Effect of Acoustic Pressure Oscillations on Burning Rate Augmentation of Composite Solid Propellants at Different Initial Grain Temperatures

ABSTRACT

The effect of acoustic pressure oscillations, besides initial grain temperature on the burning rate of composite solid propellants (AP/HTPB/RDX/Al), is experimentally investigated using a T-burner. Mean burning rate is a by-product of the T-burner experiment while screening different propellants for its stability behavior by evaluating its combustion response. Experiments at different frequencies ranging from 200 to 400 Hz and at different pressures of 2–8 MPa and at different initial grain temperatures (room temperature (303 K), hot (343 K), and cold (243 K)) by soaking the propellant samples by a novel method for two industrial-grade propellants (propellant-A and propellant-B) were conducted. Many of the tests were self-excited for propellant-A, whereas propellant-B needed to be pulsed in most of the tests. Various levels of acoustic amplitudes were imposed up to 300 kPa, which is not otherwise possible to impose it externally using any other acoustic drivers, for example, a rotary valve (up to 20 kPa max. is possible). The test results revealed that the acoustic pressure oscillations augment the burning rate tremendously. It was found that the burning rate augmentation factor of propellant-B is lower than propellant-A in most of the cases. The investigations revealed the following new findings: (a) Increasing the initial grain temperature in the presence of acoustic pressure oscillations tends to decrease the augmentation; (b) Many of the results of the augmentation showed a rising and falling trend with acoustic amplitude; (c) The burning rate augmentation factor from self-excited and pulsed tests surprisingly form a continuous trend and shown together. The positive and negative augmentation compels us to further investigate toward a better understanding about the steady combustion behavior of solid propellants since there is no model available as on today to predict the burning rate augmentation precisely due to the combined effect of acoustic pressure oscillations and initial grain temperature.

KEYWORDS:

• Composite solid propellant
• T-burner
• negative augmentation
• burning rate
• grain temperature
• acoustic pressure oscillations

Nomenclature

e	=	exponential
k	=	acoustic mode number
m	=	mass flux
$p$	=	pressure
$\tilde{p}$	=	non-dimensional pressure
R	=	response function
r	=	burning rate
t	=	time
$\alpha$	=	thermal diffusivity
$\mathrm{\Omega }$	=	non-dimensional frequency
$\omega$	=	angular frequency
Subscripts	=
${\left(\right)}_k$	=	at the frequency mode
${\left(\right)}_p$	=	pressure coupled
${\left(\right)}_0$	=	at k = 0
${\left(\right)}_{tb}$	=	at T-burner
Superscripts	=
$\overline{\left(\right)}$	=	averaged quantity over time and space
$\widehat{\left(\right)}$	=	averaged quantity over time
${\left(\right)}^{\prime }$	=	fluctuating quantity
$\stackrel{.}{()}$	=	rate with respect to time
$\widetilde{\left(\right)}$	=	non-dimensional quantity

Acknowledgements

This work was funded by Ministry of Defence, Govt. of India. The authors wish to acknowledge Dr. B.S. Subhash Chandran for his strenuous support and tremendous knowledge he had shared with us to complete this work. Thanks are due to Mr. Karthik and Mr. Annamalai who had helped in assembling the test set up to conduct the experiments.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

The work was supported by the Ministry of Defence.