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-   -   how to use finite-rate/eddy-dissipation? (https://www.cfd-online.com/Forums/fluent/46165-how-use-finite-rate-eddy-dissipation.html)

 muro September 28, 2007 00:41

how to use finite-rate/eddy-dissipation?

Dear friends,

I've got problem on using finite-rate/eddy-dissipation in the non-premixed combustion simulation. By patching some zones to the high temperature value, I also found that the combustion is not occurred in the chamber. So, please help me to fix this problem.

Thank you very much.

 pghoseju July 26, 2012 07:11

The Pressure-Based coupled algorithm which is a good alternative to density-based solvers of ANSYS
Fluent when dealing with applications involving high shear flow was employed.
The following spatial discretization schemes were used:
 For gradient – the least square cell based method was employed; and
 The second – order upwind scheme was employed for density, momentum, modified turbulent
viscosity and the energy equations.
Under relaxation factor of 0.8 was used for the species, energy and density while a value of 0.6 was
applied for the momentum, turbulent kinetic energy, turbulent kinetic dissipation rate and turbulent
viscosity.
In order to monitor convergence as the calculations (iterations) progressed, the following residual
quantities: continuity, x-velocity, y-velocity, energy, turbulent kinetic energy, k, turbulent dissipation
rate, , and all the species were monitored. The absolute convergence criteria which were set for these
quantities are: continuity 10−3, x-velocity 10−3, y-velocity 10−3 and energy 10−4 for cold flow
calculations. For the reacting flow computations, the initial temperature field was patched to initiate the
combustion process. The absolute convergence criteria for the species were then lowered to 10−6.
The initial calculations were performed assuming that all properties except density were constant.
Using constant transport properties (viscosity, thermal conductivity, and mass diffusion coefficients) is
acceptable here because the flow is fully turbulent. The molecular transport properties will play a minor
role compared to turbulent transport. The assumption of constant specific heat, in contrast, has a strong
effect on the combustion solution, and this property was represented with a polynomial function during
the reacting flow computation.
The solution was then initialized. Thereafter, the Full Multigrid (FMG) feature was run. Full
Multigrid initialization often facilitates an easier start-up, thereby, obviating the need for Courant-
Friedrichs-Lewy (CFL) condition ramping, consequently reducing the number of iterations required for
convergence.

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