Premixed flame dynamics in presence of mist

ABSTRACT

The injection of a water spray within an enclosure prone to explosion is reputed to reduce the risk. This strategy for safety improvement is at the root of numerous experiments that have concluded that premixed flame can be extinguished by a sufficient amount of a water aerosol characterized by suitable droplet sizes. On the other hand, certain experiments seemingly indicate that flame speed promotion can be observed when particular water mists are injected within the pre-mixture.

To contribute to shed light upon these less than intuitive observations, we propose to study the propagation of a nearly stoichiometric premixed flame within a 2D-lattice of water droplets. Main parameters of investigation are droplet size and droplet inter-distance (or equivalently, lattice spacing). When the droplet inter-distance is small, the results confirm that a sufficient amount of water quenches combustion. For larger droplet inter-distance, we observe a flame speed enhancement for suitable droplet size. Concomitantly, the flame front folds subjected to Darrieus–Landau (DL) instability. The final discussion, which invokes a Sivashinsky-type model equation for DL instability, interprets such a speed promotion in presence of mist as a secondary nonlinear enhancement of the flame surface.

KEYWORDS:

• Water-Sprayed Flame
• Misted Flame
• Flame Mitigation
• Premixed Flame Speed-Up
• Darrieus–Landau Instability

Acknowledgments

The present work has received the support of the Research Program “Micropesanteur Fondamentale et Appliquée”, GDR CNRS n2799, under contract CNES/170048.