any
analysis range seemed justified, since the sampling was to be accomplished at
substantially this altitude, and teaperacure effects were expected to be saall.
Furthersore, the percentage flow through the screen proved quite insensitive
to Mack mober.
To cospure the flow through the filter, an acoustically tTe-
stricted flow through the holes of the screen was assumed.
This yields about
the sane flow race as does a friction analysis, acd was accepted as being
somevhat simpler and more physically realistic.
The relation associating the
mass flow per unit ares (Q,) and reserwoir condition is
were)|
8
Here Py and p, are reservoir pressure and density, while 7 is the ratio of
specific beats.
The ratio P/P, has a walue of 0.52 for acoustical limiting.
Use of this equation, together with the values of stagnation conditions
in front of the sampler rocket, indicates that the flow through the filter
unit area is about 40 percent of the flow per unit area of the free streaa.
Impacction of particulate on screen wires proves wery effective.
Also, dif-
fusion becomes effective well before rhe cutoff of impaction efficiency.
Accordingly, of the material which flows through the screen, 66 percent is
collected while the remainder passes through the openings.
Simistic for debris molecules.)
(This is pes-
Gathering is still significant.
It takes
a@ particle with a diameter of 5 x 10 micron to penetrate the stagnation
layer, here assumed to be 25 cm thick; therefore, the gathering effect causes
variation of the owerall efficiency for a large particle-diaseter range.
The
overall performance of the screen impactor is presented in Fig. 5.4.
The screen has favorable characteristics for impaction collection but
suffers from one weakness:
air.
the screen wire is rapidly heated by the flowing
A rough computation demonstrates that it should reach ax=bient stagna-
tion temperature in a few seconds at the 125,000 foot elevation.
late sampling at a higher elevation was found to be necessary.
Therefore,
AS a pre-
cautionary measure the screen was sounted on the front of the inpaccor
arrangement, as backup in case of screen failure, and to collect fragoeats
of grease which, on overheating, would flake off the screen.