I. Introduction ombustion or thermos-acoustic instability constitute a major problem in many fields of application from aerospace propulsion systems and gas turbine engines operating in the premixed mode to boilers and radiant heaters. These unsteady behaviors can lead to structural damaging, hardware melting, high noise, flame flashback or blowoff, enhanced heat transfer to liners, and overall systems failure. Combustion instability is still one of the highly active research areas, due to the need to understand the physical processes responsible of these instabilities. Considerable research and development efforts have been invested during the past half-century to elucidate the processes responsible for the excitation of these instabilities and the development of approaches to prevent them, especially for gas turbine combustors and afterburners.

On 1956 at NASA Jet Propulsion Laboratory, Roger and Marable conducted an experiment on combustion rectangular duct to investigate the mechanism drives thermosacoustic instabilities in ramjet combustors and afterburners, in particular the high frequency oscillation [ref. 1]. The goal of the present work was to preform Computational Aeroacoustics “CAA” analysis via COMSOL® Multiphysics software to predict the frequencies, mode shapes, and behavior of the excited instabilities modes inside the duct, which were measured in the experiment.