Blower fitted with a convergent nozzle

[T C]

Recently I have some conversation with an electrical engineer and from him I cam to know about a peculiar information. We were talking about what will happen if we put a convergent nozzle before a electricity driven fan/blower. I have concentrated what will happen at the throat and other factors. But, he said that putting a convergent nozzle will put more load on the motor of the blower/fan. I have repeatedly asked him to give some examples and also about the possible theoretical explanation behind this phenomenon. He admitted that he was unable to analyse it theoretically and the only incident that he revealed is that one of his very close friend had told him that.
if he was a common man, I wouldn't bother to think about what he said. But as he is a PhD, I am now want to know more about it. Those who are reading this thread kindly tell me first whether this is true or not and if it's true, kindly give me about some real examples.
At present, I want to be with what my little knowledge of physics and engineering told me. But making this thread just for a testing of that.

 

[jrmichler]

Propeller fans (AKA panel fans) draw more power when the flow is restricted. Forward curve centrifugal blowers draw less power when the flow is restricted. For more information on flow vs power for different types of fans and blowers, two good sources are Aerovent and New York Blower.

A convergent nozzle the same diameter as the fan/blower inlet is a velocity stack. Velocity stacks smooth incoming air flow, thus increasing total air flow. See the Wikipedia entry for a good diagram: https://en.wikipedia.org/wiki/Velocity_stack. A convergent nozzle smaller than the inlet is a restriction to air flow.

So adding a nozzle can either increase or decrease motor load.

 

[T C]

Thanks for your information. Suppose, instead of just one big nozzle with the same (or bigger) diameter nozzle more than nozzles are used. The inlet area of a single nozzle is less but the inlet of all the nozzles together will be same (or more) than the area of the fan. What will happen then?
Do the load increase or decrease?

 

[anorlunda]

Thanks for your information. Suppose, instead of just one big nozzle with the same (or bigger) diameter nozzle more than nozzles are used. The inlet area of a single nozzle is less but the inlet of all the nozzles together will be same (or more) than the area of the fan. What will happen then?
Do the load increase or decrease?

That is like a single nozzle with internal walls to partition it into segments. What do you think? Well an internal partition change the answer?

 

[T C]

My common sense tells me that if the walls have little or no friction, then the outcome wouldn't be different. Whatsoever, in reality totally frictionless walls aren't possible. In short, the outcome would be more or less lesser than the scenario with a single nozzle.

 

[russ_watters]

Yes, in the real world adding seams adds friction loss, which magnifies the effects @jrmichler was referring to.

 

[T C]

@jrmichler has referred to two effects, one increasing and one decreasing. As per him, if the inlet of the nozzle is same or bigger than the outlet of the blower then the flow increases and in fact the nozzle will smooth out the flow. But adding veins to it will add friction. That doesn't mean magnifying but rather diminishing. Which one you are talking about?

 

[anorlunda]

It should be clear that the answer is "it depends on the numbers." As you dig deeper, you'll find that laminar/turbulent flow and Reynolds number can become significant. There can be no simple increase/decrease answer to your question.

 

[T C]

IMO, the veins will prevent the flow to become turbulent and will keep it laminar. Kindly explain what you want to mean by "it depends on the numbers".

 

[jrmichler]

Both Aerovent and New York Blower have resource links that answer your questions. In detail and with diagrams.

Also, please check the definition of vein and vane.

 

[russ_watters]

@jrmichler has referred to two effects, one increasing and one decreasing.

This is because not all fans are the same and you didn't specify a type.

As per him, if the inlet of the nozzle is same or bigger than the outlet of the blower then the flow increases and in fact the nozzle will smooth out the flow. But adding veins to it will add friction. That doesn't mean magnifying but rather diminishing. Which one you are talking about?

I was referring to both the first two cases in his first paragraph. However, in reareading your OP, you said " ...if we put a convergent nozzle before a electricity driven fan/blower." The word "before" in this context can mean either upstream or downstream of the fan. Which were you referring to? You've gotten answers about both...

 

[russ_watters]

IMO, the veins will prevent the flow to become turbulent and will keep it laminar.

Were you trying to describe a honeycomb configuration of nozzles in parallel? I don't see what purpose you are looking for there; flow in a duct generally isn't laminar and while your honeycomb of nozzles will create little laminar jets, they then have to combine and become turbulent, fully-developed flow.

Is this going anywhere? Can you be more specific about what you are asking? It is very vague and unfocused feeling.

 

[T C]

By "before", I clearly want to mean downstream i.e. the flow will come out of the blower and will enter the nozzle. Hope this is enough to clarify. From jrmichler, I came to know that if the inlet of the nozzle is smaller than the area of the blower, then it will put pressure on the motor of the blower. What I specifically want to know is whether using a bunch of smaller nozzles can also put pressure on the motor or not. The frictional loss on the vanes inside isn't my concern.

 

[anorlunda]

What I specifically want to know is whether using a bunch of smaller nozzles can also put pressure on the motor or not. The frictional loss on the vanes inside isn't my concern.

You already correctly said in #5 that it makes no difference.

We are going around in circles. The question has been adequately answered. Thread closed.