Specify mass transfer source term at gas liquid interface

SAM S · January 17, 2016, 08:55

I am modelling a thin film evaporation process in fluent. Here mass transfer happens only at the free surface of the liquid film and I want to specify source term for the liq to gas transfer. How to specify in udf code that the source term should apply only at the interface. Am using VOF model with species transport enabled.

Thank you

`e` · January 20, 2016, 17:14

I'm not familiar with the VOF model, but I'll try considering no one else has responded. In your case is the volume fraction completely either phase in most of the domain and then a mixture along the free surface (one cell height)? You could use a conditional statement where the source term is applied where the volume fraction is a mixture. Otherwise, what other method could you use to define the location of the free surface?

SAM S · January 21, 2016, 00:16

Thank you for your reply. As you mentioned my mixture phase is very less compared to single phase of each. Am thinking to use the below condition

vof_cutoff = 0.05;
if ( (C_VOF(c,t) > vof_cutoff ) && (C_VOF(c,t) < (1 - vof_cutoff) )

Is this correct?

`e` · January 21, 2016, 06:26

Yes, I've also now found that code and it's the approach I'd take as well. Perhaps start with analysing the volume fraction within your domain and check if there's a step change between the phases (with one intermediate step which is between zero and one). Then you can select an appropriate value for the cut off.

SAM S · January 21, 2016, 08:23

Ok thank you. I will do that and proceed further.

January 17, 2016, 08:55	Specify mass transfer source term at gas liquid interface	#1
SAM S Member Join Date: Oct 2015 Posts: 34 Rep Power: 11	I am modelling a thin film evaporation process in fluent. Here mass transfer happens only at the free surface of the liquid film and I want to specify source term for the liq to gas transfer. How to specify in udf code that the source term should apply only at the interface. Am using VOF model with species transport enabled. Thank you

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January 20, 2016, 17:14		#2
`e` Senior Member Join Date: Mar 2015 Posts: 892 Rep Power: 18	I'm not familiar with the VOF model, but I'll try considering no one else has responded. In your case is the volume fraction completely either phase in most of the domain and then a mixture along the free surface (one cell height)? You could use a conditional statement where the source term is applied where the volume fraction is a mixture. Otherwise, what other method could you use to define the location of the free surface?

January 21, 2016, 00:16		#3
SAM S Member Join Date: Oct 2015 Posts: 34 Rep Power: 11	Thank you for your reply. As you mentioned my mixture phase is very less compared to single phase of each. Am thinking to use the below condition vof_cutoff = 0.05; if ( (C_VOF(c,t) > vof_cutoff ) && (C_VOF(c,t) < (1 - vof_cutoff) ) Is this correct?

January 21, 2016, 06:26		#4
`e` Senior Member Join Date: Mar 2015 Posts: 892 Rep Power: 18	Yes, I've also now found that code and it's the approach I'd take as well. Perhaps start with analysing the volume fraction within your domain and check if there's a step change between the phases (with one intermediate step which is between zero and one). Then you can select an appropriate value for the cut off.

January 21, 2016, 08:23		#5
SAM S Member Join Date: Oct 2015 Posts: 34 Rep Power: 11	Ok thank you. I will do that and proceed further.