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Q: Shear rate in a hand mixing ( Answered,   0 Comments )
Subject: Shear rate in a hand mixing
Category: Science > Physics
Asked by: cct-ga
List Price: $10.00
Posted: 14 Jun 2006 19:38 PDT
Expires: 14 Jul 2006 19:38 PDT
Question ID: 738257
How much is the shear rate in a hand-mixing?

The shear rates in various conditions are listed in some textbooks.
The shear rates of the mixing or stirring is within the range of
10^1-10^3 (s^-1). But, the specific range of the shear rate in a
hand-mixing is not given. Some web resouces provide a number, 200
(s^-1). I am not confident with this value because I could not find
out the references on how to obtain this number. If possible, please
help to find out any reseach
paper related to determination of the shear rate in a hand-mixing.

Request for Question Clarification by hedgie-ga on 23 Jun 2006 04:20 PDT
The question is not sufficienty well defined to allow for quatitative answer.

 It depends on the container, speed of mixing, stirrer, ...

  If you really need precise data, it would be better to measure the rate.

 What is your budget?

Clarification of Question by cct-ga on 06 Jul 2006 02:50 PDT
It sounds a good idea to measure the shear rate directly.
If I am going to make cement pastes of 10 grams using a small plastic
weighting dish, how can I estimate the shear rate while I am mixing?
Subject: Re: Shear rate in a hand mixing
Answered By: hedgie-ga on 06 Jul 2006 05:42 PDT
It would be easier if you would have a transparent liquid.

Also, you, I hope, understand that what you will measure in small
container will not transfer simply to a larger container.

There are some 'scaling laws' which define similar conditions, but
they involve more then just size of the vessel, but also viscosity,
speed of rotations and size of the mixing blade. Problem is more complex
then $10 question would indicate, but I can give you some references which
I hope will be useful - at least useful for asessing the scope of the problem.

There are shear-rate meters - and whole science of Rhaology dealing
with these issues. Even though you did not indicated the size of your
budget, I am guessing that you are intending to give it some time and energy.

So, lets starts with an overview:

 People do study complex flows -  it is a demeanding discipline,
 normally needing a well eqipped laboratory and expertise.
  Shear rate is not same at different parts  vessel, it changes
  with position, and in your case with time (as viscosity is changing).

Typical velocity field looks like this:

Even if liquid is not transparent, you can imbed markers and track 
those near surface  optically, other by complex telemetry (Xrays,
magnetic sensors ..)

Markers could be small rods (rather then balls) - and then their rate
of rotation will be proportional to shear rate at that time and place,

 Here are references to the scaling laws and Reynolds number
To measure rheology of a material, it needs to deform over a period of
time, ... Scaling Laws. Strain rates in earth versus experiment are
orders of ...

 Basicaly Same number means 'similar flow' (turbulent or laminar)

I suspect you want something more simple:

I would try to estimate the shear rate from the force  needed to mix the paste 
at different rotation rates.

The viscosity of the paste is non-newtonian, and so the torque needed
to rotate the blades will depend on the shear rate.

You will have to measure fast (electronic recording) to differentiate
effect of speed and of hardening.

Here you will find more references:

good luck

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