# Sudden temperature rise in transient heat transfer simulation with S2S radiation (Fluent)

Member

Hello,

I am solving transient heat transfer problem (in Fluent) between two solids using S2S radiation model.

Solid 1 is at fixed temperature of 1073 K. The initial temperature of Solid 2 is 300 K. (Fig 1)

There is fluid body between two solids. Currently, I am comparing these results with Ansys thermal and hence I have used a fluid with very low thermal conductivity (1e-8) and low specific Heat (1e-5) so that is practically does not affect heat transfer.

In this case, since the solid-1 is at high temp, radiative heat transfer should take place and temp of solid 2 should gradually increase.

The problem is, no matter what the time step size is, the "Area weighted average temp" of solid 2 (surface) jumps to 590K just after first time step (Shown in image). This is unrealistic. The same setup in ANSYS Thermal shows gradual temp rise.

What can cause such sudden temperature increase? I would appreciate your assistance.

Some other details of the problem

Solver : 2-D transient Fluent solver with S2S Radtiation model

Properties of solid 2: Density : 1100 kg/m3, Cp : 1300 J/kg-K, Thermal Cond : 0.58 W/m-K

Mesh : 2-D quad mesh mesh with size ~ 5mm

Time step : 0.1 s

• UKForum Coordinator

How did you initialise the system?

• Member

I initialized from all zones and then patched 300 K on Solid-2 and Fluid Body.

• UKForum Coordinator

Please plot images of the temperature field at t=0 and t=next time step. Has the step converged?

• Member
edited August 2020

I ran the simulation for just one time step. It converged after 17 iterations.

Here are the plot of temp at t = 0 s and t=0.1 s.

I am plotting static temperature. (Node values)

• UKForum Coordinator

Re-plot with node values off: is it the fluid or solid that's got hot? If you work out the radiation load is that sensible? Remember that heat will travel VERY quickly as thermal radiation.

• Member

Hi @Rob

Here are two plots : one with Nodes values on and other with node values off.

They show different values. High temperature at the solid-fluid interface is very high when using node values.

What causes this difference?

The radiation flux has realistic values. (matching with analytical values)

Yes, radiation travels very quickly. But, solids cannot get hot with such unrealistic rate.

As I am writing report file of average temperature at the interface and it uses node values it shows the jump. Is there any way to use cell values in report?

• UKForum Coordinator

Not sure, change the range to see if the issue is graphics or there is a temperature rise at that location.

• Member

It shows the same results even after changing the temperature range.

And those high values are reported in the report file too.

• UKForum Coordinator

With node values off? What are you reporting?

• Member
edited August 2020

Static temperature.

In the plot, I am reporting area weighted average of Static temperature.

Wall temperature gives the same result.

• UKForum Coordinator

What happens when you run the model on? Monitor wall and volume temperatures.

• Member

As I mentioned, after sudden jump the average surface temp rises gradually. (Shown in the plot posted earlier)

Volume temperature rises gradually as it takes some time for the entire volume to get heated.

• UKForum Coordinator

So given a surface is "thin" how long would it take to warm up if exposed to sunlight?

• Member

Here is the case file.

I didnt get you point.

There in no "thin" surface. Its just interface of fluid and solid.

• UKForum Coordinator

You have a wall at that location, which is thin, and has to be there to separate the fluid & solid.

Staff aren't permitted to download or open files, so I'll leave that for someone else in the Community to have a look at your set-up.

• Member

Yes, @Rob

There is a wall -wall shadow with "0" thickness.

I hope this what you are asking.

• UKForum Coordinator
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