Technique scales flame structure linearly with gas kinetic mean free path, which increases two to three orders of magnitude as pressure decreases like amount. Kinetic and transport time scales expand in proportion so that regression rates for laser-induced flames are two to three orders of magnitude slower

Friedman, R.

Hertzberg, M.

Mc Hale, E.

Von Elbe, G.

English

NASA Technical Reports Server

September 1971	 Brief 71-10335 
NASA TECH BRIEF 
Langley Research Center	
-4r 
NASA Tech Briefs announce new technology derived from the U.S. space program. They are issued to encourage 
commercial application. Tech Briefs are available on a subscription basis from the National Technical Information 
Service, Springfield, Virginia 22151. Requests for individual copies or questions relating to the, Tech Brief program may 
be directed to the Technology Utilization Office, NASA, Code KT, Washington, D.C. 20546. 
Flame Zone of a Composite Propellant Expanded by a Laser Source 
A laser source can be used to heat the surface of 
a propellant in order to sustain combustion at 
greatly reduced pressures, expanding the flame 
zone to a scale large enough for studying its struc-
ture. The flame zone thickness of a composite pro-
pellant (an oxidizer such as ammonium perchlorate 
and a polymeric fuel binder) is normally so small 
that studying the thermal, compositional, and flow 
structures of the flame by macroscopic means such 
as probes becomes very difficult. 
One approach to flame zone studies is to ex-
pand the zone to a more convenient scale by 
reducing the pressure in the combustion chamber. 
The combustion of the propellant, however, is sus-
tained by heat conducted from the flame zone. Since 
the temperature gradient in the gas phase adjacent 
to the propellant surface decreases with expansion of 
the flame zone, quenching usually results befire the 
flame zone can be expanded to a useful size. 
A normal combustion environment for ammonium 
perchlorate propellants, composed of particles in 
the micron range, involves rapid regression rates 
(0.5 to 2.0 cm/sec) at pressures in the range of 1.01 
to 10.1 MN/m 2 . Under these conditions, vaporiza-
tion of the solid is maintained by conductive heat 
feedback from a gas flame, whose distance from the 
propellant surface is also in the micron range. Since 
analytical probes cannot be miniaturized to the 
scale of such flames, their structures have largely 
been a matter of conjecture. With decreasing pres-
sure, however, the flame zone thickness of premixed 
flames increases, increasing the feasibility of probing 
their structures. But decreasing pressure also means 
a diminished burning rate because the temperature 
gradient above the surface decreases as the flame
zone thickness increases. Few propellants burn 
freely at pressures substantially below 0.1 MN/m2. 
Even those propellants that sustain combustion at' 
low pressures have particle radii so small that the 
characteristic diffusion length is still microscopic. 
Attempts to expand the flame zone by increasing 
particle radii have been unsuccessful, since the 
propellant soon ceases to burn freely, and ex-
tinguishes at some particle radius for which the 
flame zone is still too small for convenient prob-
ing. 
The use of an external radiant CO 2 laser source 
to sustain and control the surface regression rate has 
resolved these scaling problems. The technique allows 
the heat input to the burning surface to be regu-
lated, independent of ambient pressure. The pressure 
may then be lowered to the order of 0.01 MN/m2, 
and the particle radii becomes macroscopic. Since 
the laser maintains the required heat input, the pro-
pellant continues to burn. In effect, this new technique 
scales the flame structure linearly with the gas 
kinetic mean free path, which increases two to 
three orders of magnitude as the pressure decreases a 
like amount. The kinetic and transport time scales 
are expanded in proportion so that the regression 
rates for the laser-induced flames are two to three 
orders of magnitude slower (0.02 to 0.04 .crn/sec).. 
Note: 
No additional documentation is available. Specific 
questions, however, may be directed to: 
Technology Utilization Officer 
Langley Research Center 
Hampton, Virginia 23365 
Reference: B71-10335
(continued overleaf) 
This documetct was prepared under the sponsorship of the National 
Aeronautics and Space Administration. Neither the United States 
Government nor any person acting on behalf of the United States
Government assumes any liability resulting from the use of the 
information contained in this document, or warrants that such use 
will be free from privately ownedrights.
https://ntrs.nasa.gov/search.jsp?R=19710000333 2020-03-17T02:28:39+00:00Z
Patent status: 
Inquiries about obtaining rights for the commercial 
use of this invention may be made to: 
Patent Counsel 
Mail Code 173 
Langley Research Center 
Langley Station 
Hampton, Virginia 23365
Source: Dr. M. Hertzberg, Dr. G. von Elbe,

Mr. E. McHale, and Dr. R. Friedman of

Atlantic Research Corporation 
under contract to

Langley Research Center

(LA R- 10660) 
Brief 71-10335	 Category 03
/ /


Flame zone of a composite propellant expanded by a laser source

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