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National Advisory Committee for Aeronautics, Report - Performance and Boundary Layer Data from 12° and 23° Conical Diffusers of Area Ratio 2.0 at Mach Numbers up to Chocking and Reynolds Numbers up to 7.5X10

For each of two inlet-boundary—layer thicknesses, performance
and boundary-layer characteristics have been determined for
a 12°,10-inch—inlet—diameter difuser, a 12°,21-inchrinlet-
diameter difiuser, and a 23°,21-inchrinlet—diameter diffuser.
The investigation covered an inlet Mach number range from
about 0.10 to choking. The corresponding inlet Reynolds
number, based on inlet diameter, varied from about 0.5 X 10“
to 7.6 X 10“.

Although small regions of separated flow existed in the
12° difi'users, the flow was relatively steady. In the 23° diffuser,
the flow was badly separated and very unsteady. The addition
of a uniformly rough layer of cork particles to the walls of the
23° difi'user eliminated the unsteadiness but did not improve
the pressure recovery. Total-pressure losses increased and the
static-pressure recovery decreased with increasing inlet-bound-
ary—layer thickness for all three difl'users. Increasing flow
rate (increasing Mach and Reynolds number) produced an ad-
verse efl'ect on performance which was very slight for the thinner
inlet boundary layers in the 12° diflusers,‘ but which became
more severe with increasing inlet-boundanj-layer thickness or
increased difl'user angle.

The performance of propulsion units which handle large
quantifies of air is strongly aflected by the losses incurred in
the associated duct systems. One of the most important
components of these duct systems is the diffuser in which the
performance depends upon the rate of geometric expansion,
inlet Mach numb er and Reynolds number, and inlet-b oundary-
layer conditions. Although much diffuser research has
been done, most of the data available (refs. 1 to 4) are at
Mach numbers and Reynolds numbers too low to be of
direct practical value in the design of aircraft-duct systems,
are for improbable or often unlmown inlet-boundary-layer
conditions, and are taken from configurations with diffuser
angles much nearer the optimum than can usually be ob-
tained in practice.