[Kashif Rashid]

Hi every body;
I am simulating heat exchanger, with fins inside & flow channels, 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
-Density-----incompressible ideal gas
-Cp ---------Piecewise polynomial
-Pressure based solver--- segregated/simple
-Inlet -------- Mass flow inlet
-Outlet -------- Pressure outlet
-Accuracy ------all second order
-FMG setting---see attached file
-Mesh---maximum skewness = 0.64

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 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. In actual situation, temp. between inner walls of two heat exchangers are about 1007 k.

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.