Liquid Pyrolysis and Autoignition Temperature.

[Siva D]

Hello

1) I have two blocks of Heptane Liquid and I am Introducing an auto ignition temperature of 500 Degree Celsius to one of the blocks.
2) However, the block which has an auto ignition temperature, catches fire even before the attainment of 500 C.
3) Could someone please run the file and explain this phenomenon to me.

Thank you and I have written the code below:

&HEAD CHID='Liquid Pyrolysis', TITLE='Liquid Fuel Test'  /

&MESH IJK=25,25,25 XB=0, 50, 0, 25, 0, 25 /

&TIME T_END=120./

&REAC FUEL               = 'N-HEPTANE'
      SOOT_YIELD         = 0.037 /

&MATL ID                          = 'HEPTANE LIQUID'
      EMISSIVITY                = 1
      NU_SPEC                    = 1
      SPEC_ID                     = 'N-HEPTANE'
      HEAT_OF_REACTION  = 364.9
      CONDUCTIVITY           = 0.14
      SPECIFIC_HEAT          = 2.2464
      BOILING_TEMPERATURE    = 77.1      
      DENSITY                             = 684
      ABSORPTION_COEFFICIENT = 1000. /
     

&SURF ID        = 'HEPTANE POOL'
      COLOR     = 'RED'
      MATL_ID   = 'HEPTANE LIQUID'
      THICKNESS = 0.1
      BURN_AWAY= .TRUE./
     
&SURF ID        = 'HEPTANE POOL1'
      COLOR     = 'RED'
      MATL_ID   = 'HEPTANE LIQUID'
      THICKNESS = 0.1
      BURN_AWAY= .TRUE.
      AUTO_IGNITION_TEMPERATURE = 500/
     

&OBST XB= 7,9,   14,16, 14,16 SURF_ID='HEPTANE POOL'  BULK_DENSITY = 30 /
&OBST XB= 41,43, 14,16, 14,16 SURF_ID='HEPTANE POOL1'  BULK_DENSITY = 30 /

&SLCF PBY = 15    QUANTITY = 'TEMPERATURE' /

&TAIL /

[Randy McDermott]

The AUTO_IGNITION_TEMPERATURE region you set is only for the first cells off the normal sides of the block.  Once fuel escapes that cell it sees AIT = -273 C.  Adding the AIT has clearly slowed down the HRR for the second block.

The AIT zone feature is meant to help model a piloted ignition zone.  You are doing the opposite.  This is not the intent.

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[Siva D]

My requirement is that the second block should only start burning only when at least one of its outer cells attain 500 degree Celsius. How may I achieve that?

[Randy McDermott]

I think you are confusing "burning" with "pyrolyzing".  I think you want an IGNITION_TEMPERATURE criterion on the SURF.  But that requires you to set the mass loss rate once the temperature is achieved.

On Tue, Jul 25, 2017 at 12:09 PM, Siva D <sivaa...@gmail.com> wrote:

My requirement is that the second block should only start burning only when at least one of its outer cells attain 500 degree Celsius. How may I achieve that?

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[Siva D]

Yes absolutely, I want IGNITION_TEMPERATURE criterion in the surf line. But sadly, it does not seem to work. However, if I set a Mass loss rate, it would be like a self controlled combustion. And moreover, how can I select a mass loss rate? Is it specific for a specific material? Finally, could you please explain the difference between pyrolyzing and burning?

[Siva D]

[Salah Benkorichi]

MLR is the derivative of the mass loss ( how your mass is being consumed with time). One material can have different MLR depending on the testing conditions (i.e. under oxygen or nitrogen atmosphere for example), the heat flux applied to the material, etc.. So,

MLR has to come from experiment data close to what you want to do, ideally perfectly matching  with your set up. Specifying MLR means you specified the HRR, HRR = MLR * HEAT_OF_COMBUSTION.  

Pyrolysis generally means the degradation of your material, turning from virgin material to char + gas, whereas oxidation is turning the char into an ash. As for the burning is how much your material has been burned. 

Hope this clarifies it a bit.

On 26 July 2017 at 05:11, Siva D <sivaa...@gmail.com> wrote:

Yes absolutely, I want IGNITION_TEMPERATURE criterion in the surf line. But sadly, it does not seem to work. However, if I set a Mass loss rate, it would be like a self controlled combustion. And moreover, how can I select a mass loss rate? Is it specific for a specific material? Finally, could you please explain the difference between pyrolyzing and urning?

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[Siva D]

Thank you for explaining. I have an another doubt.

I am burning a block of heptane which has a mass of 480 kg ( Density* Thickness *Length *Breadth) (60 *2*2*2). The complete fuel is consumed within 14.8 seconds. However, the output in the "Fuel test_devc.csv" file contains values more than the initial mass of fuel. How is this possible? Please explain. I have attached the "Fuel test.fds" file for your reference.

[dr_jfloyd]

See BULK_DENSITY in the User's Guide.

[Siva D]

I am still not getting 480 Kg upon completion of fire at 14 seconds. The "Fuel test_devc.csv" file gives lesser values. My code is as follows, please check it and let me know

&HEAD CHID='Fuel test', TITLE='Liquid Fuel Test'  /

&MESH IJK=25,25,50 XB=0, 25, 0, 25, 0, 25 /

&TIME T_END=30./

&REAC FUEL                   = 'N-HEPTANE'

    

&MATL ID                     = 'HEPTANE LIQUID'
      EMISSIVITY             = 1
      NU_SPEC                = 1
      SPEC_ID                = 'N-HEPTANE'
      HEAT_OF_REACTION       = 364.9
      CONDUCTIVITY           = 0.14
      SPECIFIC_HEAT          = 2.2464
      BOILING_TEMPERATURE    = 77.1      

      DENSITY                = 60

      ABSORPTION_COEFFICIENT = 1000. /
     

&SURF ID        = 'HEPTANE POOL'
      COLOR     = 'RED'
      MATL_ID   = 'HEPTANE LIQUID'

      THICKNESS = 2
      BURN_AWAY= .TRUE./
     
&OBST XB= 11.5,13.5, 14,16, 14,16
      SURF_ID='HEPTANE POOL'
      BULK_DENSITY = 60/


&DEVC   XB=0, 25, 0, 25, 0, 25 
        ID = 'Mass of fuel'
        QUANTITY = 'DENSITY'
        STATISTICS = 'VOLUME INTEGRAL'
        SPEC_ID = 'N-HEPTANE'/

[dr_jfloyd]

At each output time, your DEVC measures the amount of N-HEPTANE existing as a gas in region defined by XB at that point in time.  Some of the fuel you are releasing from the surface is burning and any N-HEPTANE that burns is no longer N-HEPTANE, but has been converted to combustion products.

[Siva D]

In that case, how may I track the combustion products for I do not know what the combustion products are actually. Thank you in advance.

[dr_jfloyd]

13.1.1 Simple Chemistry Parameters

When simple chemistry is being used, FDS will automatically create three lumped species: AIR, FUEL, and PRODUCTS. The actual name of the fuel species will be the name given on the REAC line.

The .out file will tell you what is in the species PRODUCTS.

[Siva D]

Cool, my issue is resolved.

[Salah Benkorichi]

Ok, thanks for reporting back.

On 28 July 2017 at 06:20, Siva D <sivaa...@gmail.com> wrote:

Cool, my issue is resolved.

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