Modeling turbulent transport effects on kernel formation and ?ame propagation in an ignition process

Reliable relighting is crucial for advanced low-emission aero-engine combustors. For forced ignition under highly-turbulent conditions, eddies of sizes being smaller than the flame kernel can affect its formation through penetrating and modifying the kernel structure. In this study, a one-dimensional model for small-scale turbulence on kernel formation is developed through the incorporation of turbulence-induced diffusion in governing equations. With given flow conditions,the spatial and time-dependent turbulent diffusivity is modeled using the idea of residual eddy viscosity. One-dimensional spherical flames of premixed pre-vaporized n-dodecane/air mixtures under high-altitude conditions are simulated to investigate the effects of turbulence. It is revealed that the range of the equivalence ratio for successful turbulent ignition is much narrower than that for laminar ignition. The range decreases with an increased turbulent intensity, and this effect is more pronounced for a low spark energy. In addition, turbulent transport has more pronounced effects on rich mixtures. An analysis on energy budget and species profiles shows that turbulence-induced diffusion enhances the heat loss at the very beginning of the kernel formation process that could lead to ignition failure. After the flame kernel is established, turbulent transport broadens the flame front, enhances the heat release rate, and thereafter increases the flame propagation speed.

Advances in Research on the ITCZ:Mean Position,Model Bias,and Anthropogenic Aerosol Influences

The zonal-mean position of the intertropical convergence zone(ITCZ)and its shift in the meridional direction significantly influence both the tropical and even global climate.This work reviews three aspects of the progress in ITCZ-relevant research:1)the mechanism behind the asymmetry of the ITCZ annual-and zonal-mean positions relative to the equator;2)causes of the double-ITCZ problem(pervasive in climate models)and the efforts to solve it;and 3)the physical mechanisms by which anthropogenic aerosols affect the location of the zonal-mean ITCZ.According to recent studies,the north-of-the-equator location of the annual-and zonal-mean ITCZ is mainly driven by the cross-equatorial energy transports in the ocean,induced by the Atlantic overturning circulation.A quantitative relationship between the ITCZ shift and the anomalous cross-equatorial energy transport in the atmosphere has been found.Presently,the double-ITCZ problem is still the most common and pronounced bias in tropical precipitation simulations with climate models.Recently,some studies have found that simply correcting the biases in hemispheric energy contrast does not improve the simulation of the ITCZ with climate models;whereas others have found that improving model resolutions and convective parameterizations in climate models,such as entrainment rate,raindroplet re-evaporation,and convection triggering function,can alleviate the double-ITCZ bias.Therefore,it seems that the double-ITCZ problem in climate models is rooted in the complex physics of the models,which is not yet well-understood.In addition,anthropogenic aerosols are suggested to be able to induce meridional shifts of the ITCZ,but through various physical mechanisms.Absorbing aerosols like black carbon influence the ITCZ position basically via instantaneous absorption of shortwave radiation in the atmosphere,whereas scattering aerosols like sulfate affect the location of the ITCZ through the cloud lifetime effect and the subsequent response of surface evaporation.