Acoustic boundary conditions for can-annular combustors

Authors

James Brind  - Whittle Laboratory, University of Cambridge, United Kingdom

Abstract

This paper derives and validates an analytical model for acoustic boundary conditions on a can-annular gas turbine combustion system, composed of discrete cans connected to an open annulus upstream of a turbine. The analytical model takes one empirical parameter: an impedance between adjacent cans. Extracting this impedance from time-marching computations of a representative geometry indicates a positive acoustic resistance. The simulations show acoustic resistance is a weak function of can geometry and combustor--turbine gap length. With a calibrated value of acoustic resistance, the analytical model reproduces can-to-can transfer functions predicted by full-annulus computations to within 0.025 magnitude at compact frequencies. Both the model and computations exhibit a minimum in total reflected energy at non-zero gap length, dropping by 60% compared to the datum gap length. A parametric study yields a design rule for gap length at the minimum reflected energy, allowing a designer to maximise transmission from the combustion system.