Bernoulli's Principle and Fans [split]

  • Thread starter T C
  • Start date
  • #1
T C
149
3
No (obviously). This is a direct result of Bernoulli’s principle, which is simply conservation of energy.
That's confusing. If the velocity at the exit is increased, that simply means kinetic energy is increased. But as the pressure is decreased and suppose the process is adiabatic (in case of compressible fluid), that means the temperature too is decreased. That simply means the enthalpy of the fluid is decreased. Where did the decreased enthalpy gone? It has converted into the kinetic energy. Thus the total amount of energy is conserved, so no violation of 1st law of thermodynamics.
The motor power used by the blower is converted into kinetic energy of the fluid and both before and after the fluid entering and exiting the blower, it contains its own internal enthalpy. When the speed increases at the throat, its the enthalpy that has been converted into kinetic energy as both pressure and temperature decreases at the throat. So the Kinetic energy of the fluid at the throat is certainly higher than the energy supplied to it by the motor of the blower because inside the nozzle, the enthalpy has been converted into Kinetic Energy.
The nozzle will create back pressure on the fan - and that will slow the airflow and the fan.
The nozzle will create backpressure because of the friction inside. A perfect frictionless nozzle wouldn't give any kind of backpressur. It's called the nozzle efficiency. The closer the efficiency of the nozzle to perfection i.e. 100%, lesser will the backpressure. And what the backpressure would do is to reduce the ratio. As for example, if the ratio is 2:1, then the velocity at the throat wouldn't 2 times at the inlet but 1.5 due to frictional loss i.e. backpressure.
The flaw is that you didn't do any real analysis of the fan's performance; you just multiplied the outlet velocity by 2!
Real fans have performance curves that must be used to analyze the fan's performance when attached to a given system
Can you explain how a fan's performance can be affected by just putting a nozzle before it?
 

Answers and Replies

  • #2
russ_watters
Mentor
20,284
6,864
That's confusing. If the velocity at the exit is increased, that simply means kinetic energy is increased. But as the pressure is decreased and suppose the process is adiabatic (in case of compressible fluid), that means the temperature too is decreased.
As per many, many previous threads where we've had this discussion, the assumption for a "fan" unless otherwise stated is that the flow is incompressible, so there are no density or temperature change happening here. This problem is described completely by the basic version of Bernoulli's equation.
The motor power used by the blower is converted into kinetic energy of the fluid and both before and after the fluid entering and exiting the blower, it contains its own internal enthalpy. When the speed increases at the throat, its the enthalpy that has been converted into kinetic energy as both pressure and temperature decreases at the throat. So the Kinetic energy of the fluid at the throat is certainly higher than the energy supplied to it by the motor of the blower because inside the nozzle, the enthalpy has been converted into Kinetic Energy.
No. [in a compressible situation] In order to be decompressed and cooled it first needs to be compressed and heated -- by the blower. In real life, in order to have an outlet temperature lower than the inlet, a heat exchanger is generally used.
The nozzle will create backpressure because of the friction inside. A perfect frictionless nozzle wouldn't give any kind of backpressure. It's called the nozzle efficiency.
No. Even in a totally lossless situation, the nozzle adds backpressure. The nozzle applies pressure because it's sides are angled into the flow! Or, in order to speed up the flow it has to have static pressure that can be converted to velocity pressure. Again: look at Bernoulli's equation!

The efficiency just tells us how much additional pressure is lost.
The closer the efficiency of the nozzle to perfection i.e. 100%, lesser will the backpressure. And what the backpressure would do is to reduce the ratio. As for example, if the ratio is 2:1, then the velocity at the throat wouldn't 2 times at the inlet but 1.5 due to frictional loss i.e. backpressure.
OMG, NO! You are making such a big mess here. Except in compressible flow, the velocity has to be exactly the ratio of the areas, otherwise conservation of mass is violated. If the ratio were to drop, air would have to vanish!
Can you explain how a fan's performance can be affected by just putting a nozzle before it?
When you say "before", you really mean at the outlet, right?

This was answered succinctly in post #3. To add a bit more: static pressure at the fan outlet has to go up, so airflow - per the fan curve - has to go down.

Haven't we been discussing this for something like a year?
 
  • #3
Dale
Mentor
Insights Author
2020 Award
30,860
7,463
That's confusing. If the velocity at the exit is increased, that simply means kinetic energy is increased. But as the pressure is decreased and suppose the process is adiabatic (in case of compressible fluid), that means the temperature too is decreased.
We were discussing incompressible flows here. This is a reasonable approximation for low Mach numbers, and here we are dealing with around Mach 0.1, so the incompressible flow is a good assumption.

A perfect frictionless nozzle wouldn't give any kind of backpressur.
I don’t believe this. Please cite the scientific reference that you believe teaches this.
 
  • Like
Likes russ_watters
  • #4
T C
149
3
As per many, many previous threads where we've had this discussion, the assumption for a "fan" unless otherwise stated is that the flow is incompressible, so there are no density or temperature change happening here. This problem is described completely by the basic version of Bernoulli's equation.
Bernoulli's equation means fall in pressure and that means fall in temperature too. Do you want to deny that? There is nothing that can be considered as 100% incompressible. It's an observed fact that when aeroplanes fly, if the air sufficiently humid, then fog is observed above the wings as the temperature and pressure there falls and a part of humidity has been converted into water.
220px-Cloud_over_A340_wing.jpg

This picture is clear proof that there are fall in both pressure and temperature even in case of "incompressible fluids".
No. [in a compressible situation] In order to be decompressed and cooled it first needs to be compressed and heated -- by the blower. In real life, in order to have an outlet temperature lower than the inlet, a heat exchanger is generally used.
I don't know any such theory. Kindly give me some source. And also clarify whether the air above the wings of aircrafts were first compressed and heated before being cooled or not.
No. Even in a totally lossless situation, the nozzle adds backpressure. The nozzle applies pressure because it's sides are angled into the flow! Or, in order to speed up the flow it has to have static pressure that can be converted to velocity pressure. Again: look at Bernoulli's equation!.
The efficiency just tells us how much additional pressure is lost.
In case of an ideal situation, the pressure/enthalpy lost is converted into kinetic energy.
OMG, NO! You are making such a big mess here. Except in compressible flow, the velocity has to be exactly the ratio of the areas, otherwise conservation of mass is violated. If the ratio were to drop, air would have to vanish!
Kindly give a look at this video here. I am sure that the person describing the scenario here is no less knowledgeable than you.
 

Attachments

  • #5
russ_watters
Mentor
20,284
6,864
Bernoulli's equation means fall in pressure and that means fall in temperature too. Do you want to deny that?
Yes, absolutely, completely, wholeheartedly. You are misusing Bernoulli's equation if you think that. Please read the link provided and pay particular attention to where it says: The simple form of Bernoulli's equation is valid for incompressible flows.
There is nothing that can be considered as 100% incompressible.
Flow is in fact assumed incompressible below about 220mph/mach .3 in basic fluid mechanics, including aerodynamics, and again, in particular for the basic form of Bernoulli's equation.
I don't know any such theory. Kindly give me some source.
Bernoulli's principle, provided above.
Kindly give a look at this video here. I am sure that the person describing the scenario here is no less knowledgeable than you.
It's 8 minutes long: please tell me what specific point is addressed, where.
 
  • #6
T C
149
3
When you say "before", you really mean at the outlet, right?
This was answered succinctly in post #3. To add a bit more: static pressure at the fan outlet has to go up, so airflow - per the fan curve - has to go down.
Haven't we been discussing this for something like a year?
No, Before means before entering the blower. And what I actually want to mean is the airflow contains some enthalpy before being blown by the blower and that remains unchanged.
Note, @.Scott in this case the fan is turned by a 3-phase motor and the rpm is fixed. For some motors/fans, the rpm will drop if you add a nozzle, as you said.
I just want to know any real example of this phenomenon. From the graph, it's simply not possible to understand how rpm will reduce if a nozzle is placed before a blower and all the conditions will remain unchanged.
And, by the way, for a nozzle "back pressure" means the pressure in which the flow from the nozzle is released. Kindly read this thread here. In case of a blower, there is no example as far as I know that the flow is decreased due to the effect of the nozzle.
 
  • #7
T C
149
3
We were discussing incompressible flows here. This is a reasonable approximation for low Mach numbers, and here we are dealing with around Mach 0.1, so the incompressible flow is a good assumption.
You are shifting the goalposts. What I want to mean is that the increase in velocity at the throat comes at the cost of internal enthalpy of the fluid. As the pressure decreases, so the temperature along with it and we can consider the process to be adiabatic. Just calculate the fall in pressure and from that the drop in enthalpy and at the same time rise in the kinetic energy at the throat. You can see that it's almost the same. At least close enough to be accepted scientifically.
I don’t believe this. Please cite the scientific reference that you believe teaches this.
First we have to correct the definition of back pressure for a nozzle. For a nozzle, back pressure means the pressure at which the flow is discharged. I don't any example that pressure point is created before a blower for putting a convergent nozzle before it.
 
  • #8
russ_watters
Mentor
20,284
6,864
No, Before means before entering the blower.
This thread is discussing a nozzle at the outlet of the fan.
I just want to know any real example of this phenomenon.
What phenomenon?
From the graph, it's simply not possible to understand how rpm will reduce if a nozzle is placed before a blower and all the conditions will remain unchanged.
As I said, for the scenario I gave, the rpm is fixed. It does not get reduced. Also, most industrial fans - including the one I gave - have an inlet cone/bell already. They smooth airflow entering the fan to provide for less pressure drop.
 
  • #9
russ_watters
Mentor
20,284
6,864
Off topic posts have been split from another thread.
 
  • #10
Dale
Mentor
Insights Author
2020 Award
30,860
7,463
First we have to correct the definition ...
No, first you have to provide a professional scientific reference that supports your disputed claim that “A perfect frictionless nozzle wouldn't give any kind of backpressur”.

On this forum it is mandatory to provide references on request.
 
  • #11
russ_watters
Mentor
20,284
6,864
@T C, here's what I need from you in order for this thread to be allowed to continue:

1. Explicitly acknowledge you accept and understand the incompressible flow nature of the basic form of Bernoulli's equation, per post #5.
2. Specifically describe/define the scenario you would like to discuss, since clearly it differs from what was being described in the thread this was split from.
 
  • #12
T C
149
3
No, first you have to provide a professional scientific reference that supports your disputed claim that “A perfect frictionless nozzle wouldn't give any kind of backpressur”.
In that case, I am requesting you to give a reference on how nozzles create bakcpressure and what can be considered as back pressure of a nozzle.
 
  • #13
russ_watters
Mentor
20,284
6,864
Thread locked as it is promoting misinformation.
 

Related Threads on Bernoulli's Principle and Fans [split]

  • Last Post
Replies
8
Views
11K
  • Last Post
Replies
14
Views
9K
Replies
14
Views
12K
Replies
1
Views
2K
  • Last Post
Replies
3
Views
5K
Replies
10
Views
2K
Replies
4
Views
5K
  • Last Post
Replies
15
Views
4K
Replies
4
Views
8K
  • Last Post
Replies
13
Views
4K
Top