h & Nu : counterflow concentric tube HX
 

Dear Friends,

I am simulating counterflow tube in tube type straight as well as helical HX with supercritical co2 on annular side and subcritical co2 on central tube side. I know massflow rates, temperatures, pressures for both supercritical and subcritical co2. The propertise of CO2 are entered in terms of polynomial governing equations in terms of temperature. Temp variation is very large for both fluids. I am finally going to compare CFD results in terms of either local 'h' or 'Nu' with analytical one.

Also please guide me in defining the boundary conditions for tube wall (of central) between two fluids since heat flux and temperature are also not constant.

Can anyone will explain me method to correctly get local 'h' and Nu, which will match with analytical results.