The Use of Helmholtz Resonators in a Practical Combustor

Abstract

The drive to reduce emissions has led to the development of lean premixed combustors. However, lean premixed combustion is often associated with combustion oscillations which can be so severe that they can cause structural damage to the engine. Since the associated frequencies are typically of the order of hundreds of Hertz, there is a need for a compact device to absorb the noise which drives the oscillation. Helmholtz resonators are commonly used as absorbers of incident acoustic power. In addition they are relatively compact. However, their use in combustors creates practical issues, such as placement within the chamber, neck length, and cooling, which need to be addressed. In this paper we consider these practical problems and describe how to overcome them in a real combustor.

Acoustic Models for Cooled Helmholtz Resonators

“…In this case, Eqs. (10a, 10b) agree with models which assume a continuous stagnation enthalpy oscillation across the HR [4] (see discussion on this point in Appendix VI.A). However, if the neck temperature is much lower than that in the combustor, which is often the case due to the cooling flow through the neck, Eqs.…”

“…It is worth noting that the effect of hot-gas penetration from the combustor into the neck of the resonator at large oscillation amplitudes has been found to alter the sound absorption performance of the HR [13,14]. However, the effect of the HR-combustor temperature difference on the acoustic (and entropy) waves in the combustor is also highly relevant [10,15], and has not been considered in previous studies [4,10,16].…”

“…As we are modelling the general local acoustic and entropy wave relations across the HR, no combustor acoustic boundary conditions nor the HR axial location need to be prescribed, although it is assumed in this section that the HR is installed near a pressure anti-node to clearly show its effect on the combustor acoustics [2,4,22,23]. The entropy wave strength on the upstream side of the HR, E 1 , is assumed to be zero.…”

“…This model has been successfully used by many researchers [4,5,7]. In the presence of a mean bias flow, incident acoustic waves cause unsteady vortices to be shed at the edges of the neck apertures, which are swept away by the mean flow.…”

“…1 (left)), while linear models suffice in the presence of a mean neck flow (see Fig. 1 (middle)) [2][3][4][5].…”