Learning Forum

[luna6362]

I am using fluent to simulate a compressor now.And I want to change the material in the compressor.

But I am wondering the density and viscosity setting of the material.

Should I change the density and viscosity under the inlet pressure and temperature?

Or I can just set the density and viscosity under 101325pa and 15°C?

Thank you!

[Rob]

Maybe? What operating conditions and pressure range are you expecting? 

[luna6362]

Hi,Rob!

The temperature and pressure on inlet is 90°C and 2959600pa.

And 90°C and 2824315pa on outlet.Thank you.

[Rob]

OK, so 28-29bar at around 90C.  

You need to decide if the density difference is sufficient to need to use a variable density (ideal gas) or fixed density at the mean temperature & pressure. The latter will be much easier to model. 

[luna6362]

 

So if I want to use fixed density and viscosity,I will need to set the density and viscosity at the mean temperature and pressure or at 15°C and 101325pa?

Thank you for your reply!

 

[Rob]

If you use fixed values you may want to set them at the condition you want to model (eg 90C & 28bar). Otherwise whilst the model will be correct for the values set you may find they're different to what you expect. 

[luna6362]

So your mean is if the inlet at 90°C and 2959600pa,I need to change the density and viscosity of the material to the value at 90°C and 2959600pa?

Sorry for so many question.Thank you so much.

[Rob]

If you want to represent the gas at those conditions, yes. But you may need to defend any assumptions to your Prof. 

[luna6362]

I got it! Thank you????

[luna6362]

Sorry Rob,there is one more question.Hope you still online.????

I am wondering which one is better,the fixed density and viscosity at the mean temperature and pressure ,or at 90°C and 2959600pa?

[Rob]

I'm UK based, but my frequency of posting is very dependent on who else has booked my time. 

Temperature is 90C, and pressure varies from around 28 to 29 bar. What is the density at 90C 28 bar when compared to 90C 29bar and 90C 30bar? Once you've worked that out, how much of a difference do you expect that to make in your model? 

[luna6362]

I see.I am from Taiwan.I am really appreciate that you answer me so many question! I am very new for Ansys fluent.Haha

The density at 90C 28bar is 59.712kg/m3. At 90C 29bar is 63.347kg/m3.

Actually,my goal is simulating the seperation of R32 and the oil.

And I just use R32 in my model now. I think the setting of material might influence the result.

But my model is very complexed.So I think use the fixed density is better.

[Rob]

The material properties will effect the result. The question is how much will it effect the result? That's why you need to understand the fluid dynamics parts as well as what buttons to press. 

[luna6362]

Ok I got it! I will discuss this issue with my Prof.

Thank you for your reply again! Have a nice day????