Inquiry Regarding the Burner Size and HRR Specification in the ISOHept9 FDS Validation Input File

[DAWOONG-567]

Dear FDS Developers,

I am conducting research on compartment fires and using FDS to model and simulate compartment fire scenarios.

I am currently reproducing the ISOHept9 experiment described in NIST Technical Note 1603. As part of this work, I have been referring to the publicly available FDS validation input file for “TN-1603 Test 9.” The input file and its validation results have been very helpful in understanding and reproducing the ISOHept9 experiment.

TN-1603 Test 9(ISOHept9) code:
ISOHept9.txt

However, while reviewing the input file, I came across two questions regarding the burner size and the specification of the heat release rate (HRR).

1. Question regarding the burner size

NIST Technical Note 1603 states that the free-burn pan used in the ISOHept9 experiment had internal dimensions of 70.7 cm × 70.7 cm, corresponding to a burner area of approximately 0.5 m². I also understand that the pan was positioned at the geometric center of the compartment floor.

However, in the FDS validation input file, the burner is defined as follows:

&OBST XB=0.9,1.5,1.5,2.1,0.0,0.1,
SURF_IDS='fire','STEEL SHEET','STEEL SHEET' /

Based on these coordinates, the burner dimensions in the FDS model are 0.6 m × 0.6 m, corresponding to an area of 0.36 m².

Therefore, there appears to be a difference between the experimental burner area of approximately 0.5 m² and the modeled burner area of 0.36 m².

I would like to ask whether there was a specific reason for setting the burner side length to 0.6 m in the validation input file, rather than using a value of approximately 0.7 m to represent the 0.707 m burner used in the experiment. For example, was 0.6 m selected as a modeling simplification to align the burner boundaries with the 0.1 m computational grid, or was the difference in burner area compensated for through another input parameter, such as the fuel mass flux?

2. Question regarding the HRR specification

Figure 3.3 of NIST Technical Note 1603 presents two heat release rate values for the ISOHept9 experiment:

Ideal HRR calculated from the burner mass loss rate
Measured HRR obtained using oxygen consumption calorimetry

In the FDS validation input file, I understand that the HRR was not directly prescribed. Instead, the n-heptane fuel mass flux history was defined using MASS_FLUX and RAMP_MF, as shown below:

&SURF ID='fire',
SPEC_ID(1)='N-HEPTANE',
MASS_FLUX(1)=0.0625,
RAMP_MF='fire_ramp' /

&RAMP ID='fire_ramp', T= 0., F=0.000 /
&RAMP ID='fire_ramp', T= 45., F=1.750 /
&RAMP ID='fire_ramp', T=440., F=1.850 /
&RAMP ID='fire_ramp', T=550., F=0.000 /

Regarding this setup, I would like to know whether the fuel input history defined by MASS_FLUX and RAMP_MF was based on the Ideal HRR calculated from the burner mass loss rate or on the Measured HRR obtained from oxygen consumption calorimetry.

I would also like to know whether the HRR calculated by FDS during the validation was intended to be compared primarily with the Ideal HRR or with the Measured HRR.

When I compared the fuel input history in the validation input file with the two HRR curves in Figure 3.3, it appeared that the input was closer to the Measured HRR during some time periods and closer to the Ideal HRR during others.

Comparison of HRR Graph:

Thank you very much for taking the time to review my questions.

Any information regarding the reasoning or modeling considerations behind the burner size and fuel mass flux history used in the ISOHept9 validation input file would be extremely helpful for my research.

Thank you.

[mcgratta]

Let me take a look at the case.

[mcgratta]

1. The modeled pan was 0.6 m by 0.6 m for two reasons, if I remember correctly. First, it was so that the pan would be centered on the 10 cm grid, but more importantly, because the fire was generated by a liquid spray and the actual burning area was probably a bit less than the full extent of the pan. That being said, let me know if you see significantly different results.
2. The specified HRR in the FDS simulation is based on the Ideal HRR. I chose to just draw a simple 4 point representation of the actual curve. I wanted to specify the measured fuel flow rate to see how close the model would predict the measured HRR based on oxygen consumption.

[drjfloyd]

This test ISO 9 was an initial volume of heptane in a pan. The Ideal HRR is based on the load cell.

[mcgratta]

Yes, you're right. Later tests used a spray. So my answer to question 1 is that we chose the burner dimensions to be symmetric.

[drjfloyd]

The burner was centered on the floor and the compartment dimensions are a multiple of 0.2 m.

With an 0.7 m burner the choice is preserve burner size which means the burner will no longer be centered in the model or keep the burner centered which means changing 0.7 to 0.6 or 0.8.