Quantifying the effects of thermodiffusive instabilities on flame surface wrinkling under engine conditions using an effective chemical time-scale

Combustion and Flame, vol. 289, pp. 114996

Abstract

In lean premixed hydrogen flames, thermodiffusive instabilities (TDIs) significantly alter flame structure and surface wrinkling. TDIs notably increase flame propagation speed and reduce flame thickness compared to corresponding unstretched laminar flames, thereby significantly reducing the effective chemical time-scale. The ratio between chemical and turbulent time-scales, expressed by the Damköhler and Karlovitz numbers, strongly influences turbulence-flame interactions and flame surface wrinkling. This study investigates whether TDI effects on flame surface wrinkling can be characterized using an effective chemical time-scale, which is of significant importance for modeling hydrogen flames with TDIs. Both direct numerical simulations (DNS) and experimental data are considered. In DNS, flame kernels are simulated with and without TDIs by applying different species transport assumptions. Experimentally, laser-induced fluorescence (LIF) imaging is carried out in an optically accessible internal combustion engine fueled with hydrogen and methane to represent flames with and without TDIs, respectively. It is found that flames with and without TDIs, exhibit nearly identical flame surface wrinkling as long as properly defined effective Damköhler and Karlovitz numbers are the same. Notably, flame surface area dynamics, particularly the flame surface generation and annihilation by propagation, show close agreement across cases with the same effective Damköhler and Karlovitz numbers. This is attributed to the similar correlation between local flame curvature and displacement speed. As part of the analysis, the ability of 2D-based image processing methods in experiments to represent three-dimensional flame surface wrinkling was assessed. To this end, planar slices from the DNS data were evaluated with a flame surface density method suitable for 2D experimental data. It is found that the 2D-based analysis for both numerical and experimental data corroborates the findings of the 3D-based flame surface wrinkling analysis of the DNS data. Novelty and significance statement The effects of thermodiffusive instabilities (TDIs) in hydrogen flames have been widely investigated, primarily by comparing flames with and without TDIs at the same Damköhler and Karlovitz numbers, which are defined using the unstretched laminar flame and turbulence properties. A key open question in modeling premixed turbulent hydrogen flames is whether flame surface wrinkling influenced by TDIs can be characterized by an effective chemical time-scale that accounts for TDI-driven flame thinning and acceleration. This study is the first to comprehensively address this question under hydrogen engine conditions, using both direct numerical simulations and optical engine experiments. The significance of this work lies in providing essential guidance for incorporating TDI effects into predictive combustion models for hydrogen-fueled engines, such as the G -equation approach. The results indicate that no additional model modifications are required to account for flame surface wrinkling when an effective chemical time-scale is employed.

Authors 5

1. Shibam Bose Aachen Institute for Combustion Technology

   RWTH Aachen University

   Affiliation as printed

   Institute for Combustion Technology, RWTH Aachen University, 52056 Aachen, Germany

2. Pedro Ye

   Technische Universität Darmstadt

   Affiliation as printed

   Reactive Flows and Diagnostics, Technical University of Darmstadt, 64287 Darmstadt, Germany

3. Hongchao Chu corresponding Aachen Institute for Combustion Technology

   RWTH Aachen University

   Affiliation as printed

   Institute for Combustion Technology, RWTH Aachen University, 52056 Aachen, Germany

4. Benjamin Böhm

   Technische Universität Darmstadt

   Affiliation as printed

   Reactive Flows and Diagnostics, Technical University of Darmstadt, 64287 Darmstadt, Germany

5. Heinz G. Pitsch Aachen Institute for Combustion Technology

   RWTH Aachen University

   Affiliation as printed

   Institute for Combustion Technology, RWTH Aachen University, 52056 Aachen, Germany