Investigation on ignition behaviors of pulverized coal particles in a tubular swirl burner

This paper intensively investigated the ignition of turbulent coal flames in a novel fully-mixed tubular swirl burner. The Nikon D300s digital camera was used to capture the statistical ignition behavior of dispersed coal particle streams in different ambiences. Meanwhile, the combustion dynamics of individual coal particles were also recorded by means of high-speed photography. Two low-rank coal samples, Hulunbel lignite and Zhundong coal, were tested in this study. The ignition delay times of coal particles in the swirl burner were compared with those in a flat-flame burner. In contrast to previous work on a laminar flat-flame burner, the current experimental results show that the turbulent ambience significantly enhances the ignition of all coal samples, which is exceptionally pronounced under high temperature and low oxygen conditions. In addition, the sensitivity analysis suggests that both the enhanced heat and mass transfer contribute to the early ignition in turbulence. The effect of elevated mass transfer coefficient turns prominent in low oxygen fraction ambience, wherein the volatile barrier effect is suppressed by the enhanced mixing process. The combined effect of turbulence favors the shifting of ignition modes to the heterogeneous-dominant region. Last but not least, the ceased volatile flame that visualized in turbulent low oxygen ambience further confirms the important role of heterogeneous ignition.     

An easy-to-use estimate of the energy-release rate for crack arrays

Formation of crack arrays plays an increasing role in several fields of applied physics. The energy-release rate of the cracks controls the development of the array. Therefore, following the concept of configurational forces, a simplified analytical expression is provided for the energy-release rate, which is based both on numerical studies and on a specially adapted beam model. Comparisons of this easy-to-use estimate of the energy-release rate with established results from the literature as well as detailed numerical results are presented. The provided estimate of the energy-release rate can easily be extended to non-equidistant cracks and an anisotropic material.     

Flow structures and heat transfer characteristics in arrays of submerged laminar impinging jets

This study summarizes the scaling behavior of single laminar submerged jets with circular and planar cross sections. Unified correlations for the stagnation zone heat transfer of both configurations, based on the dominant dimensionless numbers, are presented. In technical applications, impinging jets are often applied in jet array configurations. Compared to single jet impingement, jet-to-jet interaction can have a substantial influence on local heat transfer. A distinct pattern of the heat transfer coefficient was observed experimentally. Numerical simulations revealed two counter-rotating vortices in the interaction zone between two jets to be the causing mechanism of this pattern. (© 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)