Learning Forum

[dandelion]

Sorry, using fluent for the first time, so no prior knowledge here - I'm lacking some basic understanding.

I have a tank full of water. The wall of the tank is made up of different materials and doesn't have the same width everywhere so I meshed the tank and the water.

The tank is surrounded by air of room temperature. I would like to start the simulation with warm water of which I know the temperature at the wall of the tank (there are temperature sensors integrated in the wall of the tank). Of course, I can also give a guess for the temperature of the water in general. Then, I am just interested in the temperature of the water over time and would like to see how it cools down, how warm it is at different places inside the tank.

Now I'm a bit lost considering how I set the initial conditions considering the fact that the water is not a constant heat source but is heated prior before filled in the tank (prior to start of the simulation) and for the duration of the simulation, no additional energy is brought into the system.

I would really appreciate your help and sorry if I did not explain this clearly, I would be happy to include more explanation if needed. Thank you!

[Rob]

It's going to be a transient calculation, so will take a fair time to run. The initial condition will be stationary water (probably) at a set temperature. So either initialise the model to suit (standard initialisation) or use the Patch option once you've initialised the model. Note, buoyancy effects may be significant and you need to read how the various wall boundary options work for the outer skin of the domain. Do NOT mess with or alter any coupled walls without understanding why and what you're doing.

[dandelion]

Thank you! Yes, I am using transient calculation and I've noticed it takes some time to run. I have tried the Patch option but it didn't seem like the water cooled down at all, I could only notice the changes in the temperature of the tank around the water as it got heated by the water. So I wasn't sure whether this option was a correct way to do it. But in general the Patch option gives a certain domain an initial temperature (and not a static temperature), is that correct? Then it would be what I need. If I can tell correctly, I cannot give different temperatures to different parts of the model in the standard initialisation.

[Rob]

In standard initialisation you set one temperature (there are ways around this but not at this point). Then use patch to set another constant temperature in different zones/registers. If the tank is heating up but nothing is then cooling check the external boundary surface. Default is adiabatic, and I suspect you want a convection boundary: you need to estimate the external htc.

[dandelion]

Thank you so much! Another question - I put the tank inside a box of air when creating the geometry, is that a correct way to do it? Or do I not have to mesh the surrounding air?
And yes, I would like to set convection - however, so far all boundaries appear as coupled walls (so this setting is not available and plus you have already warned me not to mess with coupled walls). Can you suggest where to find explanations of these settings? I do not yet understand the meaning of it and when it should be used and unfortunately, I couldn't find any really helpful explanations.

[Rob]

Please can you sketch out what you're trying to model. Pen on paper is best, and write on all information that you know, or need to know.
Regarding the various terms. At the top of the page you'll see a Courses tab. Do a few of the fluids ones, basic flow and heat transfer plus anything else that looks relevant or interesting. Also do a few of the tutorials in the Help system, the first few are more generic and then pick a couple of the heat transfer ones. That'll help with the basics.

[dandelion]

hey, thank you so much! I think I understood now that coupled walls is exactly what I need as I want to consider the transfer of heat from the warm water to the air and from what I understood now, with a coupled wall, fluent sets all the correct coefficients itself based on the material properties of the two different materials.

[Rob]

Correct, and based on the local flow field.