Inlet temperature driven supercritical bifurcation of combustion instabilities in a lean premixed prevaporized combustor

TL;DR

This study experimentally observes a supercritical bifurcation in combustion instabilities driven by inlet temperature in a lean premixed combustor, revealing a critical temperature threshold where instability amplitude suddenly decreases.

Contribution

It provides the first experimental evidence of a supercritical bifurcation in combustion instabilities triggered by inlet temperature in a pressurized LPP combustor, supported by a low order network model.

Findings

Instability amplitude drops suddenly at Ta=570 K

Phase difference exceeds 90 degrees above 570 K

Higher inlet temperature enhances fuel evaporation and stabilizes spray

Abstract

The present article reports experimental observation and analyses of a supercritical bifurcation of combustion instabilities triggered by the air inlet temperature (Ta). The studies are performed with a pressurised kerosene fuelled Lean Premixed Prevaporized (LPP) combustor operated under elevated temperature. Unlike some previous studies, starting from an unstable condition of the system, the amplitude of combustion instabilities suddenly decrease when Ta exceeds a critical value of Ta =570 K. When the temperature is lowered back the system returns to being unstable without featuring any hysteresis behaviour, as expected in case of a supercritical bifurcation. The unstable flames feature a periodic axial motion of lift-off and re-ignition, characterized as Helmholtz mode. The phase difference between chemiluminescence and pressure signals is found to increase with Ta, exceeding 90 degrees (out of phase) for temperatures higher than 570 K. A low order network framework is conducted, illustrating that when Ta is increased a stability shift of this mode is predicted at Ta near 570 K, in good agreement with the experimental observations. The impact of Ta on the spray characteristics is also examined, finding that higher Ta promotes fuel evaporation and reduces equivalence ratio fluctuation at the exit of the swirler.