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sprayFoam Evaporation Model

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Old   October 20, 2015, 10:03
Default sprayFoam Evaporation Model
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Mr.liu
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HI
I am a new foamer and now i meet a problem that i can not solve, please help me if u know.

i use sprayFoam in OF 2.3.0 to simulate SPRAY A (900K atmosphere), PhaseChangeModel, LiquidEvaporationBoil (default setting). The liquid should evaporate quickly because atmosphere T is 900K. However, the evaporation rate is very slow.

My friend told me that, 2.3.0 sprayFoam is same as 2.0.0 dieselFoam, so i simulated it again using dieselFoam, 2.0.0, evaporationModel, standardEvaporationModel (default setting), the results seem good, evaporate and mix fast.

So, i am confused that, why there is no dieselFoam in 2.3.0 and the sprayFoam can replace the dieselFoam? is there any difference in Ecaporation Model? or it just changes another name?

Thanks u
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Old   October 21, 2015, 07:45
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Quote:
Originally Posted by lx882211 View Post
HI
I am a new foamer and now i meet a problem that i can not solve, please help me if u know.

i use sprayFoam in OF 2.3.0 to simulate SPRAY A (900K atmosphere), PhaseChangeModel, LiquidEvaporationBoil (default setting). The liquid should evaporate quickly because atmosphere T is 900K. However, the evaporation rate is very slow.

My friend told me that, 2.3.0 sprayFoam is same as 2.0.0 dieselFoam, so i simulated it again using dieselFoam, 2.0.0, evaporationModel, standardEvaporationModel (default setting), the results seem good, evaporate and mix fast.

So, i am confused that, why there is no dieselFoam in 2.3.0 and the sprayFoam can replace the dieselFoam? is there any difference in Ecaporation Model? or it just changes another name?

Thanks u
can anybody help me?
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Old   January 7, 2016, 08:33
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Hi,
do you solve your problems?
I am having similar problems with sprayFoam since I see that, considering even the aachenBomb tutorial, the spray starts to evaporate even if both gas and liquid temperature are imposed at 300K. On my opinion this is not true.
Opinions regarding this point are welcome!
Stefano
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Old   January 7, 2016, 08:40
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Quote:
Originally Posted by Stefano Puggelli View Post
Hi,
do you solve your problems?
I am having similar problems with sprayFoam since I see that, considering even the aachenBomb tutorial, the spray starts to evaporate even if both gas and liquid temperature are imposed at 300K. On my opinion this is not true.
Opinions regarding this point are welcome!
Stefano
sorry i haven't solve it, which version you use OF?
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Old   January 7, 2016, 08:42
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The version that I am using is the 2.3.1
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Old   January 7, 2016, 09:01
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Quote:
Originally Posted by Stefano Puggelli View Post
The version that I am using is the 2.3.1
in 2.3, PhaseChangeModel includs the evaporationModel in 2.0, it depends on the parameters, but i am not sure how to set it.
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Old   January 9, 2016, 09:20
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Pascal Balz
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Hi,

some time ago I had similar problems with the evaporation rate in sprayFoam.

Quote:
Originally Posted by Stefano Puggelli View Post
the spray starts to evaporate even if both gas and liquid temperature are imposed at 300K.
As long as you don't have saturated vapor in your fluid domain there will always be a mass transfer from spray to carrier phase due to evaporation (not boiling).

Have a look at the source code of the liquidEvaporationBoil model:

Code:
else
            {
                // evaporation

                // surface molar fraction - Raoult's Law
                const scalar Xs = X[lid]*pSat/pc;

                // molar ratio
                const scalar Xr = (Xs - Xc)/max(SMALL, 1.0 - Xs);

                if (Xr > 0)
                {
                    // mass transfer [kg]
                    dMassPC[lid] += pi*d*Sh*Dab*rhos*log(1.0 + Xr)*dt;
                }
            }
This is for evaporation only, so T < TBoil and no saturated vapor.
As you can see the mass transfer is strongly dependent on the 'molar ratio' (which you might not change) and 'Dab' which basically is the vapor diffusivity and therefore a liquid property.
I changed this value so that the evaporation rate fits my desired values, as I needed a completely new liquid property anyway. You can find this value in the liquidProperties source code of your specific liquid (i.e. C2H6.C). Just change the value of 'D_' there and remember to look up which polynomial interpolation is used for the specific liquid (NSRDS functions).
After the changes recompile the code and it should work

I don't know if there are any other possibilities to 'tune' the evaporation rate, but this worked perfectly for me.
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Pascal
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Old   January 10, 2016, 07:08
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Hi Pascal,
thank you for your quick answer.
After looking to the source code, I arrive to your considerations.
So I perfectly agree with you.
Thanks,
Stefano
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