[Kashif Rashid]

Hi every body;
I am simulating heat exchanger, with fins inside & flow channel, serpentine flow, 18 passes each sides. Energy of Flue gas (N2,CO2,H2O,O2) is being used to heat the process air.

1. Flue gas entering from top at 1000 k (0.002355 kg/s) leaving at the bottom 927 k (Re# 1913, Mach# 0.06045), 20 m/s at the entrance

2. Process air entering from bottom at 300 k (0.00323 kg/s) leaving at the top 996 k (Re# ~7000, Mach# 0.146), 50 m/s at the entrance

I am using following scheme for simulation
-Energy--on
-Steady
-Laminar
-Species transport
-Pressure based solver---both tried segr. & coupled
-Tried both---Velocity inlet & Mass flow inlet
-Outlet----Pressure outlet
-Mesh---maximum skewness = 0.64

After doing all above stuff, solution is not proceeding ahead, facing errors like
# Divergence detected in AMG solver: temperature -> Increasing relaxation sweeps!
# Divergence detected in AMG solver: x-momentum -> Increasing relaxation sweeps!

I am stuck, desperately looking for help, please suggest, what should i do to get converged and reasonable solution??

I have not tried density based solver, also treating problem as a incompressible flow despite large temperature variation which might effecting the density of flue gas and process air. What is your opinion on that as well.
Thanks in advanced.

[LuckyTran]

This type of occurrence is common when you have material properties that are temperature dependent. You initial guesses for the temperature & velocity fields are lousy, causing them to blow up quickly. Two suggestions:

1. What equation of state are you using for each? Constant or variable properties? I would try switching all the equations of state to constant properties (density, specific heat, etc).

2. I would also disable the energy equation entirely, and run a constant density simulation to get a reasonable fluid flow solution.

Try to see if the problem still diverges when you try both of these. Basically you're testing to see if the solution advances at all by removing the hard stuff. If it works then you can slowly add the complexities back into the problem.

[Subhasree]

Hi,
I am modeling a 2-D wedge in hyper-sonic flow having the following free-stream conditions:
Mach no-9.59
Temperature-185.6k
Pressure-36.5Pa
I have used the following setup in fluent post:
Desity based
fluid-Ideal gas
standard initialization from farfield
I tried running the 1000 iterations initially...in which the continuity and energy equations are hardly changing while the x velocity is having a negative slope(convergence), the y velocity is having a positive slope(divergence)..i having tried reducing the cfl value...it is showing the same trend as I increase the number of Iteration.....I have attached snap shots of the mesh and residual plot.
plz suggest!!

[Kashif Rashid]

Originally Posted by LuckyTran

This type of occurrence is common when you have material properties that are temperature dependent. You initial guesses for the temperature & velocity fields are lousy, causing them to blow up quickly. Two suggestions:

1. What equation of state are you using for each? Constant or variable properties? I would try switching all the equations of state to constant properties (density, specific heat, etc).

2. I would also disable the energy equation entirely, and run a constant density simulation to get a reasonable fluid flow solution.

Try to see if the problem still diverges when you try both of these. Basically you're testing to see if the solution advances at all by removing the hard stuff. If it works then you can slowly add the complexities back into the problem.

As you directed, i started with simplest one, adding complexity bit by bit. I have reached to a converged solution. But this converged solution, not fulfilling my requirements, giving reasonable value of air heating, not good for flue gas temperature variation. I have attached my work, mesh, simulated temp. contours, experimental setup with required values. Temp. variations are exist after simulation but not shown by temp. contours.
Secondly, if i put zero heat flux on the outer and inner walls, solution converged quickly, but when i specify some temp. on the walls it didn't come to convergence, iteration going on and on.
I am looking for help to get desired solution. Thanks in advance.