Oscillations in Grains of Gun Propellants · Accelerated Burning of Gun Propellants Caused by ... Q...

WTD 91 43RD NDIA Gun & Missile Systems Conference & Exhibition, New Orleans, April 2008 © 1

Accelerated Burning of Gun Propellants Caused by Pressure Oscillations Within the Perforations

of Propellant Grains

Gert Pauly – Rudolf ScheibelWehrtechnische Dienststelle für Waffen und Munition (WTD 91)D-49716 Meppen, Germany email: [email protected]

43rd Annual Armament Systems:Gun & Missile Systems Conference & ExhibitionApril 21 – 24, 2008, Sheraton New Orleans Hotel

New Orleans, LA, U.S.A.


Contents

1. Pressure oscillations in holes of propellant grains

2. Connections between pressure oscillations, accelerated burning and temperature behaviour

3. Working hypothesis explaining the effect of accelerated burning caused by pressure ocillations

4. Conclusions


Messkurve Treibladungsanzünder 120 mm Mörser bei STUB

-100

0

100

200

300

400

500

-5 0 5 10 15 20

Zeit in [ms]

Dru

ck in

[bar

]

Druck

Example for Vieille-oscillations in a closed vesselWTD 91 January 2007


Oscillations superimposing the rise of pressure

0 10 20 30 40t [ms]

0

200

400

600

800

1000

Dru

ck

[ba r

]

TLP Typ I5420Länge 50 mmTemp. -40°CSamplerate 12 usLadedichte 0,1 g/mlBombenvolumen 700 mlFile 27779b

Pressure / time curve resulting from burning a tube propellant in a closed vessel.The oscillations are found in the encircled range.


Pressure oscillations when burning a tubular propellant

0 2 4 6 8t [ms]

0

10

20

30

-10

-20

-30

Dru

ckdi

ffere

nz

[bar

]

TLP Typ I5420Länge 50 mmTemp. -40°CSamplerate 2 usLadedichte 0,2 g/mlBombenvolumen 700 mlFile 27782b Kanal 2

Dargestellter Bereich 200 bar - 768 barbei einem Pmax der Messung von 2329 bar

The picture was achieved by subtracting the original p / t - curve from the smoothed p / t - curve.


Confirmation of the existence of pressure oscillations when burning propellant grains

Final report to ISL-VE05029-1


Confirmation of the existence of pressure oscillations when burning propellant grains


Abnormal (wavelike) erosions in the holes

The picture shows a sectional view of a triple-base 19-hole-propellant grain after interrupted burning at - 46°C (- 51°F). The wavelike erosions indicate the occurrence of standing waves in the holes.


Frequency spectrum of a tubular propellant with a length of 50 mm

0 30 60 90 120[kHz]0

0,6

1,2

1,8

2,4

3

Inte

nsitä

t

X=19,7 kHz

X=50,0 kHz

X=99,8 kHz

TLP Typ I5420Länge 50 mmTemp. -40°CSamplerate 2 usLadedichte 0,1 g/mlBombenvolumen 700 mlFile 27779b Kanal 2

X=10,0 kHz

Characteristic maxima of frequencies associatedwith the fundamental frequency (10 kHz)


0 10 20 30 40 50 60 70 80 90 100 110 120 130 [kHz]

00,40,81,2

00,30,60,9

00,20,40,6

00,20,40,6

00,20,40,6

0123

5 mm

10 mm

15 mm

20 mm

30 mm

50 mm

Frequency spectra of different long propellant tubes (Loading density = 0.1 g/ccm)

Length

Frequency spectra of tubular propellants with different lenghts

( Loading density = 0.1 g/ccm )

30 mm

50 mm

20mm

5 mm

10 mm

15 mm

The shift of the frequencies proves that the oscillationsare connected with the tube lengths.


Contents




4. Conclusions



Double-base 7-hole-propellant grain after interrupted burning at - 40°C (- 40°F).


19-hole-triple-base-propellantR5730


German Armored Howitzer 2000PzH 2000 , Cal. 155 mm


R5730


R5730after

Inter-rupted burning

-46 °C = -51 °F

+60 °C = +140 °F


Pressure - time curves of the propellant R5730 in a closed vessel at – 40 (- 40°F) and +50°C (+122°F)

Both curves are temporally superimposed to improve the comparability. The cold propellant (-40°C)shows intense oscillations. The warm one (+50°C) shows only weak and short-time oscillations.



Furthermore the cold propellant shows increasing oscillations for a while connected with

accelerated burning and overtakes the warm propellant during burning.



Having overtaken - the oscillations of the cold propellant die away and the burning rate risesslower. The burning rate of the warm propellant rises faster and overtakes the cold one again.



Finally the warm propellant reaches a higher maximum pressure than the cold propellant.


The excessive burning corresponds with the intensity of pressure

oscillations

Dynamic Vivacity: V = dp/dt * 1/(pmax* p(t))


Contents




4. Conclusions


Gas Phase Reaction Zone

Unburned Propellant

Heating Zone (solid)

Foam/Liquid Reaction Zonewith bubbles – two phases

Flue Gas

Flame Zone

Q

Simple model of a burning propellant surface


Wavelike erosion on the surface of a hole after interrupted burning, Tpropellant = 20°C (= 68°F), magnification: 200 times, sample 92/02


Surrounding liquid

Increasing static pressure

Cavitation bubble imploding closeto a fixed surface generating a jet (4)of the surrounding liquid.

1 432

Ultrasound generates cavitation bubbles in liquids


Working hypothesis

The foamy layer is stressed with intense pressure oscillations. The layer is modified by pressure oscillations comparably like ultrasonic waves impact a surface covered with a liquid.


Gas Phase Reaction Zone

Unburned Propellant

Heating Zone (solid)

Foam/Liquid Reaction Zonewith bubbles – two phases

Flue Gas

Flame Zone

Q Q

The intense oscillations cause a reduced foam / liquid zone and an increased heat flux.


Relationship between pressure oscillations and accelerated burning using a triple-base propellant with and without perforations

Grains without perforations

Grains with perforations


Grains without perforations

Sample 90/02


Dependency of dynamic vivacity on temperature, non-perforated propellant.

0 0,2 0,4 0,6 0,8 1p/pmax

0

0,03

0,06

dyn.

Leb

hafti

gkei

t [1/

(bar

xs)]

0

9

18

27

-9

-18

Probe 90/02Temperatur 63°C, 21°C, -40°CLadedichte 0,20 [g/cm 3]

+63°C

-40°C

+21°C

-40°C

Druckoszillation

[bar]

From +63 (+145°F) to –40°C (- 40°F) the vivacity of the non-perforated propellant decreases with temperature. No noteworthy oscillations can be observed.


Grains with perforations

Sample 91/02


Dependency of dynamic vivacity on temperature,perforated propellant.

0 0,2 0,4 0,6 0,8 1p/pmax

0

0,04

0,08

0,12dy

n. L

ebha

ftigk

eit [

1/(b

arxs

)]

0

9

18

27

-9

-18

Druckoszillation

[bar]

Probe 91/02Temperatur 63°C, 21°C, -40°CLadedichte 0,20 [g/cm 3]

+63°C

-40°C+21°C

-40°C

Reversal of the temperature behaviour for the perforated propellant within the temperature range from +63 (+145°F) to - 40°C (- 40°F). The vivacity of the perforated propellant increases with temperature. The excessive burning corresponds with the intensity of pressure oscillations.


Expanded p/pmax- scale of the initial range of the dynamic vivacities with graphic depiction of the related pressure oscillations at different temperatures

0 0,05 0,1 0,15 0,2 0,25p/pmax

0,04

0,08

dyn.

Leb

haft

igke

it [1

/( bar

xs)]

0

20

40

Dru

cko s

zilla

tion

[bar

] - 4

0°C

0

20

-20

Dru

ckos

z illa

tion

[ba r

] 6

3 °C

0

-20

-40

Dru

ckos

zil la

tion

[bar

] 21

°CProbe 91/02Temperatur 63°C, 21°C, -40°CLadedichte 0,20 [g/cm3]

At +63°C: only weak oscillations - regular behaviour of the dynamic vivacityAt +21°C: intense oscillations - increased dyn. viv. compared to the +63°C-curveAt –40°C: intense oscillations - significant increase of the dynamic vivacity


Conclusions

• During combustion of perforated propellant grains in closed vessels a special form of pressure oscillations is generated independent from the known Vieille-oscillations.

• These oscillations have been proven at several institutes(WTD 91, ISL, Diehl).

• The oscillations often result in accelerated burning and modify the surfaces of the perforations.

• Working hypothesis: The pressure oscillations modify the two phases reaction zone and cause a reduced foam layer and anincreased heat flux.


Aim of the work

Presently ammunition with temperature independentmuzzle velocity is under development.

This work is a foundation for understanding one ofthe principle burning mechanism of propellants used for this ammunition. Pressure oscillations in the holesof perforated propellant grains are able to increase the burning rate at low temperatures (over)compensating the reduced burning rate of the energetic material at low temperatures.


Technical Test Center for Weapons and Ammunition – WTD 91 - Meppen

Oscillations in Grains of Gun Propellants · Accelerated Burning of Gun Propellants Caused by ... Q...

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