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


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


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