Acoustic instability while combusting aluminum particles in MHD facilities

Abstract

In this paper, the theoretical study of acoustic-vibration excitation while combusting aluminum particles in a gas mixture is carried out. At long distances, aluminum particles are considered to be monodisperse and immobile with respect to the gas. In determining the excitation conditions for acoustic vibrations, the gas mixture used was considered as a perfect gas. Expressions for the frequency and excitation increment of acoustic vibrations are obtained, characteristics of a fuel and of an oxidizer explicitly entering into these expressions. In [1], powders prepared on the basis of carbon and metals were suggested as a fuel for pulsed MHD generators. Air or oxygen are oxidizers in this case. Characteristics of MHD generators while operating with these fuels, in fact, do not differ from those obtained using solid rocket propellants. In this case, the powder-like fuel presents the possibility for profound control of the device power when changing the mass flow-rate of components and provides for long-term operation of powerful MHD generators. It was shown in [2] that aluminum fuel is optimal for pulsed MHD generators, provided that the air oxidizer is employed. It is known [3] that in the process of burning a disperse fuel the acoustic vibrations can be excited, which results in violation of the normal operation of combustion chambers and even in their destruction. In the practice of rocket-engine development [4], engines having pressure-vibration amplitudes of not more than 5% from the nominal value are related to those exhibiting stable combustion. It was shown in [5] that the requirements for stability of the combustion process in the MHD-generator combustion chamber should be more rigorous than in rocket engines. This is caused by the fact that for such pressure vibrations electric-power pulsation phenomena hinder the normal operation of an MHD generator. Therefore, consideration of acousticvibration excitations in the case of combusting a powder-like fuel in the combustion camber of an MHD generator is an urgent problem. Usually, in studies of acoustic-vibration excitation, it is suggested that, in the case of combusting a disperse fuel, the burning zone is substantially smaller than the length of the combustion chamber [3, 4]. In actual conditions, the burning zone has a certain extension along the longitudinal axis of the combustion chamber, which is comparable with its length [6]. Therefore, in the case of acoustic vibrations of pressure, disturbances of the velocity and temperature of the gas mixture along the length of the combustion chamber are different, which affects the stability of the combustion process [3, 4]. In this paper, in studies of the excitation of acoustic vibrations, a distributed combustion model is used. In other words, we assume that the zone of burning aluminum particles is approximately equal to the length of the combustion chamber.