Steam jet -- noise levels 3

[otaio]

When steam under pressure discharges from a vent pipe it creates a noise.

Is anyone aware of a way of calculating the likely noise level given the diameter of the pipe and pressure of dicharge?

Bob B

 

[MikeyP]

This isn't my speciality, but as a first approximation, the noise from a subsonic or sonic jet (ie no nasty shockwaves) radiating into free space is effectively a quadrapole acoustic source. Estimates of the spectrum of the noise can be made by employing "Lighthill's Acoustic Analogy". From this it can be found (and validated by experiment) that the noise power is proportional to the 8th power of the jet exit velocity and the nozzle diameter squared.

I don't really have much more on this but I may have given you a few specific terms which should help you to find the right book.

M

 

[otaio]

Mikey P

A great lead. Thankyou very much for this.

Bob B

 

[vanstoja]

There's an article by Succi,G.P.(1986), "Flow-Generated Noise in Circular Cavities or How Teapots Whistle". Noise Control Engineering Journal, Jan./Feb. 1986, pp.8-16. It doesn't provide a quick or easy (to me) answer and it looks like you have not provided all the information needed to use it. For "screech" noise it looks like the orifice L/d ratio has to be between 0.5 and 2.0 and you need the exit flow Mach number from the equation M=v/c={[2/(gam-1)]*[1-(PT/PA)^(gam-0.5)]}^0.5 where gam =ratio of specific heats, PT=Plenum pressure, PA=Atmospheric pressure. The frequency equation is f=(c/L)*(alpha*M)/(1+alpha*M).Here c=sonic velocity, L=orifice length and alpha =??? can't find it!
Anywho, there are some results plotted for 0.5 in. diameter and 0.5 in.length orifice showing 1st stage response running from 5 to 9 KHz for Mach 0.3 to 0.7. 2nd stage is from 9-16KHz over same Mach range and 3rd stage is 13.5-20KHz from M=0.3 to 0.5. There's also an L/d =2.0 plot for d=0.5 in. showing 5 stages with disconnected response points over a 0.2-0.6 Mach range. Sounds simple enough doesn't it. If I ever find out what alpha is I'll get back to you.

 

[otaio]

Vanstoja,

Perhaps alpha is the meaning of life.

Seriously -- thank you very much for this interesting reference. I enjoyed reading it and thinking.

This is one of the joys of the forum. Now and again it throws up far field stuff which gives you a 'Oh yeah' moment.

Again thanks.

Bob B

 

[Mpino]

Hi friend:
read Noise and Vibration Control (1971) by Leo Beranek, there is a chapter dealing with jet noise, I gues the chapter 9 or 10.

luck

 

[athomas236]

One simple formula I have seen in publications from a silencer supplier is:

The unsilenced noise from a steam vent in dbA refernced to 1E-12W is 17*LOG (Flow tonnes/hour) + 50*Log(Temp degree K)-15

athomas236

 

[vanstoja]

More likely alpha is the cause of the "Big Bang" and omega is the meaning of life. You've got to be pulling my leg about reading that reference. I thought I was the only one left in the world who knew about it and had a copy of it. If you really found it, pray tell me how and where.
There is some amplitude info in the article. For the L/d=1 orifice at Mach 0.48, Fig. 7 shows a frequency amplitude spectrum with peaks at about 6.5, 13.0 and 19.5 KHz with "Relative SPL (dB)" of about 49, 38 and 33. Not much help when it doesn't say relative to what. Otherwise, there is nothing there on how to calculate amplitudes.
Athomas236 has a possible direct answer to your question in A-weighted sound power level providing your geometry conforms exactly to that of a "steam vent" and you haven't got a silencer of any kind. Can you verify that?

 

[otaio]

Vanstoja,

Thanks once more for coming back. Though I haven't read the original source -- my reply was misleading - it was your synopsis I enjoyed reading.

My curiousity was prompted by another thread where someone asked for a design of such a silencer. The 'traditional' approach seemed to be very empirically based and I wondered if there were ways in which a silencer could be optimised.

Hence the original query.

Regards
Bob B

 

[otaio]

athomas236

Thankyou for sharing this predictive equation for sound power level. Did it mention the distance?

Bob B