Bernoulli s Principal would a fan matter?

[Mark D Larsen]

Bernoulli"s Principal would a fan matter?

If you have air moving through a 3' tube connected to a 1' tube with a taperd connection. Than the pressure in the 3' tube would be greater than the 1' tube due to the speed in the 1' tube being higher than the 3' tube, correct?

Then if you put a fan in the 3' tube (blowing air through the tubes) the pressures would still be higher in the 3' tube correct?

Then if you put a fan in the 1' tube (blowing air through the tubes) the pressures would still be higher in the 3' tube correct?

Then if you put a fan in between both tubes (In the tapered area blowing air through the tubes) the pressures would still be higher in the 3' tube correct?

Friend keep telling me that the pressure would be higher in the 1' tube when the fan is in the tapered area because the fan is compressing the air?

I thought that when speed increases pressure decreases and vice-versa. Would the fan in the tapered area throw this Principal out?

 

[rcgldr]

The fan increases both the speed and the pressure of the air it affects. In the immediate vicinity of the fan it's mostly an increase in pressure with little change in speed. Just fore of the fan is a low pressure area that speeds up the air further in front of the fan. just aft of the fan is a high pressure area that also speeds up the air further aft of the fan.

Whether there is an overall increase in pressure depends on the pressure increase due to the fan versus the pressure drop due to the tubes.

 

[lalbatros]

Larsen,

The static pressure variation will be:

variation(static pressure) = (fan total pressure rise) - variation(dynamic pressur)

If the fan provides only a small effect, a pressure drop will still result, as if there was no fan.
If the fan provides a large (total) pressure rise, a static pressure rise will result.
Bernouilli is engineer's best friend.

Michel

 

[cjl]

Bernoulli's principle only applies when there is no energy added to or lost from the flow. If there is a fan, you're adding energy, so it doesn't apply any more. An example of energy loss would be a shock - if the flow passes through a shock wave, then Bernoulli's principle would also not hold.

 

[lalbatros]

Strictly speaking you are right.
Bernoulli's principle only applies when there are no energy input!
Similarly the Bernoulli's principle does not apply when there are energy losses!
Therefore, strictly speaking, the Bernoulli's principle never applies in practice, since there are always some losses!

The Bernoulli's principle is usually applied by including additional terms for losses and gains of energy.
It is simply extended as follows:


pStatic2 + pDynamic2 = pStatic1 + pDynamic1 + DP(fan) - DP(losses)

or

variation(pStatic) + variation(pDynamic) = DP(fan) - DP(losses)


The specifications and testing of fans are based on this equation.
Similarly the pressure drops are defined from this equation.
The book https://www.amazon.com/dp/8179921182/?tag=pfamazon01-20 by I. E. Idelchik contains thousands of formulas to evaluate pressure drops in numerous sytems. All these formulas are referring to the above "extended Bernoulli's equation" that includes pressure drops or pressure rises.

 

[lalbatros]

... An example of energy loss would be a shock - if the flow passes through a shock wave ...

A shock is a steep variation of the flow parameters, like the static pressure or the density.
A shock, by itself, does not imply an energy loss.

The example I know the best are shocks in http://farside.ph.utexas.edu/teaching/plasma/lectures/node80.html" .
In the case of ideal MHD, the Bernouilli's equation needs to include an additional term: the magnetic pressure.
Shocks can build up even if there are no dissipative effect like those due to resistivity.
In these ideal MHD shocks, the energy conservation applies, which is the equivalent to the Bernouilli's law, except for the additional magnetic pressure.

The applicability of the Bernouilli's law or the energy conservation, depends only on the existence of dissipative effects. It does not depend on the existence of steep variations in the flow parameter.
However, steep variations may sometimes enhance dissipative effects.

 

[lalbatros]

The Bernouilli's principle can also be applied to a moving channel.
An additional term accounts for the work done by the channel on the fluid.
This leads to the modified Bernouilli equation where the energy of the fluid is not conserved, but changes because of the work done by the channel on the fluid.
This is how the theory of fans can be developped.

The same applies for energy losses by friction.

 

[kambireddy]

Dear All,

I am New to these forum. While seeing above discussions on SP, i would like to ask my problem too.

I am developing a centrifugal fan for my motor(internal).

First i need to find flow rate and pressure drop in inside of motor.
I know flow rate which 0.5 m^3/s. Next i have calculated pressure drop in motor using cfd tool(cfx).
My pressure drop is coming 600Pa (static) and 420Pa(Total).
I am feeling little bit confusion about these values? will DelPs is more than DelPt?

Pt1-Pt2=Ps1-Ps2+Pv1-Pv2,
Here my outer area(2) is 10 time lower than inlet area., these makes more velocity at outlet(2) and results decrease in DelPt compared to DelPs?

Also How could i decide a fan? based on static or total pressure drop?

I have a fan with 0.5m^3/s @350Pa(St) and 600Pa(Tot).

Plz help me out in these issue