High-frequency combustion oscillation

AP composite propellants without aluminum particles are termed reduced-smoke propellants and are employed in tactical missiles to conceal their launch site and flight trajectory. No visible smoke is formed when the relative humidity of the atmosphere is less than about 40%. However, since high-frequency combustion oscillation tends to occur in the combustion chamber in the absence of solid particles that serve to absorb the oscillatory energy, a mass fraction of 0.01-0.05 of metallic particles is still required for the reduced-smoke propellants. These particles and/or their oxide particles generate thin smoke trails. The white smoke trail includes the white fog generated by the HCl molecules and the condensed water vapor of the humid atmosphere. 

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 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. 

A monograph of Shchelkin Troshin (Ref 11) is based on studies at the Institute of Chemical Physics, Academy of Science, Moscow during the period 1952-1962, of theoretical analyses of detonation, deflagration, flame acceleration, nonsteady-state double discontinuities, and high-frequency oscillations in forced-combustion chambers. The book has been translated into English (See Ref 11a)... 

Penetration depth and wavelength increase with the thermal diffusivity of the body and the periodic time. High frequency oscillations, which appear in fast running combustion engines, have a much lower penetration depth in the cylinder... 

Premixed flames are waves of combustion (for example, see [PRU 13]), whose reference velocity (that of the adiabatic planar flame) can quite easily be calculated on the basis of the thermodynamic characteristics of the constituent parts, as long as we are deahng with flames with high activation energy. Here, we shall limit ourselves to the study of the effects of gravity and of a centrifugal acceleration field on such flames, and primarily on their res dting frequency of oscillation. 

The flammability and stability limits of Fig. 19.7 were obtained using fuel-air mixtures with the same equivalence ratio in the radial and tangential inlets, and without an axial jet. The lean flammability limit decreased from 0.57 to 0.4 as the swirl number was increased from 0.6 to 3.75, and the region of high-heat release moved closer to the swirler which represented an acoustic pressure antinode for the naturally occurring oscillations associated with a quarter wave in the entire duct, with frequency close to 200 Hz. Thus, swirl led to an increase in the amplitude of oscillations and to an earlier transition from smooth to rough combustion with antinodal RMS pressures up to 10 kPa, and initiated at an equivalence ratio of 0.5 for a swirl number of 3.75... 

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