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Drummer March 16, 2003 14:56

Evaporation problem
Hello! I'm a beginner in star-cd and I have such a problem: I'm using 2 phase lagrangian flow - air + H20 droplets (just like in tutorial...) Air flow temperature is 413K, droplets temp is for example 350K. The problem is that when droplets do not evaporate - their temperature continues raising after 373K (100 C). What do I have to do to realize droplets evaporation - temperature must be constant and mass decrease... Where am I wrong? Best regards...

Volker March 17, 2003 06:47

Re: Evaporation problem

First of all, check if the boiling model in droplet properties is switched on. Furthermore you have to provide sensible values for Tcrit and the saturation pressure. For the latter I think you should provide a temperature-dependent function via a user subroutine if you want to get realistic results.

Hope that helps,


Drummer March 17, 2003 11:38

Re: Evaporation problem
Thanks, but: 1) If I turn on boiling model, droplets instead of continius evaporation boil immediatly 1setp there droplets exist and next step there are no droplets any more:(( not realistic... and I want them to evaporate slowly... 2) I don't know what to write in subroutine - I hadn't used them... How can I do it... I mean temp-dependent function - are there some examples???

Best Reagrds

Volker March 18, 2003 03:06

Re: Evaporation problem

it shouldn't be too difficult to find values for the saturation pressure of water. Otherwise try and find an equation to calculate it (eg. Lee and Kessler equation). The crit. temperature of water is 647,14 K. To use the user-subroutine, create a directory 'ufile' in your working directory and then select 'user' under 'saturation pressure' in 'droplet properties'. The file dropro.f will be written in ufile. You have to write your equation for the saturation pressure into this file.

Hope this works,


Drummer March 18, 2003 03:24

Re: Evaporation problem
"You have to write your equation for the saturation pressure into this file. " - That's the main difficulty - I don't know what to write and how to write - I don't know fortran syntax! - That is why I ask examples... for any equation

Thank you

Volker March 18, 2003 04:04

Re: Evaporation problem
OK. if you open the file dropro.f, you will find two blank lines followed by 'return end' at the bottom of the file.

you will fill in the equation into this empty space after you have defined the droplet type for which you want to use the user-subroutine (eg. type 1):


'your equation' (PS=f(TD), e.g. PS=3.0*TD**2, this is just an example!!!)


Make sure you leave at least seven spaces between your input and the left margin.

Good luck,


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