Diagnostics of acoustic properties of the combustion zone by means of periodic changes in the nozzle throat area

Abstract

A mathematical model is proposed, which allows calculating the amplitude of forced pressure oscillations in the combustor of a liquid-propellant rocket engine, caused by periodic changes in the nozzle throat area, with allowance for acoustic properties of the combustion zone. Results calculated for some particular cases are given.The development of highly forced combustors is associated with ensuring stability of the operation process. As the combustion zone is capable of sustaining self-induced high-intensity pressure oscillations, which may lead to structural failure, it is necessary to increase the amount of validation tests and, sometimes, to avoid using the previously chosen scheme of mixing of the species. Self-induced oscillations are usually divided into low-frequency, intermediate, and high-frequency oscillations. Studying and finding means of suppression of high-frequency oscillations are the most difficult tasks. This phenomenon has been most comprehensively studied for liquid-propellant rocket engines. A large amount of experimental data has been accumulated (see the monographs [1-3]). The emergence of oscillations is dangerous not only because of higher thermal and mechanical loads on structural elements of the combustor, but also because of worse mixing, which reduces the combustion efficiency.A completed theory of this phenomenon has not yet been developed. The theoretical progress is mainly based on the phenomenological approach and involves an analysis of the delay between the moment when the species enter the combustor and the end of the combustion process. As the delay is sensitive to regime parameters of the combustor, there appears a feedback between the energy release in the combustion region and pressure oscillations. As there is no complete understanding of steady combustion of liquid propellants, there are no grounds for the development of a physical theory of vibrational combustion. It should be noted 1 Russian Research Center "Applied Chemistry," St. Petersburg 197198; office@cisp.spb.ru.that the attempts to transfer the laws of steady combustion to the processes of unsteady combustion may turn to be ineffective because a minor fraction of the total energy released in the combustion region is sufficient to initiate developed pressure oscillations in the case of low acoustic losses in the combustor. Though the available phenomenological theories [1] allowed a scientifically grounded approach to planning and analyzing experiments on vibrational combustion to be developed, the effect of physicochemical and kinetic properties of the liquid propellant on stability of the operation process has not been adequately studied. It is difficult to interpret the results of numerous available experiments because it is next to impossible to retain the regime parameters affecting the mixing processes unchanged when using another propellant. In addition, when some species of the propellant is changed, the physical parameters (evaporation heat, vapor elasticity, viscosity, and surface tension) also become...