8 replies to this topic

[polipo]

Hi there,

I've done the experiment of fluid flow in pipe and sedimentation. First, I wonder why there is negative pressure drop in sudden expansion. Is it possible that due to the diameter gets larger, and The pressure in the bigger pipe is higher than that of smaller one, so the pressure drop becomes negative? Does this make sense? Moreover, negative pressure drop also existed in orifice meter and venturi meter as well. If so, why is the pressure
proportional to the diameter of the pipe?

For sedimentation, I'm unsure that settleing velocity at t₁ is (z₁-z₀)/t₁ or (z₁-z_i)/t₁ or (z_i-z₁)/t₁.

Thank you for your assistance.

[katmar]

To understand the pressure increase through a sudden expansion (or a gradual expansion) you need to revise your notes on the Bernoulli Equation. The Bernoulli Equation is a statement of conservation of energy, explaining how kinetic energy is converted to pressure energy as the velocity decreases. The actual pressure increase is less than that predicted by Bernoulli, and this non-ideality is calculated using the k-value for the particular fitting e.g. sudden expansion, pipe reducer or conical section.

If you define the settling velocity as dz/dt at any t, then the slope at t would be (zi-z1)/t1.

[polipo]

If I change z₁ to any other point, will z_i change? Or it will be constant in any cases?

One more thing about the concentration. Is the formula: concentration at any time = (initial concentration)(z₀)/z_i correct? I'm unsure because my results were weird.

Thank you again

[katmar]

zi is the tangent line and z1 is the point of contact between the tangent and the curve. They will both change as t changes. I don't know about the concentration.

[polipo]

That's fine. Thank you, though.

I still have further questions. Does the concentration affect the settling velocity? Or only particle size that influences on settling velocity?

My result showed about the same settling velocity no matter what concentration it is.

[Dacs]

That's fine. Thank you, though.

I still have further questions. Does the concentration affect the settling velocity? Or only particle size that influences on settling velocity?

My result showed about the same settling velocity no matter what concentration it is.

This is way back college but I believe that in sedimentation, you have to consider the regime where you based your model on.

At first, the particles are far apart that you can assume that the particles are floating freely.

But as the sediments condense to form a layer, I believe the regime to model it is different since you also have to consider the neighboring particles.

[polipo]

This is way back college but I believe that in sedimentation, you have to consider the regime where you based your model on.

At first, the particles are far apart that you can assume that the particles are floating freely.

But as the sediments condense to form a layer, I believe the regime to model it is different since you also have to consider the neighboring particles.

Sorry I don't understand your point. Could you please clarify more? I really have problem with sedimentation but I've already tried to research.
By the way, I actually mean the settling velocity in the constant period.

Edited by polipo, 26 July 2010 - 09:15 PM.

[Dacs]

This is way back college but I believe that in sedimentation, you have to consider the regime where you based your model on.

At first, the particles are far apart that you can assume that the particles are floating freely.

But as the sediments condense to form a layer, I believe the regime to model it is different since you also have to consider the neighboring particles.

Sorry I don't understand your point. Could you please clarify more? I really have problem with sedimentation but I've already tried to research.
By the way, I actually mean the settling velocity in the constant period.

At the initial stage of sedimentation the sediments are scattered all around. So you can say that at this point, the particulates fall freely on their own against the liquid without any intervention from other particulates.

I believe this is the "constant" rate regime.

But at some point, the sedimentation rate will be affected by the neighboring particulates. I think this is the "hindered settling" point.

I'm just pulling it out of what I've learned back in college so my knowledge is rusty at best.

[polipo]

Thank you a lot. You did save me from this lab.