Effect of Fuel Composition on High Frequency Oscillations in Rocket Motors Burning Premixed Hydrocarbon Gases and Air

Abstract

The experiments reported herein were concerned with the determination of some of the factors which contribute to the occurrence of transverse and longitudinal modes of combustion oscillation in rocket motors burning gases. Three rocket motors having different combustion chamber geometries were employed, using gaseous hydrocarbon fuels premixed with air as propellants; the fuels were methane, ethane, superheated propane, ethylene and hydrogen. The findings indicated that the incidence and amplitude of the combustion pressure oscillations were profoundly influenced by the coupled effects of the chemical composition of the fuels and the aspect ratio of the cylindrical combustion chamber. The results also indicated that the following measures of the behavior of combustion oscillations were functions of the chemical differences of the fuels: 1) the relative ability of the different gaseous propellant combinations to support combustion oscillations; 2) the shape and origin of the instability region of the transverse modes, and the amplitudes of the oscillations within that region; 3) the combustion chamber length below which longitudinal oscillations will not occur (the lower critical length) , * 4) the tendency for interactions between transverse and longitudinal modes to occur; and 5) the mode of oscillation.

Importance of Combustion Chamber Geometry in High Frequency Oscillations in Rocket Motors

“…6 is approximately the same for the 74m diameter motor as well as for the methane-air propellant combination in both motors. Although the instability results presented are for the transverse modes of oscillation, steadily propagating longitudinal modes w&re also observed in the 74m diameter motor at chamber lengths of 8 in, (L/D -L14) 7 and above; they were observed in. the 144m diameter motor at chamber lengths of .12 in.…”

“…The increased temperature and pressure of the gases cause an accelerated chemical reaction rate (or heat release rate) behind the pressure front which drives and sustains the pressure wave. The amount of energy supplied to the pressure wave depends somewhat upon the chemical reactants (7); in this case the ethylene-air combination supplies more than, methane-air. As has been stated, that mechanism was employed (5) to explain the existence of the lower critical length and to postulate the probable existence (1) of the upper critical length, The lower critical length of the longitudinal mode was deiiimi in (5) as the combustion chamber length below which a longitudinal mode cannot exist.…”

Combustion stability in an annular gas combustor.

“…3 and Ref. 9) it is clear that as the activation energy decreases, the toe of the instability region occurs at a lower combustion pressure and the equivalence ratio limits broaden. It is evident in Fig.…”