Any idea for constant thermophysical properties for fluid (water) and P/T decoupling in governing level?

Hello.

I'm trying to modify TOUGH2 codes for constant thermophysical properties for fluid (eos1) and P/T decoupling in governing equation level.

In order to do this, I've fixed density, viscosity, specific enthalpy and added water compressibility and water thermal expansion coefficient to fluid mass term.

However, it is not working because of convergence problem in energy balance equation.

As I said, I think mass balance is not a problem, but energy balance is something wrong because of constant properties which are against to thermodynamics.

Anyone has an idea for avoiding this problem or some tricks approaching constant values in fluid flow and non-isothermal condition.