Application off springs in ducts

[JoakimHold]

Hello all engineers at Eng Tips forum, just recently joined and this is my first post.

Currently I am studyingmechanical engineer, product development in Sweden whereas I am doing a project on designing a potential grille/diffuser for a new innovative duct system.

Lets not get into more details unless you really want to...

Basically I am wondering if applying a typical axial spring, see figure below, in the path of the airflow will create noticeable drag or vortex/turbulence.

Picture taken from Lesjöfors website,

http://springs.lesjoforsab.se/tryckfjadrar/tryckfjader

// Joakim Holdeberg

 

[MintJulep]

It depends.

 

[JoakimHold]

Hi MintJulep!

what does it depend on, care to elaborate?

I am much for developing new strategies, taking phenomenon from diverse areas and applying it on HVAC, hence my curiosity.

Although my lacking knowledge about fluid mechanics is constraining me...

Thanks in before hand!

Joakim

 

[JoakimHold]

To explain my situation a little bit more ill share a little sketch

The sketch contains two views, TOP and FRONT, the AIRFLOW are supposed to come from the duct. So it follows the ducts axial length direction and passes the "hole" where the grille are supposed to be placed. The AIRFLOWS first contact are the springs after entering the hole. The springs are placed on each side, as close to the walls as possible.

Will this cause extra drag and develop unwanted turbulence?

Correct me about any fundamental fluid mechanics that I have missed out on.

Many thanks!

Joakim

 

[Lnewqban]

Regular grilles and diffusers have edges that go inside the wall/duct opening, causing more drag and turbulence that those springs located completely out of the air stream.
If the springs are positioned perpendicularly inside the airstream, you may have difficulties finding the value of the induced drag, because it is not something common in wind tunnel research.

"God will not look you over for medals, degrees or diplomas, but for scars." - Elbert Hubbard

 

[JoakimHold]

I too have walls that go inside the ducts but tried to design them airstream friendly. Next to those walls and placed accordingly to my sketch are the springs.

Am I supposed to interpret this as; so far it is unknown if it will or not cause noticble drag.

 

[EnergyProfessional]

I'm curious to know what you try to design?
Drag and noise all depends on the velocity. I think to get actual values you have to test. If velocity is not too high, it is not much compared to a diffuser. Let's say you have a 40 cm x 40 cm duct and the spring is 2 cm thick and it is kind of perforated you won't get much reduction in crossection. sure some eddies around the spring. If your spring fills half of the duct, the situation is different. It depends

One thing to consider is the spring accumulates dirt over time and needs to be maintained. so you need to make it easily accessible.

 

[dbill74]

In answer to your question, if the springs are in the airflow, yes there will be noticable drag. Although in designing ducts for an HVAC system we are more interesed in the static pressure drop of the air as it goes by an obstruction (often expressed as "equivalent length" of straight duct). Keeping the equivalent length of a duct to a minimum minimizes the amount of work a fan has to do and saves energy. To this end, get the spring out of the air flow if at all possible.

As for trying to calculate the pressure drop, check your fluid dynamic texts, search the internet for white papers and ask your professors. Might be that the best way to determine the actual pressure drop is to build a model, blow air through it and measure.

 

[Lnewqban]

Another thing to test/consider are vibration of the springs at different air velocities and the resulting noise.

"God will not look you over for medals, degrees or diplomas, but for scars." - Elbert Hubbard

 

[JoakimHold]

wow thanks for all the answers!

The application of this grille is a basic HVAC in a household. Its going to deliver fresh air directly from a duct to the room, not as a "branch" of a duct like other conventional designs.

Imagine putting a duct from one wall to another in an open room, and the cutting a hole in the duct and placing the grille in it.

@EnergyProfessional as it looks like now the grille might be 1000x60 millimeters and the spring hopefully be under 10 mm in width (diameter). Spoke with a teacher of mine and he believes this size difference will make this drag small enough to neglect.

@dbill74 wonderful information, something I will definitely do some more research on. Gratitude!

@Lnewqban answering your statement I believe the air velocity may be low enough to neglect that phenomena, although I am going to check up on what air velocities my grille is going up against, speaking with my client.

Best thing I discovered when studying engineering is all the things we can neglect, hopefully this is the case too. Learned the hard way that having a needy girlfriend is not a thing to neglect... (lets keep that off the record)​

Joakim