Farshchi - Associate Professor, Department of Aerospace Engineering, Sharif University of Technology
Fatehi - Msc Student, Department of Aerospace Engineering, Sharif University of Technology
Ramezani - Research Engineer, Air Defense System Research Center

A theoretical study analyzing three-dimensional combustion acoustic instabilities in liquid propellant rocket engines has been conducted. A linear theory base on Crocco’s time lag hypothesis is used. To apply this theory the engine is divided into two mean components, including the combustor and the converging part of the nozzle. The assumption of concentrated combustion zone is used and the governing perturbation equations describing the oscillation of the flow variables in the combustor are considered. To solve these equations appropriate boundary conditions at both ends of the combustor are required. Combustion zone boundary condition at one end and the nozzle admittance relation at other end are used. To obtain the nozzle admittance the three dimensional flow perturbation equations are solved in the converging part of the nozzle. This approach is capable of predicting acoustic stability behavior of combustor at a wide range of Mach numbers and frequencies. Also, this analysis enables the rocket engine designer to observe the effects of different parameters such as nozzle entrance Mach number, chamber geometry, nozzle geometry, gas property, etc. on the stability characteristics of an engine. In case of instability observation; one can predict the acoustic mode which causes the instability and achieve an optimum design before conducting any time and money consuming experimental tests.

Combustion instability – Liquid propellant – High frequency