condensation_2.fds Verification test case

[Muthu-V]

condensation_2_verification.txt

I tried to run the test case condensation_2.fds that comes from fds verification testcase to check the increase in temperature and pressure due to condensation of water vapor that is kept initially at -100 C in 1 cubic meter box. There is a discrepancy in the pressure and Temperatures, when I checked it with simple Ideal gas law approximation with CFD results. You may refer the python code attached above for the hand calculation. Would appreciate your feedback.

[drjfloyd]

These lines are not correct. They are going to give you too much initial mass. First determine the average MW of the mixture, then use that to get the density of the gas, then use mass fraction and volume to get the mass of air and vapor.

Mass of water vapor and air in the box

mass_air = 1 / (R * initial_temp_kelvin / initial_pressure)
mass_water_vapor = water_vapor_ratio * mass_air

This line is not correct. It is missing the condensed water mass which has a cp of 4.

temp_increase = energy_released / (mass_air * Cp)

[Muthu-V]

@drjfloyd Thank for your comments. I tried to follow your suggestion, still there is a discrepancy. Check the attached spreadsheet hand calculation.
Condensation_2_handCalculation.xlsx

[drjfloyd]

If you have 1 % mass fraction of H2O and 99 % mass fraction of air, then if you have 1 kg of gas you have 1*.1/0.0180=0.555 moles of water vapor and 1*0.99/0.0286=34.3 moles of air which gives a 1.59 % vapor mole fraction not 1 %. The mixture MW is 28.68 g/mol.

The default ambient pressure is 101325 Pa not 103250 Pa.

You are using room temperature properties. This problem is -100 to -64 C. At -100 C the latent heat of vaporization is 2.9E6 J/kg not 2.5E6.

Since this is a control volume you need to work with internal energy and not just enthalpy; you are neglecting pressure work.

[Muthu-V]

@drjfloyd Thank you for your inputs. After setting the latent heat of vaporization correctly to 2.9E6 J/kg for the condensed mass and Cv for the air 716 j/kg k , the Temperature increase is 40 K. slightly above 36 K (as reported in verification manual ). Mixture MW is also corrected. Revised spreadsheet is attached, here. If there can be any improvements let me know.
Condensation_2_handCalculation.xlsx

[drjfloyd]

After condensation you have liquid water.

[drjfloyd]

I also had a typo in my earlier post, the latent heat at -100 C is 2.7E6.