The effects of ring-shaped porous inert media on equivalence ratio oscillations in a self-excited thermoacoustic instability

Gas turbine operation increasingly relies on lean premixed (LPM) combustion to reduce harmful emissions, which is susceptible to thermoacoustic instabilities. Most combustion systems are technically premixed and exhibit a degree of equivalence ratio inhomogeneity. Thermoacoustic pressure oscillations can couple with the heat release oscillations through the generation of equivalence ratio fluctuations at fuel injection sites, which are then convected to the flame front. Previous experimental studies have shown that porous inert media (PIM) can passively mitigate these instabilities by adding acoustic damping and by reducing the thermoacoustic feedback mechanism. To understand the role of PIM on these equivalence ratio oscillations, spatially resolved, phased averaged equivalence ratio fluctuations are measured using the ratio of OH*/CH* chemiluminescence. Spatial imaging of OH* or CH* radicals produce integrated line of sight intensity values and an Abel transformation is used to obtain spatially resolved values. Phase averaged images are synced with dynamic pressure measurements, and an axisymmetric atmospheric burner is used to study the effects of ring-shaped PIM on the spatially resolved equivalence ratio field with self-excited thermoacoustic instabilities. The results show that PIM significantly reduces these fluctuations, and the effects on the stability of the system are discussed.

Swirler Effects on Passive Control of Combustion Noise and Instability in a Swirl-Stabilized Combustor

High strength porous inert media (PIM) placed in the reaction zone of a swirl-stabilized lean-premixed combustor is a passive method of controlling combustion noise and instabilities. In this study, the effect of swirler location and swirl number on combustion without and with PIM has been investigated experimentally, using a methane-fueled quartz combustor at atmospheric pressure. Three axial swirlers were designed with eight vanes, a solid centerbody, and vane angles of 30, 45, and 55 deg to yield calculated swirl numbers of 0.45, 0.78, and 1.10, respectively. Swirler location was varied to obtain recess depth in the premixer tube of 0.0 cm, 2.5 cm, and 5.0 cm. A downstream bluff body was used with the recessed swirlers to stabilize the flame at the dump plane. Experiments were conducted at constant air flow rate of 300 SLPM and equivalence ratios of 0.70, 0.75, and 0.80. PIM annular rings with increasing and decreasing cross-sectional area in the flow direction were tested, referred to as diverging and converging PIM. The performance of each test case is compared by observing the flame behavior and measuring sound pressure level (SPL) with a microphone probe. Results include total SPL and SPL in one-third octave bands. PIM proved effective in mitigating combustion noise and instability for all flush-mounted swirlers with total SPL reductions of up to 7.6 dBA. The effectiveness of the PIM generally improved with increasing equivalence ratio. Combustion instability that occurred within the frequency band centered about 630 Hz was suppressed with both PIM configurations. These results confirm that PIM is an effective method to control combustion noise and instabilities in swirl-stabilized LPM combustion.