Pressure Oscillations in Regenerative Liquid Propellant Guns

Abstract

The present paper deals with pressure oscillations in regenerative liquid propellant guns (RLPG) presenting the state‐of‐the‐art research into sources and control of these combustion instabilities. Pressure oscillations with amplitudes up to 50 % of mean pressure and frequencies up to 60 kHz or more are present in experimental data of RLPGs, especially at medium and large calibers. Amplitudes increase with the volumetric energy density of the liquid propellant and the mass flow rate during injection. Frequency analyses reveal that both acoustic modes and combustion noise are components of the recorded oscillations. Acoustic modes, in particular, have the potential to couple to resonant modes in near‐field mechanical structures. A multi‐phase, multi‐dimensional model investigation at ARL indicates that pressure waves reflected from internal boundaries are amplified as they pass the combustion zone of the highly pressure‐sensitive liquid propellant in a localized region near the injector. Experimental data lead to a similar understanding of the amplification of pressure oscillations in RLPGs. Experiments in the United States and in Germany confirm that the pressure oscillations can be mitigated by altering the combustion characteristics of the liquid propellant. Also, techniques that more effectively disperse the liquid propellant jet and thereby decrease the local accumulation of liquid propellant, may reduce pressure oscillations as shown in 30‐mm RLPG experiments. Further, energy‐absorb‐ing chamber walls or liners can serve as broad‐band filters and have been shown experimentally to be effective in reducing the amplitude of all frequencies. In addition, physical methods such as cavities and baffles reduce significantly specific acoustic frequencies of the oscillations.

Regenerative Liquid Propellant Gun of Caliber 40 mm

“…The initial variation in piston travel between 3 ms to 4.5 ms is attributed to a mass-spring reaction which can be eliminated by improving the initial ignitiona combustion reaction (6) . The potential of the lumped-parameter model is demonstrated in Fig.…”

“…However, reduction of pressure oscillations is possible by various ways. Approaches include (6) (a) mechanical devices to affect the break-up of the liquid jet, (b) chemical methods to increase the burning rate, and (c) dissipative mechanisms to absorb the pressure waves. For example, jet dispersers can dramatically lower the magnitude of the ®rst-radial mode, the dominant acoustic mode in RLPG pressure oscillations, and liners in the combustion chamber (see Fig.…”

“…As indicated in Fig. 1, a liner can be placed at the chamber wall representing one of the dissipative approaches that have shown promise in reducing pressure oscillations (6) .…”

“…The historical and extensive treatments of liquid gun propellant research and development programs which spanned over the past ®ve decades are given in two AIAA Progress Series books (1,2) . While recent publications have dealt with the fundamentals (3,4) and addressed speci®cs, such as electrical ignition (5) and pressure oscillations (6,7) , here we describe investigations carried out with a 40-mm regenerative liquid propellant gun at the Ernst-Mach-Institut (8±10) . In particular, the combination of gun ®rings at EMI (9) and the numerical lumped-parameter simulations of the ®ring data by WTD 91 (10) are described in greater detail.…”

Study on Time‐frequency Characteristics of Pressure Oscillations in a Bulk‐loaded Liquid Gun

“…The burning of droplets is dominant in a RLPG [6] or liquid propellant rocket [7], and the liquid propellant needs to be atomized to droplets firstly. G. P. Wren [8] found that the pressure oscillation in a RLPG was related to acoustic modes and combustion noises, and that the oscillation can be mitigated by altering the combustion characteristics of liquid propellant or by decreasing the local accumulation of liquid propellant. T. P. Coffee [9] studied the effects of the breakup and combustion of propellant jet on the high-frequency pressure oscillations by the numerical simulation for the interior ballistic cycle of a RLPG.…”