Learning Forum

[hardik2309]

Hello,nI am solving transient heat transfer problem (in Fluent) between two solids using S2S radiation model.nSolid 1 is at fixed temperature of 1073 K. The initial temperature of Solid 2 is 300 K. (Fig 1)nThere 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- and low specific Heat (1e-5) so that is practically does not affect heat transfer. nIn this case, since the solid-1 is at high temp, radiative heat transfer should take place and temp of solid 2 should gradually increase.nThe problem is, no matter what the time step size is, the Area weighted average temp of solid 2 jump to 400 K just after first time step. This is unrealistic. The same setup in ANSYS Thermal shows gradual temp rise.nWhat can cause such sudden temperature increase? I would appreciate your assistance.nnSome other details of the problemnSolver : 2-D transient Fluent solver with S2S Radtiation modelnProperties of solid 2: Density : 1300 kg/m3, Cp : 1100 J/kg-K, Thermal Cond : 0.58 W/m-KnMesh : 2-D quad mesh mesh with size ~ 5mmnTime step : 0.1 snTransient Formulation : First order implicitnn

[Karthik Remella]

Hi,nAre you computing the view factors in your S2S model? What settings are you using? Can you please embed a screenshot of the geometry and the S2S parameters?nWhat boundary conditions are you using?nAlso, are you converging the simulation every time-step? nAlso, are you solving any flow in your simulation? nThanks.nKarthikn

[hardik2309]

nI have a very simple geometry.nYes, I am computing view factors with default settings and and I have radiation iteration at every 10 energy iterations.nI am using using velocity inlet and pressure outlet boundary conditions. nHowever, as I stated earlier fluid does not take part in heat transfer because of its physical properties.nSolution is converging at every time step as it is very simple geometry. In the first time step it takes 15-20 iterations. Then it takes only 1-2 iterations to converge. (Image attached)nYou can see in the second image that the temp plot starts at ~590 K. Where the initial temp of the body is 300 K.nnn

[Karthik Remella]

My apologies for missing this post earlier. Can you plot the contour of your temperature on Solid 2 just after your initialization?nThank you.nKarthikn

[Rob]

Earlier info here: /forum/discussion/19439/sudden-temperature-rise-in-transient-heat-transfer-simulation-with-s2s-radiation-fluent n

[hardik2309]

nSorry for the trouble, this post was submitted twice and two copies were created simulateneoulsy. nHere is the contour after initialization. Solid 2 is at 300K.nnn

[Karthik Remella]

Can you do a quick hand calculation for this scenario? If you had two parallel plates separated by the distance which is equal to the thickness of your fluid region in your problem and say the temperatures of these plates are T1 = 1073K and T2 = 600 K, what is the overall heat transfer due to radiation between these plates? Please calculate the view factor using the analytical expression. Finally, you will have a value of Q_rad. Could you please check how this Q_rad compares with the Fluent result?nThanks.nKarthikn

[hardik2309]

ThanksArraynI have already calculated the analytical heat flux which should be 74700 W/m2 just after initialization. It will be keep on reducing as the temperature of the solid 2 rises.nFluent reports 58031 W/m2 after the first time step (t = 0.1s).nHere is another obseravation of this sudden temperatetu rise . This jump increases with increasing mesh size as shown here.nMesh 1 :Finest (temp starts at 585 K)nnMesh 2 :Finer (temp starts at 742 K)nnMesh 3 :Coarse (temp starts at 825 K)nn

[Karthik Remella]

Hello,nIt almost seems like you have a grid independence issue. Could you please refine the mesh and see if this issue goes away? nI'm suspecting that the view factor calculation is using this grid and because of the resolution, this calculation is incorrect. nCould you please refine the mesh and see if this solves your issue?nThanks.nKarthikn