Forced ignition in turbulent mixtures of sufficiently large Lewis numbers

点击翻译按钮获取中文摘要

The forced ignition of a turbulent premixed mixture is of relevance in many devices used in practice such as spark ignition engines/gas turbines, and in natural phenomena such as fires and explosions. It is therefore indispensable to understand the dynamics of ignition kernel evolution (IKE) in turbulent flows to improve the design of technical devices and to reliably control fires and explosions.A controlled forced ignition (or spark ignition) process consists of four phases: ignition kernel formation, ignition kernel growth, transition between ignition kernel and sustained flame kernel, and flame kernel development and flame stabilization. For the forced ignition of premixed reactants, the classic conclusion of turbulence effects on IKE is that turbulence renders ignition more difficult due to the turbulence-induced Kernel Dissipation (KD), which, however, neglects the effects of turbulence on the stretch of the ignition/flame kernel (Kernel Stretch, KS) during the growth and transition phases. However, facilitated ignition by turbulence for Le>1 was recently discovered in experiments. It was hypothesized that the turbulence-induced KS effect created flames with a wide range of positive and negative stretch rates. Negatively stretched flames with Le>1 are strengthened by differential diffusion, and this effect can potentially facilitate ignition. However, this important finding has not yet been analyzed using corresponding turbulent simulations that could reveal the detailed mechanism of turbulence effects on IKE when Le>1. Thus, understanding turbulence/kernel interaction in the forced ignition of turbulent premixed mixtures with Le>1 is an unresolved scientific issue. Closing this gap is the main objective of the project proposed here.The project is a close collaboration between TU Darmstadt (TUD) and Peking University (PKU). At PKU, Direct Numerical Simulations (DNSs) with detailed chemistry of laminar and turbulent forced ignition cases are performed and analyzed to gain a detailed understanding of IKE. The DNS database is used by TUD to develop, adapt and validate an advanced premixed flamelet model comprehensively accounting for negative/positive stretch, curvature, and preferential diffusion. Using this model, multidimensional flamelet tables will be created using at least progress variable, strain and curvature as parameters. The tables will be coupled with the flow solvers for simulations of turbulent forced ignition cases at both institutions: the results of Flamelet-DNS (PKU) and Flamelet-LES (TUD) will be compared to the DNS data with detailed chemistry.In summary, the project aims to contribute to the understanding of IKE in turbulent flows and to provide efficient modeling approaches for the design of next-generation high-efficiency, low-emission combustors.