Numerical Analysis of Probe Microphones Used for Thermoacoustic Measurements

Acoustic measurements within combustion chambers are expensive due to high thermal loads applied on the measurement devices at operating conditions. As a more feasible substitute, probe microphones can be used to lead acoustic waves from combustion chambers to externally mounted microphones. Since these probe microphones are purged by nitrogen at atmospheric temperature, high thermal loads are avoided. However, the acoustic signal measured by the probe microphone is altered compared to the signal within the combustion chamber. This change in the acoustic signal can be characterised by means of the acoustic transfer function of the probe microphone, which mainly depends on the probe microphone geometry and the combustion chamber temperature distribution. Both impacts are studied in the present paper. The main subject is to analyse the inuence of the combustion chamber temperature distribution on the acoustic transfer function of probe microphones. In addition, the eect of changes in the probe microphone diameter is discussed. For this scope, experiments at standard conditions and transient CFD simulations for dierent temperature distributions have been carried out.