Behaviour of a Working Fluid in an ...

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Propellants, Explosives, Pyrotechnics 23, 17±22 (1998)
17
Behaviour of a Working Fluid in an Electrothermal Launcher
Chamber
B. Baschung, M. Samirant, K. Zimmerman, C. Steinbach, and D. Mura
Institut Franco-Allemand de Recherches de Saint Louis (ISL), F-68301 Saint Louis Cedex (France)
A. Louati
Laboratoire Gestion des Risques et Environnement (GRE), F-68200 Mulhouse (France)
Das Verhalten einer Arbeits¯È ssigkeit in der Kammer einer
elektrothermischen Kanone
Der elektrothermische Antrieb, dessen Konzept es gestattet, die Grenzen
der Projektilgeschwindigkeit zu hÈheren Werten zu verschieben, bietet
gegenÈer einer konventionnellen Kanone viele Vorteile. Wie bei Pul-
verkanonen, ist die Kenntnis der thermochemischen Daten und des
Verhaltens des Treibstoffs fÈr die Optimierung des Systems notwendig.
Um die chemischen PhÈnomene zu verstehen, die wÈhrend des elek-
trothermischen Prozesses statt®nden, haben wir versucht, die Reak-
tionsprodukte von Methanol zu charakterisieren. Hierzu wurde ein
manometrischer Kessel, der hohen DrÈcken standhalten kann, zusÈtzlich
mit einem Plasmagenerator ausgestattet. Wir identi®zieren und quanti-
®zieren die Verbrennungsprodukte von Methanol mittels chromato-
gra®scher und spektometrischer Analyse. Die freigesetzte chemische
Energie wird auf der Grundlage der Analysenergebnisse berechnet. Die
Ergebnisse zeigen, daû es eine in das Plasma eingekoppelte Energie gibt,
von der ab die gesamte Masse des Methanols zerfÈllt. Diese Energie wird
Èber ein empirisches Verfahren abgesch
È
tzt. Die theoretischen Tem-
peraturberechnungen sind in guter
È
bereinstimmung mit den beob-
achteten PhÈnomenen. Es wird gezeigt, daû die im Kessel benutzten
Kunststoffe eine bedeutende Rolle bei der Verbrennung spielen.
Auûerdem ist die wÈhrend der Reaktion freigesetzte chemische Energie
kein entscheidender Parameter zur Optimierung der Leistung der Kanone
wÈhrend des elektrothermischen Beschleunigungsprozesses. Es wird
eine mathematische Beziehung zwischen der wÈhrend der Reaktion
freigesetzten chemischen Energie und der in das Plasma eingekoppelten
elektrischen Energie fÈr Methanol vorgestellt.
Comportement d'un ¯uide moteur dans une chambre de lanceur
Âlectrothermique
La propulsion
Â
lectrothermique dont le concept permet de repousser
les limites de la vitesse atteinte par le projectile, prÂsente de nombreux
avantages par rapport Á la propulsion conventionnelle. Comme pour un
lanceur Á poudre, la connaissance des donnÂes thermochimiques et du
comportement du propulseur sont nÂcessaires Á l'optimisation du
systÁme. Pour comprendre les phÂnomÁnes chimiques ayant lieu
pendant le processus Âlectrothermique, nous entreprenons de car-
actÂriser les produits de rÂaction provenant de la dÂcomposition du
mÂthanol. Dans cette perspective, une enceinte rÂsistant aux hautes
pressions est sp
Â
cialement modi®
Â
e par adjonction d'un g
Â
n
Â
rateur de
plasma. Nous identi®ons et quanti®ons les produits de combustion du
m
Â
thanol par analyse chromatographique et spectrom
Â
trique. L'
Â
ner-
gie chimique libÂrÂe est calculÂeÁ partir des rÂsultats d'analyse. Les
r
Â
sultats montrent qu'il existe une
Â
nergie inject
Â
e dans le plasma
Á
partir de laquelle la totalit du mÂthanol est dÂtruite. Cette Ânergie est
estim
Â
e par m
Â
thode empirique. Les calculs th
Â
oriques de temp
Â
rature
sont en bon accord avec les phÂnomÁnes observÂs. Il est montr que les
matiÁres plastiques employÂes dans l'enceinte jouent un rÃle important
dans la combustion. En outre, l'Ânergie chimique libÂrÂe par la rÂac-
tion n'est pas un facteur dÂterminant quant Á l'optimisation des per-
formances du lanceur pendant le processus Âlectrothermique. Une
relation entre l'Ânergie chimique libÂrÂe pendant la rÂaction et
l'Ânergie injectÂe dans le plasma est Âtablie pour le mÂthanol.
Summary
1. Introduction
Electrothermal propulsion allows higher limits of projectile
velocity. It provides many advantages over conventional
propulsion. As for a conventional gun, the knowledge of thermochemical
data and propellant behaviour is necessary to optimize the system. The
reaction products issued from the decomposition of methanol during an
electrothermal process were characterized in order to understand the
chemical process. Therefore, a plasma generator is specially added to a
closed vessel suitable for high pressures. The combustion products of
methanol were identi®ed and quanti®ed by the analysis of the gases with a
chromatograph coupled with a mass spectrometer. The released chemical
energy is calculated from the analysis results. A de®ned electrical energy
injected into the plasma is found by which the amount of methanol is
completely decomposed. This energy is estimated by an empirical
method. The theoretical calculations of temperature ®t well with the
observed phenomena. Results indicate that the plastic materials used in
the closed vessel have an important in¯uence on the combustion.
Moreover, the chemical energy released during reaction is not the deci-
sive parameter for the optimization of the launcher's performances dur-
ing the electrothermal process. A relation between the chemical energy
released during reaction and the plasma injection energy is established for
methanol.
Conventional launchers use the expansion of gases under
high pressure to accelerate a projectile. For a given initial
energy, the main parameter which controls the muzzle
velocity and the system ef®ciency is sound velocity in
gases. This velocity determines the effective projectile base
pressure during acceleration. Therefore, the gases must
have a low molecular weight and a high temperature.
Electrothermal launchers use the expansion of a plasma,
generated by an electrical discharge in a chamber possibly
with the addition of the decomposition of a working ¯uid
which allows to increase the volume of the gases and to
maintain temperature within the limits consistent with the
resistance of the metal of the launchers.
As in the case of a conventional launcher, it is necessary
to know the thermochemical data and the propellant behav-
iour (the composition of the combustion gases) to optimize
the electrothermal system.
# WILEY-VCH Verlag GmbH, D-69451 Weinheim, 1998
0721-3115/98/0102±0017 $17.50:50=0
18 Baschung, Samirant, Zimmermann, Steinbach, Mura, and Louati
Propellants, Explosives, Pyrotechnics 23, 17±22 (1998)
Numerous mixtures and pure substances have been stud-
ied and identi®ed as working ¯uids
(1,2)
. Several studies have
still been carried out, especially the thermochemical prop-
erties of working ¯uids
(3)
, and the plasma=working ¯uid
interaction in the electrothermal launcher chamber
(4)
.
Encouraged by the initial results, based on tests using
methanol as working ¯uid in the ISL 12 mm bore diameter
launcher
(5)
, the reaction products were characterized in
order to understand the chemical process occurring during
the electrical discharge. The aims of this report are (1) to
present the decomposition and reaction of methanol during
the electrothermal process, (2) to determine the chemical
energy released by the reaction, and (3) to establish a
relation between the energy injected into the plasma and the
energy released by the decomposition of methanol.
Expansion chamber
It allows the expansion of the plasma generated by the
electrical discharge. The expansion chamber is ®xed at the
downstream end of the plasma generator. It consists of three
pieces of metal, the interior walls of which are coated with a
thin polycarbonate sheath to protect them against erosion.
The chamber interior diameter is 20 mm, its length 130 mm.
Before testing, the chamber is evacuated.
Nozzle
The nozzle is a steel cylinder 20 mm in length with an
internal bore of 1 mm in diameter. It is connected with the
expansion chamber. After the electrical discharge, the
pressure increases and the gases begin to stream down the
nozzle. Erosion of the nozzle leads to a very fast enlarge-
ment of its diameter and to a rapid reduction of pressure.
Then the combustion gases stream down from the closed
vessel into the combustion gas-holder.
2. Experimental
To perform this study, a vessel suitable for high pressures
was specially modi®ed by adding a plasma generator
(Figure 1).
Gas-holder
Plasma generator
The combustion gases ejected from the nozzle are
recovered in an evacuated steel gas-holder. The gas-holder
is connected to the closed vessel with a polyethylene tap
(Figure 2) and is ®tted with a gauge to measure the pressure
after the experience. All the volumes up to the polyethylene
membrane (expansion chamber, nozzle, piping and gas-
holder) are evacuated with an oil diffusion pump to a
pressure of about 10
ÿ3
It allows the conversion of electrical energy into thermal
energy. The plasma generator consists of two electrodes, a
rear electrode and an annular electrode, separated by an
insulating tube of polyethylene. In this type of con®gura-
tion, the rear electrode is conceived as a high voltage
electrode. The second electrode is grounded. The distance
between the electrodes is 79 mm and the interior diameter of
the plasma generator is 10 mm. The arc ignition is obtained
by exploding a constantan wire (diameter 120 mm) between
the electrodes.
For each test, 2 g of methanol are located in the plasma
generator. The downstream end (level with the annular
electrode) is closed with a double polyethylene membrane.
The wire between the electrodes is adjusted along the
plasma generator axis and is immersed in methanol.
Pa.
Electrical source
The electrical source (Figure 3) is the same as the one
used for the ISL 12 mm launcher
(6)
. It consists of a
35 kV=97 kJ capacitor bank. The bank consists of 31 par-
allel connected capacitors (5.13 mF). A spark gap initiates
the discharge process. The natural inductance of the circuit
Figure 1. Representation of the closed vessel with the plasma generator.
Propellants, Explosives, Pyrotechnics 23, 17±22 (1998) Behaviour of a Working Fluid in an Electrothermal Launcher Chamber 19
analysis is proceeded at 100
C. The sample injection is
obtained with a gas-tight syringe. The mass spectrometer
(HP-5972A) acts as a detector: it characterizes and quanti-
®es the gas mixture components.
3. Results
Figure 2. Recovery system for the gases.
The current intensity in the plasma generator measured
with a Rogowski coil and the voltage in the plasma gen-
erator measured with a high-voltage gage, characterize the
closed vessel process. The electrical energy injected into
the plasma is determined from the data for intensity and
voltage.
is 43 mH. An additional 40 mH induction coil allows
tailoring of the current. A crowbar consisting of a series of
diodes (72 kA maximum intensity and 2.2 kV maximum
voltage) avoids polarity reversal on the capacitor bank. The
plasma generator resistance varies between large limits
during discharge.
By choosing the number of capacitors from 8 to 31, the
electrical energy on the capacitor bank can be varied from
25 to 97 kJ. Then, the energy injected into the plasma can be
increased from 13 to 70 kJ.
4. Interpretation
Figure 4 shows the percentage in volume of the major
constituents in the combustion gases for various plasma
injection energies. The products contained in the gaseous
mixture can be divided into two groups. The ®rst group is
formed by carbon monoxide, methane, carbon dioxide
(which is always present in small quantities) as well as
methanol in some cases. They were obtained from the
methanol decomposition. In the second group hydrocarbons
are found, such as acetylene, ethylene and ethane. They are
probably degradation products of the plastic materials
(polyethylene from the plasma generator and polycarbonate
from the expansion chamber). The quantities of hydro-
carbons, except for acetylene, are very small. The results
are shown in Table 1.
For the lower energies, the increase of carbon monoxide
and methane percentages when increasing the plasma
injection energy can be explained by the progressive dis-
appearing of methanol. Methanol can be found in the
combustion products when the plasma injection energy is
below 32±33 kJ. Then methanol disappears by increasing
the energy to a larger level, being totally decomposed.
The amounts of carbon monoxide and methane slightly
increase by increasing the energy between 35 and 50 kJ.
Gas-container
A30cm
3
stainless steel gas-container is ®tted on the gas-
holder. It allows to collect a sample of the combustion gases
after each test.
Gas analysis
The combustion gases analysis is performed using the HP
MS Chemstation system, a gas chromatograph coupled with
a mass spectrometer (GCMS). Light gases, hydrocarbons
and alcohols can be separated by using a 25 m in length
PoraPLOTQ capillary column, with an interior diameter of
0.32 mm. Helium is used as a carrier gas. The isothermal
Figure 3. Electrical circuit.
20 Baschung, Samirant, Zimmermann, Steinbach, Mura, and Louati
Propellants, Explosives, Pyrotechnics 23, 17±22 (1998)
Figure 4. Percentages (in volume) of the gaseous mixture components, (carbon monoxide, methane and methanol) versus plasma injection
energy.
Table 1. Overview
E
plasma
Gaseous products obtained (percentages in volume)
DH
reaction
(a)
%E
chemical
(b)
(kJ)
CO
CH
4
CO
2
C
2
H
2
C
2
H
4
C
2
H
6
H
2
O
3
OH
(kJ=2 g MetOH)
13.05
44.30
14.60
0.40
2.40
0.40
0.40
0.50
36.90
78.06
38.16
15.00
47.80
24.00
0.60
0.60
0.40
0.50
1.20
24.80
77.64
33.75
30.02
57.30
21.50
0.90
9.80
1.90
1.50
1.50
5.50
74.58
13.24
30.18
51.30
21.50
0.80
10.50
2.60
1.30
1.10
10.80
74.61
13.26
31.06
53.90
29.30
1.10
9.20
2.50
1.20
1.90
1.00
74.31
12.17
33.80
52.50
30.40
1.00
10.20
3.00
1.20
1.80
0.00
73.94
10.43
34.69
53.30
32.00
0.60
9.20
2.90
1.10
0.70
0.30
73.98
10.29
48.22
54.00
32.60
0.60
8.00
2.90
1.50
0.40
0.00
74.16
7.95
50.31
54.50
33.20
0.50
8.00
2.60
1.10
0.10
0.00
74.15
7.61
61.06
62.00
27.10
0.50
4.60
2.40
1.50
0.00
0.00
74.71
7.17
64.39
61.20
29.90
0.30
4.40
2.30
1.10
0.00
0.00
74.83
6.98
64.95
55.80
33.70
0.50
5.20
2.70
1.50
0.30
0.00
74.74
6.80
66.88
56.90
33.70
0.20
5.20
2.60
1.10
0.10
0.00
74.64
6.49
69.48
63.40
28.30
0.20
4.80
2.30
0.90
0.00
0.00
74.90
6.58
70.43
61.30
29.20
0.30
4.80
2.60
1.00
0.00
0.00
74.75
6.32
(a)
Chemical energy is the variation of enthalpy between ®nal products (determined from the chromatographic and spectrometric analysis) and the
initial product, methanol.
(b)
Chemical energy percentage that is [DH
reaction
=DH
reaction
E
plasma
].
Whereas above 50 kJ a progressive increase of carbon
monoxide can be observed, the quantity of methane does
not change signi®cantly above 50 kJ.
Experiences using an energy of about 65 kJ showed that
the amount of methane is larger whereas the amount of
carbon monoxide is smaller: hydrogenation is preferred to
oxygenation but the actual reasons for this phenomenon are
not well understood yet. High temperature may favour
hydrogenation against oxygenation. Moreover, a catalytic
hydrogenation may be enhanced by the presence of metal
from electrodes erosion.
The presence of methanol in the ®nal gas using a plasma
injection energy below 33 kJ can be explained as follows.
At the beginning of the electrical discharge, methanol is
ejected from the plasma generator into the expansion
chamber, since the membrane of the plasma generator
opens. Once in the expansion chamber, it reacts. The
reaction rate depends essentially on the temperature reached
in the chamber, on the localization of the methanol in the
chamber (at the entry, near the annular electrode, or near the
nozzle) and on the time the nozzle opens. The temperature
reached in the expansion chamber depends on the electrical
energy injected into the plasma. The calculations performed
by K. DarÂe for the ISL 12 mm electrothermal launcher
show that the temperature gradients are very important in
the launcher. The achieved temperature is 1500 K during
0.2 ms for 69 kJ at a distance of 15 cm from the rear elec-
trode (in the expansion chamber centre) using 2 g methanol
in the plasma generator. In the arc core, the temperature
reaches 30 000 K. Accordingly, when the plasma injection
energy is larger than 35 kJ, the high temperature zone is
suf®ciently large to totally decompose the methanol in the
expansion chamber. On the other hand, when the plasma
injection energy is below 33 kJ, the high temperature zone
in the expansion chamber is too localized to decompose all
methanol: only a part will react before the opening of the
nozzle.
Figure 5 shows the chemical energy (DH
reaction
) liberated
as a function of plasma injection energies. The total
decomposition of methanol is indicated by a bending point.
Up to 33±34 kJ, the chemical energy released by the reac-
tion diminishes gradually until methanol completely dis-
appears. Then, the chemical energy increases slightly to an
amount of 4.8 kJ as 65±70 kJ were injected into the plasma.
Propellants, Explosives, Pyrotechnics 23, 17±22 (1998) Behaviour of a Working Fluid in an Electrothermal Launcher Chamber 21
Figure 5. Chemical energy (reaction enthalpy) as a function of the plasma injection energy.
Figure 6. Percentage of the chemical energy as a function of the plasma injection energy.
The presence of methanol in the ®nal gaseous phase
explains the large amount of chemical energy obtained by
injecting small energies into the plasma (7.64 kJ chemical
energy released for 15 kJ plasma injection energy).
In Figure 6 the percentage of the released chemical
energy is plotted against the various plasma injection
energies and shows a bending point for the same abscissa as
on the diagram in Figure 5. The percentage of the released
chemical energy is very large for small plasma injection
energies (35% for an energy injected in the plasma of
15 kJ). The best results for the launcher are obtained by
injecting 65±70 kJ energy into the plasma since the
launcher's performances increase as the plasma injection
energy increases. It seems that the released chemical energy
of the electrothermal process is not completely used to
optimize the performances of the launcher.
However, a relation between the released chemical
energy of the reaction and the plasma injection energy is
established. Figure 7 shows the Naperian logarithm of the
chemical energy for various plasma injection energies. It
displays two straight lines. The abscissa of their intersection
point agrees with the abscissa of the in¯ection point. Those
two straight lines de®ne two energy intervals (the two
intervals given before: below 33 kJ and above 35 kJ). Thus,
a relation between the energies above-mentioned is estab-
lished with an exponential function:
DH
reaction
e
baE
plasma
100 ÿ e
baE
plasma
E
plasma
where a and b are, respectively, the slope and the ordinate
from the point of origin of the straight lines.
The straight lines coef®cients are the following:
8E
plasma
; such as 10 < E
plasma
< 33 kJ;
a 6:26 10
ÿ2
and b 4:459;
8E
plasma
; such as 35 < E
plasma
< 70 kJ
a 1:29 10
ÿ2
and b 2:746:
The value for which the chemical energy is the smallest is
determined. It is the value of the chemical energy for the
abscissa of the intersection point: E
plasma
34.5 kJ.
Further investigations will be carried out using different
working ¯uids in order to con®rm this relation for other
substances.
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