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Three-Dimensional (3D) swirling flow structures, generated by a counter-rotating dual-stage swirler in a confined chamber with a confinement ratio of 1.53, were experimentally investigated at Re = 2.3 × 105using Tomographic Particle Image Velocimetry (Tomo-PIV) and planar Particle Image Velocimetry (PIV). Based on the analysis of the 3D time-averaged swirling flow structures and 3D Proper Orthogonal Decomposition (POD) of the Tomo-PIV data, typical coherent flow structures, including the Corner Recirculation Zone (CRZ), Central Recirculation Zone (CTRZ), and Lip Recirculation Zone (LRZ), were extracted. The counter-rotating dual-stage swirler with a Venturi flare generates the independence process of vortex breakdown from the main stage and pilot stage, leading to the formation of an LRZ and a smaller CTRZ near the nozzle outlet. The confinement squeezes the CRZ to the corner and causes a reverse rotation flow to limit the shape of the CTRZ. A large-scale flow structure caused by the main stage features an explosive breakup, flapping, and Precessing Vortex Core (PVC). The explosive breakup mode dominates the swirling flow structures owing to the expansion and construction of the main jet, whereas the flapping mode is related to the wake perturbation. Confinement limits the expansion of PVC and causes it to contract after the impacting area.

To study statistical characteristics of the random spray autoignition, aviation kerosene was injected transiently into non-vitiated air crossflow in a flow reactor with optical accesses. The operating conditions were relevant to gas turbine combustor: the air crossflow pressure and temperature were in the range of 1.4–1.7 MPa and 830–947 K, respectively, and the jet-to-crossflow momentum flux ratios were 20, 50 and 80. Statistical distributions of random ignition delay times with adequate convergence were estimated based on histograms. The dependences of the distributions on reactor pressure, temperature, and jet-to-crossflow momentum flux ratio were studied. The results show that the resulting distributions appear more concentrated with the increase of air temperature or jet-to-crossflow momentum flux ratio. And then the correlations for the mean and standard deviation of the ignition delay time sample data were developed based on the present results. Compared with the correlations of ignition delay time of homogeneous premixed gas-phase kerosene/air mixture reported in the literature, the results show a greater significance pressure dependence and lower temperature sensitivity of the ignition delay time of non-premixed kerosene spray.

Vortex/flame interaction is an important mechanism for unsteady combustion in a swirl combustion system. Technology of low emission stirred swirl (TeLESS), which is characterized with stratified swirl flow, has been developed in Beihang University to reduce NOXemission. However, large-scale flow structure would be induced in strong swirl flow. Experiments and computational fluid dynamics (CFD) simulation were carried out to investigate the unsteady flow feature and its mechanism in TeLESS combustor. Hotwire was firstly applied to testing the unsteady flow feature and a distinct mode with 2244Hz oscillation frequency occurred at the pilot swirl outlet. The flow mode amplitude decayed convectively. Large eddy simulation (LES) was then applied to predicting this flow mode and know about its mechanism. The deviation of mode prediction compared with hotwire test was 0.8%. The spiral isobaric structure in pilot flow passage indicates that precessing vortex core (PVC) existed. The velocity spectrum and phase lag analysis suggest that the periodic movement at the pilot outlet was dominated by precessing movement. Negative tangential momentum gradient reflects that the swirl flow was unstable. Another phenomenon was found out that the PVC movement was intermittently rotated along the symmetric axis.

In order to investigate the effect of extrusion on Mg-4Zn-1Y alloy, microstructure and mechanical properties were analyzed by optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), energy dispersive spectrum (EDS) and tensile testing. The results indicated that the microstructure was obviously refined by extrusion and dynamic recrystallization. The second phases were dynamic precipitated and distributed more dispersively through extrusion. W-Phases (Mg3Zn3Y2) were twisted and broken, while I-Phases (Mg3Zn6Y) were spheroidized by deformation. Twin bands were formed to achieve the large deformation and hinder the slip of dislocations effectively to improve tensile properties. The tensile strength and elongation of extruded Mg-4Zn-1Y alloy were 254.94 MPa and 17.9% respectively which were improved greatly compared with those of as-cast alloy. The strengthening mechanisms of the extruded alloy were mainly fine-grain strengthening and distortion strengthening.

The influences of Ca and Y additions on the microstructure and corrosion resistance of vacuum die-cast AZ91 alloys were investigated by optical microscope, electron scanning microscope, weight-loss test, and electrochemical corrosion experiment. The results indicate that the Ca or Ca and Y additions refined the microstructure and decreased the amount of Mg17Al12phase on grain boundaries in the alloys. Meanwhile, the addition of Ca and Y led to the formation of network Al2Ca phase and rod-like Al2Y phase, improved the corrosion resistance of AZ91 magnesium alloy. Compared with AZ91 alloy, the corrosion rate of AZ91–1.5Ca–1.0Y alloy was decreased to 16.2%, and its corrosion current density was dropped by one order of magnitude after immersion in 3.5 wt% NaCl solution for 24 h.

Air worthiness requirements of the aircraft engine emission bring new challenges to the combustor research and design. With the motivation to design high performance and clean combustor, computational fluid dynamics (CFD) is utilized as the powerful design approach. In this paper, Reynolds averaged Navier-Stokes (RANS) equations of reactive two-phase flow in an experimental low emission combustor is performed. The numerical approach uses an implicit compressible gas solver together with a Lagrangian liquid-phase tracking method and the extended coherent flamelet model for turbulence-combustion interaction. The NOxformation is modeled by the concept of post-processing, which resolves the NOxtransport equation with the assumption of frozen temperature distribution. Both turbulence-combustion interaction model and NOxformation model are firstly evaluated by the comparison of experimental data published in open literature of a lean direct injection (LDI) combustor. The test rig studied in this paper is called low emission stirred swirl (LESS) combustor, which is a two-stage model combustor, fueled with liquid kerosene (RP-3) and designed by Beihang University (BUAA). The main stage of LESS combustor employs the principle of lean prevaporized and premixed (LPP) concept to reduce pollutant, and the pilot stage depends on a diffusion flame for flame stabilization. Detailed numerical results including species distribution, turbulence performance and burning performance are qualitatively and quantitatively evaluated. Numerical prediction of NOxemission shows a good agreement with test data at both idle condition and full power condition of LESS combustor. Preliminary results of the flame structure are shown in this paper. The flame stabilization mechanism and NOxreduction effort are also discussed with in-depth analysis.