Combusting flow with incompressible code?
 

Hi everyone, I have a question concerning the computation of a reacting flow (combustion).

Do you think it is possible to compute a flame using the SIMPLE method?

What I think is that, assuming steady state (so no acoustics going on) and for low-Mach number flows, the gases are essentially incompressible (density differences only due to temperature changes, since the pressure is almost constant accross the flame), therefore I think that it is possible to compute it with an incompressible code (I'm particularly referring to the methodology proposed in Versteeg & Malalasekera or Patankar).

Thanks in advance.

Cheers,
Michujo.

Yes. Most of the combustion CFDs are done using SIMPLE and incompressible.

the use of SIMPLE is not a characteristic of incompressible flows. You can use SIMPLE algorithm even for compressible flows.
The question is more can we use a incompressible solver for combustion problem? If your mach number is below 0.3 I guess we can...

Hi guys, thanks for your feedback.
You're right, I was referring to the use of an incompressible code for combustion, so you pretty much gave a satisfactory answer :). I still have some questions though. For instance: let's say that density depends on temperature, and I compute it as rho=function(T), would mass still be conserved?

My experience with combustion is using a non-commercial LES compressible solver so I was not familiar with the use of compressible codes for combustion. Do you guys know of any good reference that i could check in this regard?

Thanks again.

Cheers,
Michujo.

I would recommend using ideal gas option for calculating density. Most of (if not all of) them should have it.

Thanks yonchong. Actually I ask so because I have in mind developing a 2D incompressible code of my own, so no commercial. Given that pressure is constant (except for the few kilopascals that would drive the flow), the ideal gas law is reduced to a function solely of temperature, is it not? (although I'll have to deal with the change of molecular weight of the gas for a multispecies flow...). Too many doubts!

Thanks.

Cheers,
Michujo.

Pressure may not be changing but still need to calculate density.

By the way, if you are trying to write your own CFD code have a look at these posts.

http://www.cfd-online.com/Forums/mai...-language.html

http://www.cfd-online.com/Forums/mai...tml#post376764

Michujo,

Mass conservation in SIMPLE (or any pressure based algorithm) is achieved in the pressure correction step. As long as the pressure correction step includes the density variation associated with variation in temperature, the mass is "conserved" - I used inverted comma here as mass conservation depends on how good the pressure solver is and the tolerance used. As a good practice I would recommend that you implement a mass conservation check.

I would recommend doing a bit of reading on the derivation of the low Mach number approximation for pressure based equation (google dilatable incompressible low Mach number equation may give you to the right answer). Otherwise I can try to dig out the references from my PhD (unsteady simulation of reactive flow at low Mach numbers).

Julien

Dear all, thanks for your contributions. I will try and check out those references.

Cheers,
Michujo.