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-   -   Modeling Evaporation at a Heated Wall (http://www.cfd-online.com/Forums/openfoam/84866-modeling-evaporation-heated-wall.html)

 Edy February 10, 2011 10:20

Modeling Evaporation at a Heated Wall

Hi Foamers,

I want to model nucleate boiling in cylindrical pipes, and one important part in nucleate boiling is ... boiling at the wall! :)

I use the Kurul & Podowski heat flux partitioning. In my solver I calculate the three contributions. The evaporation heat flux density q_e is defined as volScalarField which is null everywhere except for near wall cells where:

q_e=Pi/6*d_dep³*freq*NSD*rho_v*h_lv (where d_dep is the departure diameter of bubbles,freq the detachment frequency, NSD the nucleation site density and h_lv is the latent heat).

Once this is known I can determine the evaporation mass flow rate at the wall, in [kg/m²/s], which is defined as:

m_e = q_e/h_lv

But all the equations solved in my solver (alphaEqn, UEqns, ...) are dealing with cells, meaning finite volumes, therefore I need a volume evaporation rate, whose unit is in [kg/m³/s]. I thought about calculating it as :

Gamma_e = m_e*(area of the cell in contact with the heated wall)/(volume of the cell)

But if x is the direction of the flow:

Gamma_e = m_e*(x_cell*z_cell)/(x_cell*y_cell*z_cell)
= m_e/y_cell

And my evaporation rate depends on the mesh refinement which is of course non physical...

Has anyone an idea how to solve this problem? Cause I've been thinking about it for a couple of days unsuccessfully...

Thanks for reading, giving a tip or helping!

Best,

/Edouard

 Edy February 10, 2011 10:28

I wrote "mass flow rate" for m_e but it is indeed rather a evaporation mass flux!

 nimasam February 11, 2011 05:32

in openFoam there is no interface reconstruction so you can not convert mass flux into mass evaporation per volume! like :
massflux * interfaceArea/ volume

you can only use mag(grad(alpha)) which seems its not compeletly true for local interface area estimation
how ever i eager in this subject, can u share ur efforts (aricles and source code)?
nimasam65 <@> gmail.com

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