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-   -   How come... the fuel is still igniting at 300K? (http://www.cfd-online.com/Forums/fluent/113361-how-come-fuel-still-igniting-300k.html)

diamondx February 18, 2013 13:17

How come... the fuel is still igniting at 300K?
 
Hello there,
I have started a couple of days ago to work on combustion simulation. i have done recently the tutorial with the liquid fuel combustion using the non-premixed model and pdf table generation. where liquid droplet of c5h12 are injected in an air stream flowing at 650K. until now everything seems ok.
Next i wanted to play more with it, so i decided to change both the fuel and the air's temperature and make it 300K. In reality it can happen. no ignition should happen there... just fuel being injected... but when i ran my simulation, i ended up with a flame ! i don't know why , cane someone please explain to me what I've done wrong ...

Thanks a lot

stuart23 February 19, 2013 11:21

Hey Ali, because you're not a CFD beginner, this may be a stupid question; but did you reinitialise the domain with a constant temp? Or just change the temps of the inlets?

Stu

diamondx February 19, 2013 11:25

thanks stuarts for answering,

I didn't patch any region with a specific temperature, also temperature inlet of both fluid is 300 K.
Long time no see btw, have you been on vacation ?

shk12345 February 20, 2013 10:05

Hi dimond
 
Basically what you are saying is not digestive as no reaction should happen at that temperature.

But still i think you should check the equation that you are using in excel or matlab .

Plot k(k=k0*e6(-E/R*T) v/s T and see after what temperature the reaction starts . May be something wrong with the equation.

Let me know the results.

Regards
shk

Jenny_W February 20, 2013 11:49

Hi,
Just wondering, did you regenerate your PDF table? It may be because the PDF that is used in the simulation is still from the old temperatures.
Good luck,
Jenny

diamondx February 20, 2013 12:13

Thanks guys for you answer, i made sure it's the new pdf table. both the air and the c5h12 enters at 300K. i didn't use any patch for ignition. could it be that the turbulence brought suffiant energy, or the result are wrong ?

https://dl.dropbox.com/u/35161486/how_come_1.PNG

And here is the temperature profile with a maximum of 1560 K:
https://dl.dropbox.com/u/35161486/how_come_2.PNG


A screenshot at the droplet:
https://dl.dropbox.com/u/35161486/how_come_3.PNG

Thanks a lot

Jenny_W February 20, 2013 12:27

Hi Ali,
One reason I can think of is that non-premixed combustion model assumes "mix is burnt", that is, reaction occurs once the fuel and oxidiser mix. That is why you don't have to patch a region of high temperature (ignition point not required). You may want to try species transport model (eg. laminar finite-rate chemistry) and compare the results with your non-premixed combustion results to see if they are the same.
Good luck,
Jenny

diamondx February 20, 2013 12:33

thanks for you answes, good luck for yout oo

msaeedsadeghi February 22, 2013 00:14

Quote:

Originally Posted by diamondx (Post 409089)
Thanks guys for you answer, i made sure it's the new pdf table. both the air and the c5h12 enters at 300K. i didn't use any patch for ignition. could it be that the turbulence brought suffiant energy, or the result are wrong ?

https://dl.dropbox.com/u/35161486/how_come_1.PNG

And here is the temperature profile with a maximum of 1560 K:
https://dl.dropbox.com/u/35161486/how_come_2.PNG


A screenshot at the droplet:
https://dl.dropbox.com/u/35161486/how_come_3.PNG

Thanks a lot

Please let me know your injection settings.

diamondx February 22, 2013 09:59

My injection setting:

i'm running steady injection . Cone (droplets) with 60 degrees, and a velocity of (20 m/s) about the velocity, i don't know if it's enough. the velocity of the air where the injection starts is 10 m/s. there is also some circulation there. (the magnitude of the velocity in the tutorial is about 80)
The mass flow rate is 0.00249 kg/s. i gave it 60 particles. i also set the number of tries to 10.
Something i don't understand, it writes in the console that 600 injection are tracked, but 600 escaped. if they escaped, it means they didn't have the time to evaporate, so why is there a flame !!!
i could have solved the problem of evaporation by reducing the diameter or increasing the max number of time step.
these setting represent a mix of tutorials and my own decision as i just started. i'm not trying to replicate any experiments because simply i don't have any data.
if somebody have some realistic setting applicable to combustion chamber please let me know...
In general i don't understand why there is a fire at first place.
Sorry for asking too much.

Thanks a lot

msaeedsadeghi February 22, 2013 10:15

Just change injection angle to 30 degree (half angle should be set).

diamondx February 22, 2013 11:10

Quote:

Originally Posted by msaeedsadeghi (Post 409522)
Just change injection angle to 30 degree (half angle should be set).

Thanks for answering.
that has to be applied to start angle and stop angle(air blast atomizer), where you use only half. but i think in my case the angle i gave it is really the one i see in particle plot. (from 0 degres to 90 from x axis to y axis, while the symmetry is along z)

msaeedsadeghi February 22, 2013 11:26

I really didn't understand. I have confronted with this problem before. The only problem is with your injection settings. Just change them to see their parameters effects.

diamondx February 22, 2013 11:28

ok i will.

sanjeetlimbu April 27, 2015 16:38

5 Attachment(s)
Hi
I am trying to do auto ignition for nheptane/Ar/O2/N2 mixture... with adiabatic wall
Can anyone suggest how to do ..ignition

1. I tried the laminer - species transport: It shows no rise in temp / pressure (I did the cold flow calculation + patch temp 1400K and species reactant), as there is no combustion even after patch temp... just increase in temperature as in attached snapshot

2. I tried the flame let model in Partially premixed but that show error in the unsteady initialisation that the mixture fraction limit is grater than the largest domain fraction even as I entered 0 in it

I tried using the cold flow+ patch at 1400K the body -part in mesh , but no combustion observed , only the temp raised....

I tried then 2900K/40atm patch but no raise observed in products _ CO2 or H20 in mole fraction...

In both cases the temperature just climbs to the patch value and there is no temp /pressure raise after than


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