Pyrolysis nHeptane

[Shivam Gupta]

Hello,

I am doing pyrolysis of n heptane. 

I have collected all the data from available on NIST and from various research paper. 

I have few queries

1) As I defined the thickness of layer 1 cm  in surface. According to calculation it should burn for 80 sec.

In simulation, it burns more than 80 sec. 

Kindly explain the possible cause ? 

2) Does the soot yield varies varies with fire size? 

If Yes, can I set the different value for different HRR from my side. 

Or else

I should use the experimental value than change absorption coefficient because 

In some paper I found

their is correlation between soot yield and absorption coefficient. 

The problem is I am not getting the desired HRR corresponding to diameter size. 

Kindly advice

[Kevin]

The first thing to do is to look in the FDS Validation Guide to see if there is an example of what you are doing. If so, then pose your question in this way, "I notice that in the latest FDS Validation Guide, Section 14.6.2, the predicted burning rate of heptane is12% greater than the experimental measurement. I ran a similar calculation with the same parameters, only varying the depth of the pool, and my prediction is X% less than the measurement. I tried varying parameters X, Y, and Z, and I noticed that ..."

If the question is posed like this, we might be able to offer some suggestions. 

[Shivam Gupta]

Thank you Kevin Sir, for your time,

As you guided, i went through the validation guide, I was definitely looking for this guidance, 

I run the case VTT Large Hall Test mentioned in the section 14.6.5

Based on the material properties in section 14.6.2 Pool Fire experiments. 

Instead of circular cross-section I made the equivalent square pool pan. 

Cell Size was 0.05 cm 

Predicted Mass loss rate does not match with available MLR graph in FDS validation. 

In FDS Validation MLR value does not exceed 0.08 kg/m^2/s

I took the surface layer of HEPTANE = 1 cm.

I have attached the image of MLR of heptane fnd FDS file for reference.

Kindly help me in this regard. 

On Fri, Feb 5, 2021 at 12:14 AM Kevin <mcgr...@gmail.com> wrote:

The first thing to do is to look in the FDS Validation Guide to see if there is an example of what you are doing. If so, then pose your question in this way, "I notice that in the latest FDS Validation Guide, Section 14.6.2, the predicted burning rate of heptane is12% greater than the experimental measurement. I ran a similar calculation with the same parameters, only varying the depth of the pool, and my prediction is X% less than the measurement. I tried varying parameters X, Y, and Z, and I noticed that ..."

If the question is posed like this, we might be able to offer some suggestions. 

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[Kevin McGrattan]

Why do you set the BACKING to 'VOID'?

&SURF ID='HEP',
      RGB=153,255,255,
      BACKING='VOID',
      MATL_ID(1,1)='HEPTANE',
      MATL_MASS_FRACTION(1,1)=1.0,
      THICKNESS(1)=0.01/

[Shivam Gupta]

Should I set to exposed ? 

Sir,

I had read about the surface but could not understand it better although I tried to read many times,

Shall you describe in simple terms what does it mean?

Backing void  and exposed ? 

I have re run the case setting surf to exposed.

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[Kevin McGrattan]

The SURF called 'HEP' has a single layer, and then a BACKING of 'VOID'. This means that the floor of the pan is an ambient temperature void. This doesn't make sense. The bottom of the pan and the floor below have to be added as additional layers of the SURF to account for heat transfer into the pan and floor.

[Shivam Gupta]

Hello Sir,

I rerun the validation case of n-heptane. I vary the thickness layer of fuel from .01 to .05 m. 

1) Predicated MLR of fuel does not match with the validation result. 

2) Numerical instability occurs in 0.05 m after 80 s while for .01 to .04 m numerical instability arises after 30 s. 

Kindly help me to resolve this issue.

I have attached the MLR of fuel of .05 m thickness and FDS file for reference.

On Fri, Feb 5, 2021 at 11:12 PM Kevin McGrattan <mcgr...@gmail.com> wrote:

The SURF called 'HEP' has a single layer, and then a BACKING of 'VOID'. This means that the floor of the pan is an ambient temperature void. This doesn't make sense. The bottom of the pan and the floor below have to be added as additional layers of the SURF to account for heat transfer into the pan and floor.

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[Kevin McGrattan]

I will run the case.

[Shivam Gupta]

Thank you sir,

I will wait for your reply. 

On Mon, 8 Feb, 2021, 19:17 Kevin McGrattan, <mcgr...@gmail.com> wrote:

I will run the case.

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[Kevin McGrattan]

I am running your case with the latest FDS source code and it has reached 200 s and is still going. If you want to try this version of FDS, download from a link here

https://github.com/firemodels/fds/wiki/FDS-Release-Notes