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November 11, 2005, 08:42 
DEFINE_MASS_TRANSFER UDF

#1 
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I want to use the DEFINE_MASS_TRANSFER UDF to compute the mass transfer across phase interface.
The documentation about that UDF is very unclear and actually doesn't provide what in fact that function compute. In chapter 4.4.4 we have: "The mass transfer rate specified using a DEFINE_MASS_TRANSFER UDF..."  so it means that it computes the mass transfer rate [kmol/(m^3s)] In chapter 24.7 in the definition equation (24.71) source/sink terms are also expressed in [kmol/(m^3s)] BUT: Also in chapter 4.4.4 (a few lines down) we have: "The following UDF, named liq_gas_source, specifies a simple mass transfer coefficient..."  so it seems to be the mass transfer coefficient  but this is different quantity and is expressed in [m/s] In appendix B.1 "VOF Model" in Table B.1.2 (3 position form the bottom): "mass transfer coefficient  DEFINE_MASS_TRANSFER  phase interaction" so again mass transfer coefficient [m/s] Does anyone know what quantity in fact that UDF computes? regards Maciej 

November 11, 2005, 09:14 
Re: DEFINE_MASS_TRANSFER UDF

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I just copied the first part of the sample udf from the manual
m_lg = 0.1*C_VOF(cell,liq)*C_R(cell,liq)* fabs(C_T(cell,liq)T_SAT)/T_SAT; from units we see C_VOF is m3_liquid/m3_cell C_R is kg_liquid/m3_liquid 0.1 is the time factor in 1/s (you could read it as 10 percent per second) so the return value of m_lg is kg_liquid/m3_cell/second which is the total mass transfer and not the transfer coefficient as stated in the manual.I think its simply an documentation error. RoM 

November 11, 2005, 10:15 
Re: DEFINE_MASS_TRANSFER UDF

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Thanks for the answer  now I am sure what I should compute in that UDF.
In fact the units for mass transfer rate can be set in Define/Units/masstransferrate so it can be kmol/m^3s or kmol/cm^3s or kmol/ft^3s  so its probably not kg/m^3s. best regards Maciej 

November 11, 2005, 10:33 
Re: DEFINE_MASS_TRANSFER UDF

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The units from define>units dont apply to udfs. UDFs only use SI units and mass transfer is usually handled as kg/m^3/s. I can remember only two udf that require kmol/m^3/s (DEFINE_VR_RATE and DEFINE_HET_RXN_RATE).
RoM 

November 11, 2005, 11:01 
Re: DEFINE_MASS_TRANSFER UDF

#5 
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Oh, I didn't know about it. Thanks for that. I was sure the units from Define menu apply to every Fluent feature. I have to study the documentation more thoroughly.
Maciej 

December 2, 2005, 21:03 
Re: DEFINE_MASS_TRANSFER UDF

#6 
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Hello,
I have a question concerning DEFINE_MASS_TRANSFER UDF. Do you have any example other of that in manual? I am trying to make simulation with mass transfer accross interface and it seems to me that I need a cell geometry (volume) to make this. Also is that UDF applied to all cells or only these which defines the interface? regards, Maciej 

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