## Fluids

#### Problem with transient thermal analysis of 2d fuel tank filled with jetfuel

• Chun-Wei Tsai
Subscriber

I am doing transient thermal analysis of jetfuel for long haul flight, I have set the boundary condition of walls with heat transfer coefficeint of 15 and temperature of -56.5degree C. And the initial temp. of jetfuel inside being 15degree celcius, the results doesnt really make sense as I expected the temperature would cool from outside to inside gradually. The I have set jetfuel properties with the default piecewise polynomial defalt for jet a 1 from the database and turn the gravity on. Also I have chosen laminar ,energy model for the simulation.

• Chun-Wei Tsai
Subscriber

This is the residuals plot

• Chun-Wei Tsai
Subscriber

and the meshing

• Rob
Ansys Employee

And density varies with temperature in a closed domain? Where is the extra volume coming from or going to?

• Chun-Wei Tsai
Subscriber

I didn't think of that at the first place, maybe that's the problem. I have tried using constant density and the result of contour plot showing something like this, I wonder how reasonable is the result.

• Rob
Ansys Employee

If it's constant density you'll not see any convective mixing. Just diffusion & conduction.  With changes in density you also need to account for volume: there's a reason for the Bousinesq approximation.

• Chun-Wei Tsai
Subscriber

Thank you for your answer!! If I'd like to use density change, what kind of change in my model would you recommend? Will it be much more complex? Or do I just need to leave some empty space

• Rob
Ansys Employee

You'd either need Bousinesq (read for why) or add a pressure boundary somewhere to let the solver add/remove a tiny amount of mass to balance the volume.

Leaving a "space" would require a multiphase model, and that's another layer of complexity that you may not want, or need, at present.