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Iterative Non-Convergence and Thermodynamic Function Errors

Posted 30.08.2025, 18:43 MESZ 0 Replies

three Yang

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I am conducting a finite element simulation of the steady-state operation of a sodium heat pipe using COMSOL, employing the built-in Non-Isothermal Flow module. For the heat pipe modeling, the liquid flow within the wick is neglected, and both the wick and the heat pipe wall are treated as pure heat conduction problems, with the primary focus on studying the vapor flow within the vapor chamber.

The details parameters of model are as follows: the length of evaporation section, adiabadic section and condention section are 0.14 m, 0.04 m, 1.1 m respectively; the radius of vapor chamber is 0.0057 m, the thickness of wick is 0.00015 m, the thickness of annular is 0.00015 m and the thickness of wall is 0.0009 m. The porosity of wick is 0.4221. I built a 2-D axisymmetric model in COMSOL and establishied a thermodynamic system to calculate ideal sodium vapor density as a function of temperature and pressure

To investigate the vapor-liquid phase change at the interface between the vapor chamber and the wick, a pressure inlet boundary condition is applied in the Laminar Flow interface, while a boundary heat source is defined in the Heat Transfer in Solids and Fluids interface. These settings are consistent with the heat pipe example in the COMSOL案例库 (Application Library).

Additionally, a fixed heat flux is applied at the evaporation section, and thermal convection cooling is set at the condensation section. Specific parameter settings and boundary conditions are provided in the MPH file. To reduce the file size, the mesh and solution data have been removed.

The current objective is to study the maximum temperature and average temperature at the outlet of the evaporation section under heat inputs ranging from 200 W to 2600 W. Converged results are achieved when the input power is not lower than 1200 W, but the simulation fails to converge when the input power is below 1200 W.

I have attempted to adjust the initial solver values, mesh density, number of iterations, and solution methods. When the maximum number of iterations is set too low, the solver reports convergence failure due to exceeding the iteration limit. When the iteration count is increased, errors similar to the following consistently occur: The simulation encountered an error during the calculation of thermodynamic functions. Error Message: Pressure must be positive real value Function: Viscositypp3

Variable Evaluation Failures:

comp1.spf.mumat

Definition: (unit_Pa_cfViscositypp3(comp1.spf.fp1.minput_temperature/unit_K_cf, comp1.spf.fp1.minput_pressure/unit_Pa_cf))unit_s_cf

comp1.spf.mu → Defined as: comp1.spf.mumat

comp1.spf.mu_eff → Defined as: comp1.spf.mu

comp1.spf.K_stress_tensorzr → Defined as: comp1.spf.mu_eff*(comp1.uz + comp1.wr)

The issue is likely related to the vapor flow velocity within the heat pipe reaching sonic speed under low power conditions, but I am uncertain how to resolve it. I would greatly appreciate your insights and suggestions on this matter. Thank you very much.

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