Flame-wall interaction of thermodiffusively unstable hydrogen/air flames, Part I: Characterization of governing physical phenomena

Hydrogen combustion systems operated under fuel-lean conditions offer great potential for low emissions. However, these operating conditions are also susceptible to intrinsic flame instabilities. Even though technical combustors are enclosed by walls that significantly influence the combustion process, intrinsic flame instabilities have mostly been investigated in canonical freely-propagating flame configurations unconfined by walls. This study aims to close this gap by investigating the flame-wall interaction of thermodiffusive unstable hydrogen/air flame through detailed numerical simulations in a two-dimensional head-on quenching configuration. It presents an in-depth qualitative and quantitative analysis of the quenching process, revealing the major impact factors of the instabilities on the quenching characteristics. The thermodiffusive instabilities result in lower quenching distances and increased wall heat fluxes compared to one-dimensional head-on quenching flames under similar operation conditions. The change in quenching characteristics is shown not to be driven by kinematic effects. Instead, the increased wall heat fluxes are caused by the enhanced flame reactivity of the unstable flame approaching the wall, which results from mixture variations associated with the instabilities. Overall, the study highlights the importance of studying flame-wall interaction in more complex domains than simple one-dimensional configurations, where such instabilities are inherently suppressed. Further, it emphasizes the need to incorporate local mixture variations induced by intrinsic combustion instabilities in combustion models for flame-wall interactions. In part II of this study, the scope is expanded to gas turbine and internal combustion engine relevant conditions through a parametric study, varying the equivalence ratio, pressure, and unburnt temperature.

Novelty and Significance Statement

This work presents novel simulations and in-depth analysis of flame-wall interaction (head-on quenching) of laminar thermodiffusively unstable hydrogen/air flames. Thermodiffusive instabilities are significant in technical combustion chambers enclosed by walls, such as gas turbines and internal combustion engines, particularly under lean conditions, where they raise safety concerns like flame flashback and increased thermal loads on walls. The study shows that these instabilities strongly affect flame-wall interaction, leading to smaller quenching distances and higher wall heat fluxes than in one-dimensional head-on quenching. Consequently, this work demonstrates that for flames susceptible to instabilities, such as lean hydrogen/air flames, one-dimensional head-on quenching simulations are inadequate for accurately determining wall heat fluxes and quenching distances. Additionally, this study highlights that differential diffusion effects induced by the intrinsic instabilities must be considered in combustion models, requiring full resolution of the local mixture distribution.