Flow Induced Vibration

[StressGuy]

Time to stir things up again. I've bit hit with a question and I'm not sure what to make of it.

I have an application of a small diameter pipe projecting into a duct to act as injection nozzle. The "duct" is actually 3/8" plate and the pipe is welded to it at the penetration.

A concern has been expressed about vibration of the nozzle due to the high velocity flow through the nozzle. No specific basis is given.

Now, I'm aware of flow induced vibration on a projection, such as a thermowell, due to flow in the main header/duct. However, that is not an issue with this application and it is not the concern being expressed.

The only possible mechanism I could even fathom would be some kind of turbulence effect at the nozzle exit, but I'm not aware of anything like that.

So, is the guy mistaken, or is there some phenomena that I'm not aware of that would create a forcing fuction on a projection like this due to the flow inside the nozzle?

Thanks,
--ED Edward L. Klein
Pipe Stress Engineer
Houston, Texas

All opinions expressed here are my own and not my company's.

 

[butelja]

Is this a liquid/liquid, gas/liquid, liquid/gas, or gas/gas system with respect to the main duct and the injection?

The only mechanism I can fathom (other than vortex shedding on the cantilever that you mentioned) would be vortexes shed by the turbulent free jet (assuming gas/gas). I've seen a correlation that gives the predominant jet noise frequency related to the Strouhal number, and involves the jet diameter and exit velocity. Unfortunately I can't find my reference at the moment. The predominant frequencies of gas discharges are often in the kHz, and are very broad band, approaching white noise. Proper modeling of the actual stresses would have to consider random vibrations and power spectral densities of the noise source.

I'm not aware of any practical process application where this is a concern. The applications where this type of excitation are a problem that I'm aware of are in the aerospace field. They usually deal with structural fatigue of aircraft structures near a jet engine exhaust or the tail end of a rocket during the initial seconds of launch.

 

[StressGuy]

Both the injector and duct are gas flows. Both are low pressure with the duct being about 50" of water column and the injector being about 9psig upstream of the nozzle. The flow velocity is high at the exit - about 660 fps. I'm not sure of the velocity of the duct, but it is very low.

The low duct velocity and the fact that the injector is shielded by internal refractory pretty well eliminate any vortex shedding induced vibration on the projection of the injector inside the duct. Edward L. Klein
Pipe Stress Engineer
Houston, Texas

All opinions expressed here are my own and not my company's.

 

[poetix99]

I agree with "butelja"; it seems extremely unlikely, in my opinion.

The characteristics of a a "free jet" discharge are not likely to include an orderly vortex street in the manner of fluid flow around a bluff body; the frequencies would be closer to white noise.

Pump vibrations would more likely cause a problem than vortex induced vibrations, and I don't mean to suggest that is a problem either (especially not knowing anything about the pump); it was just meant as a comparison.

On second thought: take a closer look at the shielding arrangement of your injector. There could be a vortex shedding phenomenon at the gap between the nozzle and the refractory. I've had some experience with pressure fluctuations at flush-wall pressure taps. The fluctuations were attributed to vortices being generated at the hole. Any lip on the pressure tap made this much worse, and I doubt that such care is being taken with your shielding.

You could calculate a Strouhal number based on the "characteristic lengths" associated with your geometry (try the hole diameter, or the gap radius) and the duct flow characteristics. Figure out the frequency and impose that upon your injector.

If there is no gap, or the gap can be made small, I would also ignore the last two paragraphs.

 

[vanstoja]

Jet engine "screech" noise from the tailpipe is related to hot jet-cold ambient gas interaction effects along the jet periphery which may involve a dicrete or narrowband excitation source. I'll have to check out my sources of papers on the subject to determine how this might affect your hardware. One or more papers by Crow and Champaigne seem to ring a bell here. If your duct is not all that wide, then there is the possibility of an "impinging jet" noise source as the jet crashes into the far wall and spreads out. Wyganski of Penn State I believe had a paper describing the critical wall distance and jet velocity parameters. Does your duct have standing waves whose frequency might be altered by a transverse high velocity jet flow. All of these potential flowtone sources will not hurt the air or gas where the sources may develop. What you really need to worry about are the resonance frequencies of the confining structures that may get excited by the fluid noise sources. Do you know what the critical structural response modes and frequencies are?

 

[abeltio]

There are some aspects that are not clear...how thick is the pipe? what size?
There is a book called: What went wrong? (sorry cannot remember the author... I think it was published by Hydrocarbon Processing) which showed the correct way to do injection of one fluid into another to avoid stress concentration and fatigue due to vibration.
The whole thing boiled down to something like this:
| |
| |
----| |---- Blind Flange, due to the very low press
-- | | -- (50in of water a packing gland can be used
| | | | compatible with the design temperature)
| | | |
| | | | Pipe nipple about 1.5D
| | | | weld
-----/ | | \------
| |
| | Distance to wall ~ 3 nozzle pipe diameters
| |
\ \ --\ Nozzle
\---/


This design avoids welding the nozzle directly to the main duct, it also facilitates maintenance and inspection...the nozzle can be removed from the service without going inside the duct.
Also: the pipe nipple welded to the main duct is not affected by flow.

HTH
Saludos
a.

 

[butelja]

I'd like to re-itterate that it is unlikely you will have a problem. The aerospace structures where the phenomena is significant are very light weight and excited by very high level pressure fluctuations. If your duct is 3/8" thick steel with refractory lining, it is hardly a light weight structure. The refractory around the pipe (I assume either castable or rope packing in the gap) will serve to stiffen and/or dampen any vibration. My recommendation would be to add gusset plates on the outside of the duct to make the customer happy. They are likely not needed, but it is cheap, easy, and puts the problem to rest.

 

[vanstoja]

I could't locate the papers I cited but found some related stuff that would worry me about the consequences of a 660 ft/sec discharge velocity of a gas jet into a plenum of (unknown to me) dimensions. The jet impingement-on -wall excitation mechanism may be for high subsonic speeds of Mach No. 0.6 and greater which you are just below with your present delta p. I found some calculations I did for a submerged water jet where I used Strouhal Numbers of 0.85, 0.09 and 0.33 for non-impinging, impinging-radial and impinging -axial cases where nozzle velocity and nozzle diameter are U and d in the equation S=fd/U which I compared to calculated frequencies of the "wall" structure.
The other area of concern is at lower subsonic speeds where discrete tones can supplant the jet "column" wideband resonance mode with a shear layer instability mode with up to four subharmonics when the exiting jet is excited by either a "whistler" extension (say a pipe fitted over the projecting pipe) or by pulsing the jet at the shear layer instability frequency or by exciting it at the open-open organ pipe frequency of the jet pipe. For details of these mechanisms see the following:
Hill,W.G.& Greene, P.R. (1977), " Increased Turbulent Mixing Rates Obtained by Self-Excited Acoustic Oscillations", Trans. ASME, J. Fluids Engineering, Sept. 1977, pp. 520-525
Kibens,V. (1980), "Discrete Noise Spectrum Generated by an Acoustically Excited Jet", AIAA Journal, Vol. 18, No. 4, April 1980, pp.434-441
Karadogan,H, & Rockwell,D.O. (1983), "Toward Attenuation of Self-Sustained Oscillations of a Turbulent Jet Through a Cavity", Trans. ASME, J. Fluids Engineering, Sept. 1983, pp. 335-340
The first two papers are by aerospace people apparently trying to improve the mixing rate of jet engine exhausts without much concern for the implications of the whistling, siren-like noise they are creating while the last is by acoustics experts dedicated to the suppression of the noise consequences. Apparently,they don't seem to know that their efforts and purposes are at complete odds. Ah... the blessings of science!
You may be able to take the Alfred E. Neumann approach of "What...me worry!! so nobly enshined by MAD magazine. I have seen enough grief from unwanted flow tones to feel queasy about your geometry and 660 fps exit velocity.