Nusselt Number in Droplet Condensation Model

Ali734 · August 8, 2014, 19:42

Hello,

I am trying to use a different equation for the Nusselt number of the continuous phase (water vapor) through CEL expression. But, I can't get any realistic results.

I even tried to define the default (Nu=2/(1+3.18*Kn)) equation through CEL, to see if I was able to reproduce same results, which I wasn't.

I myself presume that it might be due to the way that the solver report the droplet radius/diameter.

Does anyone have experience in this respect?

ghorrocks · August 9, 2014, 05:53

What definition of Nusselt number are you trying to apply? What makes you think the results are not realistic?

Have you done the basic checks on whether your simulation is accurate? http://www.cfd-online.com/Wiki/Ansys..._inaccurate.3F

Ali734 · August 9, 2014, 11:50

Hello Glenn,

1- In fact, I am trying to test different droplet growth models. So, I have to test different nusslet numbers/heat coefficients for water vapor and droplet.
In CFX, By choosing "Small Droplet" for the heat transfer equation of dispersed phase (water droplets) and for the interphase mass and heat transfer models. Then, the temperature of the droplet is calculated by an algebraic equation (i.e. No energy equation is solved for the droplet) and temperature gradient inside the droplet is ignored. In addition, the nusselt number of the vapor (according to CFX documents) should be given by Nu=2/(1+c*Kn), c=3.18, Kn: Knudsen number, which is the one that I'm talking about, and is equivalent to the droplet growth model proposed by Gyrmathy.

2-The simulation using above settings can be easily done and compared with the experiment of Nozzle B reported by Moore. The thing that I can't do is reproducing the same results by manually defining the same equation for the Nusselt number, which should give the same results. I tried to do this matter by using Two resistances model, and defining the aforesaid Nusselt number for the vapor and zero resistance for the liquid.

Regards,
Ali

ghorrocks · August 10, 2014, 06:34

Now I think I understand what you are asking.

If you replace the default Nusselt number with an identical CEL expression and get different answers then that suggests either your implementation of it is wrong or CFX has a bug in it.

I have no experience in this exact area so cannot help you. I can only recommend you carefully check your implementation and ensure it is correct. If you find it still does not match then report it to ANSYS as a question and possibly a bug.

August 8, 2014, 19:42	Nusselt Number in Droplet Condensation Model	#1
Ali734 New Member Ali Join Date: Jun 2012 Location: Finland Posts: 3 Rep Power: 13	Hello, I am trying to use a different equation for the Nusselt number of the continuous phase (water vapor) through CEL expression. But, I can't get any realistic results. I even tried to define the default (Nu=2/(1+3.18*Kn)) equation through CEL, to see if I was able to reproduce same results, which I wasn't. I myself presume that it might be due to the way that the solver report the droplet radius/diameter. Does anyone have experience in this respect?

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August 9, 2014, 05:53		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	What definition of Nusselt number are you trying to apply? What makes you think the results are not realistic? Have you done the basic checks on whether your simulation is accurate? http://www.cfd-online.com/Wiki/Ansys..._inaccurate.3F

August 9, 2014, 11:50		#3
Ali734 New Member Ali Join Date: Jun 2012 Location: Finland Posts: 3 Rep Power: 13	Hello Glenn, 1- In fact, I am trying to test different droplet growth models. So, I have to test different nusslet numbers/heat coefficients for water vapor and droplet. In CFX, By choosing "Small Droplet" for the heat transfer equation of dispersed phase (water droplets) and for the interphase mass and heat transfer models. Then, the temperature of the droplet is calculated by an algebraic equation (i.e. No energy equation is solved for the droplet) and temperature gradient inside the droplet is ignored. In addition, the nusselt number of the vapor (according to CFX documents) should be given by Nu=2/(1+c*Kn), c=3.18, Kn: Knudsen number, which is the one that I'm talking about, and is equivalent to the droplet growth model proposed by Gyrmathy. 2-The simulation using above settings can be easily done and compared with the experiment of Nozzle B reported by Moore. The thing that I can't do is reproducing the same results by manually defining the same equation for the Nusselt number, which should give the same results. I tried to do this matter by using Two resistances model, and defining the aforesaid Nusselt number for the vapor and zero resistance for the liquid. Regards, Ali

August 10, 2014, 06:34		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	Now I think I understand what you are asking. If you replace the default Nusselt number with an identical CEL expression and get different answers then that suggests either your implementation of it is wrong or CFX has a bug in it. I have no experience in this exact area so cannot help you. I can only recommend you carefully check your implementation and ensure it is correct. If you find it still does not match then report it to ANSYS as a question and possibly a bug.