Combustion Instability Test

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 

When two propellant grains with and without A1 particles are arranged along the axis of a motor, i. e., one grain without A1 particles is placed at the fore-end and the other grain is placed at the rear-end, the motor combustion instability occurs at about 0.3 s after ignition, as shown in Fig. 13.24 (a). The motor is broken by the rapid pressure increase caused by the instability and the propellant combustion is 

It is evident that the standing pressure wave in a rocket motor is suppressed by solid particles in the free volume of the combushon chamber. The effect of the pressure wave damping is dependent on the concentrahon of the solid parhcles, and the size of the parhcles is determined by the nature of the pressure wave, such as the frequency of the oscillation and the pressure level, as well as the properties of the combustion gases. Fig. 13.25 shows the results of combustion tests to determine the effechve mass fraction of A1 parhcles. When the propellant grain without A1 particles is burned, there is breakdown due to the combushon instability. When 

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Vaudrey, M. A., W. R. Saunders, and B. Eisenhower. 2000. A test-based methodology for a priori selection of gain/phase relationships in proportional phase shifting control of combustion instabilities. ASME Paper No. 2000-GT-0530. 

Combustion tests carried out for a rocket motor demonstrate a typical T combustion instability. Double-base propellants composed of NC-NG propellants with and without a catalyst (1 % nickel powder) were burned. Detailed chemical compositions of both propellants are given in Section 6.4.6 and the burning rate characteristics are shown in Fig. 6.29. The addition of nickel is seen to have no effect on burning rate and the pressure exponent is n = 0.70 for both propellants. 

The combustion tests conducted for a rocket motor show that the combustion becomes unstable below 1.7 MPa and that the burning acquires a chuffing mode in the case of the uncatalyzed propellant. However, as expected, the combustion is stable even below 0.5 MPa for the nickel-catalyzed NC-NG propellant, as shown in Fig. 13.13. Propellants for which the flame temperature decreases with decreasing pressure tend to exhibit T combustion instability. 

Substances, mixtures, or reaction masses are also evaluated and/or tested for compatibility with common process chemicals and contaminants (e.g., rust, water, air, heat transfer medium). Substances which are not energetic but show only decomposition and/or instability in the presence of oxygen (air), the so-called combustibles in Box 5 of Figure 2.3, can be included in the strategy shown on Figure 2.5. 

The stability study was verified over 12 months at -20°C, -i-20°C and +40°C, monitoring the contents of matrix elements (C, H, P, N), minor elements (Na, Cl, Fe, Mn) and trace elements (Hg, Cd, Pb, Se). Measurement methods were INAA, catharometry after combustion in He/02 followed by conversion over Cu/CuO and pressurised digestion followed by ETAAS, CVAAS or ICP-AES. No instability was detected for any of the elements tested, even at prolonged storage at -t-40 C. 

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