Learning Forum

[Xiong Lie]

Dear experts!

        I want to simulate the flow field temperature field of a heat absorber, which is arranged with spiral coils, and the work mass goes from the inlet to the outlet, I want to simulate the flow field and temperature field of the heat absorber, so I have built a large computational domain outside the absorber. My model for radiative heat transfer is the S2S model. The natural convection heat transfer of air inside the heat absorber is considered in the simulation. My question is : How can I get the radiative heat transfer losses and natural convective heat transfer losses at the lower opening of the heat absorber in my model?

                       Thank you in advance!

[Essence]

Hello,

You can try to create an interior surface at the bottom of the heat absorber, to allow heat and mass flow through it. This can be done by appending the mesh in the existing domain. Run the simulations for some iterations and use Fluxes in Results > Fluxes. There you can see the option for Radiation heat transfer rate and select the interior surface.

Now for natural convection heat loss, choose Total heat tansfer rate. Subtract Radiation heat from the Total heat, assuming there is no forced convection in the heat absorber. Total heat transfer = Natural convection + Radiation heat through the interior surface. Of course, the viscous dissipation loss will be included, but can be neglected here.

[Xiong Lie]

Dear  Essence!

    I tried it -“use Fluxes in Results > Fluxes”and found that the total heat transfer rate and radiative heat transfer rate on all interior surfaces is 0 in fluent post-processing.

       Thanks again！

                              

[Essence]

Hello,

Adding to my reply, please note that in the term "Natural convection heat", the conduction heat transfer in fluid is also included by default. This is because, while solving for the Energy equation, Fluent solves for the conduction in fluid also.

[Essence]

Hello,

Could you schare the screenshot of where did you create the interior?

[Xiong Lie]

Dear Essence!

    I built a simple model and found that the total heat transfer rate and radiative heat transfer rate on the  interior surface is 0 .

                       Thank you!

[Essence]

 

Hello,

In your geometry, could you specify where is the inlet and outlet for air flow. I am assuming that the interior face is outlet. Please mention the air inlet in the heat absorber for the heat transfer to occur.

 

[Xiong Lie]

Dear Essence!

     I think the area we are talking about should be set to INTERIOR FACE not to INLET BOUNDARY. Because there is a large computational domain outside my model. In my model the boundary conditions are set as shown in the picture, there is no air inlet.

 

[Essence]

Hello,

I got your point. By "inlet" and "outlet", I meant the passage for the air flow in and out of the heat absorber. If the hot air comes out through bottom, then, there should be cold air entering the heat absorber to maintain the continuity, right? Could you share the velocity contours and vectors?

I suppose you have also applied gravity acceleration upwards and not downwards i.e +9.81 m/s^2 instead of -9.81 m/s^2 (as hot air flows out through bottom).

[Xiong Lie]

Dear Essence!

     I didn't apply gravity acceleration.After iterating for a few steps it was found that  the total heat transfer rate and radiative heat transfer rate on the  interior surface is 0.This is a simple test case , how can I give case to you?

 

[Xiong Lie]

The case is here.https://drive.google.com/drive/folders/1g394tJQi4uJT6svlJ4oBcfuAzR5lQNTG?usp=sharing

[Essence]

Hello,

You do not need to share the case files. I am not supposed to open any links posted on the Forum.

If you have not chosen any gravity then, how can you expect the natural convection to occur? I would suggest you to go through the Ansys guide links:

https://ansyshelp.ansys.com/account/secured?returnurl=/Views/Secured/corp/v231/en/flu_th/flu_th_sec_hxfer_buoy.html?q=natural%20convection

https://ansyshelp.ansys.com/account/secured?returnurl=/Views/Secured/corp/v231/en/flu_ug/flu_ug_bcs_sec_operating.html

If you are unable to access the link, follow this Forum discussion /forum/forums/topic/using-help-with-links/#latest

[Xiong Lie]

Dear Essence!

      I'm concerned about the amount of radiant heat transfer on the  interior surface, even if there is no natural convective heat transfer, there should be radiant heat transfer and total heat, right? Why is the radiant heat transfer and total heat on the  interior surface zero?

[Essence]

Hello,

If you only want to calculate the heat transfer due to the radiation, then use Surface integrals. Select Field variable as Incident radiation and select the interior surface. As interior surface has no BCs, the incident heat will be equal to the radiative heat transferred through the interior.

[Xiong Lie]

Dear Essence!

       I tried what you said and found that all the data on the surface is 0. The temperature contourshows that the temperature on the surface is normal.

[Xiong Lie]

Dear Essence!

        Could you give me a definitive answer? Can I manipulate this way in the S2S model to get the radiative heat transfer on the interior face?

[Essence]

That's odd, I am getting non-zero value on the interior surface.

[Essence]

Could you try using different radiation models?

[Xiong Lie]

Dear Essence!

       What version of fluent are you using, I tried it on 2021R2 and it still doesn't work? Also my radiation model is S2S model, what is your radiation model?

      If I use the DO radiation model,I can I can get the same results as you.Why is the  radiant heat on the surface I'm interested in 0 in the S2S model, but there are results in the DO radiation model? Can I get results on the S2S model? Is it possible to get the radiant heat at the surface using UDF inside the S2S model?

[Essence]

Hello,

The version should not be an issue here. Please check if you applied the "View Factors and Clustering" settings properly for the S2S model. Please refer to the S2S model in user's guide.

[Xiong Lie]

Dear Essence！

              What is your radiation model?Can you also get the radiant heat on the interior faces in the S2S model?The  "View Factors and Clustering" settings  are  fluent's default setting.I didn't change the setting.

[Essence]

Here are some disadvantages of S2S model:

‐ The S2S model assumes that all surfaces are diffuse
‐ The implementation assumes gray radiation
‐ The storage and memory requirements increase as the number of surface faces increases (N x N)
‐ CPU time is independent of the number of clusters used (when Face to Face basis used for view factor calculation)
‐ Cannot be used to model participating radiation problems (absorption, emission, scattering)
‐ Does not support hanging nodes or mesh adaption
‐ Not strictly conservative

[Xiong Lie]

Dear Essence!

     What do you mean by listing the shortcomings of the S2S model?I cannot get the radiant heat of the interior face in the S2S model?

[Essence]

Please go through the Ansys user and theory guide for more information.