An experimental study on thermoacoustic instabilities of non-premixed combustion using internal flue gas recirculation

Internal flue gas recirculation (IFGR) is a commonly used low-NOx combustion technique in industrial boilers. This study experimentally investigated the nonlinear behavior of the self-excited thermoacoustic instability in non-premixed combustion using IFGR. CH* chemiluminescence imaging and Proper orthogonal decomposition (POD) analysis were conducted to study the characteristics of flame dynamics. The results showed that, as global equivalence ratio Wg decreased, the combustion system sequentially exhibited combustion noise, 90 Hz limit cycle oscillation, dual-frequency transition, and 80 Hz limit cycle oscillation, accompanied by frequency shift and mode transition phenomena. For the 90 Hz and 80 Hz oscillation modes, the first mode of POD exhibited axial oscillation patterns and global flashing oscillation patterns, respectively. Finally, combustion chamber temperature measurement and low-order thermoacoustic network model was used to analyze the mechanisms behind mode transition and frequency shift phenomena. The results showed that the decrease in combustion chamber temperature, which was caused by Wg decrease, led to a reduction in the frequency of the first natural acoustic mode, ultimately resulting in the mode transition. IFGR altered the range of Wg where thermoacoustic oscillations occur. The phenomenon of frequency shift was primarily caused by the decrease in convective time delay tau conv. This study further explores the influence of IFGR and Wg on thermoacoustic oscillation modes and flame dynamics, which contributes to the development of stable, low-NOx non-premixed combustors.