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Convergence issues for transient freezing (apparent heat capacity phase change)
Posted May 15, 2012, 3:49 p.m. EDT Heat Transfer & Phase Change, Studies & Solvers Version 4.2a 1 Reply
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Hello,
I am fairly new to COMSOL and am trying to solve a transient (over 600 seconds) freezing (phase change) problem in a gel (similar thermal properties to water). I am using temperature-dependent thermal conductivity and density (both implemented as cubic-smoothed lookup functions, f(T) ). I am simulating the phase-change with the apparent heat capacity method (also implemented as a cubic-smoothed lookup function, f(T) ).
Across the model, my temperatures vary from 140K to 300K, with fairly high temperature gradients near the cooling interface (order of 40K/mm).
I have a 2D geometry with phase change occurring over <1K, that solves without issue. However, I have a slightly more complicated 3D geometry that I am having problems converging. I can get the solution to converge, treating the phase change as a triangular function over >30K, but the solver gets hung up for any more aggressive phase change.
I would like to reduce the temperature duration over which my phase change is implemented (less than 10K). I am assuming a triangular “pulse” will provide the most gradual change for specific heat, but is this an incorrect assumption and will another function provide less of an issue for convergence? Is there any other settings I can look at to help with convergence? I am using the default settings for the transient solver.
Any assistance you can offer would be greatly appreciated.
Thanks!
Michael
I am fairly new to COMSOL and am trying to solve a transient (over 600 seconds) freezing (phase change) problem in a gel (similar thermal properties to water). I am using temperature-dependent thermal conductivity and density (both implemented as cubic-smoothed lookup functions, f(T) ). I am simulating the phase-change with the apparent heat capacity method (also implemented as a cubic-smoothed lookup function, f(T) ).
Across the model, my temperatures vary from 140K to 300K, with fairly high temperature gradients near the cooling interface (order of 40K/mm).
I have a 2D geometry with phase change occurring over <1K, that solves without issue. However, I have a slightly more complicated 3D geometry that I am having problems converging. I can get the solution to converge, treating the phase change as a triangular function over >30K, but the solver gets hung up for any more aggressive phase change.
I would like to reduce the temperature duration over which my phase change is implemented (less than 10K). I am assuming a triangular “pulse” will provide the most gradual change for specific heat, but is this an incorrect assumption and will another function provide less of an issue for convergence? Is there any other settings I can look at to help with convergence? I am using the default settings for the transient solver.
Any assistance you can offer would be greatly appreciated.
Thanks!
Michael
1 Reply Last Post May 15, 2012, 3:59 p.m. EDT
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Posted:
1 decade ago
Hi
I would rather propose to use a smooth step() or rect() function, than a sharp triangular one, as it is continuously derivable (or at least 2x)
And then probably tweak the solver to try a strict time stepping, with many small steps
--
Good luck
Ivar
I would rather propose to use a smooth step() or rect() function, than a sharp triangular one, as it is continuously derivable (or at least 2x)
And then probably tweak the solver to try a strict time stepping, with many small steps
--
Good luck
Ivar