# Convective heat transfer model and density

I am modeling a closed water tank (actually a pool) at a specific water temperature and introducing a higher temperature water at the top of the tank. Over time, I would expect the inlet to have a lower density than the rest of the water body. However, even with “Compressible” switch on, the density remains constant at the value set for the material “Water” throughout the entire pool and never changes with time. The net effect is that two different temperature water areas are mixed fairly quickly.

However, if I use a multi-phase model with two different densities of water, the model behaves as exactly as expected with a distinct separation region of the two water temperatures and as time progresses, the top contains more lower density water and the bottom, less higher density water.

Does the convective heat transfer model take into account changes in density due to temperature differences? It appears that it really doesn’t although I would have expected it to. The field density plot shows up as constant over the entire volume for all time steps.

Also, with a convection model, why is the water density specified explicitly instead of calculated? Temperature should affect the density, correct?

https://www.simscale.com/workbench/?pid=8215532880343646018&mi=spec%3Ad90041df-6f06-44a0-92d1-20c3206b07f0%2Cservice%3ASIMULATION%2Cstrategy%3A311

What am I missing?

Hey @mas985!

Does the convective heat transfer model take into account changes in density due to temperature differences? It appears that it really doesn’t although I would have expected it to.

From the documentation: “This analysis type is used when temperature changes in the fluid lead to density variations and movement of the fluid due to gravity. This solver can be used for natural convection but also for forced convection, when flow motion is induced by external forces.”

@CFD-SQUAD, want to help out our user here?

Cheers,

Jousef

Thanks, I did read that which is what prompted the question. Also, the switch is labeled as “Compressible” however water is in-compressible so I thought maybe it did not work with water or perhaps there is a parameter that I missed.

[UPDATE] I did a run with “Compressible” off vs on. In the off state, the passive species does not propagate as it should. With Compressible on, the scalar does propagate but again the density remains uniform throughout the simulation.

I figured out one problem. The default equation of state is constant rho for Water which is actually surprising given the switch for compressible is on when added. Once I changed that to perfect fluid, then the density of the fluid changed with water depth as it should.

However, I think this might make the fluid compressible? How does one take into account the density change of water due to temperature (and pressure)?

Ok, so I really simplified the geometry and now have a cylinder 0.75m by 1.5m tall. I ran two natural convection simulations one with cold water in the upper half and warm water in the lower half and visa versa. There is no forced convection in this setup and I am using the Constant RHO equation of state . The results are nearly identical after 60 min. So it is clear that the default Constant RHO is not taking any density into account even thermal. Not sure why this would be an option at all. Also, not sure why the fluid boundary creeps up on the sides in both simulations.

Project link:

https://www.simscale.com/projects/mas985/natural_convection_validation/

Working on some other equation of state.