[RaiderDoctor]

Check out this image.

[ywan168]

Originally Posted by RaiderDoctor

Okay, let's attack your issues one at a time. Sound good?


So, the first and major issue I see is that your mesh is not good at all. There is no way you can expect a usable answer when your elements are nearly as big as your geometry. That isn't really how CFD works. To give you some perspective, simulations ran on blood vessels can contain anywhere from 1 million elements, to 10 million, depending upon the simulation. And these are for structures that are significantly smaller than your described pipe-annulus setup. So, you are going to need a much finer mesh before you start showing any accurate data.


The next issue lies in your geometry. Why do you need to simulate the whole pipe? I get that you want to be as accurate as possible, but this is computationally inefficient to perform. If you wanted to run on the bottom one-foot section (where a lot of the interesting thing happens) that'd be a much more efficient simulation. I get that you want to let the computer solve the equations for you, but you need to drastically shorten your fluid domain, otherwise this will waste a lot of time. I would make the assumption that at a certain distance away from the circulating region, the fluid can be thought of as fully developed, and calculated fairly easily by hand. Not saying I want to, just saying that it doesn't seem awful.



Also, as a side not on geometry, you can actually simplify it since it's a pipe. Instead of taking the entire cross-sectional area, only model one half of that since it's axisymmetric. This way, you save on resources, and can produce a much finer mesh. Model the middle line as "symmetry" and the solver will automatically solve it appropriately.



One final note, as you will notice, your mass in and mass out graphs do are not only mismatched (meaning that you are not satisfying continuity) but they are also taken at every time step. Try to take at every iteration, as this will help you see how the values change over the course of the simulation and help you better judge convergence.

Dear Jason,

Thanks for your reply. I set this geometry because the temperature change along depth is really small. Also if I just model the bottom part, I don't know the inlet temperature though.

[RaiderDoctor]

I think you'll have to run some hand calculations to approximate the inlet temperature. Unfortunately, it's been a while since my heat transfer days, so I won't be of much help.

[ywan168]

Originally Posted by RaiderDoctor

I think you'll have to run some hand calculations to approximate the inlet temperature. Unfortunately, it's been a while since my heat transfer days, so I won't be of much help.

Hi, I have another question. If I know my surface T is 40F, and inlet T is 80F. In Operating Conditions, which value should I input for Operating Temperature in Boussinesq parameters?

Thanks,

[RaiderDoctor]

Unfortunately, I'm not sure. I don't work with the Energy model at all, and I couldn't even find the operating condition you were looking at.

[ywan168]

okay, thanks all the same.