Effects of Volume and Neck Length of Helmholtz Resonator on Thermo-acoustic Instability

Abstract

Thermo-acoustic instabilities are caused due to the in-phase combination of pressure fluctuation and uneven heat release. These instabilities are often observed in acoustically closed spaces like Jet Engines, Gas Turbine, Afterburner, etc. and have a destructive effect on the structure, combustion efficiency and heat transfer rate. To avoid these there is a need to suppress these instabilities. This work presents a way of controlling the instabilities with the help of passive control technique using Helmholtz resonator. The dimensional parameters of the Helmholtz resonator like neck length, neck diameter and cavity volume have significant effect on damping of the instability. This study focuses on assessing effects of neck length and cavity volume on damping. An equivalent system of 1-D combustor is created using Rijke Tube model of L/D ratio as 10. For the fixed equivalence ratio and burner position, second mode of instability was observed. For second mode, based on the position of anti-node, the location of the Helmholtz resonator was decided and fixed on Rijke tube. The results show Helmholtz resonator performance depends on neck length and cavity volume. Thus, the neck length and volume of the resonator are critical parts while designing Helmholtz resonator. With right combination of neck length and volume of the resonator, maximum suppression was achieved up to 23 dB.