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-   -   Exhaust Gas manifold simulation (http://www.cfd-online.com/Forums/cd-adapco/56343-exhaust-gas-manifold-simulation.html)

 shrinath November 6, 2007 06:30

Exhaust Gas manifold simulation

Dear friends, I have a doubt in starccm+ software. i have to compare the results of fluent with starccm+, so i imported the mesh from fluent and directly simulating with starccm+ without changing the mesh.

i simulated a exhaust gas manifold of 8 cylinders engine, and i used only outlet from engine 2 and engine 8 . So the geometry is 2 massflow inlet manifold(from cylinder 2 and 8) and comined to one outlet manifold ,where i set pressure outlet condition. The massflow is instationary i.e massflow varies according to the timestep , also temperature at inlets various acc. to time step. In the outlet i have set pressure outlet condition also changes according to time step, the temperature too. I have splited the inlet manifolds into 6 region, 6 for 2nd cylinder manifold and 6 for 8th cylinder, also 4 region for outlet manifold.

My problem is after 2 cycle i.e 720° of simulation , the values of massflow and temperature in various region is same as in fluent simulation, but the pressure is 2 bar less than in fluent in inlet regions and 3 to 4 bar less in outlet manifold. two possiblities are that i have set some boundary conditions wrong or the starccm+ calculating it wrong.

the other BC are fluid is air , ideal gas, intial pressure and temperature 3 bar and 1200k respectivley, and the relative pressure is 1 bar.

please tell me why i have 2 bar less pressure than actual pressure.

regards shrinath

 Donald November 6, 2007 17:54

Re: Exhaust Gas manifold simulation

one obvious point: don't use the bad tet-meshes fluent is tuned with.

- Have you selected the same wall function, turbulence model?

- Have you initialize with static, piezo or total pressure, absolute/relative? Is it the same in both?

- Shouldn't you use a non-reflective b.c.?

- you should send your .sim to the local cd-adapco office for the pre-sale guys to have a look.

D

 Pauli November 6, 2007 19:16

Re: Exhaust Gas manifold simulation

Sounds like you are solving a different problem. You do not have the same pressure at either the inlet or the outlet. Yet you are using ideal gas. So your density is also wrong throughout the domain. This sounds to me like a reference pressure issue.

Sorry but I'm not a CCM+ user. So I can't tell you where to look.

 JohnKpax November 11, 2010 13:18

Hello,

I am interested in the simulation of exahust flow's simulation. Could i make you a question? How do you apply the temperature wall of the pipe? do you use a constant value? or some kind of temperature law? Thanks.

 RobertB November 12, 2010 08:47

How can you have a 3-4 bar error in pressure in an exhaust manifold? Unless you are running a very highly loaded turbocharged diesel engine the absolute pressure in the manifold will be less than ~2bar even if turbocharged.

As a comment I doubt you will get very good results running massflow inlets and exhaust pressure outlets, all time varying. I assume you got these from a 1D code. When you start the simulation your exit will be reacting to exhaust pulses that have not yet arrived which will send pressure pulses back up the system. In addition. if your speed of sound/... is slightly different then you will alsways have problem with out of phase inlet and outlet BCs.

I believe that integrating one of the 1D codes that couples with CCM+ might help.