Total mass of the mixture in a closed domain is changing with time.

abhnv_sngh · April 21, 2022, 01:59

I am working with 2-D transient simulation with compressible flow, gravity assisted.

I am using VOF- multiphase model and evaporation condensation model as the phase change model, with saturation temperature as a function of saturation pressure.

liquid is primary phase and Vapor is secondary phase.

liquid properties - piece-wise polynomial function of temperature
vapor density - Ideal gas
vapor viscosity and thermal conductivity - piece-wise poly function of temperature.

Operating pressure - 0, gauge pressure - 76043 Pa
Operating density - 0 kg/m3
Initial temp - 75K

CONVERGANCE criteria -
(continuity - 1e-05), (velocity - 1e-04) and (energy - 1e-06)

SOLUTION METHODS
Pressure-velocity coupling - PISO
Gradient - Least squares cell based
Pressure - Body force weighted
Density - second order upwind
Momentum - Quick
Volume fraction - geo-reconstruct
Energy - first order upwind

All the URFs are default.

why is total mass changing with time and also the continuity residual falls in the range of 1e-01 to 1e+02?

LaminarFlow · April 22, 2022, 05:26

I have already noticed similar errors for open systems. In my case, the error was caused by the Volume Fraction Cutoff Value. Every volume fraction below that value is set to zero. This can lead to mass imbalances. Set the Cutoff Value to 0 for test purposes and check the simulation again for mass imbalances / convergence issues. Alternatively you can replace the VOF model by the similar mixture model which does not require a cut-off value.

April 21, 2022, 01:59	Total mass of the mixture in a closed domain is changing with time.	#1
abhnv_sngh New Member ABHINAV SINGH Join Date: Sep 2021 Posts: 4 Rep Power: 5	I am working with 2-D transient simulation with compressible flow, gravity assisted. I am using VOF- multiphase model and evaporation condensation model as the phase change model, with saturation temperature as a function of saturation pressure. liquid is primary phase and Vapor is secondary phase. liquid properties - piece-wise polynomial function of temperature vapor density - Ideal gas vapor viscosity and thermal conductivity - piece-wise poly function of temperature. Operating pressure - 0, gauge pressure - 76043 Pa Operating density - 0 kg/m3 Initial temp - 75K CONVERGANCE criteria - (continuity - 1e-05), (velocity - 1e-04) and (energy - 1e-06) SOLUTION METHODS Pressure-velocity coupling - PISO Gradient - Least squares cell based Pressure - Body force weighted Density - second order upwind Momentum - Quick Volume fraction - geo-reconstruct Energy - first order upwind All the URFs are default. why is total mass changing with time and also the continuity residual falls in the range of 1e-01 to 1e+02?

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April 22, 2022, 05:26		#2
LaminarFlow New Member Join Date: Dec 2020 Location: Germany Posts: 12 Rep Power: 6	I have already noticed similar errors for open systems. In my case, the error was caused by the Volume Fraction Cutoff Value. Every volume fraction below that value is set to zero. This can lead to mass imbalances. Set the Cutoff Value to 0 for test purposes and check the simulation again for mass imbalances / convergence issues. Alternatively you can replace the VOF model by the similar mixture model which does not require a cut-off value.