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

TL;DR

This paper investigates how thermodiffusive instabilities in hydrogen/air flames influence flame-wall interactions, revealing that instabilities lead to lower quenching distances and higher heat fluxes, emphasizing the need to include these effects in combustion models.

Contribution

It provides the first detailed numerical analysis of flame-wall interaction in unstable hydrogen/air flames, highlighting the impact of instabilities on quenching behavior and heat transfer.

Findings

Unstable flames have lower quenching distances.

Instabilities increase wall heat fluxes.

Changes are due to mixture variations, not kinematic effects.

Abstract

Hydrogen combustion systems operated under fuel-lean conditions offer great potential for low emissions. However, these operating conditions are also susceptible to intrinsic thermodiffusive combustion 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 seems 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.