Temp of dew point

nasdak · June 26, 2018, 03:13

Hello

I deal with flue gas flow in the industrial chimney. My goal is to identify zones where the condansation occurs. I use wall condensation model and defined air as an noncondansable and water as a vapor-liquid.
My problem is the temperature of dew point for this mixture is 60 C, and for the real gas is 140 C (this temperature I can approximate)
Here is the quastion how can I change the temperature of the dew point?
Places where the temperature drops below dew point temp I can verify with experiment.

Thanks for help

ghorrocks · June 26, 2018, 05:49

The dew point is a function of the amount of water vapour in the air and pressure. So if you simply want to know where a chimney design is going to condense, can't you just run a simple single phase model and look for where the temperature drops below the dew point? In other words, you don't need to model condensation at all.

To answer your question: You can change the dew point by changing the water vapour and pressure of the air. You do not set the dew point directly as an input variable.

nasdak · June 26, 2018, 06:34

In the first stage, I did as you wrote.
I solve single phase model and look for where the temperature drops below the dew point and velocity has low value. Using the wall model i get another usefull info like condensation mass flux or change in mass fraction.

Thank you for your answer.

nasdak · June 28, 2018, 06:52

Back to the topic. Vapor-Liquid equilibrium in CFX is defined with Antoin Equation. I calculated vapour pressure for the temp dew point. How can I calculte A,B,C parameters to complete Antoin equation. Thanks for help

nasdak · June 29, 2018, 15:43

Anybody solved similar problem and can help me? Come on.

ghorrocks · June 30, 2018, 05:51

I am not an expert on this stuff, but from my reading of the documentation it is not intended for users to derive the constant terms themselves. You should refer to the literature for the constants for various fluids - the CFX documentation references a textbook which apparently has values for several fluids.

If you want to derive your own constants for a fluid then you better read the literature on how other researchers have done this. This is not a CFX issue.

Gert-Jan · July 2, 2018, 03:01

The Antoine equations are fit parameters derived from experiments. You will have to look them up in literature. Go to the library and look for one of the heaviest books available:

CRC Handbook of Chemistry and Physics

June 26, 2018, 03:13	Temp of dew point	#1
nasdak New Member chris Join Date: May 2009 Posts: 18 Rep Power: 16	Hello I deal with flue gas flow in the industrial chimney. My goal is to identify zones where the condansation occurs. I use wall condensation model and defined air as an noncondansable and water as a vapor-liquid. My problem is the temperature of dew point for this mixture is 60 C, and for the real gas is 140 C (this temperature I can approximate) Here is the quastion how can I change the temperature of the dew point? Places where the temperature drops below dew point temp I can verify with experiment. Thanks for help

June 26, 2018, 05:49		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,700 Rep Power: 143	The dew point is a function of the amount of water vapour in the air and pressure. So if you simply want to know where a chimney design is going to condense, can't you just run a simple single phase model and look for where the temperature drops below the dew point? In other words, you don't need to model condensation at all. To answer your question: You can change the dew point by changing the water vapour and pressure of the air. You do not set the dew point directly as an input variable. __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

June 30, 2018, 05:51		#6
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,700 Rep Power: 143	I am not an expert on this stuff, but from my reading of the documentation it is not intended for users to derive the constant terms themselves. You should refer to the literature for the constants for various fluids - the CFX documentation references a textbook which apparently has values for several fluids. If you want to derive your own constants for a fluid then you better read the literature on how other researchers have done this. This is not a CFX issue. __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

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June 26, 2018, 06:34		#3
nasdak New Member chris Join Date: May 2009 Posts: 18 Rep Power: 16	In the first stage, I did as you wrote. I solve single phase model and look for where the temperature drops below the dew point and velocity has low value. Using the wall model i get another usefull info like condensation mass flux or change in mass fraction. Thank you for your answer.

June 28, 2018, 06:52		#4
nasdak New Member chris Join Date: May 2009 Posts: 18 Rep Power: 16	Back to the topic. Vapor-Liquid equilibrium in CFX is defined with Antoin Equation. I calculated vapour pressure for the temp dew point. How can I calculte A,B,C parameters to complete Antoin equation. Thanks for help

June 29, 2018, 15:43		#5
nasdak New Member chris Join Date: May 2009 Posts: 18 Rep Power: 16	Anybody solved similar problem and can help me? Come on.

July 2, 2018, 03:01		#7
Gert-Jan Senior Member Gert-Jan Join Date: Oct 2012 Location: Europe Posts: 1,827 Rep Power: 27	The Antoine equations are fit parameters derived from experiments. You will have to look them up in literature. Go to the library and look for one of the heaviest books available: CRC Handbook of Chemistry and Physics