# Density setting of the fluid phase

Hi, I got a little confused about the fluid density setting.
If we are simulating a cold flow, we can simply take the gas as incompressible and set
a constant density to the gas phase.
When we are simulating a gas flow with chemical reactions, there are new gas generation and the pressure variation caused by Temperature changes so we can set it as ideal gas flow.
However, when we are simulating a liquid with species transported process, the liquid is incompressible. But its density can definitely change. Here, the ideal gas flow is also not right. Should we write a UDF to caculate?

Yes. You need a UDF to calculate the density. The simplest way is to search where the EOS function is called and replace that part with your own density function. check Code Structure of CFD-DEM in MFiX

If the density change is small, a constant density should also work.

Thank you very much. This is really helpful.