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Pádraig

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Posts posted by Pádraig

  1. (Apologies if this is the wrong forum, couldn't find one I thought suited. It's technically homework, feel free to move if there's a better fit!)

     

    Hi all,

     

    I'm looking to do some mixing at home, and measuring with a viscometer. I'll be using tap water, and one other fluid, idealy with a viscosity that's a bit higher than it. Where's the best place I could purchase a fluid that's

     

    1) Reasonably cheap,

    2) Has a viscosity at least a small bit higher than water,

    3) Has its density and viscosity values available?

  2. On 3/15/2023 at 2:07 AM, npts2020 said:

    Unless the two fluids you are mixing have the same viscosity to begin with, I don't see how it is possible to have homogenous viscosity without also having homogenous mixing. Whether your program will predict the correct outcome or not is a different matter...

     

    That's exactly the logic I've been using, I've just been worried that as I haven't got a high level/"expert" level of chemistry that I am/was overlooking something

  3. 19 hours ago, sethoflagos said:

    Bottom line is that software simulators are no substitute for laboratory work and pilot plant tests however much employers may wish that they were.   

     

    I completely agree with this, I'd actually be tempted to shout it even louder again. I'm just saying, even in a lab situation, when crosslinking is included - does that affect the overall homogeneity, when the molecular viscosity is homogenous throughout after mixing, and why/why not?

     

    (Sorry if this seems like I'm asking the same thing again - it's just that the link for entropy of mixing gives "when several initially separate systems of different composition, each in a thermodynamic state of internal equilibrium, are mixed without chemical reaction"

  4. 3 hours ago, sethoflagos said:

    Many viscosity models for mixtures employ terms that resemble in form those that characterise entropy of mixing. So if your model indicates a homogenous viscosity, it implies that the entropy is also homogenous which is as good an indicator of complete mixing as you could wish for.

    You will be on fairly safe ground when dealing with say mixtures of liquid alkanes where the components are fully miscible in all proportions and where much of the research on this subject has been focussed. Don't rely on it for say a suspension of cellulose fibres.

     

    Thanks very much - I'll look into those terms you've given me, that's a big help.

     

    Edit: @sethoflagos, this is in the second link: "mixed without chemical reaction". I'm mixing with crosslinking, will that heavily impact whether the viscous homogeneity is general homogeneity?

  5. 15 hours ago, sethoflagos said:

    In the absence of any explanation of how Ansys attempts to solve the advection equation in particular, then how can we be expected to judge how reliable it is?  

    Sorry, I probably should have added - Assuming Ansys Fluent is accurate (and I am running validation tests on it), is homogenous viscosity an OK meter for finding general homogeneity in mixed fluids? 

  6. Hi all,

     

    I'm mixing two fluids, and want to find when they become homogenous. I'm using a mixing programme called Ansys, and I'm checking the mixing via the molecular viscosity. It gives this kind of result (this is after 2550 s of mixing):

     

    image.png.a759e8708fb384f6050cfcb5926243ea.png

     

    In this mix, I have the initial viscosities of the fluids set at 100 cSt (99 % of mix) and 500 cSt (1 % of mix) respectively. I'm mixing 60-180 s per step, until I'm at homogenous viscosity (so I can see at what time this occurs). Is this OK for finding the general homogeneity of the fluid? Here is the initial mix after 300 s, for interest:

     

    image.png.16bd3e600fc2411d142ac8d8818978c1.png

  7. On 1/31/2023 at 7:20 PM, sethoflagos said:

    There are, but they are usually behind paywalls.

    I'd be wary of using viscosity as your parameter. A particularly important viscosity behaviour is that of a binary mixture of water and ethanol. It is highly irregular in certain proportions and certain conditions due to specific interactions between the components. You can get the experimental data easily enough, but then the question is whether Ansys contains an appropriate mixing model for that particular system. Empirical data always trumps predictive modelling. It comes down to what you're exactly trying to achieve.

    Could you pick entropy instead? It follows a much more predictable path that's quite simple to calculate. Look for a wikipedia page on Entropy of Mixing. 

     

     

    I can do Static Pressure and Pressure Coefficient (both 0) and Absolute Pressure, I couldn't find any setting for Entropy

  8. Hi all,

     

    I have a mixing simulation drawn up on the programme Ansys, where I am mixing two fluids. I want to verify my mixing process, by using this process to mix two other materials and take a variable of each material (e.g. viscosity) and compare the actual final mixing value to the one that I get when I mix them in my simulaiton.

     

    Is there a paper, site, or other available resource that contains mixing data for any two fluids?

  9. 37 minutes ago, studiot said:

    You surely can and I agree Seth deserves one.  See below to make your choice of like (blue) or upvote (green) .

    The only restrictions are that new members are restricted to 5 posts in their first 24 hours to prevent spambombers.

    click.thumb.jpg.209b48d8c16291967f7f2682fc1c9a43.jpg

     

     

    I'm getting the issue that there's no heart for Seth's OP, so when I went looking around it I couldn't give it karma; I have one for your post (both attached).

     

    Edit: reloaded page and I was able to give him the karma, heart appeared.

     

     

    01.PNG

    02.PNG

  10. 22 hours ago, sethoflagos said:

     

    Different forces are in play with different orders of magnitude through the start up process.

    When you start rotating the mixing bar, it acts like the impeller of a centrifugal pump and creates a pressure low spot at the 'eye of the pump'. This draws water down past the calcium block while the vortex is developing its parabolic profile above.

    The calcium rich water is then propelled radially outward until the flow regime is fully established.

    Now entropy takes over and calcium slowly diffuses up through the water column until it's evenly distributed. 

     

    That's really interesting, thank you. I've never really looked at fluid flow in that "detail" (may not be detailed to ye!), I'll look into what you've talked about now.

     

    I'm assuming I can't give you reputation as I'm a new member?

  11. Hello,

     

    I'm currently doing 3D mixing in a computer programme called Ansys. I have a cylinder full of water, featuring a square cuboid of calcium (the rectaungular cuboid is a stirrer bar, and the smaller cylinder is a mixing zone. I have gravity defined as the standard 9.81 m/s2 downwards) :

     

    image.png.b9a90f8d0f8d91e44b5259af35a38a66.png

     

     

    I have the mixing set at 60 rpm at the moment.

    I find that, after 2.5 minutes, my calcium is down around the bottom (this is a volume fraction graphic, with more blue meaning higher fraction of water, and as the colour leans into red, you have more calcium) :

     

    image.png.bfad2281546fac53eb4334291d6b63c4.png

     

    Then, as the mixing continues, the homogeneity of the entire cylinder stabilises much more (This is about ten minutes in. I've calculated that 3.4671e-04 throughout the cylinder is perfectly homogenous, so it's very close now) :

     

    image.png.d3be6de06cbe67a263c847109dfe7633.png

     

    My question (a basic mechanical/chemical one, I hope) is, why does the calcium initially "stick" to the bottom, despite being set as a cuboid a bit above the bottom at zero seconds, and then is "happy" to mix as time increases?

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