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, and little is known about how nanofluid fuel drive or dampen thermoacoustic oscillations of 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 a pressure transducer. 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.
Yue Wang,Wei-Hua Cai,Tong-Zhou Wei,Hong-Na Zhang,Lu Wang,Feng-Chen Li
In this article, the experiments of two-oscillating grid turbulence with viscoelastic fluids were carried out using particle image velocimetry. Two classical drag-reducing additives with viscoelastic characteristics were chosen: polymer (polyacrylamide) and cationic surfactant (cetyltrimethyl ammonium chloride). In order to investigate the viscoelastic effect on coherent structures, proper orthogonal decomposition was performed to identify coherent structures based on particle image velocimetry data. The results show that the minimum number for eigenmodes required for capturing coherent structures, which contains 90% of total turbulent kinetic energy, is 127, 19, and 117 for the Newtonian fluid case, 25 ppm polyacrylamide solution case, and 25 ppm cetyltrimethyl ammonium chloride solution case at grid oscillating frequency f = 7.5 Hz, respectively. It means that coherent structures can be inhibited due to the addition of polyacrylamide additives but not remarkable in 25 ppm cetyltrimethyl ammonium chloride solution case, in other words, the decrease in flow complexity in 25 ppm polyacrylamide solution case. This phenomenon also appears at grid oscillating frequency f = 5 Hz. However, as cetyltrimethyl ammonium chloride solution concentration increases up to 50 ppm, the cetyltrimethyl ammonium chloride solution case shows similar trends as those in 50 ppm polyacrylamide solution case (the number for eigenmodes required for capturing coherent structures is approximate). Therefore, compared with a channel flow with cetyltrimethyl ammonium chloride solution, there exists the larger critical concentration in two-oscillating grid turbulence to show turbulence suppression effect.
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