Effect of vorticity flip-over on the premixed flame structure: First experimental observation of type I inflection flames

TL;DR

This paper presents the first experimental observation of type I inflection flames, demonstrating that vorticity dynamics can cause flame front concavity contrary to buoyancy explanations, with experiments confirming theoretical predictions.

Contribution

First experimental realization of vorticity flip-over regime in premixed flames, validating recent theoretical predictions about flame front inflection due to vorticity balance.

Findings

Flames with inflection points are produced in lean mixtures with weak ignition.

Observed flame shapes match theoretical predictions.

Vorticity effects dominate buoyancy in shaping flame fronts.

Abstract

Premixed flames propagating in horizontal tubes are observed to take on shape convex towards the fresh mixture, which is commonly explained as a buoyancy effect. A recent rigorous analysis has shown, on the contrary, that this process is driven by the balance of vorticity generated by a curved flame front with the baroclinic vorticity, and predicted existence of a regime in which the leading edge of the flame front is concave. We report first experimental realization of this regime. Our experiments on ethane and n-butane mixtures with air show that flames with an inflection point on the front are regularly produced in lean mixtures, provided that a sufficiently weak ignition is used. The observed flame shape perfectly agrees with the theoretically predicted.