Learning Forum

[efremsurbakti]

Hello. I tried to calculate the temperature in the outlet of Sudden Expansion. My working fluid is R-134a. I turn on the energy equation and i use k epsilon. I used couple with second order in solution. for the boundary condition i used flow rate at inlet and pressure outlet. In theory the fluid should decrease if the pressure decrease but in my case only the pressure is decreasing. Any answer would be appreciated.

 

[DrAmine]

Are both regions somehow connected? Please where are the bcs located in your images? I hope the working fluid is modeled as compressible fluid.

[efremsurbakti]

Dear Amine
Thank you for your answer. Both the region is connected through small area (sudden expansion). The picture above is plane view. the working fluid i input from NIST and it should be compressible, isn't it? Below I insert 3D picture of my model.

 

[DrAmine]

Yes, NIST is compressible. Temperature is decreasing towards the outlet. Are you above or below the inversion temperature? Is your case converged?

[efremsurbakti]

Dear Amine.

Based on theory, the Temperature should starting to decrease from point A, but as you can see  in my case the temperature starting to decrease in point B. The temperature inlet is 311 K and the temperature at outlet should be around 288 K and in my simulation the temperature only decrease to 300 K. 
Pordon me, can you explain what do you mean about my case is converged or not? 

[DrAmine]

Is your case converged or not? You know: convergence. Screenshot of residuals, Fluxt reports for mass and energy and monitors of temperature are of great benefit here.

[DrAmine]

What about reversal flow? Is it occuring at the outlet? Again a screenshot of the velocity vectors plotted on the same plane is helpful. 

[efremsurbakti]

Dear Amine

Here is the residual you asked before. I didn't really pay attention to reversal flow, is it exist automaticly, or should i eliminate the reversal flow. if So can you tell me how. Thank you

[DrAmine]

You should run for more iterations (much much more..) and check all other stuff I was mentioning. If the reversal flow is occuring, this might be due to numerical reasons and could disappear or even physical. What you can do is to elongate the outflow region and use realistic backflow quantities at the outlet. Other way (not like it) is to switch on "prevent backflow". 

[efremsurbakti]

Dear Amine
Thank you for your advice, I shall try your suggestion.

 

Best regards,
Efrem