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Andrew Clarke September 15, 2008 12:29

Material properties
 
Hi all

I am about to start a multiphase model in Fluent combining water and waste water sludge.

Does anybody know of a suitable equivalent material to use or of a suitable source for material properties of the sludge?

Thanks for reading..

mettler September 15, 2008 13:33

Re: Material properties
 
you should try calling some of the local sewage treatment plants..see if they can tell you.

Andrew Clarke September 15, 2008 13:49

Re: Material properties
 
I have already rang many. I have been unsuccessful so far. My options are limited to the fact that I would prefer to find the material properties after the process of anaerobic digestion.

I have found 720.9 kg/m3 for the density and 0.003 for the dynamic viscosity on the internet for sludge waste after anaeroibic digestion. 720.9 kg/m3 seems very small for sludge density.

I guess I will have to go with these values for now. Wish me luck.

Thank you for your advice and help Mettler.

mettler September 15, 2008 13:52

Re: Material properties
 
Have you tried the Civil Engineering Department - if you are in school? You could just call the local university if you aren't. Some of the professors there might have a good idea for some values.

wog September 15, 2008 16:27

Re: Material properties
 
you could try finding it out yourself, by dropping a steel sphere through a long vertical pipe full of the stuff and timing it (you need to do a force analysis on a falling sphere). You can easily work out what the velocity is a function of time and hence knowing the height of the pipe can then modify the viscosity on your correlation of drag to get your prediction to match with your experiments. I tried this some years ago, but it was difficult to get a large enough diameter pipe so that edge effects did not effect the falling of the sphere. You will have great difficulty in finding out these values as very few people know them, or be willing to tell you.

good luck!


Andrew Clarke September 16, 2008 06:45

Re: Material properties
 
I have a meeting later this week with my superviser. I will keep you all informed of the outcome in the next few weeks. Thanks guys for all your suggestions. It just comes to show that no matter what the problem there is always a solution somewhere.

andy2o September 16, 2008 13:55

Re: Material properties
 
Andrew,

The fluid properties of a sludge are likely to be non-Newtonian. E.g. the viscosity depends on the shear rate. This is true of many sludges found in waste water plants - and may well remain true after the process of anaerobic digestion. This non-Newtonian characteristic can *dramatically* affect the flow fields. Even the velocity field in a pipe can be completely altered compared to a normal constant viscosity fluid. So, don't just take a single viscosity value without checking if the fluid is non-Newtonian! Depending on the flow regime of interest, you might find that using a constant viscosity value can make your results very very wrong.

So using a single value (such as "0.003 for the dynamic viscosity" as you mention above) *could* turn out to be either OK (if you're very lucky) or very bad!

So, I suggest:

1) Do a literature review or measurements to see if your sludge is non-Newtonian. There's quite a large literature about the waste water industry!

2) If it is non-Newtonian, then you need to use a suitable viscosity model (power law, Bingham fluid, etc), so you must suitable parameters describing your sludge in terms of the chosen model. Most CFD major codes can handle this quite easily, but sometimes you need to use a user defined function.

3) At the very least, you need to consider how this will affect your flows, perform sensitivity studies, etc. It can make a big difference to the results.

Good luck, andy2o



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