Aerodynamic Stability of Branched Diffusers

Abstract

A systematic analysis is made of the potential instability of branched diffuser systems such as are inherent to the annular combustor systems of gas turbine engines. The system is modeled to include diffusion in each branch with pressure recovery characteristics which are a simple function of the fraction of total flow into the branch. Also included in each branch is a volume (i.e., an accumulator) and a resistance representing the combustor shells before the two branched streams rejoin. Analysis of the system equations is carried out in two perspectives. First, the equations are linearized and simple generalized criteria for stability are derived. Then the full nonlinear equations are programmed for digital computation in a form where they can be integrated with respect to time, and the full dynamic behavior of flows and pressure described. The computation is carried out several times for system variations of shell resistance, accumulator volumes, and diffuser characteristics, so that general conclusions may be drawn of the effects of these parameters on the frequency, wave form, and amplitudes of the systems’ oscillations.