Prediction of combustion noise for a swirling flame with low order network model

Abstract

Combustion noise has become an important noise source for gas turbines and aero engines. Efforts are needed in understanding mechanisms of combustion noise and developing prediction methods. In this work, direct combustion noise for a swirling premixed flame is predicted by low order model (LOM) and compared with experiments. In LOM, model combustor is simplified to a plenum, a swirler and a combustion chamber. Unsteady heat release rate is set to a constant value and pressure perturbation is calculated. Thermoacoustic transfer function defined by the ratio of pressure perturbation and unsteady heat release is calculated to describe noise spectrum distribution. Results show that peaks in thermoacoustic transfer function increase with the gain of downstream reflection coefficient while corresponding frequencies maintain unchanged. The phase of downstream reflection coefficient affects both peak frequency and amplitude. In experiments, unsteady heat release and pressure perturbation are measured. Thermoacoustic transfer function calculated by experimental results presents several peaks associated with thermoacoustic instability modes. With proper reflection coefficient, predicted thermoacoustic transfer function is compared with experiments and achieves reasonable agreement. Thermoacoustic transfer function provides a basic understanding for energy transfer from heat release to acoustic waves and could be used to predict combustion noise.