
DESIGN: COMBUSTION SYSTEMS  387 
analysis is based on the assumption that mixing is complete at exit and does not 
address the question of how long the mixer must be to accomplish this mixing. 
The length required for complete mixing of the core gas and bypass airstreams, 
as denoted by 
Lmix 
in Fig. 9.35, can be estimated by using the shear/mixing layer 
equations [Eqs. (9.52-9.60) from Sec. 9.1.4 (see also Figs. 9.18 and 9.19)]. How- 
ever, as Fig. 9.35 suggests, the distance required may be excessively long, as one 
stream may "mix out" while the other stream still contains unmixed gas. One way 
to reduce the length required to mix out is to reduce the annular height of both 
passages at mixer inlet, which reduces the initial scale of segregation of the two 
mixant streams. 13 This can be done by constant-area ducting (that is, by keeping 
the product of cross-sectional radial height H  and mean radius 
rm 
constant as 
r m 
is increased) both streams to the greatest permissible outer radius prior to mix- 
ing. Some other ways are to l) increase the velocity ratio of the two streams [see 
Eqs. (9.52-9.54)], 2) purposely imbalance or "un-match" the static pressures in the 
two streams so that the higher-pressure stream is pushed laterally into the lower- 
pressure stream, and 3) replace the flat-edge splitter plate illustrated in Fig. 9.35 
with a  "fluted" plate, which can be visualized as a flat plate trailing edge that is 
smoothly crimped laterally, similar to hand-crimping the top and bottom layers of 
a pie crust. The fluted splitter plate induces vertical velocity components, spatially 
alternating upward and downward from both mixant streams, which reduces the 
scale of segregation between the two mixants to the pitch of the corrugations in 
the fluted splitter plate. Of course, all of the suggested ways to shorten 
Lmix 
cause 
further increases in total pressure loss. 
In any case the rational design of the mixer is far too complicated for treatment 
at our level of description, 13  and so the properties at stations 6,  16, and 6A listed 
on the Engine Station Test Results sheet will simply be taken as given for prelim- 
inary design purposes, and the required length can only be guessed or based on 
observation of similar designs. 
An alternative approach is to not mix the two streams at all. By keeping the two 
streams separate until just upstream of the hot section, temperature modulation 
for the afterburner can be achieved by having two separately fueled afterburner 
systems, one  located  in  the  core  stream and  the  other  in  the  unmixed bypass 
fan airstream. Stage 1 would be the inner, core stream afterburner, and after the 
first stage is lit off, the separately fueled, outer, bypass fan airstream afterburner 
could be ignited for stage 2. This in turn leads to flameholding difficulties in both 
streams because although the hot core stream is oxygen poor, compared to the 
bypass stream, the oxygen-rich bypass stream (air) is cold in comparison with the 
core stream. 
9.3.2.2  Diffuser.  The flow entering the afterburner at station 6A (Fig. 9.2a) 
must be slowed to a Mach number that provides a balance between total pressure 
loss and afterburner cross-sectional area. The minimum Mach number entering 
the combustion zone of the afterburner at station 6.1 is usually fixed by a require- 
ment that the diameter of the afterburner section not exceed that of the  engine 
components located upstream, specifically the fan casing, for case of installation 
and removal. A short diffuser length is desired, of course without producing flow 
separation, in order to reduce engine weight and length. In augmented turbofan 
engines, the diffuser may be combined with a mixer so that a mixed stream enters