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Outlet boundary condition =/

hello every one:

im working in a simulation of turbulent non premixed flames with the flamelet model (its a cylindrical combustion chamber of a turbine), and im stuck, with the outle boundary condition, im really confused, this its my inlet boundary condition:

pressure inlet for gas: 14.31bar
pressure inlet for air(primary,secundary,dilution,refrigeration,swirl ing): 6.40bar

my question its, what parameter i should to set, for the outle boundaray? with that inlet condition? i donthave much information about the outlet of the chamber.. i dont know if i cant set the flux of mass?cuz i can determine the mass flux, for the all inlet...in with the conservation of mass...m(oxidante)+m(fuel)=m(products), its possible this? set pressure for inlet, and mass flux for outlet?

best regards

 ghorrocks December 2, 2009 17:47

Sounds like you know the pressure at the inlet so use a pressure inlet there. I don't understand your comments about mass flow, but if you know the mass flow you should use that as the outlet condition.

sorry for my inesperience in CFD modelling, i post this, with out no sleeping it all. this im trying to said.

For one hand, the only paramaters that i have measured its the pressure inlet for the and for the fuel (no information about the exit).For example, in thermodynamics, in a thypical brayton cycle balance for a combustor chamber, its typical to use, the pressure of descharge from the air compressor to be the inlet and outlet boundary condition of the chamber.
but here, i have 2 different pressure for both streams (air and fuel), so my question from this point of view its: how can i ,calculate or aproximate a pressure value for the outlet boundary?.

for the other hand, with thermodynamics and a little of mathematical mechanism, with the pressure's inlet i was find, temperature and mass flow, so i has:

Pressure inlet gas
temp inlet gas
mas flow gas

pressure inlet air
temp inlet air
masflow air

im working with the tutorial of combustion and radiaton documantation of cfx 10 ( so its my guide,and there the inlets are setting by velocities). for conservation of mass, i have the 2 inlet mas flow, then i have the outelet mas flow:

mas flow (air)+mass flow(fuel)=massflow (products)

so my question its, its possible, to set using flamalet model, my boundary condition for the outlet with a flow mass value? can i use this:

a)

mass flow inlet air and fuel
temp inlet air and fuel

mass flow outlet

b)

pressure inlet for air and fuel
temp inlet for air and fuel

mass flow outlet products

i think this question are very stupid, but i dont have anyone to ask this "stupid ideas", im looking answers like...yes, thats its correct, no that sucks dont use it.Cfd its a new stuff here, im the first one working with this.:p

aaaaaaaaaaaand my finally question XD: im studing very much, the flamelet model, RANS code donde solve the temperature or mixture fraction equation (just post processing). but if you do, the tutorial for combustion and radiation, and when you are in the CFX-SOLVER running this, you can see, H-EQUATION and T variance?, but flamelet dont work with the energy equation in the RANS code? what its this? (its possible to be, a equation from the discrete tansfer metod for radiation?).:(

thanks very much for your time and patience.
best regards:D

 ghorrocks December 4, 2009 06:09

Use option b. Option a will not work as the pressure level is not set. Have a read of the documentation about obtaining convergence and which combination of boundary conditions to use.

Oh, and get some sleep. Fire off a big CFD job and go home. Things will be clearer tomorrow.