Probabilistic approach in studying the effects of the external constraints on the dynamic behavior of an aircraft combustor

This paper presents a probabilistic approach to characterize the influence of external constraints on the modal behavior of an aircraft’s combustor. The needing to reduce pollutant emissions has led to the birth of a new conception of aeronautic combustors that achieve a lean burn at relatively low temperatures. Unfortunately, this environmental enhancement has created an engineering challenge: the lean burning combustors are prone to flame instability which can cause pressure pulsations large enough to trigger structural instability phenomena of the combustor thus seriously affecting flight safety. Hence the need to study the combustor’s structural dynamics and the interaction between elastic, thermal and acoustic phenomena. In this work the influence of external constraints on the normal mode frequencies of the combustor were analyzed in order to improve the level of correlation of numerical models results and experimental tests . The analyses were performed by finite elements (FE) approach while tests were carried out using experimental facilities available at AVIO Aerospace Propulsion laboratory. Finite elements models took in account the elasticity of the connection between the combustor and mounting flanges: each node of the mounting flanges was connected to combustor by means of three orthogonal springs defined in a proper cylindrical reference system. The springs’ stiffness was used as variable input for a probabilistic design simulation (PDS); a preliminary statistical analysis was at first performed via Monte-Carlo Simulation (MCS) method, obtained results were then optimized via Response Surface Method (RSM) in order to get a better correlation of most significant numerical frequencies with their experimental values