Quarter wave resonator for generator intake 1

[iandmac]

I am trying to design a quarter wave side branch resonator for a two stroke V8 GM detroit diesel genset intake. It has a 3 lobe roots blower rotating at twice the crankshaft speed of 1500rpm. I am thinking the dominant pressure wave in the intake will be at at 1500 x 2 x 3 / 60 or 150Hz. I can't run it at the moment so I am unable to verify this with a spectum analysis.

From a post by Rob45 on this subject I was able to calculate the required tube length at 1128fps/150 Hz/4 = 1.88' = 22.6" long. I'm going to make it a bit longer than needed and put a moveable piston in for tuning during commissioning.

It's going to be a side branch to a large intake pipe, nearly 8 inches diameter, so from a purely practical point of view I'd rather not insert a tee with an 8 inch side branch. I am wondering how effective a four inch side branch might be? This will be much easier to fit in around the existing equipment.

Does anyone have any suggestions for how successful this is likely to be or any relevant experience with silencing these very noisy beasts? The machine will be eventually housed in a soundproof room but I wanted to deal with the point sources first then run the machine to see what I've got left before designing the building. From previous generator installations I am familiar with most of the usual tricks but this is the first two-stroke/roots blower machine I've had to deal with.

Any help appreciated, Regards, Ian

 

[GregLocock]

A 4 inch quarter wave tuner in an 8 inch duct can only cancel 1/4 of the incident pressure wave, that's roughly a 3 dB reduction.



Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[iandmac]

Thanks Greg ... can I infer that to get the best attenuation effect I should ideally be using the same diameter tube for the side branch resonator?

Ian

 

[GregLocock]

I think so. I'm sure it is asymptotic in practice, and of course there are secondary effects.





Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[MikeHalloran]

It might be more effective to put your resonator closer to the actual blower inlet, which I think is a rectangular flange.

Notice the resonator cavities cast into these industrial blowers just inboard of the round flange:

http://www.rootsblower.com/index.cfm/go/list-products/productline/RAS-Series-Blowers-Category/

Mike Halloran
Pembroke Pines, FL, USA

 

[MikeHalloran]

Rectangular flange examples here:

http://www.hamptonblowers.com/PRICE.HTML

Mike Halloran
Pembroke Pines, FL, USA

 

[iandmac]

Thanks guys, here's a closeup of the intake system. The blower is fitted with a flap valve for shutdown which has to stay so the closest I can get will be next to the round flange after the bend. Space is not an issue and I can go straight up with a 22 inch long 8 inch resonator.

Any ideas about materials? Aluminium tube about 1/8" thick welded would be nice but I don't want to set up another noise radiator in the process. Or would I better off making it from heavy walled steel pipe then lagging the outside? Or at the other extreme, PVC (heat considerations aside). All input welcomed, Regards, Ian.

 

[GregLocock]

For cars we use some nasty blow moulded or slush moulded plastic. Al tube would be fine. For whatever reason cylindrical pipes seem a good choice in air intakes, that includes waste water pipe.



Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[iandmac]

Ok, aluminium it is. I'll make it a double flanged tee with the resonator branching from the centre. That way it can just bolt in between the inlet duct and the shutdown valve so if it needs mods later it can be easily removed. The tuning piston can be a PVC plug with a couple of O-rings for sealing.

Last time I had this machine running it had a loud drone which seemed to be coming from the air filter housing. It is a large metal can made from light gage steel and it rings when tapped on the flat end cover.

Today I did some spectrum analysis tap tests around the inlet ducting and air filter can to see what the natural frequencies were. A very dominant peak appears at 300Hz with a second harmonic at 600.

How likely is it that second and third harmonics of the primary frequency of 150Hz are exciting the can causing it to resonate, hence the very loud drone from the intake?

I'm thinking of stiffening the flat surfaces of the can by adding ribs or extra mass by rivetting on some heavy gage sheet metal. Either approach should shift the natural frequency away from the 150Hz pressure wave. Together with the quarter wave resonator I'm hopeful of getting a pretty good result with this. Does anyone have any idea what this might amount to in dB?

Am I on the right track with any of this? Regards, Ian

 

[GregLocock]

I am sure that your blower generates a nice harmonic sequence starting at bpf if not before, so yes it'll excite 300 hz.

Stiffneing ribs can work, but for a one-off test damping material is quicker and surer.

Impossible to predict the dB change without doing more work than your fix would be.

Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[iandmac]

Yes, all that makes sense. One last question before I start cutting metal. Should I measure the required quarter wavelength of tube from the centerline of the main intake tube, the outside edge, or somewhere between the two? On such a large diameter duct this difference becomes significant and I'm getting pushed for space.

Regards, Ian.

 

[GregLocock]

Theoretically I suppose to the centre of the main duct, in practice make it long and then you can easily shorten it.



Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.