Chemistry Reference
In-Depth Information
catalysts in composite solid propellant burning). A basic schematic diagram and
a photo of the main unit of the chemical arc system are shown in Fig. 2.7. In
the chemical arc, the burning of fuel and oxidizer, which play the role of the
“electrodes” (by analogy to the steady burning in the high-temperature electrical
plasma discharge between the electrodes in an electric arc), is maintained due to
the flame produced during the interaction between the pyrolysis products of these
components.
In chemical arc experiments, a fuel sample
2
(polymethyl methacrylate, PMMA,
cast or pressed from a standard powder, 6
18 mm) was fixed in a static
holder while an oxidizer sample
3
(ammonium perchlorate, AP, pressed from pow-
ders of two size fractions: 315-l60
×
18
×
m in a 1:1 ratio) of a similar size
was uniformly moved towards sample
2
by an electrical drive unit.
Burning was initiated by means of a thin ballistite powder plate placed between
the samples. The lateral surfaces of the samples were covered with a thin layer of
nonflammable material beforehand. The velocity of the movement of the oxidizer
sample,
U
mov
, was set by the pitch and rotation frequency of the actuating screw
5
(recorded by a special marker on the chart of the electronic recorder).
For steady burning of the components in the chemical arc, the width of the gap
between the burning surfaces,
h
, is constant, and
U
mov
is related to the linear rates
of burning (pyrolysis) of the oxidizer,
U
ox
, and the fuel,
U
fuel
,as
μ
m and 90
μ
U
mov
(
h
)=
U
ox
(
h
)+
U
fuel
(
h
)
.
(2.1)
The values of
h
and
U
fuel
(
h
) were recorded using a simple optical setup based on a
Jupiter-3 lens system and a powerful lamp that provided a distinct shadograph of the
gap on a graduated screen. Values of
U
ox
(
h
) were calculated from data on
U
mov
(
h
)
and
U
fuel
(
h
) using Eq. (2.1). The steady mode setting time was in the range of
15-25 s. As
U
mov
increased, the gap width decreased, while
U
ox
and
U
fuel
increased.
If the sample surfaces came into contact due to very high
U
mov
, the chemical arc
phenomenon transformed into a mode involving the periodic burning-out of an end-
to-end system. The experimental results were analyzed in the form of plots with
coordinates:
m
ox
m
fuel
=
U
ox
γ
ox
U
fuel
γ
ox
=
f
(
h
)
.
U
fuel
=
U
fuel
(
h
)
,
U
ox
=
U
ox
(
h
)
,
In contrast to methods based on regular linear pyrolysis, the chemical arc technique
does not provide information on the quantitative kinetic characteristics of the high-
temperature decompositions of the components. However, this method does allow
one to study linear pyrolysis under conditions maximally close to those that occur in
the vicinity of the burning surfaces of composite solid propellants. Thus the chem-
ical arc technique can be considered an auxiliary method for studying linear pyrol-
ysis in the burning mode. The application of information obtained by the chemical
arc technique is considered later (Sect. 3.3), where the study of the effect of using
two combustion catalysts in a composite solid propellant on the burning rates of the
individual fuel components is used as an example.
