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Conjugate Heat Transfer: Solid Time Step in Transient Simulation, DO Model

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Old   July 9, 2021, 06:19
Default Conjugate Heat Transfer: Solid Time Step in Transient Simulation, DO Model
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Hi everyone,

I've been stuck with a case I'm trying to run for a while now, and hope for some help or suggestions. The setup is a simple burner geometry, where premixed gas enters a combustion chamber through small holes in a burner plate and then burns. I am trying to find the stationary temperature distribution in the burner solid.

I have a velocity inlet upstream of the burner plate, and a pressure outlet at the end of the domain. All other outer boundaries, including the ones on the solid, are symmetric. I am using a custom-built tabulated chemistry combustion model, in which temperature, density and all properties are looked up from a table. The walls are set to coupled. I am using the transient solver, as I cannot achieve convergence with the pseudo-transient solver (might be a settings problem, I don't really have experience with steady-state simulations). The energy-equation is turned on to solve for the temperature within the solid, but temperature is set to constant in the fluid (it gets looked up from the table via udf).

Two problems/questions:

1) Solid Time Step:

This base case seems to be running fine (i.e. the flow field looks reasonable and doesn't change much anymore after some time, the combustion model works), however due to the different time scales the heating up of the solid takes a very long time. When I set the solid time step to a large value, obviously the solid temperature starts increasing much faster. However, I am not sure if the heat transfer between solid and fluid is computed correctly. The solid temperatures very quickly exceed my expected values and keep increasing. How does the coupling of heat transfer between fluid and solid work when I specify a larger solid time step? Can I expect a reasonable outcome from my setup?

2) Radiation:

I am trying to add radiation to the simulation, using the DOM. I have activated the model, and set the pixels to 3 as recommended for cases with symmetry boundaries. For the beginning, I have set the absorption coefficient of the fluid mixture to 0, to mimic an optically thin gas and investigate only solid radiation from the burner surface. The solid does not participate in radiation, and I have set the internal emissivity of the burner surface to 0.5. At the outlet, I have defined a black body radiating at 300K (in reality, cooling coils are located there).

The result is that the solid heats up faster. Despite the radiation heat flux having a positive sign on the burner surface (which I'd interpret as heat leaving the solid). I am completely lost why this might be happening. All the remaining case setup is identical to the case without radiation. I don't expect massive impact of radiation on my case, but at least radiation should somewhat cool down the burner, not triple the heat-up rate.

EDIT: I just checked again. The total surface heat flux in the case with radiation is consistently lower (meaning less negative, less heat transferred to the solid) than that of the case without radiation. The solid temperature still increases much faster. What's even happening?

Are there maybe any known issues when using either DOM with symmetry boundaries or DOM with a different solid time step?

Thanks a lot for your time, any help or suggestions are highly appreciated!
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