Entropy Generation Rate as a Marker for the Onset of Flame Instability

Abstract

A comprehensive understanding of the mechanisms of intrinsic flame instability, including hydrodynamic and thermodiffusive instabilities, is becoming more important with the move towards greater reliance on hydrogen as a zero-carbon fuel. While intrinsic flame instabilities have been studied extensively both numerically and experimentally, certain important features, including their onset, have been defined mainly by qualitative measures. This work proposes a quantitative marker to identify the onset of intrinsic flame instabilities derived from the statistics of the entropy equation. Direct numerical simulations were carried out for two-dimensional laminar premixed planar methane-air flames, with varying amounts of hydrogen addition up to 100% by volume. Entropy generation mechanisms were analysed based on contributions resulting from heat conduction, viscous dissipation, mass diffusion, and chemical reaction. Instability onset was shown to be characterised by increased data dispersion in all entropy generation terms. The dispersion was quantified by the statistical range, which increased for all locations within the flame as the flame transitioned into instability. Increasing hydrogen addition resulted in a delayed instability onset attributed to the decreasing hydrodynamic instability growth rate. The entropy generation rate due to viscous dissipation was found to be smaller in magnitude compared to other mechanisms, but it was found to be the most sensitive indicator of instability onset. This quantity is readily computed using data from numerical simulations and can be estimated from experimental data, suggesting its potential use as a marker of intrinsic flame instability.