[chemix]

hi all

I have designed a liquid N2 cooled helical coil pre cooler where He gas at 300 K flows through the helical tubes dipped in Liquid N2 bath (no solid boundaries. Fluid zone region) and get cooled to 80 K while passing through LN2 maintained at 77 K.

He gas has some heat load which will be transferred to LN2 and it results in increase in temperature of LN2. I need to determine the temperature distribution in the helix. This is a conjugate heat transfer problem & this is how I proceeded.
The geometry was created with 2 fluid zones (the helium zone inside helical coil and the Ln2 bath region) and one solid zone (helical coil pipe) (1 part 3 bodies)
The mesh was created and the following BC’s were given
1) He in (V inlet)
2) He out (P outlet)
3) Interface at the inner wall of helix where there is contact between gas and pipe wall (Coupled)
4) Interface at the outer wall of helix that is dipped in LN2 (Coupled)
5) Wall condition for helix which is not in contact with LN2 (Convection at 300 K)
I assigned fixed temperature (77 K) to LN2 cell zone condition. Apart from this no other properties were defined for the cooling bath

My difficulty

1) When assigning boundary condition for walls, I want to provide only convection (S. No.5) so can I put all other values corresponding to conditions (heat flux, radiation, temperature etc) as zero?

2) The density of LN2 will change during heating. So I considered density based solver. Divergence was detected in amg solver

3) Also there are 2 fluid cell zone conditions (For he and liquid nitrogen) and when I tried assigning different operating densities corresponding to each fluid in operating conditions, I am not able to do it.

Any suggestions? thanks