Observed dependence of characteristics of liquid-pool fires on swirl magnitude

TL;DR

This study investigates how varying swirl levels, induced by vane angles, affect the structure, stability, and burning rates of liquid-pool fires, revealing a progression from puffing to whirl and blue whirl phenomena.

Contribution

It provides detailed experimental insights into how different swirl magnitudes alter fire behavior and structure, including the formation of fire whirls and blue whirls, which was not previously well understood.

Findings

Moderate swirl suppresses puffing and creates tall fire whirls.

Higher swirl levels lead to vortex breakdown and recirculation regions.

Increasing swirl initially increases burning rates, then decreases them due to reduced liquid surface area.

Abstract

One dozen vertically oriented thin rectangular vanes, 62 cm tall and 15.2 cm wide, were placed 27 cm from the center of heptane and ethanol pool fires in continuously fed, floor-flush pans 3.2 cm and 5.1 cm in diameter in the laboratory. The vanes were all oriented at the same fixed angles from the radial direction, for 9 different angles, ranging from 0 to 85 degrees, thereby imparting 9 different levels of circulation to the air entrained by each pool fire. The different swirl levels were observed to engender dramatically different pool-fire structures. Moderate swirl suppresses the global puffing instability, replacing it by a global helical instability that generates a tall fire whirl, the height of which increases with increasing circulation. Except for the largest heptane pool, higher swirl levels produced vortex breakdown, resulting in the emergence of a bubble-like recirculation region with a ring vortex encircling the axis. Measured burning rates increase with increasing swirl levels as a consequence of the associated increasing inflow velocities reducing the thickness of the boundary layer within which combustion occurs right above the liquid surface, eventually forming detached edge flames in the boundary layer that move closer to the axis as the circulation is increased. Still higher circulation reduces the burning rate by decreasing the surface area of the liquid covered by the flame, thereby reducing the height of the fire whirl. Even higher circulation causes edge-flame detachment, resulting in formation of the blue whirl identified in recent literature, often meandering over the surface of the liquid in the present experiments. This sequence of events is documented herein.