Turbulent flame speed of thermodiffusively unstable flames: experimental investigation and scaling

TL;DR

This study experimentally investigates how turbulence and thermal-diffusive instability influence the turbulent flame speed of lean hydrogen-enriched methane-air flames, proposing a parameterization to predict these effects across different turbulence levels.

Contribution

It provides new experimental data and a novel parameterization model that captures the combined effects of turbulence and TD instability on flame propagation.

Findings

Turbulence enhances flame propagation in thermodiffusively unstable flames.

Flame sensitivity to turbulence increases below a transitional Lewis number.

A parameterization based on Karlovitz and Lewis numbers fits experimental data well.

Abstract

This work presents an experimental set of Bunsen flames characterized by a moderate Reynolds number and a variable turbulence intensity. Ten lean hydrogen-enriched methane-air mixtures at three levels of turbulence are investigated, ranging from pure methane-air to pure hydrogen-air. Such mixtures are selected in order to have an almost constant laminar flame speed while inducing the onset of thermal-diffusive (TD) instability by gradually increasing the hydrogen content of the blend. The flames are analyzed in terms of global consumption speed, stretch factor, and flame surface area. Results indicate an interplay between TD instability and turbulence that enhances the overall flame propagation. In particular, below a transitional Lewis number, flame propagation is observed to be particularly sensitive to external turbulent forcing, expressed in terms of the Karlovitz number. A parameterization is thus proposed, based on a functional form depending on both Karlovitz and Lewis numbers, able to fit the experimental results at different turbulence levels and capture the steep transition across the transitional Lewis number.