Combustion instability oscillatory

When an energetic material burns in a combustion chamber fitted with an exhaust nozzle for the combustion gas, oscillatory combustion occurs. The observed frequency of this oscillation varies widely from low frequencies below 10 Hz to high frequencies above 10 kHz. The frequency is dependent not only on the physical and chemical properties of the energetic material, but also on its size and shape. There have been numerous theoretical and experimental studies on the combustion instability of rocket motors. Experimental methods for measuring the nature of combustion instability have been developed and verified. However, the nature of combustion instability has not yet been fully understood because of the complex interactions between the combustion wave of propellant burning and the mode of acoustic waves. 

Combustion of a propellant in a rocket motor accompanied by high-frequency pressure oscillation is one of the most harmful phenomena in rocket motor operation. There have been numerous theoretical and experimental studies on the acoustic mode of oscillation, concerning both the medium-frequency range of 100 Hz-1 kHz and the high-frequency range of 1 kHz-30 kHz. The nature of oscillatory combustion instability is dependent on various physicochemical parameters, such... 

Fig. 13.21 shows another example of oscillatory burning of an RDX-AP composite propellant containing 0.40% A1 particles. The combustion pressure chosen for the burning was 4.5 MPa. The DC component trace indicates that the onset of the instability is 0.31 s after ignition, and that the instability lasts for 0.67 s. The pressure instability then suddenly ceases and the pressure returns to the designed pressure of 4.5 MPa. Close examination of the anomalous bandpass-filtered pressure traces reveals that the excited frequencies in the circular port are between 10 kHz and 30 kHz. The AC components below 10 kHz and above 30 kHz are not excited, as shown in Fig. 13.21. The frequency spectrum of the observed combustion instability is shown in Fig. 13.22. Here, the calculated frequency of the standing waves in the rocket motor is shown as a function of the inner diameter of the port and frequency. The sonic speed is assumed to be 1000 m s and I = 0.25 m. The most excited frequency is 25 kHz, followed by 18 kHz and 32 kHz. When the observed frequencies are compared with the calculated acoustic frequencies shown in Fig. 13.23, the dominant frequency is seen to be that of the first radial mode, with possible inclusion of the second and third tangential modes. The increased DC pressure between 0.31 s and 0.67 s is considered to be caused by a velocity-coupled oscillatory combustion. Such a velocity-coupled oscillation tends to induce erosive burning along the port surface. The maximum amplitude of the AC component pressure is 3.67 MPa between 20 kHz and 30 kHz. - ...

D.W. Rice, CombstnFlame 8(1), 21-8 (1964) CA 60, 14325 (1964) (Effect of compositional variables upon oscillatory combustion of solid rocket propellants) N ) R.W. Hart F.T. McClure, "Theory of Acoustic Instability in Solid Propellant Rocket Combustion , lOthSympCombstn (1965), pp 1047-65 N2) E.W. Price, "Experimental Solid Rocket Combustion Instability , Ibid, pp 1067-82 Qi) R.S. Levine, "Experimental Status of High Frequency Liquid Rocket Combustion Instability , Ibid, pp 1083-99 O2) L. Crocco,... 

Interactions between the flame and the surrounding wall (in a combustion chamber) could influence the contaminant production. This is examined by Dionisios Vlachos and his group at the University of Delaware (formerly at the University of Massachusetts at Amherst) using numerical bifurcation techniques (Chapter 26). For the first time, oscillatory instabilities have been found and control methodologies have been proposed to reduce flame temperatures and NO2 emissions. 

Hatami (1981) determined the dynamic behavior of the total combustion system theoretically for conditions with and without flame and presented an experimental procedure. Theoretical calculations were verified experimentally. It is found that the recirculation parameter, the gas chamber volume and the time lag influence the instability of the system resulting in variations in the oscillatory behavior of the pressure. 

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