heat flux limit condition

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Hello,

I'm trying to model the behavior of a hollow cylinder made of a certain material, with an inner radius r1 fixed at 0.5m and an outer radius r2 that I need to determine. The behavior I'm trying to model is that in response to combustion inside the cylinder for a certain period with a certain heat flux.

My problem depends only on the variable r (the radius). Therefore, I can reduce it to a 2D problem where instead of a cylinder, we have a hollow disk.

To model the combustion described above, I impose a heat flux at r1 = 0.5m. On the cylinder, the amount of heat provided is constant at Q=33333 [W] for 2 hours and then drops to 0. It is uniformly distributed. The total surface on which this flux applies is the inner surface of the hollow cylinder with a radius of r1 = 0.5m and a height of z=2 m (S = 2π×0.5×2 = 2π) and thus q = 5305.11 [W/m²].

Here's my question: Since I'm reducing the problem to 2D, can I still say that the inward heat flux is equal to 5301.11 [W/m²]? I feel like I need to adjust it, but I don't see how. I'm having trouble visualizing how we can talk about a surface quantity when the condition on the 2D disk applies to a contour and not a surface.

Thank you for your futur response !



1 Reply Last Post Mar 20, 2024, 3:46 a.m. EDT
Henrik Sönnerlind COMSOL Employee

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Posted: 1 month ago Mar 20, 2024, 3:46 a.m. EDT

Yes, you are doing the right thing. Heat fluxes still have the same unit, irrespective of the spatial dimension.

The thickness is not seldom important in 2D, but there is one. It is set in the Heat Transfer in Solids node. It will be important only if you for example want to enter the total input power in terms of W.

You could choose to set Flux type to Heat rate in your input, and enter 33333 [W]. But then it is essential that you have set the thickness to 2[m].

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Henrik Sönnerlind
COMSOL
Yes, you are doing the right thing. Heat fluxes still have the same unit, irrespective of the spatial dimension. The thickness is not seldom important in 2D, but there is one. It is set in the *Heat Transfer in Solids* node. It will be important only if you for example want to enter the total input power in terms of W. You could choose to set *Flux type* to *Heat rate* in your input, and enter 33333 [W]. But then it is essential that you have set the thickness to 2[m].

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