Experimental investigation on the thermoacoustic instability of boron/ethanol nanofluid fuel spray swirling flames

• Highlight 1: Our study concerns the instability of boron/ethanol nanofluid fuel spray swirling flames, which is highly relevant to practical applications. • Highlight 2: Our investigation focuses on the mechanism of thermoacoustic oscillations that drive or dampen the nanofluid fuel propulsion system, a phenomenon that is rarely explored. • Highlight 3: Our research provides evidence that the downstream boron combustion suppresses the thermoacoustic instability of the upstream ethanol flame. Nanofluid fuel has attracted the interest of researchers for decades due to its prominent combustion and propulsive properties. However, combustion instability is inevitable in propulsive systems operating with nanofluid fuel, and little is known about how thermoacoustic oscillations drive or dampen the system. A confined boron/ethanol (B/EtOH) nanofluid fuel spray swirling flame stabilized by an axisymmetric bluff body has been experimentally investigated in this work. The flame response to the varying B nanoparticles (NPs) doping concentrations was recorded and compared. 2D images and critical data were acquired using a 10 kHz repetition-rate OH* and BO 2 * chemiluminescence (CL) system, a photomultiplier tube, and pressure transducers. Meanwhile, several analysis methods, including flame visualization, Fourier/Hilbert transforms, spectrograms, proper orthogonal decomposition (POD), extended POD (EPOD), and Rayleigh's criterion analysis, are utilized to help us understand the underlying mechanism. From the averaged images, the high-intensity region of the BO 2 *-CL appears further downstream than that of OH*-CL, resulting in a broader heat-release distribution of B/EtOH nanofluid fuel spray flames than neat EtOH ones. As the B NPs doping concentration increases, the oscillation frequency of the B/EtOH spray swirling flames remains almost unchanged, but the oscillation amplitude gradually decreases. Numerous B particles and agglomerates exhibit intense combustion when the micro-explosion phenomenon occurs. Additionally, the time-resolved pressure and heat release oscillations are out of phase and are supposed to be associated with acoustic energy dissipation. The POD and EPOD analyses reveal that the primary flame oscillations are driven by the longitudinal flame-shedding motion and the entrained reaction pockets. Meanwhile, the OH* and BO 2 * radicals exhibit different local dynamics responses to the oscillation. Based on the Rayleigh index distribution coupling the fluctuation of pressure and heat release from EtOH and B, our study provides evidence that the combustion of B NPs downstream suppresses the thermoacoustic instability of the EtOH flames.