Experimental investigation of mode switching mechanisms and control strategies for combustion instabilities in a rich-lean gas water heater

This study experimentally investigated mode switching of combustion instabilities in a rich-lean gas water heater under varying thermal inputs (18 kW, 24 kW, 30 kW) and flue pipe lengths (1.3 m, 1.8 m, 2.3 m). Pressure measurements, high-speed CH* chemiluminescence imaging, Proper Orthogonal Decomposition (POD), and thermoacoustic simulations were employed. As the excess air coefficient decreases, the system abruptly switched from separated flames with a dominant frequency around 50 Hz and weak pressure amplitude to merged flames at 75–77 Hz with intense oscillations reaching nearly 500 Pa. The convective time delay, derived from the heat-release centroid of the POD modes, remains stable for separated flames, increased sharply when flames merge, and then gradually into the region of maximum growth rate, triggering self-sustained oscillations. Flue pipe length variation revealed that the high-frequency mode is a conventional thermoacoustic mode coupled to the combustor acoustics, whereas the low-frequency mode is consistent with an intrinsic thermoacoustic (ITA) mode insensitive to combustor geometry. Thermal power experiments at different thermal inputs confirm that merged flames are more prone to instability, although sufficiently high power could also drive separated flames into oscillation. To suppress the observed instability, a passive mitigation strategy was proposed: increasing the gap between the rich and lean flow passages to prevent flame merging. This simple geometric modification effectively eliminates intense thermoacoustic oscillations and provided a practical solution for stable operation.