Equalizing airflow through multi-hole orifices

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In summary, the Bernoulli equation does not make a distinction between one hole and multiple holes with the same total area, but in a practical implementation, the number of holes and the blade area can affect the flow and rotation speed of a turbine. The side with more holes and a higher blade surface area may experience more friction, resulting in a higher velocity and rotation speed. However, other factors such as pressure drop can also play a role in the observed differences.
  • #1
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TL;DR Summary
One pressure vessel has 6 holes from which air escapes, another has 8 holes, total open area of the holes is same on each vessel. Should the volume flow be the same?
I know the Bernoulli equation for calculating the air velocity through a pressurized hole, but I am wondering if one hole of a given area behaves the same as multiple holes with the same total area. The Bernoulli equation doesn't make a distinction.

Background: As a personal 3D design and printing project I designed a siren whistle patterned after a Francis turbine, in which a dual runner (two runners back to back) chops the air coming out of exit holes on each side. The number of blades on each side of the runner is different (6 and 8) with a corresponding number of exit holes. As the runner rotates, the turbine produces two tones, with the airflow interrupted 6 times per rotation on one side, and 8 times per rotation on the other. The holes on each side have the same total open area.

The device works pretty well, but if I cover the holes on one side or the other, to allow only one side of the runner to drive its rotation, the rotation speed is clearly different for the same input pressure (by my best guess by breathing into it). It's slower when air exits only the 6-hole side.

That made me wonder if several small holes behave differently than fewer holes with the same open area.

Hmm, as I write this, I realize the difference could also be due to the different number of blades. The side with the higher number has a higher surface area on which the air can impart force.
 
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  • #2
To your summary question:

No - the flow shouldn't be the same. One of the factors controlling the flow is friction. The implementation with more holes has a higher 'perimeter/area' ratio. More 'perimeter' results in more friction. How much actual difference that makes depends on how large a factor friction is in your specific case - higher velocities result in more significant friction terms.
 
  • #3
That's what I thought. However, the effect I observe is the opposite of what would be expected (higher rotation when exhausting through the 8 hole side instead of the 6 hole side) so maybe the runner blade area is having a greater effect, which may be mitigated only slightly by the greater surface area of the holes.
 
  • #4
If you drive the turbine with a constant flow, then it may have greater pressure drop with the 8 hole side, which may explain the higher velocity.
 

Related to Equalizing airflow through multi-hole orifices

What is the purpose of equalizing airflow through multi-hole orifices?

The purpose of equalizing airflow through multi-hole orifices is to ensure a uniform distribution of air across all the orifices. This is crucial in applications like combustion systems, HVAC systems, and industrial processes where consistent airflow improves efficiency, performance, and safety.

How can the airflow be measured through each orifice?

Airflow through each orifice can be measured using various methods such as pitot tubes, anemometers, or flow sensors. These instruments can provide real-time data on the velocity and volume of air passing through each orifice, allowing for adjustments to be made to achieve equalization.

What factors affect the distribution of airflow through multi-hole orifices?

Several factors can affect the distribution of airflow through multi-hole orifices, including the size and shape of each orifice, the pressure differential across the orifices, the viscosity and density of the air, and any obstructions or variations in the upstream and downstream flow paths.

What methods can be used to achieve equal airflow through multi-hole orifices?

Methods to achieve equal airflow through multi-hole orifices include adjusting the size or shape of the orifices, using flow control devices like dampers or valves, and ensuring a consistent pressure differential across all orifices. Computational fluid dynamics (CFD) simulations can also be used to model and optimize airflow distribution.

Why is it challenging to equalize airflow through multi-hole orifices?

Equalizing airflow through multi-hole orifices is challenging due to the complex interplay of factors such as pressure variations, orifice geometry, and flow dynamics. Small differences in these factors can lead to significant variations in airflow, making precise control and adjustments necessary to achieve uniform distribution.

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