Re: Engine hush kits

From: (H Andrew Chuang)
Organization: United Technologies Research Center
Date:         27 Mar 95 01:29:49 
References:   1
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In article <airliners.1995.309@ohare.Chicago.COM>, Steve Howie ( wrote:

> I've asked unsuccessfully on about what is involved in 
> installing the so-called "hush kits" on jet engines. Could someone 
> (Andrew?) enlighten me as to what makes up one of these kits, and how 
> they actually reduce engine noise.

I have very limited knowledge on the hush kits.  My understanding is
that the kit is actually an internal flow mixer (the same thing I was
talking about in another thread on engine design) and perhaps some
additional acoustic liners near the exhaust.  All the stage-2 engines
already have internal flow mixers.  Hence, I don't understand how an
improved mixer can significantly reduce the noise level and make those
engines comply with the new noise regulation.  The mixer looks like a
"daisy-wheel" or a "cookie-cutter".  If you still remember the old
turbojets (not turbofans) on early B707s, CV880s, etc, there were
external noise suppressors which look very much like the mixer I'm
talking about.

The convoluted surface of a mixer increases the area for the hot and
cold flows to mix more effectively than a straight annular duct.  By
mixing the two streams, the exhaust velocity will be reduced, but the
exhaust momentum will actually be increased (remember momentum is not
a linear function of velocity).  Therefore, for the same fuel
consumption, one will get more thrust by mixing the flows (since thrust
is proportional to the change of momentum at the inlet and the exit);
i.e., the specific fuel consumption is improved, too.  With the lower
exhaust velocity, less "violent" mixing with the ambient air will occur,
hence, less noise.

I don't have any formal training in turbomachinery flow, so, I can't
give you too much detail of how they really work (without making a fool
out of myself :-).  I know there are guys at P&W, GE, and Allison
(perhaps even at R-R) who are reading this group.  However, few have
actively participated in the group discussion.  Perhaps, some of them
will enlighten us more.

H. Andrew Chuang