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83 Views, 7 Replies

03/12/2018 11:58 AM

I everyone,

I'm new to CFD autodesk and I'm trying to set a flow+thermal analysis, but I have some troubles. I want to simulate a very simple geometry: a pipe with a diameter of 5 cm and with a length of 3 m. I set the following boundary conditions:

- inlet: mass flow rate= 0.118 kg/s, Temperature=10 °C

-Outlet: pressure=0

-External surfaces: film coefficient of 100 W/m2K at 75°C

My analytic results give me a heat flux=1973.5 W (q=h*A*ΔT) and a temperature of the outlet=50°C. CFD results (measured with the wall calculator) show that the temperature=49 °C but the heat flux is around 1700 W . I used an ADV5 and a turbulence model of K-epsilon.

Are there any errors in my settings?

Thanks

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Message 1 of 7 ( Views: 83 ) 03/12/2018 5:35 PM

Hello @a.scolari004

I tried to duplicate your model here but you didn't specify the material.

In any case, I would do the following:

1- Make sure the mesh is fine enough. Refine the mesh and see if the results change.

2- Extend the inlet a little bit so that the temperature BC is not touching the Film Coefficient BC as shown in image below. The little cylindrical surface in between will have no boundary conditions.

Marwan

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Message 2 of 7 ( Views: 69 ) 03/20/2018 1:23 PM

Hi @marwan_azzam,

thank you for your answer. I'll try to be more accurate. I'm trying to set this analysis with these parameters:

- material: water

- geometry: length=3 m, diameter=5 cm

- inlet: temperature = 10 °C, mass flow rate = 0.0118 kg/s

- Surface temperature: constant = 75 °C with a convection coefficient of 100 W/m2K

- outlet: pressure = 0 MPa

I've to calculate the temperature at the outlet. This is a forced covection analysis, so I check the "Auto forced convection" analysis, and I set a laminar flow as it is.

The analytic results show me that the temperature at the outlet has to be 50 °C and the flow rate has to be 1973 W, but my results are not the same. How I have to set the analysis to obtain the same results as the analytic ones?

Thank you

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Message 3 of 7 ( Views: 56 ) 03/20/2018 3:11 PM

Thanks for the details. By the way you had the mass flow rate in the original post at 0.118 kg/s but it should be 0.0118 kg/s as you have in the last post.

In any case, I am getting an outlet temperature of 43 C. I can get more accurate results if I change some settings.

What kind of results are you getting?

Perhaps you can send the support share file (*.cfz).

Marwan

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Message 4 of 7 ( Views: 52 ) 03/20/2018 4:13 PM

Sorry, 0,018 kg/s is the correct data.

I've changed something from the first post, but the data in the second one are the actual input of the analysis.

I obtained a final temperature of 40 degrees

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Message 5 of 7 ( Views: 48 ) 03/20/2018 5:14 PM

Hello,

In the model I made here, with flow rate of 0.0118kg/s and turbulent flow, I get an exit temperature of 44.1C and Fluid Energy Balance MdotIn x Cp x (TOut - TIn) = 1681.4 Watts in the summary file.

If I do the hand calculations for m.Cp.Delta_T I get 1682.75 W which is almost perfect.

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Message 6 of 7 ( Views: 46 ) 03/21/2018 10:26 AM

Thank you for your quick reply,

but why have you used a turbolent flow when the Reynolds number is below 2300?

One more question, that is about an other problem. When there is applied a constant Surface temperature the correct boundary conditions is "temperature" or "film coefficient" (I assigned a convection coefficient measured at the temperature of the Surface).

Thank you so much

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Message 7 of 7 ( Views: 40 )