Learning Forum

[rkoomul]

I am new to modeling radiative heat transfer using Ansys/Fluent. Any pointers to model the following problem will be greatly appreciated.

The computational domain is cylindrical cavity with 0.5 m radius and 3 m long, and the boundary surfaces are black surfaces at 300K. The medium is a participating medium with an absorption coefficient of 0.1 m^-1 (gray gas) and temperature 1200K. In this problem, I would like to calculate the radiative heat flux to the cylindrical surface.

Here is the setup that I used.

Radiation Model: Discrete Ordinate (DO) method with 4 divisions along theta and phi directions and number of bands as zero (gray gas).

Material models: Absorption coefficient as 0.1 m^-1 and scattering coefficient as 0.0 m^-1.

Cell Zone Conditions: Set as Participates in Radiation

Boundary Condition: For “Thermal” condition, selected “Radiation” and set “External Emissivity” as 1.0, and “External Radiation Temperature” as 300K. For “Radiation”, set the BC Type as opaque, and “Internal Emissivity” as 1.0 and “Diffusion Fraction” as 0.0

I tried two different options.

·        In the first case, flow, energy, and DO equations are solved. In this case, as expected, the temperature became uniform at the end of the iterations and radiative heat flux became a very small value. 

·        In the second case, I solved only DO equation (turned off the flow and energy equations under Solution -> Control -> Equations), since I wanted to calculate the radiative heat flux when the temperature inside the cavity is 1200K and wall temperature is 300K. DO method converged in a few iterations. However, the radiative heat flux at the wall is showing as zero. I guess, this is because the energy equation is not solved. Is there any way I can calculate the radiative heat flux, after DO method converges? 

Attached is an image of the heat flux distribution for this case from a published paper.

[Karthik Remella]

Hello You will need to solve the energy equation model to obtain the radiative heat flux value. DO is only solving the radiative transfer equation for the intensity. It should be solved with the energy equation in a coupled manner. If you are solving for a steady state solution, you might perhaps want to freeze the flow solution and solve the energy and DO equations in a coupled manner? Does that help?
Karthik

[rkoomul]

Thank you Karthik for your comments.
Air inside the cavity is stationary. So, I turned off the flow equations, and solved only energy and DO equations. Once the steady state conditions are reached, the temperature inside the cavity became 300K, because of the heat transfer from the hot air to the wall. As a result of the heat transfer, the air inside the cavity cools down, and reaches the wall temperature when steady state conditions are reached.
I tried another approach. I solved the DO equation first (without flow and energy equations). Then turned on the energy equation, and solved both the equations in transient mode with a very small time step (1.0E-07 seconds) for one time step. The goal is to estimate the heat transfer rate at the first time step, before a lot of energy being transferred to the wall. Attached is an image of the predicted heat transfer rate to the wall. This results matches well with the published data.
Thanks.

[Karthik Remella]

Hello Good to know that this worked and you are able to match your simulation results with the paper.
Karthik

[lei2019]

@koomullil Thank you for sharing your case setting. could you provide the source of the journal paper you mentioned? Thank you.