constant diffusivity coefficient

[gforney]

Please complete the following lines...

Application Version: 5.1.1
SVN Revision Number: 6385
Compile Date:        22 jun 2010
Operating System:    normal PC

Dear K. McGrattan et all,

In the technical reference guide equation (9,18) the binary diffusion coefficient of
vapour in gas is used to calculate the mass transfer coefficient. In the code this
is set in part.f90 on lines 1375 and 1557.
D_AIR                  = 2.6E-5_EB 
H_MASS = SHERWOOD*D_AIR/LENGTH
I was wondering why it is chosen in FDS to set this as a constant. In literature, by
example Bolz and Tuve 1976, I found that this diffusion coefficient can be almost twice
as large for a temperature of 423 K(not uncommon in smoke layers). Hereby causing the
mass transfer coefficient to become twice as large as well.

So I am wondering why FDS chooses to use a constant diffusivity for the mass transfer
coefficient of droplets when there is also a temperature dependent diffusivity defined
in FDS.

Best regards,

Romboud van de Woestijne
Master student of physics
Delft university of technology, Holland

Original issue reported on code.google.com by romboudvdw on 2010-11-02 15:27:37

[gforney]

I have always been curious about this fact -- when the NUSSELT and SHERWOOD numbers
are based on empirical correlations, does one use the ambient value of the transport
variable? Is the correlation based on the ambient or temperature-dependent value? And,
of course, the LENGTH is often arbitrarily chosen. We have chosen a LENGTH=1 m for
no particular reason. The lines of code you cite are for droplets that are attached
to surfaces. The mass and heat transfer correlations for this case have a large amount
of uncertainty because the droplets form a film and it is difficult to know exactly
how the heat/mass is exchanged. I think that the temperature-dependent D_AIR is just
one of several uncertain parameters here.

We have no plans at the moment to change our assumptions. The uncertainty is mainly
in the assumption of a droplet sticking to a wall and forming a uniform film.

Original issue reported on code.google.com by mcgratta on 2010-11-02 21:38:38

[gforney]

Dear Kevin,

Thank you for your extensive answer! Unfortunately I misqouted the line, it should
be line 1569. I am mostly thinking about the situation of a droplet in free air.

Nice to see you are also a bit in doubt which value to use for these transport coefficients.
I found myself in a similar situation. I will have a further thought about it.

Maybe I could ask my professor about his opinion. 

best regards,

Romboud

Original issue reported on code.google.com by romboudvdw on 2010-11-04 09:15:41

[gforney]

Thanks -- I'll keep this issue open. Let's see if we can clarify the use of empirical
heat/mass transfer coefficients.

Original issue reported on code.google.com by mcgratta on 2010-11-04 12:16:32

[gforney]

My experience with these correlations (from using Incropera and DeWitt) is that the
properties are evaluated at the "film" temperature, which is the average between the
free stream and the surface temperature. I have not specifically checked the case of
an evaporating droplet, but I would assume that it is similar.

Original issue reported on code.google.com by williamson.justin.wade on 2010-11-04 14:38:43

[gforney]

Yes, and that's part of the problem with evaporation. The evaporation rate is very sensitive
to the selection of film and free stream values. 

Original issue reported on code.google.com by mcgratta on 2010-11-04 15:05:45

[gforney]

http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/22473/1/97-0966.pdf

This paper in section 3.4 discusses the effect of what temperature to use as the reference
condition.  It concluded that using a temperature larger than the droplet temperature
gave larger errors than just using the droplet temperature and best was the wet bulb
temperature.  

Original issue reported on code.google.com by drjfloyd on 2010-11-04 17:56:03

[gforney]

Romboud -- has there been any progress on this issue?

Original issue reported on code.google.com by mcgratta on 2010-12-06 21:00:52

[gforney]

(No text was entered with this change)

Original issue reported on code.google.com by mcgratta on 2010-12-10 18:09:46