Negative enthalpy in simulation

  • Last Post 14 January 2019
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zakmt293 posted this 11 January 2019


I want to ask a question regarding a simulation. I am simulating flow Boiling through VOF model and when the vapor source is generated in the simulation through UDF. I checked the contours of Enthalpy and there is negative entalphy. I do not understand what could be the main reason for this.

Can some one help me and the working fluid is water 

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rwoolhou posted this 11 January 2019

If you're adding a mass source of vapour into the system, are you removing any liquid and how are you accounting for both latent heat and the energy associated with the mass source? 

Fluent assumes that any mass added via a source term is 298.15K so you may need an energy source/sink to account for that. 

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zakmt293 posted this 11 January 2019

Yes sir I am removing the liquid source as well. I will incorporate the energy source for latent heat as well. Let me check the behavior if this was not clear I will post again

zakmt293 posted this 11 January 2019

Another question that I want to ask please if you can help me.


#include "udf.h"

#include "sg_mphase.h"

#define T_SAT 373

#define k_eff 0.015

#define LAT_HT 2566e3

#define dx 0.01



real m_v;

Thread*mix_th,*sec_th;                         // declaration of threads

mix_th=THREAD_SUPER_THREAD(pri_th);           // declaration of super thread

sec_th=THREAD_SUB_THREAD(mix_th,0);          // declaration of sub thread which is now water flowing

m_v=0;                                       // In start mass of vapor is zero

if (C_T(cell,sec_th)>373)                    //As the  current cell temp rises


m_v=k_eff*(C_T(cell,sec_th)-T_SAT)*C_VOF(cell,sec_th)/(LAT_HT*dx*C_VOLUME(cell,sec_th));   // Source of vapor incorporated in secondary phase under the cell index



return m_v;




Dear sir I have written above program for generating vapor source. I cannot address directly the vapor source as it is not present in the grid. So can we have other methods and is the above mentioned program genuine for retrieving vapor phase


rwoolhou posted this 11 January 2019

You do realise that the VOF solver is designed to model a free surface? So adding mass of a second phase into a cell is likely to make the model unstable if it's not at the interface? 

Re adding in energy, if you are calculating the mass source somewhere you will know the energy sink/source based on the mass being transferred. 

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zakmt293 posted this 12 January 2019

Yes sir

You are correct. I am still searching for methods to define interfaces that can help me in my model. A simple would help me enough do you know any site which can help me devise the algorithm and implement it.

Thank you

rwoolhou posted this 14 January 2019

I'd look into the multi-fluid VOF model and see if that's suitable (it should be). Re the boiling models etc I can't help with that. Ask your supervisor if (s)he has access to the ANSYS Customer Portal as there may be some information on there. 

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