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  • Assessment of the stabilization mechanisms of turbulent lifted jet flames at elevated pressure using combined 2-D diagnostics

    Guiberti, Thibault; Boyette, Wesley; Krishna, Yedhu; Roberts, William L.; Masri, Assaad R.; Magnotti, Gaetano (Combustion and Flame, Elsevier BV, 2020-01-22) [Article]
    The stabilization mechanisms of turbulent lifted jet flames in a co-flow have been investigated at a pressure of 7 bar. The structure of the flame base was measured with combined OH and CH2O planar laser induced fluorescence (PLIF) and the spatial distribution of equivalence ratio was imaged, simultaneously, with CH4 Raman scattering. The velocity field was also measured with particle imaging velocimetry (PIV). Different bulk jet velocities Uj and co-flow velocities Uc were examined. Data show that flames with Uc = 0.6 m/s stabilize much further away from the nozzle than those with Uc = 0.3 m/s and that their structure does not resemble that of the edge-flames found closer to the nozzle. In addition, for Uc = 0.6 m/s, the measured lift-off height decreases with increasing bulk jet velocity, which is opposite to what is typically observed for lifted flames. Statistical examination of CH4 Raman images shows that the flames with Uc = 0.6 m/s propagate through regions of the flow where the equivalence ratio is not always stoichiometric but, instead, spans the whole flammability range. This is not consistent with edge-flames and is, instead, indicative of premixed burning. This is corroborated by PIV results which show that the flame base velocity exceeds that typically reported for edge-flames. Measurements of relevant flow properties were also conducted in non-reacting jets to predict the turbulent burning velocity of these lifted flames burning in a premixed mode. For Uc = 0.6 m/s and relatively large bulk jet velocities (Uj = 10 and 15 m/s), the predicted turbulent burning velocities are sufficiently high to counter the incoming flow of reactants and, in turn, allow flame stabilization. However, for a lower bulk jet velocity of Uj = 5 m/s, the predicted turbulent burning velocity is much less, leading to blow-out. This explains why the lift-off height decreases with increasing jet velocity for methane at 7 bar and Uc = 0.6 m/s. Data also shows that increasing pressure promotes transition from edge-flames to premixed flames due to reduced laminar burning velocity and enhanced mixing.
  • Phylogeographical patterns and a cryptic species provide new insights into Western Indian Ocean giant clams phylogenetic relationships and colonization history

    Fauvelot, Cécile; Zuccon, Dario; Borsa, Philippe; Grulois, Daphné; Magalon, Hélène; Riquet, Florentine; Andréfouët, Serge; Berumen, Michael L.; Sinclair-Taylor, Tane H.; Gélin, Pauline; Behivoke, Faustinato; Poorten, Jan Johan; Strong, Ellen E.; Bouchet, Philippe (Journal of Biogeography, Wiley, 2020-01-21) [Article]
    The unique biodiversity in the Red Sea is the result of complex ecological and evolutionary processes driven by Pleistocene climatic change. Here we investigate the species diversity, phylogenetic relationships and phylogeographical patterns of giant clams in the Western Indian Ocean (WIO) and the Red Sea to explore scenarios of marine speciation in this under-studied region.
  • Enhanced Quality of Wafer-Scale MoS 2 Films by a Capping Layer Annealing Process

    Xu, Xiangming; Zhang, Chenhui; Hota, Mrinal Kanti; Liu, Zhixiong; Zhang, Xixiang; Alshareef, Husam N. (Advanced Functional Materials, Wiley, 2020-01-21) [Article]
    Wafer-scale, single-crystalline 2D semiconductors without grain boundaries and defects are needed for developing reliable next-generation integrated 2D electronics. Unfortunately, few literature reports exist on the growth of 2D semiconductors with single-crystalline structure at the wafer scale. It is shown that direct sulfurization of as-deposited epitaxial MoO2 films (especially, with thicknesses more than ≈5 nm) produces textured MoS2 films. This texture is inherited from the high density of defects present in the as-prepared epitaxial MoO2 film. In order to eliminate the texture of the converted MoS2 films, a new capping layer annealing process (CLAP) is introduced to improve the crystalline quality of as-deposited MoO2 films and minimize its defects. It is demonstrated that sulfurization of the CLAP-treated MoO2 films leads to the formation of single-crystalline MoS2 films, instead of textured films. It is shown that the single-crystalline MoS2 films exhibit field-effect mobility of 6.3 cm2 V−1 s−1, which is 15 times higher than that of textured MoS2. These results can be attributed to the smaller concentration of defects in the single-crystalline films.
  • Hierarchical algorithms on hierarchical architectures

    Keyes, David E.; Ltaief, Hatem; Turkiyyah, G. (Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, The Royal Society, 2020-01-20) [Article]
    A traditional goal of algorithmic optimality, squeezing out flops, has been superseded by evolution in architecture. Flops no longer serve as a reasonable proxy for all aspects of complexity. Instead, algorithms must now squeeze memory, data transfers, and synchronizations, while extra flops on locally cached data represent only small costs in time and energy. Hierarchically low-rank matrices realize a rarely achieved combination of optimal storage complexity and high-computational intensity for a wide class of formally dense linear operators that arise in applications for which exascale computers are being constructed. They may be regarded as algebraic generalizations of the fast multipole method. Methods based on these hierarchical data structures and their simpler cousins, tile low-rank matrices, are well proportioned for early exascale computer architectures, which are provisioned for high processing power relative to memory capacity and memory bandwidth. They are ushering in a renaissance of computational linear algebra. A challenge is that emerging hardware architecture possesses hierarchies of its own that do not generally align with those of the algorithm. We describe modules of a software toolkit, hierarchical computations on manycore architectures, that illustrate these features and are intended as building blocks of applications, such as matrix-free higher-order methods in optimization and large-scale spatial statistics. Some modules of this open-source project have been adopted in the software libraries of major vendors. This article is part of a discussion meeting issue ‘Numerical algorithms for high-performance computational science’.
  • Managing grains and interfaces via ligand anchoring enables 22.3%-efficiency inverted perovskite solar cells

    Zheng, Xiaopeng; Hou, Yi; Bao, Chunxiong; Yin, Jun; Yuan, Fanglong; Huang, Ziru; Song, Kepeng; Liu, Jiakai; Troughton, Joel; Gasparini, Nicola; Zhou, Chun; Lin, Yuanbao; Xue, Ding-Jiang; Chen, Bin; Johnston, Andrew K.; Wei, Nini; Hedhili, Mohamed N.; Wei, Mingyang; Alsalloum, Abdullah Yousef; Maity, Partha; Turedi, Bekir; Yang, Chen; Baran, Derya; Anthopoulos, Thomas D.; Han, Yu; Lu, Zheng-Hong; Mohammed, Omar F.; Gao, Feng; Sargent, E.; Bakr, Osman (Nature Energy, Springer Science and Business Media LLC, 2020-01-20) [Article]
    Inverted perovskite solar cells have attracted increasing attention because they have achieved long operating lifetimes. However, they have exhibited significantly inferior power conversion efficiencies compared to regular perovskite solar cells. Here we reduce this efficiency gap using a trace amount of surface-anchoring alkylamine ligands (AALs) with different chain lengths as grain and interface modifiers. We show that long-chain AALs added to the precursor solution suppress nonradiative carrier recombination and improve the optoelectronic properties of mixed-cation mixed-halide perovskite films. The resulting AALs surface-modified films exhibit a prominent (100) orientation and lower trap-state density as well as enhanced carrier mobilities and diffusion lengths. These translate into a certified stabilized power conversion efficiency of 22.3% (23.0% power conversion efficiency for lab-measured champion devices). The devices operate for over 1,000h at the maximum power point under simulated AM1.5 illumination, without loss of efficiency.

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