On the self-excited instabilities of premixed swirl flames near blow-off limits — An experimental study using simultaneous measurements of thermal boundary conditions and core flow scalar fields

Spatially- and temporally-resolved measurements of the thermal boundary conditions and core flow scalar fields were conducted on premixed swirl flames of CH${}_4$-air near the lean and rich blow-off limits in a custom-built optical model combustor. This model combustor featured a unique and minimum-intrusion design of thermocouple probe matrices for measuring surface temperatures and heat fluxes, while allowing access to optical diagnostics for simultaneous OH-PLIF and OH*/CH* chemiluminescence imaging. A new data analysis scheme was developed that enabled automatic harmonic extraction and phase averaging for analyzing the time-varying scalar fields in swirl flames. Based on these methods, a parametric study of self-excited instabilities was performed at CH${}_4$ flow rates of 1.00–4.00 SLPM and equivalence ratios of 0.75–1.40, and a regime diagram of the observed instability modes was presented. Near the lean and rich blow-off limits, the premixed swirl flames exhibited quasi-periodic oscillations in the form of flame lift-off, partial quenching, re-ignition, and re-attachment. Example case studies under four representative conditions revealed that the detailed dynamics of flame oscillations, for example the flame re-ignition position and propagation pathway, strongly correlated with the thermal boundary conditions of the sidewall. The diagnostic method, data analysis schemes, and experimental data set presented in the current study are useful to the development and validation of swirl flame instability theories and CFD models.

Novelty and significance

This study presented, to the authors’ knowledge, the first simultaneous measurement of thermal boundary conditions and core flow scalar fields on the self-excited instabilities of premixed CH${}_4$-air swirl flames near blow-off limits. The experiment was conducted in a custom-designed model combustor that featured a unique, minimum-intrusion design of thermocouple probe matrices for measuring surface temperatures and heat fluxes, while simultaneously allowing access to optical diagnostics for OH-PLIF and OH*/CH* chemiluminescence imaging. At the near-limit conditions, large-amplitude oscillations were observed in the premixed swirl flames, the detailed dynamics of which were seen to correlate strongly with the sidewall thermal boundary conditions. A regime diagram of instabilities was obtained. The novel diagnostic and data analysis methods, as well as the experimental data set presented in this study, promise to be useful in advancing the swirl flame instability theories and validating modern CFD models.