Simulation of de-humidification of air and change in quality of refrigerant

[Thisani]

Good day Autodesk community,

I have some difficulty understanding some of the inputs on Autodesk CFD 2016 and how they affect the results, can you please assist me with answers the below:

1] I have modelled an evaporator and have used R-134a as my refrigerant. I specified the temperature (-10*C), mass flow (0.026kg/s) and quality (0.2) of the fluid and made the fluid "variable" to enable it to change state. I had initially assumed that the program knows at which pressure (200.6kPa) the refrigerant should be running based on the enthaply vs. pressure chart to achieve my specified criteria. I have ran the simulation twice, first time without specifying the pressure of the refrigerant then second time I specified the pressure of the refrigerant and the results I have got are completely different in terms of cooling capability. The results achieved when I do not specify the pressure seem more inline with what I would expect, when I specify the correct pressure the refrigerant does not change state / reach super-heat. Please can you advise which boundary conditions need to be specified for this simulation?

2] I am modelling the dehumidification of air as well as the change in quality of the refrigerant. When I enter the solver tab it only allows me to solve for one or the other, I cannot do both. I have ran this simulation with just the humidity selected can I expect the software to correctly predict the phase change of the refrigerant as well or will I have to do the two simulations separately to determine phase change and dehumidification?

Would appreciate any comments,

[yotamalfandary]

hey

could you post the simulation project?

I'm trying to set a refrigerant simulation for my model, but the simulation 'stops unexpectedly' every time

maybe i'm doing something wrong

[Amal.C]

Hi @yotamalfandary,

Would you please attach your support file (. cfz) so I can help you here correctly setup your model? 🙂

Thanks
Amal

[yotamalfandary]

Hey,

Thanks for your assistance

[Amal.C]

Hi @yotamalfandary,

Thanks for the attached file.

After investigating the model setup, I noticed that you applied your BC (boundary conditions) of the inlet and the outlet at the surface of the copper (solid) instead of the refrigerant (fluid).

You need to suppress (do not mesh) the external parts of the copper tube in a way that you make sure that the refrigerant is in contact with the external environment. Then, you need to apply the BC to your openings.

Adopting this, you will avoid applying internal BC to your model.

Please also set your refrigerant and water environments to "Variable".

I hope this helps.

Thanks,

Amal

[yotamalfandary]

Hey again, and thanks for your assistance!

I implemented your suggestions, and the analysis runs.

BUT 🙂

I was not able to suppress only the external part of the copper tube,

I could only suppress the hole Volume, Is there a way to suppress a surface?

because right now, when the volume is suppressed the coolant cannot draw heat from the environment.

why am I supposed to suppress the copper?

[Amal.C]

Hi @yotamalfandary,

This is great! 

you need to cut a small part of the tube in front of the openings in the CAD software so that the tube has in total 5 parts: Two external, two in front of the openings and one internal. In this way, you make sure to keep the tube walls necessary for your study while avoiding to apply internal BC. 

Does this help?

Thanks,

Amal

[yotamalfandary]

Hey again,

I followed your instructions, if understood right

could you look at the attached support file? and tell me why does it stops unexpectedly?

thanks in advance.

[Jon.Wilde]

I think that the issue you have is the internal boundary conditions.

You need to be able to see the inlet and outlet faces of the water without needing to hide parts, does that make sense?

Every meshed part needs an initial temperature assigned, so far you do not have this.

You will also need a better mesh through your inner fluid and solid parts but let's cross that bridge once it is running,

Hope that helps,

Jon

[yotamalfandary]

Hey @Jon.Wilde, and @Amal.

Thanks for your support,

I tried your suggestion, but again stumbled into an unexpected stop.

I noticed I had 2 parts that were overlapping in the CAD,

I thought fixing that would help, but unfortunately not.

I attached the support file.

your assistance would be much appreciated.

[Jon.Wilde · Accepted solution]

@ceylon_schwarz - please could you start up a new thread where we will investigate?

Thanks 🙂

[Jon.Wilde]

Mine runs OK with a better mesh.

Select the coolant, make the mesh uniform and refine a bit (mine was down to 0.44 randomly). 

This is running 😉

Does that solve your issue?

Jon

[yotamalfandary]

Hey,

It is running! 🙂 thanks!

just one last problem,

if I run the steady state analysis the results get not accurate, or maybe I'm just missing something,

I set the initial conditions as 25C to the whole system,

I set the refrigerant's temp to -1C,

so what I expect to see when running the steady state analysis, is that the first few iterations would start when the initial conditions still affect some parts of the system

but what I get is on the first iteration the whole system is already at -1 degrees C.

any suggestions?

thanks again.

[ceylon_schwarz]

ohh..

[Jon.Wilde]

You would need to run that transiently 🙂

Also, you will probably need a far finer mesh all over the model, see how this run goes first. Then look to do a mesh sensitivity study.

Hope that helps,

Jon