PASSIVE CONTROL OF THERMO-ACOUSTIC INSTABILITY IN DIFFERENT LENGTH COMBUSTORS USING A HIGH-STRENGTH METALLIC POROUS INSERT

摘要

Although lean premixed combustion (LPM) is very clean and basically soot-free, it has one serious drawback, i.e., tendency to develop thermo-acoustic instabilities. These instabilities can be very violent, at the least causing unwanted noise and vibration, and in more serious cases, complete engine failure. Current research has shown methods to address such instabilities using passive and active mitigation techniques. In this study, thermo-acoustic instabilities in a swirl-stabilized LPM combustion system are mitigated using a high-strength metallic porous insert fabricated by a 3D additive manufacturing technique. Although the technique has been demonstrated in our previous studies, the present focus is to utilize a given porous insert geometry to mitigate thermo-acoustic instabilities in different length combustion chambers producing different resonant frequencies, to overcome the typical limitation of passive techniques. For each combustor length, experiments are conducted over a range of equivalence ratios and reactant flow rates. In all cases, porous insert was effective in significantly reducing the sound pressure level (SPL) at the frequency of the instability, with reductions of 20 dB and higher. Time-resolved particle image velocimetry (PIV) measurements are acquired to describe flow and turbulence fields in the combustor without and with porous insert. Proper orthogonal decomposition (POD) analysis is used to quantify the energy content of turbulent modes, and harmonic reconstruction is performed to illustrate the dramatic changes in the oscillatory flow field when the porous insert is used. The ability of the porous insert to adjust to different geometric and operating conditions of the combustor is a unique capability, inherent to its fundamental operating principle.