Numerical simulation of combustion instabilities of double-base homogeneous propellants in rocket motors

A comprehensive numerical analysis has been conducted to study the interactions between acoustic oscillations and transient combustion responses of double-base homogeneous propellants in rocket motors. The formulation treats the complete conservation equations and accounts for finite-rate chemical kinetics in the gas phase and subsurface regions. Turbulence closure is achieved by means of a well-calibrated two-layer model taking into account the effect of propellant surface transport properties. One-dimensional calculations are conducted to obtain combined gas and condensed phase solution, which reveal several important features. First, steady and isobaric flow results are consistent with experimental data. Second, the nonsteady heat release in the flame zone presents a complicated mechanism for driving and suppressing acoustic oscillations. The model has been implemented to examine the detailed flow structures and heat-release mechanisms in various regions of the two-dimensional motor, including micro-scale motions near the propellant surface and macro-scale motions in the bulk of the chamber. Results indicate that strong interactions between exothermic reactions and acoustic waves occur in regions with steep temperature gradients due to the large activation energy of the associated chemical kinetics. The dynamic behavior of the luminous flame plays a decisive role in determining the motor stability characteristics. Distributed combustion response in the gas phase provides the energy for driving flow oscillations, and can be correctly treated as a combination of monopole and dipole sources based on acoustic theory. Response of the condensed phase to pressure oscillations plays an important role in changing the phases of temperature and heat-release fluctuations, resulting in opposite dipole sources in terms of Rayleigh's criterion. The oscillatory flow characteristics are significantly altered by the presence of turbulence, due to enhanced momentum and energy transport in the gas phase. The primary flame zone plays a more important role in determining motor stability characteristics in the turbulent flow region based on Rayleigh's criterion. In the condensed phase, large temperature fluctuations and deep penetration of the thermal wave take place in the downstream region, leading to a very large burning rate fluctuation. The turbulent reacting acoustic boundary layer on a solid propellant surface could be one of the primary mechanisms in velocity-coupled erosive burning.