Closed Vessel Test Influence of the Ignition Method on the Combustion Rate

7/27/2019 Closed Vessel Test Influence of the Ignition Method on the Combustion Rate

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CLOSED VESSEL TEST: INFLUENCE OF THE IGNITION METHOD ON THE

COMBUSTION RATE.

Laurence Jeunieau1, Michel H. Lefebvre1,Alexandre Papy2, Marc C. Pirlot2,

Pierre Guillaume3, Christiane Reynaud4

1Laboratory for Energetic Materials, Royal Military Academy

Av. de la Renaissance 30, 1000 Brussels, Belgium

Fax: +32(0)2 7376352

E-mail: [email protected]

2Department of Weapon Systems and Ballistics, Royal Military Academy

3PB Clermont s.a.

Rue de Clermont 176, 4480 Engis/ Belgium.

4Centre de Recherches du Bouchet, SNPE,

Rue Lavoisier 9, 91710 Vert-le-Petit/ France

AbstractA method of homogeneous ignition of deterred spherical particles has been

proposed. The ignition mixture 0.05 MPa CH4-0.075 MPa O2 permits to

discriminate the combustion properties of the two parts of the particles. This

ignition method does not introduce additional influence on the combustion rate

compared to the classical ignition by black powder.

1. IntroductionThe ballistic performance guns can be improved by the use of less

degressive burning propellants. For this purpose, deterred propellants can be

used especially in small arms. In deterred propellants, the concentration of

deterrent in the surface layer is larger than that in the core of the propellant

particles.


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In ballistic models, factors characterising the burning rate of the propellant

must be introduced. These factors are usually obtained in closed vessel

experiments. These ones must be different for the two parts of the propellant

particles. No difference between them can be observed if black powder is used

as ignition system. The aim of this work is to investigate closed vessel tests,

which permit to see clearly the difference between the two parts of the particles.

For this purpose, an homogeneous ignition system is required. This study

focuses on the influence of the ignition phase on the final experimental output

(i.e. burning rate) of these tests. In order to have a homogeneous ignition of the

propellant particles, gaseous ignition systems are investigated.

These ignition systems are not comparable to the ones used in the actual

combustion chamber of guns where ignition is done either by hot particles or by

hot gases. In fact, the ballistic properties depend on the igniter [1]. Therefore,

ballistic models must take into account the nature of the igniter and the more or

less homogeneous ignition of the propellant.

For this purpose, the following scheme of investigation is followed in this

paper.

- Different ignition mixtures of methane and oxygen are investigated. These

ignition systems are compared with the commonly used ignition by blackpowder.

- The possible influence of the ignition system on the combustion rate is

investigated. At this state, an ignition system is selected.

- This ignition system is tested on propellants with different penetration depths

of the deterrent to confirm that the two combustion processes correspond to

the two chemical formulations of the particles.

2. Experimental

Propellants used

The different ignition systems have been tested on a spherical deterred

propellant with an average diameter of 0.77mm. The selected ignition system

has been tested on different spherical propellants with different types of coating.

The characteristics of these propellants are shown in Table 1 (the percentage in


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weight of deterrent in the particle is given). For propellants A and B all the

dibutylphthalate is in the deterred portion of the particles. For the propellants C

and D, some of the dibutylphthalate is in the inner part of the particle.

Diameter (mm) % dibutylphthalate inthe powder

% dibutylphthalate inthe inner part

Propellant APropellant BPropellant CPropellant D

1.001.011.020.96

7%7.4%8.5%7.9%

--2%1.8%

Table 1: Characteristics of the different investigated propellants.

Ignition system usedThe compositions of the different ignition systems are shown in Table 2.

The gaseous ignition systems are characterized by their partial pressures.

Composition

IgniterIIgniterIIIgniterIIIIgniterIVIgniterVIgniterVI

0.1 MPa CH4-0.2 MPa O20.1 MPa CH4-0.14 MPa O20.05 MPa CH4-0.075 MPa O20.1 MPa CH4-0.1 MPa O20.05 MPa CH4-0.05 MPa O2Black powder (loading density: 0.007g/cm3)

Table 2: Characteristics of the different investigated ignition systems.

Closed vessel experimentsClosed vessel experiments were carried out in a vessel of 140cm3 using a

piezo-electric pressure transducer (Kistler 6201B4) to record the pressure. The

output voltage of the pressure gauge was transferred to a data acquisition

system (Nicolet Multipro, resolution 12 bit, sampling frequency 250KHz). The

propellant loading density is 0.20g/cm3. The ignition system consists of two

electrodes, which are connected with a nickeline hot wire, and a valve to

introduce a CH4/O2 ignition mixture. The partial pressures of these mixtures are

measured with a piezo-electric pressure transducer (Kistler 4070). When black

powder is used as ignition method, 1g of powder is used. The combustion rate

is calculated according to Stanag 4115 [2].


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3. Results

3.1 Preliminary observations

Figure 1 shows different combustion rates obtained with ignition by

different gas mixtures and with black powder. Three types of combustion rate

vs. pressure have been observed.

(1) The largest combustion rate is obtained when using igniter I. In this case,

important oscillations of the pressure are noticed.

(2) For the igniters II and III, intermediate combustion rates are obtained.

(3) When less energetic ignition mixtures are used (igniters IV and V)

combustion rates similar to the one obtained with the black powder are

observed.

The important oscillations of the pressure which are noticed for the

igniters I and II may be due to shock waves stemming from the detonating gas

mixtures.

0

5

10

15

20

25

30

35

0 50 100 150 200Pressure (MPa)

Combustionrate(cm/s

).

(I) (II)

(III)

(V)(VI)

(IV)

Figure 1: Combustion rate of the deterred propellant obtained with different ignitionsystems. (I) IgniterI , (II) IgniterII, (III) IgniterIII, (IV) IgniterIV, (V) IgniterV and (VI)

IgniterVI .


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Figure 2 shows some of the pressure derivative vs. time. The pressure

derivative permits to distinguish clearly two phases in the combustion process

when igniterI, II orIII is used as ignition system. In fact, two different positive

slopes can be observed in the pressure derivative vs. time. In the other cases,

the variation of the derivative vs. time is more regular and the combustion

process can no more be divided in two parts. The lack of discontinuity could

stem from the inhomogeneous ignition of the propellant by the black powder.

When ignition is inhomogeneous, the deterred layers of all the particles do not

burn simultaneously. When one particle has its deterred layer burnt and

therefore its combustion rate increased the other particles may have their

deterred layer not completely burnt. Thus the observed combustion rate is

more or less a smooth average of the overall combustion rate.

0

20

40

60

80

100

120

140

160

180

200

-5 -4 -3 -2 -1 0 1 2 3 4time (ms)

dP/dt(MPa/ms)

(I)

(III)

0

20

40

60

80

100

120

140

160

180

200

-5 -4 -3 -2 -1 0 1 2 3 4time (ms)

dP/dt(MPa/ms)

Figure 2: Derivative of the pressure vs. time for different ignition systems.1 (I) IgniterI, (III) IgniterIII. 2 Igniter VI.

The fact that the observed discontinuity in the derivative curve

corresponds to the burning of the deterred layer has to be confirmed. If it does,

the thickness of this layer can be calculated. This will be a first test to check

whether the discontinuity corresponds to the end of the combustion of the

deterred layer. If this is the case, the obtained thickness of the deterred layer

should be the same for different loading densities and for different ignition

1 2


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systems. To obtain the burnt thickness, the discontinuity in the derivative curve

is associated with a time and from these with a pressure. From the pressure,

the burnt mass fraction (zbreak) can be calculated using equation (1) where , ,

Pmax, Pmin, Pbreak, are the loading density, the propellant density, the maximum

experimental pressure, the pressure resulting from the igniting gaseous mixture,

the pressure corresponding to the discontinuity in the derivative curve and the

covolume, respectively.

min

minmaxbreakbreak

minmax

break

min

break

max

break

minmax

break

P

P1

PPP1PP

11

P

P1

P

P1

P

1PP

11

z

!+"#"+

!#

$

#

!#$#

!+"#"+

!#

$

#

!#

$=

K

K

(1)

This equation permits to take into account the pressure coming from the ignition

system (Pmin). As this pressure is not identical for all the ignition systems, it has

to be taken into account in order to obtain comparable results. As the burnt

mass fraction is defined by equation (2) where V is the propellant volume and

V0 its initial value, the thickness of the deterred layer can be easily calculatedfrom the propellant volume. The obtained values are given in Table 2.

0V

V1z != (2)

Igniter Loading density (g/cm3) Thickness of the deterred layer (mm)

IgniterI 0.10.150.200.25

0.0420.0480.0490.046

IgniterII 0.20 0.047IgniterIII 0.10

0.150.20

0.0440.0410.041

Table 2: Thickness of the deterred layer for different ignition systems and for differentloading densities.

These different values show that the discontinuities in the derivative curves are

related neither to the ignition composition nor to the initial pressure.


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At this stage, it must be checked that the ignition system does not

influence the combustion rate of the propellant. For this purpose, the properties

of the ignition systems have been studied thoroughly and the influence of the

ignition system on the chemical reaction products has been investigated.

3.2 Comparative study of the ignition methods

3.2.1 Study of the ignition systems

In order to characterize the different ignition systems, closed vessel

experiments of the gas mixtures alone and of the black powder have been

performed. Different pressure-time curves are shown in Figure 3.

Thermodynamic calculations have been done with the ICT code [3]. For these

calculations the presence of air in the closed vessel has been taken into

account. Table 3 resumes different characteristics of the igniters.

0

1

2

3

4

5

6

7

8

-5 -3 -1 1 3 5 7 9

time (ms)

Pressure(MPa)

(I)

(II)

(III)

(IV)(VI)

(V)

Figure 3: Pressure vs. time curves of the different ignition systems. (I) IgniterI, (II)

IgniterII, (III) IgniterIII, (IV) IgniterIV, (V) IgniterV and (VI) IgniterVI.


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Deto-nationa

Insta-bilitiesb

rp/rpVIc Pmax

d(MPa)

Pmaxe

(MPa)Texplosion,

(K)Energyf

(J)

IgniterIIgniterII

IgniterIIIIgniterIVIgniterVIgniterVI

YesYes

YesNoNo-

YesYes

NoNoNoNo

HighestHigher

HigherSimilarSimilar-

5.384.68

2.523.601.872.64

5.484.90

2.874.242.563.10

35813555

3377325432272002

32522841

1632230814002186

Table 3: Different characteristics of the igniters.

(a) The detonation limits are not known in the used experimental setting. The used

detonation limits (0.105-0.9824 OCH

P/P ) are obtained in a tube with a diameter of 1 cm

[4].(b) The combustion instabilities refer whether or not important oscillations are observedin the experiments.

(c) Comparison of the obtained burning rate with the one obtained with the blackpowder (igniterVI).(d) Experimental maximum pressure.(e) Theoretical maximum pressure.

(f) The energy has been obtained by E= Cv * T*migniter

The important oscillations observed for igniter I and II in the closed

vessel test experiments with and without propellant may be due to the

detonation of the gas mixture. The composition of igniter III is also in the

detonation limit. But these limits are for methane mixed with pure oxygen. Infact, the critical energy for direct initiation of detonation increases significantly

when the percentage of diluant increases [5].

The maximum pressure is relatively important for igniterI,II and IV. The

maximum pressure of igniterIII and VI are in the same order.

The explosion temperature is far lower for the igniter VI than for the

gaseous ignition systems.

The energy produces by the igniter is important for igniterI, II and IV. Itis also in the same order for igniterIII and VI.

The slope of the pressure-time curve (Fig.3) is lower for igniterVI and is

more important for igniterI and II.

From these results, the only factor that can be related to the change of

combustion rate as a function of the igniter is the slope of the pressure-time

curve. In fact, the gaseous ignition systems, which have a similar combustion


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rate than the black powder, have also the slowest increase in pressure. This

factor can be related to a more or less homogeneous ignition of the propellant.

The more the combustion of the igniter is rapid the more the ignition must be

homogeneous.

3.2.2 Effect of the ignition system on the chemical composition

A required important characteristic for an experimental ignition system is

that it must not affect the determination of the combustion rate. In fact, it is

considered that the first part of the pressure-time curve is somehow influenced

by the ignition system and consequently for the determination of the combustion

rate only the range between 30%-70% of the maximum pressure is used [2].

Table 4 shows the influence of the ignition system on the combustion of

the propellant. Two factors have been considered: first, the ratio between the

molar percentage of CO2 and H2O (rich oxygen species) vs. the percentage of

CO and H2 (poor oxygen species) and secondly, the percentage difference of

the explosion temperature.

The amount of rich oxygen species increases by adding the igniter. The

increases are more important for the igniter I and VI. The explosion

temperature increases when the ignition system is gaseous and decreaseswhen black powder is used. The increase due to igniterI is important.

2

22

H%CO%

OH%CO%

+

+

()%Texplosion

Propellant+IgniterI+IgniterII+IgniterIII

+IgniterIV+IgniterV+IgniterVI

0.4180.444 (5.80%)0.432 (3.35%)0.429 (2.63%)

0.424 (1.44%)0.424 (1.44%)0.436 (4.28%)

-2.610.900.95

0.610.46-0.71

Table 4: Influence of the ignition system on the propellant. The percentages used inthe calculations are the percentages in mole. The figures between brackets are the

percentages of increase of the ratio due to the igniter.


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At this point, an ignition system must be selected. Igniter I and II must

be excluded, important oscillations in the pressure-time curves are observed

and the maximum pressure of the igniter alone in the closed vessel is high.

Igniter IV has also a high maximum pressure and produces a lot of energy. It

must therefore also been excluded. The increase of rich oxygen specie due to

igniterIII is important, but the increase due to igniterVI is larger. As the igniter

III permits to see a difference between the two parts of the particles, it is

selected.

3.3 Study of propellants with different deterrent penetration depths

Now that an ignition system has been selected, one must check whether

or not the discontinuity in the curves is actually due to change in chemical

composition. For this purpose, spherical propellants with different

concentrations in deterrent are used. These propellants are described in Table

1.

As at the end of the coating, almost all the deterrent is in the particles,

the penetration depth must follow the same sequence than the percentage of

deterrent. Figure 4 shows the obtained combustion rate with the IgniterIII and

with the igniterVI.

The difference between the different propellants is noticeable when the

gas ignition mixture is used. A great difference is observed between the

propellants with and without dibutylphthalate in their inner core. A difference of

combustion rate is observed between propellants A and B. The increase of

combustion rate appears further in the case of propellant B than for propellant

A. This is to be related to the higher percentage of deterrent in propellant A

resulting in a larger coating layer. A small difference is also observed between

propellant C and D. When black powder is used, a slight difference is observed

between the propellants, whit and without dibuthylphtalate in their inner core.


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0

2

4

6

8

1012

14

16

18

20

0 50 100 150Pressure (MPa)

Combustionrate

(cm/s). (a)

(b)

(c)

(d)

0

2

4

6

8

10

12

14

16

18

20

0 50 100 150Pressure (MPa)

Combustionrate

(cm/s).

(a),(b)

(c),(d)

Figure 4: Combustion rates of (a) Propellant A, (b) Propellant B, (c) Propellant C, (d)

Propellant D.1 IgniterIII2 IgniterVI

Figure 5 shows the derivative of the pressure vs. time. If the gas mixture

is used as ignition system, a difference can be easily observed between the

different propellants (Fig. 5-1). The discontinuity occurs first for propellant A,

followed by propellant B. This is the variation expected from the different

percentages in deterrent. Propellant C and D have a more gradual increase of

their derivative curve. This comes from the deterrent in the inner core of the

particles. Therefore the difference between the two chemical compositions of

the particles is less important. It can be seen that the discontinuity occurs first

for propellant C than for propellant D, this stems from the higher deterrent

concentration of propellant C. If the black powder is used as ignition system, a

difference can be observed only between the propellants with and without

dibutylphthalate in their inner core.

1 2


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0

20

40

60

80

100

120

-5 -3 -1 1 3 5

time (ms)

dP/dt(MPa/m

s)

0

20

40

60

80

100

120

-5 -3 -1 1 3 5time (ms)

dP/dt(MPa/m

s).

(a)

(b)

(c)

(d)(a)

(b)

(c)(d)

Figure 5: Derivative curves (a) Propellant A, (b) Propellant B, (c) Propellant C, (d)

Propellant D. To clarify the graphs, the curves of propellant B and D are in gray. 1.IgniterIII2. IgniterVI.

Table 5 shows the penetration depth of the propellants obtained as

previously explained for the propellants ignited by the gas mixture. As

expected, the penetration depth increases with the deterrent concentration.

Penetration depth (mm)

Propellant APropellant BPropellant CPropellant D

0.0760.0950.1020.098

Table 5: Penetration depths corresponding to the discontinuities in the derivative

curves (see Equation (1) for calculation procedure).

4 Conclusions

This study demonstrates the possibility to observe in closed vessel

experiment the difference between the two chemical compositions of a deterred

propellant.

First, different gaseous ignition systems have been tested. It has been

clearly observed that the ignition system has a significant influence on the

combustion rate. Three of the tested ignition systems permit to see a

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Closed Vessel Test Influence of the Ignition Method on the Combustion Rate

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