A 3 microphone method to separate direct and in-direct combustion noise inside a turbofan jet engine annular combustor

Greater fuel efficiency is a key area in the development of a new generation of turbofan engines. It is currently unclear as to how modifications to the combustor to achieve higher levels of efficiency will affect the combustion noise that reaches the far field. To understand this, it is necessary to be able to identify the contribution of both the direct and the indirect combustion noise to the far field level. The contribution of these two combustion noise sources is unknown for the current generation of turbofan engines. This paper presents a 3 microphone method for the separation of the direct and indirect combustion noise signals inside the combustor of a turbofan jet engine. The effects of transfer function error and coherence of unwanted noise on the performance of the technique are examined. This technique is then used as an input to the current combustion noise measurement method, called ‘3S Array’, to examine the effect of transfer function error on the coherence with the far-field. It has been found that the separation is only weakly dependent on transfer function error and is therefore a reasonable technique to apply to real engine data. All of the data presented in this paper uses models of the problem rather than measurements.