Fluids

Fluids

Latent heat (negative value?)

    • giovanni_ianziti
      Subscriber

      I want descrive the evaporation-condensation inside a close heat pipe. I`ve 0.13 vol fraction of water and the other volume Is vapour. I want insert 2455 Kj/Kg and my Temp of saturation is 373 K. 


      I`m following these steps for insert latent heat:


       


      For liquid and vapor, use reference temperature as saturation temperature.


       


      Specify the same molecular weight for liquid and vapor phase.


       


      For liquid specify standard state enthalpy = 0, and


       


      for vapor specify standard state enthapy = latent heat (in j/kg-mol units)


       


      If you have latent heat data in j/kg then multiply it with molecular weight


       


      (unit kg/kg-mol). This will give the latent heat value in required units.


       


      This specification will ensure appropriate latent heat removal/addition.


       


      Standard state entropy has no role to play. It can be left to default value


      I have found a value of  44227316 J/Kmol (18.0152 Kg/Kmol * 2455000 J/Kg).


      My question is: Is 44227316 a negative value?


       


      thank you very much

    • DrAmine
      Ansys Employee

      From liquid to vapor you need to provide heat: That is why the latent heat is a positive quantity. This heat will be released into surrounding if you go from vapor to liquid. So put the positive value on the vapor side and keep zero on the liquid side: the difference is important. 


       


      In a phase  T-H diagram that the distance you travel under the dome between a state with 0% vapor to 100% vapor 

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