Cavity-Stabilized Rotating Flames in a Circular Hele-Shaw Burner

TL;DR

This study experimentally investigates self-organized rotating premixed flames in a circular Hele-Shaw burner, revealing how flow rate and fuel type influence flame patterns, stability, and transition to steady flames, with implications for micro-combustor design.

Contribution

It provides the first detailed experimental characterization of rotating flames in a Hele-Shaw burner, including their formation, dynamics, and transition boundaries across various conditions.

Findings

Rotating flames form spontaneously at low flow rates and exhibit stable traveling-wave patterns.

Number of flame heads and rotation frequency increase with flow rate until transition to steady flames.

Critical flow rate for flame transition is insensitive to equivalence ratio, gap distance, and fuel type.

Abstract

We report direct experimental observations of self-organized rotating flames of premixed CH4 and air in an open circular Hele-Shaw burner equipped with an annulus cavity flame holder. These flames formed spontaneously at sufficiently low flow rates, where flame flashback was counteracted by thermal quenching, resulting in a dynamic balance between the local flame speed and flow velocity. Unlike flames propagating in closed micro-channels, these flames exhibited stable traveling-wave patterns with heads gliding along the leading edge of the cavity, where rapid expansion created a low-speed zone that facilitated flame stabilization. At low flow rates, the rotating flames were single-headed, with their rotation frequencies roughly proportional to the laminar flame speeds, suggesting that the flame fronts traveled in a nearly constant-shape fashion. As the flow rate increased, the rotating flames split into multiple heads at approximately equal spacing, and the number of heads and rotation frequency increased with the flow rate, until these rotating flames transitioned into steady ring-shaped flames anchored at the cavity leading edge. Blow-off or extinction occurred at sufficiently high flow rates, where the flame front was pushed out of the rear side of the cavity. Parametric measurements were conducted over a wide range of equivalence ratios and flow rates, from which a regime diagram of different flame modes and their transition boundaries was obtained. Additional experiments were conducted on C3H8 and DME. It was found that the critical total mass flow rate at the rotating-steady flame transition boundary is insensitive to equivalence ratio, gap distance, and fuel type. These results should be useful not only for the fundamental understanding of flame dynamics in micro-channels but also for the practical design of micro-combustors and the application of micro-combustion technologies.