Question about the inconsistent Prandtl number and Cv of water in tutorial cases

chengtun · January 16, 2019, 12:35

Hi, everyone! I have a question about the inconsistent Prandtl number of water in tutorial cases. In the case of wallboiling of reactingEulerFoam solver in https://github.com/OpenFOAM/OpenFOAM...perties.liquid. The Prandtl number of water of liquid and gas was set at the same value, 2.289.(I wonder why the value was set the same? Is it not for the saturated phase?)
I looked up in the table and found for the pressure in simulation(p=100000Pa) and saturated phase, Pr is 1.757 for liquid and 1.015 for gas. For the pressure and initial temperature in simulation(p=100000Pa, T=368K) Pr is 1.848. All of them are different from the case value. How to explain the difference and how to set the value?
Another question is the Heat capacity at constant volume Cv is also inconsistent with actual value. Suppose p=100000Pa, for gas Cv was set at 4195 but actually is 1552.7; For liquid Cv was set at 12078.4 but actually is 3769.7; The unit is J/kg/K.
Thanks a lot for any hint!

ZZW · February 4, 2020, 10:53

Hi Tun,

I guess the unreal value is given to Cv because of the assumption to vapour. In the tutorial case the vapour is assumed saturated, which means that the properties remain unchanged after the vapour is generated. Therefore, an extreme high cv is given to stable the vapour temperature to suit this assumption.

Hope this would help you.

Regards,
Zao

January 16, 2019, 12:35	Question about the inconsistent Prandtl number and Cv of water in tutorial cases	#1
chengtun New Member Tun Join Date: Mar 2018 Posts: 12 Rep Power: 8	Hi, everyone! I have a question about the inconsistent Prandtl number of water in tutorial cases. In the case of wallboiling of reactingEulerFoam solver in https://github.com/OpenFOAM/OpenFOAM...perties.liquid. The Prandtl number of water of liquid and gas was set at the same value, 2.289.(I wonder why the value was set the same? Is it not for the saturated phase?) I looked up in the table and found for the pressure in simulation(p=100000Pa) and saturated phase, Pr is 1.757 for liquid and 1.015 for gas. For the pressure and initial temperature in simulation(p=100000Pa, T=368K) Pr is 1.848. All of them are different from the case value. How to explain the difference and how to set the value? Another question is the Heat capacity at constant volume Cv is also inconsistent with actual value. Suppose p=100000Pa, for gas Cv was set at 4195 but actually is 1552.7; For liquid Cv was set at 12078.4 but actually is 3769.7; The unit is J/kg/K. Thanks a lot for any hint!

February 4, 2020, 10:53		#2
ZZW New Member Zhao Join Date: Dec 2019 Posts: 4 Rep Power: 7	Hi Tun, I guess the unreal value is given to Cv because of the assumption to vapour. In the tutorial case the vapour is assumed saturated, which means that the properties remain unchanged after the vapour is generated. Therefore, an extreme high cv is given to stable the vapour temperature to suit this assumption. Hope this would help you. Regards, Zao