Blower fitted with a velocity stack

[T C]

TL;DR Summary

I want to know if a velocity stack is fitted at the upstream to a blower, whether that can enhance the performance of the blower or not.

Many of us know about velocity stacks. It's a proven fact that velocity stacks can enhance the performance of internal combustion piston-cylinder engines. I want to know if a velocity stack is added at the upstream (just before) to a blower/fan, whether it can enhance the performance of the blower or not. Velocity stacks uses Venturi effect to suck in more air. I want to know whether such phenomenon can enhance the performance of blower/fan or not.

 

[Baluncore]

Where a fluid is converging and entering a parallel walled tube, the velocity stack will reduce or eliminate the vena contractor.
https://en.wikipedia.org/wiki/Vena_contracta

 

[T C]

Kindly explain a little bit more. Vena Contracta means where the velocity is maximum. Does adding a velocity stack means the velocity wouldn't be maximum at the throat?

 

[jrmichler]

You can do some rough calculations to find out if it could make a measurable difference. The blower has a flow rate and an inlet diameter. You can treat the inlet as an orifice, and calculate the pressure drop across that orifice. Search orifice coefficient because that is an important part of orifice flow. Compare that pressure drop to the pressure rise from blower inlet to blower discharge.

A velocity stack will increase the orifice coefficient from what you found above to 1.0 (or about 0.99). Then repeat the calculation.

This is a very rough calculation, but will give you an idea of whether inlet loss is significant, and whether a velocity stack will make a measurable difference. The inlet pressure loss is normally small compared to the blower pressure rise.

 

[Baluncore]

Vena Contracta means where the velocity is maximum. Does adding a velocity stack means the velocity wouldn't be maximum at the throat?

Not exactly maximum velocity. It means that the flow section is contracted. The velocity may still be a maximum in the throat, but the throat velocity will be lower for the same flow with the velocity stack. There are savings when flow velocity² is reduced.

 

[T C]

There are savings when flow velocity2 is reduced.

What kind of savings?

 

[T C]

The inlet pressure loss is normally small compared to the blower pressure rise.

That loss in inlet pressure means increase in velocity of the flow as per Venturi Effect or Bernoulli's theorem, right? That means the flow already has some extra velocity even before being sucked by the blower. Is it some kind of enhancement? Suppose the flow is being discharge at ambient pressure and the flow is being sucked at ambient pressure. In short, the main purpose of the blower is to create velocity, not pressure. Does that means increased velocity?

 

[russ_watters]

That loss in inlet pressure means increase in velocity of the flow as per Venturi Effect or Bernoulli's theorem, right?

No, a loss is an energy loss; a violation of/excursion from Bernoulli's principle. Basically when you try to have air make a sharp turn (do anything worse than gentle/smooth transitions), there is a loss of energy. It is usually expressed as a fraction of velocity pressure. It's the "loss coefficient" I mentioned in the other thread. It then requires more static pressure from the fan to maintain the same velocity than without the loss -- that's the extra/lost energy. Here's a list of a bunch of common situations and their typical coefficients:
https://www.engineeringtoolbox.com/minor-loss-air-ducts-fittings-d_208.html
And another:
https://www.engineeringtoolbox.com/air-duct-minor-loss-diagram-d_332.html
And starting at Page 80, fan inlet effects:
https://utahashrae.org/images/meeting/011119/Slides/smacna_duct_design_fundamentals.pdf

 

[T C]

In case of a velocity stack, there is no sharp turn. What's the minor loss co-efficient for a velocity stack?

 

[Baluncore]

In effect, without the velocity stack the situation is the same as flow from a room into a duct. That has the minor loss coefficient = 0.35;
If the velocity stack is sufficiently long, it will eliminate all of that minor loss.

 

[T C]

That means some increase in power, right?

 

[Baluncore]

That means some increase in power, right?

That is an unreliable conclusion. How do you know that the power limitation is due to the air intake geometry and not due to some other parameter such as a fuel flow limit or exhaust back-pressure?

 

[T C]

If the velocity stack is sufficiently long, it will eliminate all of that minor loss.

Elimination of minor loss.

 

[Baluncore]

Elimination of minor loss.

Your one-liners are too obtuse to consider meaningful.

 

[T C]

I mean if the minor loss is eliminated, that means the lost power/pressure will be added to the flow, right?

 

[Baluncore]

If you eliminate a loss you do not make a profit, you break even.

 

[cjl]

That is an unreliable conclusion. How do you know that the power limitation is due to the air intake geometry and not due to some other parameter such as a fuel flow limit or exhaust back-pressure?

I think it's a pretty safe statement that reducing inlet restriction will basically always give at least some very minor increase in power. Even if the limitations are elsewhere, you'll still have a marginally lower pumping loss, which will increase output power.

(The magnitude of this gain may be very small though)