ratio of hrr to mlr is the effective heat of combustion of fuel
which should be a constant. However, hrr/mlr of the example couch.fds
decreases with time.

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[drjfloyd]

1) A simple test case would have given you the answer to this
question. Write an FDS input where the area of the &SURF XB that the
HRRPUA is applied to does not map perfectly to the &MESH XB and IJK.
Run the case. Do you get the HRR defined by the original SURF and
HRRPUA in the input file or do you get the HRR defined by the are of
the SURF as it is snapped to the grid.

2) HRR in .csv / MLR in .csv = heat of combustion ONLY if all the fuel
burns. If unburned fuel leaves the domain or if insufficient air is
available to burn the fuel, then HRR in .csv / MLR in .csv /= heat of
combustion. HRRPUA specifies how much fuel to put into the domain.

[jxh...@163.com]

Thank you both.
I got to know the effective buring rate in _hrr.csv and why the HRR/
MLR in _hrr.csv decreases with time.
Furthermore, I agree that the energy would still be 1kW although the
simple case has not been tested.

Regards.

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