CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > Software User Forums > ANSYS > CFX

Detonation, Unburnt Abs.Temperature and difference between BVM and ECFM model

Register Blogs Members List Search Today's Posts Mark Forums Read

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   December 12, 2018, 22:58
Red face Detonation, Unburnt Abs.Temperature and difference between BVM and ECFM model
  #1
New Member
 
Alex
Join Date: Jan 2018
Posts: 9
Rep Power: 6
Viento is on a distinguished road
Hello. I am working on a detonation wave simulation in premixed kerosene-air mixtures.
First of all, I studied publications like this (https://www.sciencedirect.com/scienc...40748916302802) and calculated ignition delay time τ(P,T) method which gave a great resemblance to theory. I used this correlation in BVM with autoignition model. It is important that as the reference temperature I chose not the temperature of the mixture, but the temperature of the unburned fractions. (Otherwise, any laminar burning immediately goes into detonation - it contains local temperatures over 2000K in flame front. But this is an average temperature. I suppose that reacted molecules heat the unreacted slowly.)

So, I got flat detonation wave with characteristics (velocity, pressure, temperature) very similar to known data. Results with ECFM and BVM were very similar.
First of all, I found out the following. Adiabatic compression to 30-50 atm heats the mixture from 300K to 800-900K. But τ(50atm,900K) = 1ms, however, the ignition time inside the detonation wave should be several microseconds. This implies much higher temperatures, 1500-1600K. Therefore, there must be some additional temperature, which is taken from incomplete combustion in a "thin supersonic flame" that exists over the autoignition wave.

Then I discovered two weird things:
1. Detonation with a BVM is significantly less stable than with a ECFM. A small expansion is enough for it to fade out, as if the critical diameter is a several meters, not 5 cm (known data). I do not know, why it is so. Perhaps, Unburnt Absolute Temperature variable is calculated incorrectly (In some simulations in expansion wave behind the reaction, it reaches weird values ​​like 1K.). Perhaps, the turbulent speed of a "thin supersonic flame" is calculated incorrectly. Perhaps a method itself to take Unburnt Absolute Temperature as a reference temperature is incorrect. With ECFM everything works well.
2. But with ECFM everything works too well! If you reproduce the structure of the detonation wave and completely turn off the auto-ignition model, the simulated detonation wave continues to exist and give exactly the same results. This is a bit confusing. (Perhaps I don't quite understand the logic of the ECFM, but can it itself predict ignition time?)

So, questions:
1. Which of these should I trust?
2. What is the actual "Unburnt Absolute Temperature" variable calculation formula for BVM and ECFM?
Viento is offline   Reply With Quote

Old   December 13, 2018, 04:16
Default
  #2
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 17,170
Rep Power: 134
ghorrocks is just really niceghorrocks is just really niceghorrocks is just really niceghorrocks is just really nice
I know little about combustion/explosion modelling so cannot help you there.

But I can give a little bit of general CFD advice which is that make sure you don't compare one model to another or come to conclusions about what is going on until you have validated your model. Do the basic checks that you are adequately converged, your mesh is fine enough and that your time steps are fine enough. You should check all these things with a sensitivity analysis.

Looking at the results of an under converged or poorly resolved simulation is meaningless, it can tell you anything.
__________________
Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.
ghorrocks is offline   Reply With Quote

Old   December 13, 2018, 23:28
Default
  #3
New Member
 
Alex
Join Date: Jan 2018
Posts: 9
Rep Power: 6
Viento is on a distinguished road
The convergence is good enough, I think. Target 1e-4 is easily achieved.
I used the following principle of proportionality between the size of the grid and the timesteps: With the expected speed of the transition process, several timesteps are required to go through one grid element. It is quite simple in the case of the detonation of the air-kerosene mixture, since its approximate speed is known in advance.
Attached Images
File Type: jpg 4e3f655a6fc42_1.jpg (132.6 KB, 13 views)
Viento is offline   Reply With Quote

Old   December 14, 2018, 17:49
Default
  #4
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 17,170
Rep Power: 134
ghorrocks is just really niceghorrocks is just really niceghorrocks is just really niceghorrocks is just really nice
Quote:
The convergence is good enough, I think.
You think? Does that mean you are guessing? If you have not done a sensitivity analysis to check then you are just guessing and you could be completely wrong. Do a sensitivity analysis to make sure your convergence is OK.

Your comment on time step implies you used a Courant number like parameter to set the time step. Are you aware that as an implicit solver, Courant number is of limited use in setting time step size? Again, a sensitivity analysis is the way to set the time step size for an implicit solver like CFX.

I apologise for being pedantic about these things, but looking at the results of an inadequately converged and poor tie resolution simulation is pointless as the results are rubbish. It is very common to find that unexpected results are just numerical errors from poor simulation setup.
__________________
Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.
ghorrocks is offline   Reply With Quote

Reply

Tags
bvm, detonation, ecfm, unburnt

Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are On
Refbacks are On



All times are GMT -4. The time now is 19:48.