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ranjit January 15, 2007 01:56

UDF for heat transfer
 
Can anyone help me with creating a udf for heat transfer between ice and air...Since this is the first time i am going to attempt writing a udf, even a basics help is welcome..

ranjit January 15, 2007 03:42

Re: UDF for heat transfer
 
please someone atleast suggest the macros that can be used , the logic, etc , i just need something to build upon..

Sujith S Nair January 15, 2007 15:15

Re: UDF for heat transfer
 
Are you simulating air flow over ice with ice as wall boundary condition. Then you can use a constant heat flux or a heat flux profile using DEFINE_PROFILE macro.May i know what is the exact physics in your model?

ranjit January 16, 2007 08:57

Re: UDF for heat transfer
 
thanks for the response

yeah basically my aim is to find the amount of chilling caused by airflow over ice. Actually an ice-water mixture is to flow inside a tube , and the air flows over the tube.. I need to find the amount of heat that the ice-water mixture can absorb.

Can such a problem be done in multiphase flows in fluent without UDF?. As of now i am treating it as a convection problem between air and ice, bur dont know how to bring the latent heat of "melting of ice" into picture..

Thanks for the suggestion to use Heat flux macros, i ll look into it.. Do you have any idea how to formulate a governing equation for this.. Any help besides that is also welcome.

Thanks!

Venkatesh V January 16, 2007 10:26

Re: UDF for heat transfer
 
What is the size of ice particles interms of tube dia or volume fraction.

ranjit January 16, 2007 10:37

Re: UDF for heat transfer
 
the volume fraction will be around 0.4 how is that going to help?

Venkatesh V January 16, 2007 12:22

Re: UDF for heat transfer
 
I thought if volume fraction is low you can consider using DPM (Multiphase) and you can control the melting using UDF is needed.

ranjit January 16, 2007 21:18

Re: UDF for heat transfer
 
@venkatesh

but how to specify three different materials in fluent, ice , water and air. Also i thought multiphase in fluent is only for immiscible fluids, water and ice are miscible. What approach do you suggest

ranjit January 17, 2007 08:25

Re: UDF for heat transfer
 
any further help please..

Hugo January 17, 2007 09:53

Re: UDF for heat transfer
 
Hi. Is this a steady state problem? is the ice/water mixture flowing down the tube? Is the amaount of heat exchanged actually melting the ice? Options: - a domain of only air and a solid tube. Define specific heat of tube to be temperature dependant, with very very high value (2.27 MJ/kg/K) in the range 272.5<T<273.5. see example 1 in 4.3.14 DEFINE_PROPERTY section of UDF guide for format. Can't find syntax for specific heat capacity there. Anyone? - hand calculate it, using heat transfer correlation from eg. Bejan, 2004. Convecton Heat Transfer. Wiley&Son. p.363. Hugo.

ranjit January 17, 2007 11:40

Re: UDF for heat transfer
 
Thanks, will give it a try surely... is it sufficient to give the define_property macros alone? i am new to writing UDF's

Hugo January 18, 2007 05:34

Re: UDF for heat transfer
 
Sorry, me last message was truncated - I was going to say have variable heat capacity, with a value of 2.27 MJ/kg/K in the range 272.5 < T < 273.5, following the format in example 1 in 4.3.14 of UDF manual, using DEFINE_PROPERTY macro:

DEFINE_PROPERTY(cell_viscosity,c,t) { real mu_lam; real temp = C_T(c,t);

if (temp > 288.)

mu_lam = 5.5e-3; else if (temp > 286.)

mu_lam = 143.2135 - 0.49725 * temp; else

mu_lam = 1.;

return mu_lam; }

obviously you'll have to change it to specific heat capacity.

Not sure I understood you question " is it sufficient to give the define_property macros alone?" - could you give more details?

Hugo.


ranjit January 18, 2007 10:43

Re: UDF for heat transfer
 
thanks for the reply, but in the same documentation, if you scroll up its given that "specific heat values can only be read and not assigned in fluent"..

does that make this method ineffective?


Sujith January 18, 2007 11:36

Re: UDF for heat transfer
 
ya... in Fluent specific heat cannot be specfied through a UDF, But you can give a temperature dependant polynonial/piecewise polynomial using the options availble in the Materials panel.

--sujith


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