Experimental investigation of low-temperature autoignition in turbulent premixed swirling flames

The role of low-temperature chemistry/ignition (LTC/LTI) in aero engine-like combustors that feature confined bluff-body and swirling flows inside has been studied in the present work. A series of CH${}_4$/DME/air mixtures were experimentally investigated at $\varphi =0.65$ with the volume fraction of DME in the fuel blend (${\alpha }_{\text{DME}}$) ranging from 0% to 100% to take LTI into account. To this end, OH-PLIF and CH${}_2$O-PLIF measurements combined with PIV and thermocouple methods are used to capture high-temperature and low-temperature flames (HTFs and LTFs) and the flow field and temperature in the outer recirculation zone (ORZ). It is found that adding DME into the lean CH${}_4$/air mixture has the potential to introduce LTF in the ORZ under certain conditions. There are three flame regimes in the bluff-body swirl burner in terms of DME enrichment (${\alpha }_{\text{DME}}$): (1) When ${\alpha }_{\text{DME}}<50\%$, neither of LTF and HTF exist in the ORZ, and only a V-shape HTF is observed between the inner shear layer (ISL) and inner recirculation zone (IRZ), denoted as regime I. (2) When $50\%\le {\alpha }_{\text{DME}}\le 70\%$, a stable LTF in the ORZ can co-exist with the above V-shape HTF, denoted as regime II. (3) When ${\alpha }_{\text{DME}}>70\%$, besides the V-shape HTF, the LTF in the ORZ and a new HTF front between the ORZ and outer shear layer (OSL) can exist intermittently, i.e., the LTF first occurs in the ORZ and then transitions to the new HTF between the ORZ and OSL, denoted as regime III. Furthermore, using an ignition Damköhler number ($D{a}_{\text{ig}}$), defined as the ratio of maximum fluid residence time and evaluated shortest first-stage or second-stage ignition delay time in the ORZ, is able to reasonably classify the flame regimes. These results open up the possibility of employing LTI to extend the stability margin of lean swirling flames.